gross alpha and gross beta activity measurements in ... · gross alpha and gross beta activity...
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Stan Chałupnik
Silesian Centre for Environmental Radioactivity
Central Mining Insitute
Katowice, Poland
Gross alpha and gross beta activity measurements
in drinking and mineral waters
Drinking Water Directive 98/83/EC
• Direct addressing tritium level only (100 Bq/l)
• Total indicative dose (TID) due to ingestion of all other
radionuclides, except K-40 and radon decay products
• TID should be estimated accordingly to the Directive
96/29/Euratom (BSS)
• Radon and decay products – Directive
2001/928/Euratom The Protection of the Public
against Exposure to Radon in Drinking Water
Supplies
• Draft of Euratom Drinking Water Directive
– Gross alpha/beta measurement as screening method
– Dose asssessment for adults only
Gross alpha/beta
• This technique is only a screening method.
• Till now there is no regulations, concerning
it’s application, only recommendations.
• A source – WHO recommendations:
– 2004 α < 0.5 Bq/l β < 1.0 Bq/l
– Previously α < 0.1 Bq/l β < 1.0 Bq/l
Gross alpha/beta
• Useful method for screening of the
contamination of the environment, caused by
man-made radioactivity.
• Seems to be that application of the method is
simple and possible for natural radioactivity, for
instance in water samples.
• LSC is a very good technique for measurements
of gross alpha/beta.
Procedures for gross α/β
– We try to test two different procedures for gross alpha/beta measurements
– ISO 11704:2010(E) „Water quality – Measurement of gross alpha and gross beta activity concentration in non-saline water – Liquid Scintillation Counting Method”
– H-α-GESAMT-TWASS-02 “Schnellverfahren zur Bestimmung der Gesamt-Alpha-Aktivitätkonzentration im Trinkwasser” Beyermann, Bünger
ISO Method
• Sample of water (up to 200 ml)
• Evaporation to dryness or very small volume (2-4 ml)
• Addition of deionised water (up to 6 ml + small amount of HCl, if necessary)
• 12 ml of LS cocktail added – preferably Ultima Gold AB
• Measurement
• Advantage – no lost nuclides
• Disadvantage – problem with K-40
Fast method
• Sample volume up to 500 ml
• Admixture of 2 ml 3,2 M (NH4) 2HPO4
• Precipitation – Important – no precititation of I group cations (K-40,
but also Cs isotopes)
• Dissolution of the precipitate in 4,4M HCl
• Addition of deionised water (up to 6 ml)
• 12 ml of Ultima Gold AB
• Measurement
• Advantage – no influence of K-40
• Disadvantage – lost Cs, if present
Detection Limit
• Decision threshold
– Ld = [k1-α/(V*η)]*SQRT[IB(1/tB+1/tp)]
• Detection Limit
– LLD =
)*(*1
/))*/((*222
2
Vsk
tpVkLd
Threshold levels
• WHO (Guidelines for drinking water quality 2004): – Gross alpha < 0.5 Bq/l (500 mBq/l)
– Gross beta < 1.0 Bq/l (1000 mBq/l)
• Poland: – Gross alpha < 0.1 Bq/l (100 mBq/l)
– Gross beta < 1.0 Bq/l (1000 mBq/l)
• New draft od Euratom Directive: – Gross alpha < 0.1 Bq/l (100 mBq/l)
– Gross beta < 1.0 Bq/l (1000 mBq/l)
• Further measurements required, if screening values are above threshold levels.
Gross alpha/beta and LSC
• LSC technique is used very commonly for this purpose
• Preparation of samples and measurements
– Evaporation of the liquid sample
– Mixing with the scintillator
– Measurement
– The most important problem – misclassified particles!!
LSC spectrometer
Detection limits:
Gross alpha =
0.01 Bq/l
Gross beta =
0,05 Bq/l
Sample volume
200 ml
Counting time 1 h
QUANTULUS
PSA and SQP(E)
• Effieciency of alpha/beta separation is also dependent on quenching;
• SQP(E) – indicator of the quenching in Quantulus, good for pure beta emitters.
• No simple correction for PSA versus SQP(E) .
• A kind of a „mysterious” behaviour of samples with obvious quenching and high SQP(E)
„Misclassified particles”
• Just a reminder – alpha/beta separation is
done electronically.
