2.7.4 - Contamination Control by Swipe Assays

Introduction

Swipe Assays (wiping tests) are performed in German nuclear facilities for laboratory contamination and leakage control according to DIN ISO-7503-1 1990.

We have investigated the following filter materials for Liquid Scintillation measurement:

• Paper (Macherey Nagel)

• Cellulose mixed ester (Schleicher & Schuell M7)

• Glass fiber (Whatman GF/A) and

• Rectangle polystyrene platelets

Swipe assays tests on the basis of homogeneous LS measurements (dissolved polystyrene platelets) have the advantage that the counting efficiency is nearly independent on the β-energy, except for very low energy β-emitter (<100 keV). This is in contrast to the widely used proportional counters. The method is used for a rapid survey of gross α- and β-contamination e.g. in leakage control.

For our invetigations, swipe assay samples have been taken from Depleted Uranium (DU) penetrators (see chapter 2.2.1.8.) with a yellowish oxidized surface.

More recently, a quantitative determination of ⁶³Ni wipe tests with glass fiber discs has been reported [Wolf 2017].

Materials and Equipment

• Polystyrene platelets (4 x 1 x 0.1 cm)

• Organic cocktail (e.g. MaxiLight+, Toluene Scint)

• PSD LS counter

Procedure

1. Using rectangle polystyrene platelets (4 x 1 x 0.1 cm), a 100 cm² surface is swiped off.

2. The platelet is dissolved in 10 mL organic cocktail and measured as homogeneous solution in the α- and β-channel by PSD.

3. Filters have been dried and rolled along the inner wall of a glass vial (fig. 44).

4. Afterwards they have been moistened with a lipophile LS cocktail and measured in a LS system by α/β-discrimination. The low amount of cocktail (1 to 1.5 mL) fixes the filter on the wall and minimizes the background [Frenzel et al. 2002].

(As the filter is only moistened it is fixed on the inner glass wall in an ideal measuring position.) [Frenzel et al. 2002]

Evaluation

The activity concentration A_C is calculated according to

R_M = Measuring rate (cps)

R₀ = Background rate (cps)

ε = Measuring efficiency (100 % for a, 90 % for b)

h = Swiping yield (10 % as conservative value)

Classical Toluene Scint (120 g naphthalene + 4 g PPO + 0.05 g POPOP + 1 L toluene = „Bray” solution) dissolved the material immediately, while MaxiLight+ and BetaPlate Scint require a longer overnight storage time.

A minimum detectable activity MDA of < 0.7 ± 0.2 mBq/cm² for homogeneous polystyrene solution and 3 ± 1 mBq/cm² for glass fiber for a mean surface of 100 cm² has been calculated from the measured values in table 8.

The counting efficiency has been determined with a ²⁴¹Am standard solution, which has been pipetted onto each filter. The calculations are based on a 2s standard deviation with a measuring time of 300 seconds and a wiped area of 100 cm² considering a conservative hold-back factor of 0.1 or 10 %.

Detection Limit (MDA):

for α-emitters < 0.002 Bq/cm²

for β-emitters < 0.02 Bq/cm²

Frenzel E., Möbius S. and Oikari T. 2002: Localization of depleted Uranium contamination; in: S. Möbius et al. “LSC2001 Liquid Scintillation Spectrometry”, pp.429-433, Radiocarbon 2002, Tucson

Wolf S. 2017: Quantitative determination of Ni-63 wipe tests by liquid scintillation analysis; Paper ID 295, “LSC2017 Advances in Liquid Scintillation Spectrometry”, Copenhagen