JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
was large. The activation method is not applicable for thermal neutron capture cross section
measurements since the residual nuclei are stable. Recently, Nakamura et al. measured the
thermal neutron capture cross sections of 93Zr and 107Pd with a prompt -ray spectroscopy
[5,6] and gave the only available datum for 107Pd. Though Pomerance also measured the
thermal capture cross section of 93Zr with a pile oscillator method [7,8], there is a large
discrepancy between two data with different methods. For 99Tc and 129I, a number of
experimental data with the activation, lead spectrometer, prompt -ray and TOF methods
were reported. However, the accuracy of experimental data for 99Tc and 129I was not enough
since those were considered as the first priority nuclei for a transmutation.
As mentioned above, the current status of experimental data is not sufficient both in
quality and in quantity. This is because it is not easy to prepare enough amount of sample
with a high purity. To overcome the difficulty, we have started a series of experimental studies
for LLFPs using the Accurate Neutron-Nucleus Reaction measurement Instrument (ANNRI)
which was installed at the neutron Beam Line No.4 (BL04) of the Material and Life science
experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC) [9].
Neutron capture rays from a sample were measured with a 4 Ge spectrometer using the
TOF method. A high intensity pulsed neutron source makes it possible to measure the capture
rays accurately with a small amount of sample. Moreover, the background due to impurities
contained in a sample can be removed from the observed events using Ge detectors with a
high energy resolution. In this paper, the preliminary results of 93Zr, 99Tc and 107Pd are
reported.
2. Experimental Procedure
The samples of 93Zr, 99Tc and 107Pd were purchased from the Oak Ridge National
Laboratory (ORNL) of USA, the Research Institute of Atomic Reactors (RIAR) of Russia and
the Nuclear Research and consultancy Group (NRG) of Netherlands, respectively. The metal
or oxide powder of 93Zr, 99Tc and 107Pd were encapsulated in aluminum disk-shaped containers.
A sintered natural boron sample was used for the measurement of the incident neutron flux
on the sample. A crystal of NaCl sample which emits well-known rays from the neutron
capture of chlorine was also used as a standard -ray source. The characteristics of samples
and containers are shown in Table 1. Isotopic purities of LLFP samples were determined with
a Thermal Ionization Mass Spectrometer (TIMS) as shown in Table 1 except for 99Tc sample
which contains no isotopic impurities. The net weight of 99Tc was 78 mg. The net weights of
93Zr and 107Pd were determined as 470 and 20 mg using the weights of powder and the
isotopic purities, respectively.
The neutron capture cross section measurements have been performed at the ANNRI.
The pulsed-neutron beam was produced by spallation of 3-GeV proton beam at the Japan
Spallation Neutron Source (JSNS) [10] of MLF. The accelerator was operated in a
double-bunched mode at a power of 120 kW and repetition rate of 25 Hz. The 4 Ge
spectrometer composed of two cluster and eight coaxial Ge detectors surrounded by BGO
Compton suppression detectors was placed at a distance of 21.5 m from the pulsed neutron
source. In this work, we used only two cluster Ge detectors. Each sample was set at the center