CN113875316B

CN113875316B - Method and system for timing electron beam injection in a multi-energy X-ray cargo inspection system

Info

Publication number: CN113875316B
Application number: CN201980096970.7A
Authority: CN
Inventors: 亚历山大·萨维尔斯基
Original assignee: American Science and Engineering Inc
Current assignee: American Science and Engineering Inc
Priority date: 2019-05-31
Filing date: 2019-08-28
Publication date: 2024-02-20
Anticipated expiration: 2039-08-28
Also published as: WO2020242511A1; CN113875316A

Abstract

Embodiments of the disclosed systems and methods for generating multi-energy X-ray pulses are provided. An electron beam is generated with an electron gun and modulated prior to injection into the acceleration structure to achieve first and second specified beam current amplitudes at least during corresponding beam current time profiles. The radio frequency field is applied to the accelerating structure at a specified RF field amplitude and a specified RF time profile. The first and second specified beam current amplitudes are injected consecutively in such a way that each after a specified delay, electrons of two different energies are achieved that are accelerated within the acceleration structure at least during the course of a single RF pulse. The electron beam is accelerated by a radio frequency field within the acceleration structure to produce accelerated electrons that strike the target for the generation of bremsstrahlung X-rays.

Description

Method and system for timing electron beam injection in a multi-energy X-ray cargo inspection system

Cross reference

The present application relies on the priority of U.S. patent provisional application No. 62/855,713 entitled "Method and System for Timing the Injections of Electron Beams in a Multi-Energy X-Ray Cargo Inspection System" filed on 5.31 of 2019.

The present application is a partial continuation-in-line application entitled "Source for Intra-Pulse Multi-Energy X-Ray Cargo Inspection" filed on day 6, 14, 2019, which is a continuation-in-line application entitled "Source for Intra-Pulse Multi-Energy X-10,368,428" filed on day 12, 5, 2019, 30, which in turn is a continuation-in-line application entitled "Source for Intra-Pulse Multi-Energy X-Ray Cargo Inspection", which is in turn a continuation-in-line application entitled "U.S. patent No. 9,867,271 filed on day 10, 28, 2016, and filed on day 1, 9, 2018, which is in turn a chapter 35 section national phase entry of PCT/US15/30716 filed on day 5, chapter 35 (c), which in turn relies on a co-named and provisional application entitled" U.S. patent application No. 61/994,484, filed on day 5, 16, 2014.

Technical Field

The present invention relates generally to systems and methods for inspecting cargo using penetrating radiation, and more particularly to systems and methods for material discrimination based on varying the energy and flux of incident radiation during the course of a single pulse.

Background

The differentiation of materials has become a standard requirement for security inspection systems. Inspection systems for cargo and container screening typically employ electron accelerators capable of interleaved dual energy operation and are capable of differentiating intervening materials of different atomic numbers using differential transmission of X-rays characterized by different energy spectra. As used herein, the term "interleaving energy" means the use of a stream of X-ray pulses, wherein successive pulses are characterized by different energy spectra. Applications for material-differentiated interlaced energy inspection are known, such as processing techniques investigated by Ogorodnikov et al, processing of interlaced images in 4-10MeV dual-energy customization systems for material identification, phys. Rev. Specialty cosmetics-Accelerators and Beams volume 5 104701 (2002), and references cited therein, are incorporated herein by reference in their entirety. Bremsstrahlung spectrum is characterized by its endpoint energy, defined by the energy of electrons striking an X-ray target to generate X-rays. The input data for identifying the type of material being inspected is provided by attenuating the radiation-transmissive inspection object by two (or in some cases, a plurality of) different energies.

Various techniques are known for generating X-rays of interleaved energy based on an electron accelerator, such as, for example, described in U.S. patent No. 7,646,851 entitled "Device and Method for generating X-Rays Having Different Energy Levels and Material Discrimination System" and U.S. patent No. 8,604,723 entitled "interleaved multi-energy radiation sources", both of which are incorporated herein by reference. Because they are not relevant to the present description, the technique of interleaving energy radiation is not discussed further herein, except to highlight its drawbacks for cargo inspection purposes.

When using sources of interlaced X-ray energy, the material discrimination function comes with several limitations. Using two pulses separated in time to generate one inspection data point effectively reduces inspection speed. Moreover, while the basic assumption of the dual energy technique is that the same region of cargo is detected by both energies, it must be kept in mind that the cargo and probe are typically in relative motion. Thus, for slowly moving cargo, the interweaving energy method is the only viable.

X-ray security inspection systems for inspecting cargo and shipping containers typically use transmission radiography techniques. Fig. 1 depicts a cargo inspection system employing this technique. The fan beam 12 of penetrating radiation emitted by the source 14, detected by the elements 18 of the detector array 16 located at the distal end of the target object (here the truck 10), is used to produce an image of the target object. The detector elements 18 generate corresponding detector signals which are processed by the processor 19 to provide information about the material composition of the cargo and an image of its spatial distribution. In some cases, the thickness of the material through which the X-rays penetrate may exceed 300mm steel equivalent. To ensure the required penetration, inspection systems typically use X-rays with a maximum energy of several MeV, currently up to about 9MeV. X-rays exceeding 1MeV are generally referred to as hard X-rays or high energy X-rays.

Based on the interaction of X-rays with a material, and more specifically, by irradiating the material with an X-ray beam having an energy spectrum with more than one distinct energy end point (peak energy), or by employing an energy discriminating detector, information about the material composition of the contents of the object (such as mass absorption coefficient, effective atomic number Z _eff Electron density, or spatial distribution of any of the above, etc.). The material-differentiated dual-energy approach is widely used in X-ray inspection systems for security control of carry-on suitcases in customs and other security checkpoints.

The dual (and often more, multiple) energy approach has been extended to high energy inspection systems for cargo containers where material discrimination is less effective due to the weaker Z-dependence of the dominant interactions.

In performing a dual energy inspection, X-ray transmission data about the inspected object for both energies is acquired and processed by a computer, wherein the resulting image is displayed on a monitor, typically in a special palette that facilitates visual identification of contraband or hazardous materials. More specifically, special computer software can identify various materials and can assign artificial colors to Z _eff Is a function of the value of each of the values of (a).

Due to photoelectric interactions (from cross section Z ⁴ -Z ⁵ Characterization) is generally at lower energies, the energy range for inspecting smaller objects is typically below 0.5MeV, taking into account the strong Z dependence of the X-ray attenuation coefficient. However, in the range of 1MeV to 10MeV, X-ray interactions are well-establishedThe bowden effect dominates and its attenuation coefficient (mass absorption) is weakly dependent on atomic number: mu (mu) _c Z/a (approximately constant and equal to 0.5), where Z represents an atomic number and a represents an atomic mass, i.e. the mass absorption coefficient is mainly Z insensitive in the energy range dominated by compton scattering.

As described in detail in U.S. patent No. 8,457,274 ("Arodzero' 274") issued on month 6 and 4 of 2013, the contents of which are incorporated herein by reference, the preferred method for material discrimination is such that the pulse energy is varied during the course of each individual pulse.

Lejow szilward conceived a linear accelerator (linac) in 1928 when he was also a professor of the university of berlin. From RolfThe linac was then independently constructed by the engineering institute of Walter Rogowski of adam (Aachen) at about the same time. The electrons accelerated by the linear accelerator were first used to generate X-rays in the mid-1950 s.

Some prior art methods for varying the emission energy during the pulsing process require x-ray flux tracking of the end point energy. For example, the Arodzero '274 patent describes "Concurrently with the sweeping of the endpoint energy, the X-ray flux may increase from a minimum to a maximum" (Arodzero' 274, column 6, lines 47-48).

U.S. published patent application 2014/0270086 (to Krasnykh), incorporated herein by reference, describes an intra-pulse multi-energy method using a traveling wave accelerator structure. It suggests using feedback of the electron gun grid voltage to compensate for X-ray flux variations during the pulsing process. Krasnykh et al, which contemplates RF Linac, SLAC Pub-15943 (day 18, 4, 2014) for multi-energy scanning within pulses, provide further description and are also incorporated herein by reference. However, for example, even if the operation is highly advantageous in a cargo inspection environment, the prior art modes of operation cannot accommodate individual customization of the flux and endpoint energy of the X-ray pulses.

Inspection speedOne limiting factor in the degree is the RF power available for acceleration. The maximum Pulse Repetition Frequency (PRF) that a linac-based X-ray source can provide is limited by the RF source. The RF source (typically a magnetron or klystron) is at maximum average P _av,max And pulse P _p,max The power aspect is limited. These two parameters define a maximum duty cycle d _max It can also be used in PRF (f) and pulse duration t _p The expression is carried out in aspects:

for example, where the selection is made by P _p，max A characterized Single Energy (SE) (non-interleaved) accelerator to generate High Energy (HE) pulses, and t _p Approximately 3.3 mus and d _max Approximately 0.001, the maximum PRF is limited to f _H ≈300Hz(pps)。

For dual energy interleaved linac, the maximum available frequency can be estimated from the equation:

wherein P is _H And P _L Representing the RF power required to generate High Energy (HE) and Low Energy (LE) pulses, respectively. If it is assumed t _p Remains the same for both energies, and P _H ＝P _p，max Then for P _L ＝P _H (e.g., by RF switching/tuning, manipulation of beam loading, and phase shifting of the accelerating field to achieve that the RF power of the two pulses remains constant),in other words, a dual energy repetition rate of at most half the single energy repetition rate can be achieved. On the other hand, if the low-energy pulse generates only half the power of the high-energy pulse, < >>(e.g., using RF generator power modulation is possible)Realizations), then->That is, 2/3 of the single energy pulse rate may be achieved based on the interleaving energy.

