CN110471101A - Impulse gamma X-ray detection X method and detection system based on laser polarization modulation - Google Patents

Abstract

In order to solve the technical problem of low sensitivity of the existing active laser probe gamma ray detection method, the present invention provides a pulsed gamma ray detection method and detection system based on laser polarization modulation. The detection method includes steps: 1) converting gamma rays into non-equilibrium free carriers through a radiation conversion crystal, so that the refractive index of the radiation conversion crystal material undergoes a transient change; 2) injecting a probe laser into the radiation conversion crystal to obtain Transmittance and/or reflectance curves of horizontal and/or vertical polarization components corresponding to different refractive indices; A certain polarized light component is used as the polarized light component to be measured, and an analyzer optical path for filtering out the polarized light component to be measured is designed; 4) measuring and recording the measured polarized light component caused by the transient change in the refractive index of the radiation conversion crystal material The amount of transient change in intensity of the polarized light component.

Description

Pulse gamma ray detection method and detection system based on laser polarization modulation

technical field

The invention relates to a pulse gamma ray detection method and detection system utilizing laser polarization and crystal refractive index modulation.

Background technique

With the emergence and development of ultrafast radiation sources such as ICF (Inertial Confinement Fusion), inverse Compton scattering sources, and free electron lasers, the conventional "radiation-electric" detection technology is limited by the space charge effect, and the time response of the system The ability is usually the fastest to the sub-ns (nanosecond) level, which can no longer meet the detection requirements of ultrafast radiation pulses.

The generation and relaxation time of the remaining carriers in the semiconductor is extremely short (<1ps), which makes the transient change process of the semiconductor refractive index induced by pulsed gamma rays very fast. This method is the main reason for obtaining the ultra-fast time response result (<10ps) of the system. The existing laser probe-based pulsed radiation detection method has two key links: one is to use semiconductor crystals to convert the ray particles into transient changes in the refractive index of the material, and then modulate the transient changes in the refractive index into the measuring arm of the interferometer; The second is to measure and record the transient changes of the laser interference signal. According to these research results, on the one hand, it is proved that using a semiconductor with a short carrier lifetime as a radiation conversion medium and using a laser as a probe can realize the measurement of the time spectrum of ultrafast pulsed gamma rays; on the other hand, the current system based on this detection method The detection sensitivity is very low, which is the biggest shortcoming of this detection technology. In addition, because the radiation detection technology based on the laser method is "active" detection, there is usually a laser DC background. When using a photomultiplier tube or an optical streak camera, etc. Gain photoelectric conversion devices, the higher the DC laser is, the more unfavorable it is for measurement, and it may cause permanent damage to devices such as large photomultiplier tubes. How to further enhance the laser modulation effect, change the technical idea of changing the refractive index of laser sensing materials, and improve the comprehensive conversion efficiency of radiation → laser is the key to achieving higher sensitivity for this type of laser active detection system. The practical significance of more diverse new "radiation-optic (Rad-Optic)" detection technology.

Contents of the invention

In order to solve the technical problem of low sensitivity of the existing active laser probe gamma ray detection method, the present invention provides a pulsed gamma ray detection method and detection system based on laser polarization modulation.

Technical solution of the present invention is:

The pulsed gamma ray detection method based on laser polarization modulation is special in that it includes steps:

1) Convert the gamma rays into non-equilibrium free carriers through the radiation conversion crystal, so that the refractive index of the radiation conversion crystal material undergoes a transient change;

2) The probe laser is incident on the radiation conversion crystal, and the transmittance and/or reflectance curves of the horizontal and/or vertical polarization components corresponding to different refractive indices when the probe laser is incident on the radiation conversion crystal are obtained according to Fresnel's theorem, so that Converting transient refractive index changes in radiation-converting crystal materials to transient changes in laser polarization components;

3) According to the transmittance and/or reflectance curves of horizontal and/or vertical polarization components corresponding to different refractive indices obtained in step 2), select a certain polarized light component as the polarized light component to be measured, and design it to filter out all Describe the analyzer optical path of the polarized light component to be measured;

4) Measuring and recording the transient change in the intensity of the polarized light component to be measured caused by the transient change in the refractive index of the radiation conversion crystal material.

Further, the radiation conversion crystal used in step 1) should have a sufficiently fast non-equilibrium free carrier relaxation time (compared with the pulse length to be measured, usually in the sub-ps order), the absorption limit of the radiation conversion crystal The wavelength of the probe laser light is smaller than the probe laser light, rendering the radiation conversion crystal transparent to the probe laser light.

Further, the incident end face of the radiation conversion crystal used in step 1) should be polished to ensure the uniformity of the laser incident interface. For the case of only using the front face to reflect light, at least ensure that the incident end face is polished; in addition , the incident end face and the exit end face should be as parallel as possible.

