Lead 208 Pb Radius Experiment PREX Elastic Scattering
Lead (208 Pb) Radius Experiment : PREX Elastic Scattering Parity-Violating Asymmetry E = 1 Ge. V, Z 0 : Clean Probe Couples Mainly to Neutrons FN (Q 2) Applications : Nuclear Physics, Neutron Stars , Atomic Parity, Heavy Ion Collisions Nuclear Theory Ran Mar 19 – June 20, 2010 ( Dany Page ) Obtained 20% statistics Main problem: radiation degradation of vacuum seals and HRS controls systems Accomplishments : PREX : density -dependence of the symmetry energy. Nuclear physics community very interested in this ! • • • Statistics limited, negligible noise Pb target ran at 50 u. A with last 3 days at 70 u. A Precision Compton and Moller polarimetry (1. 5%) Thin quartz detector for high rates (1 GHz) Septum magnet at high luminosity <Q 2> : low-current cavity monitors, GEM detectors
PREX Physics Asymmetry vs Data Set Run-averaged asymmetry placed at “ 1. 0” (approx ppm) for presentation. “Blinded” Asymmetry Good chi-square: data are statistical All data. Sign flips removed (product of Wien flip and IHWP flip) “slug” ~ 1 day Lessons of PREX • Last 3 days @ 70 u. A promising : 125 ppm detector noise @15 Hz equiv. flip rate • Raster synched to flip rate to reduce noise due to target. • Transverse asymmetry AT < 2 ppm ½ our data; the sign flips with Wien x IHWP flip Plan to release results at April 2011 APS meeting. Expect d. RN / RN ~ 2. 5% ( Ultimate goal : 1% ) Proposed new measurements with fixes to vacuum system and shielding
Apparatus was assembled correctly at the 1 mm level.
Problem: Missing acceptance. Q 2 pushed high Kiad’s HAPLOG 2450: We were underbending by 5 mrad, need to run 760 A septum (was 729 A) Non-optimal Q 2
R. Michaels, Oct 2010 (thanks, Bogdan W. ) Need GEANT simulations to design the shielding Need engineering and design support.
How good is HAMC ? Using HAMC (Monte Carlo) to predict a future running point. 1 - HRS Keep apparatus same Run septum @ 775 A Ebeam = 925 to 1000 Me. V
Future Run of PREX
Lead / Diamond Target Seem to melt after ~1 week We melted two targets A third ran for 4 days at 70 u. A. Strategy ? Run with ~12 targets 6 lead/diamond 6 lead/carbon (carbon more robust)
PREX: Things to Do / Conclusions d. RN/R N = 1 % 12 Ge. V run ? Thinking: Oct – Dec 2013 Hall A Commissioning. Asking for 8 to 10 weeks. Go to PAC in Dec 2011. Assuming approval : In ~12 month period prior to experiment: • Re-design beamline to withstand radiation & thermal cycling. • Design radiation shielding to protect equipment. • Minor mods to targets (try Pb/carbon, add more targets).
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