Calculation of airborne radioactivity in a Technegas lung ventilation unit

Airborne contamination by Tc-99(m) has been monitored in the Nuclear Medici ne Department in our hospital to assess the risk of internal contamination to occupational workers exposed to Technegas studies. An air sampler fitted with a membrane filter was used. The optimum time for air absorption for o btaining the maximum activity in the filter was calculated. Maximum activit y in the membrane filter ensures minimum uncertainty, which is especially i mportant when low-level activities are being measured. The optimum time dep ends on air absorption velocity, room volume and filter efficiency for isot ope collection. It tends to 1/lambda (lambda = disintegration constant for Tc-99(m)) for large volume and low velocity. Room activity with the air pum p switched on was related to filter activity, and its variation with time w as studied. Free activity in air for each study was approximately 7x10(-4) the activity used, and the effective half-life of the isotope in the room w as 13.9 min (decay and diffusion). For a typical study (630 MBq), the effec tive dose to staff was 0.01 mu Sv when in the room for 10 min. ((C) 1999 Li ppincott Williams & Wilkins).