5 Neutron Spectrum
127
5.1.2 Characteristics of the Target
5.1.2.1 Dimension of Solid Targets
The combined use of the heavy (tungsten, lead and bismuth: W, Pb and Bi) [3] and the
light nuclides (beryllium: Be; lithium) was considered useful in accomplishing the
study objectives related to the neutron spectrum and the neutron yield. The neutron
yield was obtained from the neutrons produced at the surface of the target. The two
targets laid with the combined use of heavy and light nuclides are termed “two-layer”
targets in this study, and their characteristics were numerically investigated by the
MCNPX [4] and SRIM codes [5] with the JENDL/HE-2007 [6] library. The aim of
these numerical analyses was to investigate the neutron spectrum in the high-energy
region and to determine the thickness of solid targets so that incident protons could
be fully stopped inside the target.
From the numerical results by MCNPX, the neutron spectrum was observed in
the high-energy region of the solid target used: W, W–Be or Pb–Bi, as shown in
Figs. A2.5 and A2.6, when 100 MeV protons were injected onto them. The neutron
spectrum was somewhat similar, regardless of the kind of solid target used as shown
in Fig. A2.5, although at each target it was comparatively different ranging from 1 to
10 MeV. Of particular interest here is the influence of the difference in the neutron
spectrum, caused by the kind of target used, on neutron multiplication in the core. In
the numerical simulations of neutron generation of the target, the neutron spectrum
of W-Be target (two-layer target) was compared remarkably with other single targets,
ranging from 85 to 100 MeV, as shown in Fig. A2.6, and the difference between the
two spectra was attributed to the scattering reactions of Be to high-energy protons.
The aim in using the two-layer target was to acquire the neutron spectrum in the
high-energy region and the neutron yield of high-energy neutrons. Consequently, the
proton beams actually penetrated the Be target, and conversely stopped inside the W
target. The thickness of the solid targets was correctly determined by the numerical
results of the range of high-energy protons with the use of the SRIM code. Lastly, the
dimensions of the solid targets were determined as shown in Table A2.6, and since
the size of the proton beam spot was 40 mm in diameter, the targets were adequately
covered with the proton beams and satisfactorily penetrated and stopped fully in the
solid targets.
5.1.2.2 Experimental Settings
The ADS experiments were carried out in the A-core (Fig. A2.1) with the combined
use of fuel and polyethylene reflector rods. In the A-core, the fuel assembly shown
in Fig. A2.2 is composed of 60 unit cells, and upper and lower polyethylene blocks
about 25
and 20
long, respectively, in an aluminum (Al) sheath 2.1 × 2.1 × 60
.
In the A-core, the neutron flux information was acquired from
115 In(n, γ)
116m In
reactions using the indium (In) wire (1 mm diameter and 800 mm length), under
127
5.1.2 Characteristics of the Target
5.1.2.1 Dimension of Solid Targets
The combined use of the heavy (tungsten, lead and bismuth: W, Pb and Bi) [3] and the
light nuclides (beryllium: Be; lithium) was considered useful in accomplishing the
study objectives related to the neutron spectrum and the neutron yield. The neutron
yield was obtained from the neutrons produced at the surface of the target. The two
targets laid with the combined use of heavy and light nuclides are termed “two-layer”
targets in this study, and their characteristics were numerically investigated by the
MCNPX [4] and SRIM codes [5] with the JENDL/HE-2007 [6] library. The aim of
these numerical analyses was to investigate the neutron spectrum in the high-energy
region and to determine the thickness of solid targets so that incident protons could
be fully stopped inside the target.
From the numerical results by MCNPX, the neutron spectrum was observed in
the high-energy region of the solid target used: W, W–Be or Pb–Bi, as shown in
Figs. A2.5 and A2.6, when 100 MeV protons were injected onto them. The neutron
spectrum was somewhat similar, regardless of the kind of solid target used as shown
in Fig. A2.5, although at each target it was comparatively different ranging from 1 to
10 MeV. Of particular interest here is the influence of the difference in the neutron
spectrum, caused by the kind of target used, on neutron multiplication in the core. In
the numerical simulations of neutron generation of the target, the neutron spectrum
of W-Be target (two-layer target) was compared remarkably with other single targets,
ranging from 85 to 100 MeV, as shown in Fig. A2.6, and the difference between the
two spectra was attributed to the scattering reactions of Be to high-energy protons.
The aim in using the two-layer target was to acquire the neutron spectrum in the
high-energy region and the neutron yield of high-energy neutrons. Consequently, the
proton beams actually penetrated the Be target, and conversely stopped inside the W
target. The thickness of the solid targets was correctly determined by the numerical
results of the range of high-energy protons with the use of the SRIM code. Lastly, the
dimensions of the solid targets were determined as shown in Table A2.6, and since
the size of the proton beam spot was 40 mm in diameter, the targets were adequately
covered with the proton beams and satisfactorily penetrated and stopped fully in the
solid targets.
5.1.2.2 Experimental Settings
The ADS experiments were carried out in the A-core (Fig. A2.1) with the combined
use of fuel and polyethylene reflector rods. In the A-core, the fuel assembly shown
in Fig. A2.2 is composed of 60 unit cells, and upper and lower polyethylene blocks
about 25
and 20
long, respectively, in an aluminum (Al) sheath 2.1 × 2.1 × 60
.
In the A-core, the neutron flux information was acquired from
115 In(n, γ)
116m In
reactions using the indium (In) wire (1 mm diameter and 800 mm length), under