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 188.8.131.52.) 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
- Using rectangle polystyrene platelets (4 x 1 x 0.1 cm), a 100 cm2 surface is swiped off.
- The platelet is dissolved in 10 mL organic cocktail and measured as homogeneous solution in the α- and β-channel by PSD.
- Filters have been dried and rolled along the inner wall of a glass vial (fig. 44).
- 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].
Figure 44: Filter material (25 mm) in 20 mL standard counting vial with 1.5 mL organic cocktail
(As the filter is only moistened it is fixed on the inner glass wall in an ideal measuring position.) [Frenzel et al. 2002]
Table 8: Swipe assay measurements with 241Am
The activity concentration AC is calculated according to
RM = 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