Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Simulation study of neutron measurement using NEBULA
simulation package for SπRIT project
G. Jhang∗1,∗2 and T. Isobe∗2 for the SπRIT Collaboration
Neutron and proton emission is one of many reaction
observables that can be used to constrain the densitydependent nuclear symmetry energy1) , which is important to describe isospin-asymmetric nuclear matter.
In the SπRIT project, we plan to measure the n/pratio complementary to the t/3 He ratio measurementby making use of the TPC and the NEBULA array. Inorder to investigate the response of the NEBULA array before the actual experiment, we are performing
a simulation with the NEBULA simulation packagev2.0.52) developed by Nakamura group at Tokyo Institute of Technology on the basis of GEANT4.9.2p02
and ROOT. We are using the Particle and Heavy Ion
Transport code System(PHITS)3) v2.60 as an event
generator to produce collision events with 132 Sn projectiles and 124 Sn targets. Detailed information on thephysics processes used can be found in the link of Ref.
4.

mary neutron signals as shown in Fig. 2, in order toobtain precise information on as many neutrons as possible. Such information can reveal whether we can distinguish one theoretical model from the others in theactual experiment.

Table 1. Information on generated events

Fig. 1. Number of neutrons in generated events and in accepted the range of the NEBULA arraybeam energy (AMeV)
200300neutron events
522,665
523,058

105

numNeutAccept_200

Entries
Mean
RMS

4

10522665
38.99
23.06

Entries
Mean
RMS

5226657.184
5.287

Entries
Mean
RMS

52305837.82
20.26

numNeutAccept_300

103Counts

numNeut_300

523058 Entries
6.678 Mean
4.431 RMS

numNeut_200

102300 AMeV
300 AMeV Accepted

10200 AMeV
200 AMeV Accepted

10204060

80100120140160

Number of Neutrons

total events
100,000,000

detectedNToF_300

In the simulation setup, the NEBULA array isplaced 2 m away to the left side of the beam direction so that charged reaction particles having high pz
and beam remnants do not enter the array.
In the distribution of the number of neutrons foreach beam energy shown in Fig. 1, lines with “Accepted” in the legend indicate that most of the neutrons generated by the collision are going outside of
the acceptance range, and less than 40 neutrons enter
the array in our setup.
Figure 2 shows the momentum distribution of neutrons detected by the NEBULA array. The distribution of neutrons with the assumption of 100% detection eﬃciency(blue) is slightly diﬀerent from that obtained by taking the time of ﬂight of each neutron’sﬁrst hit(pink). The diﬀerence originates from the exclusion algorithms that excludes some number of scintillator bars near the ﬁrst detection. This result implies
that if we can distinguish the ﬁrst hit only from thesecondary, tertiary, and so on, we can measure mostneutrons’ momenta with precision. The red line shows
the added backgrounds, which should be eliminated toobtain the proper neutron information.
We are building an algorithm to eliminate the background noise, which is about 10 times larger than pri∗1∗2

Entries 8219436
Mean
660.3
RMS
229.2

104

CountsdetectedN_300

103

Entries 1144867
Mean
747.6
RMS
197.4

102

Entries 1052683
Mean
747.8
RMS
201.8detectedNToF1_300

Neutron(ToF)
Neutron

10Neutron(ToF,1st)
10200

400

600

800

1000

p (MeV/c)

1200

1400

1600

1800

Fig. 2. Momentum distribution of neutrons with beam energy. Blue line is the result with an assumption of 100%detection eﬃciency of the NEBULA array. Pink line isthe result with the time of ﬂight method for the primary
reaction. Red line shows the signal plus noise expected
in a real experiment without any cut parameter.

References
1) M. A. Famiano et al.: Phys. Rev. Lett. 97, 052701,(2006).
2) NEBULA simulation package:
http://be.nucl.ap.titech.ac.jp/˜nebula/index.php
3) T. Sato et al.: J. Nucl. Sci. Technol. 50:9, pp. 913-923,(2013).
4) PHITS(Particle and Heavy Ion Transport code System):
http://phits.jaea.go.jp/OvMapOfModels.html

Department of Physics, Korea University
RIKEN Nishina Center- 33 -