Design of a compact electron accelerator-driven pulsed neutron facility at AIST

Accepted Manuscript Design of a compact electron accelerator-driven pulsed neutron facility at AIST Koichi Kino, Takeshi Fujiwara, Michihiro Furusaka, Noriyosu Hayashizaki, Ryunosuke Kuroda, Koji Michishio, Takemi Muroga, Hiroshi Ogawa, Brian E. O’Rourke, Nagayasu Oshima, Daisuke Satoh, Norihiro Sei, Tamao Shishido, Ryoichi Suzuki, Masahito Tanaka, Hiroyuki Toyokawa, Akira Watazu

PII: DOI: Reference:

S0168-9002(19)30250-5 https://doi.org/10.1016/j.nima.2019.02.062 NIMA 61935

To appear in:

Nuclear Inst. and Methods in Physics Research, A

Received date : 12 September 2018 Revised date : 14 February 2019 Accepted date : 21 February 2019 Please cite this article as: K. Kino, T. Fujiwara, M. Furusaka et al., Design of a compact electron accelerator-driven pulsed neutron facility at AIST, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.02.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Design of a compact electron accelerator-driven pulsed neutron facility at AIST Koichi Kinoa,b,＊, Takeshi Fujiwaraa,b, Michihiro Furusakaa,b, Noriyosu Hayashizakia,b,c, Ryunosuke Kurodaa,b, Koji Michishioa,b, Takemi Murogab, Hiroshi Ogawaa,b, Brian E. O'Rourkea,b, Nagayasu Oshimaa,b, Daisuke Satoha,b, Norihiro Seia,b, Tamao Shishidob, Ryoichi Suzukia,b, Masahito Tanakaa,b, Hiroyuki Toyokawaa,b, Akira Watazua,b a

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba,

Ibaraki, Japan b

Innovative Structural Materials Association (ISMA), Chiyoda-ku, Tokyo, Japan

c

Tokyo Institute of Technology (Tokyo Tech), Meguro-ku, Tokyo, Japan

*

Corresponding author at: National Institute of Advanced Industrial Science and

Technology (AIST), Tsukuba, Ibaraki, Japan. E-mail address: [email protected] (K. Kino) Abstract We have designed and been constructing a compact accelerator-driven neutron facility for Bragg edge transmission imaging of structural materials to obtain position dependent crystallographic information. In order to combine a high neutron wavelength resolution, which is especially required for strain measurements, and an intense neutron beam flux at the sample position, which is required for obtaining high quality statistics within practical measurement times, the components of the neutron facility are optimized. To achieve a high wavelength resolution while maintaining an intense neutron beam flux at the sample position, a solid methane decoupled cold moderator at about 20 K coupled with neutron guide tubes is employed. The target-moderator system, especially the moderator thickness was optimized using the Monte-Carlo simulation code PHITS. Neutrons will be produced using a 35-MeV electron accelerator which is currently under construction. The accelerator has a design maximum beam power of 10 kW, a maximum repetition rate of 100 Hz and a maximum pulse width of 10 μs. The combination of the flight path length of about 8 m and the repetition rate is suitable for efficient Bragg edge measurements. The pulse width of the electron beam is suitable to achieve a high wavelength resolution with the solid methane decoupled moderator. The 1

estimated flux of the neutron beam at the sample position is 1.2×104~4.5×104 /cm2/s depending on the usage of super mirror guide tubes. The wavelength resolution at the sample is 0.5~0.7 % in the wavelength region from 0.3 to 0.4 nm. 1

Introduction Man-made global warming is perhaps the biggest concern facing humanity today. To

minimize the expected warming our society needs to reduce emissions of CO2 gas, which is a greenhouse gas emitted both through the consumption of fossil fuels and other industrial activities. A considerable amount of CO2 gas is produced by transportation vehicles and there are currently two major methods to mitigate it: motorization (electrification) and weight saving. Our research focuses on weight saving, in particular the development of innovative steels and joining techniques of dissimilar materials. In order to push forward these developments for actual products, measurements of strain, crystalline phase and so on for assembled products are necessary. One of the useful methods is the non-destructive imaging of crystalline information. The strength and durability of products are related to crystallographic structure. The non-destructive method is also useful for rapid measurements, which does not need sample preparation. Furthermore, on-demand measurement facilities are essential to satisfy the needs of rapid-cycle development and analysis of products in industry. X-ray beams can provide images of crystalline information and desktop type X-ray diffractometry is suitable for on-demand measurements. However, low energy X-ray based techniques are limited to analysis of the sample surface [1]. A high-energy X-ray beam from a large facility, which is difficult for on-demand measurements, is needed for imaging of the internal structure of larger samples. On the other hand, the penetration depths of steel and other typical metals for neutrons are the order of 10 mm, in contrast to several µm in case of laboratory generated X-rays. Moreover, the neutron energy spectrum from a neutron moderator is independent of the production method, therefore penetration depths of neutrons produced by a compact accelerator-driven neutron source (CANS) or a large accelerator driven neutron source are identical. Therefore, CANS are useful candidates for on-demand measurements. The use of neutron beams and associated imaging techniques, especially Bragg edge imaging, are particularly promising in this area. Bragg edge imaging has been used for 2

the analysis of materials, devices, important cultural artifacts etc. [2] and its usefulness has been demonstrated mainly at the large accelerator-based and atomic reactor neutron sources in recent years. The advantage of the technique is that the crystalline strain, phase, orientation, sizes, and so on can be visualized from a single measurement. In the recent years, the use of compact neutron sources for industrial applications has become very active. Low energy electron accelerator-based neutron sources have been used for neutron cross section measurements, which requires a pulse width of 1 µs or less in an epi-thermal resonance region. Typical examples include the Kyoto University Institute for Integrated Radiation and Nuclear Science (KURNS) linear accelerator (LINAC) in Japan, Gaerttner LINAC in the US, GELINA in Belgium and Bariloche electron LINAC in Argentina to name a few [3]. The Hokkaido University Neutron Source (HUNS) is also based on an electron LINAC, which has been used for cold neutron source development and research on neutron optics and detectors. In the last decade or so, it has been used to demonstrate the usefulness of CANS for Bragg edge imaging and small-angle neutron scattering type of experiments using the intermediate-angle neutron scattering instrument, iANS [4,5]. Another type of CANSs are driven by a low energy proton LINAC, such as, LENS in the US, the RIKEN Accelerator-driven

