CE75 Experiment at the Indiana University Cooler Facility

In the early days of the RHIC facility, people began discussing the possibility of polarizing the planned proton beams in order to study the spin structure of the partons (quarks and gluons) inside of the proton. One of the many technical challenges of doing this was to find a way to measure the polarization of the proton beams at high energies. There were several proposals, most of which were either too expensive, or had too low figures of merit (not enough rate). One proposal seemed cheap and had a high figure of merit, but had never been attempted.

p +12C elastic scattering in the Coulomb-Nuclear Interference region could be used to measure the polarization of high-energy protons. Such a polarimeter must be capable of detecting recoiling Carbon nuclei of energies as low as 0.1 MeV. In order to successfully detect these low energy carbons, we needed thin targets and detectors with low noise and dead-layers. Microribbon Carbon targets (developed at IUCF) would be thin enough to allow the recoils to exit without high energy loss and multiple scattering, and silicon surface barrier detectors with low-noise co-located preamplifiers (AMPTec) could detect such low-energy heavy ions. (See a picture of the detector module).

We proposed to use the Indiana Cooler to study the feasibility of using the low-energy recoil nuclei to measure p +12C scattering in an accelerator environment (proposal talk). At 200 MeV in the IUCF Cooler a clean sample of scattering events would be prepared by requiring a coincidence with the forward scattered proton. This allowed us to investigate the energy spread of the recoil nuclei due to the thickness of the target and the insensitive entrance window of the detector.

The experiment was successful (see plot of Carbon recoils) and lead to the E950 experiment at the BNL AGS, which measured the assymetry at the RHIC injection energy (~25GeV).