EP3111250A1

EP3111250A1 - Tool for detecting photonic radiation particularly suitable for high-flux radiation

Info

Publication number: EP3111250A1
Application number: EP15713201.0A
Authority: EP
Inventors: Francis Glasser
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2014-02-27
Filing date: 2015-02-24
Publication date: 2017-01-04
Also published as: JP2017512984A; WO2015128574A1; FR3017962A1; US10175367B2; US20170017000A1; FR3017962B1

Abstract

The invention relates to a tool for detecting radiation, comprising: a semiconductor detector material (10) able to interact with ionising radiation; an electrode (100) for collecting charge carriers generated in the detector material under the effect of an interaction with the ionising radiation; a shaping circuit (11) for forming an electrical pulse having a shape that depends on the amount of charge collected; a counting circuit (15, 16, 17, 22) for counting the number of pulses formed, comprising a counter and an incrementing element, characterised in that it comprises: a duration-measuring element (17) for measuring a pulse duration (f) for each pulse formed; a peak-detecting element (15) for determining a maximum amplitude (H) of each pulse formed; and a combining element (16) for combining said maximum amplitude H and said pulse duration (f), in order to establish said parameter for comparison. Preferably, the parameter for comparison is the product (H × t) of a maximum amplitude measured for the pulse and the corresponding pulse duration, the counting threshold having a fixed preset value.

Description

Photon radiation detection tool particularly suitable for high-flux radiation

The invention relates to a tool for detecting photonic radiation. Such a detection tool is particularly used in the fields of medical imaging, including radiology or X-ray scanner, astronomical imaging, nuclear and industrial inspection.

The known detection tools consist essentially of a detector, a collimator and computer processing means.

The collimator makes it possible to select the photons arriving on the detector, it is formed of channels delimited by septas.

The detector may comprise a scintiilator material, such as Cesium Plodide, Sodium Hodide, Lanthanum Bromide (LaBr ₃ ) or Bismuth Germanate (BGO), associated with photodetectors, for example a matrix of photodiodes. Alternatively, the detector comprises at least one semiconductor detector material, for example CdTe or CdZnTe, capable of being polarized by a cathode and an anode, these electrodes being generally arranged on two opposite sides of a block of semiconductor material . We bet then of semiconductor detector.

When a photon enters and interacts with the semiconductor material, all or part of its energy is transferred to charge carriers (electron-hole pairs) in the semiconductor material. Since the detector is polarized, the charge carriers migrate towards the electrodes (including the anode). They are collected there to produce an electrical signal. This electrical signal, which is a succession of pulses whose amplitude is proportional to the energy deposited by a photon during an interaction, is then processed. Depending on the nature of the detector, the signal is collected only at the anode (the most common case), only at the cathode, or both electrodes. A semiconductor detector usually comprises a plurality of physical pixels, each physical pixel corresponding to a charge collection circuit by an electrode.

The invention is particularly suitable for high-flux X-ray radiation, such as that of an X-ray scanner.

Three modes of operation are known for detection tools semiconductor detector: integration mode, ie spectrometric mode, photon counting mode.

In integration mode, integration type electronics measure the current from each electrode for a given period of time, typically a few hundred με. This current is the sum of the dark current, a part of the current created by the incident radiation during this period and a part of the current created during the previous period, the latter being called signal drag. is well suited for radiology although hampered by dark current and signal drag. In X-ray scanning, with rapid variations in incident flux of a few decades, the signal drag is unacceptable and the integration mode can not work.

In the spectrometric mode, the output current of each electrode is amplified by a charge preamplifier and shaped with a time constant of the order of 1 s. The measurement of this charge is representative of the energy of the incident photon. The spectrometric mode allows a precise measurement of the energy of the incident photons but it is not fast enough for an X-ray scanner application or the incident flow of the photons is greater than 10 ⁹ photons / s.mm ²

In the photon counting mode, the output current of each electrode is amplified by a current preamplifier and is compared with a threshold, called the counting threshold. This count threshold makes it possible to discriminate a low amplitude interaction, which will be rejected, of a significant amplitude interaction, which will be taken into account. Typically, the count threshold can be equivalent to an energy of 25 keV, only the interactions releasing a higher energy are taken into account, therefore counted,