Neutron

Source

(RANS)

in

Japan,

Kyoto

University

Accelerator-driven Neutron Source (KUANS) in Japan and CHPS in China [3]. In recent years the number of CANS’s projects is increasing quite rapidly. At RANS, the measurement of phase and texture analysis in steel samples have been demonstrated by neutron diffraction [6]. The combination of Bragg edge imaging and a compact accelerator-driven neutron source, which has a high usage flexibility, should have a large range of potential industrial applications. Strain imaging, which is one of the useful applications for Bragg edge imaging technique, is thought to be very difficult to realize at a CANS type facility, because this type of imaging needs a high wavelength resolution. To realize high wavelength resolution a short neutron pulse length, determined by the neutron emission time distribution from the moderator, together with a long flight path is required. These conditions result in a reduced neutron beam intensity. Fortunately, Bragg edge transmission measurement allows a large incoming beam divergence without sacrificing wavelength resolution, because the Bragg cut-off corresponds to the 180° reflection for polycrystal materials. Of course, the spatial resolution of the image becomes worse 3

when the beam divergence is large. The use of neutron guide tubes to enhance the intensity is therefore very effective in this case, which has been demonstrated at HUNS [7]. At HUNS, a decoupled polyethylene moderator is used to generate a temporally sharp neutron pulse. The result was relatively high wavelength resolution, only about 2 times worse than that obtained at the Japan Proton Accelerator Research Complex (J-PARC) [7]. In addition, the neutron flux at the sample position was increased by a 3.6-Qc guide tube, which was installed very close to the moderator [7]. We have designed and are now constructing a new CANS facility at the National Institute of Advanced Industrial Science and Technology (AIST), in Tsukuba, Japan, which can produce about one order of magnitude higher neutron flux compared to that of HUNS. 2

Concept and grand design of the neutron facility We have been designing a Bragg edge imaging facility together with a CANS

optimized for the analysis of structural materials. The design of the present system is shown in Fig. 1. The major components of the system are; an electron linear accelerator, a neutron source, and a measurement beam line. The Bragg edges, which are observed in metals used for structural materials, are around 0.4 nm. For example, Figure 2 shows transmission spectra as a function of neutron wavelength for 1-mm thick steel and 2-mm thick aluminum, which are among our target materials. These spectra are calculated by the RITS code [4,8-10]; they include the contribution from coherent elastic scattering, incoherent elastic scattering, inelastic scattering, and absorption reactions. Our target neutron wavelength resolution is 0.6 % at a neutron wavelength of around 0.4 nm due to the constraints of neutronic performance of the neutron moderators and the flight path length of the experimental room where the present facility will be constructed. This value is about 3 times larger than that of the imaging beam line (BL22) at J-PARC [11]. There are basically two types of neutron moderators; coupled and decoupled. Moreover, there are several moderator materials including solid methane, liquid hydrogen and mesitylene and so on. We have chosen to use a decoupled solid-methane moderator among them because the decoupled type can provide much shorter pulse width than the coupled type and solid methane is the best material at around 0.4 nm wavelength for optimizing the pulse width and neutron flux [12]. The 4

flight pathlength is limited to about 8 m due to the size of the experimental room. The neutron wavelength resolution is Δt/t, where Δt is the neutron pulse width from the moderator and t is the Time of Flight (TOF). For the case of the decoupled solid-methane moderator, Δt was reported to be about 70 μs at 0.4 nm [12]. The neutron wavelength resolution at 0.4 nm with a flight pathlength of 8 m is therefore estimated to be around 0.87 %. In this study, we optimized the moderator thickness for the target neutron wavelength resolution by Monte-Carlo simulation taking into account the neutron flux. The neutron flux at the sample position is another important factor for Bragg edge imaging. The flux varies depending on the initial neutron production rate, the target-moderator-reflector (TMR) system, and the beam line. We set the target value to that which the two-dimensional counting-type neutron detector, located behind the sample and with an effective area of 100 mm×100 mm and a spatial resolution of 1 mm, can maximally accept at the moment. The maximum counting rate of this detector is of the order of 105 per second. If a more intense neutron flux than the target value is available with the present facility, more rapid measurements will be possible with improved neutron detectors in the future. In this paper, we discuss the use of neutron guide tubes with a high Qc supermirror to increase the neutron flux at the sample position. For the present facility, we chose to use an electron linear accelerator because peak current can be very high with sufficiently short pulses of 1~10 µs, which is optimum for Bragg edge measurement. The effect of the 10 µs pulse width of the electron beam is 0.1 % at 0.4 nm with a flight pathlength of 8 m and does not largely deteriorate the performance of the decoupled solid-methane moderator. The design electron energy of the accelerator is about 35 MeV, increasing the energy much above this value does not bring a large increase in flux since the rate of increase of the neutron yield per unit power decreases above about 30 MeV when using a heavy metal target [13]. A low energy proton accelerator was also considered but the neutron production rate of a 7-MeV proton beam is roughly one order of magnitude smaller than that of an electron linear accelerator with the same beam pulse width and repetition rate as those of our facility. This is due to the limitation of the peak current of the proton beam with current accelerator technology [14].