In practical use, at the same time (t _b =0) turn on the RF power and implant beam. The results of this practical application are shown in dashed curve 30 drawn in fig. 3, representing the beam energy W versus time t for a 6-MeV acceleration structure designed for safety applications. Fill time, i.e. the electric field in the accelerator structure decays to its initial value e ^-1 The time taken is t _f,95％ ≈1μs。

In comparison to standing and travelling wave structures, SLAC linac conference, SLAC-PUB-3935, pp.216-21 (1986) (hereinafter "Miller (1968)"), the contents of which are incorporated herein by reference, roger Miller describes a well-known solution for reducing fill time. The Miller scheme allows beam pulses with constant energy to be created for the pulse duration. To attenuate t _b Turning on the acceleration beam, i.e., is defined as:

β is the coupling coefficient between the RF power feed waveguide 222 (shown in fig. 2) and the accelerating structure 22 (shown in fig. 2, also referred to herein as the "accelerating cavity structure"), r is the shunt impedance of the accelerating structure 22, L is the length of the accelerating structure 22, and P is the power dissipated in the accelerating structure 22, and τ is the decay time constant of the accelerating structure 22. Strictly speaking (whereby the numerator and denominator of the logarithm are in voltage.) β is defined as the ratio of the power lost outside the accelerating cavity structure 22 (i.e., in the feed waveguide 222) to the power dissipated inside the accelerating cavity structure 22. When beam 220 is on, if β=β is adjusted ₀ So that there is no RF power reflection from the accelerating structure 22, the above equation can be calculated as:

Wherein beta is ₀ Is the optimal coupling coefficient

And, as above, τ is the decay time constant of the acceleration structure 22.

It is known to those of ordinary skill in the art that the coupling coefficient (β) of the accelerating structure 220 (also referred to as an "accelerating resonator," or "RF accelerating structure") to external circuitry (feed waveguide 222) depends on the current accelerated (and interacted) in the resonator 220. In general, the presence of current causes a reduction in the coupling coefficient measured by VSWR (voltage standing wave ratio) and the phase of the reflected signal from resonator 220. Initially (no current), resonator 220 needs to be over-coupled and has a coupling coefficient greater than β=1. Optimal coupling coefficient beta ₀ Is a value that allows matching resonator 220 with external waveguide 222 at accelerating current I. When the coupling coefficient beta is equal to beta ₀ The coupling is referred to herein as "optimal". By reference to Sobenin et al, the electrodynamic properties of the accelerating cavity (Electrodynamic Characteristics of Accelerating Cavities) (transliteration), CRC Press, especially page 121 (equation 4.49), (1999), collin, microwave engineering foundation (Foundations for Microwave Engineering), mcGraw-Hill (first edition, 1992), and the analytical formula for the coupling coefficient β of the Gao. Cavity waveguide coupling System (Analytical formula for the coupling coefficient β of a cavity waveguide coupling system), physics research A, volume 309, pages 5-10 (1991), the best β can be found ₀ Is incorporated by reference in its entirety.

In implementing the design of the acceleration system, the resonator 220 must be over-coupled (coupling coefficient beta) for all values of the acceleration current (I) and the RF power (P) in view of operation _c >1, beta _c ≥β ₀ ). Otherwise, the operation of the acceleration structure 22 becomes unstable. In some cases, a waveguide is providedNear optimal coupling is calculated but there are cases where waveguides are designed for a wide range of applications and there is significant over-coupling when operating in "low current" applications.

The embodiments in this specification provide improvements to conventional interleaving systems for material discrimination using dynamic dose control as employed. U.S. patent No. 8,054,937, entitled "Systems and methods for using an intensity-modulated X-ray source," assigned to the applicant and incorporated herein by reference, describes an embodiment of a conventional system utilizing dynamic dose controlled material differentiation.

In general, material discrimination is achieved by applying a dual/multiple energy approach, using interleaved pulses of different energies, or by forming dual/multiple energy structures within a single microwave pulse. Typically, dynamic dose control is achieved by varying the pulse duration of each spectral component individually. There is a need for successful operation of such a system (with material discrimination and dynamic dose control) where the energy spectrum of each energy component must be constant, while the dose changes due to variations in pulse duration within a range of defined values (from minimum to maximum).

U.S. patent No. 9,867,271, assigned to the applicant and related to the present specification, entitled "Source for intra-pulse multi-energy X-ray cargo inspection", describes a method of forming a beam that satisfies these conditions by injecting the beam with a specified time delay. One key assumption of the method is to match the system that is optimally coupled with the low energy/first (highest) beam current. Methods for material discrimination are provided based on variations in energy and flux of incident radiation during the course of a single pulse. An electron beam is generated and modulated with an electron gun prior to injection into the acceleration structure to achieve at least a first and a specified beam current amplitude during a corresponding beam current time profile. The radio frequency field is applied to the accelerating structure at a specified RF field amplitude and a specified RF time profile. The first and second specified beam current amplitudes are injected consecutively in such a way that at least two different end point energies of electrons accelerated within the acceleration structure are each achieved after a specified delay, i.e. during the course of a single RF pulse. The electron beam is accelerated by a radio frequency field within the acceleration structure to produce accelerated electrons that strike the target to generate bremsstrahlung X-rays. There is a further need to address the application aspects of forming X-ray beams that can support both material discrimination and dynamic dose control functions.

Disclosure of Invention

The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools, and methods, which are meant to be exemplary and illustrative, and not limiting in scope. Various embodiments are disclosed.

In an embodiment, the present specification discloses a method for generating multi-energy X-ray pulses using an acceleration structure comprising a standing wave resonator, the method comprising: generating an electron beam using an electron gun; modulating the electron beam prior to injection into the acceleration structure, wherein modulating the electron beam produces at least 1) a first beam current amplitude and a first beam current time profile and 2) a second beam current amplitude and a second beam current time profile, and wherein the electron beam is characterized by an electron beam pulse duration; applying a radio frequency field to the accelerating structure, wherein the radio frequency field is defined by an RF field amplitude, a specified time profile, and an RF pulse duration; determining a time profile of the amount of power reflected from the acceleration structure; determining a first delay and a second delay, wherein each of the first delay and the second delay is determined based at least in part on a time profile of an amount of power reflected from the acceleration structure; injecting the modulated electron beam at a first beam current amplitude after a first delay to achieve a first sub-pulse, and then, after a second delay, injecting the modulated electron beam at a second beam current amplitude to achieve a second sub-pulse, wherein the injecting is configured to achieve a first steady state energy level of the first sub-pulse and a second steady state energy level of the second sub-pulse, and wherein the first steady state energy level is different from the second steady state energy level; accelerating the modulated electron beam with a radio frequency field within the accelerating structure to produce accelerated electrons; and causing the accelerated electrons to strike the target to generate X-rays by bremsstrahlung.

Optionally, the method further comprises: the coupling coefficient of the acceleration structure is optimized, wherein the coupling coefficient is optimized to be larger than the critical coupling of any beam current.

Alternatively, the modulated electron beam is injected with a first beam current amplitude and then with a second beam current amplitude within a single RF pulse. Optionally, the injection of the modulated electron beam is performed at a time based at least in part on achieving a minimized deviation of the energy level from the first steady state energy level during the first sub-pulse and a minimized deviation of the energy level from the second steady state energy level during the second sub-pulse.

Optionally, the duration of the first sub-pulse is different from the duration of the second sub-pulse. Optionally, at least one of the duration of the first sub-pulse or the duration of the second sub-pulse is variable, wherein the minimum duration of any sub-pulse may be zero, and wherein the maximum duration of the first sub-pulse or the maximum duration of the second sub-pulse is defined by a function of the RF pulse duration, the time of injecting the modulated electron beam, and the duration of one or more other sub-pulses.

Optionally, the injection of the modulated electron beam is performed at a time based at least in part on a minimized deviation of the power amplitude reflected from the accelerating structure from a first steady state level of the reflected power amplitude during the first sub-pulse and a minimized deviation of the power amplitude reflected from the accelerating structure from a second steady state level of the reflected power amplitude during the second sub-pulse.

Optionally, the injection of the modulated electron beam is performed at a time based at least in part on minimizing a deviation of a normalized X-ray beam intensity of the first sub-pulse over a pulse duration from an X-ray beam intensity corresponding to a steady state energy level of the first sub-pulse during the first sub-pulse and minimizing a deviation of a normalized X-ray beam intensity of the second sub-pulse over a pulse duration from an X-ray beam intensity corresponding to a steady state energy level of the second sub-pulse during the second sub-pulse.

In an embodiment, the present specification discloses a system for generating multi-energy X-ray pulses, the system comprising: an electron gun configured to generate an electron beam; a standing wave resonator; an RF source configured to apply a radio frequency field to the standing wave resonator, wherein the radio frequency field is characterized by an RF field amplitude, a specified time profile, and an RF pulse duration, and wherein the standing wave resonator is configured to receive the electron beam and accelerate the electron beam with the radio frequency field to produce accelerated electrons; at least one detector configured to generate data indicative of a time profile of an amount of power reflected from the acceleration structure and to generate a value indicative of the amount of power reflected; a controller configured to: 1) Receiving a value from at least one detector indicative of the amount of reflected power; 2) Determining a time profile of the amount of reflected power; 3) Determining a first delay and a second delay, wherein each of the first delay and the second delay is determined based at least in part on a time profile of the amount of reflected power; and 4) injecting an electron beam into the standing wave resonator to produce accelerated electrons and form at least a first sub-pulse defined by a first beam current amplitude and a first RF field amplitude and a second sub-pulse defined by a second beam current amplitude and a second RF field amplitude, wherein the injecting is performed to achieve a first steady state energy level of the first sub-pulse and a second steady state energy level of the second sub-pulse that are different; and a target configured to receive the accelerated electrons and generate a multi-energy X-ray pulse.

Optionally, the at least one detector comprises a directional coupler and a microwave detector.

