Proceedings of the International Conference on Neutron Optics (NOP2017)
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Proc. Int. Conf. Neutron Optics (NOP2017)
JPS Conf. Proc. 22, 011034 (2018)https://doi.org/10.7566/JPSCP.22.011034

Fundamental physics using high-performance optics
Masaaki Kitaguchi1,2 , Koichiro Asahi3 , Shunsuke Endo2 , Christopher C. Haddock2 ,Masahiro Hino4 , Katsuya Hirota2 , Takashi Ino5 , Ikuya Ito2 , Takahiro Iwata6 , Jun Koga7 ,Yoshiyuki Miyachi6 , Takamasa Momose8 , Noriko Oi2 , Takuya Okudaira2 , Kenji Sakai9 ,Tatsushi Shima10 , Hirohiko M. Shimizu2 , Shusuke Takada7 , Yutaka Yamagata11 , Tomoki
Yamamoto2 , Tamaki Yoshioka7,121

Center for Experimental Studies, KMI, Nagoya University, Furo-cho, Chikusa-ku, Nagoya-city,
Aichi 464-8601, Japan2
Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya-city, Aichi 464-8601,
Japan
3
Department of Physics, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro, Tokyo
152-8551, Japan
4
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
5
Institute of Material Structure Science, High Energy Accelerator Organization (KEK), Oho 1-1,
Tsukuba, Ibaraki 305-0801 Japan
6
Department of Earth and Environmental Sciences, Yamagata University, 1-4-12 Kojirakawa,
Yamagata 990-8560, Japan
7
Department of Physics, Kyushu University, 744 Motooka, Nishi, Fukuoka, Fukuoka, 819-0395
Japan
8
Department of Chemistry, UBC Faculty of Science, University of British Columbia, Vancouver,
British Columbia V6R1Z1, Canada
9
J-PARC center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1184 Japan
10
Research Center for Nuclear Physics, Osaka University, Osaka 567-0047, Japan
11
Center for Advanced Photonics, RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan
12
Research Center of Advanced Particle Physics, Kyushu University, 744 Motooka, Nishi, Fukuoka,
Fukuoka, 819-0395 Japan
E-mail: kitaguchi@phi.phys.nagoya-u.ac.jp(Received September 30, 2017)
The neutron is suitable for precision measurements to search for new physics. Combination of thepulsed neutrons provided by J-PARC and the advanced optical devices enables us to perform new
types of high precision measurements. The large CP violation should exist to explain the matteruniverse. Now we are discussing the search for large T-violation by using nuclear reaction. The
enhancement of the violation of time-reversal symmetry is predicted in the neutron capture reaction
for some nuclei. In this experiment, spin-controlling devices and optics for epithermal neutrons mustbe essential. We have started feasibility studies to search for T-violation in compound nuclear systems
at J-PARC.
KEYWORDS: CP violation, Epithermal neutron optics, Nuclear polarization

1.

Introduction

Neutron is a chargeless massive particle with the lifetime in the macroscopic range, which is suitable for precision measurement of the small influence of new physics. The combination of acceleratordriven high-intensity neutron source and high-performance neutron optics enables us to perform various physics experiments with far superior statistics. Now we stand at the portals of a new era ofneutron physics.
1■■■011034-1©2018 The Author(s)This article is available under the terms of the Creative Commons Attribution 4.0 License. Any further distribution of this work must maintain attribution to theauthor(s) and the title of the article, journal citation, and DOI.

Proceedings of the International Conference on Neutron Optics (NOP2017)
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Today’s matter universe requires CP-violation from new physics, which is related to asymmetry
between matter and antimatter, larger than that from standard model of particle physics. One of thestrongest constraints in the search for large CP-violation comes from measurements of the neutron
static electric dipole moment (nEDM). Non-zero value of the permanent nEDM signals the violationof time-reversal (T) invariance directly. Although experimental searches have been pursued in theworld, the nEDM has not yet been observed. The present upper limit is |dn | < 3 × 10−26 e·cm (90%C.L.), which is at, our below, predictions for EDMs which arise from new physics beyond the standard
model of particle physics (from supersymmetry, for example) [1]. This value was obtained throughmeasurements of the spin precession frequency in both magnetic field and electric field of confined
ultra-cold neutrons (UCNs). It is internationally competition to develop the new type of neutron
source for high-density UCNs, which enables us to search the neutron EDM of the order from 10−27
to 10−28 e·cm. Although next-generation UCN sources are being developed intensively, it is valuable
and very important to develop diﬀerent methods to search the CP-violation with the other methods
with diﬀerent systematic uncertainties and hopefully to improve the experimental sensitivity.
Now we are discussing another approach to search for the large T-violation. The enhancement of
the violation of time-reversal symmetry is predicted in the neutron capture reaction for some nuclei.
In this experiment, spin-controlling devices and optics for epithermal neutrons must be essential.

