H. Fergani

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

An array of Neutral-Current Detectors (NCDs) has been built in order to make a unique measurement of the total active ux 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 lled with a mixture of 3He and CF4 gas capable of detecting the neutrons liberated by the neutrino-deuteron neutral current reaction in the D2O, and four strings lled with a mixture of 4He and CF4 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.

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 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 2 years of data collection of SNO are presented.

Measurement of radium concentration in water with Mn coated beads at the Sudbury Neutrino Observatory

We describe a method to measure the concentration of and in the heavy water target used to detect solar neutrinos at the Sudbury Neutrino Observatory and in the surrounding light water shielding. A water volume of 50– from the detector is passed through columns which contain beads coated with a compound of manganese oxide onto which the Ra dissolved in the water is adsorbed. The columns are removed, dried, and mounted below an electrostatic chamber into which the Rn from the decay of trapped Ra is continuously flowed by a stream of N2 gas. The subsequent decay of Rn gives charged Po ions which are swept by the electric field onto a solid-state α counter. The content of Ra in the water is inferred from the measured decay rates of , , , and . The Ra extraction efficiency is >95%, the counting efficiency is 24% for and 6% for , and the method can detect a few atoms of per m3 and a few tens of thousands of atoms of per m3. Converted to equivalent equilibrium values of the topmost elements of the natural radioactive chains, the detection limit in a single assay is a few times Th or U/cm3. The results of some typical assays are presented and the contributions to the systematic error are discussed.

Measurement of 222Rn dissolved in water at the Sudbury Neutrino Observatory

The technique used at the Sudbury Neutrino Observatory (SNO) to measure the concentration of in water is described. Water from the SNO detector is passed through a vacuum degasser (in the light water system) or a membrane contact degasser (in the heavy water system) where dissolved gases, including radon, are liberated. The degasser is connected to a vacuum system which collects the radon on a cold trap and removes most other gases, such as water vapor and N2. After roughly of H2O or of D2O have been sampled, the accumulated radon is transferred to a Lucas cell. The cell is mounted on a photomultiplier tube which detects the α-particles from the decay of and its progeny. The overall degassing and concentration efficiency is about 38% and the single-α counting efficiency is approximately 75%. The sensitivity of the radon assay system for D2O is equivalent to water. The radon concentration in both the H2O and D2O is sufficiently low that the rate of background events from U-chain elements is a small fraction of the interaction rate of solar neutrinos by the neutral current reaction.