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ᧄ࡟ࡐ࡯࠻ߪᣣᧄේሶജ⎇ⓥ㐿⊒ᯏ᭴߇ਇቯᦼߦ⊒ⴕߔࠆᚑᨐႎ๔ᦠߢߔ‫ޕ‬ᧄ࡟ࡐ࡯࠻ߩ౉ᚻਗ߮ߦ⪺૞ᮭ೑↪ߦ㑐ߔࠆ߅໧޿วࠊߖߪ‫ޔ‬ਅ⸥޽ߡߦ߅໧޿วࠊߖਅߐ޿‫ޕ‬ߥ߅‫ోߩ࠻࡯ࡐ࡟ᧄޔ‬ᢥߪᣣᧄේሶജ⎇ⓥ㐿⊒ᯏ᭴ࡎ࡯ࡓࡍ࡯ࠫ㧔http://www.jaea.go.jp/index.shtml㧕
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JAEA-Research 2006-090
Measurements of Spatial Distribution of Intensity and Energy Spectrum of Neutrons from
Coupled Hydrogen Moderators by using a Position Sensitive Detector(Cooperative Research)
Tetsuya KAI, Takashi KAMIYAMA∗ , Fujio HIRAGA∗ , Takashi KATO
and Yoshiaki KIYANAGI∗
Proton Accelerator Development Facility Unit
Quantum Beam Science Directorate
Japan Atomic Energy Agency
Tokai-mura, Naka-gun, Ibaraki-ken(Received December 5, 2006)
An experiment was performed by utilizing a neutron source of the Hokkaido University
45-MeV electron linear accelerator facility to conﬁrm a peculiar spatial distribution of neutron
intensities below 15 meV for a para-hydrogen moderator that has been suggested by neutrontransportation calculations. For comparison, both para-hydrogen and ortho-rich-hydrogen (ortho/para ratio was 60:40∼70:30) were utilized for moderator materials. Moderator dimensions
were 50 mm in thickness and 120 mm in width and height. The spatial distributions of the neutron intensity at the moderator surfaces were measured based on the pinhole camera principle
by using position sensitive helium-3 detectors and by putting a cadmium plate with a pinhole
at the middle of the ﬂight-path (6m). As a result, the para-hydrogen moderator exhibits anintensity enhancement toward fringe parts nears premoderator, while the intensity decreases at
the fringe parts for the ortho-rich-hydrogen moderator. These results indicated consistency ofthe peculiar spatial distribution obtained by the calculations.
Position dependences of neutron energy spectra on the moderator surface were measured
to discuss the reason for the peculiar distribution. In the energy region between 15 meV and
100 meV, both moderators exhibit the intensity enhancements at the fringe parts. At energies
below 15 meV, the para-hydrogen moderator exhibits higher intensity at the fringe parts, while
the intensity decreases at the fringe parts of the ortho-rich-hydrogen moderator.
The results provide us a basis for reliability of the neutron transportation calculation used
for the J-PARC spallation neutron source design.
Keywords: Spatial Distribution, Energy Spectrum, Cold Neutron, Coupled Hydrogen Moderator, Para Hydrogen, Ortho Rich Hydrogen, Position Sensitive Detector, Measurement∗

This research was performed by cooperative research with Hokkaido University
Quantum Science and Engineering, Graduate School of Engineering, Hokkaido UniversityiiJAEA-Research 2006-090
位置敏感型検出器を用いた結合型水素モデレータの低エネルギー中性子空間分布の測定(協力研究)
日本原子力研究開発機構
量子ビーム応用研究部門 陽子加速器施設開発ユニット
甲斐 哲也、加美山 隆∗ 、平賀 富士夫 ∗ 、加藤 崇、鬼柳 善明 ∗(2006 年 12 月 5 日 受理)
中性子輸送計算によって示されたパラ水素モデレータに対する 15 meV 以下の中性子強度の特異な空間分布を確認するため、北海道大学 45 MeV 電子線形加速器施設の中性子源を利用して、実験を行った。モデレータ材料には、比較のためパラ水素とオルソリッチ水素 (オルソ/パラ比は60:40∼70:30) の両者を使用した。モデレータ寸法は厚さ 50 mm、縦横 120 mm とした。ピンホー
ルカメラの原理により、位置敏感型ヘリウム 3 検出器を用い、飛行経路 (6m) の中間に直径 2 mmのピンホールのあいたカドミウム板を設置し、モデレータ表面における中性子強度分布を測定した。その結果、パラ水素モデレータでは、プリモデレータが隣接するモデレータの周辺部に向かって中性子強度の増加を示していたが、オルソリッチ水素モデレータでは、周辺で強度の低下が見られた。これらの実験結果は、計算で示された特異性と矛盾のないものであった。
さらに、特異な分布の原因を議論するため、モデレータ表面上の位置に対する中性子エネル
ギースペクトルの依存性を測定した。15 meV〜100 meV のエネルギー領域では、両モデレータとも周辺部で高い強度を示した。15 meV 以下では、パラ水素モデレータでは、同様に周辺部で高い強度を示したが、オルソリッチ水素モデレータでは、周辺部で強度の減少が認められた。
本研究で得られた結果は、J-PARC 核破砕中性子源で使用されている中性子輸送計算の信頼性を確認する根拠の一つである。

