A. L. Hallin

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.

Optical calibration hardware for the Sudbury Neutrino Observatory

The optical properties of the Sudbury Neutrino Observatory (SNO) heavy water Cherenkov neutrino detector are measured in situ using a light diffusing sphere (“laserball”). This diffuser is connected to a pulsed nitrogen/dye laser via specially developed underwater optical fibre umbilical cables. The umbilical cables are designed to have a small bending radius, and can be easily adapted for a variety of calibration sources in SNO. The laserball is remotely manipulated to many positions in the D2O and H2O volumes, where data at six different wavelengths are acquired. These data are analysed to determine the absorption and scattering of light in the heavy water and light water, and the angular dependence of the response of the detector’s photomultiplier tubes. This paper gives details of the physical properties, construction, and optical characteristics of the laserball and its associated hardware.

Positron production in heavy ion collisions: Current status of the problem

Narrow peaks have been observed at GSI Darmstadt in the energy spectra of positrons and sum-energy spectra of positron-electron pairs, produced in collisions of very heavy ions. To date, there is no satisfactory explanation of the origin of these lines although many differing models have been proposed. In this contribution, the authors describe the features of a new experiment aimed at the study of the line phenomenon and present the results of their first experiments. The specific goals of their experiment are to clarify the experimental situation regarding the lines through high-resolution, high-statistics data and, by direct measurement of the vector momenta of the peak pairs, to determine their kinematics.

The photomultiplier tube testing facility for the Sudbury Neutrino Observatory

Abstract A facility to test the photomultiplier tubes (PMTs) for the Sudbury Neutrino Observatory (SNO) was constructed at Queens University. The facility measured the noise rate, relative efficiency, and charge and time spectra of single photoelectron anode signals for 44 PMTs daily. The system is described, and detailed results of the testing for two particular PMTs are presented as well as statistics for 9829 Hamamatsu R1408 PMTs accepted for use in SNO.

["Measurement of the Λ→n+γ branching ratio"]

The branching ratio for the {Lambda} weak radiative decay has been measured to be {ital B}({Lambda}{r arrow}{ital n}+{gamma})/({Lambda}{r arrow}anything) =(1.78{plus minus}0.24(stat){plus minus}{sub 0.16}{sup 0.14}(syst)){times}10{sup {minus}3}. A low-energy kaon beam was used to produce the {Lambda} hyperons via the reaction {ital K}{sup {minus}}+{ital p}{r arrow}{Lambda}+{pi}{sup 0} at rest. Photons from the signal channel and {pi}{sup 0} decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

A solenoidal spectrometer for positron-electron pairs produced in heavy-ion collisions

Abstract A new solenoidal spectrometer, designed to study the production mechanism of electrons and positrons in heavy-ion collisions, has been constructed at Argonne National Laboratory. The spectrometer uses a 300 G magnetic field to transport the leptons to two highly segmented silicon arrays that are centered on the solenoid axis, 1.2 m from the target. Positrons are identified by detecting their annihilation radiation with two arrays of position sensitive NaI(Tl) crystals that surround the silicon arrays. A novel design feature of the spectrometer is the ability to measure the angles of emission of the leptons relative to the solenoid axis. The measured response of the apparatus to electrons and positrons is in very good agreement with the calculated response obtained from Monte Carlo simulations.

A large solid-angle array for heavy ions from APEX

A large solid-angle array of Low Pressure Multi-Wire Proportional Counters (LPMWPCs) has been constructed as part of the ATLAS Positron Experiment (APEX). Eight three-element trapezoidal counters span 360-degrees in phi and 20-68-degrees in laboratory theta. Angle sensitivity in theta is provided by a transmission-line delay cathode, while the 24-fold segmentation provides angle sensitivity in phi. Details are given on the design of the counters and associated hardware. Performance of the counters is described, based on tests with scattered heavy ions ranging from Ni-58 to U-238.

Measurement of the. Lambda. r arrow n +. gamma. branching ratio

The branching ratio for the {Lambda} weak radiative decay has been measured to be {ital B}({Lambda}{r arrow}{ital n}+{gamma})/({Lambda}{r arrow}anything) =(1.78{plus minus}0.24(stat){plus minus}{sub 0.16}{sup 0.14}(syst)){times}10{sup {minus}3}. A low-energy kaon beam was used to produce the {Lambda} hyperons via the reaction {ital K}{sup {minus}}+{ital p}{r arrow}{Lambda}+{pi}{sup 0} at rest. Photons from the signal channel and {pi}{sup 0} decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

Apex and the e+/e− Puzzle: Recent Results

Narrow structures first reported in positron singles energy spectra [1, 2, 3, 4, 5, 6, 7] and later in electron-positron energy distributions [5, 8, 9, 10, 11, 12] associated with collisions of very heavy ions near the Coulomb barrier have been an outstanding puzzle in nuclear physics for nearly fifteen years. In a series of positron and positron/electron experiments by three different groups (EPOS, ORANGE and TORI) ([5] and citations therein) at the GSI UNILAC at Darmstadt, Germany narrow structures (‘lines’) were observed by EPOS and ORANGE for several heavy-ion collision systems, with some features such as the line energies and widths having apparently similar values for the different collision systems.

APEX heavy ion counters

A large solid-angle array of low pressure multiwire proportional counters has been constructed as part of the Atlas Positron EXperiment (APEX). Eight three-element trapezoidal counters provide 360-degrees coverage in phi and 20-degrees-68-degrees coverage in theta. Angle sensitivity in theta is provided by a transmission-line delay cathode, while the segmentation provides angle sensitivity in phi. Sample data are presented from the 1450 MeV U-238 + Ta-181 reaction.