A relativistic ultrashort intense laser pulse (1018 W/cm2) interacting with a target can produce a large number of high-energy electrons, and these can then drive a high-flux continuous gamma-ray source.
Matter and Radiation at Extremes, Volume. 6, Issue 1, 014401(2021)
Compact broadband high-resolution Compton spectroscopy for laser-driven high-flux gamma rays
A compact broadband Compton spectrometer with high spectral resolution has been designed to detect spectra of laser-driven high-flux gamma rays. The primary detection range of the gamma-ray spectrum is 0.5 MeV–13 MeV, although a secondary harder gamma-ray region of 13 MeV–30 MeV can also be covered. The Compton-scattered electrons are spectrally resolved using a curved surface detector and a nonuniform magnetic field produced by a pair of step-like magnets. This design allows a compact structure, a wider bandwidth, especially in the lower-energy region of 0.5 MeV–2 MeV, and optimum spectral resolution. The spectral resolution is 5%–10% in the range 4 MeV–13 MeV and better than 25% in the range 0.5 MeV–4 MeV (with an Al converter of 0.25 mm thickness and a collimator of 1 cm inner diameter). Low-Z plastic materials are used on the inner surface of the spectrometer to suppress noise due to secondary X-ray fluorescence. The spectrometer can be adjusted flexibly via a specially designed mechanical component. An algorithm based on a regularization method has also been developed to reconstruct the gamma-ray spectrum from the scattered electrons.
I. INTRODUCTION
A relativistic ultrashort intense laser pulse (1018 W/cm2) interacting with a target can produce a large number of high-energy electrons, and these can then drive a high-flux continuous gamma-ray source.
Kim et al.
In the present study, we develop a new Compton spectrometer using permanent magnets and incorporating an improved design of electron magnetic spectrometer to further enhance spectral resolution, expand the measurement bandwidth (especially toward the lower-energy region), and reduce the size and weight of the spectrometer so that it can be manipulated by one person. A nonuniform magnetic field generated by a pair of step-like magnets is used to extend the measurement bandwidth. The primary measurement range is 0.5 MeV–13 MeV with higher spectral resolution, and a secondary higher-energy range of 13 MeV–30 MeV can also be covered, but with lower spectral resolution. A curved surface detector, which is placed at the imaging points of the electron beams after they have passed through the magnetic spectrometer, is designed to improve the spectral resolution. In the range 4 MeV–13 MeV, the spectral resolution is 5%–10%, and in the range 0.5 MeV–4 MeV, it is 10%–25% (using an Al converter of thickness 0.25 mm and a collimator of inner diameter 1 cm). The size of the electron magnetic spectrometer is reduced to 175 × 270 × 145 mm3, and its weight is reduced to 24 kg. Among all the Compton spectrometers that have been described in the literature, the present spectrometer has the smallest volume, the lowest energy (0.5 MeV) at the lower boundary of spectral coverage, and the best spectral resolution (at the same size and similar bandwidth). In the inner surface of the electron magnetic spectrometer, low-Z materials are used to suppress noise due to secondary X-ray fluorescence. The separated design of the scattered electron magnetic spectrometer and the shielding/collimating components mean that this Compton spectrometer can be easily installed and adjusted with the help of an adjusting mechanism. The performance of the spectrometer has been verified by Monte Carlo simulation, and a spectral reconstruction method based on Tikhonov regularization has been developed.
II. DESIGN OF COMPTON SPECTROMETER
A. Principle of Compton spectrometer
The Compton spectrometer is based on the Compton scattering process. The extranuclear electrons of the converter collide with incident photons, gaining energy and escaping from the nucleus as free electrons. The energy of a scattered free electron is
Based on Eq.
It should be noted that Eq.
B. Mechanical design of the Compton spectrometer
The overall mechanical structure of the spectrometer is shown in
Figure 1.Mechanical structure of the Compton spectrometer. (a) Top view. (b) Side view. (c) Adjusting mechanism, allowing fine adjustment of translation, rotation, and pitch angle. The flanges around this mechanism limit the position and prevent the spectrometer from sliding.
