1. Accurate  Spectroscopy for Ultracold Neutrons Jeff Martin University of Winnipeg See also: J.W. Martin et al, Phys. Rev. C 73 015501 (2006) J.W. Martin et al, Phys. Rev. C 68, 055503 (2003) T.M. Ito et al, NIM A, in preparation. M.J. Betancourt, B.W. Filippone, B. Plaster, J. Yuan Caltech S.A. Hoedl U Washington A.R. Young NCSU T.M. Ito LANL and the UCNA Collaboration

2. Physics: V ud Beta-Asymmetry Parameter:

3. Experimental Method to Measure A Two important recent achievements in electron detection (for UCNA): 1.electron backscattering. 2.detector performance results.

4. 1. Electron Backscattering 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%

5. 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

6. Experimental Setup: A small accelerator to measure backscattering Electron gun Beam diagnostics Backscattering chamber Electron Beam

7. Experimental Setup grid Two modes: Silicon detector mode (det on rotating arm) Current integration mode (with grid) Used in 2003 for Be and Si targets

8. New in 2005: Scintillator Target Results Geant 4 Penelope Lines = data Histo = simulation Additional systematics: - charging - deterioration at high current

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

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

11. 2. Detector Performance

12. UCNA Spectrometer detector mount points UCN Source field uniformity to 1e-4 (spec: 5e-4)

13. β-Detector Package  MWPC: position information, capture gamma rejection, low threshold for identification of backscattering  (163 × 163) mm 2 active area  100 Torr neopentane gas  thin entrance/exit windows  Plastic scintillator: energy and timing information  15-cm diameter, 3.5-mm thickness  adiabatic light guides around edge of disk T.M. Ito et al., in preparation for NIM A MWPC entrance window (25-micron) facing decay trap MWPC 100 Torr nitrogen vacuum housing for scintillator and light guides 4 PMTs with magnetic shields (~300 Gauss) neopentane and nitrogen gas-handling system

14. NEW: On-line performance tests  Conducted with conversion line sources during January 2006  113 Sn: 364 keV  207 Bi: 481 keV, 975/1047 keV  Motion vacuum feedthrough used to move thin point sources throughout fiducial region  Confirms energy calibration of the spectrometer, suppression of background gammas. neutron β-decay end-point = 782 keV

15. MWPC position reconstruction  Reconstruction with source near edge of fiducial volume important for rejection of events near edge of UCN trap

16. Conclusions New dataset on electron backscattering: –Fit gives normalization scale factors in agreement with unity to within systematic uncertainties of 12% and 9%. UCNA spectrometer commissioned in detail using radioactive sources. Upcoming work (beam on target last Thurs.): –UCN source commissioning –detailed UCN guide tests –construction of cosmic muon veto –spectrometer cooldown for more tests late summer (radioactive Xe calibration system)

17. Summary  On-line calibration studies of the β-spectrometer for the UCNA experiment conducted with conversion-line sources  Shown feasibility of extracting position information from the scintillator and measured the gain as a function of position in the fiducial volume  MWPC  Reconstructs (x,y) position distributions with widths of ~few mm  Requiring coincidence between MWPC and scintillator greatly reduces ambient room backgrounds  Using information from opposite-side MWPC provides identification of backscattering events  Calibration using gaseous source of radioactive Xe isotopes under development

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

19. UCNA progress and schedule  June 2005 – December 2005  Experiment commissioning and UCN source studies  Short β-decay run in late-December 2005  Extracted β-decay rate consistent with known UCN production and transport to spectrometer  May 2006 – …  May 1: LANSCE proton beam returns  May 2006 – July 2006: source commissioning and UCN guide transport studies  Fall 2006: first physics run for A measurement