Biomedical Engineering Reference
In-Depth Information
Fig. 10.45
Pulse-height spectrum from
3
He proportional
counter for monoenergetic neutrons of energy
T
.
in the center-of-mass system for neutron laboratory energies up to
∼10
MeV, the
average energy imparted to protons by neutrons in this energy range is
T
/2
(Sec-
tion 9.6).
Organic proton-recoil scintillators are available for neutron spectrometry in a va-
riety of crystal, plastic, and liquid materials. The full proton recoil energies can
be caught in these scintillators. Complications in the use of proton-recoil scintilla-
tors include nonlinearity of response, multiple neutron scattering, and competing
nuclear reactions. For applications in mixed fields, the gamma response can, in
principle, be separated electronically from the neutron response on the basis of
quicker scintillation.
Proportional counters have been designed with hydrocarbon gases, such as CH
4
.
These have inherently lower detection efficiencies than solid-state devices, but offer
the potential for better gamma discrimination. Wall effects can be important. Pro-
portional counters have also been constructed with polyethylene or other hydroge-
nous material surrounding the tube. One such device, based on the Bragg-Gray
principle, will be discussed in Section 12.6.
A proton-recoil telescope, illustrated in Fig. 10.46, can be used to accurately mea-
sure the spectrum of neutrons in a collimated beam. At an angle
θ
, the energy
T
p
of a recoil proton from a thin target struck by a neutron of incident energy
T
is, by
Eq. (9.5),
T
p
=
T
cos
2
θ
.
(10.11)
The
E
(-d
E
/d
x
)
coincidence particle identifier (Fig. 10.27) can be used to reduce
background, eliminate competing events, and measure
T
p
.