• When quenching occurs, some alpha
particles are classified as beta pulses.
• Theoretically – we should apply low PSA,
but as a result some beta particles are
classified as alpha ones!!!!
Am-241 + Sr-90/Y-90
...AL\LIN5\S53.sum 11
...AL\LIN5\S53.sum 21
...AL\LIN5\S53.sum 22
Sample Spectrum
1 000950900850800750700650600550500450400350300250200150100500
5
4
3
2
1
0
UG AB
PSA=90
Misclassified... cd.
• Another problem is related to the fact, that
alpha/beta separation is also dependent
on „generation” of LS spectrometer.
• For instance, in case of older Quantuli,
separation is energy dependent (big
problem is proper separation for K-40)
• Much better for „new vintage” of
spectrometers.
Misclassified particles (at least 5-7%)
Missclassified particles for Quantulus (1990)
0
5
10
15
20
25
30
35
40
90 100 110 120
PSA
[%]
UG alfa UG beta Optiscint alfa
Optiscint beta LumaGel alfa LumaGel beta
Misclassified particles (1,5 % only)
Misclassified particles Quantulus (2006)
0
5
10
15
20
25
30
35
40
60 65 70 75 80 85 90 95 100
PSA
[%]
UG alfa UG beta Opti alfa Opti beta LumaGel alfa LumaGel beta
Linearity – O.K.!
Linear response Sr-90
-1
0
1
2
3
4
5
6
00,
30,
60,
91,
21,
51,
82,
12,
42,
7 33,
33,
63,
94,
24,
54,
85,
15,
45,
7 6
objętość [ml]
często
ść [
cp
s]
Sr-90 beta Liniowy (Sr-90 beta)
Linear response Am-214
0
0,5
1
1,5
2
2,5
3
3,5
0
0,0
2
0,0
4
0,0
6
0,0
8
0,1
0,1
2
0,1
4
0,1
6
0,1
8
0,2
0,2
2
0,2
4
0,2
6
0,2
8
0,3
0,3
2
0,3
4
0,3
6
0,3
8
0,4
0,4
2
0,4
4
0,4
6
0,4
8
0,5
0,5
2
0,5
4
objętość [ml]
często
ść [
cp
s]
Am-214 Liniowy (Am-214)
Misclassified....
• Again a reminder – alpha/beta separation
is dependent on the type of coctail.
• For any type of sample PSA must be set
separately!!
• Moreover, even for the same type of the
spectrometer it must be done separately,
„vintage” is important again.
Lost nuclide(s) Pb-210
...AL\LIN5\S49.sum 11
...AL\LIN5\S49.sum 21
...AL\LIN5\S49.sum 22
Sample Spectrum
1 000950900850800750700650600550500450400350300250200150100500
1
0
Lost nuclide(s) Ra-228
...\QUICK1\S25.sum 11
...\QUICK1\S25.sum 21
...\QUICK1\S25.sum 22
...\QUICK1\S26.sum 11
...\QUICK1\S26.sum 21
...\QUICK1\S26.sum 22
Sample Spectrum
1 000950900850800750700650600550500450400350300250200150100500
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Fast method – additional problem
• Too much acid or big amount of precipitate
leads to significant quenching
• Alpha/beta separation is different
• SQP(E) characteristics is a bit peculiar
– The highest value found for the most
quenched sample???
Fast method – additional problem
...\SR_AM1\S41.sum 11
...\SR_AM1\S41.sum 21
...\SR_AM1\S41.sum 22
...\SR_AM1\S46.sum 11
...\SR_AM1\S46.sum 21
...\SR_AM1\S46.sum 22
Sample Spectrum
1 000950900850800750700650600550500450400350300250200150100500
4
3
2
1
0
Conclusions
• Application of LSC for measurements of gross alpha/beta activities must be done carefully.
• ISO method is reliable but K-40 influence is important, especially for mineral waters.
• Fast method seems to be better for screening, except cases, when Cs presence is predicted.
• Some radionuclides are simply excluded by gross alpha/beta (Ra-228, Pb-210).
• Concerning the dose, Ra-228 is the most important radionuclide.
• Therefore gross alpha/beta measurements should be treated only as an additional tool!!