2.5.1.4 - Ra-226 through Radon Emanation
Introduction
226Ra and 222Rn are formed from the Uranium decay series according to
226Ra is considered as tracer and key nuclide for natural radionuclides in water samples. As a long-lived α-emitter, it plays a dominant role for the radiation exposure when consumed in drinking or mineral waters.
Maximum values for 226Ra itself or for the gross α-activity concentrations are either fixed by national legislation or are recommended in most countries.
For the consumption of drinking water, a maximum value of 0.1 mSv/a corresponds to a maximum activity concentration of 125 mBq/L, if for 226Ra, 4.7 x 10-6 SvBq-1 as ingestion factor for small children according to BfS (ICRP 68 recommends 2.8 x 10-7 SvBq-1 [Magill 1999]) and a daily consummation of 500 mL is assumed.
The median value for 226Ra in German drinking waters is 4.8 mBq/L, maximum values have been measured from 200 to 300 mBq/L. In mineral water samples values of up to 1 Bq/L have been found in Germany.
Quick methods for the determination of 226Ra in drinking and mineral water are therefore of practical importance in order to observe and control the national regulations world-wide.
As Radium can be found in environmental water samples mostly in Milli-Becquerel concentrations, and often in presence of high amounts of Radon and progenies, it has to be concentrated and separated before head.
The most often reported method for Radium determination in the intercomparison studies, organized by us along with the LSC2001 LS Conference, was the in-vial extraction of equilibrated Radon after sufficient storage (20 to 25 days) followed by LS measurement [Möbius and Salonen 2002]. The classical procedure is described below.
Materials and Equipment
Organic cocktail
PSD LS counter
Procedure
(according to [Messanleitungen 2000], [Schönhofer 1989] and [Schönhofer et al. 1991])
8 mL organic cocktail are under layered by 12 ml water sample in a glass counting vial.
The vial is closed (tight cover!) and stored with the cover downwards in a refrigerator in order to avoid Radon losses through the plastic cap.
After equilibrium has been established between 226Ra and 222Rn (minimum 20 days: > 5 half-lives, > 97 % equilibrium) the vial is shaken vigorously (time t0) and stored for another 3 hours.
Measurement is done either directly in an open counting channel with 480 % efficiency (100 % per α-emitter, 95 % per β-emitter) or is measured in the α-channel with better α/β-PSD after separation of an aliquot of the organic phase into a mini-vial in analogy to method 2.2.1.2.
Evaluation
The activity concentration AC of 226Ra is calculated according to
RN = Net rate (in cps)
e = Counting efficiency (480 % if total volume is measured)
VS = Sample volume (12 mL)
ft1 = Correction factor for 222Rn ingrowth (in case of unequilibrium) 1 / (1 – exp-(t1 / T1/2(222Rn)) * ln2
ft2 = Decay correction 1 / (1 – exp-(t2 / T½ (222Rn)) * ln2)
t1 = Time difference between sample preparation and 222Rn extraction
t2 = Time difference between extraction and mean measuring time
The aliquot volume (e.g. 7 of 8 mL) of organic phase has to be taken into consideration for data evaluation.
Detection Limit (MDA) with PSD: 180 mBq/L
Analysis uncertainty: 6.5 %
A MDA of 150 mBq/L 226Ra can be reached already after 24 hours storage (see 3.3.). In this case the surplus Radon has to be eliminated before head by vigorous shaking, or by flushing with Radon-free air. The equilibrium/unequilibrium conditions have to be taken into account for the data evaluation [Möbius and Zeisberg 2002]. This concerns all 226Ra determination procedures followed by 222Rn measurement.
Other radionuclides do not interfere.
Magill J. 1999: Nuclides 2000, Joint Research Centre, Karlsruhe
Möbius S. and Salonen L. 2002: Pilot intercomparison and workshop for Radon and Radium in water; in: S. Möbius et al. “LSC2001 Advances in Liquid Scintillation Spectrometry”, pp 433-444, Radiocarbon 2002, Tucson
Möbius S. and Zeisberg H. 2002: Investigations on instrumental Radium analysis by alpha/beta LS spectrometry; in: S. Möbius et al. “LSC2001 Liquid Scintillation Spectrometry”, pp 273-280, Radiocarbon 2002, Tucson
Schönhofer F. 1989: Determination of Radon-222 and Radium-226 in mineral water and drinking water - a survey in Austria; Analyst 114 (1989) 1345-1347
Schönhofer F., Barnet M. and McKlveen J. 1991: Determination of 222Rn and 226Ra in drinking water by low-level liquid scintillation counting – surveys in Austria and Arizona; in: H. Ross et al. “Liquid Scintillation Counting and Organic Scintillators”, pp 537-545, Lewis Publishers, Chelsea 1991, Michigan
