Speaker
Hiromitsu Takahashi
(Hiroshima University)
Description
After the Fukushima Daiichi nuclear accident, the public necessity
increases to measure and identify the radioactive isotopes from the
environment. Although Ge detectors are powerful with the high energy
resolution, it is too expensive (and heavy) to be used widely.
Therefore, we developed an inexpensive portable compact gamma-ray
spectrometer with the combination of CsI(Tl) scintillator and
Multi-Pixel Photon Counter (MPPC) detector, a silicon photomultiplier
developed by Hamamatsu Photonics. In this paper, we present (1) how we
chose the sizes of CsI(Tl) and MPPC, (2) the development of the
feedback circuit to keep the MPPC gain stable against the temperature
variation, and the calibration of the spectrometer at Facility of
Radiation Standards of Japan Atomic Energy Agency, and (3) some
example of actual environment measurements.
Among many photon readout detectors, MPPC, photo-multiplier,
photo-diode, avalanche photo-diode, we chose the 6 x 6 mm2 MPPC
according to the high quantum efficiency, small detector size, light
weight, and low operational high voltage. CsI(Tl) scintillator was
selected from the relatively high stopping power with the low price.
The size of CsI(Tl) crystal was determined as large as 1-inch cubic to
identify 137Cs and 134Cs lines (667 keV and 605 keV) separately with
keeping the energy resolution better than 8% at 662 keV (FWHM).
There is small deviation (~7%) from the linear correlation between the
gamma-ray energy and output pulse height above ~1 MeV. It is
reasonable due to the pileup effect of some photons into one pixel,
since the number of CsI(Tl) scintillation photons is 56000/MeV with
the decay time constant of ~1 us.
The temperature dependence of MPPC gain was measured for 10 units of
1-inch cubic CsI(Tl) and MPPC. The results show that there is almost
no difference about the slope of the relation, while the offset
changes for unit by unit. The spectrometer includes the feedback
circuit of the high voltage with temperature monitor.
Author
Hiromitsu Takahashi
(Hiroshima University)
Co-authors
Takashi Ohsugi
(Hiroshima University)
Tatsuya Nakaoka
(Hiroshima Univ.)
Toru Takagaki
(TAC Co. Ltd)
Tsunefumi Mizuno
(Hiroshima University)
Yasushi Fukazawa
(Hiroshima University)