Optionally, the controller is configured to: within a single RF pulse, the electron beam is injected at a first beam current amplitude after a first delay to achieve a first sub-pulse, and then, after a second delay, at a second beam current amplitude to achieve a second sub-pulse. Optionally, the controller is configured to inject the electron beam at a time based at least in part on achieving a minimized deviation of the energy level from the first steady state energy level during the first sub-pulse and a minimized deviation of the energy level from the second steady state energy level during the second sub-pulse.

Optionally, the duration of the first sub-pulse is different from the duration of the second sub-pulse. Optionally, at least one of the duration of the first sub-pulse or the duration of the second sub-pulse is variable, wherein the minimum duration of any sub-pulse may be zero, and wherein the maximum duration of the first sub-pulse or the maximum duration of the second sub-pulse is defined by a function of the RF pulse duration, the time of injection of the electron beam, and the duration of one or more other sub-pulses.

Optionally, the controller is configured to inject the electron beam at a time based at least in part on minimizing a deviation of the reflected power amplitude from the standing wave resonator from a first steady state level of the reflected power amplitude during the first sub-pulse and minimizing a deviation of the reflected power amplitude from the standing wave resonator from a second steady state level of the reflected power amplitude during the second sub-pulse.

Optionally, the controller is configured to inject the electron beam at a time based at least in part on minimizing a deviation of the normalized X-ray beam intensity of the first sub-pulse over the pulse duration relative to the X-ray beam intensity corresponding to the steady state energy level of the first sub-pulse during the first sub-pulse and minimizing a deviation of the normalized X-ray beam intensity of the second sub-pulse over the pulse duration relative to the X-ray beam intensity corresponding to the steady state energy level of the second sub-pulse during the second sub-pulse.

The foregoing and other embodiments of the present specification will be described in greater depth in the following drawings and detailed description.

Drawings

These and other features and advantages of the present description will be further appreciated as a result of a better understanding of the following detailed description considered in conjunction with the accompanying drawings.

FIG. 1 illustrates a typical high energy transmission X-ray inspection system in accordance with some embodiments of the present description;

FIG. 2 illustrates a block diagram of an X-ray source employing an acceleration structure and modulating within microwave pulse injection current and RF excitation, according to some embodiments of the present description;

FIG. 3 illustrates the dependence of energy within a microwave pulse on current according to some embodiments of the present description;

FIG. 4 illustrates a block diagram of an X-ray source employing a standing wave accelerating structure modulated within microwave pulse injection current and RF excitation, in accordance with some embodiments of the present description;

FIG. 5 shows an application I by utilizing an optimal delay according to an embodiment of the present specification _L And I _H Current-created linac implementations with dual energy pulses;

FIG. 6 illustrates a block diagram of an X-ray source employing an acceleration structure and modulating within microwave pulse injection current and RF excitation, according to some embodiments of the present description;

FIG. 7 illustrates waveforms of reflected power from an over-coupled microwave cavity detectable by a detector and absence of accelerating beam current, according to some embodiments of the present description;

FIG. 8 is a graph illustrating reflected power waveforms acquired with low and high energy injection currents of successive injections to form a dual energy beam, according to some embodiments of the present description;

FIG. 9 is a graph illustrating an exemplary reflected power waveform obtained from an industrial linac when a beam current injection pulse is injected at an optimal time;

FIG. 10A is a graph showing an exemplary reflected power waveform when the injection beam current injection pulse is later than the optimal time (overshoot);

FIG. 10B is another graph showing an exemplary reflected power waveform when the injection beam current injection pulse is earlier than the optimal time (undershoot);

FIG. 11 is a graph showing the X-ray beam intensity (D) for a pulse width on the y-axis and the pulse width duration (t) on the X-axis _p ) A chart of an exemplary set of measurements that are normalized;

fig. 12A illustrates an exemplary waveform of an intra-pulse dual energy structure when the second (high energy) beam pulse is injected later than the optimal time (high energy overshoot), according to some embodiments of the present description;

FIG. 12B illustrates exemplary waveforms of an intra-pulse dual energy structure when a second (high energy) beam pulse is injected at an optimal time, according to some embodiments of the present disclosure;

FIG. 13 is an exemplary flow chart illustrating a method of intra-pulse tuning according to some embodiments of the present description;

FIG. 14 is a graph showing beam energy curves in a region around an optimal injection delay according to some embodiments of the present description;

FIG. 15 is a flowchart illustrating an exemplary process for evaluating acceptable ranges of parameters and refining the need for dual energy discrimination for dynamic dose control according to some embodiments of the present description;

FIG. 16 is a graph showing the deviation of average each pulse from energy versus steady state values over pulse duration and a plot of injection time error typically for low energy in accordance with some embodiments of the present description;

FIG. 17 is a graph showing a plot of average per pulse versus energy versus steady state value over pulse duration and injection time error typically for low energy, according to some embodiments of the present description;

FIG. 18 is an exemplary chart showing how acceptable ranges of parameters used in material discrimination utilizing dynamic dose control are selected according to some embodiments of the present disclosure; and

fig. 19 is a flowchart illustrating an exemplary process of generating multi-energy X-ray pulses according to some embodiments of the present description.

Detailed Description

The present description is directed to various embodiments. The following disclosure is provided to enable any person of ordinary skill in the art to practice the present description. No language used in the specification should be construed as indicating any single embodiment as essential to the general description or the meaning of the terms used herein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Furthermore, the terms and phrases are for the purpose of describing the exemplary embodiments and should not be taken as limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications, and equivalents consistent with the principles and features disclosed. For the purpose of clarity, details concerning technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.

In the description and claims of the present application, each word "include", and "have", and forms thereof, are not necessarily limited to the elements in the list that may be associated with the word. Here, it should be noted that any feature or component described in association with a particular embodiment may be used and implemented with any other embodiment unless clearly indicated otherwise.

As used herein, the indefinite articles "a" and "an" mean "at least one" or "one or more" unless the context clearly indicates otherwise.

Definition:

the term "multi-energy" shall refer to an X-ray examination modality in which X-rays of different spectral composition are used to characterize a medium by differential transmission of the medium.

The term "pulse duration", denoted t _RF Refers to the duration of the application of RF excitation to the linac accelerating structure.

Duration t _L +t _H The term "break point" of the current pulse of (a) is defined as t _L /t _H Wherein t is a value of _L And t _H Respectively the duration of the current pulse during which the electron beam emitted from the acceleration structure is characterized by a low energy and a high energy, respectively.

In the case of a current pulse with multiple sub-pulses, this can be defined as t _L /t _H The break point of the value of (1) characterizes any pair of sub-pulses, where t _L And t _H Respectively the duration of the current sub-pulses.

If a breakpoint is adapted to change from one pair of current pulses or sub-pulses to another pair of current pulses or sub-pulses, the breakpoint of a set of current pulses or sub-pulses should be considered "dynamically variable". Likewise, if the endpoints are adapted to change from one current pulse to another, the endpoints of the pulses may also be characterized as "dynamically variable".

As related to x-ray sources, the term "current level" refers to the average flux of electrons incident on a target, expressed in milliamperes (mA), and refers to the average over a specified duration. Unless otherwise indicated, the specified duration of the average value employed is the duration of the pulse.

As related to x-ray sources, the term "current amplitude" refers to the value of the instantaneous flux of electrons incident on the target, expressed in mA.

The term "start delay" refers to the period between the application of an RF field to an accelerating structure and the injection of a pulse or sub-pulse of electron current into the accelerating structure. If there are multiple sub-pulses of electron current, the cumulative start delays of the respective sub-pulses constitute the sum of the start delays.

As used herein, the term "bremsstrahlung" may be used to denote the emission of X-rays by the impact of energetic electrons on a metal target, and furthermore, the phenomenon involves a physical process.

The term "steady state" generally refers to a period of substantially constant or unchanged condition, such as previous or subsequent values of a given wave or signal within + -n% of each other over a predefined period. The steady state condition is typically after a period such as a basic change or change condition where the previous or subsequent values of a given wave or signal exceed + -n% of each other within a predefined period. The value of n depends on the specific application and generally varies from more than 10% to less than 1%.

There are two substantially different processes related to the present invention to which the term "steady state" is applicable. The exponential method of the first process-steady state value is described by the separate first or separate second term in equation (10). Mathematically, it takes an infinite amount of time to reach a steady state value. Technically, it takes tζ2.3τ to reach 90% or tζ4.6τ to reach a steady state value of 99%. The second pass is described by equation (10) as a wholeAnd (5) processing. The two exponential processes (first term and second term in equation (10)) are characterized by different amplitudes of the same time constant τ, but with opposite signs. When the sum of the two exponents becomes time independent, there is a unique time t _b0 And mathematically speaking, at time t _b0 The "steady state" value described by equation (11) is accurately reached. Technically, the transition time from the exponential process to the steady state value (on the leading edge of the current pulse) and back to the exponent (on the trailing edge of the current pulse) is reached in a time comparable to the leading/trailing edge of the actual current pulse. The control of beam energy stability with respect to time t is further described in the subsequent section _b0 More details of the control of the necessary precision.

The focus of this study is on the time course of the interaction of the acceleration field with the acceleration beam in the resonant structure. Signal noise, reflection of the linac RF network, and/or measurement lines are not considered in detail. When the actual waveforms from the operational linacs are discussed, corresponding annotations are made.

The systems and methods described herein may be described in terms of X-rays, however, the teachings are clear of applicability to other spectral ranges, and all manner of penetrating radiation is contemplated within the scope of the invention.

The various embodiments of the invention described herein employ the variation of the spectral content of an X-ray pulse during the pulse process to distinguish differences in X-ray transmission of a medium under different energy conditions. The solution according to the teachings of the present invention is particularly advantageous in cases where high speed scanning is required, such as trains or high throughput scanners.

