Before `beta-`decay neutron is at rest Hence `E_(n) =m_(n)c^(2), p_(n)=0`
`p_(n) = p_(p) +p_(e)`
`" or " " " p_(p) +p_(e) =0 rArr |p_(p)| =|p_(e)| = p`
`"Also"" " E_(p) =(m_(p)^(2) c^(4) +p_(p)^(2) c^(2))^(1/2)`
`E_(p) =(m_(p)^(2) c^(4) +p_(p)^(2) c^(2))^(1/2)`
`=(m_(e)^(2)c^(4) +p_(e)^(2)c^(2))^(1/2)`
From conservation of energy
`(m_(p)^(2)c^(2)+p^(2)c^(2))^(1/2) + =(m_(e)^(2) c^(4) +p^(2)c^(2))^(1/2) =m_(n)c^(2)`
`m_(p)c^(2) ~~936MeV , m_(n)c^(2) ~~ 938MeV , m_(e)c^(2) =0.51MeV`
since the energy difference between n and p is small pc will be small `pc lt lt lt m_(p)c^(2)` while pc may be breater than `m_(e)c^(2)`
`rArr" "m_(p)c^(2) + (p^(2)c^(2))/(2m_(p)^(2)c^(4)) ~~ m_(n)c^(2) =pc`
To first order `pc~~ m_(n)c^(2) -m_(p)c^(2) =938MeV -936MeV = 2MeV`
this gives the momentum of proton or neutron. then
`E_(p) =(m_(p)^(2) c^(4) +p^(2)c^(2))^(1/2) =sqrt(936^(2)+2^(2))`
`~~936 MeV`
`E_(e) =(m_(e)^(2)c^(4)=p^(2)c^(2))^(1/2) =sqrt((0.51)^(2)+2^(2))`
`=2.06MeV`