The typical pulse duration of a signal for an X-ray scanner application is of the order of 5 to 15 ns. If the amplitude of the studied pulse is greater than the counting threshold, a counter is incremented. The photon counting mode is compatible with strong fluxes, the count rate of the detector being in particular greater than 1 shots / s / mm ² . At such counting rates, there is no question of making an accurate measurement of the energy deposited by each interaction. The usual devices are limited to making a measurement of the amplitude, that is to say the maximum level, of each pulse produced by the detector. However, at such counting rates, disturbances can affect the detector, affecting the response stability of the detector. Thus, for the same energy released in the detector, the shape of the pulses can be derived, the pulses being less high and longer. We understand that a simple thresholding amplitude reaches its limits. Indeed, interactions releasing the same energy can give rise to pulses whose maximum amplitude is different, which leads to a degradation of its energy resolution.

It has been found, for example, that when a detector is exposed to an intense and constant incident flux, the number of pulses counted, that is to say the number of pulses exceeding a predetermined amplitude threshold, decreases.

Such a drift can be critical, especially in the case of an X-ray scanner where small variations of one electrode relative to another of a few per thousand cause artifacts during its reconstruction of an image.

The aim of the invention is to propose a radiation detection device adapted to strong incident photon fluxes, that is to say a tool that allows rapid counting of each photon, similar to devices operating according to the counting mode. photons, and which is more reliable, especially at high counting rates.

In terms of reliability, an object of the invention is to provide a detection tool having a low drift, for example less than 1%.

To this end, the invention proposes a radiation detection tool comprising:

a detector material capable of interacting with ionizing radiation; it should be noted that the detector material may be a scintillator material or, preferably, a semiconductor material,

an electrode for collecting charge carriers generated in the detector material under the effect of an interaction with the ionizing radiation; note that the expression "charge carriers" here covers, exceptionally, not only electron-hole pairs generated in the case of a semiconductor detector material but also photons generated in the case of a scintillator detector material ( the term "electrode" then designating a photodiode),

a shaping circuit for forming an electrical pulse having a shape which depends on the amount of charges collected, a counting circuit for counting the number of pulses formed, comprising a counter and an incrementing element for incrementing the counter when a comparison parameter exceeds a count threshold,

Of course, the tool is defined and described with an electrode for the sake of simplicity; but in the usual way, it comprises a multitude of electrodes (pixels) organized in a matrix, and the characteristics defined in relation to the electrode are found for each of the electrodes of the matrix.

The radiation detection device according to the invention is characterized in that it comprises:

a duration measuring element for measuring a pulse duration for each pulse formed,

a peak detection element, for determining a maximum amplitude of each pulse formed,

a combination element, able to combine said maximum amplitude and said pulse duration, to determine a comparison parameter,

an element able to determine said comparison parameter, as a function of said pulse duration and said maximum amplitude. The invention therefore differs from the known counting mode in that the counting is not done solely as a function of a maximum amplitude of a pulse, but as a function of both the maximum amplitude of the pulse formed and the additional information on the temporal shape of the pulse.

The use of this additional temporal information makes it possible to compensate for the previously mentioned drifts. Indeed, the inventor has established that in the case of a semiconductor detector, such a drift could be explained, at least partially, by the following phenomenon. A space charge created by a strong fux will very slightly modify the electric field close to the anodes and thus distort the temporal response. For the same energy deposited by a photon, the pulse will then be slower and therefore the amplitude less important. Since the energy spectrum of the photons that we are trying to count is continuous, a portion of the pulses whose amplitude is close to the threshold will fall below the counting threshold and will therefore no longer be taken into account.

In other words, the invention consists in counting pulses, not not on the basis of their amplitude, but on the basis of a parameter of comparison as close as possible to the integral of the pulse formed (the integral of the pulse formed for a photon having interacted with the detector material being representative of the energy deposited by the photon during the interaction) while keeping a dead time between two detected pulses as short as possible.

