Low Level Liquid Scintillation Counting of Radioactivity - PerkinElmer

application Note
Liquid Scintillation Counting
Low Level Liquid
Scintillation Counting
of Radioactivity in
Food Products
Author
Lauri Kaihola
Wallac Oy, P.O.Box 10, SF-20101 Turku, FINLAND
Low level liquid scintillation spectroscopy can be successfully used for counting alpha and beta activity in food products
to monitor the natural radioactivity, contamination by nuclear fallouts, contaminants from nuclear power stations or
fuel reprocessing plants. It also provides means for surveying the synthetic additives in food on the basis of the natural
radioactivity to be found in the products.
Cosmogenic carbon isotope is first discussed as an example. This isotope is a natural ingredient of biological food products
and acts as an important tracer, revealing any deviations from the origin of the food.
C-14 IN NATURE
Cosmic radiation produces C-14 ('radiocarbon') in stratosphere by neutron bombardment of nitrogen
14 1 14 1
N + n --> C + p
7 0 6 1
The production rate is 7.5 kg isotope C-14 yearly. The C-14 concentration stays approximately constant due to rapid mixing
of the atmosphere, although the cosmic intensity is higher at the poles due to the deflection of charged cosmic particles
along the magnetic field lines of the earth (corresponding to neutron intensities in the ratio 5:1 at the poles and the equator,
respectively). Consequently, C-14 atoms combine to form 'heavy' CO2, which, except in the radioactive decay (and isotopic
fractionation effects), is indistinguishable from the ordinary carbon dioxide. The total amount of C-14 on earth in equilibrium
is 62 tons, which is 10exp(-10) per cent of all carbon in biosphere, atmosphere and oceans.
C-14 circulates through the reservoirs in the same pattern as the ordinary carbon.

Decay of C-14 is through beta emission, where simultaneously emitted neutrino takes part of the decay energy
and therefore beta particle is not monoenergetic but has a long tailed energy spectrum with maximum energy
150 keV and the mean energy 30 keV:
14 14 -
C --> N + b + y
6 7
Half life in the above decay is 5730 +- 40 a.
PRINCIPLE OF C-14 DATING
All the living matter becomes marked with C-14, which decays exponentially after the death of the object. One
is then able to calculate the age t of the object by taking a sample and comparing its present activity C with the
activity at its death Co
5730 Co
t = ----- ln (-----).
ln2 C
Obviously one has to make the assumption that the C-14 concentration in the exchange reservoir has been
constant or is known at the time of death of the object.
This principle of radiocarbon dating was proposed by W.F. Libby in 1946 and he was conferred a Nobel prize in
chemistry for it in 1960. The same principle has then been applied using other cosmogenic isotopes as tracers
(K-Ar dating etc.).
MAN-MADE FLUCTUATIONS IN C-14 CONCENTRATION
The constancy of the C-14 content is quite a reasonable assumption to make and from measurements on known
age samples it appears that within a few percent the assumption is valid back to 1500 B.C. There have been
fluctuations caused by climate as well as short and long term variations due to the sunspot activity and changes
of the geomagnetic moment. There are, however, also man-made fluctuations in the C-14 content:
1) 'Fossil effect' or the Suess effect
The combustion of coal and oil releases into the atmosphere large quantities of CO2 in which the C14 has
decayed long ago. This 'dead' carbon dilutes the C-14 concentration in the air. Therefore, the activity of wood
samples, grown say in 1950 (prior to hydrogen bomb testing) is in fact lower than in samples grown in 1850
prior to the industrial revolution despite the decay that has occurred in the latter.
2) Effect of nuclear weapon tests
It has been estimated that neutrons released in fission and fusion explosions until 1962 have caused the
formation of about 2 tons of C-14. If this were distributed uniformly, there would be an excess of 3 % of C-14.
However, there is a 'hold-up' in the atmosphere and so the effect is much greater and the concentration was
twice the prebomb one in 1963 (see fig. 2) and has since decreased to 30 % excess. (The bomb effect has
been useful in studying the carbon cycle because of the prohibition of the atmospheric and ocean tests in 1963
by an agreement between the superpowers.)
2

COUNTING OF C-14 BY LOW LEVEL LIQUID SCINTILLATION SPECTROMETRY
In most cases for radiocarbon dating the organic samples are converted into benzene (C6H6) for liquid
scintillation counting by first combusting into CO2, which is further, synthesized into lithium carbide with lithium
above 750C
2CO 2
+ 10Li ---> Li2C 2
+ 4Li 2
O.
This is then hydrolized into acetylene in reaction
Li2C 2
+ 2H 2
O ---> C 2
H 2
+ 2 LiOH.
Acetylene is converted in the presence
of a catalyst into benzene.
Counting of C-14 activity is accomplished by adding scintillation agent, e.g. butyl-PBD in the benzene sample.
LIQUID SCINTILLATION COUNTING OF FOOD
A) Disclosing adulteration of food
There are legislative requirements that synthetic food or alcohol can not be distributed for use. In most cases
standard chemical methods are satisfactory to monitor unauthorized use of synthetic substituents. In some other
cases they are not identifying synthetic materials of identical composition. Synthetic food may be made out of
petroleum derived raw material having minimal content of C-14 activity. In those cases it is possible to enforce
the law by analysing food for the content of C-14 (the same principle can be used for dating of wines). Normal
C-14 activity for fresh food will be around 18 dpm/g carbon (= modern carbon). Lower activities decline due to
dilution by synthetic raw materials.
Counting can be accomplished on CO2 produced by combustion and dissolved in the scintillation liquid or
by synthesizing the food into benzene like in radiocarbon dating. The former method does not allow large
concentrations of carbon to be introduced in the cocktail and the latter one means quite elaborate synthesis.
Another way is to ferment the product, starch, sugar; etc. into alcohol with can be counted directly in a cocktail.
Measurement of the C-14 content of ethanol (or gasohol in some countries, which must originate from
renewable sources, i.e. contains only 'modern' carbon) can be done directly mixing with the scintillation liquid.
The mixing ratios may be up to 3:2 in ethanol:scintillation liquid ratio. It is advantageous to use alcohol with less
than 10 % water in it.
Lower limit of detection in Quantulus is 0.1 dpm/g carbon for 100 min counting time and 3 sigma resolution
criterion (Schönhofer). This means that less than a 1 per cent dilution of 'modern' alcohol by synthetic one is
revealed at 99.5 % probability in 100 min counting.
Direct counting of caffeine and cinnamic aldehyde has been carried out by Noakes and Hoffman to evaluate the
presence and degree of adulteration.
B) Counting of radioactive contamination of food
Recent nuclear fallout has clearly shown the need to be able to monitor the radioactivity of food by reasonably
fast methods. In most instances gamma spectrometry is the appropriate method to apply. The semiconductor
detectors have excellent energy resolution enabling efficient identification of the often numerous low activity
isotopes. Low level liquid scintillation spectrometry has its areas of application as is seen in the following
examples.
3

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.
4

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