Physics and Engineering of Radiation Detection

348 Chapter 6. Scintillation Detectors and Photodetectors

large volume liquid scintillator detectors are already in operation and others are
being planned (see, for example (9; 1)).
One of the features of liquid scintillators is their high dependence on the relative
densities of different solvents in the base liquid. Generally the amounts of primary
and secondary fluors are selected according to the application requirements. Since
the properties of organic liquid scintillators vary considerably from one type to
another and from one fluor concentration to another therefore it is not possible to
assign any typical values to them. Table.6.2.2 lists properties of some on of the
organic liquid scintillators that have been investigated. The reader should, however,
be warned that these values should not be taken to represent most organic liquid
scintillators.

Table 6.2.2: Wavelength of maximum emissionλmaxand decay constantτof some
organic liquid scintillators (39).

Liquid λmax(nm) τ(ns)

Toluene with 5gm/lofp-terphenyl 355 2.2

Toluene with 5gm/lofp-terphenyl and

0.02gm/lof 2-(1-naphthyl)-5-phenyloxazole 415 ≤3.2

Toluene with 3gm/lof 2,5-diphenyloxazole 382 ≤3.0

Phenylcyclobexane with 3gm/lofp-terphenyl 355 <2.9

Phenylcyclobexane with 3gm/lofp-terphenyl and

0.01gm/lof 1,6-diphenyl-1,3,5-hexatriene 450 <8.0

6.2.D Crystalline Scintillators

Organic scintillating crystals have been found to be advantageous over their inor-
ganic counterparts due to the following two reasons.

Non-hygroscopicity

Small back-scattering
We will see later that absorption of moisture from atmosphere is one of the
major problems of inorganic crystalline scintillators. Organic crystals do not have
this vulnerability to moisture and therefore do not required to be stored and used
in sealed containers. The other good thing is that organic crystals are mostly made
of hydrogen, which makes the probability of backscattering extremely small. Also
presence of dense hydrogen makes them suitable for use for neutron spectroscopy.
In the following we will look at some commonly used crystalline organic scintil-
lators and discuss their properties and applications.