2. Nuclear Physics and Nuclear Data

概要

CO2-1 β-decay spectroscopy of rare fission products with a 4π clover detector using an Isotope
Separator On-Line KUR-ISOL

Graduate School of Engineering, Nagoya University
1
School of Engineering, Nagoya University
2
Radioisotope Research Center, Nagoya University
3
Institute for Integrated Radiation and Nuclear Science,
Kyoto University
INTRODUCTION: The decay data of the fission
products are important for evaluating decay heat and
determining the structure of neutron-rich nuclei. Many
neutron-rich nuclei with mass numbers near 150 do not
have detailed decay schemes due to their short half-lives
and low fission yields. The nuclide 157Nd was proposed to
have a half-life of 1.15 s by Wu et al. with β-particle
measurements [1] and a level scheme of the daughter
nuclide 157Pm was reported by Bhattacharyya et al. by the
prompt γ-ray measurements of spontaneous fission
fragment of 252Cf [2], but, no γ rays associated with the
β-decay of 157Nd were reported. To identify the γ rays
associated with the β-decay of 157Nd, β-γ coincidence
measurements were performed using a high-efficiency
clover detector coupled with β-ray detectors with On-line
Isotope Separator KUR-ISOL.
EXPERIMENTS: 72 mg of 93% enriched 235UF4 target
was inserted at the through-hole facility in Kyoto
University Reactor. The nuclei of interest were produced
by thermal neutron-induced fission of 235U. The nuclei
were transported by He-N2 gas jets and ionized in a
thermal ionization ion source. The mass-separated
radioactive beams were collected on a thin Mylar tape
and periodically transported to the center of the detector
by a computer-controlled tape transport system, and were
measured with detectors. The clover detector has four
large Ge crystals with a diameter of 80 mm and a length
of 90 mm arranged in the shape of a four−leaf clover
around a through hole with a diameter of 15 mm. Two
identical β-ray detectors were made of plastic scintillators
105 mm long, 12.6 mm wide, and 1 mm thick, contacted
with a semi-cylindrical light guide with a radius of 6.5
mm. A 3×3 mm2 MPPC (Multi-Pixel Photon Counter)
module C13367 made by Hamamatsu Photonics was
mounted on the end of the light guide. The β-detectors
were inserted in a through-hole of the clover detector.
The whole detector was shielded with 10 cm thick lead
bricks and 10 cm thick boron-doped polyethylene blocks
outside them to reduce background neutrons and γ-rays.
Data were recorded on APV8008 and APV8016 DSP data
acquisition systems made by Techno AP Corporation with
list mode including time information. The nuclide 157Nd
was measured for 39 hours with the both periods of
collection and measurement were set 3.0 s. After the
experiments, time dependent spectra were extracted to
analyze the decay properties of γ rays and KX-rays.
RESULTS and DISCUSSION: To identify the γ rays
associated with the β-decay of 157Nd, the decay properties

of the γ rays and Pm KX-rays, and also their coincidence
relations were analyzed. The coincidence time was set to
700 ns. Fig.1 shows the γ-ray singles and β-γ coincidense
spectra for the A= 157 radioactivities. It was confirmed
that nuclides with adjacent mass numbers 156 and 158
did not mix in the mass-separated A= 157 beams. In the
β-γ coincidence spectrum, the background radiation such
as the γ-ray associated with the decay of 41Ar produced
with 40Ar(n, γ) reaction or capture γ-rays of Ge crystals
by the neutron in the reactor room were reduced
effectively. The peaks were analyzed using the peak
fitting program developed by Yamada et al. [3] Most γ
rays and KX-rays were originated from the β-decay of
daughter nuclide 157Pm (T1/2=10.56 s) [1] and ground
daughter nuclide 157Sm (T1/2=8.03 m) [4]. However, from
the analysis of the time dependent β-γ coincidence
spectra each 1 s as shown in Fig.1(b), and also the
add-back of four crystals, and singles spectra, the 66 keV
γ-ray and the KαX-ray region correspond to Pm were
observed to disintegrate with a half-life shorter than
157
Pm. In the prompt γ-ray measurements with 252Cf [2],
the 66 keV γ-ray were proposed as the transition from the
first excited state to the ground state in 157Pm. The γ ray
is possible to be associated with the β-decay of 157Nd. In
addition, the KαX-ray region of Pm also disintegrate
with almost the same half-life. The precise analyses are in
progress.
REFERENCES:
[1] J. Wu et al., Phys. Rev. Lett., 118 (2017) 072701.
[2] S. Bhattacharyya et al., Phys. Rev., C98 (2018) 04316.
[3] S. Yamada, KURRI-TR-430.
[4] N. Nica, NDS. 132 (2016)1.

