"Kerma Rate" is the kerma per unit time.
Applicable units are those of quantitykind:KermaRate
http://en.wikipedia.org/wiki/Half-value_layer
"Kerma Rate" is the kerma per unit time.
$\dot{K} = \frac{dK}{dt}$, where $K$ is the increment of kerma during time interval with duration $t$.
http://www.iso.org/iso/catalogue_detail?csnumber=31895