2.5.1.3 - Radon by Gel Counting

Introduction

Direct gel counting avoids Radon losses during extraction, but allows only a fare α/β-discrimination. In-situ measurements give the present concentration of both Rn and daughters. If Rn should be quantified from gel counting, the equilibrium of Rn with daughters has to be assured. The absence of other radionuclides in comparable concentrations is precondition.

Materials and Equipment

• Gelating cocktail (AB)

• PSD LS counter

Procedure

(according to [Salonen 1993])

1. 12 mL gelating cocktail (Ultima Gold™ (AB), alternatively AquaLight™, OptiPhase HiSafe™ III, AquaSafe™ or others) are under layered with 8 mL water sample.

2. The counting vial is closed and shaken vigorously.

3. Radon is measured as homogeneous gel preferably in a α-channel.

Evaluation

The activity concentration A_C of ²²²Rn is calculated according to

with f(t) = 1 / exp (-t / T_½ (²²²Rn) _* ln2)

R_N = Net rate (in cps)

ε_α = α-counting efficiency (290 - 300 % for clear α/β-discrimination)

V_S = Sample volume (8 mL)

t = Time difference between sampling and measurement (in s)

Detection Limit (MDA): 0.4 Bq/L (PSD-α-channel)

Analysis uncertainty: 4.2 %; however 5 % Rn losses from extraction must be taken into account.

Both α- and β-pulses may be considered, however, the lowest limit of detection is obtained when only α-counts are registered using PSD. Calibration of the LS counter for Radon measurement preferentially is done with a ²²⁶Ra-standard solution. Therefore, equal volume of sample and cocktails as well as identical PSD and pulse height channel settings for standard and real samples have to be used. In case of poor α/β-separation the PLI level should be lifted in that way, that no β-interferences in the a-channel appear. The losses of α-counts in the β-channel and consequently the lower ε_α must be taken into consideration. Alternatively the total α/β integral without PSD with a total α/β efficiency of 480 % (3α, 2β) in case of equilibrium conditions can be used (less sensitive but minor source of errors).

When the sample is measured directly after sampling, equilibrium conditions between Radon and progenies may be identified in combination with Rn extraction according to 2.2.1.2. [Möbius 2002].

Möbius S. 2002: Fast methods for field analysis of natural radionuclides - new approaches; in: S. Möbius et al. „LSC2001 Advances in Liquid Scintillation Spectrometry”, pp 235-246, Radiocarbon 2002, Tucson

Salonen L. 1993: Measurement of low levels of ²²²Rn in water with different commercial liquid scintillation counters and pulse-shape analysis; in: J.E. Noakes et al. “Liquid Scintillation Spectrometry 1992”, pp 361-372, Radiocarbon 1993, Tucson