Further, the radiation conversion crystal used in step 1) is a photorefractive semiconductor with certain radiation resistance to ensure the repeatability of the measurement process and results, such as GaAs, InP, GaN, InGaAsP, CdTe, ZnO Wait.

Further, between steps 1-2), beam expansion processing is performed on the probe laser, and the beam spot size after beam expansion should be smaller than the beam spot size of pulsed gamma rays, so as to improve detection sensitivity.

Further, in step 2), the incident angle of the probe laser is determined according to the transmittance and/or reflectance curves of the radiation conversion crystal.

Further, the principle of selecting a certain polarized light component as the polarized light component to be measured in step 3) is:

If the refractive index of the radiation conversion crystal decreases, the reflection amount of the horizontal polarization component or the transmission amount of the vertical polarization component should be selected as the polarization component to be measured;

If the refractive index of the radiation conversion crystal increases, the transmission amount of the horizontal polarization component or the reflection amount of the vertical polarization component should be selected as the polarized light component to be measured.

Further, in step 4), there are two ways to measure and record the transient change of the intensity of the polarized light component to be measured: one is to use a photodetector equipped with an oscilloscope or an optical streak camera to perform a single pulse of the polarized light component to be measured. Measurement and recording; the other is to use the laser pulse autocorrelation method to obtain the laser pulse waveform of the polarized light component to be measured through multiple scans, which is mostly realized by Michelson interference optical path.

Further, in step 2), the radiation conversion crystal is placed in the laser resonator, the probe laser is incident at the Brewster angle of the radiation replacement crystal, and according to Fresnel's theorem, when the probe laser is incident on the radiation conversion crystal The reflectance curves of the horizontal polarization components corresponding to different refractive indices; in step 3), the reflection amount of the horizontal polarization component is selected as the polarization component to be measured.

The present invention also provides a pulsed gamma-ray detection system for realizing the above pulsed gamma-ray detection method based on laser polarization modulation, which is special in that it includes: lasers and radiation conversion crystals arranged in sequence along the optical path; A diaphragm, a half-wave plate, a polarization beam splitter and a Glan prism are sequentially arranged on the reflection or transmission optical path of the crystal;

It also includes a laser pulse measurement and recording system for recording the laser signal reflected by the polarization beam splitter prism and the laser signal output by the Glan prism.

Further, a beam expander mirror is also arranged between the laser and the radiation conversion crystal.

Further, the laser pulse measurement and recording system includes a first detector, a second detector and an oscilloscope; the first detector is used to detect the reflected laser light of the polarization beam splitter prism, and the second detector is used to detect the laser signal output by the Glan prism; the first Both the detector and the second detector are connected with an oscilloscope; the first detector and the second detector are used to convert the laser signal into an electrical pulse signal, and the oscilloscope is used to record the electrical pulse signal.

Further, the first detector and the second detector use a photomultiplier tube, a phototube, an ultrafast photodiode or a microchannel plate.

Further, when the Brewster's angle is used to incident the radiation conversion crystal, and the horizontal polarization component of the reflected light is selected as the laser quantity to be measured, the laser pulse measurement and recording system uses an optical streak camera.

The advantages of the present invention are:

1. Pulsed X and gamma rays are incident on semiconductor crystals to generate non-equilibrium free carriers, which can change the refractive index of semiconductor crystals by changing the local carrier number density. Some studies have found that for gallium arsenide, indium phosphide, and nitride For semiconductor crystals such as gallium and compound semiconductors based on them, the change in refractive index of semiconductors introduced by pulsed gamma rays is only at the level of 0.01-1. In order to solve the problem of low sensitivity of the existing active laser probe gamma ray detection method, the present invention starts from laser polarization and crystal Fresnel principle, takes the transient change of refractive index as the intermediate quantity, and successfully establishes the pulsed gamma ray The functional relationship S(E) between the signal and the laser polarization component.

2. The present invention has ultra-fast time response capability and high detection sensitivity.

The present invention utilizes the relationship between the laser polarization characteristics and the crystal refractive index to convert the change of the crystal refractive index into the change of the transmittance or reflectance of the laser polarization characteristic component at the interface of the semiconductor crystal, which makes it very convenient to improve the sensitivity of the system. The direct way is to increase the initial power of the probe light, and the output signal increases linearly with the initial light intensity I ₀ under the same transmittance or reflectance, so as to realize the ultrafast pulsed radiation particle detection target.

3. The present invention can realize a single measurement of the lifetime of non-equilibrium free carriers in a semiconductor.

The current method of measuring semiconductor dynamics mainly adopts the standard laser pumping-probe experimental method. The biggest advantage of the present invention is that a single measurement can more accurately reflect the dynamic change process of the entire carrier. In addition, regardless of whether the refractive index variation is positive or negative, the present invention can select different polarization components to convert into laser increments, so the refractive index variation can be measured when it is positive or negative.

