Page 543 - Instrumentation Reference Book 3E

526 Nuclear instrumentation technology

introduced, and now the range of organic scintil- which cause them to scintillate when bombarded
lators available is very great. The plastic and by ionizing particles or ionizing radiation. An
liquid organic scintillators are generally cheaper acrylic such as methyl methacrylate can also be
to manufacture than inorganic scintillators, and doped to produce a scintillating material but not
can be made in relatively large sizes. They are with the same high light output as the polyvinyl-
generally much faster in response time than most toluene-based scintillators. It can be produced,
inorganic scintillators, and being transparent to however, much more cheaply and it can be used
their own scintillation light can be used in very for many high-energy applications.
large sizes. Table 22.6 gives the essential details of Plastic scintillators have the ability to be
a large range or plastic and liquid scintillators. molded into the most intricate shapes in order
The widest use of organic scintillators is prob- to suit a particular experiment, and their inert-
ably in the field of l~quid-scint~llation counting, ness to water, normal air, and many chemical
where the internal counting of tritium, I4C, 55Fe substances aIiows their use in direct contact
and other emitters of low-energy beta particles at with the activity to be measured. Being of low
low activity levels is being carried out on an atomic number constituents, the organic scintil-
increasing scale. Biological samples of many lators are preferred to iiiorgan~cs such as Nal:
types have to be incorporated into the scintillator (TI) or CsX (T1> for beta counting, since the
and it is necessary to do this with the minimum number of beta particles which are scattered
quenching of the fight emitted. minimum chemi- out of an organic scintillator without causing
luminescence, as well as minimum effort-the last an intcraction are about 8 percent, whereas in
being an important factor where large numbers of a similar Nal (Tl) crystal the number scattered
samples are involved. out would be 80-90 percent.
At low beta energies the counting equipment is When used to detect X- or gamma-rays organic
just as important as the scintillator. Phosphores- scintillators differ in their response compared
cence is rduced to a minimurn by the use of special with inorganic scintillators. Where inorganic
vials and reflectors. Chemiluminescence, another scintillators in general have basically three main
problem with biological and other samples, is not types of response, called photoelectric, Compton,
completely solved by the use of two photomulti- and pair production, because of the high 2
pliers in coincidence viewing the sample. This must (atomic weight) of the materials of the inorganic
be renioved or reduced, for example, by removing scintillators, the lour-Z characteristics of the basic
afkalinity andor peroxides by acidifying the solu- carbon and similar components in organic scin-
tion before mixing with the scintillator. tillators lead only to Compton reactions, except
at very low energies, with the result that for a
monoencr~etic gamma emitter, the spectrum pro-
duced is a Compton distribution^
22.2.2.3 Loaded wgdc ~c~~~~il~~~ps
For study of the basic int~i-actions between
fn order to improve the detection efficiency of gamma and X-rays, and scintillation materials,
scintillatars for certain types of particles or see Price (1964), as these reaction studies are
ionizing or non-ionizing radiations small quan- beyond the scope of this chapter.
tities of some substances can be added to scin- The ability to produce simple Compton dis-
tillators without degrading greatly the light tribution spectra has proved of considerable use
output, It must be borne in mind that in nearly in cases where one or two isotopes have to be
all cases there is a loss of light output when measured at low intensities, and a large inor-
foreign substances are added to an organic ganic Nal (TI) detector might be prohibitively
scintillator, but the gain in detection efficiency expensive. Such is the case with whole-body
may be worth a slight drop in this output. counters used to measure the 40K and 137Cs
Suitable loading materials are boron, both nat- present in the human body-the 40K being the
ural boron and boron enriched in 'OB and natural activity present in all potassium, the
gadolinium -these are to increase the detection 137 Cs the result of fallout from the many atomic
efficiency for neutrons. Tin and lead have been bomb tesKS. Similarly, when measuring the
used to improve the detection efficiency for potassium content of fertilizer a plastic scintil-
gamma rays and have been used in both liquid lator can carry this out more cheaply than an
and plastic scintillators. inorganic detector. The measur~ment of moist-
ure in soil by gamma-ray ~i-a~ismission through
a suitable sample is also perforn~ed more easily
12.2.2.4 Plrrstic ~&~i~f~~~~€Ol~S
with a plastic scintillator, since the fast decay
Certain plastics such as polystyrene and poly- time of the organics compared with, say, Naf
vinyltoluene can be loaded with small quantities (Ti) al1cw.s higher counting rates to be used,
of certain substances such as p-terphenyl, etc., with consequent reduction of statistical errors.