T.H. Burritt

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.

Focal-plane detector system for the KATRIN experiment

Abstract The focal-plane detector system for the KArlsruhe TRItium Neutrino (KATRIN) experiment consists of a multi-pixel silicon p-i-n-diode array, custom readout electronics, two superconducting solenoid magnets, an ultra high-vacuum system, a high-vacuum system, calibration and monitoring devices, a scintillating veto, and a custom data-acquisition system. It is designed to detect the low-energy electrons selected by the KATRIN main spectrometer. We describe the system and summarize its performance after its final installation.

Dead layer on silicon p–i–n diode charged-particle detectors

Semiconductor detectors in general have a dead layer at their surfaces that is either a result of natural or induced passivation, or is formed during the process of making a contact. Charged particles passing through this region produce ionization that is incompletely collected and recorded, which leads to departures from the ideal in both energy deposition and resolution. The silicon p–i–n diode used in the KATRIN neutrino-mass experiment has such a dead layer. We have constructed a detailed Monte Carlo model for the passage of electrons from vacuum into a silicon detector, and compared the measured energy spectra to the predicted ones for a range of energies from 12 to 20 keV. The comparison provides experimental evidence that a substantial fraction of the ionization produced in the “dead” layer evidently escapes by diffusion, with 46% being collected in the depletion zone and the balance being neutralized at the contact or by bulk recombination. The most elementary model of a thinner dead layer from which no charge is collected is strongly disfavored.

Dead layer measurements for KATRIN prototype PIN diode array

Dead layer measurements for a prototype PIN diode array for the KATRIN (KArlsruhe TRItium Neutrino) experiment are presented. KATRIN is a direct neutrino mass measurement of tritium beta decay near the electron endpoint energy of 18.6 keV. A neutrino mass sensitivity goal for KATRIN is m(ve) < 0.2 eV. The methods used to determine the dead layer include the standard angle method, commonly found in texts, and the newly developed energy method. An average dead layer across the entire array of 119 plusmn 3 nm and 109 plusmn 3 nm, was found respectively for the two methods.

Four methods for determining the composition of trace radioactive surface contamination of low-radioactivity metal

Four methods for determining the composition of low-level uranium- and thorium-chain surface contamination are presented. One method is the observation of Cherenkov light production in water. In two additional methods a position-sensitive proportional counter surrounding the surface is used to make both a measurement of the energy spectrum of alpha particle emissions and also coincidence measurements to derive the thorium-chain content based on the presence of short-lived isotopes in that decay chain. The fourth method is a radiochemical technique in which the surface is eluted with a weak acid, the eluate is concentrated, added to liquid scintillator and assayed by recording beta–alpha coincidences. These methods were used to characterize two ‘hotspots’ on the outer surface of one of the 3He proportional counters in the Neutral Current Detection array of the Sudbury Neutrino Observatory experiment. The methods have similar sensitivities, of order tens of ng, to both thorium- and uranium-chain contamination.

Alpha backgrounds for HPGe detectors in neutrinoless double-beta decay experiments

The Majorana Experiment will use arrays of enriched HPGe detectors to search for the neutrinoless double-beta decay of 76Ge. Such a decay, if found, would show lepton-number violation and confirm the Majorana nature of the neutrino. Searches for such rare events are hindered by obscuring backgrounds which must be understood and mitigated as much as possible. A potentially important background contribution to this and other double-beta decay experiments could come from decays of alpha-emitting isotopes in the 232Th and 238U decay chains on or near the surfaces of the detectors. An alpha particle emitted external to an HPGe crystal can lose energy before entering the active region of the detector, either in some external-bulk material or within the dead region of the crystal. The measured energy of the event will only correspond to a partial amount of the total kinetic energy of the alpha and might obscure the signal from neutrinoless double-beta decay. A test stand was built and measurements were performed to quantitatively assess this background. We present results from these measurements and compare them to simulations using Geant4. These results are then used to measure the alpha backgrounds in an underground detector in situ. We also make estimates of surface contamination tolerances for double-beta decay experiments using solid-state detectors.

Surface-alpha backgrounds for the Majorana neutrinoless double-beta decay experiment

The Majorana Experiment will use arrays of enriched HPGe detectors to search for the neutrinoless double-beta decay of 76Ge. Such a decay, if found, would show lepton-number violation, confirm the Majorana nature of the neutrino, and help determine the effective Majorana neutrino mass. A potentially important background contribution to this and other double-beta decay experiments arises from decays of alpha-emitting isotopes in the 232Th and 238U decay chains on and near the surfaces of the detectors. An alpha particle emitted from the surface can lose energy within the dead region of a detector, depositing only a partial amount of its kinetic energy within the active region and possibly mimicking the energy signal from neutrinoless double-beta decay. Cleanliness, exposure to radon, detector design, and analysis techniques all contribute to the effect from surface alphas. Our experimental and simulation efforts to understand and mitigate surface alpha backgrounds for both n-type and p-type HPGe detectors will be presented.

Background simulations and detector design for the KATRIN experiment

The Karlsruhe Tritium Neutrino Experiment (KATRIN) aims to measure directly the mass of the neutrino, an open question in physics. The experiment will measure the integrated tritium beta-decay electron energy spectrum near the 18.6 keV endpoint, where the shape is most sensitive to neutrino mass. In order to reach the proposed sensitivity to neutrino mass of 0.2 eV (90% CL) the detector backgrounds in the endpoint energy region must be limited to 1 mHz. Extensive Geant4 simulations of the KATRIN detector region have identified the largest contributions to the background and guided the detector design. For this experiment, at the surface of the earth, the major backgrounds will be cosmic ray induced photons as well as betas and high-energy gammas from natural radioactivity. Cosmic rays and their secondaries can be vetoed with an active shield. Careful material selection can reduce natural radioactivity, and a passive shield can mitigate radioactivity-induced backgrounds. Post-acceleration of electrons emerging from the KATRIN spectrometer can raise the signal energy to lower background regions. Armed with an understanding of the major background mechanisms, the detector design has been optimized to reduce the total background to the 1 mHz goal.