JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
17. Thermal / epi-thermal neutron spectrometer with
a 3 He position sensitive proportional counter
Masao ITO*, Isao MURATA
Division of Electronic and Information Engineering, Osaka University
Yamada-oka 2-1, Suita, Osaka, 565-0871 Japan
*E-mail: ito@ef.eie.eng.osaka-u.ac.jp
At present, measurement of energy spectrum for thermal and epi-thermal neutrons becomes crucial to apply
to the medical, as well as physical applications. In the present paper, we described the preliminary result of
ongoing development of a new thermal and epi-thermal neutron spectrometer using the detection depth
information measured by a 3 He position sensitive proportional counter.
1. Introduction
Spectrum measurement of thermal and epi-thermal neutrons is becoming crucial recently considering possible
applications such as boron neutron capture therapy (BNCT). But there is no direct technique to measure the
low-energy neutron spectrum. In the low-energy neutron measurement, there are some conventional ways. The
most frequently used ways are a gas counter like BF3, 3 He and fission chamber. However, they have too large
Q-values compared with the incident neutron energy to detect the energy correctly. Next is the foil activation
method, which uses several types of foils and unfolding code to estimate the neutron spectrum. This method is
available only by off-line analysis and the unfolding process is regarded more-or-less as an underdetermined
problem. If the neutron source is pulsed, the time-of-flight method can be used. It needs a complicated technique
in processing detector signals with electronic modules. However, the resulting accuracy is known to be
commonly high.
In BNCT facilities, the major problem in the neutron spectrum measurement is how precisely the energy could
be determined. So in this study we have been developing a new spectrometer which aimed at realization of
measurement of the low-energy neutrons from thermal to 10keV. We have developed a prototype detector of 3 He
position sensitive proportional counter. The spectrometer acquires the information as to how deep an incident
neutron has reached in the detector and where the nuclear reaction with the detection gas by the neutron has
occurred. The multi-parameter MCA system was set-up for measuring signals from the detector in the two
dimensional space. The response function of the detector, that is, the detection position distribution of each
energetic neutron, is calculated from the neutron cross section data.
The neutron spectrum can be derived by unfolding the measured detection position (depth) distribution with
the response function of the detector. This time, we measured thermal neutrons which are produced by