The decay constant is
In the following, 
will be assumed as the effect of non-zero
neutrino mass is extremely small.
The phase space volume element is
where 
It is usual to write
where
f goes as 
for low 
and 
he decay constant can then be written as
Rearranging,
Using 
this can be written as
an `ft' value
Because of the wide range of beta lifetimes,
transitions are classified by 
into allowed, etc.
Superalowed Fermi transitions have 
and
hence 
s; of course, measurements of the
lifetimes of these yields rather a value for g.