6.12 β-counting
111
It is worth remembering that the characteristic obtained for β-counting by the
scintillation counter is not as universal as one gets with G.M. counter. The settings
obtained with the scintillation counter correspond to the isotope for which the particular amplification and EHT settings have been observed. When isotope or amplification or EHT values are changed, maxima of the parabola also changes. Therefore, it is
necessary to find the best operating condition, i.e., the parameters like amplification
and EHT for the β-particles emitted by the radioactive isotope to be counted.
6.13 Quenching Corrections
In liquid scintillation counting, one has to ensure whether sample or solvent absorbs
visible photons produced by interaction of radiation with scintillator. As mentioned
earlier, all colored samples, water, halogenated solvents (CCl 4 , CHCl 3 , etc.), alcohols, acetone, etc., are known to absorb visible photons produced from the scintillator. These chemicals/solvents are designated as quenchers. In order to appreciate
the effect of quenching on the spectrum of β-particles, spectrums of β-particles of
Chlorine-36, (LiCl labeled with Chlorine-36) dissolved in toluene (Fig. 6.8A) as a
solvent and in acetone as a solvent are shown in Fig. 6.8B. These spectrums are
taken under similar condition (i.e., at the same operating conditions of the scintillation counter). It can be seen that the addition of acetone and water reduces β-spectrum
as well as shifts the whole spectrum toward lower energy. These two figures suggest
that it is essential to find out the exact percentage loss of activity due to the quencher
(acetone/water mixture) for getting an accurate activity of the sample. Many methods have been devised to correct the count rate for loss of the activity due to such
quenching effects. Among these, some of them are discussed here.
One of the method of quenching correction is by drawing a graph of percentage
loss of sample’s activity against the volume of quencher added to the solution containing known but fixed quantity of the radioactive sample. A typical nature of such
graph showing the decrease in activity recorded due to the presence of quencher is
shown in Fig. 6.9.
This graph can be used for quenching correction. By knowing the volume of
quencher added into counting sample, the corresponding percentage loss of activity
due to quencher can be calculated from the graph shown as f 1 , for example, of
0.2 ml of added quencher (Fig. 6.9). Water was used as a quencher solvent for this
experiment. This correction is valid only for the isotope for which this type of graph
is drawn and for operating condition of scintillation counting system. If there is any
change in the isotope, solvent or operating condition, a fresh quenching correction
has to be determined. This method is also known as internal standard technique.
Another method for quenching correction is when the amount of quencher present
in the counting sample is not known (known as external standard method). Both
these methods are discussed in detail in the foregoing sections.
111
It is worth remembering that the characteristic obtained for β-counting by the
scintillation counter is not as universal as one gets with G.M. counter. The settings
obtained with the scintillation counter correspond to the isotope for which the particular amplification and EHT settings have been observed. When isotope or amplification or EHT values are changed, maxima of the parabola also changes. Therefore, it is
necessary to find the best operating condition, i.e., the parameters like amplification
and EHT for the β-particles emitted by the radioactive isotope to be counted.
6.13 Quenching Corrections
In liquid scintillation counting, one has to ensure whether sample or solvent absorbs
visible photons produced by interaction of radiation with scintillator. As mentioned
earlier, all colored samples, water, halogenated solvents (CCl 4 , CHCl 3 , etc.), alcohols, acetone, etc., are known to absorb visible photons produced from the scintillator. These chemicals/solvents are designated as quenchers. In order to appreciate
the effect of quenching on the spectrum of β-particles, spectrums of β-particles of
Chlorine-36, (LiCl labeled with Chlorine-36) dissolved in toluene (Fig. 6.8A) as a
solvent and in acetone as a solvent are shown in Fig. 6.8B. These spectrums are
taken under similar condition (i.e., at the same operating conditions of the scintillation counter). It can be seen that the addition of acetone and water reduces β-spectrum
as well as shifts the whole spectrum toward lower energy. These two figures suggest
that it is essential to find out the exact percentage loss of activity due to the quencher
(acetone/water mixture) for getting an accurate activity of the sample. Many methods have been devised to correct the count rate for loss of the activity due to such
quenching effects. Among these, some of them are discussed here.
One of the method of quenching correction is by drawing a graph of percentage
loss of sample’s activity against the volume of quencher added to the solution containing known but fixed quantity of the radioactive sample. A typical nature of such
graph showing the decrease in activity recorded due to the presence of quencher is
shown in Fig. 6.9.
This graph can be used for quenching correction. By knowing the volume of
quencher added into counting sample, the corresponding percentage loss of activity
due to quencher can be calculated from the graph shown as f 1 , for example, of
0.2 ml of added quencher (Fig. 6.9). Water was used as a quencher solvent for this
experiment. This correction is valid only for the isotope for which this type of graph
is drawn and for operating condition of scintillation counting system. If there is any
change in the isotope, solvent or operating condition, a fresh quenching correction
has to be determined. This method is also known as internal standard technique.
Another method for quenching correction is when the amount of quencher present
in the counting sample is not known (known as external standard method). Both
these methods are discussed in detail in the foregoing sections.