According to an embodiment of the invention, the novel apparatus and the novel method are arranged such that a pulse profile of the multi-energy beam can be advantageously created in such a way that material discrimination is improved while retaining the highest possible scanning speed and that an optimized dose to cargo and environment is allowed. Furthermore, embodiments of the present description provide a method of fine tuning of the optimal delay for low and high energy pulses of a single energy, interleaved, or intra-pulse operation.

A novel penetrating radiation source, generally designated by the numeral 20, in accordance with an embodiment of the present invention, will now be described with reference to fig. 2. The linac 21 depicted in fig. 2 includes an acceleration structure 22 and an electron gun 23 that acts as an injector of electrons emitted by a cathode 235. An X-ray target 24, an RF source 25, an RF source modulator 26, an RF isolator 27, and an electron gun modulator 28 are used in conjunction with linac 21. RF circuit 29, including RF source 25 and RF source modulator 26, provides microwave power 250 for a pulse duration t _RF A constant level within. The electron gun 23 driven by the electron gun modulator 28 is operated for a total duration t _p ≤t _RF An accelerated electron beam 220 characterized by a two-stage injection current pulse 210 (also referred to herein as a "pulse") is provided into the acceleration structure 22. The injection current pulse 210 may also be referred to herein as an "injection current" and, as described above, has an amplitude corresponding to the instantaneous value of the electron flux in mA, denoted I _L And I _H 。

By injection current I of higher amplitude _L The first portion 212 of the characterized pulse 210 creates a low energy portion of the beam pulse due to higher beam loading, where W _L Representing the low energy portion. (to avoid ambiguity, note, W _L Both the low energy portion of the pulse and the value of the instantaneous endpoint energy that characterizes the low energy portion of the pulse. The same applies (mutatis mutandis) to the high-energy part W of the pulse _H ). With lower amplitude I _H A second portion 214 of the injection current pulse of (2) generates a beam pulse W _H Is a high energy portion of (2). As defined above, the "break point" of the pulse has t _L /t _H The specific meaning of the values of (c) can vary within the scope of the invention, whereby a dynamic control of the dose of emitted X-rays over the cargo and the environment can be achieved. According to particular embodiments of the present invention, the breakpoint may advantageously vary from pulse to pulse, such that the breakpoint dynamically changes.

Referring now to fig. 3, wherein the dependence of energy and current within a microwave pulse is depicted. The accelerating beam current is shown by dashed line 32, while beam energy is shown by solid line 34. As previously discussed, the dashed line 30 shows the constant current I _H Energy dependence of (2). Due to the front and rear of the pulse Has two different energy levels, and in one embodiment: w (W) _L(0.-1.5)μs =3.9 MeV and W _{H(1.8-3.3)μs} =5.8 MeV. As used herein, "energy level" refers to the average value of each sub-pulse of the electron beam energy duration. The endpoint energies of the set of photons distributed in the energy of the basic bremsstrahlung spectrum according to photon energy may be suitably referenced based on the electron beam energy spectrum.

Optimized coupling coefficient beta of acceleration structure with respect to parameters of single energy beam is known in the art and has been described in the background above ₀ . In fact, for all values of the acceleration current (I) and the RF power (P) considered for the operation, the acceleration structure 22 is over-coupled (coupling coefficient β _c >1, beta _c ≥β ₀ )

In fig. 4, a linac 41 similar to the linac 21 shown in fig. 2 is shown. Acceleration system 20 is based on a standing wave structure 42 (otherwise referred to herein as a "standing wave resonator"), and Miller has proposed a distinction of standing wave structure 42 from a traveling wave structure (1986). Selecting the current I using the algorithm set forth in detail above _L The best coupling coefficient beta ₀ . Selecting the value of IL to provide energy W _L And if I _L Is delayed t with respect to the beginning of the RF pulse _bL Then the energy value W _L Remain constant throughout the duration of the sub-pulse. The low energy current delay t is defined by equation (4) above _bL . At the end of the low energy pulse (at t _L Thereafter), the current is turned off. Select I _H To provide W _H And if I _H The current is applied from a low energy pulse t _L Delay t of end start _bH The energy level remains constant. The high energy pulse delay is defined by the following equation:

according to a particular embodiment of the invention, t may be allowed _L Point change, thereby allowing the ratio t _L /t _H (defined herein as "break points") and thereby advantageously provide dynamic control of the X-ray dose over cargo and environment.

The average current during the low energy portion of the pulse is referred to herein as the low energy current and, mutatis mutandis, the average current during the high energy portion of the pulse is referred to herein as the high energy current.

As long as at delay t _bL The "low energy" current is then started to be applied, and as long as the delay t is applied to the "high energy" current _bL The energy in each part of the pulse will remain constant. Energy constancy within each of the low and high energy portions of the pulse benefits from material discrimination: the energy spectrum of the X-ray beam remains constant and therefore no additional calibration points are needed.

In FIG. 5, a pass-through pair I is shown _L And I _H Examples of linac implementations of dual energy pulses created by current application of optimal delay. The parameters of Linac are the same as those already shown as an embodiment in fig. 3. Applying an optimal delay to the low energy current 51And at->Thereafter, the low-energy current 51 is turned off. At->Applying a delay to the high-energy current 52>And at the end of the microwave pulse the high-energy current 52 is turned off.

In fact, it is not possible to learn, such as beta c, r, L, P, for calculating the optimal delay with sufficient accuracy to ensure the necessary constant values of the beam energy at low and high energy levels _H 、P _L 、I _H 、I _L Is a parameter of (a). While dynamically changing the pulse duration of the beam, if the energy spectrum is changed, then the optimum delay is requiredThe value is adjusted. The adjustment may be performed based on parameters commonly available to monitor most accelerators. Such adjustment may be performed based on an evaluation of parameters commonly available for monitoring particle accelerator and beam performance. For example, the parameters (variables) are: an instantaneous value of the electron beam energy, an average energy per pulse (sub-pulse), an instantaneous value of the RF power reflected from the acceleration cavity into which the single or multiple current pulses are injected, and the X-ray beam intensity integrated over the duration of the pulse or sub-pulse. Linear electron accelerators for applied applications, such as safety, non-destructive testing (NDT) medical treatments, typically do not have sophisticated equipment that directly monitors beam energy, energy spectrum of the beam, or even electron beam output. In this case, the analysis of the reflected power waveform may be based and the injection delay (t _bL And t _bH ) The adjustment of the optimal delay value is performed by repeating the adjustment. Exemplary embodiments of adjusting the injection delay are further described in the following sections. Information about the waveform of the power reflected from the accelerating structure is generally available for most RF accelerators. Furthermore, analysis of normalized intensity dependence versus pulse duration may be performed after fine readjustment of the injection delay. X-ray beam intensity measurements are also commonly used for linac with X-ray output.

In a further refinement, the injection of the electron beam pulses is timed by applying a predefined delay such that a first sub-pulse is realized with a first beam current amplitude and a second sub-pulse is realized with a second beam current amplitude, the first beam current amplitude being different from the second beam current amplitude and having a minimized deviation from a predefined steady state energy level. Referring to fig. 6, a system for determining a desired pulse delay and achieving optimized injection of an electron beam pulse is shown, including an exemplary linac system 600 with intra-pulse operation and a directional coupler 602 for extracting reflected power waveforms.

The linac 61 includes a standing wave accelerating structure 62 and an electron gun 63 that serves as an injector of electrons emitted by a cathode 635. Further comprising an X-ray target 64, an RF source 65, an RF source modulator 66, and an RF isolator 67. Standing wave accelerating junction The structure 62 is connected to the RF source 65 by a feed waveguide 622 and is characterized by a coupling coefficient beta _c In the presence of a signal having the highest amplitude I _max Coupling coefficient beta at acceleration current of (2) _c The overcoupling condition (beta) being selected to provide an accelerating structure _c >1). Another linac includes an electron gun modulator 68. In the embodiments of the present description, the electron gun modulator 68 may also be referred to herein as a gun controller 68, as it controls the injection of electron beam current. An RF circuit 69 comprising an RF source modulator 66 and an RF source 65 is pulsed for a duration t _RF In which a constant level of microwave power (P) 600a is provided or a two (multi) level microwave power function (P) for enhancing the energy difference between the low energy 614 and high energy 642 sub-pulses _L ,P _H ) 600b. Here, the RF circuit 69 may also be referred to as an RF controller 69 due to its structure to control microwave power feeding the accelerating structure 62. The electron gun 63 driven by the electron gun modulator 68 is for a total duration t _p ≤t _RF Is provided into the acceleration system 62, characterized by a secondary injection current pulse 610 (also referred to herein as a "pulse"). The injection current pulse 610 may also be referred to herein as an "injection current" and, as defined above, has an amplitude corresponding to the instantaneous value of its electron flux in mA, denoted I _L And I _H 。

By injection current I of higher amplitude _L The first portion 612 of the characterized pulse 610 creates a low energy portion 641 of the beam pulse 640 due to higher beam loading, where W _L Representing the low energy portion. (to avoid ambiguity, note, W _L Both the low energy portion of the pulse and the value of the instantaneous endpoint energy that characterizes the low energy portion of the pulse. The same applies (mutatis mutandis) to the high-energy part W of the pulse _H ). With lower amplitude I _H Generates a beam pulse W from the second portion 614 of the injection current pulse of (2) _H Is included in the high energy portion 642 of (a). Duration t of sub-pulse _L And t _H Respectively can refer to duration I _L And I _H And then W _L And W is _H 。

In various embodiments, RF circuit 69 is pulsed for a duration of time t _RF The internal supply is made of low-energy microwave power level P _L And the duration of the level (at least t _bL +t _L ) With RF pulses t _RF High energy microwave power level P for the remaining duration of (2) _H A secondary pulse of the characterized microwave power 600 b. The electron gun modulator 68 is at a total duration t _p ≤t _RF An electron beam characterized by a secondary injection current pulse is provided. By injection current I of higher amplitude _L The first pulse is characterized. Due to the higher beam loading and lower microwave power, a low energy portion of the beam pulse is created, where, by W _L Representing a low energy portion; with lower amplitude I _H The following second part of the injection current pulse of (2) in combination with the higher microwave power generates the beam pulse W _H Is a high energy portion of (2). In another embodiment, the microwave power level P can be determined by first using the appropriate microwave power level _L And injection current I _L Created low energy pulse W _L The high-energy microwave power level P is placed at the beginning of the subsequent microwave pulse _H And a suitable injection current I _H Creating a high-energy pulse W _H . Thus, in an embodiment, electron gun modulator 68 and RF circuit 69 are configured to generate X-ray energy levels in descending or ascending order.

