Development of New Tracking Detector with Fine-grained Nuclear Emulsion for sub-MeV Neutron Measurement
T. Shiraishi, I. Todoroki, T. Naka, A. Umemoto, R. Kobayashi, O. Sato
PProg. Theor. Exp. Phys. , 00000 (16 pages)DOI: 10.1093 / ptep/0000000000 Development of New Tracking Detector withFine-grained Nuclear Emulsion for sub-MeV
Neutron Measurement
T. Shiraishi † , I. Todoroki , T. Naka , A. Umemoto , R. Kobayashi , and O. Sato Department of Physics, Toho University, Chiba 274-8510, Japan ∗ E-mail: [email protected] Graduate School of Science, Nagoya University, Aichi 464-8602, Japan Kobayashi-Maskawa Institute, Nagoya University, Aichi 464-8602, Japan Institute of Materials and Systems for Sustainability, Nagoya University, Aichi464-8602, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
In this study, we have developed a new sub-MeV neutron detector that has a highposition resolution, energy resolution, directional sensitivity, and low background. Thedetector is based on a super-fine-grained nuclear emulsion, called the Nano ImagingTracker (NIT), and it is capable of detecting neutron induced proton recoils as tracksthrough topological analysis with sub-micrometric accuracy. We used a type of NITwith AgBr:I crystals of (98 ±
10) nm size dispersed in the gelatin. First, we calibratedthe performance of NIT device for detecting monochromatic neutrons with sub-MeVenergy generated by nuclear fusion reactions, and the detection efficiency for recoilproton tracks of more than 2 µ m range was consistently 100% (the 1 σ lower limitwas 83%) in accordance with expectations by manual based analysis. In addition, recoilenergy and angle distribution obtained good agreement with kinematical expectation.The primary neutron energy was reconstructed by using them, and it was evaluatedas 42% with FWHM at 540 keV. Furthermore, we demonstrated newly developed anautomatic track recognition system dedicated to the track range of more than a fewmicrometers. It achieved a recognition efficiency of (74 ± γ -rays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Subject Index
1. Introduction
Neutron measurement is an important subject in a wide range of studies such as low back-ground experiments in the particle physics (e.g., dark matter search [1, 2], neutrinoless doublebeta-decay search [3, 4]), neutron imaging from nuclear fusion or fission reactors [5–7], andneutron radiography [8]. In these measurements, the He proportional counter and the liq-uid scintillator are often utilized. For example, the He proportional counter is well suitedfor the measurement of thermal neutrons, however, it has no energy resolution (surely, nodirection sensitivity) because it detects protons by the neutron absorption reaction He(n,p) † Corresponding author © The Author(s) 2012. Published by Oxford University Press on behalf of the Physical Society of Japan.This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited. a r X i v : . [ phy s i c s . i n s - d e t ] J a n fter deceleration by the moderator. For the liquid scintillator, γ -rays become the back-ground in a large γ /neutron ratio environment; in particular, it difficult to discriminatebetween neutrons and γ -rays in the sub-MeV or less region. Each of these methods has somedisadvantages, and furthermore, they are not suitable for neutron imaging or directionaldetection because of the lack of spatial resolution. In this study, we propose a super-fine-grained nuclear emulsion, called the Nano Imaging Tracker (NIT) [9, 10], as a new neutrondetector with measurement capability of sub-MeV or more. This detector is expected wellworking in, for example, very high γ /neutron environment, neutron imaging and so on withboth energy and spatial resolution.In nuclear-emulsion-based neutron measurement, hydrogen, which is one of its components,is the main target for neutron detection. In an early approach to neutron detection with anuclear emulsion by Y. Nomura et al. [11], they manually measured neutrons in the several-MeV region by tracking recoil protons > µ m with an optical microscope. The automaticanalysis was developed by S. Machii et al. [12], and the measurement of surface environmentalneutrons was performed at energies greater than 2 MeV.For neutrons in the sub-MeV region, the recoil proton tracks are several micrometers, andtherefore sufficient studies have not been conducted up to now. In the directional dark mattersearch experiment [13], which is the primary objective of NIT detector development, detec-tion technology for 100 nm tracks has been implemented [14], and an adequate environmentfor track detection and analysis > µ m has been provided. The underground environmentfor the dark matter search has the neutron flux of approximately 5 orders of magnitudeless than the γ -ray flux, therefore the properties of neutrons in such environment have notbeen well understood yet, and high γ -ray separation capability is required for rare eventsearch experiments working in such environment. The NIT detector is expected to have anextremely high γ -ray rejection power compared to existing neutron detectors owing to itsdetection principle, not only high precision tracking of protons induced by neutrons.In this study, our primary contribution is a new technique for measuring sub-MeV neutronsand evaluating its performance. In section 2, we describe the details of the NIT device usedin this study. To verify the detection capability for sub-MeV neutrons, an experiment withmonochromatic neutrons from the fusion reaction was conducted, and the performance of itsprinciple of sub-MeV neutron detection is evaluated in section 3. In section 4, we propose anautomatic analysis method with an optical microscope system for more practical use, andevaluate its implementation and performance. Further, we evaluate the rejection power for γ -rays as a background event, and examine its feasibility in the future high γ /neutron ratioenvironment.
