We are developing a beta spectrometer at the Lawrence Berkeley National Laboratory 88-inch cyclotron. This spectrometer will be used to make a precision measurement of the beta energy spectrum from ¹⁴O beta decay [1] as part of a test of the Conserved Vector Current Hypothesis in the A=14 system [2,3]. Considering the rapid development of radioactive beam facilities at the 88-inch cyclotron, the development of a reliable beta spectrometer will be crucial for future experiments. The spectrometer consists of two parallel, dipole magnets which bend incident charged particles by 180^o. The field between the magnets has been measured and is constant in space to within 0.05% at 1200 gauss.  Attention was given to the rise of the field in the region between the source position and the magnet opening. The momentum separated particles are detected by a wire chamber which was constructed in 1999 and  improves on a previous design.  The wire chamber runs in the proportional regime, and has a detection region of 6" vertical by 1.5" horizontal.  The detection wires are located on two separate sense planes, offering 64 channels of vertical resolution and 16 channels of horizontal resolution.  The detection wires are 25 micron gold plated tungsten spaced at 2 mm intervals, which corresponds to a momentum resolution of Dp/p = 0.01 at the 1200 gauss setting expected for the ¹⁴O experiment. We have performed a Monte Carlo simulation of the spectrometer to determine the size of false shape factor introduced by the spectrometer itself, i.e. an "instrumental" shape factor. This Monte Carlo takes into account the wire spacing, magnetic field variations, source-to-collimator distance, finite source size and collimator size. Early results indicate that the instrumental shape factor is negligible and this wire chamber will be suitable for a measurement of the shape factor at the desired level of accuracy. We have performed an EGS4 Monte Carlo study to optimize for the collimating geometry by taking into account beta scattering from the collimator.  Current results indicate the collimator should have a solid angle of approximately 3x10^-5. Preliminary analysis of pulse shapes for various sources of radiation has been completed. This is be our present area of concentration and the wire chamber will be modified to include the possibility of identify various radiation types according to pulse height discrimination. The wire chamber will be characterized so that its response to various types of radiation will be understood. Most important is the response to 2.3 MeV gamma rays (from the 0⁺ to 0⁺ branch of ¹⁴O decay) and 0.511 MeV annihilation radiation which are the major sources of background in this experiment. Instrumental shape factors due to energy dependent detector responses are also a primary concern.

A transfer arm with a catch foil will be constructed which will move ions between the beam line and the spectrometer mouth.  This transfer process will occur in a vacuum of better than 10^-7 torr.  We expect to begin measuring ¹⁴O beta decay spectrum by late 2000.