In an embodiment, the RF controller provides microwave power pulses of constant amplitude 600a, the gun controller at amplitude I _H <I _L <I _max Providing a descending order of injection current 610. The gun controller also generates a low energy sub-pulse optimum delay t _bL And high energy sub-pulse optimum delay t _bH To maintain a constant amplitude of the corresponding beam energy within low energy sub-pulse 641 and high energy sub-pulse 642. For this case, the optimal delay t is defined by equation (7) _bL And t _bH ，

In an embodiment, the RF controller follows an ascending order 600b with an amplitude P _L <P _H Providing two microwave power pulses of different power levels, wherein the low energy sub-pulses P _L With duration t _bL +t _L Gun controller with amplitude I _H <I _L <I _max Providing a descending order of injection current 610. The gun controller also generates a low energy sub-pulse optimum delay t _bL And high energy sub-pulse optimum delay t _{bH_a} To maintain a constant amplitude of the corresponding beam energy within low energy sub-pulse 641 and high energy sub-pulse 642. For this case, the optimal delay t is defined by equation (8) _bL And t _bH ，

Sometimes, it is beneficial to correspond to the amplitude W _H And W is _L Sub-pulses 641 and 642 of decreasing order form dual energy pulse 640. In an embodiment, the RF controller follows the descending order with amplitude P _H >P _L Providing microwave power pulses having two different power levels, wherein the high energy sub-pulses P _H With duration t _bH +t _H Gun controller with amplitude I _H <I _L <I _max An ascending sequence of injection currents 610 is provided. The gun controller also generates a first high energy sub-pulse optimum delay t _bH And a second low energy sub-pulse optimum delay t _{bL_d} To maintain a constant amplitude of the corresponding beam energy within the low energy and high energy sub-pulses. For this case, the optimal delay t is defined by equation (9) _bH And t _bL ，

According to an embodiment of the present specification, at each appropriate delay t _bL 、t _{bL_d} And t _bH 、t _{bH_a} Thereafter, the electron gun modulator 68 further tunes the appropriate delay based on the amount of power reflected from the acceleration structure 62. In an embodiment, the directional coupler and microwave detector 602 is configured to determine the amount of power reflected from the acceleration structure 62. In some embodiments, detector 602 is in a near-square low region Operates to produce a signal proportional to the microwave power.

In an embodiment, the purpose of tuning each delay is to accelerate the beam current pulse I _L And/or I _H The amplitude of the power reflected from the accelerating structure 62 during this time is maintained within a predefined deviation of the steady state values of the corresponding reflected power of the low energy and high energy pulses (or sub-pulses). Further, the electron gun modulator 68 modifies the tuning delay (t) based on determining the time required to maintain the normalized X-ray beam intensities of the low-energy pulses and/or the high-energy pulses within the corresponding predefined ranges _bL 、t _{bL_d} And t _bH 、t _{bH_a} ) Wherein the normalized X-ray beam intensity is the X-ray beam intensity produced by the low energy pulse and/or the high energy pulse relative to the corresponding pulse width (t _L Or t _H ) Is a function of (2). The subsequent section entitled "control Beam energy stability (Controlling Beam Energy Stability)" gives more details about setting and maintaining constant values of reflected power and X-ray beam intensity, as well as values of parameters of a predefined range.

The following portions of this specification, including fig. 7-12, provide specific details regarding the use of reflected power and X-ray beam intensity data as a means for setting and maintaining constant values of acceleration beam energy within pulses/sub-pulses of different energy levels.

In interleaving, intra-pulse, or single energy operation, fine tuning of the injection point of a single high-energy or low-energy pulse can be achieved by first evaluating the optimal delay of the high-energy and low-energy components assuming a single beam pulse structure. After this, the reflected power waveform shape can be analyzed and the injection time within the injected beam pulse adjusted to achieve an acceptable beam energy level deviation. This step may be performed for each energy component as a single energy beam pulse structure or an intra-pulse dual energy structure. Finally, a standardized X-ray beam intensity distribution can be acquired and analyzed. The injection time can be fine-tuned accordingly.

FIG. 7 illustrates the absence of an accelerated beam detected by detector 602 in accordance with some embodiments of the present disclosureIs provided, the reflected power 704 of the microwave cavity of (a). In an embodiment, the detector 602 operates in a near-square low region, producing a signal proportional to the microwave power. In an embodiment, the coupling coefficient is greater than one for any considered combination of microwave power P and accelerating current I, i.e., meaning that the accelerating structure 62 is over-coupled for any considered mode of operation (low energy or high energy pulse). In an embodiment, the value of the reflected power is represented by a reflection coefficient (Γ) and a Voltage Standing Wave Ratio (VSWR). The VSWR of the over-coupling cavity is also equal to the coupling coefficient beta _c And may be calculated from the reflected power waveform.

At a first instant 708 (t=0) of the pulse of RF power 702 being applied, the resonator acts as a short circuit (Γ= -1) and reflects full RF power from the microwave cavity. Since the intracavity is built up at a lower index, VSWR transitions through the matching point 706, where Γ=0, vswr=1, and then approaches the steady state value 710, and vswr=β _c ，Γ＝(β _c -1)/(β _c +1). The acceleration current injected into the waveguide provides an "extra match" effectively reducing the VSWR of the acceleration cavity to the current. At accelerating current I _opt Vswr=1. The highest value of the accelerating waveguide efficiency reaches these parameters. In this case, if the point 706{ Γ=0, vswr=1 } on the reflected waveform in fig. 7 is accurately injected into the beam, then the field in the cavity will be at the beam duration (t _Iopt ) The inner part remains constant. In the usual case, I _L <I _opt It is necessary to pulse the first low energy pulse (I _L ) Or high energy pulses of decreasing energy level (I _H ) Injected (and thus clocked or delayed) to the right of point 706 (Γ=0, vswr=1). If the delay is equal to the optimum delay defined by the first equation in the appropriate system of equations (7), (8), or (9), then the reflected signal will be at a first beam pulse duration (t _L Or t _H ) The inner part remains constant.

Then, the corresponding optimal delay t can be obtained _bH /t _{bH_a} /t _{bL_d} High-energy acceleration current I of time injection ascending order energy level _H Or low energy pulses of decreasing energy level (I _L ) So that the reflected signal remains in the beam pulse duration (t _H Or t _L ) During which the pulse specified value of VSWR is kept constant.

FIG. 8 is a graph illustrating a method of using low energy (I) with continuous implantation with optimal delay in accordance with some embodiments of the present disclosure _L ) 812 and high energy (I) _H ) 814 accelerate the current drawn simulated reflected power waveform 802 to form a graph of the dual energy beam within the pulse.

In this case, the field in the cavity is still in the beam duration (t _L ) The inner part remains constant. On the reflected waveform, a low energy pulse (and thus timing or delay) is injected slightly to the right of point 806, where Γ=0, vswr=1, such that the reflected signal is at the beam duration (t _L ) The "low energy" VSWR amplitude remains constant. At the best delay t _bH Is accelerated in the waveguide _H <I _L So that the reflected signal is reflected in the beam duration (t _H ) The higher value of the "high energy" VSWR amplitude remains constant.

It is critical that the material with dynamic dose control distinguish between the smallest possible energy deviations from steady state energy levels while varying the dose as the pulse duration varies. To a large extent, energy stability depends on the accuracy of setting and maintaining the optimal implantation time.

It should also be noted that it is possible to implement "typical" reflected power waveforms such as those shown in fig. 7 and 8 with higher restrictions on parameters, such as but not limited to: the RF pulse shape is such that it is perfectly square, there is noise, loss of RF lines, etc. In general, a "typical" reflected power waveform can be achieved with a lower RF power signal (with "sharp" front and back edges).

Fig. 9 is a graph showing reflected power waveforms corresponding to beam current pulse 912 lines acquired from industrial linac. As described above, reflected power waveform 902, shown in yellow, corresponds to beam current pulse 912, shown in red, injected at a time near the optimal time delay. Another reflected power waveform 904 ("unloaded") shown in gray corresponds to the same RF pulse, except for the scenario when there is no accelerating beam current pulse. Reflection ofThe power waveforms 902 and 904 coincide until a beam current pulse 912 is applied. The reflected power waveform 902 deviates from the "unloaded" waveform 904 and exhibits a constant amplitude level for the duration of the beam pulse. Reference line 903 on the waveform is shown to represent reflected power signals (for specifying acceleration beam and acceleration structure parameters P, I, L, r, beta _c ) Steady state values of (2) and signal deviations from steady state level 903. Reflected power waveforms 902 and 904 are affected by some reflection and noise that is common at the leading edge of beam current pulse 912. The area of influence is delineated within the dashed circle 914. The reflected power waveforms 902 and 904 are also sloped due to the long trailing edge of the magnetron pulse. The portion of waveform 902 that is within the beam current pulse duration ("loading" waveform) is technically minimally biased from reflected power steady state level 903 that is achievable. The "loaded" portion of the reflected power waveform (beginning with an initial point 902a where the "loaded" waveform begins to deviate from "unloaded") has a constant amplitude equal to steady state value 903. Thus, this procedure of moving the beam pulse within the reflected power waveform (between point Γ=0, vswr=1 } and the end of the RF pulse) to achieve "loading" a constant level of reflected power 902 amplitude (equal to steady state value 903) can be used to achieve a constant amplitude of beam energy within the beam pulse, even without calculating the value of the optimal time delay.

