JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
3. Results and discussion
3.1 Multi parameter MCA measurement
A two dimensional spectrum of signals measured by the 3 He position sensitive proportional counter is shown in
Fig.5. In the figure, the horizontal axis shows the sum of the two signals and the vertical axis shows one of the
two signals. It was confirmed that the detection position information could successfully be extracted. The
vertical axis value means the detection depth position from the detector edge. There are no cadmium collimators
in the measurement, meaning that neutrons are entering the detector from the side surface uniformly. Full energy
peaks marked with a red circle show the detection position distribution, y(r), in the detector by projecting on the
y-axis. But, y(r) is not a clear detection position distribution as shown in the figure. In Fig.5, there is a zigzag
curve formed with the full energy peaks and thus y(r) seems to be not a uniform distribution. One of the reasons
is existence of the copper pipe of the detector. So we must make the identification system connecting between
the two-dimensional figure and the detection depth spectrum. It means that it is necessary to make one-to-one
correspondence between the position of the full energy peak in the figure and the real detection position in the
detector. And it is quite important to fix the detectable reaction depth to define the measurable neutron energy
range. For these requirements, in the next step, we carried out measurements with a cadmium collimator put
around the detector.
Fig.5 Two dimensional MCA result.
3.2 Detection position identification
For the collimator we used a cadmium cover, which has a large (n,γ) reaction cross section for thermal
neutrons. Incident thermal neutrons are mostly blocked by the collimator and detected only from an open
window of 1cm width. The schematic view for the test measurement is shown in the left figure in Fig.6. To
establish the one-to-one correspondence, we have measured several tens times by moving the open window from
one edge to another at every 1cm. Incident neutrons are perpendicular to the detector axis as shown in the figure.
In the right figure, several cadmium collimators were rolled around the detector to demonstrate the position
sensitive detector performance. The center positions of 7 open windows were 4cm, 9cm, 16cm, 25cm, 32cm,