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The NPDG Motion System for Detector Array
AlignmentChristopher B. CrawfordUniversity of Tennessee
NPDG Collaboration
DNP Meeting, Nashville, TN
2006-10-27
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IntroductionNPDG probes the Parity Violating (PV)weak hadronic interactionInterference of strong (PC) and weak (PV) vertex
Low backgrounds: -5 x 10-8 expected asymmetry
insensitive to MOST background physics processes (PC) UP-DOWN
Asymmetry
LEFT-RIGHT
Asymmetry
Cartesian invariants
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Left-Right AsymmetriesThree processes lead to LR-asymmetryPC npdg asymmetry
Csoto, Gibson, and Payne, PRC 56, 631 (1997)
np->np elastic scatteringbeam steered by analyzing power of LH2
eg. 12C used in p,n polarimetry at higher energiesP-wave contribution vanishes as k3 at low energy
Mott-Schwinger scatteringinteraction of neutron spin with Coulomb field of nucleus
electromagnetic spin-orbit interactionanalyzing power: 10-7 at 45 deg
0.23 x 10-8
2 x 10-8
~ 10-8
at 2 MeV
(Michael Gericke et al.)
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Motivation U-D and L-R asymmetry mixing
from misalignment (rotation) of detectors goal: suppression LR asymmetry by factor of
100-1 = sin(0.57 deg)
Geometry factors account for extent of target and detector
can be measuredinstead of MC calculation
Detector systematics demonstrate understanding of rates
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detector yield depends on distance
extract positions from the gradient
Formalism
geometry factor 3-d position
target
detector
explicit z dependence
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CsI(Tl) Detector Array 4 rings of 12 detectors
15 x 15 x 15 cm3 each VPD’s insensitive to B
field detection efficiency: 84% current-mode operation
5 x 107 gammas/pulse counting statistics
limited
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Detector Motion Stand constructed at TRIUMF < 0.001” position precision extensive safety features LabVIEW computer interface
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Data and Analysisyields measured over 5x5 grid in x and yover-determined for study of linearitytargets: B4C, Cd, LH2
analysisdetector rates normalized by beam monitors
pedestals and empty target subtracted
Y(dx,dy) fit to obtain absolute detector positions
relative detector positions compared with physical survey of array
need to be corrected for extended geometry by Monte Carlo
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Results - Cd target, ring 2
x = 5.6 mm y = 2.8 mm
x position (mm)
x po
sitio
n (m
m)
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Conclusions
technique for measuringdetector positions demonstratedwith success on Cd target5 mm precision ~ 1 deg.50 x suppression of L-R asymmetry
LH2 target measurements in progressfirst run affected by venting of target
next run in December