本研究は北海道大学との協力研究のもと実施された。
原子力科学研究所 (駐在): 〒 319-1195 茨城県那珂郡東海村白方白根 2-4∗北海道大学 大学院工学研究科 量子理工学専攻

ii
ii

JAEA-Research 2006-090

Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Experimental arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Calibrations and corrections of detector system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3 Para hydrogen concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Spatial distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 Neutron energy spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

iii
iii

JAEA-Research 2006-090

目次
1 はじめに . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 実験 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 実験配置 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 検出器システムの較正と補正 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3 水素のオルソ/パラ比 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 結果 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 空間分布 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 中性子エネルギースペクトル . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 結論 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
謝辞 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
参考文献 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9iv
iv

JAEA-Research 2006-090

1

Introduction
The Japanese Spallation Neutron Source (JSNS) is one of the most important facilities at

the J-PARC (Japan Proton Accelerator Research Complex) project [1] conducted by the collaboration between the Japan Atomic Energy Agency (JAEA) and the High Energy Accelerator
Research Organization (KEK). At J-PARC, pulsed protons (3 GeV, 1 MW, 25 Hz) bombard a
mercury (Hg) target to produce spallation neutrons, and emitted neutrons from the target areconverted into cold neutrons through moderators. The neutrons are utilized mainly for neutron
scattering experiments. The neutron source has one coupled and two decoupled supercriticalhydrogen (H2 ) moderators with light water (H2 O) premoderators. The coupled H2 moderator
aims at providing intense pulsed cold neutrons with maximizing time-integrated and pulse-peakintensities, while the decoupled moderators are to deliver narrower pulses for high-resolution
experiments.
Extensive studies have been carried out to obtain optimal parameters (dimensions, materials, arrangement, etc.) for the moderators to attain superior neutronic performance by meansof Monte Carlo calculations. Although such calculations are very useful, it is indispensable to
conﬁrm consistency between calculated results and existing experimental data. In an H2 moderator, a para-H2 concentration plays an important role to determine the neutronic performance,
especially in the energy range below several tens meV. Figure 1 shows scattering cross sections
for ortho- and para-H2 at 20 K calculated for ENDF/B-VI.3 [2]. The sharp drop in the paraH2 curve below 50 meV is due to spin coherence and the second drop below 3 meV is due tointermolecular interference. The main energy transfer mechanism for neutrons (neutron energyloss) at low energies is the para to ortho spin-ﬂip transition. The neutron energy loss in thetransition is 14.7 meV.
Whittemore et al.[3, 4] measured neutron energy spectra from ortho-para-H2 and para-H2
moderators at 20 K in an energy range between 0.2 and 100 meV. The moderators (6 inches (152mm) in diameter and about 6 inches in height) were set beside a lead (Pb) target irradiated byabout 20 MeV electrons from a Linac. Neutrons were extracted from both a reentrant hole atthe center of the moderator and an outer surface of the moderator. In the case of the para-H2
moderator, the neutron spectrum intensities from the reentrant hole and the outer surface werevery similar, while the latter was much higher than the former in the ortho-para-H2 moderator
case. It was concluded that the geometrical shape of a moderator is more sensitive on theintensity at ortho-para-H2 moderator than that at the para-H2 moderator.
Würz [5] measured neutron energy spectra and pulse characteristics from a cylindrical H2moderator of 160 mm in height and 148 mm in diameter for various para-H2 concentration
between 33 and 98%, utilizing 14-MeV neutrons that was produced by the D-T reaction as asource. The para-H2 concentration was determined based on the diﬀerence of heat conductivity
between ortho- and para-H2 . In practice, diﬀerence between electric resistance of tungsten wires
in the normal-H2 (reference) and analyzing H2 gas was measured by means of a Wheatstone－1
1－

