Characteristics of Annihilation Photons Emitted From New Types of Point-Like $^{22}{\rm Na}$ Radioactive Sources With Symmetric Absorber Designs

Purpose: Point-like 22Na radioactive sources have been widely used for checking radiation detectors that measure annihilation photons and for evaluating PET scanners. We developed new types of 22Na point-like radioactive sources that are expected to be more symmetric than conventional source types in the angular distribution of the emitted photons. The purpose of this study is to use Monte Carlo simulation to investigate the basic characteristics of the photons emitted from the new type 22Na point-like radioactive sources. Methods: Two new types of 22Na point-like sources, a cylinder type and a sphere type, were considered in comparison with two conventional source types, a disk type and a cube type. The angular distribution and energy spectra of the emitted 0.511 MeV annihilation photons and 1.275 MeV gamma rays were calculated using a Monte Carlo code based on Geant4. Results: The angular distribution of the emitted photons was more symmetric in the new type point-like sources than in the conventional types. The asymmetry of the angular distribution of the unscattered 0.511 MeV annihilation photon pairs counted in coincidence was 0.1 ±0.1% and 1.6±0.1% in the sphere and cylinder types, compared with 4.1±0.1% and 22.6±0.1% for the cube and disk types. The scatter components around the 0.511 MeV photopeaks in the new source types were typically less than 2/3 and 1/2 of those in the disk and cube types, respectively. Conclusion: The new point-like 22Na sources offer a more symmetric angular distribution of the emitted photons with fewer scatter components. They are useful for applications where the symmetry of the emitted photons is crucial.