5

The maximum repetition rate of the electron beam is determined by consideration of the neutron flight path length and the required neutron wavelength bandwidth. The relation of the flight time and flight path length of neutrons is shown in Fig. 3. Assuming a maximum required wavelength of 0.5~0.6 nm, well above the Bragg edge position, with a total flight path of 8 m, the maximum repetition rate is around 100 Hz. A disk chopper is required to eliminate the frame overlap effect. The electron beam power is a product of the peak current, pulse width, repetition rate and energy. For the present accelerator, with a design peak current of around 0.275 A during the pulse, the estimated maximum beam power is approximately 10 kW. This energy load is quite challenging for the design of the target system from the stand point of heat removal and other foreseeable engineering difficulties (material activation by radiation etc.). Under these conditions the neutron production rate by the heavy metal target is about 1013 /s. A cross section of the TMR system is shown in Fig. 1. The neutron production target is composed of thin layers of tantalum separated by layers of flowing water to bring away the heat generated. Through irradiation with a 35 MeV electron beam an electro-magnetic shower is produced in the target and the produced γ-rays create evaporation neutrons through photonuclear reactions with tantalum nuclei. Two solid methane cold moderators at about 20 K are used, a decoupled one above and a coupled one below the target. The surface area of the both moderator vessels in the beamline direction is 120 mm×120 mm. The thicknesses of the upper (decoupled) moderator is 30 mm whereas that of the lower (coupled) moderator are is 50 mm. The decoupled moderator is designed for Bragg edge imaging and is completely wrapped (except for the neutron extraction area) with a 1-mm thick cadmium sheet which acts as a decoupler. The lower vessel is a coupled moderator designed for possible future applications which prioritize the neutron flux over the wavelength resolution. The electron target and moderators are surrounded by a graphite reflector, which reflect neutrons back into the moderators. Surrounding this are layers of lead, borated resin, and concrete shielding. Beam shutters, which are mainly made of concrete, are installed at each of the extraction holes of the neutron beams from the moderators. The beam shutters move upward and downward and thus open and close the neutron beamlines. The neutron production target, the moderators and a part of the reflector are installed on

6

a moveable trolley and can be extracted from the center of the shielding area for maintenance. We plan to use a Gaseous Electron Multiplier (GEM) type two-dimensional neutron detector [15]. Other 2D neutron counting type detectors like microchannel plates (MCP) [16] and micro-pixel chamber (μPIC) [17] types are available. However, we chose the GEM type because it has a neutron effective area of 100 mm×100 mm and is easy to enlarge the effective area to 300 mm×300 mm for future measurements. 3

Key components of the neutron facility 3.1 Electron accelerator The electron linear accelerator is made up of an electron injector (Mitsubishi Heavy Industry Systems, Japan) and three 2.7 m long S-band (2856 MHz) accelerator cavities (Mitsubishi, Japan). The electron injector consists of a dispenser cathode (EIMAC Y646B) and S-band pre-acceleration cavity to give a maximum output electron energy of 3 MeV. RF power is supplied to the pre-acceleration and accelerating cavities by three high power RF klystrons (E37338, Toshiba Electron Tubes and Devices Co., Ltd., Japan). Each klystron produces up to 7 MW of power in a 10 μs pulse at a repetition rate of up to 100 Hz using high voltage charging power supplies (Nihon Koshuha Co., Ltd., Japan). At maximum power the electron beam is designed to achieve a beam energy of 35 MeV with an average beam current of around 275 μA (2.75 μC per pulse), to give a maximum beam power of around 10 kW. The beam diameter is adjusted by pairs of quadrupole lens magnets located along the beamline. The beam diameter on target will be adjusted to around 20 mm to avoid excessive localized heating of the target. The electron accelerator is located in a shielded room separated from the neutron source and beamline room by a 5 m concrete wall. Accelerated electrons are directed through a beampipe located in a hole in this wall and are directed onto the neutron production target. The electron beam is extracted from the vacuum pipe through a thin (30 μm) Ti window. To prevent activation of the air around the target this region will be flushed with He gas. The location of the electron beam and its size on the target will be continuously monitored by observing the light emitted from the inner surface of the Ti window. This light is 7

incident on a mirror placed in a specially designed beam duct and directed onto a camera. 3.2 Neutron production target The neutron production target is composed of a number of 50 mm×50 mm Ta plates separated by layers of flowing cooling water as shown in Fig. 4. The target is enclosed in a Ti vessel with a length of 108 mm and a cross section of 90 mm×60 mm. The electron beam is incident on the target through a 1-mm thick Ti entrance window. The energy deposition of the electron beam in the target is concentrated at the front side of the target, since the electron beam is mainly stopped there. To lower localized heating by the beam energy deposition, the metal plates are stacked and cooled by water flow in the gap between the plates [18]. The target was designed for electron beam irradiation with a beam power of up to 10 kW. The Ta plates get progressively thicker moving from the front to the back of the target, changing from 1 mm to 5 mm. There are a total of 13 Ta plates with a total thickness of 29 mm, with each plate separated by 1.5 mm for water channels. The 1 mm and 2 mm thin Ta plates are located in the higher heat density region and the 3 mm and 5mm thick Ta plates at the back part of the target where heat density is lower. To reduce local radiation from the target to the reflector and shield on the rear side of the target, two Ti plates with thickness of 20 mm are also installed behind the Ta plates in the vessel for absorbing the radiation. We used Ti instead of Ta in order to reduce radiation exposure from the radioactive nucleus