It should also be noted that most of the fluctuations around the steady state level present on waveform 904 are a consequence of the linac technology defect, not an incorrect time delay setting. Tuning the accelerating beam to achieve a constant level 903 of reflected power 902 within accelerating beam current pulse 912 also helps achieve a more constant level of accelerating current itself. The reflected power level mirrored by the linac accelerating field is interdependent with the accelerating beam current amplitude, especially in a beamformer configuration (which is typically the case for industrial linac). The constant acceleration field produces a constant injection beam capture, and therefore a more uniform acceleration beam current amplitude is observed on the front of the beam current pulses in fig. 9, and thereafter fig. 10A and 10B.

Fig. 10A and 10B show two examples of inaccurate beam injection times. Fig. 10A is a graph illustrating an exemplary reflected power waveform 1002a when the injection acceleration beam current pulse 1012a is later than an optimal time ("overshoot"). The reflected power waveform 1002a and the "unloaded" waveform 1004a associated with the beam current pulse 1012a coincide until the beam pulse 1012a is applied. Subsequently, the reflected power amplitude decreases exponentially approaching the steady state level 1003a having the same value as in fig. 9 and marked with reference line 903. The amplitude of reflected power waveform 1002a is not constant over beam pulse duration 1012 a; the front portion of the waveform lies above steady state value 1003a and the initial point 1042a of the "load" portion of the waveform exceeds steady state value 1003a at least twice. The amplitude of the point 1042a is depicted with reference line 1005 a. In this scenario, at the beginning of the current pulse 1012a, the electron beam is accelerated by a high electric field and gets a higher energy. The energy amplitude changes during the beam pulse 1012a duration. The energy spectrum as well as the beam intensity spectrum varies with the pulse duration of the beam (i.e., the intensity is not linear with the pulse duration). Thus, it can be inferred that the current pulse 1012a needs to be moved to the left of the point, where the reflected power amplitude is equal to the steady state value 1003a, thereby achieving a constant level of reflected power 1002a and, therefore, a constant amplitude of beam energy within the beam pulse 1012a.

Fig. 10B is another chart illustrating an exemplary reflected power waveform 1002B when the injection acceleration beam current pulse 1012B is earlier than the optimal time ("undershoot"). In this example, the front portion of reflected waveform 1002b within the beam pulse is below steady state value 1003b. The value of the front portion 1042b of the "load" portion of waveform 1002b is less than steady state value 1003b at least twice. In this scenario, the beam current pulse 1012b needs to be moved to the right towards the point, where the reflected power amplitude is equal to the steady state value, thereby achieving a constant level of reflected power and thus beam energy within the beam pulse.

In an embodiment, the reflected power waveform may be continuously monitored and analyzed to determine the adjustment range of the injection time of the beam current pulse. Furthermore, by each analysis, the beam current pulse injection time can be adjusted to an optimal level required to achieve a constant level of reflected power within each injection pulse or to maintain the amount of power reflected from the accelerating structure within a predefined deviation range, such as outlined in the subsequent section "control beam energy stability (Controlling Beam Energy Stability)". Analysis and adjustment may be performed on each energy component as a single energy beam pulse structure within an RF pulse or as a dual (multi) energy beam structure with a single RF pulse (within a pulse).

Thus, standardized X-ray beam intensity distribution information can be collected and analyzed to make further fine adjustments to the beam current pulse injection time. The fine adjustment may be based on determining the time required to maintain the normalized X-ray beam intensity within a predefined range, wherein the normalized X-ray beam intensity is a function of the X-ray beam intensity relative to the corresponding pulse width. For linac with X-ray output, normalized X-ray beam intensities within the pulse can be used for such measurements. The intensity of the X-ray beam (e.g., dose rate D) is a function of beam energy and current. It can be assumed that the beam current is at pulse width t _p Internal constant, therefore, the normalized intensity D/t is measured as the beam pulse duration changes _p And pulse width t _p The dependence of (c) provides information about the energy stability.

FIG. 11 is a graph showing the intensity (D) 1105 of a pulsed X-ray beam on the y-axis versus the pulse width (t) on the X-axis _p ) 1110 to make a normalized set of exemplary measured graphs 1110. Data is collected for a single energy beam, wherein the starting point of the beam pulse is kept constant. The beam intensity is normalized over the pulse duration and then for the average of all data points. From the graph, it can be seen that the normalization of the pulse width X-ray beam intensity (D) is performed for a lower pulse duration (t _p ) Higher (up to about 7%), implying that the energy amplitude of the accelerating beam is higher at the beginning of the pulse (as in the example of fig. 10A). If the deviation amplitude is acceptable as defined using the method outlined in the subsequent section of this specification in connection with "control beam energy stability (Controlling Beam Energy Stability)"Out of range, it may be desirable to reduce the beam injection time (pulse shift to the left).

Fig. 12A and 12B illustrate exemplary graphs of intra-pulse dual energy structures according to some embodiments of the present description. Referring also to fig. 12A and 12B, "loaded" reflected power waveforms 1202A and 1202B are shown corresponding to low energy beam current pulse 1212 and high energy beam current pulses 1214a and 1214B, respectively. The low energy beam current pulse 1212 may be placed at an implantation time found during the preliminary adjustment set forth above for the single energy beam pulse structure in the context of fig. 9. Also shown is an "unloaded" reflected waveform 1204. The loaded reflected power waveforms 1202a and 1202b exhibit a constant level of reflected power during the low energy beam current pulse 1212 and have an amplitude equal to the reflected power steady state value of the low energy pulse. Referring to fig. 12A, the high energy beam current 1214a is placed at a higher delay than the optimal implantation delay. At the beginning 1242a of the high energy beam pulse, the "loading" reflected waveform 1202a exceeds the high energy reflected power steady state level 1203. The highest amplitude of reflected power within the high energy beam pulse is depicted by line 1205a and exceeds the steady state level by about 10%. Referring to fig. 12B, in accordance with various embodiments of the present description, the injection time of the high energy beam pulse is reduced by ≡0.2 mus (pulse moves to the left) and the level of reflected power waveform 1202B remains constant and equal to the high energy steady state value during the high energy beam pulse 1214B.

In an embodiment, sub-pulse durations of the normalized X-ray beam intensity distribution and high energy component may be acquired and analyzed. The injection time can be adjusted accordingly. Alternatively, the total intensity of the dual energy pulse can be measured when the low energy pulse time is fixed and equal to the value of one pulse duration employed in the preliminary low energy tuning process. Using the data acquired for several high energy sub-pulse durations, the intensity of the high energy sub-pulse component can be calculated as the difference of the total intensity and the known intensity of the low energy sub-pulse and then normalized using the high energy sub-pulse duration.

FIG. 13 is a diagram illustrating pulses according to some embodiments of the present descriptionAn exemplary flow chart of a method of internal tuning. Referring to both fig. 6 and 13, at 1302, an electron gun 63 controlled by modulator 68 generates an electron beam within standing wave accelerating structure 62. At 1304, an electron gun 63 controlled by an electron gun modulator 68 is configured to modulate the electron beam prior to injection into the acceleration structure 62 to achieve at least a first beam current amplitude (I _L ) And a duration of t _L And at a first delay t _bL First beam current time profile of injection, and second beam current amplitude (I _H ) And a duration of t _H And at a second delay t _bH And then a second beam current time profile is implanted. For purposes of description, it is assumed herein that descending beam current pulses are injected into acceleration structure 62 to generate a sum W _L And W is _H Corresponding ascending energy beams. In alternative embodiments, the reverse sequence may be employed. Here, a first beam current amplitude (I _H ) To form a high-energy beam pulse W _H Thereafter, a second beam current amplitude (I _L ) To form a low energy beam pulse W _L 。

At 1306, the RF source 65 utilizes the RF power amplitude (P) and the RF pulse duration (t _RF ) The characterized RF power time profile applies a radio frequency field to the acceleration structure 62. For purposes of description, it is assumed herein that the RF power remains constant within a single RF pulse. In alternative embodiments, power levels (P) in ascending or descending order with appropriate timing structures may be employed _H ,P _L )。

The electron beam is accelerated within an acceleration structure 62 having an RF field to generate accelerated electrons for generating X-rays toward a target 64. At 1308, the amount of RF power reflected from the acceleration structure 62 is determined by means of the directional coupler and the microwave detector 602.

At 1310, the electron gun 63 controlled by the electron gun modulator 68 is controlled at a first beam current amplitude (I after a first time delay _L ) And then after a second time delay at a second beam current amplitude (I _H ) An electron beam is injected. The time delay of each beam current pulse may be determined using equations (7), (8), or (9). As described above, at the time of determinationAnd a delay to maintain the energy deviation from the corresponding steady state energy level less than a predefined amount. Furthermore, the time delay is determined such that the amount of RF power reflected from the accelerating structure 62 during each beam sub-pulse is maintained within a predefined deviation of the corresponding steady state reflected power level.

Further, the time delay is adjusted to ensure that the normalized X-ray beam intensity, which is the X-ray beam intensity relative to the corresponding beam pulse width (t _L Or t _H ) Is a function of (2).

At 1312, the electron beam is accelerated with the RF field within the acceleration structure 62 to produce an accelerated beam having a dual energy structure within a single RF pulse. At 1314, the accelerated electrons strike the target 64 for generating X-rays by bremsstrahlung.

