7-11 July 2019
EV36 - PEP 1G: Radiation Protection Consideration during Construction, Commissioning and Production of Mo-99 with a 40 kW 35 MeV Electron Linac (Chowdhury)Lake Sheen A 08:00 - 10:00 |
| Our experience during construction, commissioning and production of 99Mo with a high power linac (40 kW, 35 MeV Electrons) will be shared. The electrons bombarded on a heavy metal converter generate Bremsstrahlung photons that undergo photonuclear reaction 100Mo(?,n)99Mo with a Mo Target placed in the forward direction. Gamma spectroscopy detected 46Sc as an activated product from Ti. Converters and targets are water cooled, and the radiation protection due to Bremsstrahlung and neutrons are achieved using iron, lead, polyethylene, concrete and earth as shielding materials. Monte Carlo simulations are performed with FLUKA to generate the dose profiles for the electron, gamma and neutron. We have explored the possibility of using a high power linac (40 kW and 35 MeV Electrons) to produce medical isotopes such as 99Mo, as a cost-effective alternative method. The electrons bombarded on a Tantalum heavy metal converter generates Bremsstrahlung photons that undergo photonuclear reaction 100Mo(?,n)99Mo with an enriched Mo target placed in the forward direction. The high intensity Bremsstrahlung and neutrons generated require significant shielding. The dose rate calculated for Bremsstrahlung from a thick Tantalum converter in the forward direction is 4 x 105 Sv/h at one meter, and in the perpendicular direction 2 x 103 Sv/h at one meter. By assuming that the 35 MeV, 40 kW electron beam is stopped entirely on a thick target, the neutron yield would be about 5 x 1013 n/s. To keep the dose rate in the public occupied areas ALARA, the number of tenth value layer of shielding required in the perpendicular direction are 8.3 for Bremsstrahlung and 6 for the neutrons, respectively. The shielding is achieved by using the following materials: Iron, Lead, Polyethylene, Concrete and Earth. The cooling water at the converter and target as well as the room air will be activated and may produce ozone and hydrogen. The expected radioactive gases produced in air are 15O, 13N, and 41Ar, and in water are 15O, 11C, 7Be, and 3H (tritium). Adequate precautions are taken to mitigate these hazards. The tritium generated in the cooling water for the converter and target after 58.5 kW-hour of Linac operation was found to be only 2.5 Bq/L. Similarly, there was no Be-7 in the converter and target cooling water, nor any ozone production in the room air could be observed during the early phase of commissioning. However, we have experienced an elevated level of radiation from the converter and target holder’s material - titanium that has undergone nuclear reaction 48Ti(?, pn)46Sc generating Sc-46, which emits two cascading gamma photons ~1 MeV detected by gamma spectroscopy, with a longer half-life of 83.8 days. The Ti was replaced by Zr and Cu at the converter and target holders, respectively. Monte Carlo simulations were performed with FLUKA to calculate the dose at the converter, target, beam dump and shielding structures, as well as independent dose profiles for the electron, gamma and neutron. We have commissioned and produced Medical Isotope with a linac (40 kW and 35 MeV Electrons) where the electrons bombarded on a heavy metal converter generating Bremsstrahlung photons that undergo 100Mo(?,n)99Mo photonuclear reaction. Issues of providing adequate radiation shielding and containment of the hazards due to activated products will be presented. Monte Carlo simulations were performed with FLUKA to generate the dose profiles of electron, gamma and neutron. |