JAEA-Research 2006-090

bridge. As a result, the energy spectrum measurements indicated that the spectral intensity
below about 20 meV increased with the para-H2 concentration. For instance, it was reported thatthe 98% para-H2 moderator exhibited 45% higher intensity than the 45% para-H2 moderator.
Pulse characteristics were measured at the para-H2 concentration of 77 and 98%. It was indicatedthat the pulse width from the 77% para-H2 moderator was rapidly broadened below about 40
meV compared to that from the 98% one.
Recently, Ooi et al.[6] measured neutron energy spectra and pulse shapes from a moderatorof 50 mm in thickness and 120 mm in width and height in a graphite reﬂector, as a functionof the para-H2 concentration, and compared the experimental results with calculations. At
the neutron energies below 100 meV, the pulse peak intensities increased and the pulse width
decreased with the increase of the para-H2 concentration. They found that the calculations wereconsistent with the experimental results in terms of the dependence of the para-H2 concentrationon the neutronic performance although there were some discrepancies in the absolute values.
It was concluded that the calculation is reliable to optimize parameters of a moderator system
although there are some discrepancies between the measurements and the calculations.
In JSNS, the optimal para-H2 concentration was obtained to be 100% together with the
optimal moderator dimension as a result of calculation studies [7, 8]. The eﬀects of these parameters on the time-integrated neutron intensity and the pulse characteristics are also consistent
with the existing experimental results. In addition, a spatial distribution of low-energy (<15meV) neutron intensity was calculated on a viewed-surface of the coupled moderator. A 100%
para-H2 moderator exhibited a peculiar distribution having an intensity-enhanced region at afringe part near premoderator, while the intensity decreased at the fringe part in a normal(25%para)-H2 moderator. For the distribution of the normal H2 moderator, an experimental result
had been reported by Ogawa et al.[9]. Ogawa measured neutron intensities on a viewed-surface
at 5 Å (3.3 meV) and 10 Å (0.82 meV) from a ortho-rich-H2 moderator as a function of thevertical distance from the moderator bottom. Judging from a natural conversion speed fromortho- to para-H2 and a holding time in the experiment, the para-H2 concentration is estimatedto be about 35%. It was conﬁrmed that the calculated distribution for normal-H2 is consistent
with the measured one. There is still no experimental data exhibiting such a peculiar spatial
distribution at the 100% para-H2 moderator. Therefore, it is indispensable to measure the spatial distribution at 100% para-H2 moderator in order to discuss a reliability of the calculated
result.
In this report, the authors measured spatial distributions of neutron intensity and energy
spectra on an ortho-rich- and a para-H2 moderators using position sensitive helium-3 (3 He)
detectors through a pinhole. The results are to be utilized to conﬁrm the calculation reliability.

－22－

JAEA-Research 2006-090

2
2.1

Experiment
Experimental arrangement
This experiment was performed at the Hokkaido University 45-MeV electron linear ac-

celerator facility.

A lead target (80W ×75H ×150D mm) and a liquid hydrogen moderator(120W ×120H ×50T mm) with 20 mm-thick polyethylene premoderator were surrounded by agraphite reﬂector, the volume of about 1 m3 , as shown in Figs. 2 and 3. A neutron beam extrac-

tion hole, the area of 120×120 mm, was equipped in the reﬂector. Neutrons were detected bya position sensitive detector system located at 6 m from the moderator viewed-surface througha cadmium (Cd) plate (0.5 mm thick) with a pinhole (2 mmφ ). An inverted distribution of
the neutron intensity on the moderator viewed-surface was observed by the detector as shownin Fig. 4. A small piece of Cd (30×8 mm) was attached on the moderator viewed-surface formarking.
The position sensitive detector system PSD2K (Fig. 5) [10] consists of eight 3 He gas counters(12.5φ ×600L mm) and data acquisition components. As shown in Figs. 6 and 7, a Cd plate withslits of 2 mm in width was attached in front of the 3 He counters aiming at improving verticalresolution. Figure 8 shows an example of the obtained data by one of the 3 He counters. A
higher intensity region around 4 ms between 90 and 145 ch corresponds to the Maxwellian peakof cold neutrons. Each 3 He counter produces two-dimensional data like this.
2.2

Calibrations and corrections of detector system
In a position sensitive 3 He counter, the produced charges by the 3 He(n,α) reaction werecollected at both ends of the counter. A relative neutron position on the 3 He counter wasdetermined from a ratio of the charges at both ends, and was stored as 256 channel digital
data by the PSD2K module [10]. To obtain absolute position, neutrons were measured throughthe Cd slits plate (2 mm in width at 12.5 mm interval) perpendicular to the counters. As an
example, Fig. 9 shows the neutron intensities as a function of relative horizontal position andtime-of-ﬂight (TOF) obtained by a counter, and Fig. 10 shows those integrated below 100 meV(>1.4 ms in TOF). A coeﬃcient to convert the relative position (channel) to the absolute one
was derived from the relations between the peak channel and the slit position. As shown in
Fig. 11, the coeﬃcient was obtained for each counter as a result of ﬁtting by the least squares
method. The average of the coeﬃcients was 2.075±0.009 (mm/ch).
It is necessary to correct diﬀerences of the detection eﬃciency between the 3 He counters.
To obtain a ﬂat distribution at the counters, neutrons were observed after removing the Cd platewith the pinhole in the ﬂight-path. The corrected neutron intensity I(X, d, E) was derived asfollows;

