  To provide a comprehensive data set for track structure-based simulations of
radiation damage in DNA, doubly differential electron-impact ionization cross
sections of pyrimidine, a building block of the nucleobases cytosine and
thymine, were measured for primary electron energies between 30 eV and 1 keV as
a function of emission angle and secondary electron energy. The measurements
were performed for secondary electron energies from 4 eV to about half of the
primary electron energy and for emission angles between 25{\deg} and 135{\deg}.
Based on the experimental doubly differential ionization cross sections, singly
differential and total ionization cross sections of pyrimidine were determined
and compared to calculations using the BEB model. In addition to the
measurements, a theoretical approach for calculating triply and doubly
differential ionization cross section was developed, which is based on the
distorted wave Born approximation, a single center expansion of molecular
orbitals and an averaging of the T-matrix over different molecular
orientations. The calculated doubly differential ionization cross sections of
pyrimidine show a qualitatively good agreement with the experimental results.
