  The bulk damage of p-type silicon detectors caused by high doses of gamma
irradiation has been studied. The study was carried out on three types of
n$^{+}$-in-p silicon diodes with comparable geometries but different initial
resistivities. This allowed to determine how different initial parameters of
studied samples influence radiation-induced changes in the measured
characteristics. The diodes were irradiated by a Cobalt-60 gamma source to
total ionizing doses ranging from 0.50 up to 8.28 MGy, and annealed for 80
minutes at 60 {\deg}C. The Geant4 toolkit for simulation of the passage of
particles through matter was used to simulate the deposited energy homogeneity,
to verify the equal distribution of total deposited energies through all the
layers of irradiated samples, and to calculate the secondary electron spectra
in the irradiation box. The main goal of the study was to characterize the
gamma-radiation induced displacement damage by measuring current-voltage
characteristics (IV), and the evolution of the full depletion voltage with the
total ionizing dose, by measuring capacitance-voltage characteristics (CV). It
has been observed that the bulk leakage current increases linearly with total
ionizing dose, and the damage coefficient depends on the initial resistivity of
the silicon diode. The effective doping concentration and therefore full
depletion voltage significantly decreases with increasing total ionizing dose,
before starting to increase again at a specific dose. We assume that this
decrease is caused by the effect of acceptor removal. Another noteworthy
observation of this study is that the IV and CV measurements of the gamma
irradiated diodes do not reveal any annealing effect.
