Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Detection eﬃciency of segmented neutron detector at 200 MeV
Y. Kubota,∗1,∗2 M. Sasano,∗2 T. Uesaka,∗2 M. Dozono,∗2 M. Itoh,∗3 M. Kobayashi,∗1 C.S. Lee,∗1,∗2 K. Miki,∗4S. Ota,∗1 T. Shima,∗4 A. Tamii,∗4 T. Wakasa,∗5 J. Yasuda,∗5 and J. Zenihiro∗2
Nucleon-knockout (p, pN ) reactions at intermediate
energies (200–300 MeV) provide a powerful probe of
the nature of single particle states (SPSs) in nuclei1) .
The goal of our study is to determine the neutron separation energy with a high resolution of about 500 keV
at full-width at half-maximum (FWHM) via the (p, pn)reaction. This is technically challenging because a highposition resolution is necessary for neutron detection
which cannot be achieved with a conventional design.
At RIKEN RIBF, we are developing a segmented
neutron detector consisting of 64 scintillating ﬁbers.
Each ﬁber has dimensions of 3.75 mm (W) × 3.75 mm(D) × 1 m (H) and has two multi-anode photomultiplier tubes (Hamamatsu H7546B) at both ends. Using
this setup, we conﬁrmed that the position resolutionof 3.75 mm in the total width, corresponding to theﬁber size, was actually realized by performing a neutron irradiation experiment at the Cyclotron Radioisotope Center (CYRIC), Tohoku University in November
2012. Furthermore, we determined the neutron detection eﬃciencies at 50 and 68 MeV to be 1.6 ± 0.4% and
2.0 ± 0.5%, respectively2) .
For higher neutron energies, we performed anotherexperiment using neutron beams at 200 MeV at theResearch Center for Nuclear Physics (RCNP), Osaka
University in November 2013. Monoenergetic neutron beams at 199 and 181 MeV were produced from7Li(p, n) and 12 C(p, n) reactions using a proton beamat 200 MeV. The Li and C targets, each with natural isotopic abundance, had thicknesses of 0.94 and2.1 mm, respectively. Neutrons ﬂew in the neutron
time-of-ﬂight (NTOF) tunnel and, were then detectedby the segmented neutron detector placed at a distanceof 50 m from the target position. The intensity of theproton beam was about 100 nA and the rate of neutrons bombarding on the detector was typically 104
particles per second.
A preliminary analysis shows that the detection eﬃciencies for 181- and 199-MeV neutrons were 2.6±0.4%
and 2.5 ± 0.4%, respectively. The threshold for neutron detection, 4.2 MeV electron equivalent (MeVee )was applied to the light output information obtained
from the charge amplitude of the dynode signal.
Figure 1 shows the distribution of the detection position, which is deﬁned as the most upstream ﬁber hit.
When there are several hit ﬁbers at the same depth,
the ﬁber with largest light output is selected. Here,the threshold of each channel was set to 0.5 MeVee .
The distribution was found not to be uniform near thesurface. Along the beam direction, most events wereconcentrated on the ﬁrst plane, and the ﬁbers on theleft and right sides had a larger number of events than
the inner ﬁbers. This enhancement in the number ofevents near the surface, which was not observed clearlyin the previous experiment because of the lack of theuniformity on the threshold of each channel, is still notunderstood and is being investigated for details. Theinner array of 6 × 6 ﬁbers had a uniform distribution
within 10%, possibly reﬂecting the uniformity of theneutron ﬂux.
The most downstream ﬁbers had a smaller number of events compared to upstream ﬁbers. For theseﬁbers, the light output deposit by recoil protons canbe small because the protons go out of the detector
volume within a short distance.
In summary, we are developing the segmented neutron detector with a high position resolution for thestudy of SPSs via the (p, pn) reaction. The detection
eﬃciencies of neutrons at 181 and 199 MeV were determined to be 2.6 ± 0.4% and 2.5 ± 0.4%, respectively.
The distribution of the detection position shows a large
enhancement in the number of events at the surface
which is being analyzed.
We acknowledge the staﬀ at the RCNP for their efforts and support.

∗11) G. Jacob and TH. A. J. Maris, Rev. Mod. Phys. 45, 6(1973).
2) M. Sasano et al., RIKEN Accel. Prog. Rep. 46, 161(2013).

∗2∗3∗4∗5Center for Nuclear Study, University of Tokyo
RIKEN Nishina Center
Cyclotron and Radioisotope Center, Tohoku UniversityResearch Center for Nuclear Physics, Osaka University
Department of Physics, Kyushu University- 220 -

1000
800600400
2000876543210012345678Fig. 1. Distribution of detecting positions indicated by theheights of the 8 × 8 blocks. The neutron beam travelledfrom top left to right bottom.

References