Status report of the In-Beam Polarimeter
R. Bieber, A.M. van den Berg, K. Ermisch, V.M. Hannen, H. Huisman, M.A. de Huu, N. Kalantar-Nayestanaki, J.G. Messchendorp, M. Volkerts, S.Y. van der Werf, H.W. Wilschut
This year several measurements with the In-Beam Polarimeter (IBP) were performed in order to have a well-established procedure for the determination of the beam polarization. First of all, we investigated the influence of some high-energy beam-line parameters on the instrumental asymmetry of the IBP. Due to the fact that at the IBP position, the beam-profile can be rather broad (up to 2 cm), we mounted CH2 spot-targets with a diameter of 2 mm and various thicknesses in the scattering chamber. As a consequence, the instrumental asymmetry of the IBP became almost independent of normal operational changes from the nominal settings of all the beam-line magnets.
To achieve minimal instrumental asymmetries we scanned the spot target across the beam profile using an unpolarized beam (by either switching off the hexapoles of POLIS or by using the CUSP source). We verified that both methods, producing an unpolarized beam, resulted in the same instrumental asymmetry. At the nominal target position the asymmetries for all planes were close to zero. The residual asymmetries were taken into account in the determination of the beam polarization.
We also checked if the beam position at the IBP depends on the polarization state. This was investigated by monitoring the beam profile via beam-position monitors, which have an accuracy of 1 mm. Within this resolution, no beam movements correlated with the state of the polarization were detected. Moreover, a variation of the polarization within the beam profile was investigated. This was done by putting different collimators in the beam. No changes of the polarization outside the uncertainty of the measurements were observed.
The most reliable procedure for an online determination of the beam polarization turned out to be as follows: first, one determines the instrumental asymmetries of the IBP by using an unpolarized beam (in order to correct for them). Then, one measures the different polarization states. The method, based on kinematical coincidences, is described in Ref. .
The determined proton beam polarization (averaged over the different planes) over a period of one week is depicted in Figure 1. The background, originating from accidental coincidences, was observed to be negligible for the proton-proton measurements. The statistical uncertainty is generally smaller than 0.005 while the systematical error is about 0.02. This systematic uncertainty is obtained by forcing the c2-value to go to unity when calculating the average value of all planes. Typically, the polarization of the beam for up and down populated states (strong and weak fields in the transition units) reaches values of ±0.75. A non-zero polarization after turning off both transition units was measured to be 0.077±0.002±0.015.
In addition to the above tests, first measurements of the vector and tensor polarization of a 130 MeV deuteron beam were performed. The measured polarizations were also in the order of 75% of the maximum theoretical value.
-  R. Bieber et al., KVI Annual Report 1998, p. 52.
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