Gross alpha and beta measurements by LSC in waters: advantages, problems and limitations. Maurizio Forte Arpa Lombardia

Transcription

1 Gross alpha and beta measurements by LSC in waters: advantages, problems and limitations Maurizio Forte Arpa Lombardia

2 Gross α/β by LSC in waters: quick and effective screening method, widely used in monitoring campaign Our method: - Acidification (HNO 3 ) - Concentration by heathing (1:10) - Cocktail addition (8:12) - LS counting (1000 min, α/β discrimination) (Rusconi, Forte et al. J Rad. Nucl. Chem. 260, 421 (2004) Internal validation (ISO accreditation) Italian Standard (UNI 11260/2008) International Standard (ISO 11704/2010) Many different methods described in literature US Standard: ASTM D

3 SOME TOPICS WILL BE DISCUSSED: Radionuclides under measurement unknown Counting parameters: windows? counting efficiency? Dependence on chemical composition of the sample (effect of acids and dissolved salts) quenching α/β misclassification (spillover) Method robustness Repeatability evaluation

4 1 - COUNTING EFFICIENCIES 238 U 90 Sr 234m Pa 234 Th 90 Y 40 K 200 kev Eff α 100% ; Eff β energy dependent

5 Gross α/β values should be always referred to calibration radionuclides, but often they are not (even in intercomparison excercises) - Comparability of analytical outputs Cal. RN Beta emitters Eff % ( kev) Eff % ( kev) 40 K 40 K 102±3 86±3 90 Sr 90 Sr, 90 Y 100±3* 68±2* 238 U 234 Th 234 Pa 102±3* 49±1* * Average efficiency (total/2) Rectangular distribution: Eff β = 68 ± 11 (u eff = 16 %, k =1)

6 2 - SPILLOVER α/β MISCLASSIFICATION Alpha emitting radionuclide: τ α % = α counts in β window / total α counts * 100 Same for beta τ β: pure beta emitter Minimum interference % τ α = τ β working point τ % 40 τ α: pure alpha emitter PSA Best discrimination parameter

7 U (4.5 MeV) 40 τ % K 90 Sr τ α τ β τ α = 15 % 241 Am (5.5 MeV) PSA PSA =100 τ = 6 % PSA =115 τ = 1.5 %

8 Correlations between sample + cocktail composition and optimum counting parameters have been studied: Sanchez-Cabeza, Pujol: Radiocarbon 43 (1993) Pates: Radiocarbon 75 (1996); Analyst 123, 2201 (1998) Salonen: Radiocarbon 135 (2006) DeVol: Health Physics 92, 105 (2007) in this presentation: - Constant discrimination parameter (PSA) setting - Effects of acid and salt content on results Robustness

9 Sample pre-treatment: acidification step (HNO 3 ) HNO 3 is normally used to acidify and preserve samples (ISO Guidance on the preservation and handling of water samples) τ % Quenching param. SQP(E) SQP(E) τ α strongly dependent on HNO 3 concentration τ β slightly decreases τ α τ β Variations of quenching parameter are negligible if its experimental uncertainty is considered M (HNO 3 )

10 Dependance of α spillover on concentration of different acids τ α % CHCl 3 HNO 3 Metane sulphonic acid HCl M (H + ) Cl - H + O = = O Cl _ Cl C -H _ Cl N O - O = = CH 3 S-O - O H + H +

11 Alpha interference vs quenching parameter 100 CAUTION!!! τ α % CHCl 3 HNO 3 Correlations made with different quenching agents may be misleading SQP(E) SQP(E) is not a useful indicator if HNO 3 is used

12 Choice of test conditions: acidification with HNO ph 1.75 ± 0.25 τ α % ph

13 Sample composition: effect of dissolved salts τ % τ α Ca(NO 3 ) 2 ph 1.5 Samples were acidified with HNO 3 at ph 1.5 or τ α Ca(NO 3 ) 2 ph 2.5 The nitrate anion cause a raise of τ α Ca 2+ is much less effective τ α CaCl 2 ph τ α CaCl 2 ph 2.5 M Ca 2+

14 Sample composition: effect of dissolved salts g/l ph 1.5 (HNO 3 ) τ α The increase of salt concentration does not cause a relevant spillover raise τ % τ β Working conditions: ph = 1.75 ± 0.25 salt conc. = 0 6 g/l M (CaCl 2 ) Interfence Parameter: τ α = 4 % ± 1.5 τ β = 4 % ± 0.5

15 A α = (C Gα C (1 τ Alpha activity Beta activity Sample mass Bα Counting time Alpha efficiency Beta efficiency )(1 τ α τ ) (C ) ε 0.2 Bq/L 0.2 Bq/L 80 g (to 8 g) 1000 min 101 % ± 3 86 % ± 3 Gβ C Q T τ α 4.0 % (± 1.5) τ β 4.0 % (± 0.5) β β Rusconi, Forte et al: Appl. Radiat. Isot. 64, 1124 (2006) Unc. α activity = 5.2 % Unc. β activity = 7.2 % α (k=1) τ contribution to uncertainty small but not negligible Bβ ) τ β % contribution to variance 37.5 Eff α 0.6 τ β Eff β Error propagation (partial derivatives) Q 1.9 Counts β Q τ α τ β τ α Counts α α β

16 Internal robustness evaluation: U 241 Am 236 U 90 Sr u k=2 90 Sr HNO 3 ph HNO 3 ph CaCl g/l HNO 3 ph CaCl g/l - Comparison with thick source standard method - Intercomparison exercises

17 3 - Repeatability Tap water sample spiked with natural uranium standard ( 238 U, 234 U, 235 U, 234 Th, 234m Pa) Relatively high activity (negligible Poisson contribution) 10 separate tests by 5 operators along 1 month: intermediate repeatability Std. deviation of raw alpha and beta counts

18 Date Operator Alpha counts Beta counts 27-mar A apr B apr C apr D apr E apr A apr B apr C apr D apr E α β Avg. Counts: Poisson (C ½ ): 180 (0.55 %) Std.Dev.: 226 (0.70 %) Fisher test: no statistical diff. Overall Repeatability: 0.70 % Avg. Counts: Poisson (C ½ ): 106 (0.94 %) Std.Dev.: 1012 (9.0 %) Fisher test: statistical diff. Overall Repeatability: 9.0 %

19 CONCLUSIONS It must be clear that gross alpha/beta counting should be used as a preliminary screening method. The sample composition, from both a radiometric and chemical point of view, has an influence on gross alpha/beta measurement results. EFFICIENCY: When performing calibration, test conditions (e.g. counting windows) and radionuclides should be carefully chosen (especially for beta emitters) and clearly reported.

20 SPILLOVER: As an acidification of the sample is performed, a careful control should be kept especially if HNO 3 is employed. High acid concentrations should be avoided anyhow. Quenching parameter may be not sensitive enough for an effective control and/or correction of spillover A range of acceptability for sample composition (ph, residue) should be set (i.e. ph 1.5 2, residue 0-6 g/l). Robustness with respect to relevant chemical parameters has been verified within this range. Is a spillover correction necessary? Experimental repeatability has been measured How to use it? FULL UNCERTAINTY EVALUATION

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