Cumulative alpha-decay event density ($D(t)={\int }_0^{t}R(s)ds$)

The number of alpha decay events per material volume accumulated over time. For constant alpha-decay event rate density $R$, $D=Rt$; otherwise $D(t)={\int }_0^{t}R(s)ds$. Multiplying by a damaged volume per event gives a dimensionless overlap parameter.