A Current Mode Detector Array for Gamma-Ray Asymmetry Measurements

Abstract

We have built a CsI(Tl) gamma-ray detector array for the NPDGamma experiment to search for a small parity-violating directional asymmetry in the angular distribution of 2.2 MeV gamma-rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion-nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires a high cold neutron flux, control of systematic errors at the ppb level, and the use of current mode gamma-ray detection with vacuum photo diodes and low-noise solid-state preamplifiers. The average detector photoelectron yield was determined to be 1300 photoelectrons per MeV. The RMS width seen in the measurement is therefore dominated by the fluctuations in the number of gamma rays absorbed in the detector (counting statistics) rather than the intrinsic detector noise. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of $10^{-9}$ in a few hours. We report on the design, operating criteria, and the results of measurements performed to test the detector array.

33 pages, 20 figures, 2 tables

References in corpus (5)

• A measurement of parity-violating gamma-ray asymmetries in polarized cold neutron capture on 35Cl, 113Cd, and 139La
• Energy calibration of the NaI(Tl) calorimeter of the SND detector using cosmic muons
• Optics: general-purpose scintillator light response simulation code
• Parity conserving gamma asymmetry in n-p radiative capture
• Progress toward a new measurement of the parity violating asymmetry in n+p-->d+gamma

Cited by in corpus (8)

• Hadronic Parity Violation: a New View through the Looking Glass
• Experiments in Fundamental Neutron Physics
• First Observation of $P$-odd $\gamma$ Asymmetry in Polarized Neutron Capture on Hydrogen
• High-Efficiency Resonant RF Spin Rotator with Broad Phase Space Acceptance for Pulsed Polarized Cold Neutron Beams
• Upper Bounds on Parity Violating Gamma-Ray Asymmetries in Compound Nuclei from Polarized Cold Neutron Capture
• Monte Carlo calculation and verification of the geometrical factors for the NPDGamma experiment
• Measurement of the Parity-Odd Angular Distribution of Gamma Rays From Polarized Neutron Capture on ${}^{35}$Cl
• Monte Carlo calculation of the average neutron depolarization for the NPDGamma experiment