In the embodiment, as long as at delay t _bL The "low energy" current is then started to be applied, and provided that t is a delay _bH Applying a "high energy" current, the energy amplitude within each portion of the RF pulse will remain constant. The constancy of the energy within each of the low and high energy portions of the pulse is beneficial in the sub-pulse t _L And/or t _H Material discrimination occurs when the duration of (a) changes. Further, the energy spectrum of the X-ray beam remains constant within each pulse, and therefore, no additional calibration points are required.

Controlling beam energy stability

According to embodiments of the present specification, it is important to minimize the beam energy variation of each energy level in the dual/multi-energy method using material discrimination.

The energy dependence of the accelerating beam in terms of time is given by the following equation:

wherein,

w is electron beam energy;

p is the RF pulse power required to provide the acceleration level;

i is the accelerating beam current;

r is the effective shunt impedance;

l is the acceleration system length;

β,β ₀ is the optimal coupling coefficient for coupling, RF cavity;

t,t _p is the time from the start of the RF pulse, and the duration of the RF pulse;

t _b ,t _b0 delta is time, optimal time to turn on acceleration current (in the injection acceleration system), and deviation from optimal time.

τ is defined asIs an index of (c).

In equation (10), at time t=t _b0 And performing beam implantation by +delta.

If delta=0, andthat is, for a given set of parameters, referred to as the optimal injection time, then the dependent energy is shown in terms of time and equation (10) reduces to: / >

This is the Steady State (SS) value of the energy of the accelerated beam for the given set of beam parameters and standing wave cavity characteristics.

For a pair ofNormalization, the absolute deviation of the beam energy from the steady state value can be expressed by the following equation, assuming that the beam injection time is not optimal,

the relative deviation of the beam energy from the steady state value can be calculated from the following equation:

fig. 14 is a graph showing the plot of energy dependence versus time (pulse duration) according to equation (10) for four cases. Referring to the graph, dashed line 1402 shows the electric field built up without injecting any current; the green line 1404 shows the energy of the beam injected at the optimal time and a constant level of beam energy independent of time; red line 1406 shows the energy level of the previous implant beam pulse ("undershoot"); and blue line 1408 shows the energy level of the subsequent implant beam pulse ("overshoot"). In the case of undershoot and overshoot, reference lines 1406 and 1408, at injection time t=t _b0 The +delta beam energy approaches the SS value as an exponential function characterized by an exponent τ and an energy deviation amplitude from the SS level. If at time t=t _b0 +δ starts an X-ray pulse of variable duration, the energy spectrum of the pulse is exponentially dependent on the pulse duration. This energy-and-time dependence is minimized by reducing the amplitude of the energy deviation from the SS level, which in turn can be achieved by adjusting the injection time of the beam pulse.

Examples of the quantization characteristics of the beam energy variation are provided below. The average electron beam energy per pulse is used to derive the beam energy variation and the interdependence of the timing parameters of the beam.

The integration of equation (12) over the pulse duration can be represented by the following equation (limit t= [ t) _b0 +δ,t _b0 +δ+t _p ]) Pulse duration t _p Due to the optimal time t _b0 To produce an average normalized absolute energy deviation from the steady state value:

the average energy deviation depends on three timing parameters: optimum delay t _b0 Deviation from the optimum delay delta, and pulse duration t _p . The relative energy deviation is given by:

as part of the dual energy material differentiation, the need for energy stability depends on a number of specific factors derived from the hardware, algorithms, and software used. FIG. 15 is a flowchart illustrating an exemplary set of steps for assessing the need for dual energy material differentiation using dynamic dose control. At step 1502, acceptable energy variations may be defined based on the details of the material discrimination method and hardware. In an embodiment, it is assumed that the relative energy deviation should be within ±1% for successful material discrimination. Thus, the requirement determines the range and interdependence of timing parameters sufficient to maintain the relative energy variation within the required limits (1%) while employing dynamic dose variation.

At step 1504, a deviation from the optimal delay (delta) and pulse duration (t _p ) Constraint of the range. Optimum delay t _b0 Is the "external" timing parameter for this task, i.e., is primarily defined by the energy and current corresponding to the dual energy component. The relative energy deviation from the two values of the optimal delay (considered in the example of a single pulse operation) is shown in the form of a surface map in fig. 16 and 17. FIG. 16 is a graph of T and _b0 plot of low energy component corresponding to =0.5 μs. FIG. 17 is a graph of T and _b0 plot of high-energy component corresponding to =1.0 μs. The horizontal axis 1602/1702 of the graphs shown in FIGS. 16 and 17 represents the deviation from the optimal delay delta and the vertical axis 1604/1704 represents the pulse duration t _p . The values of the relative energy deviation are shown in fig. 16 and 17 as colors. Green region 1606 represents a relative energy deviation of 1%; yellow region 1608 represents the relative energy deviation within the range (1-2)%; and red region 1610 represents a relative energy deviation of 2% or more. Light green zoneDomain 1706 represents a relative energy deviation of 0.5% or less; dark green region 1707 represents (0.5-1.0)% relative energy deviation; yellow region 1708 represents the relative energy deviation within the range (1-1.5)%. As can be seen from the plots of fig. 16 and 17, the performance of the material discrimination method is constrained by the low energy component. The minimum pulse width is limited by the accuracy with which the injection delay value is adjusted and maintained. For example, to enable the use of pulse duration t _p =0.1 μs, the adjustment accuracy (δ) should not exceed about 20ns. In other words, the accuracy of the timing adjustment (including timing jitter) constrains the minimum pulse duration to select the accuracy of the energy deviation. The high energy component requirements are less restrictive and do not impose additional challenges on the discrimination performance of dual energy materials with dynamic dose variation.

At step 1506, energy stability may be adjusted and controlled by adjusting and maintaining the injection time within the desired boundaries determined at step 1504. At step 1508, the actual performance of the material differentiation may be verified for a given set of hardware and software for the requirements of the material differentiation and dose variation, the acceptable beam energy variation range is further corrected at step 1510 and the iterative requirement assessment process is repeated.

In some embodiments, tools are used for the purpose of adjusting and controlling energy stability. In embodiments, the means for directly measuring the beam energy may be used to measure the instantaneous or average energy value per pulse for monitoring, correcting, or adjusting energy deviations. Because the energy of each pulse on average is chosen as the primary parameter for defining the limits of the energy deviation, the accuracy of the adjustment is linear with the desired deviation. The value and accuracy (and the required deviation) of the adjustment are defined by equations (12), (13), (14), and (15).

Industrial linac may not have built-in beam energy measuring tools. In this case, the reflected power (P _Refl ) As a tool for monitoring the microwave process in a standing wave cavity and adjusting necessary parameters. Because of delta P _Refl ～W ² Reflected power is a relatively more sensitive parameter to measure the necessary adjustment. Using P _Refl Monitoring and correcting energy deviationsHas the obvious advantages that P _Refl The signal (waveform) provides a "field signal" that accelerates the RF field (including beam loading) in the cavity. This signal requires very little processing.

In some cases, because of delta D-W ^2.7 The X-ray beam intensity (D) can even be a more relatively sensitive tool for adjusting/controlling the beam energy.

These three parameters δ W, P can be found using the following equation _Refl Sum of sensitivity of D:

δW:δP _Refl :δD＝W:W ² :W ^2.7 (16)

if the number of allocated colors is scaled according to equation (16), the power (P _Refl ) The plot of the X-ray beam intensity/dose rate (D) deviation looks the same.

Fig. 18 is an exemplary chart showing how to select an acceptable range of parameters for use in material discrimination with dynamic dose control according to some embodiments of the present description. The graph shows the relative energy (δW) 1802 and the reflected power (δP) _Refl ) 1804, and dose rate (δd) 1806 deviation from the corresponding steady state value, pulse duration (t) from the low energy component _b0 =0.5 μ) and the optimum delay deviation δ=25 ns. From the graph, it is seen that the minimum pulse duration t, which satisfies the range of the beam energy deviation δW.ltoreq.1%, is t _pmin And approximately 0.24 mus. Line 1808 shows reflected power (P _Refl 2%) and the dose rate (δD 2.7%) to maintain δW 1%.

Fig. 19 is a flowchart illustrating an exemplary process of generating multi-energy X-ray pulses according to some embodiments of the present description. At 1902, an electron beam is generated using an electron gun. The electron beam is characterized by an electron beam pulse duration. At 1904, the generated electron beam is modulated to achieve at least a first beam current amplitude and a first beam current time profile and a second beam current amplitude and a second beam current time profile prior to injection into the acceleration structure. In an embodiment, the accelerating structure comprises a standing wave resonator. In an embodiment, for an acceleration structureThe coupling coefficient is optimized to achieve a greater critical coupling (beta _c >1, beta _c ≥β ₀ ). At 1906, a Radio Frequency (RF) field is applied to the acceleration structure. The RF field is defined by the RF field amplitude, a specified time profile, and the RF pulse duration. At 1908, a time profile of the power reflected from the acceleration structure is determined. In an embodiment, a detector is used to determine a time profile of the power reflected from the acceleration structure to generate a signal indicative of the reflected power (P _Refl ) Is a value of the quantity of (a). The detector may include a directional coupler and a microwave detector. At 1910, a first delay and a second delay for injecting a current pulse are determined. The delay is determined based at least in part on a time profile of an amount of power reflected from the acceleration structure. In some embodiments, the delay is determined using the method described in the context of fig. 15.