C(X, d, E),(1)
Cref (X, d, E)
where X is the horizontal position, d is the detector identiﬁcation number, E is the neutron
I(X, d, E) =－3
3－

JAEA-Research 2006-090

energy, C is the neutron counting rate through the Cd plate with the pinhole, Cref is thecounting rate without the Cd plate.
Background measurements were carried out by blocking up the pinhole of the Cd plate.
Figure 12 compares the foreground and the background I(X, d, E) integrated below 15 meV,I15 (X, d), for a 3 He counter. The background was 5∼13% of the foreground.
2.3

Para hydrogen concentration
Figure 13 shows a diagram of an H2 gas supply system to the moderator. The supplied H2

gas is condensed at the cold head above the moderator chamber (Fig. 2). This system consistsof an H2 gas bottle, ﬂow and pressure gauges, valves and an ortho to para converter (Fig. 14),which convert ortho-H2 to para-H2 using an iron hydroxide (Fe(OH)3 ) catalyst. The equilibrium
para-H2 concentration depends on temperature as shown in Fig. 15 cited from a reference [11]. In
order to discuss eﬀects of the para-H2 concentration, both para-H2 and ortho-rich-H2 were ﬁlled
in the moderator chamber. In the case of para-H2 , H2 was introduced to the moderator chamber
through the converter at 24 K. As shown in Fig. 15, the equilibrium para-H2 concentration was
expected to be ∼99%. In the case of ortho-rich-H2 , the H2 gas was introduced directly to themoderator chamber without passing through the converter. The para-H2 concentration is 25%
at a room temperature, and increases as the following equation after H2 liquefaction.
Qp (t) = 1 −Qo (0),
1 + ktQo (0)(2)

where Qp (t) is the para-H2 concentration at time t, Qo (0) the ortho-H2 concentration at t=0
and k the natural conversion constant. Figure 16 is a plot for k = 0.0114/hr [12].
The para-H2 concentration was measured by the Raman spectroscopy, which used an inelastic scattering and subsequent deexcitation of a laser light. A photograph of the Raman
spectrometer is shown in Fig. 17. The H2 gas was sampled into a sampling bottle, and was ﬁlled
into a transparent glass cell for the Raman spectroscopy as shown in Fig. 18. A Raman spectrum
for 25% para-H2 is shown in Fig. 19. A peak around 360 cm−1 of Raman shift corresponds topara-H2 , that around 590 cm−1 ortho-H2 . A peak area for para-H2 (Ip ) is deﬁned as a productof the para-H2 population (Pp ) and the constant Cp determined by such parameters as the crosssection of Raman scattering, the detection eﬃciency for the scattered light, etc. Using thosevalues for ortho-H2 (Io , Po and Co ), the ortho/para ratio (r) is obtained as follows,r=

Io /Co
Io Cp
Po
=
=
×
.
Pp
Ip /Cp
Ip Co(3)The Cp /Co value of 1.2 ± 0.1 was obtained by measuring the Raman spectrum of the ortho-richH2 (25% para-H2 ) (r=3), which was kept at room temperature for a long time. The para-H2
concentration is derived as 1/(1+r).
－4
4－

JAEA-Research 2006-090

A time table of the present experiment is shown in Fig. 20. H2 was sampled before andafter the neutron measurements for the para-H2 moderator, and after those for the ortho-richH2 one. Raman spectra of the para-H2 moderator samples are shown in Fig. 21. Large peaks
from para-H2 and small peaks from ortho-H2 were recognized. Expanded views of these peaks
denoted by ‘view-1’ and ‘view-2’ are shown in Figs. 22 and 23, respectively. A Raman spectrumof the ortho-rich-H2 moderator sample is shown in Fig. 24. Peaks from para-H2 and orthoH2 were clearly seen. The value of r in eq.(3) and the para-H2 concentrations were derived
from integral counts corresponding to the para- and ortho-H2 peaks as shown in Table 1. The
uncertainties of the para-H2 sample were mainly caused by statistical error of the ortho-H2 peakcounts, while those of the ortho-rich-H2 one the error of the para-H2 peak counts of the orthorich-H2 (reference). The para-H2 concentrations of the para-H2 samples (sampling (1) and (2)in Fig. 20) agreed within their uncertainties. In the case of the ortho-rich-H2 sample, the paraH2 concentration increased during the neutron measurements due to the natural conversion as
mentioned in this section. Figure 25 compares the measured para-H2 concentration with thatobtained by eq.(2). It was assumed that t=0 was the middle of the H2 liquefaction duration(6 h), which was indicated as a horizontal error-bar in Fig. 25. The para-H2 concentration
was estimated to be between 30 to 40% during the neutron measurements for the ortho-rich-H2
moderator.

