Speaker
Description
For the Electron-Ion Collider, a polarized ${}^3\mathrm{He}^{++}$ ion source is being constructed at the Electron Beam Ionization Source (EBIS) of the Brookhaven National Laboratory. For precision determination of the ${}^3\mathrm{He}$ polarization, the ${}^3\mathrm{He}$ beam, after acceleration to 6 MeV at EBIS, will be elastically scattered off a gas ${}^4\mathrm{He}$ target. For such a scattering, the analyzing power $A_\mathrm{N}(E_\mathrm{beam},\theta_\mathrm{CM})$ can reach absolute, $100\%$, maximum at some points in the beam energy / center of mass scattering angle plane [1]. Several such points were found in Refs. [2, 3] including $(E_\mathrm{beam}=5.3\,\mathrm{MeV}, \theta_\mathrm{CM}=91^\circ)$.
The vertically polarized $6\,\mathrm{MeV}$ ${}^3\mathrm{He}^{++}$ ion beam will enter, through a thin window (to minimize the energy loss of the beam), to the scattering chamber filled with ${}^4\mathrm{He}$ gas at a pressure of $\sim5\,\mathrm{Torr}$. Two left-right symmetric Si strip detectors (with vertically oriented strips) will be used to detect both scattered $^3\mathrm{He}$ and recoil $^4\mathrm{He}$ particles in every event. Good energy and time resolution of the detectors will allow us to recognize $^3\mathrm{He}$ and $^4\mathrm{He}$ signals and to eliminate background events. A spin rotator will provide the beam spin-flip to suppress the acceptance and intensity related systematic errors.
For the polarimeter calibration, we plan to scan the $^3\mathrm{He}$ energy (by variation the entrance window thickness) and to measure the spin-correlated asymmetry dependence on scattering angle $\theta_\mathrm{CM}$. Analyzing power $A_\mathrm{N}=100\%$ can be attributed to the absolute maximum found in these measurements.
[1] G. Plattner et al., “Absolute calibration of spin – 1/2 polarization”, Phys. Lett. B 36, 211 (1971)
[2] D.M. Hardy et al., “Polarization in $^3\mathrm{He}$ + $^4\mathrm{He}$ elastic scattering”, Phys. Lett. B 31, 355 (1970)
[3] W.R. Boykin, S.D. Baker, D.M. Hardy, “Scattering of $^3\mathrm{He}$ and $^4\mathrm{He}$ from polarized $^3\mathrm{He}$ between 4 and 10 MeV”, Nucl. Phys. A 195, 241 (1972)