The shielding/collimator component consists of five lead bricks with a total thickness of 185 mm. Of these bricks, four have masses of 20 kg and dimensions of 270 mm (width) × 145 mm (height) × 45 mm (thickness), while the thinner brick in the middle is only 5 mm thick. There is a 5 × 20 mm2 square hole through the tops of the bricks to facilitate placement of the converter from above. The purpose of these lead bricks is to provide shielding from direct gamma-ray penetration. The residual intensity of the 20 MeV gamma-ray beam behind the lead shielding is three orders of magnitude lower than that of the scattered electron signal. A collimator with an inner diameter of 10 mm in the center of the lead bricks is used to constrain the solid angle of the incident photons and the scattered electrons such that the condition φ ≈ 0 is satisfied.
The shell of the electron magnetic spectrometer is made of iron of thickness 10 mm. The spectrometer has dimensions 175 × 270 × 145 mm3 and mass 24 kg. Compared with the spectrometers described previously in the literature,
As shown in
We found that secondary X-ray fluorescence is produced when the electrons strike the surfaces of the magnets, which generates background noise on the IP. To suppress this X-ray fluorescence noise, four measures are adopted. First, either a plastic sheet of thickness 1 mm or an additional pair of magnets is placed in front of the converter to block externally incident electrons. Second, a PTFE electron beam collimator (inner diameter 10 mm, outer diameter 20 mm, and length 100 mm) is used to prevent excitation of stray X-ray fluorescence in the hole of the collimator by scattered electrons with larger scattering angles. Third, as shown in
Although our electron magnetic spectrometer is very light compared with others,
C. Simulation of magnetic field profile and electron beam trajectory
The electron magnetic spectrometer component is the core of the Compton spectrometer. We simulated the magnetic field distribution and electron beam deflection using the COMSOL Multiphysics code
Conventional Compton spectrometers
Figure 2.Magnetic field profile in the central (
Figure 3.Trajectories of electron beams dispersed in the nonuniform magnetic field. These trajectories represent incident electron beams with energies 0.5 MeV and 1 MeV–13 MeV at 1 MeV intervals (“first IP”). The trajectories of electron beams with energies 14 MeV–30 MeV are also shown (“second IP”).
It can also be seen from
The imaging points of higher-energy electrons (>13 MeV) are outside the magnetic field region. To retain the compactness of the electron magnetic spectrometer, we give up the requirement of high spectral resolution for these higher-energy electron beams. We measure the dispersed electrons with energies above 13 MeV at the out-of-focus position (the second IP), and this allows coverage of a broadband energy range up to 30 MeV. Although the resolution is poor in this secondary region, it can still be used to provide reference data for analysis. The scattered electron spectrum above 30 MeV will be smeared by stray gamma rays directly incident on the converter, which places an upper limit on the detection region.
D. Design of the converter and the collimator
To determine the thickness of the Al converter and the size of the Pb collimator, we used the GEANT4 Monte Carlo code
Figure 4.Simulation of the scattered electron energy spectrum emitted by aluminum converters of various thicknesses from 0.1 mm to 3 mm. The gamma rays are vertically incident on the converter with a photon energy of 6 MeV and a photon number of 1 × 107. The collection angle of the scattered electrons is 0.0078 sr.
Figure 5.Scattered electron spectrum at different electron collection angles. The gamma rays are vertically incident on the 1 mm aluminum converter with a photon energy of 6 MeV and a photon number of 1 × 107.
E. Response function and spectral resolution
To calculate the response matrix R from the gamma-ray spectrum to the electron spectral curve, we used the GEANT4 Monte Carlo code to simulate the entire process of operation of the Compton spectrometer, including Compton scattering in the converter, electron beam dispersion in the magnetic field, and deposition of the energy of the scattered electrons in the IP. The standard electromagnetic process library was used in the GEANT4 simulation, which considered the photoelectric effect, Compton scattering, and the electron pair effect for the gamma photons, the ionization, bremsstrahlung, and multiple scattering effects for the electrons, and the annihilation effect for the positrons.
Figure 6.(a) and (b) Spectral curves (solid lines) of scattered electrons produced by a 1 mm aluminum converter for monoenergetic incident gamma rays with energies in the ranges 0.5 MeV–13 MeV and 15 MeV–30 MeV, respectively. Scattered positron signals for incident gamma rays with energies 12 MeV, 13 MeV, and 30 MeV are also shown (dashed lines). (c) and (d) Scattered electron spectral curves with the positron–electron pair signal subtracted for monoenergetic incident gamma rays with energies in the ranges 0.5 MeV–13 MeV and 15 MeV–30 MeV, respectively. In the simulations, the photon number in each monoenergetic incident beam was 5 × 107. Note that the spectral curves for 15 MeV–30 MeV were recorded on the second IP.
