We are developing
a beta spectrometer at the Lawrence Berkeley National Laboratory 88-inch
Cyclotron. The spectrometer is a 180°
flatfield dipole magnet, and is mounted with a proportional wire chamber.
The magnet can produce fields of up to 6 kG, and has been previously described
in the literature[1]. Beta particles with radii of curvature between 4.8
and 11.2cm may be observed with the wire chamber, which gives a momentum
bite of dP/P = 0.57. The wire chamber was constructed at LBL in 1999 and
offers a larger momentum bite than previous detectors which have been used
with this spectrometer. Precise spectral measurements demand a solid
angle acceptance of roughly 10-5. The wire chamber is composed
of five wire planes with a planar spacing of 3mm and offers position resolution
in two dimensions. There are 64 sense wires for vertical resolution and
16 wires for horizontal resolution. The sense wire spacing is 2mm.
A 50/50 Argon/Ethane gas mixture is bubbled through isopropyl alcohol and
continuously flowed through the chamber at atmospheric pressure. The two
sense planes are held at ground while the three high voltage planes have
potentials up to 3kV which for proportional chamber operation. Signals
collected from the sense wires are amplified and discriminated using custom
electronics. Three measurements with this spectrometer are under consideration.
One is a precise measurement of the spectral shape
factor in 14O beta decay as part of a test of the Conserved
Vector Current Hypothesis in the A=14 system. A measurement of the
shape factor requires 107 observed decays, which demands a rate
of over 106 14O produced per
second. Such production rates have recently been achieved using
the IRIS ECR source. Plans to mount the spectrometer on the
14O
beamline are being established. Another experiment is a measurement
of the 8B beta spectrum. This experiment would utilize a large
magnetic field (5 kG) to observe the high energy
portion of the 8B beta spectrum (which has an enpoint energy
of about 15 MeV). Such a measurement would indirectly probe the
shape of the neutrino spectrum of 8B, and decrease the error
in interpreting the results of observed 8B solar neutrino spectra
in water Cerenkov detectors. The 8B could be produced online
at the 88-inch Cyclotron. The final experiment is a measurement of the
shape factor in
22Na beta decay. Measurements
of decay properties of 22Na have produced discrepant data, possibly
due to source scattering effects. Provisions are being made to produce
thin beta sources using electrodeposition which may reduce the effect of
source scattering. An accurate measurement of the 22Na spectral
shape would be helpful in understanding the effect of induced corrections
on the decay, which some experiments suggest could be anomalously large.
A 22Na spectral measurement would require very high statistics
and a counting time of around 30 days with a source intensity of 50 uCi.
Figure 1. Photograph of the beta spectrometer. The wire chamber is
in the foreground. The dipole magnets are located behind the stainless
steel plate.