Transcript Electron Backscattering - Experimental Subatomic Physics

Accurate  Spectroscopy
for Ultracold Neutrons
Jeff Martin
University of Winnipeg
M.J. Betancourt, B.W. Filippone, B. Plaster, J. Yuan
Caltech
S.A. Hoedl
U Washington
T.M. Ito
LANL
A.R. Young
NCSU
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.
Physics: Vud
2  
Beta-Asymmetry Parameter: A  2
1  32
Experimental Method to
Measure A
dW  1  PA cos dE 
N  N 1
Aexp E   
 AP

2
N N
Two important recent
achievements in electron
detection (for UCNA):
1. electron backscattering.
2. detector performance results.
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%
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
Experimental Setup:
A small accelerator to measure backscattering
Electron gun
Beam diagnostics
Backscattering chamber
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
New in 2005:
Scintillator
Target Results
Geant 4
Lines = data
Histo = simulation
Penelope
Additional systematics:
- charging
- deterioration at high current
Current Mode and
Si Mode Compared
total
systematic
uncertainty
shown
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)
2. Detector Performance
UCN Source
detector mount points
field uniformity to 1e-4 (spec: 5e-4)
β-Detector Package
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MWPC: position information, capture gamma rejection, low threshold
for identification of backscattering
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(163 × 163) mm2 active area
100 Torr neopentane gas
thin entrance/exit windows
Plastic scintillator: energy and timing information


T.M. Ito et al., in
preparation for NIM A
15-cm diameter, 3.5-mm thickness
adiabatic light guides around edge of disk
MWPC entrance
window (25-micron)
facing decay trap
4 PMTs with
magnetic shields
(~300 Gauss)
MWPC
100 Torr nitrogen vacuum
housing for scintillator
and light guides
neopentane and
nitrogen gas-handling
system
NEW: On-line performance tests

Conducted with conversion line sources during January 2006
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113Sn:
364 keV
207Bi: 481 keV, 975/1047 keV
neutron β-decay end-point = 782 keV
Motion vacuum feedthrough used to move thin point sources
throughout fiducial region
Confirms energy calibration of the spectrometer, suppression of
background gammas.
MWPC position reconstruction

Reconstruction with source
near edge of fiducial volume
important for rejection of events near
edge of UCN trap
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.):
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UCN source commissioning
detailed UCN guide tests
construction of cosmic muon veto
spectrometer cooldown for more tests late summer
(radioactive Xe calibration system)
Summary
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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
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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
Si Det: Final Results
Geant 4
Lines = data
Histo = simulation
Penelope
UCNA progress and schedule
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June 2005 – December 2005
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Experiment commissioning and UCN source studies
Short β-decay run in late-December 2005
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Extracted β-decay rate consistent with known UCN production
and transport to spectrometer
May 2006 – …
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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