The NPDG Motion System for Detector Array

AlignmentChristopher B. CrawfordUniversity of Tennessee

NPDG Collaboration

DNP Meeting, Nashville, TN

2006-10-27

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

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.)

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

detector yield depends on distance

extract positions from the gradient

Formalism

geometry factor 3-d position

target

detector

explicit z dependence

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

Detector Motion Stand constructed at TRIUMF < 0.001” position precision extensive safety features LabVIEW computer interface

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

Results - Cd target, ring 2

x = 5.6 mm y = 2.8 mm

x position (mm)

x po

sitio

n (m

m)

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