In one embodiment of the invention, which is particularly simple and effective, the counting circuit comprises a multiplying element for multiplying, for each pulse, a maximum amplitude H measured for the pulse by the corresponding pulse duration ΐ and in FIG. the comparison parameter is the product (H xt) coming from the multiplication element, the counting threshold having a fixed predetermined value, that is to say that remains the same at least during the same detection operation .

This fixed predetermined value can be adjusted experimentally, as a function of the evolution of the counting rate, that is to say the number of pulses counted per unit time,

The inventor has established that the product H x t from the abovementioned multiplication element provides an approximation of the integral of the pulse sufficient to make it possible to reduce the drift of the measurement to less than 1%.

Moreover, several embodiments are possible for the measurement of the pulse duration. According to a first embodiment, the measured pulse duration corresponds to the time elapsing between the moment when the pulse exceeds a detection threshold and the moment it falls below this same threshold. The detection threshold can in particular correspond to the amplitude below which the signal formed corresponds to the only noise of the detector and beyond which it can be considered that the pulse formed reflects the presence of an interaction in the semiconductor material. driver. To implement this first embodiment, the measuring element is selected from a fixed frequency clock, or a capacitance powered by a DC power source.

According to a second embodiment, the measured pulse duration corresponds to the time elapsing between the moment when the pulse exceeds the detection threshold, which may be that defined in the preceding paragraph, and the moment it reaches its maximum level ( when the signal has an inversion). In in other words, the pulse duration corresponds to the rise time of the signal. This second embodiment is advantageous when the incident flux is very high. Indeed, this embodiment takes into account only the first part of the pulse, corresponding to the rise towards the maximum level, this first part being less subject to drag phenomena. To implement this second embodiment, the measuring element may be similar to those previously mentioned.

The dynamics of measuring the duration of the pulse in the above two embodiments can reach 20 ns with a temporal resolution of 20 μs.

The shaping circuit may comprise a charge pre-amplifier, downstream of which a delay line circuit is arranged, the pulse being shaped by subtracting the signal from the charge pre-amplifier to the signal from the delay line circuit. . Such a shaping circuit is described in application EP2071722. However, the performance deteriorates when the counting rate increases, in particular because of the delay imposed by the delay line.

Preferably, the shaping circuit comprises a current preamplifier. In general, when seeking to determine the integral of a pulse precisely, especially for spectrometry type applications, this type of preamplifier is not used because it is deemed noisy. But a current preamplifier is well suited to the detection tool according to the invention, since a simple estimate of the integral of each pulse is sufficient for the purpose. In addition, this type of preamplifier is better suited to high flux because it produces short pulses of the same order of magnitude as the pulses produced by the electrodes, typically 5 to 15 ns of width.

The invention extends to a detection tool characterized in combination by all or some of the characteristics described above and below.

Other details and advantages of the present invention will appear on reading the following description, which refers to the attached schematic drawings and relates to a preferred embodiment, provided by way of non-limiting example. On these drawings:

FIG. 1 is a schematic representation of a shaping and counting circuit of a detection tool according to the invention. FIG. 2 is a schematic representation of a pulse formed for a photon interaction in the detection tool of FIG. 1.

FIG. 3 is a schematic representation of the evolution of the number of peaks counted as a function of time in a prior art detection tool operating according to the conventional counting mode using as a comparison parameter the single amplitude of the pulses.

FIG. 4 is a schematic representation of the evolution of the number of peaks counted as a function of time in a detection tool according to the invention, such as that of FIG. 1, operating according to a counting mode according to the invention using as parameter of compare the product H xt of the maximum amplitude by the pulse duration.

Fig. 5 is a schematic representation of a known measuring element for measuring the maximum amplitude of the pulses of a signal.