2.

Search for T-violating eﬀect in resonance capture reaction

The large enhancement of parity (P) violation of the weak interaction contained in the nuclear
interaction was discovered in the 1980’s [2, 3, 7] and later explored in detail in the 2000’s[3]. The
asymmetry of the capture cross section with respect to the helicity of incident neutrons was enhanced
by at most 106 times larger in a several nuclei. (Fig. 1) The enhancement is explained as the result of the entrance channel interference between neighboring s- and p-wave amplitudes [6, 7]. The
mechanism of the P-violating eﬀect has been proposed theoretically to be applicable to enhance theexperimental sensitivity to search the time-reversal (T) violation eﬀect in the p-wave resonances [8,9].
The advantage to apply the enhancement mechanism is that takes T-odd eﬀects due to the final-stateinteraction is expected to be negligibly small as the neutron propagation through the target material
can be described by the neutron optics. The cross section to be measured is given as∆σCP = κ(J)

WT
∆σP ,
W

(1)

where ∆σCP and ∆σP are the CP-violating and P-violating cross section, κ(J) is the spin factor, WT
and W denote the CP- and P-violating matrix elements. This means that the candidate nuclei for Tviolation search are the nuclei with large enhancement of P-violation, however, the parameter κ(J)must be measured. When we assume s-p mixing, the enhancement factor κ(J) can be written with thepp
15 MeV-(1.7±0.8)×10-7
s-waveresonance

×106

A

n
139La

En = 0.734 eV

0.097±0.003

Fig. 1. Enhancement of parity (P) violation around p-wave resonance.
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σ

σ · (Iˆ × k̂)

Analyzing PowerIˆ ×Polarizationk̂

Fig. 2. Configuration for time reversal (T) violation measurement. Asymmetry of capture cross section withpolarized incident beam A x and polarization of transmitted beam with unpolarized incident beam P x are measured for extract ReA∗ D.

partial neutron widths of p-wave resonance.
In addition, some theories predict much larger enhancement of T-violation due to the eﬀects
from the calculations of the eﬀective interactions. Even if measured κ value is the order of 1, thisexperiment with compound nuclei system is quite significant not only for T-violation search but also
for verify the theories.
The neutron-nucleus forward scattering amplitude can be described as)((2)
f = A + B σ · Î + C σ · k̂ + D σ · Î × k̂
where σ, Î and k̂ are the spin of incident neutron, the momentum of the neutron, and the spin oftarget nuclei, respectively. (Figure 2) A is the spin independent (P-even, T-even) term. B is the spin
dependent (P-even, T-even) term, which is corresponding to neutron spin rotation through the polarized target called as ‘pseudomagnetic eﬀect’. C is the P-violating (P-odd, T-even) term, which isenhanced for the case of some nuclei discussed previously. D is the T-violating (P-odd, T-odd) term.
The measured forward intensity of the beam is proportional to f ∗ f , with the value of A∗ D, which isfrom interference of A and D, gives us the largest measurement value as a combination of analyzing
power and polarization of transmitted beam. When the D value is estimated by using measured κvalue, the feasibility of the experiment to search for T-violation can be quantified.

3.

Measurement of enhancement factor

The candidates of the target nuclei are 139 La, 131 Xe, 115 In, and so on, which were the nuclei withlarge enhancements of P-violation. These nuclei have the p-wave resonance peak of neutron capture
with the energy of the order of 1 eV. This has the advantage to use the relatively-high intensity ofincident neutron beam at the accelerator-driven neutron source. In order to estimate the enhancement
factor κ(J), we have started the measurements of (n, γ) reactions with these nuclei at the beamline
BL04 ANNRI in Material and Life science experimental Facility (MLF) in J-PARC.
At first we measured the resonance parameters of the (n, γ) reactions for the candidate nuclei byusing germanium γ-ray detectors. The germanium detectors enabled us to separate the γ-ray peaks
due to the high energy-resolution. We also established the simulations which reproduce the detector
responses very well in wide energy-range of γ-rays. These enable us to measure the angular distribution of γ-rays from a certain transition between the states of the compound nuclei.
The diﬀerential cross section of the (n, γ) reaction with unpolarized incident neutron and unpolarized nuclear target can be expanded as
dσ 1= (a0 + a1 cos θ) ,
dΩ 2

