Accurate ? Spectroscopy for Ultracold Neutrons

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Accurate ? Spectroscopyfor Ultracold Neutrons

• Jeff Martin
• University of Winnipeg

M.J. Betancourt, B.W. Filippone, B. Plaster, J.
Yuan Caltech
A.R. Young NCSU
T.M. Ito LANL
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.
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Physics Vud
Beta-Asymmetry Parameter
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Experimental Method to Measure A

• Two important recent achievements in electron
  detection (for UCNA)
• electron backscattering.
• detector performance results.

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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
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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

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Experimental SetupA small accelerator to
measure backscattering
Electron gun
Beam diagnostics
Backscattering chamber
Electron Beam
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Experimental Setup

• Two modes
• Silicon detector mode (det on rotating arm)
• Current integration mode (with grid)
• Used in 2003 for Be and Si targets

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New in 2005 Scintillator Target Results
Geant 4
Lines data Histo simulation
Penelope

• Additional systematics
• charging
• deterioration at high current

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Current Mode andSi Mode Compared
total systematic uncertainty shown
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New Statistical Analysis with Floating
Normalization Factor

• Tends to confirm visual comparison
• In general ?2(G4) gt ?2(Penelope)
• For observables free of extrapolation
  uncertainty, Penelope always within 16
• Normalization uncertainty is 12 (double-diff.)
  and 9 (current int)

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2. Detector Performance
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UCN Source
UCNA Spectrometer
detector mount points
field uniformity to 1e-4 (spec 5e-4)
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ß-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
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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.

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MWPC position reconstruction

• Reconstruction with source near edge of fiducial
  volume

important for rejection of events near edge of
UCN trap
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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)

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Summary

• On-line calibration studies of the ß-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

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Si Det Final Results
Lines data Histo simulation
Geant 4
Penelope
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UCNA progress and schedule

• June 2005 December 2005
• Experiment commissioning and UCN source studies
• Short ß-decay run in late-December 2005
• Extracted ß-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