At 1912, after a first delay, a modulated electron beam of a first beam current amplitude is injected to achieve a first sub-pulse. Then, after a second delay, a second beam current pulse of a second beam current amplitude is injected to achieve a second sub-pulse. In an embodiment, the controller is configured to perform injection of the modulated electron beam. The beam current pulse is injected to achieve electrons of a first steady state energy level of the first sub-pulse and electrons of a second steady state energy level of the second sub-pulse, wherein the first and second steady state energy levels are different. Furthermore, the first and second beam current pulses are injected within a single RF pulse. In some embodiments, and as described in the context of fig. 15-18, the injection of the modulated electron beam is performed at a time based at least in part on achieving a minimized deviation of the energy level from the first steady state energy level during the first sub-pulse and a minimized deviation of the energy level from the second steady state energy level during the second sub-pulse. In an embodiment, the duration of the first sub-pulse is different from the duration of the second sub-pulse. The duration of the first sub-pulse or the second sub-pulse is variable. The minimum duration of any sub-pulse may be zero and the maximum duration may be defined by a function of the RF pulse duration, the time delay of injection of the modulated electron beam, and the duration of one or more other sub-pulses.

In some embodiments, the power reflected from the acceleration structure (P) is based at least in part on the power reflected from the acceleration structure during the first sub-pulse and during the second sub-pulse _Refl ) The modulated electron beam is injected at a time that minimizes the deviation of the steady state level of reflected power amplitude during each sub-pulse. In some embodiments, the modulated electron beam is injected at a time based at least in part on minimizing deviation of the X-ray beam intensities during the first sub-pulse and during the second sub-pulse from the respective normalized pulse durations of the first sub-pulse and the second sub-pulse relative to the X-ray beam intensities corresponding to the first/second sub-pulse steady state energy levels.

At 1914, the modulated electron beam is accelerated with an RF field within an acceleration structure to produce accelerated electrons. At 1916, the accelerated electrons strike the target to generate X-rays by bremsstrahlung.

The above examples illustrate only a number of applications of the methods and systems of the present description. Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the present invention. The present examples and embodiments are, therefore, to be considered as illustrative and not restrictive, and the invention may be modified within the scope of the appended claims.

Claims

1. A method for generating multi-energy X-ray pulses using an acceleration structure comprising a standing wave resonator, the method comprising:

generating an electron beam using an electron gun;

modulating the electron beam prior to injection into the acceleration structure, wherein modulating the electron beam produces at least 1) a first beam current amplitude and a first beam current time profile and 2) a second beam current amplitude and a second beam current time profile, and wherein the electron beam is characterized by an electron beam pulse duration;

applying a radio frequency field to the accelerating structure, wherein the radio frequency field is defined by an RF field amplitude, a specified time profile, and an RF pulse duration;

determining a time profile of the amount of power reflected from the acceleration structure;

determining a first delay and a second delay, wherein each of the first delay and the second delay is determined based at least in part on the time profile of the amount of power reflected from the acceleration structure;

injecting the modulated electron beam at the first beam current amplitude after the first delay to achieve a first sub-pulse and then injecting the modulated electron beam at the second beam current amplitude after the second delay to achieve a second sub-pulse, wherein a first steady state energy level configured to achieve the first sub-pulse and a second steady state energy level of the second sub-pulse are injected, and wherein the first steady state energy level is different from the second steady state energy level;

Accelerating the modulated electron beam with the radio frequency field within the acceleration structure to produce accelerated electrons; and

the accelerated electrons are caused to strike a target for the generation of X-rays by bremsstrahlung.

2. The method of claim 1, further comprising: the coupling coefficient of the acceleration structure is optimized, wherein the coupling coefficient is optimized to achieve coupling greater than a threshold at any beam current.

3. The method of claim 1, wherein the injection of the modulated electron beam at the first beam current amplitude and then at the second beam current amplitude occurs on a single RF pulse.

4. The method of claim 1, wherein the injection of the modulated electron beam is performed at a time based at least in part on a minimized deviation of an energy level from the first steady state energy level achieved during the first sub-pulse and a minimized deviation of an energy level from the second steady state energy level achieved during the second sub-pulse.

5. The method of claim 4, wherein the duration of the first sub-pulse is different from the duration of the second sub-pulse.

6. The method of claim 4, wherein at least one of the duration of the first sub-pulse or the duration of the second sub-pulse is variable, and wherein the maximum duration of the first sub-pulse or the maximum duration of the second sub-pulse is defined by a function of the RF pulse duration, the time of injection of the modulated electron beam, and the duration of one or more other sub-pulses.

7. The method of claim 1, wherein the injecting of the modulated electron beam is performed at a time based at least in part on a time that a minimized deviation of a reflected power amplitude from the accelerating structure from a first steady state level of reflected power amplitude is achieved during the first sub-pulse and a minimized deviation of a reflected power amplitude from the accelerating structure from a second steady state level of reflected power amplitude is achieved during the second sub-pulse.

8. The method of claim 7, wherein the duration of the first sub-pulse is different from the duration of the second sub-pulse.

9. The method of claim 7, wherein at least one of the duration of the first sub-pulse or the duration of the second sub-pulse is variable, and wherein the maximum duration of the first sub-pulse or the maximum duration of the second sub-pulse is defined by a function of the RF pulse duration, the time of injection of the modulated electron beam, and the duration of one or more other sub-pulses.

10. The method of claim 1, wherein the injection of the modulated electron beam is performed at a time based at least in part on a minimized deviation of a normalized X-ray beam intensity of the first sub-pulse over the pulse duration relative to an X-ray beam intensity corresponding to the steady state energy level of the first sub-pulse during the first sub-pulse and a minimized deviation of a normalized X-ray beam intensity of the second sub-pulse over the pulse duration relative to an X-ray beam intensity corresponding to the steady state energy level of the second sub-pulse during the second sub-pulse.

11. The method of claim 10, wherein the duration of the first sub-pulse is different from the duration of the second sub-pulse.

12. The method of claim 10, wherein at least one of the duration of the first sub-pulse or the duration of the second sub-pulse is variable, and wherein the maximum duration of the first sub-pulse or the maximum duration of the second sub-pulse is defined by a function of the RF pulse duration, the time of injection of the modulated electron beam, and the duration of one or more other sub-pulses.

13. A system for generating multi-energy X-ray pulses, the system comprising:

an electron gun configured to generate an electron beam;

a standing wave resonator;

an RF source configured to apply a radio frequency field to the standing wave resonator, wherein the radio frequency field is characterized by an RF field amplitude, a specified time profile, and an RF pulse duration, and wherein the standing wave resonator is configured to receive the electron beam and accelerate the electron beam with the radio frequency field to produce accelerated electrons;

at least one detector configured to generate data indicative of a time profile of an amount of power reflected from the acceleration structure and to generate a value indicative of the amount of power reflected;

a controller configured to: 1) Receiving from at least one of the detectors the value indicative of the amount of power reflected; 2) Determining the time profile of the amount of power reflected; 3) Determining a first delay and a second delay, wherein each of the first delay and the second delay is determined based at least in part on the time profile of the amount of reflected power; and 4) injecting the electron beam into the standing wave resonator to generate the accelerated electrons and form at least a first sub-pulse defined by a first beam current amplitude and a first RF field amplitude and a second sub-pulse defined by a second beam current amplitude and a second RF field amplitude, wherein the injecting is performed to achieve a first steady state energy level of the first sub-pulse and a second steady state energy level of the second sub-pulse being different; and

A target configured to receive the accelerated electrons and generate the multi-energy X-ray pulses.

14. The system of claim 13, wherein at least one of the detectors comprises a directional coupler and a microwave detector.

15. The system of claim 13, wherein the controller is configured to inject the electron beam at the first beam current amplitude after the first delay to achieve the first sub-pulse and then to inject the electron beam at the second beam current amplitude after the second delay to achieve the second sub-pulse over a single RF pulse.

16. The system of claim 13, wherein the controller is configured to inject the electron beam at a time based at least in part on achieving a minimized deviation of energy level from the first steady state energy level during the first sub-pulse and an energy level from the second steady state energy level during the second sub-pulse.

17. The system of claim 16, wherein a duration of the first sub-pulse is different from a duration of the second sub-pulse.

18. The system of claim 16, wherein at least one of the duration of the first sub-pulse or the duration of the second sub-pulse is variable, and wherein the maximum duration of the first sub-pulse or the maximum duration of the second sub-pulse is defined by a function of the RF pulse duration, the time of injection of the electron beam, and the duration of one or more other sub-pulses.

19. The system of claim 13, wherein the controller is configured to inject the electron beam at a time based at least in part on minimizing a deviation of a power amplitude reflected from the standing wave resonator from a first steady state level of reflected power amplitude during the first sub-pulse and minimizing a deviation of a power amplitude reflected from the standing wave resonator from a second steady state level of reflected power amplitude during the second sub-pulse.

20. The system of claim 19, wherein a duration of the first sub-pulse is different from a duration of the second sub-pulse.

21. The system of claim 19, wherein at least one of the duration of the first sub-pulse or the duration of the second sub-pulse is variable, and wherein the maximum duration of the first sub-pulse or the maximum duration of the second sub-pulse is defined by a function of the RF pulse duration, the time of injection of the electron beam, and the duration of one or more other sub-pulses.

22. The system of claim 13, wherein the controller is configured to inject the electron beam at a time based at least in part on minimizing a deviation of a normalized X-ray beam intensity of the first sub-pulse over the pulse duration relative to an X-ray beam intensity corresponding to the steady state energy level of the first sub-pulse during the first sub-pulse and minimizing a deviation of a normalized X-ray beam intensity of the second sub-pulse over the pulse duration relative to an X-ray beam intensity corresponding to the steady state energy level of the second sub-pulse during the second sub-pulse.

23. The system of claim 22, wherein the duration of the first sub-pulse is different from the duration of the second sub-pulse.

24. The system of claim 22, wherein at least one of the duration of the first sub-pulse or the duration of the second sub-pulse is variable, and wherein the maximum duration of the first sub-pulse or the maximum duration of the second sub-pulse is defined by a function of the RF pulse duration, the time of injection of the electron beam, and the duration of one or more other sub-pulses.