Phillip D. Ferguson

Proposal for a new integrated circuit and electronics neutron experiment source at oak ridge national laboratory

Government and customer specifications increasingly require assessments of the single event effects probability in electronics from atmospheric neutrons. The accelerator that best simulates this neutron spectrum is the WNR facility (Los Alamos), but it is underfunded and oversubscribed for present and future needs. A new beam-line is proposed at the Oak Ridge National Laboratory, as part of the Spallation Neutron Source (SNS).

Performance of a Clad Tungsten Rod Spallation Neutron Source Target

Abstract Tungsten rods, slip-clad with Type 304L stainless steel, performed successfully as a spallation neutron source target operating to a peak fluence of ~4 × 1021 p/cm2. The target was used as a neutron source during the Accelerator Production of Tritium (APT) materials irradiation program at the Los Alamos Neutron Science Center. Tungsten rods of 2.642-mm diameter were slip-fit in Type 304L stainless steel tubes that had an inner diameter of 2.667 mm. The radial gap was filled with helium at atmospheric pressure and room temperature. Los Alamos High Energy Transport (LAHET) calculations suggest a time-averaged peak power deposition in the W of 2.25 kW/cm3. Thermal-hydraulic calculations indicate that the peak centerline W temperature reached 271°C. The LAHET calculations were also used to predict neutron and proton fluxes and spectra for the complex geometry used in the irradiation program. Activation foil sets distributed throughout the experiment were used to determine target neutronics performance as a comparison to the LAHET calculations. Examination of the irradiated target assemblies revealed no significant surface degradation or corrosion on either the Type 304L or the W surfaces. However, it was clear that the irradiation changed material properties because post-proton-irradiation measurements on Type 304L test samples from the APT program demonstrated increases in the yield strength and decreases in the ductility and fracture toughness with increasing dose, and the wrought W rod samples became brittle. Fortunately, the slip-clad target design subjects the materials to very low stress.

The Second Target Station at the ORNL Spallation Neutron Source

After the Spallation Neutron Source (SNS) construction project was completed in June, 2006, development of plans to construct a second target station (STS) at SNS began. These plans have evolved to the establishment of a reference concept for a STS and associated neutron beam instruments, and the evaluation of the expected performance for this station, all of which have been documented in a White Paper. Based on this White Paper, the Department of Energy has approved development of a detailed conceptual design leading to a construction project for the STS. The STS reference design is based on pulse stealing from the 60 Hz SNS accelerator system, with one pulse of every three going to the STS and the other two going to the first target station. The STS would operate in long-proton-pulse mode with no pulse compression in the accumulator ring, and would be optimized for production of intense beams of cold neutrons. The reference concept for the STS and the estimated performance for this concept will be discussed

A SAMPLE ACTIVATION PROGRAM FOR NEUTRON-SCATTERING EXPERIMENTS

Abstract Upon reaching 180 kW, approximately one-eighth of its designed full power, the Spallation Neutron Source (SNS) became the brightest pulsed neutron source in the world in August 2007. This state-of-the-art neutron-scattering facility is expected to attract 1000 to 2000 scientists and engineers each year from universities, industries, and laboratories around the world. The activation level of users’ samples must be estimated before the experiment for proper sample preparation, storage, and postexperiment treatment in compliance with the safety regulations at SNS. A program written in Perl, SAPEU (Sample Activation Program for Easy Use), was developed to serve such requests from the SNS user community. The CINDER’90 library was implemented within the program for tracking the transmutation products of the irradiated sample. The SAPEU program assumes that the incident neutron flux attenuates with the total absorption cross section and calculates the radionuclide inventory, radiotoxicity categories, radiation dose rate, and gamma spectrum during each irradiation period from a simple user input. The SAPEU program can estimate the sample activation due to a cold neutron spectrum, not limited by the 5-meV lowest energy boundary of the CINDER’90 cross-section library. For validation, the SAPEU program methodology was compared to a full analysis involving MCNPX for the flux calculation and CINDER’90 for the activation analysis for typical sample activation cases. The results were in good agreement. Although this program was developed for SNS, it may be useful as a general sample activation prediction tool at any neutron-scattering facility.

MEASURED AND CALCULATED HEATING AND DOSE RATES FOR THE HFIR HB4 BEAM TUBE AND COLD SOURCE

The High Flux Isotope Reactor at the Oak Ridge National Laboratory was upgraded to install a cold source in horizontal beam tube number 4. Calculations were performed and measurements were made to determine heating within the cold source and dose rates within and outside a shield tunnel surrounding the beam tube. This report briefly describes the calculations and presents comparisons of the measured and calculated results. Some calculated dose rates are in fair to good agreement with the measured results while others, particularly those at the shield interfaces, differ greatly from the measured results. Calculated neutron exposure to the Teflon seals in the hydrogen transfer line is about one fourth of the measured value, underpredicting the lifetime by a factor of four. The calculated cold source heating is in good agreement with the measured heating.

Detection of iron overload with the ORNL Spallation Neutron Source: An MCNPX simulation study

In previous work we have demonstrated the use of neutrons to detect iron overload in the liver. We are developing a non-invasive technique to measure liver iron concentration in the human body through neutron inelastic scatter spectroscopy. The measurement is performed using an incident neutron beam that scatters inelastically with iron nuclei in the liver, causing characteristic gamma emission that is used to quantify the tissue iron content. Due to its high neutron flux, the Spallation Neutron Source (SNS) at ORNL presents an attractive option for initial development and optimization of the technique. In this manuscript we describe a simulation study to evaluate feasibility of the SNS beam for iron overload detection in the liver. An MCNPX simulation was developed to model the parameters of the SNS beam and scan a liver phantom with tissue iron content varying from 2 mg/g (mild iron overloaded) to 10 mg/g (severe iron overload). A torso phantom filled with water was placed around the liver and used to simulate scattering effects of the human torso. The emitted gamma spectrum was acquired with a simulated ring detector. Background subtraction was performed by substituting the liver with a water phantom. Background corrected spectra were analyzed to identify gamma lines corresponding to iron in the liver tissue. Statistically significant differences with p ≪ 0.05 were identified for the 56Fe gamma line at 847 keV. Counts in the gamma line were found to be higher in the 10 mg/g sample by a factor of 4.72, differing by less than 6% from the expected value of 5. These results demonstrate the feasibility of the SNS beam to determine iron content in liver tissue.

Mesh Tally Radiation Damage Calculations and Application to the SNS Target System

A new method for the calculation of radiation damage parameters in large geometries encompassing multiple materials using the MCNPX mesh tally has been developed. The method has been tested against previously published calculations of the displacement rate for protons and neutrons at the center of the SNS 316LN stainless steel target vessel nose. Displacement rates for neutrons, protons, and the total using the mesh tally method are shown to agree with previous work. The mesh tally method is also applied to the SNS aluminum moderator vessels and to the SNS inner reflector plug composed of aluminum, beryllium, and stainless steel. Results are given for displacement, helium, and silicon production rates.