The spectral curves for the secondary detection range from 13 MeV to 30 MeV are also provided in
We also present the spectral curves of scattered electrons generated by a 0.25 mm Al converter in
Figure 7.Spectral curve of scattered electrons produced by a 0.25 mm aluminum converter and 0.5 MeV–13 MeV monoenergetic gamma rays (the positron–electron pair signal has been subtracted). The parameter settings are the same as in
The sensitivity of the Compton spectrometer can be inferred using the simulated response matrix. We integrate the scattered electron spectrum to obtain the number of scattered electron numbers by monoenergetic incident gamma rays, which is an immediate result from calculation of the spectral curves of
Figure 8.The minimum numbers of incident photons required to produce an effective scattered electron.
The sensitivity with a thin converter of 0.25 mm exhibits an interesting characteristic that will be of benefit to low-energy gamma-ray spectrum detection. As expected, in the high-energy gamma-ray region, the sensitivity with a thin converter is used is lower than that with a thick one. For example, at gamma-ray energies of 6 MeV and 2 MeV, respectively, 1.2 × 104 photons and 3.1 × 104 photons are needed, which are respectively 2.4 and 1.9 times the numbers required with a 1 mm converter. However, in the lower-energy gamma-ray region below 2 MeV, the sensitivity decreases more slowly than in the case of a thick converter. At 0.5 MeV, the sensitivity with a thin converter is not significantly different from that with a thick converter: 1.2 × 105 photons are needed to produce one scattered electron in the case of a 0.25 mm converter, which is close to the 1.6 × 105 photons required with a 1 mm converter. Thus, thin converters preferred for the detection of low-energy gamma rays in the range 0.5 MeV–2 MeV, since they have similar sensitivity to thick converters, but higher spectral resolution (see below).
The spectral resolution of the Compton spectrometer in
Figure 9.(a) Spectral resolution of the Compton spectrometer with 0.25 mm and 1 mm Al converters in the primary detection region of 0.5 MeV–13 MeV. The results for other spectrometers
It is likely that the resolution of the spectrometer can be further improved in three aspects. First, as we mentioned earlier, the use of a thinner converter can reduce the multiple scattering of photons and thereby increase the spectral resolution. As shown in
F. Spectral reconstruction method
Using the response matrix from
Compton spectrometers are mainly applied to the measurement of continuous spectra, usually bremsstrahlung spectra. Therefore, we assumed an exponential decay profile for the incident gamma-ray spectrum Sγ. We then calculated the corresponding electron spectrum Se using Eq.
Figure 10.Numerical experiments on spectral reconstruction. (a) Original (blue line) and reconstructed (red line) gamma-ray spectra
III. SUMMARY
We have designed a compact high-resolution broadband Compton spectrometer to measure gamma-ray energy spectra primarily in the range from 0.5 MeV to 13 MeV but also in the secondary higher-energy range from 13 MeV to 30 MeV. The spectral resolution is better than 10%–22% for 0.5 MeV–4 MeV and 5%–10% for 4 MeV–13 MeV, and could be further optimized by using a thinner converter and a smaller collection angle. The spectrometer is smaller and lighter than previous Compton spectrometers and can be operated by a single person. It provides broadband spectral coverage, especially at lower energies, and high resolution with a limited volume. It also has the advantages of easy adjustment and low X-ray fluorescence noise, which make it particularly suitable for detection of gamma rays driven by intense lasers or electron beams. This spectrometer has already been successfully used for the detection of high-flux gamma-ray sources.
AUTHORS’ CONTRIBUTIONS
Z.-C.Z. and T.Y. contributed equally to this work.
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Zhen-Chi Zhang, Tao Yang, Guang-Yue Hu, Meng-Ting Li, Wen Luo, Ning An, Jian Zheng. Compact broadband high-resolution Compton spectroscopy for laser-driven high-flux gamma rays[J]. Matter and Radiation at Extremes, 2021, 6(1): 014401
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Received: Aug. 22, 2020
Accepted: Nov. 5, 2020
Published Online: Apr. 22, 2021
The Author Email: Hu Guang-Yue (gyhu@ustc.edu.cn)