2.5.1.1 - Gross Alpha/Beta Survey in Drinking Water

Introduction

WHO and European drinking water regulations consider maximum values for the radiation exposure of the population by consumption of drinking water. A value of 0.1 mSv/a has been fixed as Total Indicative Dose for natural radionuclides except Tritium, ⁴⁰K and Rn with progenies.

The first approach for its survey foresees screening by gross α/β-measurement to limit the maximum activity concentration to 0.1 mBq/L for α- and 1 Bq/L for β-activity. A widely used procedure applies radionuclide enrichment by evaporation or precipitation by adsorption (“scavenging”) with final proportional counting. Detailed procedures can be found in the loose-leaf collection of the "Association for Radiation Protection" [Fachverband für Strahlenschutz 2004] and in the measuring instructions for the monitoring of radioactivity in the environment [BMU 2000].

Applying α/β-PSD with its low background in the α-channel, liquid scintillation provides a suitable alternative with easy sample preparation and effective counting. The advantage compared to proportional counting is the stability of the counting efficiency of around 90 % for β-emitters above 0.1 MeV maximum energy.

For enrichment the sample is evaporated or freeze dried directly in the counting vial. If the presence of solid materials and organic compounds in the sample cannot be neglected, fuming with nitric acid and further sulfuric acid (add 1.5 M H₂SO₄) is recommended.

The procedure is suitable for drinking water as well as spring and mineral waters with lower Fe- and salt content (Ca + Mg < 0.5 g/L). The method is simpler compared to ‘Messanleitungen’ (H-α-Gesamt-TWASS-02-01) [BMU 2000], but cannot be applied when solid material appears after dissolution with HCl.

Materials and Equipment

• Gelating cocktail (AB) (e.g. AquaLight+, Ultima Gold AB)

• Heater, vacuum evaporator or freeze dryer

• 0.5 M HCl

• PSD LS Counter

Procedure

1. 100 mL of the water sample are evaporated to dryness in a small beaker (50 mL).

2. The residue is dissolved with 1 mL 0.5 M HCl (alternatively some 2 M HCl followed by neutralisation with 6 M NH₄OH) and transferred to a 20 mL counting vial.

3. After addition of 19 mL gelating cocktail (AB) the sample is measured in the α-channel by PSD.

A longer storage of the original sample should be avoided as ²¹⁰Po might be formed from ²¹⁰Pb in case of unequilibrium. This applies as well to the measuring sample as ²²⁶Ra progenies may ingrow.

Evaluation

The total α- and β-activity concentration A_C may be estimated from the net count rates R_N in the corresponding α- and β-channel according to

whereas

R_N = Net count rate (cps)

ε = Counting efficiency

V = Sample volume (L)

For complete α-PSD separation, a counting efficiency of 95 to 100 % may be assumed in the α-channel, and less when α- and β-pulses overlap. For the β-channel, a quench correction procedure is obligatory.

Detection Limit (MDA) for 5 hours counting time

• gross α: 35 mBq/L

• gross β: 100 mBq/L (550 mBq/L for Triathler)

Analysis uncertainty (α): 4.5 %

 If the recommended maximum values of 0.1 Bq/L for gross α and 1.0 Bq/L for gross β are exceeded, a nuclide specific analysis is necessary e.g. for Radium and Uranium isotopes and ²¹⁰Pb.

If the sample is remeasured after some days storage, the presence of ²²⁶Ra may be identified.

Fachverband 2004: Empfehlungen zur Überwachung der Umweltradioaktivität FS-78-15- AKU, Fachverband für Strahlenschutz Dezember 1979/März 2004, Blatt 3.1.12.7

BMU 2000: Messanleitungen für die Überwachung der Radioaktivität in der Umwelt und zur Erfassung radioaktiver Emissionen aus kerntechnischen Anlagen; Der Minister für Umwelt, Naturschutz und Reaktorsicherheit, Urban und Fischer, München, H-α-Gesamt-AWASS-01

BMU 2000: Messanleitungen Umweltradioaktivität H-Rn-222-TWASS-01, 4.1 Kalibrierung; see also Sr-analysis and others