Electron Backscattering

1
Electron Backscattering

• Jeff Martin
• University of Winnipeg

Collaborators M.J. Betancourt1, B.W.
Filippone1, S.A. Hoedl2, T.M. Ito3, B. Plaster1,
A.R. Young4, J. Yuan1 U Winnipeg, 1 Caltech, 2 U
Washington, 3 LANL, 4 NCSU

• Outline
• Motivation
• Experimental Setup
• Results and Comparisons

See also nucl-ex/0508004 Phys. Rev. C 68, 055503
(2003)
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

• 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 ? gt 90? is due to
rescattering from chamber walls. (chamber
background)
7
New Scintillator Target Results
Geant 4
Lines data Histo simulation
Penelope

• Additional systematics
• charging
• deterioration at high current

8
Current Mode andSi Mode Compared
total systematic uncertainty shown
9
Statistical Analysis with Single Normalization
Factor

• Tends to confirm visual comparison
• In general ?2(G4) gt ?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
Lines data Histo simulation
Geant 4
Penelope