introduction-weak-interaction-volume-one

occured, but the observation of the Gamow-Teller deca y
6He > 6Li + e + v (3 .4 .20 )
invalidated this hypothesis . We note that decays need not be either pure
Fermi or pure Gamow-Teller, but may be mixed . Since the spin of both
initial and final states in the neutron decay is ± 1,
this is an exampl e
of a mixed transition 1 . Denoting the Fermi-transition (L J = 0) couplin g
constant by CF , and the Gamow-Teller (L1J = 1) one by C GT ,
we find tha t
the neutron decay rate is proportional to C2 + 3 CGT , the factor of
three entering because there are (2 J + 1) — 3 possible spi n
orientations for the lepton pair when its total spin is one .
We now attempt to calculate the values of our coupling constant s
CF and CGT . We assume I
l`i f l 2
= 1, and consider the decay
140 14N e + v , {3 .4 .21 )
which is a pure Fermi transition, since both initial and final states have
zero spin . In fact, it is a 'superallowed' decay, since it simply involve s
the transformation of a proton into a neutron, and requires no rearrangemen t
of the nucleus 2 . Hendrie and Gerhart measured the half-life of this decay ,
and found (31 )
t = 70.91 ± 0 .04 s, (3 .4 .22a )
while Bardin et al . found (32 )
t = 71.00 ± 0 .13 s, (3 .4 .22b )
in excellent agreement . By listing and evaluating correction factors,
as nuclear form factors 3 , screening of the
electron wave function, and
such
competition from K-electron capture decay, Durant at al . have estimated (33 )
the total correction factor as +0 .289% . The necessary radiative correction s
have also been calculated to greater accuracy by Kinoshita and Sirlin (34) .
Thus, by measuring the maximum electron energy in the decay (3 .4 .21), i t
has been deduced that the corrected value of ht is (33 )
3043 a, (3 .4 .2.3 )
1 . Since I -1` 11-1 _ 1-+l - 1 -
14 This may be expressed alternatively by saying that the isobars 140 and
N belong to the same isospin multiplet (see 5 .1), i.e . they have the
same mass except for slight electromagnetic self-mass corrections due to
their differing charges .
3 . i.e . the finite spatial extent of the nucleus .