JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Evaluation of the Response Characteristics of
a Portable Cosmic-ray Neutron Monitor
H. Yasuda a) , K. Yajima a) and A. Endo b)
a) National Institute of Radiological Sciences (NIRS),
b) Division of Environment and Radiation Sciences, NSED, JAEA
Introduction: Measurement-based dosimetry of cosmic
radiation exposure is still difficult at high altitude because
the radiation field in space and aviation is complex as the
field comprises various particle species: protons, neutrons,
heavy ions, electrons, photons, etc. In such fields, detector
signals attributing to different radiation components are
inseparably mixed.
Precise measurements of cosmic radiation usually need
large detectors to determine particle species and energies.
For example, cosmic-ray neutrons have been measured with
proportional counters having thick moderators called
“Bonner spheres”. However, such bulky instruments are
hardly employed for in-flight monitoring because of the
space and power limitations. Liquid organic scintillators,
which have been widely used for neutron/gamma separation
in nuclear facilities, are not allowed in civilian aircraft
mainly due to safety concerns.
Thus, we have started to develop a practical portable
monitor which can measure cosmic radiation precisely in
both spacecraft and aircraft.
Materials and methods: The monitoring system consists of
a detector probe, a signal processing unit, a high-voltage
supplier and a data analyzing program which runs on
Windows PC. The detector probe, which can be flexibly
replaced, is an original phoswich-type scintillation detector
composed of cylindrical stillbene scintillator (special order
product; Oyo-koken, Japan) with dimensions of 50 mm ×
50 mm covered with 15mm-thick plastic scintillator
(EJ-299; Eljen Technology, USA). The decay time of
scintillation from stillbene is 3.5 ns and that of the plastic
scintillator is 320 ns. The probe is coupled with a 2-inch
photomultiplier tube (R6041; Hamamatsu Photonics, Japan).
The phoswich scintillation probe gives different pulse
shapes for different particle species; for example, long decay
signal is unseen for energetic neutrons, whereas it is well
recognized for charged particles. The pulse shape is
analyzed with an original signal processing unit with
dimensions of W150 mm × D150 mm × H50 mm. This
unit has a flash ADC which converts an analogue signal (0
to 5 V, negative) from the detector probe to the digital data
with 12-bit dynamic range from -20 to 400 ns from the
trigger point at 2 ns intervals. The maximum processing
rate is about 400 events per second. Signal voltage levels
integrated for 2 ns each is transferred to a personal computer
via USB2.0 interface. The total weight of the system is
about 10 kg.
The characteristics of the monitor responses to
cosmic-ray neutrons were partly examined using energetic
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neutron beams supplied at JAEA-TIARA. The responses
to heavy charged particles were also examined using
~ 230 MeV protons and ~ 230 MeV/u helium ions at the
Heavy Ion Medical Accelerator in Chiba (NIRS-HIMAC).
Results and Discussion: Figure 1 shows the results
obtained with the vertical incident beams of 75 MeV
neutrons in comparison with protons and helium ions. The
X-axis of the graph is the signal integral from -20 to 50 ns
(the trigger point is 0) and the Y-axis is that from 200 to
400 ns. According to the plot figure, it is considered that
the newly developed monitor can detect neutrons and heavy
charged particles separately. The good ability for
separation between neutrons and -rays has already been
confirmed for the stillbene scintillator.
It should be noted, however, a considerable incident
angle dependence was observed, which is probably due to
the physical feature of stillbene crystal and the cylindrical
scintillator structure. Separation of muons and energetic
electrons including calibration methods for those particles is
the subjects in ongoing research. Determination of the
precise energy spectra of cosmic-ray particles calls for
further investigation.
Fig. 1 Two dimensional plots of the scintillation signals
integrated for different ROI ranges. The data obtained
at TIARA using 75 MeV neutrons are seen in the
left-lower part. The plots of neutrons were clearly
separated from those of protons (~ 230 MeV) and helium
ions (~ 230 MeV) obtained at NIRS-HIMAC.
Acknowledgments: The authors appreciate Dr. Yoshiaki
Shikaze and other staffs of JAEA for their great help in
neutron beam irradiation. A part of this study was
supported by the Japan Space Forum / JAXA.