140
C. H. Pyeon
5.4 Spallation Neutrons
5.4.1 Neutron Spectrum Analyses
5.4.1.1 Experimental Settings
High-energy protons were generated by the FFAG accelerator under the following
parameters: 100 MeV energy, 30 pA intensity, 30 Hz repetition rate and 200 ns beam
width. On the downstream of the FFAG beam line, the W was set at the location (15,
A
; Fig. A2.1) of the original target (80 mm diameter and 10 mm thick); the thickness
was determined on the basis of previous experimental and numerical analyses [13]
for the injection of high-energy proton beams onto the W target. For the proton beam
configuration modeled by numerical simulations, the size of the proton beam spot
was requisite experimentally and precisely, when 100 MeV protons were injected
onto the tungsten target where the spallation neutrons are generated. The Gafchromic
film [14], which is very sensitive to the charged particles, was then used to evaluate
the size of the proton beam spot injected onto the W target, since a graphic image on
the film is acquired quickly after the irradiation of charged particles for a short time.
The reaction rates for threshold energy of high-energy neutrons and the continuous
energy distribution of the spallation neutrons at the target were acquired by the foil
activation method and the organic liquid scintillator, respectively [15]. The highenergy neutrons (spallation neutrons) of threshold reactions
209 Bi(n, xn)
210−x Bi (n =
3, to 12) over 15 MeV have been generated by the injection of high-energy protons
over 100 MeV. Here, to obtain the reaction rates by high-energy neutrons at the
target,
209 Bi was selected as an activation foil (Table 5.9) to cover threshold energies
over 15 MeV, and
115 In was selected as a normalization factor for monitoring the
spallation neutrons at the target to cover threshold reactions
115 In(n, n
)
115m In over
0.3 MeV. Foil dimensions at the target were as follows:
209 Bi, 50 mm in diameter
and 3 mm thick,
115 In, 10 × 10 × 1 mm, and two foils were set around the target
region as shown in Fig. 5.5. Additionally, nine other
115 In foils (10 × 10 × 3 mm)
were placed in a circle (100 mm radius) around the target at 30° intervals on an
acryl plate, to investigate the angular distribution of spallation neutrons as shown in
Fig. 5.6. The irradiation time of
209 Bi and
115 In foils was four hours for measuring
the neutron yield of spallation neutrons, and their reaction rates were measured by
the high-purity germanium detector (ORTEC, GEM60P). Besides the previous study
[12, 13, 16], the detection efficiency of the germanium detector was determined by
the fitting line obtained from the energy calibration with the use of several γ-ray
standard sources.
The continuous energy distribution of spallation neutrons was determined by
the organic liquid scintillator (Nuclear Enterprises Ltd., NE213 Scintillator; 5
in
diameter and 5
long) set directly facing the W target without any reactor components
as shown in Fig. 5.7. The measurement circulation of the organic liquid scintillator
was as indicated in Fig. 5.8. The main advantage of the measurement system is that
the two signals (rise time and light output of γ-ray and neutron) acquired coincidently
C. H. Pyeon
5.4 Spallation Neutrons
5.4.1 Neutron Spectrum Analyses
5.4.1.1 Experimental Settings
High-energy protons were generated by the FFAG accelerator under the following
parameters: 100 MeV energy, 30 pA intensity, 30 Hz repetition rate and 200 ns beam
width. On the downstream of the FFAG beam line, the W was set at the location (15,
A
; Fig. A2.1) of the original target (80 mm diameter and 10 mm thick); the thickness
was determined on the basis of previous experimental and numerical analyses [13]
for the injection of high-energy proton beams onto the W target. For the proton beam
configuration modeled by numerical simulations, the size of the proton beam spot
was requisite experimentally and precisely, when 100 MeV protons were injected
onto the tungsten target where the spallation neutrons are generated. The Gafchromic
film [14], which is very sensitive to the charged particles, was then used to evaluate
the size of the proton beam spot injected onto the W target, since a graphic image on
the film is acquired quickly after the irradiation of charged particles for a short time.
The reaction rates for threshold energy of high-energy neutrons and the continuous
energy distribution of the spallation neutrons at the target were acquired by the foil
activation method and the organic liquid scintillator, respectively [15]. The highenergy neutrons (spallation neutrons) of threshold reactions
209 Bi(n, xn)
210−x Bi (n =
3, to 12) over 15 MeV have been generated by the injection of high-energy protons
over 100 MeV. Here, to obtain the reaction rates by high-energy neutrons at the
target,
209 Bi was selected as an activation foil (Table 5.9) to cover threshold energies
over 15 MeV, and
115 In was selected as a normalization factor for monitoring the
spallation neutrons at the target to cover threshold reactions
115 In(n, n
)
115m In over
0.3 MeV. Foil dimensions at the target were as follows:
209 Bi, 50 mm in diameter
and 3 mm thick,
115 In, 10 × 10 × 1 mm, and two foils were set around the target
region as shown in Fig. 5.5. Additionally, nine other
115 In foils (10 × 10 × 3 mm)
were placed in a circle (100 mm radius) around the target at 30° intervals on an
acryl plate, to investigate the angular distribution of spallation neutrons as shown in
Fig. 5.6. The irradiation time of
209 Bi and
115 In foils was four hours for measuring
the neutron yield of spallation neutrons, and their reaction rates were measured by
the high-purity germanium detector (ORTEC, GEM60P). Besides the previous study
[12, 13, 16], the detection efficiency of the germanium detector was determined by
the fitting line obtained from the energy calibration with the use of several γ-ray
standard sources.
The continuous energy distribution of spallation neutrons was determined by
the organic liquid scintillator (Nuclear Enterprises Ltd., NE213 Scintillator; 5
in
diameter and 5
long) set directly facing the W target without any reactor components
as shown in Fig. 5.7. The measurement circulation of the organic liquid scintillator
was as indicated in Fig. 5.8. The main advantage of the measurement system is that
the two signals (rise time and light output of γ-ray and neutron) acquired coincidently