JAEA-Conf 2011-002 - 日本原子力研究開発機構

moderating fast neutrons emitted from an AmBe source using a graphite column. The thermal column was
designed by MCNP5[1] and constructed for the present test measurement with the prototype detector. In this
paper, the details of the developed detector are presented and ongoing test measurement results are briefly
summarized.
2. Measurements Technique and Equipments
2.1 Unfolding
There is no direct method to measure the low energy neutron spectrum as mentioned before. So this time we
employed an unfolding process. It requires a certain difference of physical quantity to expand and view the
energy difference in the low-energy region. The difference can make the neutron to be identified via one-to-one
correspondence. Practically in the present method it is based on the fact that lower energy neutrons have a larger
reaction cross section value and high energy neutrons have a smaller one. The reaction cross section changes are
exhibited as the detection position (depth) changes. The response matrix R(E,r)dr can theoretically be deduced
as ∑(E)exp(-∑(E)r)dr, where ∑(E)[1/cm] is the macroscopic cross section and E is the neutron energy.
Measured detection depth distribution y(r) and neutron energy spectrum x(E) are related by the next equation,
y(r)= R(E,r)x(E). Thus, the measured reaction position distribution could be unfolded with an appropriate
unfolding code in order to estimate the neutron energy spectrum. For the unfolding process, we adopted the
Bayes theorem to derive neutron spectrum[2,3].
Normalized detector response
10 -1
10 -2
10 -3
JAEA-Conf 2011-002
: 0.01 eV
: 1 eV
: 100 eV
: 10 keV
10 20 30 40 50
Distance from the detector entrance (cm)
Fig.1 Response function of the detector.
2.2 Detector
In this study, we have to know the detection position (depth) distribution of an incident low-energy neutron.
The position sensitive proportional counter can identify where the incident neutron reacts with the inner gaseous
material. 3 He is selected as the detection gas, which has an extremely large reaction cross section in the low
energy region. The gas pressure and detector length are deeply related to each other. To make the detector length
short, the gas pressure must be increased so that the measurable energy range expands widely. This time we
developed a new position sensitive proportional counter shown in Fig.2. The length of detector is 50cm, 2.53cm
in diameter. A copper pipe is seen at the center for bottling 3 He into the counter. The 3 He gas pressure is 0.5Mpa.
In Fig.2, the block diagram of the measurement circuit is shown together with the photo. The induced charge