－55－

JAEA-Research 2006-090

3
3.1

Results
Spatial distribution
Due to the 3 He counter size, spatial distribution measurements were performed by separat-

ing the moderator viewed surface into two regions where a region is near the target part of themoderator viewed surface, called as a near-target part, and is far from the target part, called
as a far-target part. Figure 26 shows two-dimensional views of the spatial distribution of theneutron intensity below 15 meV, I15 (X, Y ), where Y is the vertical position of the slit in frontof the counter relative to the moderator center. Although I15 (X, Y ) is shown in an arbitrary
unit, those from the para-H2 and the ortho-rich-H2 moderators can be directly compared since
I15 (X, Y ) is normalized to the electron beam current of the accelerator. The horizontal distributions are shown in Figs. 27 and 28, the vertical ones are shown in Figs. 29 and 30 for thecases of the para-H2 and the ortho-rich-H2 moderators, respectively. An intensity depression
in the upper right part of each contour map (Y =30-60 mm, X≥40 mm) corresponds to thesmall piece of Cd for marking. Through the results, it is clear that I15 (X, Y ) at the para-H2
moderator is enhanced at the side and bottom fringe parts near premoderator, while I15 (X, Y )at the ortho-rich-H2 one is depressed at the fringe parts. This provides us that the calculated
distribution [7] is qualitatively reasonable.
In the viewings of the far-target parts (upper ﬁgures of Figs. 26, 27, 28), lower intensity
regions are observed at Y =60 and 72 mm in the para-H2 case and only at Y =72 mm in theortho-rich-H2 case. This suggests that there is no liquid H2 at Y =72 mm in both cases, referring
the moderator height of 120 mm corresponds to Y = -60 ∼ 60 mm. The para-H2 case suggeststhat the level of liquid H2 in the moderator chamber was less than that of Y =60 mm since theintensity at Y =60 mm is almost equal to that at Y =72 mm.
The horizontal distributions exhibit symmetrical shapes about center except the case of
Y =−38 mm, in which I15 (X, Y ) depresses unnaturally in the region of X<0. Then, let deﬁne
an asymmetric index that indicates the degree of the asymmetricity of the distribution as follows:
As(X) = 1 −I15 (−X, Y )I15 (X, Y )(X ≥ 0).

(4)

Figure 31 shows As(X) at Y =−38 mm for both moderators. From the ﬁgure, a position dependence of As(X) is almost same. Such the depressions are not seen at the far-target viewings.
Accordingly, the asymmetricity is restricted at the part very close to the target. Reviewing the
preparation of the measurements, a path to the moderator center was checked by using a laser
light, but the paths to the moderator fringe parts were not checked. And, as shown in Fig. 32,
there are some shields inside the neutron ﬂight tube. Therefore, it is considered that the neutron
ﬂight path was partly shaded by one of the shields. Then I15 (X, Y ) data below X<0 for Y =−38mm should be omitted in the following discussions.
Figure 33 shows projections of the horizontal distributions of I15 (X, Y ) at the near-target
part of the moderator for both the para-H2 and the ortho-rich-H2 cases, where the vertical－6
6－

JAEA-Research 2006-090

direction intensities at a horizontal position is integrated along the vertical direction and an
average intensity such as {(I15 (−X, Y ) + I15 (X, Y )}/2 is used at the horizontal position since

the horizontal distributions exhibits the symmetrical shape about the center. In the central
part (0<|X|<∼30 mm), I15 (X, Y ) is almost unchanged with X in both cases. From |X|∼30

mm to the fringe, I15 (X, Y ) in the para-H2 case gradually increases with X by 16% relative tothat at X=0, while I15 (X, Y ) in the ortho-rich-H2 case decreases by 13%. The calculated resultin a reference [7] shows that I15 (X, Y ) at the fringe is higher than that at the central part by
about 50% in a pure para-H2 moderator. The gain factors at the fringe part in the para-H2
moderators are diﬀerent between the calculation (50%) and the experimental result (16%). One
of reasons for this discrepancy is due to the moderator thickness diﬀerence such as 50 mm inthis experiment and 140 mm in the calculation, respectively.
Figure 34 shows projections of vertical distributions of I15 (X, Y ), corrected data as wellas the horizontal distribution case. I15 (X, Y ) in the para-H2 case monotonously decreases withincreasing Y , while I15 (X, Y ) in the ortho-rich-H2 case has a maximum around Y =−25 mm andthen decreases with increasing Y . The intensity enhancement is recognized around the fringe
part near premoderator of the para-H2 moderator in both vertical and horizontal projections.
3.2

