Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 51 (2018)

Neutron spin filter with dynamic nuclear polarization
using photo-excited triplet electron for T-violation searchin a compound nucleus
S. Takada,∗1,∗2 K. Tateishi,∗1 Y. Wakabayashi,∗3 Y. Ikeda,∗3 Y. Otake,∗3 T. Yoshioka,∗2 and T. Uesaka∗1
In a compound nucleus (e.g. 139 La+n, 81 Br+n), parity violation is enhanced due to the interference between s-wave and p-wave amplitudes. It is theoretically predicted that time reversal (T-) violation can bealso enhanced by the same mechanism.1) We are planning an experiment for sensitive T-violation search using a polarized neutron beam. The p-wave resonancesof the candidate nuclei are observed in the energy rangeof 0.1–100 eV. For this purpose, we need a neutron spinﬁlter that is suitable for the energy region.
A neutron spin ﬁlter can polarize the neutron beamby passing it through a spin-polarized media, for example 1 H or 3 He because their cross-sections have a large
helicity dependence. In particular, 1 H has ﬂat crosssection of 20 barn in the wide energy range mentionedabove. Therefore, we selected neutron spin ﬁlter using
1H media.
A solid-state of 1 H doped media is often polarized by
Dynamic Nuclear Polarization (DNP). DNP is a technique of transferring spin polarization from electrons
to nuclei with microwave irradiation. We applied DNP
with photo-exited triplet electron spin (Triplet-DNP)2)
because it can be used at a relatively high temperature (> 77 K) and in a low magnetic ﬁeld (< 1 T)
compared to the conventional DNP method. The neutron spin ﬁlter with Triplet-DNP was ﬁrst developed at
the Paul Scherrer Institut (PSI) in Switzerland. They
achieved 1 H polarization of 70% using a naphthalene
crystal with the size of 5 × 5 × 5 mm3 in 0.36 T and at
25 K,3,4) and evaluated its performance using a polarized neutron beam in the meV region.
The neutron spin ﬁlter is characterized with a neutron transmittance T , a neutron polarization Pn , anda ﬁgure of merit (FOM) which is deﬁned as follows:
T = exp (−(nσ0 + n′ σ0′ )d) cosh (P1 H n∆σd),Pn = tanh (P1 H n∆σd),

(1)
(2)

FOM = Pn2 T,(3)where σ0 and ∆σ are the spin-independent and spindependent cross section of 1 H, respectively. σ0′ is thespin-independent cross section of other nuclei. n andn′ are the density of 1 H and other nuclei. d and P1 H
are the thickness and polarization of the ﬁlter. Here,the naphthalene crystal (C10 H8 ) is assumed as a ﬁlter
media. Based on the expression, a 15 mm thicknessof the neutron spin ﬁlter can conform to the wide en∗1
∗2
∗3

RIKEN Nishina CenterDepartment of Physics, Kyushu University
RIKEN Center for Advanced Photonics- 183 -ergy range. In addition, we need large acceptance ofthe spin ﬁlter to obtain high statistics. Therefore, wedevelop ϕ15 × 15 mm3 of the spin ﬁlter.
A setup of the neutron spin ﬁlter with Triplet-DNP
is shown in Fig. 1. Triplet-DNP is carried out at 0.3 T
and 100 K. A single crystal of naphthalene doped with
deuterated pentacene with a size of ϕ15 × 15 mm3 isused as a ﬁlter. To polarize such a huge crystal, ahigh-power laser was implemented.

Fig. 1. Setup of the neutron polarization system using
Triplet-DNP.

We are planning to check the performance of theneutron spin ﬁlter at RIKEN Accelerator-driven compact Neutron source (RANS) next year. We will measure the position dependence of polarization in the spinﬁlter. In November and December 2017, we carried theexisting polarization system to RANS and measured
the position resolution to check whether performancemeasurement is possible. Since the position resolution
was about 2 mm, it is possible to suﬃciently measure
the position dependence of the polarization and thetransmittance of the spin ﬁlter.
References
1) V. P. Gudkov, Phys. Rep. 212, 77 (1992).
2) A. Henstra, P. Dirksen, W. Th. Wenckebach, Phys.
Lett. A 134, 134 (1988).
3) M. Haag, B. van den Brandt, T. R. Eichhorn, P. Hautle,
W. Th. Wenckebach, Nucl. Instrum. Methods Phys.
Res. A 678, 91 (2012).
4) T. R. Eichhorn, N. Niketic, B. van den Brandt,
U. Filges, T. Panzner, E. Rantsiou, W. Th. Wenckebach, P. Hautle, Nucl. Instrum. Methods Phys. Res. A754, 10 (2014).