Abstract
Keywords
1 Introduction
Fast neutron absorption spectroscopy (FNAS) based upon traditional accelerators or reactors has applications in many regions, including the non-destructive testing of goods[1–3], the measurement of nucleus fine structures[4,5], the study of material characteristics[6,7], etc. The energy resolution of FNAS is mainly limited by the duration of the neutron source, and the traditional fast neutron source is usually longer than 1 nanosecond (ns)[8–10]. So far, the energy resolution of the state-of-the-art traditional source is approximately 100 keV at 3 MeV, which makes it impossible for this source to accurately measure the width or shape of resonance absorption peaks of nuclei, for example,
The laser plasma wakefield accelerator (LWFA) has attracted significant interest in the past decades[13–15], due to the extremely high acceleration gradient and beam current[16,17], thus enabling GeV electron accelerators at the tabletop level, as well as driving secondary radiation/particle sources with ultrahigh brightness/flux[11,18]. LWFAs have achieved many successes in recent years, including record beam energy of up to 8 GeV in cm length[19], beam duration down to femtoseconds (fs), driven X-ray free-electron lasers[20–22], etc. Moreover, the LWFA fs duration electron beam can drive ultra-fast Bremsstrahlung
However, in experiments, it is difficult for the laser-driven ps duration pulsed fast neutron source to realize a fine absorption spectrum in a single shot, due to the low yield[23–26] and, in particular, the slow temporal response of the neutron time-of-flight (nTOF) detector[27]. In recent years, the technology of using a kilohertz (kHz) laser facility to drive high repetition rate electron beams has become mature[28–30] and the electron energy could be up to 15 MeV, which is suitable to drive photo-nuclear neutrons[30]. Although a kHz laser facility can be used to drive electron beams with suitable energy, the beam charge is only several picocoulombs, which is limited by the laser energy of several millijoules (mJ)[29]. The yield of the neutron driven by this electron beam would be too low for the application of FNAS. Notably, Papp et al.[31] have theoretically realized a hundreds of pC electron beam via a 100 mJ repetition rate laser, which could greatly enhance the neutron yield. In addition, the temporal resolution of the nTOF method is about tens of ns due to the rising edge and afterglow of the scintillator detector, with which it is hard to distinguish two fast neutron pulses with a small difference in arrival time. Thus, it is difficult to acquire a high-resolution neutron energy spectrum from the single-shot nTOF[27].
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In order to solve the above two key problems simultaneously, firstly, we proposed a method to enhance the LWFA electron beam charge to several nanocoulombs (nC) by utilizing a 100-mJ 100-hertz (hHz) laser system. In general, a faster response or longer nTOF detection distance means higher energy resolution. Secondly, we proposed a method named single-neutron-count (SNC) TOF to reduce the influence of the detector response speed and detection distance, as separated single neutron signals are easy to diagnose. Finally, we showed the potential applications of ultrahigh FNAS based upon an hHz LWFA neutron source.
2 Design of an efficient laser plasma electron accelerator
The laser plasma accelerator is based on a commercial hHz laser system, which has been equipped in many laboratories, for example, it can deliver laser pulses with energy of 50 mJ, pulse duration of 30 fs in FWHM and repetition rate of 100 Hz. The laser pulses can be focused into the gas jet with an f/3 off-axis paraboloid mirror to a spot size
Figure 1.Design of the experimental setup for the generation of an ultra-short pulsed neutron source and the fast neutron absorption spectroscopy.
In order to exhibit the characteristics of the electron beam accelerated from the above system, we have carried out 3D particle-in-cell simulations using EPOCH code[34]. The simulation window size is set to
To acquire a large charge and high conversion-efficiency electron beam, we scanned the gas density. In order to meet the matching criteria for maintaining laser intensity and overcoming quick defocus[36], that is,
Figure 2.Three-dimensional particle-in-cell simulations of laser plasma acceleration. (a)–(d) represent four different times at 0.25, 0.5, 0.75 and 1 ps, respectively. The nitrogen atom density is and the laser = 2.
