An array of low-background $^3$He proportional counters for the Sudbury Neutrino Observatory

TL;DR

This paper details the design, construction, and deployment of a low-background array of $^3$He proportional counters used in the SNO experiment to measure solar neutrinos via neutral-current interactions.

Contribution

It introduces the first large array of ultra-low-background $^3$He proportional counters for neutrino detection in the SNO experiment.

Findings

Successfully deployed 36 strings of $^3$He counters with low radioactivity

Enhanced the neutral-current sensitivity of the SNO detector

Provided detailed characterization and background analysis of the array

Abstract

An array of Neutral-Current Detectors (NCDs) has been built in order to make a unique measurement of the total active flux of solar neutrinos in the Sudbury Neutrino Observatory (SNO). Data in the third phase of the SNO experiment were collected between November 2004 and November 2006, after the NCD array was added to improve the neutral-current sensitivity of the SNO detector. This array consisted of 36 strings of proportional counters filled with a mixture of $^3$He and CF$_4$ gas capable of detecting the neutrons liberated by the neutrino-deuteron neutral current reaction in the D$_2$O, and four strings filled with a mixture of $^4$He and CF$_4$ gas for background measurements. The proportional counter diameter is 5 cm. The total deployed array length was 398 m. The SNO NCD array is the lowest-radioactivity large array of proportional counters ever produced. This article describes the design, construction, deployment, and characterization of the NCD array, discusses the electronics and data acquisition system, and considers event signatures and backgrounds.