4. The present invention realizes the fidelity conversion of pulsed gamma rays to laser signals, easily realizes the separation of "detection unit" and photoelectric conversion device, and can use optical fiber for long-distance lossless transmission of signals. When the signal is strong, it can directly Ultrafast photodiodes (products with a response time of about 10ps are sold) are used to measure the laser signal, and then recorded by a high-bandwidth digital oscilloscope, so that it is easy to measure the time spectrum of ultrafast ray pulses below 100ps.

5. When the extinction ratio of the polarizing device (polarization beam splitter, Glan prism) adopted in the present invention is high enough, the background light can be eliminated to close to 0, thereby when adopting laser measurement equipment such as photomultiplier tube, can exempt from direct current light length The problem of time exposure.

6. Since the radiation conversion crystal material can be selected and designed according to the conditions of the particles to be measured, it has the characteristics of flexible design and strong universality. With suitable materials, it can also be used to measure neutrons, protons and other radiation particles.

7. The angle range of the probe light incident on the radiation conversion crystal of the present invention is wide (may not be Brewster's angle), so in practical application, the optical path is easy to align.

Description of drawings

Fig. 1 is a schematic diagram of the principle of the detection method of the present invention.

Fig. 2 is the optical path structure of the detection system of the present invention (optical path design based on transmission polarization component modulation).

Fig. 3 is the optical path structure of the detection system of the present invention (optical path design based on reflection polarization component modulation).

Fig. 4 is a schematic diagram of a verification experiment layout of an intracavity modulation system based on the detection method of the present invention (only reflection is measured).

Figure 5 is the reflectance curve of the vertical polarization component of the reflected light corresponding to different Δn of the ZnO crystal. The dotted line in the figure indicates the reflectance curve of the vertical polarization component of the laser after the ZnO refractive index increases by 0.5, and the dotted line indicates the laser after the ZnO refractive index decreases by 0.5. Reflectance curves for vertically polarized components.

Figure 6 is the reflectance curve of the horizontal polarization component of the reflected light corresponding to different Δn of the ZnO crystal. The dotted line in the figure indicates the reflectance curve of the horizontal polarization component of the laser after the ZnO refractive index increases by 0.5, and the dotted line indicates the laser after the ZnO refractive index decreases by 0.5. Reflectance curve for the horizontally polarized component.

Figure 7 is the transmittance curve of the vertical polarization component of the transmitted light corresponding to different Δn of the ZnO crystal. The dotted line in the figure indicates the transmittance curve of the vertical polarization component of the laser after the ZnO refractive index is reduced by 0.5, and the dotted line indicates the laser after the ZnO refractive index is increased by 0.5. Transmission curve for the vertically polarized component.

Figure 8 is the transmittance curve of the horizontal polarization component of the transmitted light corresponding to different Δn of the ZnO crystal. The dotted line in the figure indicates the transmittance curve of the horizontal polarization component of the laser after the ZnO refraction is reduced by 0.5, and the dotted line indicates the laser after the ZnO refractive index is increased by 0.5 Transmittance curve for the horizontally polarized component.

Figure 9 is a comparison of the verification experiment results obtained when ZnO:Ga crystal is selected as the radiation converter, and the 632.8nm He-Ne laser is used as the probe light, and the monitoring signal is directly irradiated by the same type of photomultiplier tube used to measure the laser waveform. Signal.

Figure 10 is a comparison of the verification experiment results and monitoring signals obtained when GaAs semiconductor crystals are selected as radiation converters and 1064nm Nd:YAG solid-state laser beams are used as probe light. The monitoring signals are obtained by PIN semiconductor detectors.

Explanation of the reference signs in Fig. 2 and 3:

1-beam expander, 2-radiation conversion crystal, 3-diaphragm, 4-half-wave plate, 5-polarization beam splitter, 6-Glan prism, 7-oscilloscope.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing.

As shown in Figure 1, the detection method provided by the present invention involves five key links: (A) gamma rays are converted into non-equilibrium free carriers through radiation conversion crystals, (B) free carriers cause radiation conversion crystals The transient change of the refractive index, the (C ₁ ) or (C ₂ ) refractive index change causes the different polarization component amplitudes in the reflected and transmitted components of the probe laser to change, and the (D ₁ ) or (D ₂ ) polarization component changes by The resonant cavity realizes the amplification, and (E) realizes the measurement of the change amount of the specific polarized light component through the external optical path, and then realizes the measurement of the time spectrum of the pulsed radiation field.

If the refractive index of the radiation conversion crystal decreases (the direction of the change in the refractive index of the radiation-induced semiconductor is mostly decreased), then the reflection amount of the horizontal polarization or the transmission amount of the vertical polarization component should be used as the modulated amount (that is, the measured polarized light component) ; If the refractive index of the radiation conversion crystal increases, then the transmission amount of the horizontal polarization or the reflection amount of the vertical polarization component should be adopted as the modulated amount (the measured polarized light component);

If the refractive index of the radiation conversion crystal is reduced, and when the incident angle is smaller than the Brewster's angle (eg 10°), the amplitude change of the horizontal polarization component of the transmitted light can also be measured.