(3)

for the first order, where θ is the direction of the γ-ray, a0 and a1 are the coeﬃcients, respectovely.
According to s-p mixing theory with statistical treatments in the compound nuclei, the coeﬃcients3■■■Proceedings of the International Conference on Neutron Optics (NOP2017)
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Ge Detectors

targetpulsed neutrons

gamma ray

5161keV (gnd. 3-)5098keV (Ex 63keV 4-)4903keV (27Al)

4842keV(Ex 318keV 3-)5126keV (5-)+5131keV (2-)γ ray spectrum

γ-ray energy spectrum
4694keV(Ex 467keV 1-)
4730keV (58Fe)

4580keV(Ex 581keV 0-)

CountsFig. 3. Measurement of the angular distribution of γ-rays at ANNRI in J-PARC

neutron Energy_Spectrum_Neutron(eV)energy spectrum
p-waveresonance

300

Energy_Spectrum_Neutron

Entries
Mean
RMS

15351
0.98610.5203

1.8

2

250

200

150

gated

100

50

0
0

0.74eV 139La
0.2

0.4

0.6

0.8

gamma energy [keV]

1

1.2

1.4

1.6

Neutron Energy [eV]Fig. 4. Example of measured γ-ray spectrum and p-wave resonance peak.

can be described by using the total angular momentum and the spin of incident neutron, the spin of
the initial and final states of the nuclei, and the partial neutron width for the incident neutrons to
the p-wave resonance [10]. This means that we can extract the partial neutron width to calculate κvalue by measuring the angular correlation term a1 only with the gated γ-rays corresponding to thewell-known spin-states and to the p-wave resonance.
In ANNRI at J-PARC, the fifteen germanium detectors were arranged surrounding the nuclear
target to measure the angular distribution of γ-rays (Fig. 3). According to the energy resolution of
the detectors and the intense pulsed neutrons, we can analyze the data for each γ-ray individually.
We have already taken the physics data for some targets, especially 139 La in 2016. Figure 4 showsthe example of measured γ-ray energy spectrum (Left) and the neutron energy spectrum gated for the
single peak of γ-ray (Right). The p-wave resonance was observed clearly. Detailed analysis is now inprogress.

4.

Neutron polarization

To measure the T-odd correlation term by using the configuration we discussed, the incident neutrons must be polarized. 3 He spin filter is a well-established technique to provide polarized neutrons
with low energy [11]. This uses the strongly spin-dependent absorption cross section of 3 He. Today
K-Rb ‘Hybrid SEOP’ (Spin Exchange Optical Pumping) method can achieve the polarization of 3 He
better than 0.8. In order to apply the 3 He spin filter to the experiments with the target nuclei whichhave p-wave resonance at the order of 1 eV, for example, 139 La and 131 Xe, the large scale of 3 He
gas cell is required. 3 He gas with the eﬀective thickness of the order of 100 atm·cm is needed for a
few eV resonance. Large cross section is also required to use the number of neutrons. Now we have4■■■Proceedings of the International Conference on Neutron Optics (NOP2017)
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unpolarized neutrons

3He cellLaser for pumping

3He cellpolarized neutrons

Laser for pumping

Fig. 5. Conceptual design of in-situ 3 He spin filter for neutrons with the energy of the order of 1 eV.

started research and development for the large scale of 3 He spin filter (Fig. 5).
We are also trying to apply the 3 He spin filter to the (n,γ) measurement at J-PARC. According to
the s-p mixing theory, the angular distribution of γ-rays from the reaction with ‘polarized’ neutronsand ‘unpolarized’ target can be expanded by using many correlation terms. By measuring each term,a complete phenomenological picture for the capture of polarized neutrons in nuclei can be developed, step by step. This is quite important to verify the theoretical basis of not only the T-violation
search but also nuclear physics. We have already demonstrated (n,γ) measurement with the polarized
neutrons through a small 3 He spin filter installed at ANNRI in J-PARC. The relaxation time of about
thirty hours was achieved by using of the optimization of magnetic field environment against thesurrounding radiation shields.
Proton spin filter is the other candidate of the polarizer for relatively-high energy neutrons. This isrequired for the T-voilation experiment with the candidate nuclei like 115 In, 107 Ag. This uses strongly
spin-dependent scattering cross section of proton. In order to polarize the proton, triplet-DNP (Dynamic Nuclear Polarization) method is now been developing [14]. Electrons which are photo-excitedto triplet state in pentacene molecule are polarized to 70 % , independently of the temperature andmagnetic field. Protons can be polarized at relatively high temperature of 25-300 K and in a weakmagnetic field of 0.3-0.7 T.

