Measurement of R Parameter at ~ 140 MeV

Measurements of the Wolfenstein R parameter for p-p scattering at 140 MeV have been made for scattering angles between 11° and 35° lab. The results reported are from a preliminary analysis of the data. The 46 per cent polarized beam from the cyclotron passed down the axis of a solenoid which rotated the spin vector about the beam direction into the horizontal plane. It was then focused on to an H2 target. The state of polarization of the beam scattered in the horizontal plane was analysed in an up-down asymmetry measurement by scattering from Be at 12°. Successive measurements were made with the magnetic field adjusted to process the spin through 90°, either in a clockwise or anti-clockwise direction. Ten counter telescopes were used and this allowed measurements to be made simultaneously for 5 scattering angles. Counting rates were typically 4-10 counts/min. Since measurements were made at the same time in both the up and down telescopes, the system was self-monitoring and therefore independent of changes in the intensity when the solenoid magnetic field was reversed. However, since an up and down movement of the centre of gravity of the beam at the target position produces a false asymmetry, the solenoid was positioned so that this movement was less than 0-25 mm. (author)