The detection method of the pulsed gamma ray of the present invention is mostly a single measurement, and for the measurement of the time spectrum of the pulsed gamma source with repetition frequency characteristics, the method of the present invention can also use the technical route of laser interference to obtain ultra-fast (below 100ps) pulsed gamma ray time spectrum waveform.

Fig. 4 is a schematic layout diagram of placing a radiation conversion crystal (GaAs semiconductor) in a laser resonator (only reflection is measured), which is one of the ways for the detection method of the present invention to improve the detection sensitivity of the system. Through the optical signal gain in the laser resonator cavity, the purpose of further improving the gamma ray detection sensitivity is achieved. In order to reduce the insertion loss of the radiation conversion crystal in the laser resonator, the incident angle between the radiation conversion crystal and the laser must strictly follow the Brewster's angle. When incident at Brewster's angle, the amplitude change of the horizontal polarization component in the light reflected by the radiation conversion crystal is generally measured, and the amplitude change of the vertical polarization component transmitted out of the radiation conversion crystal can also be measured.

In addition, since the detection sensitivity can not only be improved through the resonant cavity, it is also possible to directly measure the change of the laser polarization characteristics without using the resonant cavity, so as to achieve the measurement goal of the low-intensity pulsed gamma-ray time spectrum.

From the point of view of the system implementation of the above detection method, there are two key links in technology: one is the effective modulation of the polarization component of the reflected light by the transient effect of the refractive index of the radiation conversion crystal, and the other is the control of the transient change process of the polarization component to be measured. Reliable measurement and recording.

Like other "radiation-light" detection methods based on semiconductor refractive index modulation, the time response of the detection system is determined by the generation and relaxation time of non-equilibrium free carriers in the semiconductor, and the relaxation time of the carriers actually reflects What is important is the lifetime of free carriers, that is, by using semiconductors with ultra-short carrier lifetimes, the system time response capability of less than 10 ps can be achieved. However, the existing "radiation-light" detection method is based on the external interferometry scheme. Since the thickness of the radiation conversion crystal and the time response of the detection system are contradictory quantities, the system sensitivity of the existing method is very low, and it is difficult to achieve the measurement goal of the MeV-level low-intensity pulsed gamma-ray time spectrum. The main difference between the present invention and the existing "radiation-light" detection method is reflected in the measurement method of the refractive index change. We modulate the refractive index change to the polarization transmittance/reflectivity of the probe laser, thereby increasing the lifting system Effective approach to sensitivity.

Based on the above theory, the system implementation of the detection method of the present invention has the following two modes:

As shown in Figure 2, it is an implementation of the pulsed gamma-ray detection system of the present invention. This scheme is based on the modulation of the laser transmission polarization component, including a laser, a beam expander 1 (not necessary), and a radiation conversion crystal arranged in sequence along the same optical path. 2, diaphragm 3, half-wave plate 4, polarization beam splitter prism 5 and Glan prism 6; Diaphragm 3 is located on the transmission optical path of radiation conversion crystal 2; Glan prism 6 is located on the transmission optical path of polarization beam splitter prism 5; also includes for A laser pulse measuring and recording system for recording the laser signal reflected by the polarization beam splitter 5 and the laser signal output by the Glan prism 6.

The principle of the program:

The pulsed gamma ray to be measured is vertically incident on the radiation conversion crystal 2, and non-equilibrium free carriers are generated in the radiation conversion crystal 2 to realize transient modulation of the refractive index of the radiation conversion crystal. The probe laser (horizontal linearly polarized light or non-polarized light) whose wavelength is greater than the absorption limit of the radiation conversion crystal 2 output by the laser is expanded by the beam expander 1, and the beam is expanded at an appropriate angle (the angle depends on the radiation conversion crystal material itself). The characteristic refractive index is determined) incident into the radiation conversion crystal 2, according to Fresnel's theorem, there are two polarization components of the horizontal polarization component T _p and the vertical polarization component T _s in the transmitted light of the radiation conversion crystal 2, which are filtered out by the aperture 3 For the stray light in the transmitted light, use the half-wave plate 4 to control the polarization direction, and then use the polarization beam splitter 5 to separate the two polarization components; because the Glan prism 6 has a high extinction ratio, it can be used to improve the "purity" of the polarization component , and then the probe laser light can be transmitted to the laser pulse measurement and recording system by using the image transfer optical path.

