The Neutrino Source ISIS
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'Beam Stop' Neutrinos
A tantalum/heavy water target is bombarded by 800 MeV protons coming from the
ISIS synchrotron. The interaction of this high energy protons in the target
produces neutrons and a large number of pions. The neutral pions decay into
to gammas and do not contribute to the neutrino production. Charged pions are
stopped rapidly within the target material (within a tenth of a nanosecond), so
that only every one out of 1000 pions decays in flight. The negative pions are
captured by the target nuclei, thus only the decay of positive pions plays a
role as a source for 'Beam Stop' neutrinos: muon-neutrinos, electron-neutrinos
and muon-antineutrinos. These neutrinos are produced in a decay chain, so that
the flux is exactly the same for all three flavours.
Time sturcture of the neutrinos
The unique time structure of the ISIS neutrino source lets it stand out against
all other sources. The synchrotron produces the high energy protons with a
repetition rate of 50 Hz in two pulses, each 100 ns wide. They are separated
from each other by 250 ns. The positive pions have a liftime of only 26 ns,
resulting in two muon neutrino bursts within the first 0.5 microseconds after
beam on target. The successive, much slower decay of the muons (decay time: 2.2
microseconds) produces then the electron and muon-antineutrinos in a time
window from about 0.5 microseconds to 10 microseconds after beam on target.
This neutrinos have the characterisic 2.2 microsecond shape of the muon
decay. Due to this fact, muon-neutrinos can be separated from electron- and
muon-antineutrinos by a time measurement. ISIS is the only existing neutrino
source with this possibility.
The repetition rate of 50 Hz implies a beam free interval of 20 ms between
every pair of neutrino pulses. This gives rise to duty factors of 1:100000 for
muon neutrinos and about 1:5000 for electron-neutrinos and muon-antineutrinos,
enabling a very effective suppression of cosmic induced background.
Additionally the background can be measured in the beam pauses with high
statistical precision.
Energy of the neutrinos
The energy spectrum of the muon-neutrinos is monoenergetic with an energy of
29.8 MeV due to the two body decay of the positive pion.
The successive three body decay of
the positive muon produces the electron-neutrinos and muon-antineutrinos with
continous energies of up to 52.8 MeV.
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