6.13 Quenching Corrections
117
measurements are being made ( i.e., one should use a long-lived energetic γ -emitter
of a life greater than one week at least). This technique is very useful in making
correction for quenching due to colored samples.
6.14 Effect of Multiple Type Radiations on Counting
Most of the radioactive isotopes decay by emitting more than one type of radiation
(Fig. 2.7). For example,
60 Co, decays by emitting β- and γ -rays. Should we set the
counter to measure either γ -rays or β-particles of this isotope? Is it possible to set
the counter such that we can count the activity due to one of these radiations such
that the effect of other does not influence the total activity? Because, for example,
when γ -rays are counted then β-particle will also get recorded, unless some special
effort is not made to prevent β-particles entering the counter. If no special effort is
made to prevent β-particles entering into the counter, then the percentage contribution of β-particles into the recorded count rate would depend upon the nature of
setting of the counting condition. In counting of such isotopes, if the instrument is
set at the photopeak, then the contribution due to β-particles would be negligible.
Whereas, if setting is made to count the entire spectra of γ -radiation, contribution
due to β-particle would influence the counting rate considerably. In the latter case,
there is likelihood of interference of β-particles unless, effort is made to isolate the
penetration of β-particles into the counter. However, it is not feasible with liquid
samples using liquid scintillation counting system.
For liquid sample, elimination of β-particle can be done by counting with solid
phosphor (NaI) counter by setting to count at the photopeak of γ -ray. Now, we shall
discuss various conditions of counting to measure the activity of isotopes which emit
more than one type of radiation.
• The influence of β-particle can be reduced by placing a metallic screen (which
would stop β-particles but not the γ -rays) between the source and the detector. This screen is made of a low atomic number substance to cut down the
external Bremsstrahlung radiation, as well as β-particles. However, effect of
Bremsstrahlung’s radiation can be eliminated by the pulse height analyzer, because
they appear as an electromagnetic radiation of low energy and low intensity.
• The influence of β-particles can be eliminated by allowing it to annihilate near
the source, so that the latter may be considered responsible for getting 0.511
MeV photon emission in the β-spectrum. This is accomplished by placing the
source in between two aluminum plates thick enough for the positrons to lose all
their energies. Under these conditions, emission of 0.511 MeV photons would be
observed. By making the appropriate setting in pulse height analyzer, the effect of
0.511 MeV photon can be eliminated.
• For materials which decay by γ -rays, the emitted photons compete with internal
conversion process. For radio-nuclides of low or medium atomic number elements,
117
measurements are being made ( i.e., one should use a long-lived energetic γ -emitter
of a life greater than one week at least). This technique is very useful in making
correction for quenching due to colored samples.
6.14 Effect of Multiple Type Radiations on Counting
Most of the radioactive isotopes decay by emitting more than one type of radiation
(Fig. 2.7). For example,
60 Co, decays by emitting β- and γ -rays. Should we set the
counter to measure either γ -rays or β-particles of this isotope? Is it possible to set
the counter such that we can count the activity due to one of these radiations such
that the effect of other does not influence the total activity? Because, for example,
when γ -rays are counted then β-particle will also get recorded, unless some special
effort is not made to prevent β-particles entering the counter. If no special effort is
made to prevent β-particles entering into the counter, then the percentage contribution of β-particles into the recorded count rate would depend upon the nature of
setting of the counting condition. In counting of such isotopes, if the instrument is
set at the photopeak, then the contribution due to β-particles would be negligible.
Whereas, if setting is made to count the entire spectra of γ -radiation, contribution
due to β-particle would influence the counting rate considerably. In the latter case,
there is likelihood of interference of β-particles unless, effort is made to isolate the
penetration of β-particles into the counter. However, it is not feasible with liquid
samples using liquid scintillation counting system.
For liquid sample, elimination of β-particle can be done by counting with solid
phosphor (NaI) counter by setting to count at the photopeak of γ -ray. Now, we shall
discuss various conditions of counting to measure the activity of isotopes which emit
more than one type of radiation.
• The influence of β-particle can be reduced by placing a metallic screen (which
would stop β-particles but not the γ -rays) between the source and the detector. This screen is made of a low atomic number substance to cut down the
external Bremsstrahlung radiation, as well as β-particles. However, effect of
Bremsstrahlung’s radiation can be eliminated by the pulse height analyzer, because
they appear as an electromagnetic radiation of low energy and low intensity.
• The influence of β-particles can be eliminated by allowing it to annihilate near
the source, so that the latter may be considered responsible for getting 0.511
MeV photon emission in the β-spectrum. This is accomplished by placing the
source in between two aluminum plates thick enough for the positrons to lose all
their energies. Under these conditions, emission of 0.511 MeV photons would be
observed. By making the appropriate setting in pulse height analyzer, the effect of
0.511 MeV photon can be eliminated.
• For materials which decay by γ -rays, the emitted photons compete with internal
conversion process. For radio-nuclides of low or medium atomic number elements,