10
General Features of Radioisotopic Methodology
which is convenient for counting radioactivity of low-energy emitters. Such
a count is referred to as integral counting, while the count within the selected
window of voltages is called differential counting. Scalers-spectrometers
using modem liquid scintillation counting may count in two (or even three)
channels simultaneously. One of the advantages of such counting is the
ability to discriminate between sIgnals produced by the sample or appearing
due to the background on the basis of their different energy spectra. The
second channel can also be used for quenching correction (see below).
The use 0 two channels has the advantage of counting simultaneously two isotopes in one sample with a different radiation energy and, correspondingly,
with different spectra.
The background in normal modem scale counts is between 12 and 20cpm.
The background counts are induced mainly by cosmic rays. When reacting with
the air and other matter they produce high-energy mesons, electron a-rays,
which bombard the counting sample fluors and induce the emission of
photons. The lead protection reduces the flow of these particles and y-rays, but
not completely. The counting samples themselves can contain unknown nonradioactive phosphorescent materials. Therefore there is in some cases reason
to count the background in nonradioactive blanks of identical composition.
Definite noise could also be produced as thermoionic emission from the
photosensitive layer of the photomultiplier tubes.
1.2.2.2 Practical Use of Liquid Scintillation Counting
The counting samples for the liquid scintillation measurement of radioactivity are prepared in standard vials of borosilicate glass or polyethylene. The
latter are completely transparent for photons emitted by the counting sample,
but are less resistant to heat or chemical treatment of radioactive material
before counting. The vials have standard volumes of 30 and 10 ml. In larger
vials, the maximal volume of the cocktail is 20ml and the minimal5ml. In small
vials, minimal volume can be about 2 ml.
The choice of practical scintillation cocktails is rather limited. They are
produced ready for use by companies specialized in the fabrication of instruments and chemicals for nuclear laboratories, such as Amersham, Searle,
Beckman, or Packard. The list includes mostly toluene- and dioxane-based
cocktails. As primary fluor, PPO (2.5 diphenyloxazole) is used in them at a concentration of 3-6 g I-I, and as second fluor POPOP (p-bis[2(5-phenyloxazole)]benzene) at 0.2-1 gl-I. To bring radioactive sample materials which are
not otherwise soluble in the above solvents toluene or dioxane into a homogenous system, other solvent mixture are used, which are called secondary solvents. They contain dioxane or toluene with addition of substances which
increase their ability to dissolve and mix with water (ethanol, methanol,
methoxyethanol, ethylene glycol) or which increase the counting efficiency by
General Features of Radioisotopic Methodology
which is convenient for counting radioactivity of low-energy emitters. Such
a count is referred to as integral counting, while the count within the selected
window of voltages is called differential counting. Scalers-spectrometers
using modem liquid scintillation counting may count in two (or even three)
channels simultaneously. One of the advantages of such counting is the
ability to discriminate between sIgnals produced by the sample or appearing
due to the background on the basis of their different energy spectra. The
second channel can also be used for quenching correction (see below).
The use 0 two channels has the advantage of counting simultaneously two isotopes in one sample with a different radiation energy and, correspondingly,
with different spectra.
The background in normal modem scale counts is between 12 and 20cpm.
The background counts are induced mainly by cosmic rays. When reacting with
the air and other matter they produce high-energy mesons, electron a-rays,
which bombard the counting sample fluors and induce the emission of
photons. The lead protection reduces the flow of these particles and y-rays, but
not completely. The counting samples themselves can contain unknown nonradioactive phosphorescent materials. Therefore there is in some cases reason
to count the background in nonradioactive blanks of identical composition.
Definite noise could also be produced as thermoionic emission from the
photosensitive layer of the photomultiplier tubes.
1.2.2.2 Practical Use of Liquid Scintillation Counting
The counting samples for the liquid scintillation measurement of radioactivity are prepared in standard vials of borosilicate glass or polyethylene. The
latter are completely transparent for photons emitted by the counting sample,
but are less resistant to heat or chemical treatment of radioactive material
before counting. The vials have standard volumes of 30 and 10 ml. In larger
vials, the maximal volume of the cocktail is 20ml and the minimal5ml. In small
vials, minimal volume can be about 2 ml.
The choice of practical scintillation cocktails is rather limited. They are
produced ready for use by companies specialized in the fabrication of instruments and chemicals for nuclear laboratories, such as Amersham, Searle,
Beckman, or Packard. The list includes mostly toluene- and dioxane-based
cocktails. As primary fluor, PPO (2.5 diphenyloxazole) is used in them at a concentration of 3-6 g I-I, and as second fluor POPOP (p-bis[2(5-phenyloxazole)]benzene) at 0.2-1 gl-I. To bring radioactive sample materials which are
not otherwise soluble in the above solvents toluene or dioxane into a homogenous system, other solvent mixture are used, which are called secondary solvents. They contain dioxane or toluene with addition of substances which
increase their ability to dissolve and mix with water (ethanol, methanol,
methoxyethanol, ethylene glycol) or which increase the counting efficiency by