Development of a new tracking detector with fine-grained nuclear emulsion for sub-MeV neutron measurement

Abstract

In this study, we have developed a new sub-MeV neutron detector that has a high position resolution, energy resolution, directional sensitivity, and low background. The detector is based on a super-fine-grained nuclear emulsion, called the Nano Imaging Tracker (NIT), and it is capable of detecting neutron induced proton recoils as tracks through topological analysis with sub-micrometric accuracy. We used a type of NIT with AgBr:I crystals of (98 ± 10) nm size dispersed in the gelatin. First, we calibrated the performance of NIT device for detecting monochromatic neutrons with sub-MeV energy generated by nuclear fusion reactions, and the detection efficiency for recoil proton tracks of more than 2 μm range was consistently 100% (the 1 σ lower limit was 83%) in accordance with expectations by manual based analysis. In addition, recoil energy and angle distribution obtained good agreement with kinematical expectation. The primary neutron energy was reconstructed by using them, and it was evaluated as 42% with FWHM at 540 keV. Furthermore, we demonstrated newly developed an automatic track recognition system dedicated to the track range of more than a few micrometers. It achieved a recognition efficiency of (74 ± 4)%, and recoil energy and angle distribution obtained good agreement with manual analysis. Finally, it indicated the very high rejection power for γ-rays.

Environmental sub-MeV neutron measurement at the Gran Sasso surface laboratory with a super-fine-grained nuclear emulsion detector

“…4(c) reports the neutron energy reconstructed through the recoil-proton energy and the scattering angle, with a peak value at (864 ± 46) keV, consistent with the exposure energy. The obtained energy resolution is ∆E n,FWHM /E n = 0.31 for 880 keV neutrons, comparable to the value of 0.42 measured in the previous calibration run for 540 keV neutrons 2 .…”

“…In a previous work 2 , we reported that the energy measurement through Eq. 2 showed an accuracy of ∆E n,FWHM /E n = 0.42 for 540 keV neutrons.…”

“…We have developed a new direct detection method for neutrons with energies down to the sub-MeV domain, by using a super-fine-grained nuclear emulsion 2 , called Nano Imaging Tracker (NIT) 3,4 . Owing to its unprecedented spatial resolution at the nanometric scale, this device provides the three-dimensional reconstruction of proton tracks induced by the neutron scattering, thus being sensitive to the sub-MeV neutron energy region and providing measurements of both the neutron energy and direction.…”

Directional Dark Matter Search with Super-Resolution Nuclear Emulsion

“…Among all dark matter detectors, the presence of H in NIT is a unique feature. This makes NIT a particularly efficient neutron detector using proton recoil tracking [19,20]. The tracking capability of detecting nuclear recoils corresponding to 30 keV Carbons [21] and 10 keV protons has been demonstrated by ion doping with an ion-implantation system in Japan (to be published for low-energy proton tracking around 10 keV), while the use of Ultra-NIT (U-NIT) with 20 nm AgBr diameters can lower the threshold for carbon recoils down to 10 keV.…”

Achieving Submicrometer Spatial Resolution for Neutron Imaging With Nuclear Emulsion

“…Fine-grained nuclear emulsions with silver halide crystals having a diameter of less than 0.1 µm have been developed [31][32][33] to detect the tracks of the nuclei recoiled by the so-called weakly interacting massive particles. The sensitivity of the nuclear emulsion was optimised to track the nuclei but was maintained sufficiently low for electrons and γ-rays 34 . This type of fine-grained nuclear emulsion can be used for neutron imaging by combining it with the 10 B 4 C thin layer, which absorbs neutrons and emits charged particles heading in the opposite direction via the following process: 10 B + n → 7 Li + 4 He.…”