Alpha
spectrometry sample preparation
Heinz Surbeck
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In Switzerland we have limits for radionuclides in food not only for artificial
radionuclides but also for naturally occurring ones. Members of the uranium
(
238 U,
235 U
) and
thorium
series are classed according to their radiotoxicity.
The first class contains 224 Ra, 228 Th, 234 U, 235 U and 238 U. The limit for e.g. liquid food for this class is at 10 Bq/l( units ) for the sum of all radionuclides in this class. The more radiotoxic nuclides 210 Pb, 210 Po, 226 Ra, 228 Ra, 230 Th, 232 Th and 231 Pa are in class 2. The limit for this class is set to 1 Bq/l( units ) (for liquid food). The limits for food in general are a factor of 5 higher. For seafood that naturally may contain high 210 Po levels and for some other food of minor importance for Swiss consumers higher levels are tolerated. When considering the most important food, drinking water, fortunately not all the nuclides mentioned are to be determined. Large differences in solubility and natural abundance make that 234U and 238 U dominate in class 1. For class 2 these main nuclides are 226 Ra and 228 Ra. The latter can be easily determined by gamma spectrometry (via 228 Ac). There remain 3 alpha emitters to be analysed. Whereas many environmental radioactivity labs are well equipped with alpha spectrometers they frequently hesitate to do all the painful and expensive classical radiochemistry. To make enforcement by regional (cantonal) labs more attractive, simplified analytical methods had to be offered, at least for drinking water. |
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The ions of the nuclides to be analysed in general make only a very small part of the total number of ions present in an aqueous sample. One thus needs a method concentrating the ions of interest selectively. Selective extraction can be used with liquid scintillation counting (LSC) or as a first step to make thin films by electrodeposition. Direct selective adsorption on thin films and subsequent measurement with a semiconductor detector is the possibility shown here. One thus avoids time consuming and expensive chemical steps still having a far better energy resolution than with liquid scintillation measurements. |
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MnO 2 efficiently adsorbs radium even at high Ca concentration. After having improved methods described earlier ( Moore and Reid, 1973 , Bland 1979 , Glöbel and Berlich, 1983 , Surbeck et al.. 1989 , Surbeck, 1995 ) we now are able to produce selectively adsorbing MnO 2 thin films on polyamide substrates. Exposing a 20mm x 20mm sheet for 6 h to an untreated, stirred 100 ml sample extracts typically more than 80% of the radium. The dried film is then measured with a solid state alpha detector. Energy resolution is nearly as good as for electroplated sources. After counting times of one day one typically gets a detection limit of 10 mBq/l( units ) for 226 Ra. ( application note ) |
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Films are produced by exposing a polyamide sheet to a hot KMnO 4 solution. There are some doubts that the deposit on the polyamide is actually MnO 2 ; it appears similar but a detailed analysis is still required Adsorption efficiency depends on the sample's chemical composition. At mg/l levels, barium competes with radium for the adsorption sites. Exposing at least two films subsequently to the same sample solves the problem. Each film (from the same production batch) adsorbs the same fraction of radium still present in solution . |
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For low sample activities ( tens of mBq/l(
units
)) count rates are too low to use this
method. In this case a known
226
Ra activity (some 10 mBq(
units
)) is added to the
sample before exposing the second film. It is essential to have a low Ba
(<< mg/l)
226
Ra standard solution. To strongly mineralised sparkling
waters some Na
2
EDTA is added to complex calcium. This prevents precipitation when CO
2
is
lost. There is no visible effect of this complexation on radium adsorption
fficiency (up to 1g/l Na
2
EDTA added).
Apart from radium, 210 Po is also adsorbed with a high efficiency. Uranium adsorption efficiency depends on the film preparation method. High efficiencies have been reported ( Crespo et al., 1993 ), but our films adsorb only few uranium. In general, less than 5% of the 238 U or 234 U activity are adsorbed, but there are large variations in this adsorption efficiency. These variations may be due to differences in the chemical form in which uranium is present ( Langmuir 1978 ). CO 2 forms quite stable anionic or neutral complexes with the cationic uranyl (UO 2 ). As the MnO 2 film seems to act like a cation exchanger this complexed uranium fraction is probably not adsorbed. Lowering the pH to below 4 would help to destroy these complexes, but at this pH the MnO 2 film adsorbs neither uranium nor radium. Even at the good energy resolution with these MnO 2 films, 234 U alpha energies are too close to the 226 Ra energies to be resolved. Unfortunately one can't use the well separated 238 U lines to correct for the 234 U contribution to the 226 Ra line. 234 Uand 238 U are rarely in equilibrium in drinking water samples. To correct for this contribution one has to measure the 234U / 238 U ratio by some other method. This need and the fact that uranium adsorption on MnO 2 turned out to be low and very variable has led us to look for a thin film adsorbing uranium far better than radium. In addition the film has to work at low pH to avoid uranyl complexation. |
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A
commercially available cation exchanger containing diphosphonate and
sulfonate
groups showed to have the required selectivity for uranium (
Horwitz et al., 1993
). Radium
adsorption is low and the resin is intended for low pH applications. This
resin thus would
be the ideal material for uranium adsorbing thin films. Unfortunately it is
only available as
beads. To produce nevertheless some sort of thin film we grind the dried beads
to a fine
powder. This powder is then fixed with a binder to a flat substrate. Polishing
removes part of
the binder covering the grains thus exposing them to the surface. By this
procedure films with
an active thickness of some µm can be made.
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Uranium adsorption on these films is considerably slower than radium adsorption on MnO 2 films. It takes about 20h until equilibrium is reached (4h to 50% equilibrium). After 20h a 20mm x 20mm sheet exposed to an acidified, stirred 100ml sample has taken up more than 80% of the uranium. pH is adjusted with formic or nitric acid to about 2.5. |
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After drying the exposed sheet is measured with a solid state alpha detector. Energy resolution is by far not as good as for the MnO 2 films. However it is better than achievable with liquid scintillation alpha spectrometry. For a 100 ml sample and counting times of one day one gets a detection limit of typically 10 mBq/l ( units ). ( application note ) |
| 1 pCi = 37 mBq | 10 mBq = 0.27 pCi |
| 2 pCi =74 mBq | 20 mBq = 0.54 pCi |
| 5 pCi = 185 mBq | 50 mBq = 1.35 pCi |
| 10 pCi = 370 mBq | 100 mBq = 2.7 pCi |
| 20 pCi = 740 mBq | 200 mBq = 5.4 pCi |
| 50 pCi = 1.85 Bq | 500 mBq = 13.5 pCi |
| 100 pCi = 3.7 Bq | 1 Bq = 27 pCi |
| 200 pCi = 7.4 Bq | 2 Bq = 54 pCi |
| 500 pCi = 18.5 Bq | 5 Bq = 135 pCi |
| 1 nCi = 37 Bq | 10 Bq = 270 pCi |
| 10 nCi = 370 Bq | 100 Bq = 2.7 nCi |