E.D. Blakeman

Active-mask coded-aperture imaging

Using MCNP5 code, we modeled an existing coded-aperture, gamma-ray imaging system in order to optimize the thicknesses of its mask and detector elements. We also explored the feasibility of designing an active-mask version of the instrument where the lead elements of the mask are replaced by alkali-halide detectors. The results of these simulations indicate that increasing the detector thickness will improve the instruments sensitivity, but, surprisingly, the active-mask system is only slightly more sensitive than the passive version.

High-efficiency scintillation detector for thermal and high-energy neutrons and gamma radiation

Subcriticality experiments require high-efficiency radiation detectors that are sensitive to neutrons (thermal and fast) over a wide energy range and to gamma radiation. A scintillation detector design that meets these criteria is described. It consists of two scintillators assembled on a single photomultiplier tube. One scintillator is a glass, loaded with lithium enriched in /sup 6/Li, and has high efficiency for low-energy neutrons. The second scintillator is a plastic having very good efficiency for higher-energy neutrons. Efficiency values are reported for various neutron energy ranges and for gamma radiation. >

Neutronic Analyses in Support of the HFIR Beamline Modifications and Lifetime Extension

At the High Flux Isotope Reactor, in operation since 1966 at the Oak Ridge National Laboratory, a larger HB-2 beam tube was installed to enhance capabilities for neutron science research. Neutronic analyses, including dosimetry measurements, radiation transport simulations, and simultaneous neutron and gamma spectrum adjustment calculations, performed to assess the impact of modifications on the PV lifetime are presented.

Characterization of the Neutron Field in the HSSI Reusable Irradiation Facility at the Ford Nuclear Reactor

The characterization of the neutron environment in a reusable facility for irradiation of metallurgical specimens, located at the Ford Nuclear reactor in Michigan, was performed with a combination of transport calculations, extensive dosimeter measurements, and neutron spectrum adjustment. The average difference between the calculated and “measured” reaction rates was ∼7% (the calculations underpredicted the measurements), and the average absolute difference was ∼10%. Therefore, the calculations and measurements agreed very well. The flexible facility performed excellently during several years of operation but was discontinued when the Ford reactor ceased operation. The heavy-section steel irradiation (HSSI) program is taking the initial steps toward selecting a site for a new irradiation facility, similar to the one described here.

Optical design of the SNS liquids reflectometer

Barring a monumental failure of design execution or of performance estimation, the liquids reflectometer at the SNS will provide unprecedented capabilities for the study of liquid and solid surfaces. Design of the instrument is well underway and procurement of the guide components has begun. Nuetrons from a coupled 20-K supercritical hydrogen moderator will be delivered via a multi-channel supermirror bender and tapered guide onto either a horizontal or tilted surface. Collimating slits select the beam incident angle from a 0-5° vertical intensity distribution provided by optics. With the SNS running at 2 MW, the instrument will be able to accumulate a complete specular reflectivity scan from D2O (R < 10-7, Q > 0.5 Å-1) in less than 10 minutes. We will describe the optical design of the SNS liquids reflectometer, compare it with a conventional instrument, and estimate its time resolution for a model kinetic system.

A portable measurement system for subcriticality measurements by the CF-source-driven neutron noise analysis method

A description is given of a portable measurement system consisting of a personal computer used as a Fourier analyzer and three detection channels (with associated electronics that provide the signals to signal-to-digital converters) that has been assembled to measure subcriticality by the /sup 252/Cf-source-driven neutron noise analysis method. The system contains a parallel-plate ionization chamber containing /sup 252/Cf, and two /sup 3/He proportional counters. It is capable of sampling signals at rates up to 80 kHz and processing these data at rates (to form the appropriate spectra) of 2 kHz. >