The NPDG Motion System for Detector Array Alignment

1
The NPDG Motion System for Detector Array
Alignment

• Christopher B. Crawford
• University of Tennessee
• NPDG Collaboration
• DNP Meeting, Nashville, TN
• 2006-10-27

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Introduction

• NPDG probes the Parity Violating (PV)weak
  hadronic interaction
• Interference 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 Asymmetries

• Three processes lead to LR-asymmetry
• PC npdg asymmetry
• Csoto, Gibson, and Payne, PRC 56, 631 (1997)
• np-gtnp elastic scattering
• beam steered by analyzing power of LH2
• eg. 12C used in p,n polarimetry at higher
  energies
• P-wave contribution vanishes as k3 at low energy
• Mott-Schwinger scattering
• interaction of neutron spin with Coulomb field of
  nucleus
• electromagnetic spin-orbit interaction
• analyzing 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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Formalism

• detector yield depends on distance
• extract positions from the gradient

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CsI(Tl) Detector Array

• 4 rings of 12 detectors
• 15 x 15 x 15 cm3 each
• VPDs 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
• lt 0.001 position precision
• extensive safety features
• LabVIEW computer interface

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Data and Analysis

• yields measured over 5x5 grid in x and y
• over-determined for study of linearity
• targets B4C, Cd, LH2
• analysis
• detector 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 position (mm)
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Preliminary Results, LH2

• taken after target started warming up

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Conclusions

• technique for measuringdetector positions
  demonstratedwith success on Cd target
• 5 mm precision 1 deg.
• 50 x suppression of L-R asymmetry
• LH2 target measurements in progress
• first run affected by venting of target
• next run in December