introduction-weak-interaction-volume-one

necessary to have a high flux of high-energy neutrinos . Danby et al . decided to us e
half their available beam energy : 15 GeV . The 15 GeV proton beam, which ha d
an intensity of 2 x 1011 particles per second, was made to strike a berylliu m
target . The pions and other particles produced which were within a 14 cone
were allowed to decay in a 21m straight decay tube, At the end of this, whe n
about 10 5 of the pions had decayed, the beam was filtered through 13 m of iron ,
thus stopping all particles except neutrinos . The latter were unaffected
by the iron, and continued in their original straight trajectories . They then
encountered a number of spark chambers with dimensions 1(1/3) m X 1(1/3) m X
(1/3) m, each containing nine 25 mm-thick aluminium plates . The total mass o f
material in the spark chambers was about 10 i• :g . Since the number of true
neutrino events was expected to be very small, it was essential that all possible
spurious events should be avoided . This was done in two ways . First, th e
spark chambers were surrounded by scintillation counters which were in anti -
coincidence with those placed between the spar's chambers . The neutrinos entering
the spark chambers would not interact with the external scintillation counters ,
but when they materialized into muons or electrons, they would trigger the
internal ones . The requirement for triggering the spark chamber was no coun t
in the external scintillation counters, and counts in two of the internal ones .
This eliminated a considerable number of cosmic ray events. The second method
for avoiding spurious events was to activate the spark chambers only for a
very short time while the neutrinos were actually in their vicinity. This wa s
achieved by pulsing the main synchrotron beam every 25 'As . Within each pulse ,
, there were twelve smaller 20 ns pulses, each separated by 220 ns from the next .
The spark chambers were switched on by a signal from a Cerenkov counter pointing
at the proton beam target . Over 1 700 000 beam pulses had been accepted fro m
the synchrotron, but only a few hundred photographs had been taken by the en d
of the experiment, demonstrating that the methods for selecting desired event s
were very efficient .
The next problem was to distinguish which photographs showed the effect s
of neutrino absorption, and which were simply spurious cosmic ray and main beam
events . By running the spark chamber apparatus on its own, it was establishe d
that the muons in cosmic rays had momenta of about 300 MeV/c, and hence produced