1. Electron Backscattering Jeff Martin University of Winnipeg Outline: Motivation Experimental Setup Results and Comparisons See also: nucl-ex/0508004 Phys. Rev. C 68, 055503 (2003) Collaborators: M.J. Betancourt 1, B.W. Filippone 1, S.A. Hoedl 2, T.M. Ito 3, B. Plaster 1, A.R. Young 4, J. Yuan 1 U Winnipeg, 1 Caltech, 2 U Washington, 3 LANL, 4 NCSU

2. Motivation Electron backscattering is an important systematic effect in many low-energy electroweak experiments. E.g. Asymmetries in Neutron Beta-Decay (UCNA) UCNA Experimental Goal: Asymmetry to 0.2% Residual correction due to backscattering 0.1%

3. Backscattering Data Below 40 keV: lots of data on variety of targets, oblique/normal incidence, integration of current, silicon detectors, secondary electrons, etc. Above 1 MeV: detailed Monte Carlo simulations, relatively well-calibrated. In between: only measurements of normal incidence using integration of current. Our goal: to link the two regimes with detailed measurements, focus on low Z

4. Experimental Setup Electron gun Beam diagnostics Backscattering chamber Electron Beam

5. Experimental Setup grid Two modes: Silicon detector mode (det on rotating arm) Current integration mode (with grid)

6. Silicon detector data Normalized Yield vs. energy deposition in silicon detector Rate at  > 90  is due to rescattering from chamber walls. (“chamber background”)

7. New: Scintillator Target Results Geant 4 Penelope Lines = data Histo = simulation Additional systematics: - charging - deterioration at high current

8. total systematic uncertainty shown Current Mode and Si Mode Compared

9. Statistical Analysis with Single Normalization Factor Tends to confirm visual comparison –In general  2 (G4) >  2 (Penelope) For observables free of extrapolation uncertainty, always within 16% Normalization uncertainty is 12% (double-diff.) and 9% (current int)

10. Conclusions New detailed data set for normal incidence backscattering from 43.5 to 124 keV. Good agreement with Geant4 and Penelope; Penelope gives superior description of relative distributions and normalization. Fit gives normalization scale factors in agreement with unity to within systematic uncertainties of 12% and 9% (mostly). Results will be used to assign systematic uncertainty in UCNA.

11. Si Det: Final Results Geant 4Penelope Lines = data Histo = simulation