The possibility to separate signals caused by 511 keV photons created in annihilation of electron-positron pairs and the so-called prompt photons from nuclei de- excitation is investigated. It could potentially be used to improve the quality of reconstructed images in the J-PET scanner in 2+1 photon tomography. Firstly, a research is conducted for several radioisotopes that decay via b{eta}+ decay followed by de-excitation of an excited nucleus. Efficiency, purity and false positive rate are calculated for each isotope as a function of energy deposited threshold, with a hypothesis that signals caused by 511 keV photons deposit smaller values of energy than 1 z 13the selected threshold, while prompt photons deposit larger energy than the threshold. Analysis of the results accompanied with physical properties of radioisotopes suggests using 44 Sc, which is the most promising candidate for medical applications. With the use of GATE and J-POS simulation software, in-phantom scattering was introduced and the best energy deposited threshold value was estimated to be approximately 375 keV. It corresponds to almost 100% efficiency for 511 keV signals, 75% purity for 511 keV photons, and approximately 70% efficiency and purity for prompt photons.
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