Neutron energy spectrum
The neutron energy spectra were measured by means of the time-of-ﬂight method. Thoseat the central part (|X|=0 mm), the fringe parts (|X|=42 and 52 mm), and the outside part(|X|=62 mm) are shown in Figs. 35 and 36 for the para-H2 and the ortho-rich-H2 moderators,
respectively. The energy spectra are averaged over the data between Y =−26 and 25 mm (seeFigs. 27 and 28) where the intensity along the vertical direction exhibits stable property.
The energy spectra at the outside part are composed as mixtures of neutrons from the H2moderator and the polyethylene premoderator that provides thermal neutrons. Therefore, theneutron intensity in the thermal energy region (>20 meV) at the outside part is higher thanthose at the other parts, while that in the cold energy region (<20 meV) is lower.
At the fringe part, both moderators have the intensity enhancements in the energy region
between 15 meV and 100 meV. At energies below 15 meV, the para-H2 moderator brings about
higher intensity, while the ortho-rich-H2 exhibits an intensity decrease. The intensity enhancements between 15 and 100 meV is considered to be caused by a short scattering mean-free-pathof the source thermal neutrons (above 15 meV) provided from the premoderator. The scattering mean-free path is 10 mm (at 50 meV) ∼ 20 mm (at 25 meV) in the para-H2 moderator,
and is shorter than 10 mm in the ortho-rich-H2 one. Accordingly, the both moderators exhibit

higher intensity at the fringe part at neutron energies above 15 meV. In the energy region below
15 meV, the mean-free-path for para-H2 is very long such as more than 100 mm resulting ina small probability of additional scattering. Namely, the para-H2 moderator is transparent inthis region. Therefore, the neutron intensity below 15 meV also exhibits the fringe-enhanced－7
7－

JAEA-Research 2006-090

distribution. While in the ortho-rich-H2 , the spatial distribution is approaching to a cosine like
distribution getting to equilibrium with liquid H2 since the mean-free-path is much shorter, lessthan 6 mm below 15 meV. Therefore, the spatial distribution on the ortho-rich-H2 moderator
is expressed by a combination of the fringe-enhanced distribution and a cosine like distribution.
From the above consideration, it is revealed that the fringe-enhanced distribution of neutron
intensity below 15 meV is caused by the unique property of the para-H2 cross section.
The energy spectra of the para-H2 and ortho-rich-H2 moderators are compared in Fig. 37
at various positions, |X|. For detail comparisons, right ﬁgures show the ratios of the neutron
intensities between the para- and the ortho-rich-H2 moderators. The energy dependence ofthe neutron intensity ratio is similar among all positions as having a intensity enhancement
around 10 meV. One of the reasons for this enhancement is the neutron slowing down reaction
accompanied to the para-H2 to ortho-H2 spin-ﬂip transition. The energy of neutrons aroundseveral tens meV are more eﬃciently converted to about 10 meV in the para-H2 moderator.
Another reason is the leaky characteristics of para-H2 due to the much smaller cross section,
which decreases rapidly between 50 and 15 meV down to 1/50 comparing to ortho-H2 . Therefore,
the neutrons, which are slowed down to around 10 meV, are easy to be emitted. It is noted that
this energy dependence of the ratio is consistent with the calculation [7].

－88－

JAEA-Research 2006-090

4Conclusion
The distributions of low-energy neutron intensity were measured for the para-H2 and ortho-rich-H2 moderators together with their energy spectra. The measured results demonstrate thatthe calculated distribution of low-energy neutrons in the JSNS design study [7] is qualitatively
reasonable although the moderator dimensions are not same. This experimental study validates
the JSNS design strategy in which 100% para-H2 concentration is assumed.
Acknowledgement
The authors thank Prof. H. Habasaki for the measurement of para-H2 concentration by
Raman spectroscopy. We thank Mr. S. Sato for providing us the position sensitive detector.
We thank Mr. H. Iwasa for the arrangement and operation of the cryogenic system, and Mr.
K. Sato for the accelerator operation.
References[1] The Joint Project Team of JAERI and KEK, The Joint Project for High-Intensity Proton
Accelerators, JAERI-Tech 99-056, Japan Atomic Energy Research Institute, KEK Report99-4, JHF-99-3, High Energy Accelerator Research Organization, 1999. (Latest information
is available at http://j-parc.jp/index-e.html)[2] R. E. MacFarlane, Cold-Moderator Scattering Kernel Methods, LA-UR-98-655 , Los Alamos
National Laboratory (1998).
[3] W. L. Whittemore, A. W. McReynodls, “Diﬀerential neutron thermalization”, GA-2305,
General Dynamics Corporation (San Diego, California) (1961).
[4] W. L. Whittemore, “Inelastic Neutron Scattering in Liquid Methane and Liquid Parahydrogen”, Nucl. Sci. Eng., 18, 182 (1964).
[5] H. Würz, “Untersuchungen zur Neutronnenthermalisierung an ﬂussigen Ortho- und Para
Wasserstoﬀ”, KFK-1697 (1964). [in Germany][6] M. Ooi, H. Ogawa, H. Iwasa, Y.Kiyanagi, “Experimental studies of neutronic performanceof a hydrogen moderator depending on para hydrogen concentrations”, Proceedings of the
16th Meeting of the International Collaboration on Advanced Neutron Sources (ICANSXVI), Düsseldorf-Neuss, Germany, 2003, p. 791.
[7] T. Kai, M. Harada, M. Teshigawara, N. Watanabe, Y. Ikeda, “Coupled hydrogen moderator
optimization with ortho/para hydrogen ratio”, Nucl. Instr. and Meth. A 523 (2004) 398.

