Neutron and gamma dose equivalent rates in the vicinity of a self-shielded PET cyclotron after an upgrade with increased beam current

1429 Objectives The increased demand of PET-radiopharmaceuticals has been satisfied mainly due to the upgrading of existing cyclotrons to enable higher beam currents for the production of 18F. In these cases it is also necessary to determine if the cyclotron shielding is adequate to maintain the radiation levels below safety limits. The objective of this work was to measure the neutron and gamma dose equivalent rates of a self-shielded PET cyclotron after upgrading the beam current from 40 to 60 µA. Methods The device employed was a RDS 111 cyclotron. Thermoluminiscent dosemeters (TLD) LiF:Mg,Ti chips (TLD-600, 3×3×1 mm) and LiF:Mg,Cu,P rods (GR-207, 1 mm diameter, 6 mm length) were used to evaluate the neutron and gamma radiation fields, respectively. The response of the TLD-600 was calibrated for thermal neutron flux in a bonner-sphere using a 241Am-Be source while the GR-207 gamma-response was calibrated with a 60Co source. Dosemeters were irradiated at various locations around the shielding during 18F production via the 18O(p,n) reaction. Results With the cyclotron operating at 40 µA, the average gamma and neutron dose equivalent rates, at the different locations of measurement, varied from 4.9±1.5 to 14.0±2.0 µSv/h and 1.4±0.7 to 2.4±1.1 µSv/h, respectively. After the upgrade of the cyclotron, with capability to irradiate at 60 µA, these values varied from 6.5±3.8 to 14.0±3.0 and 2.9±1.2 to 3.7±0.9 µSv/h, respectively. According to specifications of the manufacturer the original shielding of the cyclotron was designed to maintain the total radiation levels below 20 µSv/h (16 µSv/h for gammas and 4 µSv/h for neutrons) on a box-shaped surface analogous to a real room boundary. Conclusions The results of this work indicate that the shielding of the cyclotron after the upgrade with increased current capability is still adequate to maintain the radiation levels below 20 µSv/h. Research Support Research partially founded by CONACYT Grant 121652