JAEA-Conf 2011-002 - 日本原子力研究開発機構

2 Experiment
Figure 1 shows schematic view of experimental arrangement. The 120 GeV proton from the main
injector are delivered to an area of 8×20 m2 . The number of incident protons were counted by three
thin NE102A plastic scintillators (Beam monitors - BM 1, 2 and 3) which were located at upstream the
target. The beam profile and position were monitored by a multi-wire proportional chamber. The sigma
of beam radius was about φ 5 mm at the target position. The copper block, the dimension of which was
60 cm long and 5×5 cm2cross section, was employed as the neutron production target. The neutron
detector was located at 5.5 m from the target and 30◦ with respect to the beam axis.
An NE213 liquid scintillator with 12.7 cm diameter, 12.7 cm long was employed as the neutron detector.
The scintillator is suitable for neutron time-of-flight measurement due to pulse shape discrimination
capability and fast decay time of its scintillation. To eliminate charged particles, a 2 mm thick NE102A
plastic scintillator as the veto detector was placed in front of the NE213 scintillator.
Figure 2 shows a block diagram of data acquisition electronics. The electronics consisted of standard
NIM and CAMAC modules. Trigger signal for data accumulation was generated from coincidence among
the NE213 scintillator and the BMs. The data were recorded event by event. The time difference
between the BMs and the NE213 scintillator was recorded with a TDC for determination of neutron
time-of-flight. Charge-sensitive ADCs were employed in order to record total component charge of pulses
from the NE213 scintillator, the BM1, and the veto detector. Slow component charge of pulse from the
NE213 scintillator was also recorded for pulse shape discrimination. In addition, the number of protons
during 200 ns before neutron signal was recorded (scaler in flight in Fig. 2) to ensure that time-of-flight
was determined properly, as described in the next section.
Target-in measurement was carried out with beam intensity of 2×105 protons /min, during 3.5 hours.
The counts of the BMs and the NE213 scintillator were 5×107 and 8×106 , respectively. The dead time
of data acquisition was about 62 %. Target-out measurement was also performed to check contribution
of background neutron from the dump since the dump was closer than one from the target, as shown in
Fig. 1.
3 Analysis
JAEA-Conf 2011-002
The energy spectra of neutrons, i.e. double differential thick target neutron yield (TTNY), d2Y (E)/dEdΩ,
was deduced by the following equation,
d2Y (E)
dEdΩ =
C(E)
φ · ε(E) · Ω · ΔE
where E is the neutron energy, C(E) the neutron counts in an energy bin, φ the number of protons,
ε(E) the neutron detection efficiency, Ω the solid angle subtended by neutron detector, and ΔE the
width of energy bin. Neutron events were identified by charged particle discrimination based on the
veto detector signal, and gamma-ray discrimination based on the pulse shape of the NE213 scintillator.
Neutron energy was determined by time-of-flight technique. We eliminated neutron event which could
not uniquely determine its time-of-flight since two or more protons were counted by BM1 during neutron
flight. The elimination was performed using data of ”scalar in flight” and ADC BM1 shown in Fig.2.
The ”scalar in flight” was effective when the counts belong in different beam bunch. The ADC BM1 was
effective when the counts belong in a same bunch. The count loss from the eliminations was corrected
through the correction factor for the number of protons, as described in the next paragraph.
The number of protons was determined using the following equation,
(1)