182

Ta when treating the neutron production target for

maintenance. Cooling water with a flow rate of 50 L/min is injected into the vessel and flows parallel to each of the Ta and Ti plates. The inlet and outlet for the cooling water are located towards the front of the vessel in order to increase the cooling efficiency at the front part of the target where the heat load is highest. 3.3 Solid methane moderator A pair of cold neutron moderator vessels are located above and below the neutron production target (Figure 5). The moderator on the upper side is a decoupled-moderator (DM), and the moderator on the bottom side is a coupled moderator (CM), respectively [12]. Detailed drawings of the DM and CM 8

vessels are shown in Fig. 6. The moderator vessels are wrapped with metallized superinsulation film (model KC-50S, KANEKA CORPORATION), and each vessel is suspended in the middle of a vacuum chamber with thin stainless-steel wires. The moderator vessels and the vacuum chamber are made of A5052 aluminum alloy. Heat load by ionizing radiation on each vacuum chamber during full-beam operation is estimated to be about 60~70 W, and the outer surface of the vacuum chamber is cooled by continuous water flow. Figure 7 shows a block diagram of the solid-methane cold moderator cryogenic system. As discussed in the previous section, solid-methane at a temperature of 20 K is used as a moderator. The moderator vessels are cooled by a continuous flow of cold helium gas, and the solid methane moderators are kept at 20 K via a heat exchanger on the rear of the moderator vessels as is shown in Fig. 6. The combined heat load on the solid methane and the moderator vessel is estimated to be around 15~20 W, and three GM-refrigerators (SUMITOMO SRDK-500B) will be used to bring this heat away. High purity methane gas of N6.5 grade is introduced to the DM and the CM vessels via 280-L and 470-L buffer tanks respectively. The methane gas is charged at atmospheric pressure to obtain a block of solid methane with dimensions of 120 mm×120 mm×30 mm and 120 mm×120 mm×50 mm for the DM and CM, respectively. Methane gas in the moderator vessels is condensed by keeping the vessels temperature around 90 K for several hours, and is frozen by gradually decreasing the temperature down to 20 K. Periodically the neutron production target and moderator vessels will need to be removed for maintenance. The moderator vessel, vacuum chamber, neutron production target, and other associated parts, such as the coolant pipes, methane pipes and radiation shield, are assembled in a block, and can be easily removed from the concrete radiation-shielding structure. 3.4 Reflector and shield Graphite was chosen as the reflector material because it is composed of light atoms, has weak neutron absorption, and is easy to handle. Hydrogenous material such as water could also be used for the reflector. However, water was not used due to the low intensity of the thermal neutron beam from a decoupled moderator due to hydrogen’s large average logarithmic energy loss [19]. The 9

graphite used as a reflector has a density of more than 1.80 g/cm3 and is placed into three 0.5-mm thick aluminum vessels. The thickness of the graphite is 120 mm to 170 mm with two 120 mm×120 mm holes along the neutron beam lines. There is also a 40 mm×40 mm hole in the reflector for extracting fast neutrons from the neutron production target. For the radiation shield, lead, boric acid resin, and concrete are used. Lead is for absorption of γ-rays, boric acid resin is for neutrons, and concrete for both. A large number of γ-rays are emitted from electro-magnetic showers in the neutron production target. A lead shield is used to shield these γ-rays locally around the graphite reflector. The basic thickness of the lead is 50 mm although the upper side is thicker (150 mm) to suppress the upward radiation leakage. Boric acid resin with a minimum thickness of 550 mm is placed around the lead shield. The boric acid resin deaccelerates neutrons by elastic scattering with hydrogen and

10

B absorbs slow neutrons. This shield is further surrounded by

additional lead shields since a 478-keV γ-ray is emitted with a probability of 94 % if a neutron is absorbed by 10B. The remaining γ-rays and neutrons after these shields are further shielded by an outer layer of thick concrete. The outer size of the concrete shield is about 4-m wide and 3-m high. The calculated radiation dose rate outside of the concrete shield is below the Japanese regulation value (1 mSv/week) so radiation workers can stay outside the concrete shielding during operation. 3.5 Beam line The direction of the decoupled moderator neutron beam port is perpendicular to the electron beam line. The height of the neutron beam line is 156.5 mm above the electron beam line. A super mirror guide tube (3Qc), which has inner dimensions of 130 mm×130 mm and a length of 970 mm, is installed in the beam extraction hole of the beam shutter. A double disk chopper with a rotation frequency 25 Hz is set at about 3.5 m from the moderator and eliminates the frame overlapped slow neutrons. Two 2-m long super mirror guide tubes with 3Qc, with inner dimensions of 130 mm×130 mm are set upstream and downstream of the disk chopper. Hereafter, we call them the upstream and downstream mirrors. In total, almost 5 m of flight path out of 8 m of total flight path can be covered with the supermirror guide tube. They 10

are designed so that a part of it can be removed and replaced with vacuum tubes. These guide tubes are used for measurements using a high-flux 100 mm×100 mm beam at the sample position. In case of using a larger size beam, the guide tube can be removed from the beam line. Sample will be mounted on a goniometer system which is set around 8 m from the moderator and a two-dimensional neutron detector will be installed just behind the goniometer system. The whole neutron beam line is surrounded by 250-mm and 400-mm thick concrete panel shielding blocks to prevent radiation leakage. Experimental workers can enter the beam line hutch if the beam is shut off by the beam shutter. 3.6 Neutron detector The neutron detector is one of the most important components in the system. For Bragg-edge imaging, the neutron detector is required to be 2-D position sensitive, have a high spatial resolution, low gamma-ray sensitivity, and high count-rate capability. For those reasons, gaseous detectors with solid thin film neutron converters readout by a micro-patterned gas detector have been proposed as an appropriate choice for Bragg-edge imaging. So far, successful results of neutron Bragg-edge imaging are reported with GEM detectors coupled with 10B convertors [4]. Here, neutrons are detected using a nuclear reaction with 10