2. Experimental Setup
The NIT is a charged particle tracking detector in which silver iodobromide (AgBr:I) crystalsof several tens of nanometers are dispersed with high density in a gelatin and polyvinylalcohol medium with a stable crystal size distribution of approximately 15%. Each AgBr:Inanocrystal behaves as a semiconductor sensor for charged particles. When electrons areexcited by the charged particles, they repeatedly trap surface defects and desorb by thermalenergy. A silver atom is created by attracting interstitial silver ions while electrons remain inthe trap. When this reaction occurs more than four times at the same trap site, silver clusters alled latent image specks (LISs) are created. LISs grow to approximately the original crystalsize scale by chemical development, as shown in Fig. 1. These grown LISs, which form a track,are called developed silver particles.In general, because a larger crystal exhibits a higher reflection intensity of developed silverparticles with an epi-illumination microscope, it is easier to recognize a track. In this study,a NIT with a AgBr:I crystal size of (98 ±
10) nm, mass density of (3.2 ± ,and crystal density of approximately 1000 crystals/ µ m was produced to consider protontracking accuracy and γ -ray rejection power. The fractional mass of hydrogen contained inthe NIT is (1.75 ± ×
26 mm , thickness 1 mm) as a basematerial, a NIT of approximately 35 µ m thickness was applied to prepare a sample.In fact, this initial condition of the NIT leads to low sensitivity to high dE/dx particles suchas recoil protons. This makes track reconstruction difficult because the developed silver graindensity is too low. Therefore, we always apply chemical sensitization treatment to such anNIT device for more efficient formation of latent image specks. The chemical sensitizationtreatment adopted was Halogen-Acceptor sensitization [15]. Here, sodium sulfite of 3.97mol/L was utilized, and the dried NIT film after pouring was soaked in that solution for 15min at 20.0 ◦ C, and dried again.Development treatment after exposure was performed with MAA-1 Developer [16], whichis a very popular developer in photographic science for surface development of the AgBr:Icrystal, and it is the current standard development used for the NIT device. The treatmentcondition was set as development for 10 min at 5.0 ◦ C. The details are shown in the Appendix.The undeveloped AgBr:I crystals are dissolved by the fixing treatment. As a result, the NITthickness up on analysis is shrunk (0.53 ± Fast neutrons are primarily generated by nuclear fusion reactions or the spontaneous fissionof radioisotope sources such as
Cf. In particular, fusion reactions can generate neutronswith monochromatic energy, and therefore they are often used to calibrate detectors. In thisstudy, the following two types of fusion reactions (endothermic reactions, whose targets arelithium and tritium) provided by the National Institute of Advanced Industrial Science andTechnology (AIST) as the standard neutron field [17] were used to verify the detection of onochromatic neutrons with sub-MeV energy: Li + p + 2 . M eV −→ Be + n + 565 keV, (1) T + p + 1 . M eV −→ He + n + 880 keV. (2)The emitted neutron energies are respectively 565 and 880 keV at an angle of 0 ◦ . Theenergy can be adjusted by changing their emission angle with respect to the sample. Thesetup of neutron exposure at AIST is shown in Fig. 2. Table 1 shows the neutron flux and theemitted energy( E n,AIST ) for each sample. Here, the value of flux is the evaluated ones fromcalibrated neutron monitoring system and beam current by the AIST, and the E n,AIST arethe calculated ones from beam angle and the proton beam energy. In addition, an unexposedsample was prepared as the reference for comparison with the exposed samples.Fig. 2: Experimental setup for monochromatic sub-MeV neutron exposure at AIST.Table 1: Sample conditions of monochromatic sub-MeV neutron exposure at AIST. Neutronenergy and flux were calculated and measured by AIST, respectively [17].Nuetron Distance Angle E n,AIST Flux Exposure timesource (cm) (keV) (n cm − s − ) (hour)Sample1 Li(p,n)Be 100 60 ◦ ± ±