FIG. 1 illustrates an embodiment of a detection tool according to the invention, comprising a detector 10, a shaping circuit and a counting circuit. The shaping circuit comprises at the output of each electrode 100 of the detector 10, a current / voltage converter 1 1 formed for example by an operational amplifier 12 and a resistor 13 connected in parallel. This current / voltage converter 11 constitutes a current preamplifier. The signal 1 (illustrated in FIG. 2) obtained at the output of this current / voltage converter 1 1 is translated, for each photon interaction with the detector material, by a pulse. This pulse is usually not symmetrical. Its amplitude varies according to the energy but also, for the same energy, depending on the count rate, due to the drifts affecting the detection tool mentioned above. The shaping and counting circuits then comprise a peak detecting element 15, which makes it possible to measure the maximum amplitude H of each pulse of the signal 1. Optionally, the peak detection element integrates a tracing circuit. correction of the shape of the pulse, so that the latter has for example a shape close to that of a Gaussian. The peak detection element 15 may be that illustrated in FIG. 5. When the maximum amplitude H is reached, the module 15 delivers a signal equal to this maximum amplitude. The switch of FIG. 5 is closed so as to supply the capacitance after the amplitude of a pulse has reached its maximum. The measured value H is entered in a multiplication element (or "multiplier") 16. In parallel, the signal 1 leaving the current / voltage converter is inputted to a comparator 18 where its amplitude is compared with a threshold H _s , the so-called detection threshold. , to measure the duration t, the so-called pulse duration, flowing between the moment when the (rising) pulse of the signal 1 exceeds the detection threshold H _s and the moment when the pulse (downward) falls below the threshold detection H _s . This measurement of duration is carried out by a measuring element of duration 17. The measured value t is entered in the multiplication element 16.

At the output of the multiplication element 16, the potential depends on the product H x i. When the pulse amplitude drops below the detection threshold H _s, ^the switch is open. Thus, the potential difference across the capacitance C depends on H xt, where t is the duration of the pulse. Downstream, the counting circuit comprises a counter 22. This counter only takes into account the pulses whose product H xt exceeds a threshold, said counting threshold.

FIG. 3 illustrates the evolution of the number of peaks counted as a function of time in a detection tool of the prior art operating according to the conventional counting mode using as a comparison parameter the only amplitude of the pulses. There is a drift of 2.5%.

FIG. 4 illustrates the evolution of the number of peaks counted as a function of time in a detection tool according to the invention such as that of FIG. 1, using as a parameter of comparison the product H xt of the amplitude and the duration of 'impuision. There is a drift of 0.7%.

The invention can be subject to many variations with respect to the illustrated embodiment, since these variants fall within the scope of the appended claims.

Claims

A radiation detection device comprising: a detector material (10), capable of interacting with ionizing radiation,

an electrode (100) for collecting charge carriers generated in the detector material (10) by interaction with the ionizing radiation,

a shaping circuit (1 1,) for forming, for each interaction, an electrical pulse having a shape which depends on the quantity of charges collected,

a counting circuit (15, 16, 17, 22) for counting the number of pulses formed, comprising a counter and an incrementing element for incrementing the counter when a comparison parameter exceeds a count threshold,

characterized in that it comprises

a duration measuring element (17) for measuring a pulse duration (0 for each pulse formed,

a peak detection element, (15) for determining a maximum amplitude () of each pulse formed,

a combination element (16) for combining said maximum amplitude H and said pulse duration (t) to establish said comparison parameter.

2. Detection device according to claim 1, characterized in that Se the combination element (16) is a multiplying element for multiplying, for each pulse, the maximum amplitude (H) measured for the pulse by its duration. corresponding pulse (f) and that the comparison parameter is the product (H xt) from the multiplication element.

3. Detection tool according to one of claims 1 or 2, characterized in that the measured pulse duration (t) corresponds to the time elapsing between the moment when the pulse exceeds a detection threshold (Hs) and At that moment, it goes down below this detection threshold.

4. Detection tool according to claim 3, characterized in that the duration measuring element is selected from: a fixed frequency clock, a capacitor powered by a current source.

5. Detection tool according to one of claims 1 or 2, characterized in that the measured pulse duration corresponds to the time elapsing between the moment when the pulse exceeds a threshold amplitude (Hs) and the moment when it reaches a maximum amplitude (H).

6. Detection tool according to one of claims 1 to 5, characterized in that the shaping circuit comprises a charge preamplifier (1 1) and a delay line (14).

7. Detection tool according to one of claims 1 to 5, characterized in that the shaping circuit comprises a current preamplifier.