As shown in Figure 3, it is another implementation of the pulsed gamma-ray detection system of the present invention. This scheme is based on laser reflection polarization component modulation, including lasers, beam expanders 1 (not necessary) and radiation conversion arranged in sequence along the optical path. Crystal 2; an aperture 3, a half-wave plate 4, a polarization beamsplitter prism 5, and a Glan prism 6 are sequentially arranged on the reflection optical path of the radiation conversion crystal 2; the laser signal reflected by the polarization beamsplitter prism 5 and the Glan prism 6 are also included Laser pulse measurement and recording system for the output laser signal.

The principle of the program:

The pulsed gamma rays to be measured are vertically incident on the radiation conversion crystal 2 to generate non-equilibrium free carriers in the radiation conversion crystal 2 to realize transient modulation of the refractive index of the radiation conversion crystal 2 . The probe laser output by the laser is incident on the surface of the radiation conversion crystal 2 at a certain angle, especially when it is incident on the radiation conversion crystal 2 at the Brewster angle of the radiation conversion crystal 2, according to Fresnel's theorem, in principle The reflected light of the radiation conversion crystal 2 only contains the vertical polarization component R _s , and the horizontal polarization component R _p is 0, but when the free carriers generated by the pulse gamma change the refractive index of the radiation conversion crystal 2, the radiation The horizontal polarization component R _p in the probe laser reflected by the conversion crystal 2 is no longer 0, and the increase of the reflectivity of the horizontal polarization component R _p is only related to pulsed gamma rays. In this case, the direct current of the probe laser In principle, the bottom can be 0, which cannot be realized by the existing "radiation-light" detection method. The diaphragm 3 in the optical path is used to filter out stray light and reflected light from the rear end surface of the radiation conversion crystal 2 . The functions and principles of other components are consistent with those shown in FIG. 1 , and will not be repeated here.

There are two schemes for the laser pulse measurement and recording system in the pulsed gamma ray detection system shown in Figure 2 and Figure 3:

The first kind of scheme is to comprise first detector D1, second detector D2 and oscilloscope 7; Laser signal; both the first detector D1 and the second detector D2 are connected to the oscilloscope 7 . The first detector D1 and the second detector D2 are used to convert the laser signal into an electrical pulse signal, and the oscilloscope 7 is used to record the electrical pulse signal. The first detector D1 and the second detector D2 may use photomultiplier tubes, phototubes, ultrafast photodiodes or microchannel plates.

The second solution is to use an optical streak camera to directly record laser signals, which is suitable for measuring ultrafast pulse X/γ time spectra below 10ps.

In the implementation shown in Figure 2 and Figure 3:

The output wavelength of the laser is selected according to the absorption spectrum of the radiation conversion crystal 2 , and the output wavelength of the laser should be within the light transmission range of the radiation conversion crystal 2 . The type of laser can be a DC laser or a pulsed laser. When a pulsed laser is used, its pulse width must be greater than that of the pulsed gamma ray to be measured. The higher the laser power or energy stability, the better. In addition, the higher the laser power, the higher the detection sensitivity of the system. When the laser output is linearly polarized, the higher the degree of polarization the better (at least greater than 100:1). The output of most linearly polarized lasers is horizontally polarized.

The polarization beam splitter 5 is mainly used in the case of branching measurement. For the case where the detection sensitivity is high, the beam expander 1 can be used to properly bracket the laser beam before the incident laser beam, thereby increasing the volume of the sensitive area.

When the initial laser is not polarized light output, it is necessary to add a laser polarization device for laser polarization, that is, to generate linearly polarized light. The main principle of selection is that the device should have a high extinction ratio and low laser absorption loss.

The radiation conversion crystal 2 is a semiconductor or other optical crystal. In addition to matching the output wavelength of the laser, it is also necessary to consider the cross-section of the interaction between the semiconductor or optical crystal and the pulsed gamma ray according to the specific requirements of the sensitivity of the detection system; in addition, the semiconductor Or the time characteristics and specific specifications of the optical crystal need to be specifically designed according to the characteristics of the radiation particles to be measured. The incident light side of the semiconductor or optical crystal must be polished, and the other side can be used whether it is polished or not. In particular, if intracavity modulation is to be used (i.e. placing a radiation conversion crystal in the laser resonator), only Brewster's angle of incidence can be used, and the amount to be modulated is the reflectivity of the horizontal polarization of the laser.

Therefore, the selection of radiation conversion crystal 2 mainly follows the following principles:

(1) It has a high gamma cross-section, and there is a radiation transient refractive index effect to generate enough non-equilibrium free carriers, such as GaAs, InP, GaN:Mg, CdTe and other photorefractive semiconductors;

(2) The absorption limit corresponding to the forbidden band width is smaller than the wavelength of the probe light, so that the radiation conversion crystal itself is transparent to the probe light;

(3) The radiation conversion crystal should generally be polished on both sides to ensure the uniformity of the laser incident interface. For the case of only using the front surface to reflect light, at least ensure that the incident end surface is polished. In addition, the two end surfaces of the radiation conversion crystal should be as far as possible. parallel;

(4) The lifetime of the remaining carriers in the radiation conversion crystal is short enough to ensure that the time response of the system is fast enough;

(5) It has a certain anti-radiation performance to ensure the repeatability of the measurement process and results of the detection system.