12 5.68Sample2 Li(p,n)Be 100 20 ◦ ± ±
26 5.68Sample3 T(p,n)He 50 16 ◦ ± ±
30 6.92Reference - - - - 0 0sample
For the analysis of recoil proton tracks in the NIT, we used the Post Track Selector (PTS)[18], which is an automatic scanning system with an epi-illumination optical microscope. As or the development status of the PTS system, PTS-2 has been reported up to now [18], anda faster system is currently being developed. In PTS data acquisition, as shown in Table 2,the field of view (FOV) of 112 µ m × µ m is taken 116 times by a CMOS camera every 0.3 µ m thickness, which approximately corresponds to the depth of field, and high-resolutiontomographic images (three-dimensional image) of 112 µ m × µ m × µ m are obtained.These image data are transferred from the camera to the SDRAM on the PC at high speed,after which high-speed scanning can be performed by image processing on the SDRAM(online analysis). When performing manual analysis or offline analysis, tomographic imagesare saved for each FOV.Table 2: Specification of current Post Track Selector (PTS) system.Objective lens N.A. 1.45, 100 × Light source LED, 455 ±
27 nm, 17 WPixel resolution 0.055 µ mNumber of pixels 2048 × µ m × µ mLayer pitch 0.30 µ mNumber of layers per FOV 116
3. Performance Evaluation by Manual Analysis
The recoil proton tracks in the NIT can be easily recognized with the human eye from thetomographic images acquired by PTS (manual analysis). First, to demonstrate the prin-ciple of sub-MeV neutron detection, we evaluated the performance of manual analysis asbenchmark data, and compared it with the simulation to evaluate its reliability.
The information obtained in this measurement is the range of the recoil proton track ( R p [ µ m]) and the scattering angle from the neutron beam axis ( θ p ). The relation between R p and the kinetic energy of the proton ( E p [keV]) is shown in Fig. 3a from the simulation inthe NIT with Geant4. In this measurement, Eq. 3 was used as an approximate curve whenobtaining the proton energy: E p = 41 . × R / p − × R / p . (3)If the arrival direction of the neutrons is known, the neutron energy ( E n,mes ) can bereconstructed from classical kinematics as Eq. 4: E n,mes = E p cos θ p . (4)Because the NIT can also accurately determine the scattering angle θ p , the neutron energy E n,mes can be reconstructed for each event. Reconstructing the energy of monochromaticneutrons from the signals obtained by manual analysis provides objective evidence of sub-MeV neutron detection. a) (b) Fig. 3: (a) Correlation of proton energy (Ep) and range (Rp) from the simulation in theNIT with Geant4; red line is fitted by Eq. 3. (b) An example of proton recoil event foundby manual analysis in the NIT.The proton tracks were searched by manual analysis from the tomographic image capturedby the PTS, the three-dimensional coordinates of the head and tail points were measuredas shown in Fig. 3b, and the range R p and scattering angle θ p were calculated from the linesegment. The position accuracy of the developed silvers that form the track is approximately0.15 µ m for X and Y, and 0.57 µ m for Z. In the emulsion layer of approximately 30 µ m, 3 µ m each from the upper and lower surfaces were cut, and approximately 24 µ m inside wasused for analysis as a fiducial volume. This cut is applied to prevent recoil proton tracksfrom going out of the analysis area because the range cannot be calculated accurately, andthe external α -ray background. Fig. 4 shows the results of 586 events of recoil proton tracksobtained by manual analysis of a 1.3 mg volume for Sample2. This analysis applied a protontrack range cut of more than 2 µ m, which is equivalent to a proton energy of 240 keV. Wedid not analyze below the 2 µ m range because of the lack of device purification.Fig. 4a shows the scatter plot of the scattering angle and recoil energy of the detectedprotons. Most of them are distributed around the curve corresponding to Eq. 4, and thecomponents outside this curve are expected to be protons recoiled by neutrons that arriveat the NIT after scattering by the surrounding materials. The distribution of E n,mes recon-structed by Eq. 4 is shown in Fig. 4b. The mean value of the measured E n,mes in Sample2 is561 ±
