Calibration of the absolute Energy Scale of Super-Kamiokande
Super-Kamiokande observes 8B neutrinos originating from the
sun by the Cherenkov cone of the recoiling electrons from
neutrino-electron elastic scattering in water.
The angular
resolution of the reconstructed recoil electron is defined as
the opening angle of a cone around the electron's true direction
that contains with 68% probability the reconstructed direction.
At electron energies between 5 MeV and 15 MeV, it is dominated by
multiple Coulomb scattering and lies between 20 and 35 degrees.
Due to this large resolution, the
energy of the neutrino cannot be reconstructed kinematically, so
only the recoil electron spectrum of the 8B neutrinos can be
observed. The recoil electron's energy being only a lower limit
for the neutrino energy, this spectrum falls
steeply with increasing energy, and small uncertainties in the
calibration of the energy scale lead to large uncertainties
in the measured flux and energy spectrum.
Electron Linear Accelerator
Super-Kamiokande's most precise calibration was achieved with a
Mitsubishi ML-14MIII electron linear accelerator. It injects single
electron pulses with an energy range between 5 and 16 MeV into
Super-Kamiokande's water tank.
Schematic view of the linear accelerator setup and the Super-Kamiokande
tank. The eight positions where data was taken are marked A through
H. Data was also taken at a ninth position I between G and H.
Linear accelerator beam end positions.
|
Position | A | B | C |
D | E | F | G | H |
|
X (m) | -3.88 | -3.88 | -8.13 |
-8.13 | -12.37 | -12.37 | -3.88 | -12.37 |
|
Y (m) | -0.71 | -0.71 | -0.71 |
-0.71 | -0.71 | -0.71 | -0.71 | -0.71 |
|
Z (m) | 12.28 | 0.27 | 12.28 |
0.27 | 12.28 | 0.27 | -11.73 | -11.73 |
Linear accelerator beam momentum and associated energy.
|
beam mom. (MeV/c) | 5.08 | 6.03 | 7.00 |
8.86 | 10.99 | 13.65 | 16.31 |
|
Ge energy (MeV) | 4.25 | 5.21 | 6.17 |
8.03 | 10.14 | 12.80 | 15.44 |
|
in-tank energy (MeV) | 4.89 | 5.84 | 6.79 |
8.67 | 10.78 | 13.44 | 16.09 |
LINAC Calibration
The details of
this calibration are described in
Nucl.Instrum.Meth. A421: 113-129, (1999).
The experimental
setup is shown in the figure above. At nine positions in the water
tank the energy spectrum of the downward-going monochromatic beam was
studied at seven fixed beam momenta.
For each beam momentum the energy of the outgoing electrons is determined
by a Germanium detector in a separate run. By its nature, this measurement
takes into account the energy loss of the beam going through a 100mu m
titanium window at the end of the beam pipe (called end cap). The
energy loss in the Beryllium window in front of the Ge crystal and
the inactive layer at the crystal surface is adjusted for.
This method of calibration has the
advantage that electrons of a well-known energy and direction are
used. Also, the end of the beam pipe contains a scintillation counter
which allows the study of the detector response to electrons on an
event-by-event
basis. However, for each change of position the beam pipe has to be
disassembled at the old position and reassembled at the new position.
Two complete scans of the detector were done in 1997 and 1999, as well
as a scan of the positions A, B and G in 1998. At the positions E and
A, four alternative directions were measured to study the directional
dependence of the energy scale using a permament magnet which
bends the beam (at 12 MeV/c) by about 45 degrees.
Deuterium-Tritium Neutron Generator
A suitable energy calibration source in the solar neutrino
energy range is 16N which decays into 16O with
a half-life of 7.13 sec. The endpoint
of the spectrum is 10.4 MeV. For 72% of the decays
a 6.1 MeV (66%) or 7.1 MeV (5%) photon is produced in addition
to an electron and a neutrino. Capture of cosmic-ray muons
on 16O
produces 16N `naturally'. It can be produced
`artificially' by a reaction of neutrons with 16O.
Super-Kamiokande uses a deuterium accelerator with a tritium
target (D-T Generator)
to produce large numbers of 16N.
Schematic view of the D-T generator setup. The generator
is suspended from a computer-controlled crane lowering it
into position (a), generating 14.2 MeV neutrons (b) and
retracting it (c) leaving behind a cloud of 16N.
The D-T generator are accelerates deuterium ions produced
by a Penning source with a voltage between 80 and 180 kV into a
Tritium target.
D-T Generator Calibration
The details of
this calibration are described in
hep-ex/0005014
to be published in Nucl.Instrum.Meth.
The D-T generator is small enough to be immersed in the Super-Kamiokande tank,
the depth is controlled
with a computer-operated crane. A high voltage pulse accelerates
a bunch of deuterium ions into the tritium target and produces roughly
a million 14.2 MeV neutrons.
The D-T generator is automatically pulled
upwards leaving behind a cloud of 20,000 16N atoms. During the next
45 seconds, the 16N decay events are collected. Then,
the crane automatically lowers the D-T generator to fire the next pulse.
Each pulse leads to 5,000 usable 16N calibration events. At the
end of March 1999, the first D-T generator data was taken.
A D-T generator energy calibration with high statistics can be done quickly.
In July 1999, a combined
LINAC and D-T festival took LINAC and D-T data at the same position
to directly compare both calibrations and study the directional
dependence of the energy scale.
Result of the Calibration
The goal of the calibration was to tune the energy scale of
Super-Kamiokande's Monte Carlo simulation with linear accelerator data
to a precision of better than 1%.
A precision of 0.64% for the energy scale was achieved.
D-T generator data agrees within better than 0.1% with the
linear accelerator data and the absolute energy scale does not
depend on the event direction within +-0.5%.
