Diffusion equation

The first term of the r.h.s. of equation 3.22reads: $-div(\overrightarrow{J} (\overrightarrow{r} ,v,t))=D\nabla^{2}\varphi (\overrightarrow{r} ,v,t)$. The diffusion equation is obtained from the Boltzmann equation when neutrons are assumed to be monocinetic, or, in other words, to belong to a single group. This allows to drop the integration^4.8 over velocities in 3.22, thus

$$\frac{\partial\varphi(\overrightarrow{r} ,t)}{v\partial t}=D\... ...a}^{(j)}(\overrightarrow{r} )\right) +S(\overrightarrow{r} ,t)$$ (4.24)