－99－

JAEA-Research 2006-090[8] M. Harada, N. Watanabe, M. Teshigawara, T. Kai, Y. Ikeda, “Neutronic studies on decoupled hydrogen moderator for a short-pulse spallation source”, Nucl. Instr. and Meth. A 539(2005) 345.
[9] Y. Ogawa, Y. Kiyanagi, N. Kosugi, H. Iwasa, M. Furusaka, N. Watanabe, “Intensity enhancement of cold neutrons from a coupled liquid-hydrogen moderator for pulsed cold
neutron sources”, Nucl. Instr. and Meth. A 432 (1999) 415.
[10] S. Satoh, H. Sagehashi, T. Ino, M. Furusaka, Development of position sensitive detector,
PSD2K system for neutron scattering experiment, KEK-report 2001-9, High energy accelerator research organization (KEK) 2001. [in Japanese][11] B. N. Esel’son, Ch Nisenbaum, D. Slutzkin, “Properties of liquid and solid hydrogen”, Israel
Program for Scientiﬁc Translations 1971. [Translated from Russian][12] R. B. Scott, F. G. Brickwedde, Harold C. Urey, M. H. Wahl, “The Vapor Pressures andDerived Thermal Properties of Hydrogen and Deuterium”, J. Chem. Phys. 2 (1934) 454.

Table 1: Peak counts in Raman spectra and para H2 concentration
Para-H2
Ortho-H2
Para H2 concentration
Sampling Total
BG
Total
BG
r
(%)
(1)
94430 59501 32111 31565 0.018±0.009
98.2±0.9(2)78343 46003 28723 28081 0.023±0.009
97.7±0.9(3)60781 46014 77755 59140
1.5±0.2
40.2±3.3

－ 10
10 －

11

JAEA-Research 2006-090

Cross Section (b)100Ortho-H2

101

Para-H

2

1

10
100
Neutron Energy (meV)

1000

Fig. 1: Scattering cross sections for liquid ortho-hydrogen (upper curve)
and liquid para-hydrogen (lower curve) at 20 K calculated for
ENDF/B-VI.3 [2].

Fig. 2: Experimental arrangement of target, moderator and reﬂector.

－ 11 －

12

JAEA-Research 2006-090








Fig. 3: Photograph of electron beam line, reﬂector and neutron beam line.

Fig. 4: Principle of distribution measurement using a pinhole.

13－ 12 －

JAEA-Research 2006-090

Fig. 5: Components of position sensitive detector system, reproduced from Ref. [10]Fig. 6: Cross sectional view of slit and detectors.

14－ 13 －

JAEA-Research 2006-090

Fig. 7: Photograph of a Cd slit and 3 He counters. B4 C shielding-blocks
were put in front of the 3 He counters except the slit.

Fig. 8: An example of measured data by a 3 He counter. Horizontal axis
shows the horizontal position of neutrons, vertical one shows thetime-of-ﬂight of neutrons.

－ 14 －

15

JAEA-Research 2006-090

Fig. 9: Neutron intensity with vertical slits for calibration of horizontal position.

Neutron Intensity (counts/s/ch)40
35

En<100 meV

30
25
20
15
10
5
0
80

90

100
110
120
130
Horizontal Position (ch)

140

150

Fig. 10: Neutron intensity below 100 meV with vertical slits for calibration of horizontal position.

－ 16
15 －

JAEA-Research 2006-090


































Fig. 11: Relations of channel number in PSD system and absolute position at each detector.

































Fig. 12: Comparison of neutron intensities below 15 meV between theforeground and background measurements.

－ 17
16 －

JAEA-Research 2006-090

Fig. 13: Diagram of hydrogen gas supply system.

Fig. 14: Schematic view of ortho to para H2 converter.
－ 17 －

18

JAEA-Research 2006-090

Para hydrogen concentration (%)

100

97.021%(30K)90

99.821%(20K)
99.9999%(10K)8077.054%(50K)

70
60
50

38.620%(100K)

40

25.072%(300K)

30
10

20

50

100

200 300

Temperature (K)

Fig. 15: Equilibrium para-H2 concentration as a function of temperature [11].




























