JAEA-Conf 2011-002 - 日本原子力研究開発機構

3.1 Experimental Set up
JAEA-Conf 2011-002
A target in a 244Cm sample was 0.6-mg curium oxide, its activity was 1.8GBq and isotopic
abundance was 89.6%. That in 246Cm sample was 2.1-mg curium oxide with a 27.5% contamination
of 244Cm and its activity was 12.1 MBq from 246Cm and 1.7 GBq from 244Cm. [14] The samples were
sealed in aluminum capsules 9 mm in diameter and 0.5 mm thick walls. Using the “4π Ge
spectrometer” with a neutron time-of-flight method, both energy of neutrons and prompt-γ rays
from the samples were measured at the same time. The measurement time for the 244Cm sample
was about 80 hours and that for the 246Cm sample was about 100 hours. For the background
estimation, a measurement with a dummy sample of the aluminum case without curium oxide
powder was done for 40 hours and a measurement with neither of a sample material nor the
sample case (only with a sample holder) was done for 32 hours. In the measurements, MLF was
operated at an average beam power of 120 kW, and a repetition rate of 25 Hz. For dead-time
correction, pulses from a random-pulse generator (Berkeley Nucleonics : DB-2) were input through
the “test input” of the pre-amplifier of every Ge crystal.
3.2 Data Processing and Analysis
Fig. 4 shows neutron-capture -ray yields of the 244Cm, the 246Cm sample, and the dummy
case. In these experiments, neutron energies were calibrated with resonances in 197Au(n,) reaction. Resonance peaks of 244Cm, 246Cm, and 248Cm are clearly observed.
In the experiments, pulses from the random-pulse generator were input and measured
with the DAQ. The stored counts were used for the dead time correction by comparing the counts
of input pulses with actually stored pulses. Time dependent dead time was obtained from this
method. [15]
Neutron-energy dependent backgrounds were mainly caused by prompt γ rays from the
Al case and scattered neutrons by the Al case and the helium in the beam duct. The backgrounds
were derived from the neutron-capture γ-ray yields of the Al case and the blank measurement
using the full-energy peaks of 7724-keV prompt γ rays from 27Al(n, γ) reactions.
The effect from the decay γ rays from 244Cm, 28Al, and the other activated nuclides was
also deduced from the counts in
the TOF time range after 30 ms by
extrapolation with exponential
and constant function.
Self-shielding and
multi-scattering factors of the Cm
samples and the Al case were
calculated with the MCNP code.
The incident neutron flux shape
was deduced from neutron-capture
γ-ray yields of the 478-keV γ ray
from 10B(n,αγ) reactions. [16]
Fig. 4. Neutron-capture γ-ray yields of the 244Cm sample
(dashed line), the 246Cm sample (solid line), and the
dummy Al case (dashed-dotted line).