Add units

Co-authored-by: Vladimir Kulagin <[email protected]>

docs/source/theory.rst

@@ -123,7 +123,7 @@ Radioactive decay can be included in FESTIM. Neglecting other terms (diffusion,
 
     \frac{\partial c}{ \partial t} = -\lambda c
 
-where :math:`\lambda` is the decay constant.
+where :math:`\lambda\,[\mathrm{s}^{-1}]` is the decay constant.
 In FESTIM it is possible to include radioactive decay in the mobile hydrogen concentration and/or the trapped hydrogen concentrations.
 
 Heat transfer