Low Level Liquid Scintillation Counting of Radioactivity - PerkinElmer

Cs-137
Cs-137 can be monitored on the basis of its 662 keV gamma emission, which is actually from the metastable
daughter Ba-137m. Actually Cs-137 first decays by beta emission, with 82 % probability through 500 keV beta
emission into Ba-137m and with 8 % probability directly to Ba-137. Beta emission can be very well measured by
liquid scintillation spectrometry. Identification of this isotope is further determined by the monoenergetic conversion
electron peak at 625 keV.
Sr-90/Y-90
These isotopes have only beta decay mode and therefore liquid scintillation method is a very suitable one to use.
Alpha counting
Pulse shape analysis provides a means for alpha/beta separation on the basis of their different pulse lengths (shapes).
Fluorescent decay of scintillation light is composed of prompt and delayed components; most of the light is
produced in the prompt component. The amount of the light in the delayed component has long been known
to be dependent on the particle or decay type: electrons originating from beta decay, gamma and X-rays are the
predominant sources of prompt fluorescence, whereas alpha decay contributes more to the delayed component
giving longer pulses than those produced by electrons. Beta particles are about ten times more effective in producing
light than alpha particles. This is why the beta spectrum covers the alpha spectrum range in a scintillation counter
although alpha particle energies are higher, typically 4 to 9 MeV.
The Pulse Shape Analyser works by producing a PSA value, which relates the pulse length to the pulse amplitude.
Thus the amplitude dependence of the pulse length is minimized. Counting of radon in water is a good example of
the application of the pulse shape analyser. Direct mixing of the sample is also in this case a sufficient method of
sample preparation.
Rn-222 is a daughter of radium-226 in the uranium decay series. In Finland the bedrock is radioactive to some extent
to introduce alpha activity in waters from drilled wells. (Problematic is radon from soil in some areas penetrating into
houses and representing the major radioactive load.)
The alpha spectrum of Rn-222 shows four peaks, Rn-222 and Po-214 intermixed and Po-218 very well separated at
a higher energy. In the equilibrium we then have three times of the alpha activity of radon present in the sample.
Radium and its predecessors do not very much dissolve in water to be seen in the spectrum. The beta emissions in
the series are by Pb-214 and Bi-214 and by the next isotope in the series after Po-214 is Pb-210 with 22 a half-life,
thus being of low activity.
Very low backgrounds are achieved in alpha counting, typically 0.05 cpm in teflon counting vial and 0.3 in glass vials.
At Am-241 alpha energy (5.5 MeV) these values lead to LLD = 0.1 and 0.3 mBq/sample. When one measures higher
alpha energies like Po-214 (7.6 MeV), the background is even less, 0.005 cpm in a teflon vial and LLD = 0.03 mBq.
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