Production and characterization of a custom-made 228Th source with reduced neutron source strength for the Borexino experiment

TL;DR

This paper details the production and characterization of a custom 228Th source with reduced neutron emission for the Borexino experiment, including activity transfer efficiency, dose rate, and neutron source strength measurements.

Contribution

A novel method for producing a 228Th source with lower neutron emission, including detailed characterization and activity transfer analysis.

Findings

Activity transfer efficiency >91%

Final activity of 5.41 MBq with uncertainty

Neutron source strength measured as (6.59±0.85)/sec

Abstract

A custom-made 228Th source of several MBq activity was produced for the Borexino experiment for studying the external background of the detector. The aim was to reduce the unwanted neutron emission produced via (alpha,n) reactions in ceramics used typically for commercial 228Th sources. For this purpose a ThCl4 solution was converted chemically into ThO2 and embedded into a gold foil. The paper describes the production and the characterization of the custom-made source by means of gamma-activity, dose rate and neutron source strength measurements. From gamma-spectroscopic measurements it was deduced that the activity transfer from the initial solution to the final source was >91% (at 68% C.L.) and the final activity was (5.41+-0.30) MBq. The dose rate was measured by two dosimeters yielding 12.1 mSv/h and 14.3 mSv/h in 1 cm distance. The neutron source strength of the 5.41 MBq 228Th source was determined as (6.59+-0.85)/sec.