Page 310 - Radiochemistry and nuclear chemistry

Cosmic Radiation and Elementary Particles 293

i, + IH ~ n(fast) + e +

e + + e- ~ 2~, 1

n (thermal) + ll3cd --, ll4cd + "Y2

The "y's emitted are of different energy; the "tl is 0.51 MeV, but "Y2 much higher. There
is also a time lag between the "y's because of the time required for the fast neutrons to be
slowed down to thermal energy. The detection system allowed a delay time to ascertain a
relation between ~'l and "Y2 (delayed coincidence arrangement). When the reactor was on,
0.2 cpm were observed, while it was practically zero a short time after the reactor had been
turned off thereby demonstrating the formation of neutrinos during reactor operation.
Since the 1950s it has become clear that neutrinos exist as several types. In/3- decay an
"anti-neutrino" is formed, while a "neutrino" is emitted in/3+ decay. Both these neutrinos
are now referred to as electron neutrinos, i, e and l, e, respectively.
The pions formed in nuclear particle reactions are unstable and decay with a life-time of
3 x 10 -8 s into a muon and a # neutrino:

7r+ --, ~ -+ +~,

The mass of the muon is 0.1135 u (105.7 MeV). The muon is also unstable and has a
life-time of 2 x 10 -6 s; it decays into an electron, an e neutrino and a/~ anti-neutrino:

In 1979 Reines, Sobel and Pasierb made new neutrino measurements with a detector
containing heavy water so that the neutrinos would either split the 2H atom into a proton
and a neutron, or convert it into two neutrons. Both reactions would only be sensitive to
the ~'e; by measuring the neutron yield, the number of pe's could be calculated and
compared to the known ~'e flux from the reactor. The two different decays could be
followed by measuring the time delays between neutron capture and "t emission (see Ch.
8). The ratio of these two measurements was only 0.43 • 0.17, i.e. half of that expected.
The explanation proposed was that the two types of neutrinos interchange, or oscillate
between the Pe and the J,~ states, thus only 50% of the expected number would be
observed.
Since then a third kind of neutrino, the tau neutrino, Jr, has been postulated. Thus it is
now believed that here are three types of neutrinos: (i) the electron neutrino, ~'e, which
accompanies/~ decay, (ii) the muon neutrino, ~,~, which accompanies pion decay, and (iii)
the tau neutrino, ~,r, which is only involved in very high energy nuclear reactions. The
three kinds of neutrinos all have their anti particles. They have spin, no charge ('quark
charge zero'), but possibly a small mass. They react very weakly with matter, the reaction
cross section (cf. Ch. 16) being of the order of 10 -43 cm 2, depending on neutrino energy
(cross section increases as the square of the energy).
Various attempts have been made to determine the neutrino rest mass, for example by
measuring the decay of soft beta emitters like tritium, or double/3 decay as 82Se -~ 82Kr.
The present limit for the mass is set as < 18 eV. The three kinds of neutrinos are in a