The theoretical basis of the present invention is the optical Fresnel formula, which is deduced and analyzed according to different polarization components, and it is easy to obtain different laser polarization components when the probe laser is incident on the zinc oxide crystal (one of the radiation conversion crystals) shown in Figure 5-8. These curves are the basic basis for selecting the amount of probe laser to be modulated, where R _p and T _p represent the horizontal polarization component, R _s and T _s represent the vertical polarization component, and there are reports in the literature that gamma rays introduce The change in the refractive index of the zinc oxide crystal is usually in the range of 0.01 to 1. In order to clearly show the relative relationship between the curves before and after the change in the refractive index of the zinc oxide crystal, a change of 0.5 is used in the calculation. For example, it can be seen from Figure 6 that when the incident angle of the probe laser is Brewster's angle, the reflectivity of the horizontal polarization is 0, and when the pulsed gamma ray particles are incident into the zinc oxide crystal, due to the change of the refractive index (Reduced) causes the actual Brewster angle of the zinc oxide crystal to change before and after the incident, and the dotted line curve is above the solid line curve, so the reflectivity of the horizontal polarization component during the change of the refractive index of the zinc oxide crystal It will increase with the decrease of the zinc oxide crystal refractive index, and the time spectrum waveform of the "time-intensity" of the pulsed radiation particles can be obtained by measuring the change of the reflectivity of the horizontal polarization component.

In the verification experiment, a linearly polarized He-Ne laser was used as the probe laser with a laser wavelength of 623.8nm and a power of 17mW. The semiconductor crystal ZnO:Ga is used as the radiation conversion crystal, the refractive index of the crystal is 2.008, and the corresponding Brewster angle is 63.5°, and the ray source is the high-current gamma source produced by the "Qiangguang No. 1" accelerator. As shown in Figure 2, the semiconductor crystal ZnO:Ga is fixed on the rotating stage, the probe laser is incident according to the Brewster angle of the semiconductor crystal ZnO:Ga, and the horizontal polarization component in the reflected light is measured according to the principle. In principle, the reflected light at this time The horizontal polarization component becomes 0, that is, when the horizontally polarized light is incident, the reflected light is 0, but due to the limited degree of polarization of the laser, the reflected light is actually at the minimum value, and the ordinary photomultiplier tube is used for photoelectric conversion, and then transmitted to A digital oscilloscope with a bandwidth of 1GHz is used for recording, and the radiation source monitoring signal is given by direct illumination (appropriately shielded) of a photomultiplier tube of the same type.

The results of the verification experiment are shown in Figure 9. It can be seen that the leading edge of the pulse is in good agreement, the three peak positions of the peak position can be clearly distinguished, and the performance of noise on the trailing edge of the pulse is also basically the same. The trailing edge part of the measured signal is slower than the actual gamma source signal. The main reason is that the semiconductor crystal used in the experiment is gallium-doped zinc oxide crystal. In theory, the carrier generation is very fast, but the relaxation time is relatively pure. Zinc oxide crystals are much slower, so we directly verified the viability of the method experimentally. When performing ultrafast pulse measurements, it is necessary to select radiation conversion crystals with carrier generation relaxation times below sub-ps, such as GaAs and InP.

Figure 10 shows the experimental results based on GaAs crystals. Due to the crystal absorption limit, the probe laser is changed to horizontally polarized light with a wavelength of 1064nm. The X-ray source uses X-ray pulses with a peak energy of about 370keV, and the GaAs semiconductor is vertically incident. The pulse intensity >10 ⁹ ph./pulse, the nominal pulse width is 15ns, and the pulse dose at the exit end is 4mR-7mR. The reflected light coupled out by the crystal is firstly filtered out by a polarization beam splitter to filter out the horizontally polarized component. In order to increase the extinction ratio, a Glan (Glan) prism is added to the transmission path of the polarization beam splitter, and finally the coupled out horizontally polarized component is transmitted to the lead Among the shielded photomultiplier tubes, the photomultiplier tube used in the experiment is 9850B, and the negative high voltage is set to 1kV. The X-ray source waveform monitoring and oscilloscope trigger signals are provided by standard PIN detectors, with a time response of about 10ns and a voltage of -400V. It can be seen that the system waveform is in good agreement with the source characteristics, and the "secondary peak" superimposed on the trailing edge is introduced by the gain medium of the solid-state laser irradiated by pulsed X-rays, which can be eliminated by strengthening the shield or the background scale in practical applications. From Figure 10, it can be roughly obtained that the detection sensitivity is 5.43×10 ^-17 C·cm ² (the photon energy is 150KeV).