30 keV, which includes errors of statistics, the shrinkage factor, and the NIT density.It is almost the same as the neutron energy E n,AIST exposed to Sample2, and the energyresolution of FWHM is 42%. In addition, as is evident from the results of performing thesame analysis on Sample1 and Sample3 exposed with different monochromatic energies, asshown in Fig. 5, both of them can be reconstructed accurately. To evaluate the reliability of the manual analysis results, a comparison with Monte Carlo(MC) simulation was made using Geant4. A geometry very similar to that shown in Fig. 2was created for the MC simulation, as shown in Fig. 6.The number of detected recoil proton tracks is 451 ±
87 event/mg (the statistical error is4.1%, systematic errors are included as the NIT density error 6.5%, the neutron dose error is3.3%, the analysis volume error is 3.8% due to the shrinkage factor, and the hydrogen content a) (b)
Fig. 4: Results of manual analysis of 1.3 mg volume for Sample2. (a) Recoil energy vs.scattering angle. (b) Distribution of reconstructed neutron energy by using proton recoilenergy and angle.Fig. 5: Comparison of E n,AIST and reconstructed neutron energies for all samples.error is 17.1%), and the expected number of recoil proton tracks by the MC simulation is442 ±
16 event/mg. The detection efficiency of the manual analysis is 100% consistent (the1 σ lower limit was 83%).Fig. 7 compares the proton recoil energy and scattering angle for data and MC. E p and θ p in the MC simulation are calculated by smearing the head and tail points of the trackwith the measurement errors. Both the calculated values are in good agreement with thekinematical prediction. This result shows that the reliability of manual analysis is high, thereis no detection bias, and almost no noise is present.
4. Automatic Analysis
As verification of the principle of detection in section 3, sub-MeV neutron detection bymanual analysis of the NIT is shown to have high detection efficiency and energy resolution ig. 6: Geometry of MC simulation with Geant4 for the monochromatic neutron measure-ment at AIST. (a) (b)
Fig. 7: Comparison of the selected event data with MC simulation. (a) Recoil energy(Sample2), and (b) Scattering angle (Sample2).for proton tracks > µ m. In manual analysis, the recoil proton tracks are searched from thetomographic images, and the track range and angle are measured manually, which limitsthe analysis speed, and this analysis can be applied up to the 10 mg scale. In this section,we developed an automatic analysis system to improve the event selection capability byautomatically recognizing tracks and measuring the track range and angle according to theimage-data-taking speed of the PTS. This system aims to trigger event candidates andautomatically measures the track range and angle, and assumes that manual analysis isperformed after automatic analysis as the final confirmation. To analyze the high-resolution tomographic images acquired by PTS on SDRAM with highspeed, we have developed a three-dimensional tracking system called chain analysis. Thissystem can reconstruct three-dimensional tracks of more than a few micrometers length byconnecting the developed silver grains like a chain, using the following procedures:(1) Image filtering such as smoothing and subtracting the background brightness areperformed for each layer of the tomographic images, and the contour shapes of thedeveloped silver, which are called grains, are extracted (Fig. 8).
