JAEA-Conf 2011-002 - 日本原子力研究開発機構

In the case of most compact cascade such as those produced in Au, with increasing irradiation
temperatures, above 473K, well separated subcascades (SFTs) fuse into a large SFT as shown in Fig. 5. Average
SFT sizes irradiated at 300K and 473K were 1.6nm and 3.7nm, respectively. Above 623K only few SFTs
remained due to their thermal unstableness. The SFT size is larger for larger PKA energy. Figure 6 shows the
comparison of defect structures between fusion neutron irradiation (the RTNS-II, 563K, 0.017dpa) and fission
neutron irradiation (the JMTR, 573K, 0.044dpa). Though the total displacement damage for the fission neutron
irradiation was higher than that of the fusion neutron irradiation, the remaining number of SFTs was higher in
the fusion neutron irradiation.
The cross-section for the formation of SFTs was 1.7×10 -1 barns and 9.4×10 -4 barns for fusion neutrons and
fission neutrons, respectively. The average size of SFTs by fusion neutron irradiation was also larger than that of
fission neutron irradiation. If there exists a threshold energy for SFT formation, ESFT, it is evaluated using the
following equation,
N
SFT
E
d
( E
MAX
dEp
, (1)
E SFT
dE
p
p
)
JAEA-Conf 2011-002
PKA energy (keV)
Figure 4. Variation of the number of subcascades in a group with the PKA energy.
where NSFT is the concentration of SFTs observed, α is the SFT formation efficiency. We adopted the cross
section calculated by the SPECTER code [5] using the neutron spectrum of the JMTR and RTNS-II as shown in
Fig. 7. α and ESFT were obtained to be 0.05 and 80keV, respectively, using the number density of SFTs formed
by fission neutron irradiation and fusion neutron irradiation. It is concluded that SFTs are formed for PKA
energies larger than 80keV with a cascade efficiency of 0.05 at 563-573K. Actually α depends on the PKA
energy, so the value is an average one above 80keV.
Figure 5. Temperature dependence of subcascade structures in Au. Ovals indicate one cascade. Above 473 K
SFTs formed in a cascade fuse into a large SFT.