"Particle Fluence Rate" can be defined as the total number of particles (typically Gamma Ray Photons ) crossing over a sphere of unit cross section which surrounds a Point Source of Ionising Radiation.
$\theta = \frac{d\Phi}{dt}$, where $d\Phi$ is the increment of the particle fluence during an infinitesimal time interval with duration $dt$.
Applicable units are those of quantitykind:ParticleFluenceRate
Particle Fluence Rate