JAEA-Conf 2011-002 - 日本原子力研究開発機構

distribution of the neutron beam at the 21.5-m sample position. Using the rotary collimator,
neutron beams with diameters of 22, 7, and 3 mm to suit samples of different sizes are provided at
the 21.5-m sample position.
2.2 Characteristics of the neutron beam at ANNRI
The energy resolution of the neutron beam at the 21.5-m sample position depends on the
moderator system and the proton-beam operation of MLF. The proton beam usually consists of two
bunches with a distance of 600 ns. The width of each bunch increases up to 185 ns depending on an
incident proton beam power to MLF. [9] In the case of two bunches in the proton beam, the
resolution deteriorates gradually above 10 eV as the energy increases and reaches to about 10% at
10 keV. 9) The energy-integrated neutron intensities under 120 kW operation are 4.5106 n/s/cm2 in the neutron energy range of 1.5-25 meV, and 6.6105 n/s/cm2 in 0.9-1.1 keV at the 21.5-m
sample position. Under the future 1-MW operation, these intensities are expected to increase to
4.3107 , and 6.3×106 n/s/cm2 . [10] The proton intensity of shots was stable within 1% in FWHM.
2.3 The “4π Ge spectrometer”
JAEA-Conf 2011-002
The “4π Ge spectrometer” is composed of two cluster-Ge detectors and eight
coaxial-shaped Ge detectors as seen in Fig. 3. The Ge detectors were covered with BGO Compton
suppression detectors, which eliminated background Compton
events. The peak efficiency of the “4π Ge spectrometer” is 3.64 ±
0.11 % for 1.33-MeV γ rays. A typical energy resolution is
9.8-keV (on beam) and 2.4keV (off beam) in FWHM for
1.33-MeV γ rays. [11]
2.4 Data Acquisition System
A data acquisition system (DAQ) for the “4π Ge
spectrometer” is required to deal with a large amount of signals
from the spectrometer. Since MA samples are highly radioactive
and the DAQ has to handle high event rates, a high
performance DAQ system based on a digital data processing
technique is developed. [12] The time resolution of the DAQ is
10 ns. The dead time of this system is only 3.3 μs per event at
50k events/s and the maximum event rate is more than 200k Fig.3. An outward appearance
events/s.
of the“4π Ge spectrometer”
3. Measurements of Neutron-Capture Cross Sections of 244Cm and 246Cm 244Cm and 246Cm are the most important nuclei in MAs. However, only one
neutron-capture cross-section data was made in the past. [13] Furthermore, this previous
measurement was performed by the neutron time-of-flight (TOF) method using the nuclear
explosion “Physics 8” as a pulsed neutron source in 1969. The accuracy in this neutron-capture
cross-section measurement is not enough for these demands of 10%. [1] In the ND2010, very
preliminary neutron-capture cross sections of 244Cm and 246Cm were reported. [4] In this paper,
having additional measurements and analyses, preliminary but more accurate neutron-capture
cross sections of 244Cm and 246Cm are reported.