for counting. The primary fluors include 2,5-diphenyloxazole (PPO), 2,5-
bis-2-(5-T-butyl-benzoxazolyl)-thiophene (BBOT), and p-terphenyl, of
which PPO is most commonly used in a concentration of 5 g/L.
Toluene, xylene, and dioxane are the most common solvents that easily
dissolve the primary fluor and often the radioactive sample, which is a
requirement for a good solvent. These solvents, however, are poorly misci-
ble in water, and therefore their disposal in the sewer system is restricted.
For this reason, biodegradable solvents such as linear alkylbenzene and
phenylxylylethane are widely used. Counting vials are usually glass or
plastic, but the latter is not used when toluene or xylene is used as a solvent
because the solvent tends to dissolve plastic.
When radiations pass through the solvent, electrons are released from
the solvent molecules after absorption of radiation energies. These elec-
trons transfer energy to primary fluor molecules, which then emit light
photons for further processing by PM tubes and associated electronics. The
wavelength of these light photons may be somewhat shorter than required
for the spectral sensitivity of the photocathode of the PM tube. This mis-
match is rectified by adding a secondary fluor or solute, called the wave-
length shifter, to the scintillating solution. The wavelength shifter absorbs
the light photons emitted by the primary fluor and reemits them with a
longer wavelength, which is more suitable for the photocathode of the PM
tube. The compound 1,4-bis-2-(5-phenyloxazolyl)-benzene (POPOP) is
most commonly used as a secondary solute in a concentration of about
0.1%.
An attempt is always made to keep the radioactive sample in solution in
the liquid scintillator. Solubilizing agents are added to improve dissolution
of specific samples, and the common example is the hydroxide of Hyamine
10-X used in counting tissue samples.
In liquid scintillation counting, quenching is a problem caused by inter-
ference with the production and transmission of light, which ultimately
reduces the detection efficiency of the system. Quenching can be of the fol-
lowing types:
1.Chemical type, resulting from interference in energy transfer by sub-
stances such as samples or extraneous materials (e.g., dissolved O 2 )
2.Color type, resulting from absorption of light photons by colored sub-
stances, such as hemoglobin, before striking the PM tube
3.Dilution type, resulting from relatively large dilution of the scintillation
mixture, in which case many light photons may be absorbed by the
diluted sample.
4.Optical type, resulting from absorption of light by a dirty vial containing
frost or fingerprints.
Quenching must be corrected to obtain accurate counting of samples,
and three methods have been adopted for this purpose, namely, internal
standard method, channel ratio method, and external standard method.
94 8. Scintillation and Semiconductor Detectors