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Establishment of Neutron Fluence Monitoring
Techniques for Quasi-monoenergetic Neutron
Calibration Fields of High Energy at TIARA
Y. Shikaze a) , Y. Tanimura a) , J. Saegusa a) , M. Tsutsumi a) , Y. Uchita a) , M. Yoshizawa a) ,
H. Harano b) , T. Matsumoto b) and K. Mizuhashi c)
a) Department of Radiation Protection, NSRI, JAEA, b) National Metrology Institute of Japan,
National Institute of Advanced Industrial Science and Technology,
c) Department of Advanced Radiation Technology, TARRI, JAEA
The neutron standard fields above 20 MeV have not been
established in Japan. Therefore, the calibration fields have
been developed by using the quasi-monoenergetic neutron
irradiation fields with 45, 60 and 75 MeV peaks at
1-3)
TIARA of Takasaki Advanced Radiation Research
Institute, Japan Atomic Energy Agency. Establishment of
the neutron fluence monitoring techniques was needed for
use of the calibration fields. Several works were
performed for the establishment as described below.
Firstly, the transmission type neutron fluence monitor 4)
was developed with a thin plastic scintillator and two
photomultiplier tubes on both sides of the scintillator to
monitor the neutron fluence directly. The performance test
results indicated enough sensitivity and good stability of the
sensitivity within ±2.7, ±1.7 and ±1.3% for the three neutron
fields with 45, 60 and 75 MeV peaks, respectively.
Secondly, a user-friendly counting system was developed
with PC card type fast counters (Interface CSI-632106) for
the neutron fluence and beam current monitoring in
calibrations and experiments. The counting system
consists of the scaler and the multi-channel-scaler (MCS).
The scaler is used to record the monitor counts and the
integrated beam current for each measurement. The MCS
is used to record time variation of them during the
irradiation time. The MCS is useful to know stability of
the beam and monitors, and may help off-line analysis.
The incoming data are displayed by the software on the
Windows PC screen (Fig. 1). The obtained data are
recorded in the Microsoft Excel formatted files.
Thirdly, the calibration procedure of the neutron fluence
monitor for each experiment was decided. The neutron
fluence, Ф is obtained from
Ф = k fm C fm, (1)
where k fm and C fm are the conversion factor and the
fluence monitor counts. However, the gain of the fluence
monitor may slightly change every experiment although the
fluence monitor shows good stability during beam time of
one experiment. Therefore, the k fm must be evaluated
every experiment. To evaluate the k fm, the 232 Th fission
chamber with very stable gain and ease of use was employed
as a standard. The conversion factors of the fission
chamber, k fc had been evaluated from the absolute
measurements using the proton recoil counter telescope 5) .
The k fc were found to be 1.63 10 8 , 1.22 10 8 and 1.14
JAEA-Review 2010-065
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10 8 [n/sr/(fission chamber count)] for the three neutron
fields with 45, 60 and 75 MeV peaks, respectively. Using
the correlation with the fission chamber, the k fm can be
obtained from
k fm = k fc R (fc/fm), …(2)
where R (fc/fm) is the count ratio of the fission chamber to
the fluence monitor. As the fluence monitor is stable
during an experiment, reliable monitoring of the neutron
fluence was realized by the calibration procedure.
In conclusion, the development of the neutron fluence
monitor, the counting system and the calibration procedure
established the neutron fluence monitoring techniques with
good usability. The monitoring techniques together with
the precise evaluation of the neutron fluence by the absolute
measurement contributed to the establishment of the neutron
calibration fields of high energy at TIARA.
References
1) M. Baba et al., Nucl. Instrum. Meth. Phys. Res. A 428
(1999) 454.
2) Y. Shikaze et al., Radiat. Prot. Dosim. 126 (2007) 163.
3) Y. Shikaze et al., J. Nucl. Sci. Tech. Suppl. 5 (2008)
209.
4) Y. Shikaze et al., JAEA Takasaki Ann. Rep. 2008
(2009) 152.
5) Y. Shikaze et al., Nucl. Instrum. Meth. Phys. Res. A 615
(2010) 211.
Fig. 1 The counting system for the neutron fluence and
beam current monitoring. The scaler (upper left) and
the multi-channel-scaler (lower right) are displayed.