Detection sensitivity analysis of the detection system based on the detection method of the present invention:

Gamma detection sensitivity is one of the key characteristic quantities of the detection system, and higher sensitivity is one of the goals of detection system design optimization. According to the output characteristics of the detection system, combined with the general definition of detection sensitivity, the general expression of the sensitivity of the pulsed gamma ray detection system based on laser polarization modulation can be derived. It may be assumed that the horizontal polarization component of the reflected light is the polarized light component to be measured, and the incident laser is horizontal linearly polarized light incident at Brewster’s angle. Then the instantaneous power change of the vertical polarization component of the laser caused by pulsed gamma rays is:

The refractive index change Δn(t) is determined by the non-equilibrium free carrier concentration ΔN(t) introduced by gamma rays, which is used as the output of laser polarization modulation, combined with the optical gain coefficient of the laser resonator and the laser measurement equipment The output of the detection system can be obtained by the signal amplification factor. Then the general expression of detection method sensitivity can be obtained:

make is the particle energy conversion efficiency of the interaction between gamma rays and radiation conversion crystals, then:

where G is the gain coefficient of the laser resonator, G _A is the signal amplification factor of the laser measuring equipment, P _i,|| is the incident laser power, η _γ is the energy conversion efficiency of pulsed gamma rays in the semiconductor, that is, a single gamma photon The number of non-equilibrium free carriers generated can be theoretically estimated by the Monte Carlo method, θ ₁ is the incident angle of the probe laser, and similar expressions can be used for other polarization components.

Therefore, according to the general expression S(E) of detection system sensitivity based on horizontal polarization reflectance modulation, combined with the working principle of the detection method, it can be seen that if the signal gain of the laser measurement and recording system is not considered for the time being, the detection method of the present invention can also Improve the detection sensitivity of the system through other means, mainly involving two factors: laser and material:

(1) The optical output power of the probe is the most direct way to improve the detection sensitivity of the system. According to the principle of the method of the present invention, the actual modulation effect of the pulsed gamma ray signal is to affect the transmittance or reflectance of the polarization component. Therefore, under the condition of certain transmittance or reflectance, increasing the incident light power can increase the signal amplitude. To achieve the purpose of improving the detection sensitivity of the system, it should be noted that the laser power cannot be increased indefinitely, which is mainly limited by the thermal effect of the semiconductor and the limited extinction ratio of the polarizing device.

(2) The incident angle of the laser is also one of the ways to improve the sensitivity of the system. According to the change of the transmittance and reflectance curves of each polarization component, it can be seen that not every polarization component can obtain the maximum modulation amount at the Brewster angle position. Therefore, according to different materials, the calculated curve can be Optimizing the laser incident angle.

(3) Utilizing the modulation method in the resonant cavity is another advantage of the high sensitivity of the detection method. Semiconductors can be used as the coupling output mirror of the laser resonator, and the modulation of the polarized light coupling output efficiency can be realized through the modulation of the refractive index. The modulation signal can be amplified through the intracavity gain of the laser, and higher detection sensitivity can be obtained than that of the extracavity system.

(4) Semiconductor radiation conversion efficiency is the determining factor of the non-equilibrium free carrier concentration. The use of semiconductor materials with a higher gamma cross-section can improve the sensitivity of the detection system to a certain extent. The non-equilibrium free carrier concentration generated by pulsed gamma rays It can be estimated using a Monte Carlo method. Obviously, this factor is universal, and due to the limited thickness, it is not the advantage of this detection method to improve the system sensitivity by increasing the energy deposition efficiency, and it is difficult to implement due to the limitation of the wavelength of the probe light.

Claims (15)

1. the impulse gamma X-ray detection X method based on laser polarization modulation, which is characterized in that comprising steps of

1) gamma ray is converted into non-equilibrium free carrier by radiating conversion crystal, makes the folding for radiating conversion crystal material It penetrates rate and transient changing occurs；

2) probe laser is incident to radiation conversion crystal, probe laser is obtained according to Fresnel theorem and is incident to radiation conversion crystalline substance Different refractivity corresponding horizontal and/or vertical polarisation component transmissivity and/or reflectance curve when body, so that radiation be turned The transient state variations in refractive index for changing crystalline material is converted to the transient changing of laser polarization component；

3) according to the resulting different refractivity of step 2) corresponding horizontal and/or vertical polarisation component transmissivity and/or reflection Rate curve chooses a certain polarized light component as polarized light component to be measured, and designed for filtering out the polarized light component to be measured Analyzing optical path；

4) the polarized light component intensity to be measured caused by radiation conversion crystal Refractive Index of Material transient changing is measured and recorded Transient state knots modification.

2. the impulse gamma X-ray detection X method according to claim 1 based on laser polarization modulation, it is characterised in that: step It is rapid 1) employed in radiation conversion crystal should have a sufficiently fast non-equilibrium free carrier relaxation time, radiate conversion crystal Absorption limit be less than the probe laser wavelength.

