"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.
Applicable units are those of quantitykind:ParticleFluenceRate
http://www.encyclo.co.uk/define/Fluence%20Rate
"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 $dt$.
http://www.iso.org/iso/catalogue_detail?csnumber=31895