B based on the following neutron reactions: [94%]

(1)

[6%], (2) where, 7Li* is an excited nuclear state which spontaneously emits a 0.48 MeV gamma ray. Charged particles produced after the nuclear reaction, i.e. either Heor Li- particles, are emitted from the boron foil and ionize gas molecules in the gas volume creating ions and primary electrons. The electrons created in the gas volume are multiplied via electron avalanche and the position and deposited energy are readout with XY strips (Fig. 8). However, there is still room for improvement of the GEM detector, especially with our compact accelerator driven neutron source. Comparing to J-PARC, our source is expected to have 11

lower neutron flux and higher X-ray/gamma-ray background. The measurement time for a single Bragg-edge imaging experiment is expected to be a few hours, but sometimes, longer measuring time may be required. As such the detector is also required to have long-term stability, robustness, and a high neutron detection efficiency. Therefore, we are planning to adopt a micro-structured 10B foil for the neutron convertor which was developed to overcome the issue of low detection efficiency of the boron foil [20]. The detector uses a specially shaped boron converter which enables a higher neutron detection efficiency [20]. The effective area is 100 mm×100 mm and the neutron detection efficiency for a neutron wavelength of 0.4 nm is about 20 %. A high efficiency single convertor improves the complexity of the detector, which results in improved stability. In addition, a glass substrate will be used for the electron multiplication in the GEM. The glass GEM has many advantages to the conventional Kapton foil based GEMs, such as robustness, high gain and simplicity, since the glass GEM has high enough gain without cascading [21]. Furthermore, since the glass GEM is fabricated with hydrogen free materials, it is expected to reduce the background caused by scattered neutrons. 4

Monte Carlo simulation of the neutron beam 4.1 Method The neutron beam was optimized using the Monte-Carlo simulation code PHITS (Version 3.02) [22]. The kernel used for the calculations is based on the nuclear data library JENDL-4.0 [23] and we applied the JENDL-4.0 thermal scattering law on solid methane. A simulated system, which reproduces the actual neutron source and beam line, was used as a model as shown in Fig. 1. In order to obtain data with sufficiently high statistics, the calculation was performed in several steps. The neutron production rate due to the electron irradiation of the target is too low to perform the simulation calculation in a single stage (i.e. from neutron generation at the neutron production target to neutron transportation in the moderator system and the beam line). Therefore, we performed the calculation in three steps. The first is to make the initial neutron data in the neutron production target. A 20 mm diameter electron beam, with an energy of 35 MeV and power of 10 kW was irradiated on the neutron 12

production target and emitted neutrons were recorded with their position, direction, and energy properties. Next, neutrons are generated at the neutron production target in a way which reproduces the previously calculated properties and characteristics. We used these generated neutrons as the source data for the solid methane simulation to study the optimal thickness of the moderator. Finally, the energy and angular distributions of neutrons just downstream of the decoupled moderator surface were used to evaluate the performance of the super mirror guide tubes. 4.2 Results and discussion 4.2.1 Neutron energy spectrum from the neutron production target The energy spectrum of neutrons emitted from the neutron production target is shown in Fig. 9 (black line). This spectrum has a bump at about 1 MeV and has a long tail towards the low energy side. For comparison, the initial energy spectrum of neutrons produced in Ta, H2O and Ti, which are components of the neutron production target, is also plotted (red line). The ratio of amounts of neutrons produced in these components are 98.0 %, 0.3 %, 1.7 % for Ta, H2O, and Ti, respectively. Integral values of the emitted and produced neutron spectra are 1.23×1013 and 1.31×1013 neutrons per second in the energy region from 1 eV to 10 MeV and the reduction due to the absorption process is 6 %. The emitted spectrum shifts towards the lower energy compared to the incident spectrum and this is due to slowing down mainly by hydrogen in water. Moreover, the emitted spectrum has a dip at about 18 keV which is not seen in the incident spectrum. This dip originates from interactions between neutrons and materials after the neutron generation. One possible explanation is the neutron capture resonance at 17.6 keV of 48Ti. Under this assumption, we obtained the energy spectrum after substitution of

48

Ti to other Ti isotopes, this spectrum is also plotted in

Fig. 9 (blue line). There is no dip at 18 keV in this spectrum, confirming that the dip can be attributed to the neutron capture resonance of 48Ti. 4.2.2 Energy spectra and pulse shapes of the neutron beam from the decoupled moderator

