In-situ low-energy background measurement in D2O+salt phase of the Sudbury Neutrino Observatory, using the isotropy of ß-? Cerenkov light

In the second phase of the Sudbury Neutrino Observatory (SNO), salt was added to the D2O, increasing the sensitivity to measurement of the 8B solar neutrino flux through the neutral current reaction, in which the deuteron is split into its proton and its neutron. This reaction is detected by looking for the Cerenkov light which is produced when the neutron is captured on the salt or on the D2O. In this article, I will describe the in-situ method used for the determination of the neutron background from radioactivity within the D2O, as presented in [SNO Collaboration, ArXiv:nucl-ex/0309004, submitted to Phys. Rev. Lett.] (see also R.G.H. Robertson's contribution to this conference). This background is, when combined with ex-situ methods, found to be 0.72 +0.24 -0.23 neutrons per day.