Design, Commissioning and Performance of the PIBETA Detector at PSI

TL;DR

The paper details the design, construction, and calibration of the PIBETA detector at PSI, optimized for precise measurement of pion beta decay, including its components, calibration procedures, and performance results.

Contribution

It introduces a comprehensive detector system specifically designed for high-precision pion beta decay measurements, with detailed performance analysis and calibration results.

Findings

High energy and timing resolution achieved for photons and positrons.

Effective background suppression demonstrated with cosmic muon veto counters.

Detector performance validated during beam periods with various pion stopping rates.

Abstract

We describe the design, construction and performance of the PIBETA detector built for the precise measurement of the branching ratio of pion beta decay, pi+ -> pi0 e+ nu, at the Paul Scherrer Institute. The central part of the detector is a 240-module spherical pure CsI calorimeter covering 3*pi sr solid angle. The calorimeter is supplemented with an active collimator/beam degrader system, an active segmented plastic target, a pair of low-mass cylindrical wire chambers and a 20-element cylindrical plastic scintillator hodoscope. The whole detector system is housed inside a temperature-controlled lead brick enclosure which in turn is lined with cosmic muon plastic veto counters. Commissioning and calibration data were taken during two three-month beam periods in 1999/2000 with pi+ stopping rates between 1.3*E3 pi+/s and 1.3*E6 pi+/s. We examine the timing, energy and angular detector resolution for photons, positrons and protons in the energy range of 5-150 MeV, as well as the response of the detector to cosmic muons. We illustrate the detector signatures for the assorted rare pion and muon decays and their associated backgrounds.