13

The energy spectra and pulse shape of the neutron beam from the decoupled moderator were calculated at 8 m from the moderator using PHITS. Neutron flux spectra as a function of neutron energy at 8 m from the moderator for various thickness of solid methane are shown in Fig. 10. There is a thermal peak at an energy just above 1.72 meV, which corresponds to the thermal energy of the temperature of solid methane, 20-K. The thermal peak intensity increases as the solid methane thickness increases. Figure 11 shows various emission time distributions of neutrons coming out from the moderator surface, with four neutron energies and five moderator thicknesses. This time distribution, or pulse, originates from the neutron slowing down process in the moderator and reflector. The spectra in Fig. 11 are normalized to the peak intensity at each neutron energy. At 3.6 and 5.7 meV, the pulse width in FWHM (Full Width at Half Maximum) increases and the tails become longer as the moderator thickness increases. At the higher energies of 9.0 and 18 meV, the pulse widths tend to be constant even though the tail of the 9.0 meV spectra increases as the moderator thickness increases. The wavelength resolution of neutrons calculated by the time of flight method becomes worse as the pulse width increases. Therefore, as the neutron flux increases the wavelength resolution decreases and vice versa as summarized in Fig. 12. The flux increases almost linearly in the range from solid methane thicknesses of 1 cm to 3 cm and it tends to saturate above 3 cm. On the other hand, most of the pulse width data increases or is constant as the moderator thickness increases as shown in Fig. 12 (b). The FWHM of 3.6 meV decreases below 4 cm. The FWHM of the other energies are almost flat. In order to check the pulse tail quantitatively, the FWTM (Full Width at Tenth Maximum) are also plotted in Fig. 12 (b). They increase as the moderator thickness increases except for the data at 18 meV. This data at 18 meV contradicts the expected behavior for the decoupled moderator since the pulse shape of neutrons from the decoupled moderator is usually mainly determined by the moderator characteristics. This result might indicate a leakage of thermal neutrons 14

from the reflector to the moderator and the leakage could become pronounced for the thin moderator at the higher neutron energy. To determine the optimum moderator thickness, we calculated Flux/FWHM2 and Flux/FWTM2 as a function of the moderator thickness at 5.7 meV (= 0.38 nm), these quantities are plotted in Fig. 13. The dependence of the Flux/FWHM2 value on the moderator thickness is almost the same as that of the Flux. This is because the FWHM value is almost constant. On the other hand, the Flux/FWTM2 has a peak at 3 cm. Not only the FWHM value but also the FWTM value is important for obtaining sharp Bragg edge spectra. From these results, the moderator thickness was set to be 3 cm. The filled data points in Fig. 14 show the dependence of the pulse width with neutron wavelength for a 3 cm thick moderator. The pulse width increases as the neutron wavelength increases although the rate of increase is reduced above a wavelength of 0.6 nm. The actual pulse width includes the pulse width of the electron beam. The electron beam pulse in the present neutron source is 10 μs. The open data points in Fig. 14 show the actual pulse width including the contribution of the electron pulse width. The contribution of the electron beam pulse is not negligible at short wavelengths but is negligible at longer wavelengths. The neutron wavelength resolution converted from the data in Fig. 14 under the flight path length of 8 m is shown in Fig. 15. The best resolution in the present wavelength region is about 0.45 % at around 0.2 nm. At shorter wavelengths, the resolution degrades (∆λ/λ increases) due to the electron beam pulse width. Also, at longer wavelengths, the resolution degrades due to the neutron slowing down process and becomes worst (maximum ∆λ/λ) around 0.5 nm. As shown in Fig. 2, intense Bragg edges of metal structural materials such as iron and aluminum appear around 0.3~0.4 nm and the estimated resolution is 0.5~0.7 % in this wavelength region. The flight path length from the moderator to the neutron detector becomes longer if neutrons are reflected by the super mirror guide tubes. However, the increase rate of the flight path length compared to direct neutrons from the moderator is 0.05 % under the twice reflections by the 15

super mirror guide tube with the cross section of 130 mm×130 mm and the effect by the super mirror guide tubes on the wavelength resolution is very small compared to the resolution shown in Fig. 15. 4.2.3 Effect of the super mirror guide tube To evaluate the effect of the super mirror guide tube on the neutron beam with sufficient statistics, neutrons were generated just downstream of the moderator surface and detected at a distance of 8 m in the PHITS Monte-Carlo simulation. Neutron flux spectra with the super mirror guide tubes are shown in Fig. 16 (a) as a function of neutron wavelength. The flux has thermal peak around 0.4 nm and this is suitable for Bragg edge imaging of structural materials like iron and aluminum since these materials have large Bragg edges around 0.4 nm. The integral intensities in the region from 0.2 to 0.5 nm are 1.2×104, 2.6×104 and 4.5×104 /cm2/s for the neutron guides for shutter only, shutter and upstream, and shutter, upstream and downstream cases, respectively. With the addition of the super mirror guide tubes the flux increases above about 0.15 nm as compared to the shutter only case, and the peaks of the flux shifts towards a longer wavelength as the super mirror guide tubes gets longer. As shown in Fig. 16 (b), increase in flux with respect to the shutter mirror only case increases up to 0.5 nm and saturates to a flux ratio about 2.75 in the case that the shutter and upstream mirrors are used. The flux increases even further if the downstream mirror is also used. From the point of the neutron flux, the super mirror guide tubes are very effective. Figure 17 shows the flux spatial distributions at a distance of 8 m from the moderator for some selected wavelengths and including all the super mirror guide tubes. It is clear that the regions of maximum intensity change depending on the neutron wavelength. However, the majority of the intense regions are inside an area of 100 mm×100 mm, which is the effective area of the present GEM detector. Figure 18 shows plots of neutron flux as a function of arrival angle at a distance of 8m from the moderator in four wavelength regions. Here, the arrival angle is direction in which neutrons arrive within the 100 mm×100 mm detection area with respect to the beam line axis. As before, 16

