introduction-weak-interaction-volume-one

For cases in which the neutrino has almost the same energy as the photon ,
the resonance condition is fulfilled only for those photons emitted opposit e
to the neutrino . Thus, by measuring the circular polarization of the
gamma rays inducing resonance, we may determine the helicity of th e
neutrino .
In 1958, Goldhaber et al .
helicity using the proces s
152 3u(0 - )
+ e
152 s .(0 + )
(70) attempted to measure the neutrin o
152 Sz ` (1 ) + v(900 keV )
+ 1(961 keV) . (3 .8 .8 )
The lifetime of the 1 state is (7 ± 2) x 10-14 s, which is short
enough to assume that the nucleus will not lose any recoil energy betwee n
neutrino and photon emission . Gamma rays from the 152E . source impinged
on a ring scatterer consisting of 1700 g of Sa 20 3 . Any photons arising
from resonant scattering were detected by means of an Nal (Ti) scintillation
counter, which was shielded from the primary gamma rays by a lead block .
In order to determine the sense of gamma-ray polarization, the photon s
were made to traverse 3 mean free paths of magnetized iron before hittin g
the ring scatterer. An electron whose spin is antiparallel to that of
photon may absorb the latter's unit of angular momentum by spin-flip ;
when it is parallel, it may not . By measuring the change in counting rat e
when the direction of the magnetic field was reversed, it was deduce d
that the gamma-rays were (67 ± 15) % left-handed (h = -1), in agreemen t
with the theoretical prediction of 84 % for h = -1 .
v
There exists one further method of deriving the two-componen t
theory of the neutrino . This requires a new representation for the gamm a
matrices :
Y 5
1
Yr =
_0 -1-
Y4
0 -j 6 r
j 6r 0
Defining two two-component wave functions v and w, the Dirac equatio n
for zero mass now read s
a
(3 .8 .9)