Compton Backscattering Polarimeter Research Articles

The Compton backscattering polarimeter of the A4 experiment

The A4 collaboration at the Institut für Kernphysik, University of Mainz, is conducting experiments on single-spin asymmetries in the elastic electron–nucleon-scattering which require polarized beams. In order to measure the absolute beam polarization, we have installed a Compton backscattering polarimeter in front of the target, using for the first time the internal cavity concept. A maximum intra-cavity intensity of 90 W has been measured, and in August 2003, first backscattered photons have been detected. Recently, first Compton asymmetries have been measured.

Progress report on the A4 Compton backscattering polarimeter

The A4 collaboration at the Dept. of Nuclear Physics, University of Mainz, is conducting experiments on parity violation in the elastic electron-nucleon-scattering which require the use of polarized beams. In order to measure the absolute beam polarization, we have installed a Compton backscattering polarimeter in front of the target, using for the first time the internal cavity concept. A maximum intracavity intensity of 90 W has been measured, and in August 2003, first backscattered photons have been detected. This article describes the new design concept and the current status and results.

Stabilization system of the laser system of the A4 Compton backscattering polarimeter

The A4 Compton backscattering polarimeter is used to measure the degree of spin polarization of the 855 MeV electron beam provided by the MAMI accelerator facility. Therefore special care must be taken to optimize and stabilize the overlap of electron and laser beam in order to get the highest luminosity for shortest measurement times. For this purpose an active stabilization system for the laser beam position of our intra-cavity polarimeter’s optical resonator has been developed and commissioned.

Electron beam line design of A4 Compton backscattering polarimeter

The new beam line has been built to measure the polarization of the electron. The transverse position and emittance of the MAMI beam are determined from measurements of the beam size on three wire scanners. The position measurement has been done and the emittance measurement is still progressing.

Performance of a hydrogen/deuterium polarized gas target in a storage ring

The performance of a high-density polarized hydrogen/deuterium gas target internal to a medium-energy electron storage ring is presented. Compared to our previous electron scattering experiments with tensor-polarized deuterium at NIKHEF (Zhou et al., Nucl. Instr. and Meth. A 378 (1996) 40; Ferro-Luzzi et al., Phys. Rev. Lett. 77 (1996) 2630; Van den Brand et al., Phys. Rev. Lett. 78 (1997) 1235; Bouwhuis et al., Phys. Rev. Lett. 82 (1999) 687; Zhou et al., Phys. Rev. Lett. 82 (1999) 687) the target figure of merit, ( polarization) 2× luminosity, was improved by more than an order of magnitude. The target density was increased by upgrading the flux of nuclear-polarized atoms injected into the storage cell and by using a longer (60 cm) and colder (∼70 K) storage cell. A maximal target thickness of 1.2 (1.1)±0.1×10 14 nuclei/ cm 2 was achieved with deuterium (hydrogen). With typical beam currents of 110 mA , this corresponds to a luminosity of about 8.4 (7.8)±0.8×10 31 e − nuclei cm −2 s −1 . By reducing the molecular background and using a stronger target guide field, a higher polarization was achieved. The target was used in combination with a 720 MeV polarized electron beam stored in the AmPS ring (NIKHEF) to measure spin observables in electron–proton and electron–deuteron scattering. Scattered electrons were detected in a large acceptance magnetic spectrometer. Ejected hadrons were detected in a single time-of-flight scintillator array. The product of beam and target vector polarization, P e P t, was determined from the known spin-correlation parameters of e′ p quasi-elastic (or elastic) scattering. With the deuterium (hydrogen) target, values up to P e P t=0.49±0.03 (0.32±0.03) were obtained with an electron beam polarization of P e=0.62±0.04 (0.56±0.03) as measured with a Compton backscattering polarimeter (Passchier et al., Nucl. Instr. and Meth. A 414 (1998) 4988). From this, we deduce a cell-averaged target polarization of P t=0.78±0.07 (0.58±0.07), including the dilution by unpolarized molecules.

Investigation on the parity mixing of the 482 keV transition in181Ta by use of a backscattering polarimeter

The circular polarization of the 482 keV gamma radiation in181Ta was remeasured by use of a Compton backscattering polarimeter with the single pulse counting technique. The measurements yieldedPc=−(7.0±6.6)·10−6 which is in good agreement with the results of other groups. The larger value previously measured with the same apparatus has not been confirmed. The erroneous result was caused by an asymmetry in the magnet power supply.

Measurement of the parity mixing of the 501 keV transition in180Hf by use of a backscattering polarimeter

The circular polarization of the 501 keV gamma radiation in180Hf was remeasured by use of a Compton backscattering polarimeter with the single pulse counting technique. The resultPc=−(1.9±0.6) · 10−3 confirms the strong parity mixing which was previously reported by other groups.