M. M. Fowler

Cosmogenic 36Cl accumulation in unstable landforms: 1. Effects of the thermal neutron distribution

Cosmogenic nuclides produced in situ within minerals at the surface of the Earth are proving to be an effective means of assessing geomorphic histories. The use of multiple cosmogenic nuclides permits both exposure times and erosion rates to be determined. However, if two nuclides are produced only by spallation reactions, the systematic differences in their accumulation rates depend only on the differences in their production rates and half-lives. The relatively small differences that result require a high degree of analytical precision to yield useful results. In contrast to other spallogenic nuclides, {sup 36}Cl is also produced by low-energy neutron, absorption, which creates a different pattern of production as a function of depth. We have measured the thermal flux with depth in a concrete block using {sup 3}He-filled neutron detectors. The measured thermal neutron profile agrees well with predictions from a simple diffusion-based thermal neutron distribution model. Calculations of {sup 36}Cl production using the model suggest that the use of {sup 36}Cl along with a purely spallogenic nuclide to determine erosion rates and exposure times should be less sensitive to analytical error than are determinations from two purely spallogenic nuclides. 31 refs., 7 figs., 3 tabs.

A detector for (n,γ) cross-section measurements at a spallation neutron source

Abstract The quest for improved neutron capture cross-sections for advanced reactor concepts, transmutation of radioactive wastes as well as for astrophysical scenarios of neutron capture nucleosynthesis has motivated new experimental efforts based on modern techniques. Recent measurements in the keV region have shown that a 4 π BaF 2 detector represents an accurate and versatile instrument for such studies. The present work deals with the potential of such a 4 π BaF 2 detector in combination with spallation neutron sources, which offer large neutron fluxes over a wide energy range. Detailed Monte Carlo simulations with the GEANT package have been performed to investigate the critical backgrounds at a spallation facility, to optimize the detector design, and to discuss alternative solutions.

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.

Activation rates and chemical recovery of67Cu produced with low energy proton irradiation of enriched70Zn targets

Copper-67 is a radioisotope with significant potential for diagnostic and therapeutic applications in nuclear medicine. Despite its promise,67Cu has failed to make an impact in clinical nuclear medicine, primarily because it is available sporadically, and in limited quantities. Common methods of production rely on high energy proton irradiation of natural zinc targets or on induced reactions using high energy neutrons at nuclear reactors. We have evaluated alternative production methods that could provide year-round adequate supply of this isotope. Using a low energy accelerator, we have studied the production of67Cu by proton reactions on enriched70Zn. Our results indicate that it is possible to produce useful quantities of67Cu from the irradiation of enriched70Zn with protons that have energies of less than 20 MeV. Production rates are higher than currently used methods at high energy accelerators or reactors. This isotope can be made available throughout the year as a result of this research.

Determination of deuterated methanes for use as atmospheric tracers

The deuterated methanes, /sup 13/CD/sub 4/ and /sup 12/CD/sub 4/, are useful as tracers for long-range (>500-km) atmospheric transport. They are easily synthesized and released. Sampling is accomplished by collecting about 330 l of air and chromatographically separating the methane fraction, which is then analyzed by mass spectrometry for the /sup 13/CD/sub 4///sup 12/CH/sub 4/ and /sup 12/CD/sub 4///sup 12/CH/sub 4/ ratios. Detection limits in air are about 7 x 10/sup -16/ STP mol m/sup -3/ for /sup 13/CD/sub 4/ and 2 x 10/sup -15/ STP mol m/sup -3/ for /sup 12/CD/sub 4/. 11 refs., 5 figs.

Prompt radiochemistry at the National Ignition Facility (invited).

Understanding mix in inertial confinement fusion (ICF) experiments at the National Ignition Facility requires the diagnosis of charged-particle reactions within an imploded target. Radiochemical diagnostics of these reactions are currently under study by scientists at Los Alamos and Lawrence Livermore National Laboratories. Measurement of these reactions requires assay of activated debris and tracer gases from the target. Presented below is an overview of the prompt radiochemistry diagnostic development efforts, including a discussion of the reactions of interest as well as the progress being made to collect and count activated material.

Field studies of transport and dispersion of atmospheric tracers in nocturnal drainage flows

Abstract A series of tracer experiments were carried out as part of the Atmospheric Studies in Complex Terrain (ASCOT) program to evaluate pollutant transport and dispersion characteristics of nocturnal drainage flows within a valley in northern California. The results indicate that the degree of interaction of the drainage flows with the larger scale regional flows are strongly dependent on how well the shallow drainage flows are shielded by the surrounding topography from the external environment. For the valley under study, the drainage flows from about mid-slope elevations and below were generally decoupled from the externally generated flows; as evidenced by the similarity of the surface tracer distributions produced during widely varying regional flow conditions. However, tracers released immediately above the drainage flows near the ridge top did reveal considerable mixing between the transition layer flows and the underlying surface drainage flows. Likewise, the transport and dispersion of the tracers at elevated heights within the valley basin were extremely dependent on the influences of the regional scale flows on the valley circulations. The dispersion rates associated with the transition layer flows were dependent on topographic constraints but were appreciably higher than those reported for homogeneous flat terrain situations.

A long-range atmospheric tracer field test

Abstract A long-range application of a heavy methane atmospheric tracer system has been carried out using both 12 CD 4 and 13 CD 4 . The experiment had several objectives including testing a newly developed cyrogenic air sampler, testing a new sample handling apparatus and demonstrating the use of heavy methanes over transport distances up to 2500 km and travel times of more than 100 h. A simple diffusion estimation method applied in conjunction with trajectories produced by NOAA from upper air meteorological data yields good agreement with the tracer data on dosage and arrival time. The duration of observed tracer at the long-range stations was longer than estimated with simple concepts.