Biomedical Engineering Reference
In-Depth Information
As mentioned in Section 10.7, the proportional counter provides a direct method
of measuring neutron dose, and it has the advantage of excellent gamma discrim-
ination. The pulse height produced by a charged recoil particle is proportional to
the energy that the particle deposits in the gas. The Hurst fast-neutron proportional
counter is shown in Fig. 12.6. To satisfy the Bragg-Gray principle, the polyethylene
walls are made thicker than the range of a 20-MeV proton. The counter gas can
be either ethylene (C
2
H
4
)
or cyclopropane (C
3
H
6
)
, both having the same
H/C
2
ratio as the walls. A recoil proton or carbon nucleus from the wall or gas has high
LET. Unless only a small portion of its path is in the gas it will deposit much more
energy in the gas than a low-LET secondary electron produced by a gamma ray. Re-
jection of the small gamma pulses can be accomplished by electronic discrimina-
tion. Fast-neutron dose rates as low as 10
-5
Gy h
-1
can be measured in the presence
of gamma fields with dose rates up to 1 Gy h
-1
. In very intense fields signals from
multiple gamma rays can “pile up” and give pulses comparable in size to those
from neutrons.
The LET spectra of the recoil particles produced by neutrons (and hence neutron
quality factors) depend on neutron energy. Table 12.5 gives the mean quality fac-
tors (based on Table 12.1) and fluence rates for monoenergetic neutrons that give a
dose equivalent of 1 mSv in a 40-h work week. The quality factors have been com-
puted by averaging over the LET spectra of all
c
harged recoil nuclei produced by
the neutrons. For
p
ractical applications, using
Q
=
=
3
for neutrons of energi
es
less
than 10 keV and
Q
10
for all neutrons is acceptable, but may be overly conservative. Thus, in monitoring
neutrons for radiation-protection purposes, one should generally know or estimate
the neutron energy spectrum or LET spectrum (i.e., the LET spectrum of the re-
coil particles). Measurement of LET spectra is discussed in Section 12.8. Several
methods of obtaining neutron energy spectra were described in Section 10.7. The
neutron rem meter, shown in Fig. 10.44, was discussed previously.
Figure 12.7 shows an experimental setup for exposing anthropomorphic phan-
toms, wearing various types of dosimeters, to fission neutrons. A bare reactor was
positioned above the circle, drawn on the floor, with an intervening shield placed
between it and the phantoms, located 3 m away. In this Health Physics Research
Reactor facility, the responses of dosimeters to neutrons with a known energy spec-
trum and fluence were studied.
Intermediate and fast neutrons incident on the body are subsequently moder-
ated and can be backscattered at slow or epithermal energies through the surface
they entered. Exposure to these neutrons can therefore be monitored by wearing
a device, such as a thermoluminescent dosimeter (TLD) enriched in
6
Li, that is
sensitive to slow neutrons. Such a device is called an albedo-type neutron dosime-
ter. (For a medium A that contains a neutron source and an adjoining medium B
that does not, the albedo is defined in reactor physics as the fraction of neutrons
entering B that are reflected or scattered back into A.)
=
10
for higher energies will result in little error. Using
Q
=
