Performance characteristics of a commercial dedicated contamination counter

The dedicated wipe test counter (DWTC) is a commercial geiger-counter type instrument especially designed for wipe test counting. All samples are counted under the same physical conditions within 0.3 cm from a 2-mg cm-2 mica end-window geiger tube of 1.27 cm in diameter. The counting capabilities of the DWTC were tested for different radioisotopes in common use in the nuclear medicine laboratory. Since most of the imaging agents are 99mTc-based radiopharmaceuticals, an experimental efficiency factor, J = 3 cpm kdpm-1, was determined for this radioisotope by the manufacturer and stored in the counting system for default use. In addition, the formulation for the counting time needed to achieve a measurement with adequate level of significance (two standard deviations) and a background measurement time of 20 min was programmed in the firmware of the DWTC. With these parameters, the counting time for a 99mTc wipe (threshold of 2000 dpm) becomes 1.88 min for a background level of 10 cpm. The counting time increases to 3.25 and 4.80 min for the respective background levels of 20 and 30 cpm while the threshold is kept the same. In practice, we were able to measure activities as low as 0.7 kdpm for 99mTc and 0.09 kdpm for 131I. Linearity was maintained for a wide range of activities for all tested radioisotopes. The DWTC was found to be simple to operate and satisfies all requirements for performing wipe tests in the nuclear medicine laboratory.