Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

PANDORA project - low-energy neutron detector with real-timeneutron-gamma discrimination
L. Stuhl,∗1,2 M. Sasano,∗2 K. Yako,∗1 J. Yasuda,∗3 H. Baba,∗2 S. Ota,∗1 and T. Uesaka∗2
Recent nuclear physics studies are increasingly focusing on the region far from the valley of stability, requiring an increase of the intensity of available exotic
isotopes. This advancement opens up the possibility
to investigate phenomena with low cross sections, suchas charge-exchange reactions.
The (p, n) charge-exchange reaction at intermediate
energies is a powerful tool to study the spin-isospin
excitations of nuclei. The technique of inverse kinematics1,2) enables us to study the (p, n) reactions on
exotic nuclei with high luminosity. In this technique,neutron detectors are used for measuring the time ofﬂight (ToF) of low-energy (0.1–10 MeV) recoil neutrons. This method was successfully applied to study
the Gamow–Teller strength distribution from 56 Ni2,3)
and 132 Sn4) isotopes. In the latter experiment, the secondary beam intensity was 104 particles/s, the (p, n)reaction event rate was a few Hz, and the random
gamma background produced an additional trigger
rate of about 1 kHz.4) For future experiments withhigher beam intensities, this background event rate
will lead to higher trigger rates and a large dead timeof data acquisition. To solve this problem, clear tagging of recoil neutrons in real time i necessary before
recording the event data. Our work focuses on the development of the particle analyzer neutron detector ofreal-time acquisition (PANDORA). Our goals were• in online data processing: to reduce the trigger rate by one order of magnitude by removing the random gamma background, leading toa smaller data size and the capability to handle
high-intensity beams• in oﬄine analysis: to fully remove the gamma ray
background using the pulse shape discrimination(PSD) parameter.
PANDORA consists of an EJ-299-34 plastic scintillator bar5) with dimensions of 2.5×5×30 cm3 coupled
to a Hamamatsu H7195 photomultiplier tube on eachend. Detector signals from the anode output of bothPMTs are read out with a digital data acquisition system (CAEN V1730 digitizer and digital pulse processing for pulse shape discrimination ﬁrmware). In order
to measure the neutron kinetic energies using the ToF
technique with an energy resolution of ∼ 5% ΔE/E forneutron energies below 5 MeV, a ToF resolution better than 0.6 ns (FWHM) is required, while the angularresolution must be less then 1.5◦ .
∗1
∗2
∗3

Center for Nuclear Study, University of Tokyo
RIKEN Nishina Center
Department of Physics, Kyushu University- 211 -The PSD parameter is deﬁned as follows:
PSD =QLong −QShort,QLong(1)where QLong is the charge integrated in the long gate.
Typically, it contains all charges in the signal. QShortis the charge integrated in the short gate. Deﬁning
PSDmean value as the arithmetic mean of PSD valuesof two single-end readouts, PANDORA exhibits PSD
capability comparable to those presented in literature.

Fig. 1. PSDmean vs. ToF spectrum showing the separationof neutron- and gamma-like events. The sharp peak
below 4 ns corresponds to prompt gamma rays, while
the distribution in the higher ToF and PSDmean region corresponds to neutron-like events. A large random gamma background can also be observed.

The selection eﬃciency of neutrons or gammas canbe deﬁned as the ratio of the number of selected particles to the total number of detected particles. The
ToF distributions acquired using a 252 Cf ﬁssion source
proved that at a given PSD threshold (PSDmean =0.15) and 10-keVee detection threshold, the selection
eﬃciency of PANDORA is 91±1% for all detected neutrons. It decreases to about 87±2% in the neutron
kinetic energy region below 1 MeV. About 91±1% of
the detected gamma-rays can be excluded resulting inbackground reduction by one order of magnitude.6)
References
1) G. Perdikakis et al. IEEE Trans. Nucl. Sci. 56, 3 (2009).
2) M. Sasano et al., Phys. Rev. Lett. 107, 202501 (2011).
3) M. Sasano et al., Phys. Rev. C 86, 034324 (2012).
4) J. Yasuda, Study of Gamow-Teller transitions from132
Sn via the (p,n) reaction in inverse kinematics, PhD
thesis, Dept. of Physics, Kyushu University (2017).
5) N. Zaitseva et al., Nucl. Instr. Meth. A 668, 88 (2012).
6) L. Stuhl et al., Nucl. Instr. Meth. A 866, 164 (2017).