Radiation protection aspects of positrons: spectra, photon production, and photon dose estimation

Shielding of positron sources are accomplished by applying a low Z-material to absorb the positrons, and a high Z-material to attenuate photons from annihilation, bremsstrahlung, and decay processes. The concern here is mainly with resulted photons. Aluminium is taken here as the positron shielding. When the aluminium thickness is equal to or more than the range of the endpoint energy of the nuclide, the positrons are totally absorbed. Photon transmission from aluminium is studied for 1, 2, and 3 MeV positrons and for few selected nuclides by Monte Carlo simulation. Transmitted photon spectra, and transmitted angular distribution are calculated. The effect of Z-value on the production of annihilation and bremsstrahlung photons is verified experimentally. Photon dose rate as air kerma rate constant from these sources is presented. Positron emitting nuclides are studied from the radiation protection point of view, their spectra are calculated and presented as graphs and absolute intensity per energy bin. Air kerma rate constant from bremsstrahlung, annihilation, and decay photons for selected nuclides is given. The differences between electrons and positrons in stopping powers, radiation yields and other points are discussed. It is suggested that coincidence technique be used for designing a selective positron detector in a mixed radiation field. In the present work the electron mean bremsstrahlung energy for few elements are calculated. (author)