Here, we have scanned the nitrogen density, and the results are shown in Figure 3. On increasing the density, the accelerated electron beam charge is quickly increased to approximately 2.8 nC (
Figure 3.Simulation results of the electron beam. (a) Variation of electron beam charge ( > 1 MeV) with the nitrogen atom density. (b) Electron energy spectra for different nitrogen atom densities.
3 Generation of the ultra-short pulsed fast neutron source
To generate a fast neutron source, we have utilized the optimal electron beam to bombard a thin metal target for inducing photo-nuclear reactions[38]. Here, we carried out Monte Carlo simulation with Geant4 code. In the simulation setup, the metal target is a piece of tantalum (Ta) sheet with the thickness of 500 μm, which is set at 500 μm away from the electron beam source. The parameters of the electron beam, including the source size, distribution, energy spectrum and transversal momentum distribution, are imported into the input text of the Geant4 code, and
The simulation results of the Bremsstrahlung source are shown in Figure 4. According to the
Figure 4.Simulation results of the Bremsstrahlung source driven by the laser plasma electron accelerator. (a) -ray spectrum (red line) photo-nuclear reactions of , and Ta. (b) -ray source transversal distribution, which is detected on the plane of the rear surface of the Ta converter.
The simulation results of the photo-nuclear neutron are shown in Figure 5. The fast neutron source has near-Gaussian transversal distribution and its source size is approximately 250 μm in FWHM, which is larger than that of the
Figure 5.Simulation results of the neutron source driven by the laser plasma electron accelerator. (a) Neutron spatial distribution. (b) Neutron angular distribution. (c) Neutron energy spectrum. (d) Neutron yield and pulse duration.
4 Design of ultrahigh energy-resolution neutron absorption spectroscopy
The FNAS energy resolution is limited by three factors, the neutron pulse duration, detection distance and detector temporal resolution. At present, the doped
In order to further decrease the influence of scintillator fluorescence tailing on the energy resolution of FNAS, we proposed a method named SNC absorption spectroscopy, for which the setup is shown in Figure 1. The generated photo-nuclear neutron source has near-uniform distribution in the entire
As the results show in Figure 6, we present the potential of this kind of FNAS. The sample is composed of two materials, namely graphite and C7H5N3O6 (TNT), and the neutron transmittances of the two materials are shown in Figure 6(a). In the simulation, the neutron beam with the energy spectrum of Figure 5(c) passes through the sample; then one transmitted neutron is randomly selected via MATLAB code to be recorded and the recorded energy has corresponding uncertainty, which is shown in Ref. [12] (Figure 5(b)); finally, we repeat the selection process to accumulate enough transmitted neutrons to acquire a fine absorption spectrum, whose energy resolution is determined by the calculated result in Ref. [12]; the repetitive process is the same to accumulate laser shots. The absorption spectra are shown in Figures 6(b)–6(d). For the case of accumulating
Figure 6.Single-neutron-count fast neutron absorption spectroscopy. (a) Transmissivities for two materials, that is, graphite and TNT. (b)–(d) Simulated neutron absorption spectrum for the pulse duration of 36 ps, where the total neutron counts are (b), (c) and (d), respectively. (e) Absorption spectrum for the pulse duration of 1 ns, where the total count is .
5 Discussion
FNAS is widely used in the field of neutron resonance[42], in which the neutrons are absorbed by the nuclei, forming a new compound nuclei, especially when the neutron energy close to the nuclei excited state energy has a large nuclear reaction cross-section. The compound nuclei are unstable, and they would release a neutron to decay to the ground state or by emitting
6 Conclusion
In conclusion, we have presented a method for realizing FNAS with an ultrahigh energy resolution based on a high repetition rate laser plasma electron accelerator. Firstly, we designed the electron accelerator by utilizing a 100-Hz terawatt laser system and a high-density nitrogen gas jet to drive the tightly focused laser wakefield acceleration, where the optimal electron beam has the maximum energy of approximately 25 MeV and the single-shot charge of approximately 2.5 nC. Secondly, we utilized the optimal electron beam to drive the photo-nuclear neutron source in a thin metal target, where the optimal neutron source has the yield of
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