3. the impulse gamma X-ray detection X method according to claim 1 based on laser polarization modulation, it is characterised in that: step It is rapid 1) employed in radiation conversion crystal, incident end face should make polishing treatment, and its incident end face should use up with outgoing end face Amount is parallel.

4. the impulse gamma X-ray detection X method according to claim 1 based on laser polarization modulation, it is characterised in that: step It is rapid 1) employed in radiation conversion crystal be Preset grating semiconductor, and have certain radiation resistance.

5. the impulse gamma X-ray detection X method according to claim 1 to 4 based on laser polarization modulation, feature It is: between step 1-2), probe laser is carried out to expand processing, the beam spot after expanding should be less than the beam of impulse gamma ray Spot size.

6. the impulse gamma X-ray detection X method according to claim 5 based on laser polarization modulation, it is characterised in that: step It is rapid 2) in, the incident angle of probe laser is determined according to the transmissivity and/or reflectance curve of radiation conversion crystal.

7. the impulse gamma X-ray detection X method according to claim 6 based on laser polarization modulation, it is characterised in that: step Rapid 3) the middle a certain polarized light component of selection is as the principle of polarized light component to be measured:

If the refractive index of radiation conversion crystal reduces, the volume reflection or vertical polarisation component of horizontal polarisation component should be chosen Transmission amount is as polarized light component to be measured；

If the refractive index of radiation conversion crystal increases, the transmission amount or vertical polarisation component of horizontal polarisation component should be chosen Volume reflection is as polarized light component to be measured.

8. the impulse gamma X-ray detection X method according to claim 7 based on laser polarization modulation, it is characterised in that: step It is rapid 4) in, measuring and recording the transient state knots modification of polarized light component intensity to be measured, there are two types of modes:

One is the lists that polarized light component pulse to be measured is carried out using photodetector outfit oscillograph or optic streak camera Secondary measurement and record；

Another kind is the laser pulse for obtaining polarized light component to be measured by Multiple-Scan using the autocorrelative mode of laser pulse Waveform.

9. the impulse gamma X-ray detection X method according to claim 1 based on laser polarization modulation, it is characterised in that:

In step 2), radiation conversion crystal is located at that laser resonance is intracavitary, probe laser filled using radiation change crystal cloth scholar this Special angle is incident, and it is corresponding horizontal inclined to obtain different refractivity when probe laser is incident to radiation conversion crystal according to Fresnel theorem The reflectance curve of vibration component；

In step 3), the volume reflection of horizontal polarisation component is chosen as polarized light component to be measured.

10. the impulse gamma X-ray detection X method according to claim 1 based on laser polarization modulation, it is characterised in that:

If the probe laser for being incident to radiation conversion crystal in step 2) is horizontal linear polarization light, with brewster angle incidence, step It is rapid 3) in the reflected light of horizontal polarisation component for polarized light component to be measured, then the incident angle θ of probe laser₁, probe laser Incident power P_i,||It chooses according to the following formula:

In above formula:

η_γFor the particle energy transfer efficiency of gamma ray and radiation conversion crystal interaction；

G is laser resonator gain coefficient；

G_AFor laser measuring equipment signal amplification factor；

Δ n (t) is refractive index variable quantity；

Δ N (t) is the non-equilibrium free carrier concentration that gamma ray introduces.

11. realizing the pulse of any impulse gamma X-ray detection X method based on laser polarization modulation of claim 1-10 Gamma-ray detection system characterized by comprising the laser and radiation conversion crystal set gradually along optical path；It is radiating Diaphragm, half-wave plate, polarization splitting prism and Glan prism are disposed in the reflection of conversion crystal or transmitted light path；

It further include the laser of the laser signal of the laser signal for recording polarization splitting prism reflection and the output of Glan prism Impulsive measurement and record system.

12. impulse gamma ray detecting system according to claim 11, it is characterised in that: converted in laser and radiation Beam expanding lens is additionally provided between crystal.

13. impulse gamma ray detecting system according to claim 11, it is characterised in that: laser pulse measurement and record System includes the first detector, the second detector and oscillograph；Reflection of first detector for detecting polarization Amici prism swashs Light, the second detector are used to detect the laser signal of Glan prism output；First detector and the second detector are and oscillograph It is connected；First detector, the second detector are used to be converted to laser signal electric impulse signal, and oscillograph is for recording the electricity arteries and veins Rush signal.

14. impulse gamma ray detecting system according to claim 13, it is characterised in that: the first detector and second is visited It surveys device and uses photomultiplier tube, photoelectric tube, ultrafast photodiode or microchannel plate.

15. impulse gamma ray detecting system according to claim 11, it is characterised in that: laser pulse measurement and record System uses optic streak camera.