Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

Count rate per unit flux
10
1
.1
.01 10 −8 10 −6 10 −4 10 −2 10 0
Neutron energy (MeV)
HEALTH PHYSICS INSTRUMENTATION 475
Figure 9-34. Energy response curve
(solid line) of a neutron rem counter.
The broken line is the dose equivalent
per unit flux. Ideally, the response
should be 7.6 counts/s/mrem/h.
(Reprinted with permission from
Andersson IO, Braun J. A neutron
rem counter with uniform sensitivity
from 0.025 eV to 10 MeV. In: Neutron
Dosimetry: Proceedings of the Symposium
on Neutron Detection, Dosimetry,
and Standardization. Vol 2. Vienna:
International Atomic Energy Agency
(IAEA); 1963:87–95.)
mrems) per hour utilize a thermal-neutron detector surrounded by a spherical or
semispherical moderator (remball detector). Either a small (about 4 mm × 4 mm)
6 LiI(Eu) scintillating crystal located in the center of the sphere or a 10 BF3 counter
inserted into the moderator can be used as the neutron detector. However, because
the 10 BF3 tube is less sensitive to gamma rays than is the scintillation crystal, the
10 BF3 counter is the most widely used neutron detector in neutron dose-equivalent
meters. However, BF3 tubes are being replaced by tubes filled with 3 He. Because of
new restrictions on the air transport of BF3, shipping companies are reluctant to
ship devices containing BF3 despite the fact that a tube contains only about 0.02 g
BF3. The energy response of 3 He tubes is the same as that of the BF3 tubes, and the
neutron sensitivity of 3 He tubes is greater than that of BF3 tubes. Replacement of
the BF3 tubes therefore will not lead to any changes in the use of remball detectors.
The response of a spherical neutron dosimeter, when the sphere is 30 cm in
diameter, is approximately proportional to the neutron dose-equivalent rate from
thermal energies to about 15 MeV. This type of neutron dose-equivalent meter thus
may be calibrated with neutrons of any energy within this range. Spheres smaller than
30 cm in diameter are relatively more sensitive to lower energy neutrons. Since the
energy response of the instrument depends on the size of the spherical moderator, it
is possible to determine the energy distribution in a neutron field by making a series
of measurements with different-sized spheres. The spheres used for this method of
neutron spectroscopy are commonly called “Bonner spheres”; they range in diameter
from5to30cm.
Superheated Emulsion (Bubble) Dosimeter
A superheated emulsion (bubble) dosimeter (Fig. 9-36) is a sensitive passive neutron
dosimeter about the size of a fountain pen, and hence is useful for personal monitoring
as well as for area monitoring for stray neutron radiation at a radiotherapy facility
using a high-energy particle accelerator. It is completely unresponsive to gamma
radiation, and its energy response is such that it approximates the ICRP 60 weighting
factors for neutrons, thus allowing calibration and readout directly in microsieverts
or in millirems of neutron dose. The bubble detector consists of numerous microscopic
droplets of a superheated liquid of a hydrocarbon or halocarbon dispersed in