In the unit housing of a compact cyclotron and positron emission CT (PET), positron emitting gas such as¹⁵O, ¹¹C, C¹⁵O₂, C¹⁵O etc. is supplied from a cyclotron to a PET room through a transportation pipe with an appropriate shield to reduce positron annihilation radiation. This paper discribes the effect of lead and concrete shields with various thickness.
Using lead or concrete shield blocks with various thicknesses, radiation leakage through the shield was measured by an ionization chamber type survey meter during continuous and constant supply of¹⁵Ogas of 1.85 GBq/min concentration which is the maximum dose for clinical use. The leakage radiation measured was 213.7, 56.0, 15.3, 5.0 μSv/week for lead shield with 1, 2, 3, and 4 cm thickness, respectively, and 193. 3, 30.5 and 5.1 μSv/ week for concrete shields with thickness of 10, 20, and 30 cm, respectively. The present study shows that to keep less than 300μSv/week, which is the permissible dose rate of the boundary zone around the radiation controlled area by Japan Science and Technology Agency, it is required to use more than 8 mm thick lead shield or 7 cm thick concrete for continuous supply of 1.85 GBq/min¹⁵Ogas.