JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
in the counter is divided according to the ratio of distances from the side edges. Two signals from both ends of
the detector are measured by two MCAs to obtain a two dimensional counter for one output and the sum of the
two by the coincidence counting technique.
Fig.2 Schematic block diagram and 3 He detector.
2.3 Calculation and Setting
The experiment was carried out at OKTAVIAN facility of Osaka University, Japan. For measurement of
thermal/epithermal neutrons, we designed a graphite thermal column with an AmBe source by MCNP5
calculations. The calculation model is shown in Fig.3. Using graphite as neutron moderator, a rectangular shape
(100 x 100 x 100 cm 3 ) graphite thermal column was constructed. The AmBe neutron source was located at
several distances, D [cm], from the thermal column surface. The neutron spectrum at the 3 He proportional
counter positioned at 100cm from the thermal column was calculated and the most reasonable setting was
examined from the calculation results. The key point is that the 3 He proportional counter was so set that the
neutron beam direction became perpendicular to the axis of the detector in the measurement for an aim to
ascertain the detection position of neutron in the detector. But in an real application, the incident particle
direction must be parallel to the detector axis. We have calculated both of these situations.
Fig.4 shows the MCNP calculation results in some distances, D (20cm,40cm,60cm,80cm and 100cm). From
the results, we set the moderator thickness D to 50cm. At 50cm, the ratio between Thermal/MeV is enough high
and the neutron number density is not so low. The calculated spectrum will be used to verify the unfolding
result obtained from a parallel incidence experiment.
Fig.3 Calculation model. Fig.4 MCNP calculation results.