the flux was calculated with the shutter guide only, the shutter and upstream guides and the shutter and both upstream and downstream guides respectively. As a result, the flux at large angle increases as the length of the super mirror guide tubes increases. The arrows in Fig. 18 indicate the critical angles of the 3Qc super mirror for the center wavelengths of the four wavelength regions. The flux spectra with upstream and/or downstream guides obey these critical angles depending on the neutron wavelength. There are dips or flat regions around 14 mrad in spectra with longer neutron wavelength in Fig. 18. These structures originate from the space between the moderator surface and super mirror guide tube in the beam shutter. Neutrons with small beam divergence are suitable for neutron transmission imaging as such neutrons will minimize the blur of the transmission image. The degree of beam divergence is usually indicated by the L/D value. The blur is calculated by dividing the distance between the object and neutron detector by the L/D value. An L/D value of 50 corresponds to an incident angle of 20 mrad. For such an L/D value the blur is 0.2 mm and 1.0 mm for the object – neutron detector distances of 10 mm and 50 mm, respectively. Clearly there is a trade-off between the neutron flux and degree of image blur. Therefore, implementation of the super mirror guide tubes will depend on the required spatial resolution of transmission images. 4.2.4 Estimated counting rate The neutron flux spectrum at a distance of 8 m from the moderator as a function of the neutron wavelength is shown in Fig. 16 (a). The count rate of the neutron detector is estimated by multiplying the effective area and detection efficiency of the neutron detector and a dividing by the pulse repetition rate after converting the horizontal axis unit from wavelength to TOF. The effective area of the GEM neutron detector is 100 mm×100 mm. The neutron detection efficiency depends on the

10

B

converter thickness and the threshold of the electric signals originating from the nuclear reaction between a neutron and a

10

B nuclei. Here, a

2-μm thick flat 10B converter, detection of charged particles (4He and 7Li) from one surface of the converter and a threshold of 0.2 MeV were 17

assumed. This threshold is reasonable since it is low compared to the maximum energy deposition and high compared to noise level. The neutron detection efficiency obtained by a PHITS Monte-Carlo simulation is shown in Fig. 19. The estimated efficiency is about 10 % at 5 meV (0.4 nm). Estimated count rates using a GEM neutron detector with the conditions assumed above are shown in Fig. 20. The maximum count rates in the wavelength region from 0.4 to 0.5 nm are about 200, 500 and 900 kcps for shutter mirror only, shutter and upstream mirror, and shutter and both upstream and downstream mirrors respectively. These values are comparable with the maximum count rates of typical GEM neutron detectors. 5

Conclusion We have designed and are now constructing a CANS for the characterization of

innovative structural materials for transportation vehicles. The specifications of the electron accelerator, neutron source, neutron beam line and neutron detector have been optimized for Bragg edge imaging. The properties of the pulsed neutron beam produced by the neutron source were estimated using Monte-Carlo simulations. The wavelength resolution was estimated to be 0.5~0.7 % in the wavelength region from 0.3 to 0.4 nm. The flux of the neutron beam at 8 m was estimated to be 1.2×104~4.5×104 /cm2/s depending on the usage of super mirror guide tubes. The neutron beam flux at the sample position is high enough to achieve the maximum count rate from our proposed two-dimensional neutron detector. The newly designed compact neutron source is now under construction with first neutron beams expected in 2019. Acknowledgements The authors thank Prof. J. Hori, Mr. N. Abe, Mr. K. Takami, Dr. H. Nonaka, Dr. S. Gonda, and Prof. Y. Tomota for helpful discussions and advice. The new neutron facility is being developed under the auspices of the Innovative Structural Materials Association (ISMA), which is promoting weight reduction in automobiles and other transportation vehicles. This paper is based on results obtained from a project

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commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References [1] Y. Tomota, N. Sekido, S. Harjo, T. Kawasaki, W. Gong and A. Taniyama, ISIJ Int., 57 (2017) 237-2244. [2] R. Woracek, J. Santisteban, A. Fedrigo and M. Strobl, Nucl. Instrm. Methods Phys. Res., Sect. A 878 (2018) 141-158. [3] I.S. Anderson, C. Andreani, J.M. Carpenter, G. Festa, G. Gorini, C.-K. Loong and R. Senesi, Physics Reports 654 (2016) 1-58. [4] H. Sato, T. Kamiyama and Y. Kiyanagi, Materials Transactions 52 (2011) 1294-1302. [5] T. Ishida, M. Ohnuma, B.S. Seong and M. Furusaka, ISIJ Int., 57 (2017) 1831-1837. [6] Y. Ikeda, A. Taketani, M. Takamura, H. Suganuma, M. Kumagai, Y. Oba, Y. Otake and H. Suzuki, Nucl. Instrm. Methods Phys. Res., Sect. A 833 (2016) 61-67. [7] H. Sato, T. Sasaki, T. Moriya, H. Ishikawa, T. Kamiyama and M. Furusaka, Physica B 551 (2018) 452-459. [8] H. Sato, T. Kamiyama, K. Iwase, T. Ishigaki and Y. Kiyanagi, Nucl. Instrm. Methods Phys. Res., Sect. A 651 (2011) 216-220. [9] Y. Kiyanagi, H. Sato, T. Kamiyama and T. Shinohara, Journal of Physics: Conference Series 340 (2012) 012010. [10] H. Sato, T. Shinohara, R. Kiyanagi, K. Aizawa, M. Ooi, M. Harada, K. Oikawa, F. Maekawa, K. Iwase, T. Kamiyama and Y. Kiyanagi, Physics Procedia 43 (2013) 186-195. [11] Y. Kiyanagi, T. Kamiyama, H. Sato, T. Shinohara, T. Kai, K. Aizawa, M. Arai, M. Harada, K. Sakai, K. Oikawa, M. Ohi, F. Maekawa, T. Sakai, M. Matsubayashi, M. Segawa and M. Kureta, Nucl. Instrm. Methods Phys. Res., Sect. A 651 (2011) 16-20. [12] Y. Kiyanagi, Nucl. Instrm. Methods Phys. Res., Sect. A 562 (2006) 561-564. [13] W.P. Swanson, Health Physics 35 (1978) 353-367. [14] T. Iga, T. Seki, and S. Hara, Proceedings of IPAC’10, http://accelconf.web.cern.ch/AccelConf/IPAC10/papers/mopea012.pdf.