10

6

● 157 Nd→ 157 Pm
○ 157 Pm→ 157 Sm

0~1 s
1~2 s
2~3 s

singles
β -γ

(b)
●

(a)

○
○

10

○

5

○
●

Counts/energy(0.5keV)

S. Sakakibara, Y. Miyazawa, T. Kuga1, M. Shibata2 and A.
Taniguchi3

10

○

○

○
○

○

○

○

4

10 0

50

5

○

○

○

100

150

250

○

○

10

200

○

○

○

○

4

3

10
250

300

350

400

Gamma-ray energy [keV]

450

500

Fig. 1. Singles and β-γ coincidence spectra for the massseparated beam of A= 157 (a). The γ rays marked as ○ are
associated with the decay of 157Pm and that of ● is possible
to be associated with the decay of 157Nd. The inset (b) shows
the time dependent β-γ coincidence spectra each 1 s.

R4087
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Development and test of a current-mode 3He gas neutron detectors for an intense neutron
beam

T. Matsumoto, S. Manabe, A. Masuda, H. Harano, J.
Hori1, K. Terada1
National Metrology Institute of Japan, National Institute
of Advanced Industrial Science and Technology
1
Institute for Integrated Radiation and Nuclear Science,
Kyoto University
INTRODUCTION: It is necessary to measure the neutron flux for a large dynamic range to connect between a
BNCT field in a hospital and a neutron calibration field in
the National Institute of Advanced Industrial Science and
Technology (AIST). The difference of neutron fluxes between the BNCT field and the calibration field is more than
5 orders of magnitude. We have developed a new 3He gas
detector with a thimble ion chamber with a 10-mm diameter and a 10-mm length. Unlike ordinary ionization chambers that are used for gamma-ray measurements, the neutron detector has a structure that allows for gas replacement and gas sealing. The gas detector is expected to be
high radiation resistance in comparison with a photo-multiplier tube in the scintillation detector [1]. In the present
study, the sensitivity of the neutron detector to thermal and
epi-thermal neutrons will be experimentally confirmed
from time-of flight (TOF) measurements as a first step.
EXPERIMENTS: A collimated neutron beam with 30mm diameter was obtained by the photo neutron reaction
using a tantalum target with a water moderator at the
KURNS Linac [2]. A BF3 proportional counter was used
as a neutron monitor. Figure 1 shows a typical experimental setup. The chamber of neutron detector was filled
with 3He gas at 1 atm and N2 gas at 0.2 atm. A relative gas
monitor was also installed to check for gas pressure fluctuations due to gas leakage. The neutron detector was
measured by means of the TOF method to confirm that
thermal and epi-thermal neutrons were detected. The
measurements were performed for both pulse and current
modes. In the pulse mode, signals from the 3He proportional counter were obtained using a pre-amplifier (ORTEC 142PC) and main amplifier (ORTEC 570). High voltage (+500 V) was applied to the center electrode. In the
current mode, the signals were obtained using a current integrator (ORTEC 439). Hight voltage (+500) was applied
to the outer electrode to suppress the dark current. Finally,
TOF data were extracted using a multi-stop time to digital
converter and a multi-channel analyzer (Fast Com Tec
MPA3).
RESULTS: Figure 2 shows TOF results obtained from
the measurements in the pulse mode for conditions with
and without neutron beam. From figure 2, the structure of
thermal bump is clearly observed. Thermal and epi-thermal neutrons were successfully detected. Therefore, it was
confirmed that it was not a mistake as the design of the
neutron detector structure. On the other hand, it is also
found that the background including gamma rays and electric noise observed without the neutron beam is very large.
Furthermore, in the current mode, only a very small current on the order of pA was obtained. Because of the small
size of the chamber of the neutron detector being

developed, with the gas composition used in the present
experiments, many of the protons and tritons produced by
the 3He(n,p)T reaction were not completely stopped inside
the chamber. Therefore, the pulse height output in the
pulse mode was very small and the background to signal
ratio was poor. For the same reason, the output current was
very small in the current mode. We will obtain heavier
mass number gases such as Kr and Ar, next year. By filling
the chamber with heavier mass number gases instead of N2
gas, the outputs will be larger than those obtained in the
present experiments because many of the protons and tritons produced by the 3He(n,p)T reaction will stop inside
the chamber. We plan to use the neutron detector in the
current mode. In that case, it is impossible to discriminate
gamma rays from output signals. Therefore, a chamber
filled with 4He instead of 3He will prepared to subtract the
gamma components.

Fig. 1. Experimental setup for the neutron detector at
at approximately 12 m away from the target.
100
Count rate/channel (s-1)

CO2-2

10

with neutron beam
without neutron beam

-1

10-2
10-3
10-4
0

2s /channel
2000

4000 6000
TOF channel

8000

10000

Fig. 2. TOF spectra obtained from the measurements in
the pulse mode for conditions with and without neutron
beam.

REFERENCES:
[1] T. Matsumoto et al., Radiat. Prot. Dosim., 188 (1)
(2020) 142.
[2] K. Kobayashi et al., Annu. Rep. Res. Reactorinst.
Kyoto Univ., 22 (1989) 142.
This work was supported by JSPS KAKENHI (21H03755
and 19K12638).

R4125
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CO2-3

Development of neutron resonance analysis technique using a neutron time-of-flight
method

J. Lee1, K. Hironaka1, M. ...