2) The unfocused grains are erased by selecting the best-focused grains from the top andbottom tomographic images (Fig. 9a).(3) Among the best-focused grains, those with a distance of less than 1.1 µ m are connectedto form a pair (Fig. 9b).(4) The angle of the pair is extended, and the pair is connected to another grain with adistance of less than 1.7 µ m and an angle difference of less than 30 ◦ (Fig. 9c).(5) All connections are attempted like a chain, and the longest one is selected as therepresentative track (Fig. 9d).(6) Among the detected chain tracks, those with low brightness or that are composed ofgrains larger than the developed silver are cut during offline analysis. In addition, afiducial volume cut as in manual analysis is applied.Fig. 8: Schematic image and example data for filtering and track contour extraction of chainanalysis. The neutron-exposed samples were automatically analyzed using chain analysis, and itsperformance was evaluated. Fig. 10 shows the range distribution of candidate events fromchain analysis of a mass of 1.3 mg for Sample2. The same analysis of a mass of 19.4 mgfor reference sample was performed to estimate the amount of noise misrecognized by chainanalysis.In neutron-exposed Sample2, a significant number of candidates triggered by chain analysiswere detected against the reference sample, and most of them were identified as proton recoiltracks, as shown in the actual image (Fig. 10a). In addition, the same amount of noises as inthe reference sample is present in the candidate events of Sample2. The main cause of noiseis the connection of several small dust particles (Fig. 10b), and there are also a few othercomponents misidentified the part of the huge dust particle (Fig. 10c). Such dust particles arepredicted to be generated during the manufacturing process of the NIT. The noise densityis 2.22 ± µ m cut, and 0.05 ± µ m cut. These noises are not an essential background, and they can be discriminatedfrom tracks by manual analysis. Fig. 11 shows the range distribution after manual analysis ig. 9: Algorithm of chain analysis. The blue clusters show the best-focused grains, the greenones show the pairs, and the red ones show the chain tracks.Fig. 10: Track range distribution of candidate events obtained by chain analysis. Analyzedmass of 1.3 mg for Sample2, and 19.4 mg for reference sample were normalized to 1 mg. (a) isa recoil proton track triggered by chain analysis for Sample2. (b) and (c) are misrecognizednoises for reference sample.for the events triggered by chain analysis. Most of them were identified as proton tracks in ample2; on the other hand, no tracks were observed in the reference sample. The numberof events obtained for each analysis is summarized in Table 3.Fig. 11: Track range distribution of proton tracks confirmed by manual analysis amongevents triggered by chain analysis. Analyzed masses of 1.3 mg for Sample2 and 19.4 mg forthe reference sample were normalized to 1 mg.Table 3: Number of candidates triggered by chain analysis, and number of proton tracksconfirmed by manual analysis.Range cut Sample Candidates triggered by After manual analysischain analysis (/mg) (/mg)2 µ m Sample2 370 ±
17 348 ± p >
220 keV) Reference 2.32 ± < µ m Sample2 155 ±
11 155 ± p >
380 keV) Reference 0.05 ± < µ m and that of the scattering angle is 5.7 ◦ . In chain analysis, the chaintrack may be broken when the distance between grains is large or when the brightness ofthe grain is low, and the track range may be recognized as shorter. Further, the recognitionaccuracy for the Z position is worse than that for the XY position due to the depth of fieldof the optical system, and therefore the angle accuracy in the Z direction is relatively poor.Figs. 12c, 12d show the result of the neutron energy reconstruction with the recognitionaccuracy of chain analysis. Sub-MeV neutron energies can be sufficiently reconstructed withthe accuracy of automatic analysis.Fig. 13 shows the recognition efficiency, which is defined as the relative efficiency of chainanalysis to that of manual analysis, for proton tracks longer than 2 µ m. The average recog-nition efficiency is approximately 90%. However, by applying a cut for the proton range a) (b)(c) (d) Fig. 12: Performance evaluation of (a) track range, (b) scattering angle, and (c) recon-structed neutron energy by comparing manual analysis and chain analysis for Sample2. (d)Reconstructed neutron energy distribution with measurement accuracy of chain analysis.automatically measured by chain analysis with > µ m ( R p,auto cut), it becomes (83 ± ± γ -ray Rejection Power In case of a high γ /neutron ratio environment, a high rejection power for γ -rays is requiredfor sub-MeV neutron detection. In the standard nuclear emulsions sensitive to minimum ion-izing particles (MIPs), secondary electrons from the γ -rays are recorded as tracks. However,because the super-fine-grained AgBr:I crystals in the NIT drastically reduce the sensitivityto particles with small energy losses such as MIPs, most electrons cannot create LISs. Onlythe electrons immediately before stopping, with an energy of approximately 10 keV, canmake a few LISs because their energy loss increases to approximately 10 keV/ µ m accordingto the Bethe-Bloch formula [19].To quantitatively evaluate the γ -ray rejection power of the NIT, we first considered thepossibility that sub-MeV electrons recoiled by the Compton effect could be detected as ig. 13: Recognition efficiency of proton tracks with chain analysis for Sample2. Horizontalaxis corresponds to the proton track range measured by manual analysis. The black dottedline represents ”without offline cut”, the blue dotted line represents ”after R p,auto cut”, andthe red line represents ”after R p,auto , brightness and shape cut”.tracks. Using Co as a γ -ray source with energies of 1.17 and 1.33 MeV, a NIT sampleexposed to them with 10 γ /cm flux, which is equivalent to one year’s accumulation ofenvironmental γ -rays, was prepared. The expected number of Compton electrons in thissample is approximately 10 in the analyzed volume of 3.3 mg, as shown in Table 4. Chainanalysis on this sample did not detect a significant electron signal, from the result, thedetection efficiency of electron tracks is lower than 0.1%, and secondary electrons generatedfrom the γ -rays cannot be the background, considering the reaction probability.Next, if LISs created by secondary electrons from low-energy γ -rays with a large crosssection accumulate, chance coincidence events associated with them may be misrecognizedby chain analysis. To evaluate this possibility, we also prepared a NIT sample exposed to10 −
60 keV γ -rays from the Am source with 10 γ /cm flux. The expected number ofsecondary electrons generated in this NIT sample is approximately 1 × in the analyzedmass volume of 3.3 mg. Chain analysis for this sample did not detect a significant signal,from the result, the number of chance coincidence events is less than the detection sensitivityeven when secondary electrons generated from low-energy γ -rays accumulated.This performance shows that γ -rays in any energy band cannot be the background whenaccumulated for less than one year.Table 4: Expected number of reactions in NIT samples exposed to γ -rays of Co and
Am. γ -ray source Energy Exposed flux Expected number of reacted γ -rays(keV) (/cm ) in analyzed volume of 3.3 mg Co 1170, 1330 10 Am 10 −
60 10 . Discussion and Prospects As shown in section 4.3, because this neutron measurement technique has a high γ -rayrejection power, environmental γ -rays cannot be the background even after one year ofaccumulation. Dust events that are misrecognized by chain analysis also cannot be a funda-mental background because it can easily be identified by manual analysis. However, becausethe capacity of manual analysis is limited, device purification is required when the analysisscale increases or when the current proton energy threshold (240 keV) decreases. α -rays should also be considered as the background when applying this analysis to low fluxneutron measurement such as environmental neutron measurement. External α -rays can berejected with the current fiducial volume cut. We consider radioisotopes of U and
Thincluded in the NIT as the intrinsic background; they are measured by germanium detectorsas respectively 27 mBq/kg and 6 mBq/kg [20], and the number of α -rays emitted by themis expected to be 8.6 × event/(kg month). Because their track range exceeds 20 µ mand that of recoil protons induced by 1 MeV neutrons is 15 µ m, when we target sub-MeVneutron measurement, α -rays can be rejected by the track range cut. Furthermore, because α -rays have a larger dE/dx than protons, proton/ α discrimination may be possible by thesensitivity control of AgBr:I crystals in the NIT. Because environmental neutrons, especially underground neutrons, can be background eventsin direct dark matter search and neutrinoless double beta-decay search experiments, adetailed understanding of environmental neutrons is important for making those discover-ies. In particular, environmental neutrons in the sub-MeV region have not yet been directlymeasured due to technical difficulties.A study has been conducted by A. Rindi et al. [21] on the measurement of environmentalneutrons at the Gran Sasso Science Institute. The He proportional counter used in thestudy is well suited for the measurement of thermal neutrons. However, in the case of sub-MeV neutrons, because it detects protons by the neutron absorption reaction He(n,p) afterdeceleration by the moderator, it has no energy resolution, and the systematic error inthe deceleration process is large. Furthermore, it has no directional sensitivity for neutronsbecause of the lack of spatial resolution.Considering the neutron flux predicted by EXPACS [22], approximately 300 recoil pro-ton tracks induced by environmental sub-MeV neutrons are accumulated by installing a1 g NIT for 1 month on the Gran Sasso surface. Concerning the underground sub-MeVenvironmental neutron measurement, 1-kg-scale NIT analysis capability is required for a 1month installation. The tracks recorded in the NIT will fade with time in high-temperatureand high-humidity conditions. Because there is no significant fading within 1 month at 0 ◦ C,we assume the current running period to be 1 month. With a longer running period, thelong-term stability of the NIT should be confirmed at low temperature.