Fig. 16: Para hydrogen concentration as a function of time at 20 K.

－ 19
18 －

JAEA-Research 2006-090

Fig. 17: Raman spectrometer

Fig. 18: Glass cell and sampling bottle.

－ 20
19 －

JAEA-Research 2006-090



























 

Fig. 19: Raman spectrum for 25% para-H2 .

Fig. 20: Time table of experiment.

21－ 20 －

JAEA-Research 2006-090

 





















 





Fig. 21: Raman spectra from para-H2 samples taken before and after neutron measurements.

 




















 

Fig. 22: Enlarged view of Raman spectra of ‘view-1 indicated in Fig. 21.

－ 22
21 －

JAEA-Research 2006-090

 




















 

Fig. 23: Enlarged view of Raman spectra of ‘view-2 indicated Fig. 21.

 
















 





Fig. 24: Raman spectrum from ortho-rich-H2 sample after neutron measurements.

－ 23
22 －



JAEA-Research 2006-090

























Fig. 25: Time evolution of para H2 concentration in the case of ortho-rich-H2 .

24－ 23 －

JAEA-Research 2006-090

Para-H2, Energy: <15 meV

Ortho-rich-H2, Energy: <15 meV80

1.5
Far-target-part

0.8

40

1.0

0.8

20

1.0

0

1.0

1.2

0.5

−20
−40
−60

Near-target-part

−60

−40

−20

0

Near-target-part

20

40

60

−60

−40

−20

0

20

40

60

80

0.0

1.5
Far-target-partVertical Distance from center, Y (mm)Neutron Intensity (arb. unit)

60

Far-target-part

60
40

1.0

0.8

20

1.0

1.0

0

1.2

−20

0.5

1.4

1.2

Neutron Intensity (arb. unit)

Vertical Distance from center, Y (mm)Far-target-part

−40
−60

Near-target-part

Near-target-part−60 −40 −20
0
20
40
60
Horizontal Distance from center, X (mm)−60 −40 −20
0
20
40
60
Horizontal Distance from center, X (mm)0.0

Fig. 26: Spatial distributions of neutron intensity below 15 meV frompara- (left) and ortho-rich-H2 (right) moderators. Upper ﬁgures
show the results viewing a far-target part of the moderator.
Lower the results viewing a near-target part. Contour line is
not depicted outside H2 .

25

－ 24 －

JAEA-Research 2006-090



































































Fig. 27: Horizontal distribution of neutron intensity below 15 meV frompara-H2 moderator as a function of vertical length from moderator center.

26－ 25 －

JAEA-Research 2006-090



































































Fig. 28: Horizontal distribution of neutron intensity below 15 meV fromortho-rich-H2 moderator as a function of vertical length frommoderator center.

27－ 26 －

JAEA-Research 2006-090



































































Fig. 29: Vertical distribution of neutron intensity below 15 meV frompara-H2 as a function of horizontal length from moderator center.

28－ 27 －

JAEA-Research 2006-090






































































Fig. 29: Vertical distribution of neutron intensity below 15 meV frompara-H2 as a function of horizontal length from moderator center. (cont’d)29－ 28 －

JAEA-Research 2006-090








































































Fig. 30: Vertical distribution of neutron intensity below 15 meV fromortho-rich-H2 as a function of horizontal length from moderator
center.

30－ 29 －

JAEA-Research 2006-090








































































Fig. 30: Vertical distribution of neutron intensity below 15 meV fromortho-rich-H2 as a function of horizontal length from moderator
center. (cont’d)31－ 30 －

JAEA-Research 2006-090






























Fig. 31: Asymmetricity index for the horizontal distribution of I15 (X, Y ) at Y =−38 mm.

Fig. 32: Photograph of components inside neutron ﬂight tube.

－ 31 －

32

JAEA-Research 2006-090






















Fig. 33: Comparison of horizontal distributions of neutron intensity
below 15 meV between the ortho-rich- and para-H2 moderators.


















Fig. 34: Comparison of vertical distribution of neutron intensity
below 15 meV between the ortho-rich- and para-H2 moderators.

－ 32 －

33



JAEA-Research 2006-090



































































Fig. 35: Energy spectra for the para-H2 moderator at various horizontal
position (|X|).

34－ 33 －



JAEA-Research 2006-090

































































Fig. 36: Energy spectra for the ortho-rich-H2 moderator at various horizontal position (|X|).

35－ 34 －

JAEA-Research 2006-090

















































































































































































Fig. 37: Comparison of neutron energy spectra between the para andortho-rich-H2 moderator at various |X|. Right ﬁgures show ratios of intensities.

－ 35 －

36































































































































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