Hall C Moeller Measurements and the Contamination Problem J.Roche, T.Reichelt December 2000 Gen data are being taken with currents around 50 mikroA. The Moeller measurements require currents ~ 1 mikroA. During the November Moeller measurements the runs taken with currents > 0.5 mikroA showed a stable polarization.The reduction of current was done with an attenuator being placed in the Hall C laser beam. Some runs with low currents were considerably low ( ~ 55 % instead of 65 % ) . At first it was believed to be problem of the BCM's being operated out if their nominal range.Then D.Mack pointed out,that this might be related to a conta= mination of the Hall C beam with electrons from the Hall A laser having an opposite polarization.The relative contribution of these "wrong" electrons is negligible during Gen data taking,but may be important for the Moeller runs.A contamination from the Hall B laser is not relevant,because these electrons have the same polarization. Before the December runs a systematic study was made to measure the contamination and look for a remedy.The optical set-up was different,but as in November Hall B and C lasers had the same,the Hall A laser an opposite polarization. Results : Date Run # Lamda/2 Polarization Lasers (%) A B C 12/1 37157 out -70.1+-0.4 off on on 37158 out -53.0+-6 off on off 37159 out -69.9+-0.4 off on on 37160 out +38.4+-2 on on off 12/3 37200 out -62.6+-0.4 on on on 37201 out -71.1+-0.4 off on on 37202 out +51.1+-0.8 on on off During this run it was noticed that the contamination was not stable in time (40 nA ... 160 nA ). Data taken with all three laser "on" gave results between 60 % and 70 %. 37204 in +71.5+-0.4 off on on Conclusions: 1. There is a contamination from the A laser with opposite sign in polarization and from the B laser with the same sign in polarization. 2. The contamination from the A laser is larger and makes the Moller measurement useless,when no counter measures are taken. Possible ways out of the problem: 1. Moller runs only,when Hall A laser is strongly reduced or off 2. Reduce the current not with the attenuator, but with the slit.The slit is placed into the 100 keV electron beam and reduces the "right" and the "wrong" electrons in the same way,leaving the relative contribution unchanged. More Moeller runs were made to clarify this (the last column indicates the method of current reduction): Date Run # Lamda/2 Polarization Lasers (%) A B C 12/4 37246 out -72.9+-0.4 off on on att. 37247 out -72.4+-0.4 on on on slit 12/6 37282 out -72.2+-0.4 on on on slit 37283 in +71.1+-0.4 on on on slit 12/7 37323 in +72.4+-0.4 off on on slit 37324 in +72.6+-0.4 off on on att. 37325 out -73.1+-0.4 off on on att. Conclusions: 1. The polarization measured using the current reduction with the slit is the same as measured when the attenuator is used and the Hall A laser is off. 2. When the Hall a laser is off the measurements made with both methods of current reduction - slit and attenuator - give the same results. The electron beam is moved over the slit in phase with the RF. Only a fraction of the electron puls passes the slit.The phase between laser and RF determines which fraction - central,edge - is used. The polarization of the electrons which finally get accelerated may depend on the phase ( see measurements made in Hall A in 1999 ). When the slit is fully open,the total pulse gets accelerated. A down time of Hall A was used to study this dependence: with an attenuator setting at 380 and the slit being at 12.75, the Moeller coincidence rate and the polarization was measured in the range of 74 degrees to 114 degrees (see also HCLOG #31450 of 12/04/2000 ). The initial phase was at 94 degrees. While the coincidence rate has a maximum at 94 degrees, the polarization remains flat over +- 10 degrees,the acceptance of the fully open slit.This means,that the polarization measured with the Moeller polarimeter and a narrow slit is the same as during Gen running with the slit fully open.