The analysis speed for environmental neutron measurement with the NIT is limited by theimage-taking speed of the PTS and image-processing speed of chain analysis. At present, an nalysis speed of 30 g/year per PTS machine has been achieved. 1-g-scale analysis of theNIT can be performed in half a month with the one current PTS system.For the underground neutron measurement, it is necessary to further improve the analysisspeed. In the development stage, we have achieved an analysis speed of 85 g/(year · machine)using a wide F.O.V. objective lens with an optical magnification of 60 × . In addition, imagefiltering processes such as smoothing and subtracting background brightness are currentlyvery time consuming, and the analysis speed can be improved to 150 g/(year · machine) byprocessing them with a GPU. Furthermore, to improve the image-taking speed of the PTS,we are planning to introduce a new high-speed wide-imaging CMOS camera, and a piezodevice for high-speed driving. An analysis speed of scale of 1 kg/(year · machine) is expectedwith these improvements.Currently, three PTS machines are in operation, and we plan to further increase the numberof machines in the future.
6. Conclusion
We proposed a new sub-MeV neutron detection technique with NIT and PTS systems. Weprepared a sample exposed to monochromatic neutrons at AIST, and evaluated the detectionperformance using manual analysis. The detection efficiency for recoil proton tracks wasdemonstrated to be 100% consistent, and it was also shown that the energy of sub-MeVneutrons can be reconstructed from the recoil proton range and scattering angle.We developed a new algorithm for automatically recognizing recoil proton tracks of morethan a few micrometers for large-scale analysis. In this analysis, γ -rays are not detected as abackground, which is an appropriate detection method with energy resolution for sub-MeVneutron measurement. In the future, we will continue to further improve the recognitionmethod and speed up the analysis, and measure environmental neutrons both at the surfaceand underground. Acknowledgment
This work was supported by JSPS KAKENHI Grant Numbers JP18H03699, JP19H05806.Neutron Sources were supported by Dr. Tetsuro Matsumoto and Dr. Akihiko Masuda of theNational Metrology Institute of Japan (NMIJ), the National Institute of Advance IndustrialScience and Technology (AIST). The work is also supported by JSPS Core- to-Core Program(grant number: JPJSCCA20200002).
Appendix
First, the NIT film was treated with 0.49 mo/L sodium sulfate solution for 15 min as thepre-soak solution to stabilize the temperature of the film. This was followed by developmenttreatment using MAA-1 Developer. The detailed prescription is shown in Table 5. Thistreatment was performed for 10 min. Stop and fixation treatment followed, and the solutionutilized was acetic acid and NF-1 of Fujifilm. These treatments were performed for 10 minand 20 min, respectively. All processes were conducted at 5.0 ◦ C. References [1] DAMA/LIBRA, R. Bernabei et al., Nucl. Phys. At. Energy (2018) 307-325.[2] XENON Collaboration, E. Aprile et al., Phys. Rev. Lett. (2019) 071301.