A radium assay technique using hydrous titanium oxide adsorbent for the Sudbury Neutrino Observatory

Abstract

As photodisintegration of deuterons mimics the disintegration of deuterons by neutrinos, the accurate measurement of the radioactivity from thorium and uranium decay chains in the heavy water in the Sudbury Neutrino Observatory (SNO) is essential for the determination of the total solar neutrino flux. A radium assay technique of the required sensitivity is described that uses hydrous titanium oxide (HTiO) adsorbent on a filtration membrane together with a β − α delayed coincidence counting system. For a 200 tonne assay the detection limit for 232 Th is a concentration of ∼ 3 × 10 −16 g Th/g water and for 238 U of ∼ 3 × 10 −16 g U/g water. Results of assays of both the heavy and light water carried out during the first two years of data collection of SNO are presented.

Low-energy-threshold analysis of the Phase I and Phase II data sets of the Sudbury Neutrino Observatory

“…Ex situ measurements [34,35] of background levels in the D 2 O and H 2 O provided a priori information for several of them, which were used as constraints in the signal extraction fits. In addition, corrections were applied after the signal extraction fits to account for a number of background event types that contributed much smaller levels of contamination.…”

“…Several radioassays were performed during data taking to measure the concentrations of radon and radium in the D 2 O and H 2 O regions, as described in previous publications [7,34,35]. Although equilibrium was broken in the decay chains, the results are expressed in terms of equivalent amounts of 238 U and 232 Th assuming equilibrium for ease of comparison with 055504-27 other measurements.…”

A scintillator purification plant and fluid handling system for SNO+

“…A feature of QuadraSil-AP is that it can be regenerated with HCl acid, and the metals recovered and analyzed with coincidence beta-alpha counting. This provides a method for ex-situ radioactivity assay for 224 Ra and 226 Ra, as was done in SNO [12]. The method may also allow separate determination for 210 Pb and 210 Bi, to overcome the spectral degeneracy of 210 Bi with CNO neutrinos.…”

Sudbury Neutrino Observatory

“…The H 2 O and D 2 O purification plants were designed to remove Rn, Ra, Th and Pb from the water, thereby eliminating sources giving rise to the high energy γ rays. Two of the main elements of the SNO H 2 O and D 2 O purification plants consisted of newly developed ion-exchange processes using MnOx [9] and HTiO [10], which targeted Ra, Th, and Pb nuclei in the water. With the removal of these elements, secular equilibrium was broken and the short lived daughters quickly decayed away.…”

“…The most dangerous source of such neutrons was those from the photodisintegration of deuterons by γ rays, resulting from decays in the 238 U and 232 Th chains. The levels of U and Th were measured in two ways: ex situ assays of the heavy and light water [9,10], and in situ measurements of 208 Tl and 214 Bi concentrations using the differences in the isotropy of their Cherenkov-light distributions. Both methods agreed well, and by combining them the levels of U and Th in the heavy water were found to be: With these measurements, and those of radioactivity in the light water and acrylic vessel, the total number of background neutrons from photodisintegration in the Phase-I data set was 38.2 +9.4 −9.5 from the 232 Th chain and 33.1 +6.7 −7.1 from 238 U chain.…”