Observation of 511 keV peak high count rate in studying (n,x) and (g,x) reactions on terbium

Experimental investigation of (n, x) and (g, x) reactions on Tb-159 with activation technique was carried out. Tb specimens of natural composition were irradiated with (d-d) and (d-t) neutrons using NG-300 neutron generator. Additionally the series of experiments were performed with application of M-30 microtrone as a source of electrons for bremsstrahlung spectra production with end point energies 7.5, 9.5, 11, 11.5, 12, 12.5, 16.5, and 18.5 MeV. Instrumental spectra of Tb specimens were measured with HPGe and Ge(Li) spectrometers. Within the main scope of nuclear reactions research and accurate {\gamma}-spectrometry of Tb specimens a high count rate in 511 keV {\gamma}-line peak was observed. The first-priority analysis of Tb specimen impurities was done with further attempts to explain a result of observations with reference to the specific nuclear properties of Tb which could appear due to complex GDR structure. The energy threshold of the process detected was determined around 12.2 MeV. The lower estimate of cross section value for this process was assumed and calculated.