5.

Polarized nuclear target

We are also discussing the polarization of the target nuclei by using spin exchange optical pumping (SEOP) and/or dynamical nuclear polarization (DNP).
The SEOP is a well-established technique to polarize xenon nuclei, especially 129 Xe. It will
be utilized as co-magnetometer for EDM measurement experiment with UCNs. It can be applied
to 131 Xe for T-violation experiment. In order to increase the interaction, eﬀectively thick target isrequired. Solid xenon target, which is made by freezing polarized xenon gas, is now discussed.
On the other hand, the DNP can be utilized for lanthanum nuclei. The lanthanum nuclei in
LaAl2 O3 crystal were successfully polarized by using DNP method [15]. We are now discussing
the application of the DNP for lanthanum target for T-violation experiment.
Because the B term in Eq. 2, which makes pseudomagnetic rotation of neutron spin through thepolarized target, is much larger than C and D, the compensation and/or correction of the spin-rotationmust be developed. The magnetic field environment not only to keep the nuclear polarization but also
to cancel the pseudomagnetic rotation of neutrons is required. In addition, because the pseudomagnetic rotation depends on neutron velocity, information of the phase due to time-of-flight of pulsed
neutrons may be used for correction the eﬀect from B term.
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Neutron detector

We must measure the tiny asymmetry of transmitted neutrons in T-violation experiment. Finallyneutron detectors which can count the neutron with the rate of the order from 108 to 109 cps/cm2 aredesirable. One of possible candidates is segmented liquid scintillator detector. Now we have started
research and development of the new types of neutron detectors. At first lithium glass scintillator
detector with the counting rate of about 1 Mcps was demonstrated at beamline BL10 NOBORU in
J-PARC. We also tested boron-loaded liquid scintillators. It had the decay-time of 3.7 ns, which was
faster than the glass scintillator. We are continuing on the development of the detector with muchhigher counting rate including the electric circuits step by step.

7.

Feasibility at J-PARC

To assess feasibility for a T-violation search in neutron resonance reactions at J-PARC, we assume
a 3 He spin filter used to polarize the incident neutrons and the target nuclei is solid 131 Xe to permit theuse of SEOP to produce nuclear polarization [16]. When the beamline at MLF in J-PARC is assumed,
the measurement for the asymmetry of the order of 10−5 can be achieved by using of five days (Fig. 6).
We are also discussing many options to apply the other promising techniques, for example, lanthanum
target with DNP, proton spin filter for neutron polarizer, and so on.
3He spin ﬁlter

J-PARC Moderator

2.2x10-5 sr

100 atm cm
Polarization 0.7
Transmission 0.4

Solid natXe
4cm x 4cm x 20cm
Polarization 0.25
epi-thermal
neutron detector

7 x 106 n s-1 at sample position

15 m

Fig. 6. Conceptual setup for T-violation measurement at J-PARC.

8.

Summary

The new type of T-violation search experiment is discussed. The enhancement of the violationof time-reversal symmetry is predicted in the neutron capture reaction for some nuclei. The measurements of (n,γ) reactions of candidate nuclei were performed at J-PARC in order to estimate theenhancement factor of the T-violation. Polarization technique for both of incident neutron beam andtarget nuclei is been developing. The development of high-speed detectors is continuing for the experiments using extremely-intense neutrons at J-PARC.
This kind of devices enable us to perform various physics experiments with far superior statistics.
New era of neutron physics is now open with the combination of high-intensity neutron source andhigh-performance neutron optics.

Acknowledgments
The neutron scattering experiment was approved by the Neutron Scattering Program Advisory
Committee of IMSS, KEK (Proposal No. 2014S03, 2015S12). The neutron experiment at the Materials and Life Science Experimental Facility of the J-PARC was performed under a user pro6■■■

Proceedings of the International Conference on Neutron Optics (NOP2017)
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JPS Conf. Proc. 22, 011034 (2018)gram (Proposal No. 2016B0200, 2016B0202, 2017A0158, 2017A0170, 2017A0203). This work was
supported by MEXT KAKENHI Grant Number JP19GS0210 and JSPS KAKENHI Grant Number
JP17H02889.
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