  The photon count rate of superconducting single photon detectors made of MoSi
films shaped as a $2\,\mu$m-wide strip and a 115 nm-wide meander stripline is
studied experimentally as a function of the dc biasing current at different
values of the perpendicular magnetic field. For the wide strip a crossover
current $I_\textrm{cross}$ is observed, below which the photon count rate
increases with increasing magnetic field and above which it decreases. This
behavior contrasts with the narrow MoSi meander for which no crossover current
is observed, thus suggesting different photon detection mechanisms in the wide
and narrow strips. Namely, we argue that in the wide strip the absorbed photon
destroys superconductivity locally via the vortex-antivortex mechanism for the
emergence of resistance while in the narrow meander superconductivity is
destroyed across the whole stripline, forming a hot belt. Accordingly, the
different photon detection mechanisms associated with vortices and the hot belt
stipulate the qualitative difference in the dependence of the photon count rate
on the magnetic field.