[3] KamLAND-Zen Collaboration, A. Gando et al., Phys. Rev. Lett. (2019) 192501. able 5: Prescription of MAA-1 Developerp-methylaminophenol hydrochloride 2.5 gL(+)-ascorbic acid 10 gPotassium bromide 1 gSodium metaborate tetrahydrate 47.2 gWater to make 1000 mL [4] K. Nakajima, T. Iida, K. Akutagawa, T. Batpurev, W.M. Chan, F. Dokaku, K. Fushimi, H. Kakubata,K. Kanagawa, S. Katagiri, K. Kawasaki, B.T. Khai, H. Kino, E. Kinoshita, T. Kishimoto, R. Hazama,H. Hiraoka, T. Hiyama, M. Ishikawa, X. Li, T. Maeda, K. Matsuoka, M. Moser, M. Nomachi, I. Ogawa,T. Ohata, H. Sato, K. Shamoto, M. Shimada, M. Shokati, N. Takahashi, Y. Takemoto, Y. Takihira, Y.Tamagawa, M. Tozawa, K. Teranishi, K. Tetsuno, V.T.T. Trang, M. Tsuzuki, S. Umehara, W. Wang,S.Yoshida, and N. Yotsunaga, Astropart. Phys. (2018) 54-60.[5] E. Takada, A. Fujisaki, N. Nakada, M. Isobe, K. Ogawa, T. Nishitani, and H. Tomita, Plasma and FusionResearch (2016) 2405020.[6] Y. Izumi, H. Tomita, Y, Nakayama, S. Hayashi, K. Morishima, M. Isobe, M.S. Cheon, K. Ogawa, T.Nishitani, T. Naka, T. Nakano, M. Nakamura, and T. Iguchi, Rev. Sci. Instrum. (2016) 11D840.[7] P. Kandlakunta, P. Mulligan, D. Turkoglu, and L. Cao, 2012 IEEE Nuclear Science Symposium andMedical Imaging Conference Record (NSS/MIC) (2012) 1991-1995.[8] A. Taketani, Y. Wakabayashi, Y. Otake, Y. Ikeda, T. Wakabayashi, K. Kono, T. Kai, K. Oikawa, H.Sunaga, M. Yamada, and T. Nakayama, MATERIALS TRANSACTIONS (2018) 976-983.[9] T. Naka, T. Asada, T. Katsuragawa, K. Hakamata, M. Yoshimoto, K. Kuwabara, M. Nakamura, O. Sato,T. Nakano, Y. Tawara, G. De Lellis, C. Sirignano, and N. D’Ambrossio, Nucl. Inst. Meth. A (2013)519-521.[10] T. Asada, T. Naka, K. Kuwabara, and M. Yoshimoto, Prog. Theor. Exp. Phys. (2017).[11] Y. Nomura, H. Tomita, J. Kawarabayashi, T. Iguchi, M. Isobe, K. Morishima, T. Nakano, M. Nakamura,and S. Ohnishi, Plasma and Fusion Research (2011) 2402148.[12] S. Machii, Master’s thesis, Nagoya University, (2016).[13] NEWSdm collaboration, arXiv:1604.04199v1.[14] A. Umemoto, T. Naka, T. Nakano, R. Kobayashi, T. Shiraishi, and T. Asada, Prog. Theor. Exp. Phys. (2020).[15] T. Tsutsumi, K. Morimoto, S. Kimura, T. Suzuki, T. Mitsuhashi, K. Kuge, and A. Hasegawa, J. ImagingSci. Technol. (2009) 10507.[16] T. H. James, W. Vanselow, and R. F. Quirk, Photogr. Sci. Tech. (1953) 170.[17] H. Harano, T. Matsumoto, J. Nishiyama, A. Uritani, and K. Kudo, AIP Conference Proceedings (2009) 915.[18] T. Katsuragawa, A. Umemoto, M. Yoshimoto, T. Naka, and T. Asada, JINST (2017) T04002.[19] M. S. Livingston, and H. A. Bethe, Rev. Mod. Phys. 9 (1937) 263.[20] A. Alexandrov, T. Asada, A. Buonaura, L. Consiglio, N. D’Ambrosio, G. De Lellis, A. Di Crescenzo, N.Di Marco, M.L. Di Vacri, S. Furuya, G. Galati, V. Gentile, T. Katsuragawa, M. Laubenstein, A. Lauria,P.F. Loverre, S. Machii, P. Monacelli, M.C. Montesi, T. Naka, F. Pupilli, G. Rosa, O. Sato, P. Strolin,V. Tioukov, A. Umemoto, and M. Yoshimoto, Astropart. Phys. A (2016) 16-21.[21] A. Rindi, F. Celani, M. Lindozzi, and S. Miozzi, Nucl. Inst. Meth. A (1988) 871-874.[22] T. Sato, PLOS ONE (2015) e0144679.(2015) e0144679.