A solid sphere of radius $\mathrm{R}$ has a charge $\mathrm{Q}$ distributed in its volume with a charge density $\mathrm{\rho }={\mathrm{kr}}^{\mathrm{a}}$, where $\mathrm{k}$ and $\mathrm{a}$ are constants and $\mathrm{r}$ is the distance from its centre. If the electric field at $\mathrm{r}=\frac{\mathrm{R}}{2}$ is $\frac{1}{8}$ times that at $\mathrm{r}=\mathrm{R}$, then find the value of $\mathrm{a}$