Page 548 - Instrumentation Reference Book 3E

530 Nuclear instrumentation technology
the source directly into the gas of an ionization
chamber or proportional counter. The most pop-
ular method at present uses a semiconductor
detector, and this allows the energies to be deter-
mined very accurately when the detector and
source are operated in a small evacuated cell. This
I is especially important for alphas.
Detector window
Figure 22.6 Geometry of radiation collection.
Self-absorption If visible amounts of solid are
present in a source, losses in counting rate may
in radiations per unit time per unit sphere surface be expected because of self-absorption of the
area is constant over the entire surface. The particles emitted from the lower levels of the
geometry factor, G, can be therefore written as source which are unable to leave the surface of
the fraction of total 47r solid angle subtended by the source. Nader et al. give an expression for the
source and detector. For the case of the point self-absorption factor for alpha particles in a
source at a distance d from a circular window of counter with 27r geometry (this is the gas counter
radius r, we have the following relation: with source inside the chamber):
G 0.5(1 COS^) (22.13) fS=l-- S for s<pR (22.15)
2PR
and
0.5R
A=- S for s>pR (22.16)
Scattering Particles and photons are scattered
by material through which they pass, and this where s is the source thickness, R the maximum
effect depends on the type of particle or photon, range of alpha particles in source material and p
its energy, its mass, the type of material traversed, the maximum fraction of R which particles can
its mass, and density. What we are concerned spend in source and still be counted.
with here is not the loss of particle energy or
particles themselves as they pass through a sub-
stance but the effects caused by particles deflected Radiation shield The detector may be housed in
from the direct path between radioactive source a thick radiation shield to reduce the natural
and detector. It is found that some particles are background which is found everywhere to a lower
absorbed into the material surrounding the level where the required measurements can be
source. Others are deflected away from it but made. The design of such natural radiation
are later re-scattered into the detector, so increas- shields is a subject in itself, and can range from
ing the number of particles in the beam. Some of a few centimeters of lead to the massive battleship
these deflected particles can be scattered at more steel or purified lead used for whole-body moni-
than 90” to the beam striking the detector-these tors, where the object is to measure the natural
are called “backscattered” since they are scattered radiation from a human body.
back in the direction from which they came. Scat-
tering also occurs with photons, and this is parti- 22.2.4.1 Alpha-detector systems
cularly demonstrated by the increase in counting
rate of a beam of gamma rays when a high-Z The simplest alpha detector is the air-filled ion-
material such as lead is inserted into the beam. ization chamber, used extensively in early work
on radioactivity but now only used for alpha detect-
ion in health physics surveys of spilled activities on
Backscattering Backscattering increases with benches, etc. Even this application is seldom
increasing atomic number Z and with decreasing used, as more sensitive semiconductor or scintil-
energy of the primary particle. For the most com- lation counters are usual for this purpose. Thin-
monly used sample planchets (platinum) the window ionization or gas-proportional counters
backscattering factor has been determined as 1.04 can also be used, or internal counters in which the
for an ionization chamber inside which the sam- sample is inserted into the active volume of a gas
ple is placed, known as a 50 percent chamber. counter. Due to the intense ionization produced
by alpha particles it is possible to count them
Absorption As the particles to be detected may in high backgrounds from other radiation such
be easily absorbed it is preferred to mount source as betas and gamma rays by means of suitable
and detector in an evacuated chamber, or insert discrimination circuits.