The radionuclide solutions used to contaminate these coupons are NIST traceable. The Am-241 solution was prepared from AmCl3 dissolved 1 N HCl. The Pu-239 solution was prepared from Pu(NO3)4 dissolved in 4 M HNO3. The Cs-137 solution was prepared from CsCl dissolved in 0.1 M HCl. I have included all Certificates of Calibration for the stock solutions. For all solutions, a 1 μCi/mL dilution in deionized water was prepared from the stock solutions.
This testing matrix utilized four types of coupons including concrete, carbon steel, stainless steel and Plexiglas. The concrete coupons were made from construction grade concrete cores. As concrete is poured, contractors are required to pour additional cores that are submitted for testing of the overall strength. We obtained these cores from a local testing company and had them uniformly sectioned for our testing.
The steel coupons were obtained from a local metal working company. The carbon steel was cut into coupons measuring 3” x 3” x 1/8”. A 300-series stainless steel was also cut into coupons measuring 3” x 3” x ¼”.
The Plexiglas was purchased from a local home improvement store and a single sheet was cut into 3” x 3” coupons.
The uniformity of the thicknesses for all materials was checked by the lab worker due to the importance in the alpha counting method.
Coupons were contaminated using the radionuclide solutions described above. The coupons were placed on wire racks (a) which were placed in secondary containment in the fume hood. The solutions were deposited on the surface using a pipettor. The carbon steel showed visible signs of corrosion (b). Duplicate samples were prepared for each material and radionuclide.
Coupon Coating Application and Removal
After initial counts were taken, the coupons were coated with the Decon 1101 Gel. The gel was poured from the bottle to the surface of the coupon. The gel was spread over the surface using a spatula and the excess gel was allowed to drip off (c).
Prior to the radiological testing, two tests were performed to determine optimum drying time both inside and outside of a fume hood. It was determined that 24 hours would be sufficient for the gel to dry completely without any residue sticking to the surface. The coupons in this test were allowed to dry for 24 hours before being stripped.
All of the coupons were easy to peel, with the most difficult being concrete. The coatings were all removed in a single sheet. None of the coatings fractured during removal. The Plexiglas coupons had a visible oily residue on the surface after the coating was removed. The carbon steel coupons no longer had visible corrosion on the surface.
Analytical Method and Data Workup
The Cs-137 coupons were counted on a Canberra high purity germanium detector. Photographs of the detector (d) and sample holder (e) are shown below. Each coupon was counted for 1000 seconds (16.67 minutes) and the peak area was determined to calculate the activity of the sample. Cs-137 was counted using the 662 keV energy line. Calibration coupons were prepared to validate the data. The calibration coupons were 1 μCi, 0.5 μCi and 0.1 μCi.
The Am-241 and Pu-239 coupons were counted using a Ludlum 43-1 alpha scintillator (f) connected to an Eberline E600 programmed to take a 2 minute scalar count. A platform was manufactured to maintain consistent orientation and spacing for each coupon from the detector face. Calibration coupons were prepared for each type of coupon material with 1 μCi, 0.5 μCi and 0.1 μCi of Am-241 or Pu-239.