by the electron burst from the target, a lead block, 7 cm in diameter and 10 cm long,
was set in front of the entrance of the flight tube.
First, a gold foil 10, 20, 30, 40, or 50 μm thick was located as a sample at a flight
length of 11.0 m, where the neutron beam was well collimated to a diameter of
24 mm. An indicator located at a flight length of 12.7 m was surrounded by a
Bi 4 Ge 3 O 12 (BGO) assembly, which consists of 12 scintillation bricks each
5 Â 5 Â 7.5 cm
3 [6]. Prompt-capture γ-rays from the indicator were detected with
the BGO assembly in the TOF measurement. A
10 B plug 8 mm thick or gold foil
50 μm thick was used as an indicator. Because the former thick indicator can absorb
most neutrons with energies below the epi-thermal region, it was equivalent to the
conventional NRTA. In the latter case, it was the self-indication measurement. The
area densities of the samples with different thickness were estimated by area
analysis for the 4.9 eV resonance of
197 Au.
As the next step, a 50-μm-thick silver foil was added to a 10-μm-thick gold foil
to form a sample and the area density of the gold foil was measured. It is worth
noting that silver has a large resonance at 5.2 eV, close to the 4.9 eV resonance of
197 Au. The
10 B plug 8 mm thick or a gold foil 50 μm thick was also used as an
indicator. Moreover, we demonstrated a nondestructive assay for nuclear materials
using a mixture composed of a natural uranium plate and sealed minor actinide
samples of
237 Np and
243 Am. The natural uranium plate was 10 Â 20 mm
2 and
weighed 5.8 g. The samples of
237 Np and
243
Am were oxide powder, which was
pressed into a pellet 20 mm in diameter and encapsulated in an aluminum diskshaped container 30 mm in diameter with 0.5-mm-thick walls. The activities of
237 Np and
243 Am were 26 and 868 MBq, respectively. In the third measurement, the
10 B plug 8 mm thick or a natural uranium plate of 10 Â 20 mm
2 and weight 5.8 g
was used as an indicator.
KURRI LINAC
(pulsed electron beam)
–e
135º
n
Pb-Shadow
bar
beam)
ow
º
2
.
0
m
Concrete wall
Pb
Paraffin-Li 2 CO 3
Flight tube
B 4 C+H 3 BO 3
BF 3 Counter
BGO Detector
Water
Tank
B 4 C
Pb
Pb sheet
12.7+0.02 m
_
Sample
Indicator
Ta target
Water
moderator
Fig. 3.1 Experimental arrangement for the time-of-flight (TOF) measurement
3 Development of Nondestructive Assay to Fuel Debris of Fukushima Daiichi. . .
23
was set in front of the entrance of the flight tube.
First, a gold foil 10, 20, 30, 40, or 50 μm thick was located as a sample at a flight
length of 11.0 m, where the neutron beam was well collimated to a diameter of
24 mm. An indicator located at a flight length of 12.7 m was surrounded by a
Bi 4 Ge 3 O 12 (BGO) assembly, which consists of 12 scintillation bricks each
5 Â 5 Â 7.5 cm
3 [6]. Prompt-capture γ-rays from the indicator were detected with
the BGO assembly in the TOF measurement. A
10 B plug 8 mm thick or gold foil
50 μm thick was used as an indicator. Because the former thick indicator can absorb
most neutrons with energies below the epi-thermal region, it was equivalent to the
conventional NRTA. In the latter case, it was the self-indication measurement. The
area densities of the samples with different thickness were estimated by area
analysis for the 4.9 eV resonance of
197 Au.
As the next step, a 50-μm-thick silver foil was added to a 10-μm-thick gold foil
to form a sample and the area density of the gold foil was measured. It is worth
noting that silver has a large resonance at 5.2 eV, close to the 4.9 eV resonance of
197 Au. The
10 B plug 8 mm thick or a gold foil 50 μm thick was also used as an
indicator. Moreover, we demonstrated a nondestructive assay for nuclear materials
using a mixture composed of a natural uranium plate and sealed minor actinide
samples of
237 Np and
243 Am. The natural uranium plate was 10 Â 20 mm
2 and
weighed 5.8 g. The samples of
237 Np and
243
Am were oxide powder, which was
pressed into a pellet 20 mm in diameter and encapsulated in an aluminum diskshaped container 30 mm in diameter with 0.5-mm-thick walls. The activities of
237 Np and
243 Am were 26 and 868 MBq, respectively. In the third measurement, the
10 B plug 8 mm thick or a natural uranium plate of 10 Â 20 mm
2 and weight 5.8 g
was used as an indicator.
KURRI LINAC
(pulsed electron beam)
–e
135º
n
Pb-Shadow
bar
beam)
ow
º
2
.
0
m
Concrete wall
Pb
Paraffin-Li 2 CO 3
Flight tube
B 4 C+H 3 BO 3
BF 3 Counter
BGO Detector
Water
Tank
B 4 C
Pb
Pb sheet
12.7+0.02 m
_
Sample
Indicator
Ta target
Water
moderator
Fig. 3.1 Experimental arrangement for the time-of-flight (TOF) measurement
3 Development of Nondestructive Assay to Fuel Debris of Fukushima Daiichi. . .
23