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[15] S. Uno, T. Uchida, M. Sekimoto, T. Murakami, K. Miyama, M. Shoji, E. Nakano, T.Koike, K. Morita, H. Satoh, T.Kamiyama, Y. Kiyanagi, Physics Procedia 26 (2012) 142-152. [16] A.S. Tremsin, J.B. McPhate, W. Kockelmann, J.V. Vallerga, O.H.W. Siegmund, W.B. Feller, Nucl. Instrm. Methods Phys. Res., Sect. A 633 (2011) S235-S238. [17] J.D. Parker, K. Hattori, H. Fujioka, M. Harada, S. Iwaki, S. Kabuki, Y. Kishimoto, H. Kubo, S. Kurosawa, K. Miuchi, T. Nagae, H. Nishimura, T. Oku, T. Sawano, T. Shinohara, J. Suzuki, A. Takada, T. Tanimori, K. Ueno, Nucl. Instrm. Methods Phys. Res., Sect. A 697 (2013) 23-31. [18] K. Kobayashi, G. Jin, S. Yamamoto, K. Takami, Y. Kimura, T. Kozuka and Y. Fujita, Annu. Rep, Res. Reactor Inst. Kyoto Univ. 22, 142 (1989). [19] Y. Kiyanagi, J. Nucl. Sci. Technol. 24 (1987) 490-497. [20] T. Fujiwara, U. Bautista, Y. Mitsuya, H. Takahashi, N.L. Yamada, Y. Otake, A. Taketani, M. Uesaka and H. Toyokawa, Nucl. Instrm. Methods Phys. Res., Sect. A 838 (2016) 124–128. [21] T. Fujiwara, Y. Mitsuya, H. Takahashi, T. Fushie, S. Kishimito, B. Guerard, et al., J. Inst. 9 (2014) P11007–P11007. [22] T. Sato, Y. Iwamoto, S. Hashimoto, T. Ogawa, T. Furuta, S. Abe, T. Kai, P-E. Tsai, N. Matsuda, H. Iwase, N. Shigyo, L. Sihver and K. Niita, J. Nucl. Sci. Technol. 55 (2018) 684-690. [23] K. Shibata, O. Iwamoto, T. Nakagawa, N. Iwamoto, A. Ichihara, S. Kunieda, S. Chiba, K. Furutaka, N. Otuka, T. Ohsawa, T. Murata, H. Matsunobu, A. Zukeran, S. Kamada, and J. Katakura, J. Nucl. Sci. Technol. 48(1) (2011) 1-30. Figures Fig. 1 Overview of the present compact accelerator-driven neutron facility. Fig. 2 Calculated transmission spectra of 1-mm thick α-iron and 2-mm thick aluminum. Fig. 3 Relation of the flight time and flight path length of neutron beams at the present facility. The bold black line indicates the neutron cut region of the disk chopper. Fig. 4 Cross section of the neutron production target. Fig. 5 Schematic drawing of neutron moderator vessels and vacuum chambers. Fig. 6 Moderator vessels for (a) DM and (b) CM. Fig. 7 Block diagram of the solid-methane moderator system. 20

Fig. 8 Neutron detection process of the GEM detector used in the present neutron source. Fig. 9 Energy spectrum of neutrons at the neutron production target. The black spectrum is for neutrons emitted from the target. The red spectrum is for neutrons produced in the target. The blue spectrum is for neutrons from the target without 48

Ti.

Fig. 10 Neutron energy spectra at 8 m from the decoupled moderator for several moderator thicknesses. Fig. 11 Emission time distributions of (a) 3.6 meV, (b) 5.7 meV, (c) 9.0 meV and (d) 18 meV neutrons from the decoupled moderator surface for several moderator thicknesses. Neutron wavelengths of (a)~(d) are 0.48, 0.38, 0.30 and 0.21 nm, respectively. Fig. 12 The flux and pulse width as a function of the moderator thickness. (a) The flux values are expressed as ratios compared to the flux with the 1-cm moderator thickness. (b) The FWHM and FWTM are shown as a function of the moderator thickness. Fig. 13 The values of (a) Flux/FWHM2 and (b) Flux/FWTM2 as a function of the moderator thickness at 5.7 meV (= 0.38 nm). Fig. 14 FWHM values of the neutron pulse. The filled data points include only the neutron pulse. The open data points include the electron beam pulse in addition to the neutron pulse. Fig. 15 Neutron wavelength resolution at 8 m from the moderator. The filled data points include only the neutron pulse. The open data points include the contribution from the electron beam pulse in addition to the neutron pulse. Fig. 16 The neutron flux spectra with the super mirror guide tubes. (a) Flux spectra for the three mirror settings. (b) The ratios of the flux compared to the flux with only the shutter mirror. The data with the neutron wavelength longer than 0.0286 nm (=1.0 eV) is plotted. Fig. 17 The flux spatial distributions at 8 m for some wavelengths with all the super mirror guide tubes. The red squares indicate the area of 100 mm × 100 mm. Fig. 18 The relations of wavelength and angle of neutrons at 8m for four wavelength regions. The arrows indicate the critical angles of the super mirror for the center wavelengths of the four wavelength regions. 21

Fig. 19 The neutron detection efficiency obtained by a PHITS Monte-Carlo simulation. The simulation condition is explained in the text. Fig. 20 The count rate spectra of neutrons by a GEM neutron detector under the condition written in the text. The data with TOF longer than 0.578 ms (=0.0286 nm) is plotted.

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