Accurate  Spectroscopy for Ultracold Neutrons

1. Accurate  Spectroscopyfor Ultracold Neutrons M.J. Betancourt, B.W. Filippone, B. Plaster,J. Yuan Caltech T.M. Ito LANL A.R. Young NCSU Jeff Martin University of Winnipeg S.A. Hoedl U Washington and the UCNA Collaboration 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.

2. Physics: Vud Beta-Asymmetry Parameter:

3. Experimental Method to Measure A • Two important recent achievements in electron detection (for UCNA): • electron backscattering. • 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. grid Experimental Setup 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 Lines = data Histo = simulation Penelope • Additional systematics: • charging • deterioration at high current

9. Current Mode andSi Mode Compared total systematic uncertainty shown

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. UCN Source UCNA Spectrometer detector mount points 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) mm2 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 4 PMTs with magnetic shields (~300 Gauss) MWPC neopentane and nitrogen gas-handling system 100 Torr nitrogen vacuum housing for scintillator and light guides

14. neutron β-decay end-point = 782 keV NEW: On-line performance tests • Conducted with conversion line sources during January 2006 • 113Sn: 364 keV • 207Bi: 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.

15. Reconstruction with source near edge of fiducial volume MWPC position reconstruction 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. Lines = data Histo = simulation Si Det: Final Results Geant 4 Penelope

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