A Low‐Noise CsI Detector Array for the Precision Measurement of Parity Nonconservation in n⃗ + p → d + γ

We have built a CsI(Tl) γ‐ray detector array for the NPDGamma experiment to search for a small parity‐violating directional asymmetry in the angular distribution of 2.2 MeV γ‐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 control of systematic errors at the ppb level, and the use of current‐mode γ‐ray detection with vacuum photo diodes and low‐noise solid‐state preamplifiers. 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 show that the detector array operates at counting statistics and present asymmetry results for B4C , CCl4 , 27Al, Cu, and In. B4C , 27Al, Cu, and In are used throughout the experiment...