Robert G. Downing

Conduction processes in boron- and nitrogen-doped diamond-like carbon films prepared by mass-separated ion beam deposition

Abstract Boron- and nitrogen-doped diamond-like amorphous carbon (DLC) films were prepared by alternating direct deposition of low energy mass-separated 12C+ and dopant ions. Concentration vs. depth profiles for N and B dopants were determined by neutron depth profiling. The measured current-voltage characteristics of these films, which were deposited on polished stainless steel, are explained best by Frenkel-Poole emission for high electric fields. Two different trap states Φ1 and Φ2 were found to contribute to the conduction process. At low electric fields our results suggest that conduction is due to variable-range hopping via localized states at the Fermi level. The doped DLC films show a higher electrical conductivity, indicative of an increased density of localized states, rather than a shift in the Fermi level. A diode-like device was prepared, but the measured I-V curves did not indicate that a p-n junction had formed. DLC/Si heterojunctions were also prepared and their current-voltage characteristics are presented and discussed.

Neutron depth profiling at the National Bureau of Standards

Abstract The recently established neutron depth profiling facility at the 10 MW reactor of the National Bureau of Standards in Gaithersburg, MD is described. The facility uses a 9.5 mm diameter neutron beam filtered through 200 mm of single crystal sapphire to give an intensity of 3 × 10 8 n/s, a cadmium ratio for gold of 10 4 and a gamma dose of only 200 mrad/h. Examples of initial work are: range measurements with boron implanted in silicon; observations on near-surface boron in glass; and boron concentration profiles in thin layers of borophosphosilicate glass.

Neutron focusing optic for submillimeter materials analysis

A neutron lens constructed with polycapillary glass fibers is used to focus a 50×45 mm2 beam exiting a cold neutron guide onto a spot of 0.53 mm (full width at half maximum) with a current density gain of 80. The characteristics of the lens are presented. This lens is designed to enhance the detection limit and lateral resolution for prompt gamma activation analysis using cold neutron beams.

Prompt gamma activation analysis enhanced by a neutron focusing capillary lens

Abstract A focusing neutron lens using glass polycapillary fibers has been introduced successfully into a prompt gamma activation analysis (PGAA) instrument placed at the exit of a cold neutron guide. The neutron current density gain of the lens is 80, averaged over the focused beam size of 0.53 mm diameter. PGAA measurements have been made on submillimeter particles of gadolinium and cadmium. The results indicate that elemental sensitivities of measurements are increased by ∼ 60, and that particles of sizes smaller than 0.5 mm can be discerned using the focusing lens. The measured gain in prompt gamma signals for these particles is less than anticipated, probably due to alignment difficulties. Gamma ray background associated with the lens is discussed and improvements are suggested.

Analytical applications of neutron depth profiling

Using a low-energy neutron beam as an isotopic probe, neutron depth profiling (NDP) provides quantitative depth profiles in nearly all solid matrix materials. Several of the light elements, such as He, Li, B, and N can be nondestructively analyzed by NDP. The information obtained using NDP is difficult if not impossible to determine by non-nuclear techniques. As a result, NDP is used collaboratively with techniques such as SIMS, RBS, FTIR, PGAA, and AES. Profiles measured by NDP are given for semiconductor and optical processing materials, and light weight alloys. Improvements in the technique are discussed with emphasis on the use of intense cold neutron beams.

High-resolution charged particle and neutron imaging using charge injection devices

A charge injection device (CID) camera and image processing system have been used as a position sensitive detector for energetic charged particles and low energy neutrons. This video radiation detector (VRD) is simple in design but highly effective for real-time radiography and dosimetry with many advantages characteristics. The VRD currently has a dynamic range of 65,000 intensity levels for a 755 X 484 pixel matrix, an active area of 7 mm X 9 mm, a spatial mapping resolution of about 14 micrometers for single detected events (7 micrometers for radiation from a point source), and is sufficiently radiation-hard to be operated in a neutron beam for extended periods of time. Radiation images are updated at a rate of thirty frames per second. The VRD is sensitive to fission fragments, alpha particles, and slow neutrons. Using commercially available image processing hardware and software and an off-the-shelf camera, the system is inexpensive, easy to use with simple interpretation of data, and is capable of performing radiography with only minimal adaptations. Applications in our laboratory include the characterization of focused cold neutron beams, the mapping of uranium and lithium distributions in samples by the detection of neutron absorption reaction products, and the mapping of spontaneous alpha radioactivity from environmental samples. Results provide information on x-y position, counts received, and energy deposited per count, each as a function of time.

The efficiency of thermal neutron detection and collimation with microchannel plates of square and circular geometry

Detectors with microchannel plates (MCPs) are currently widely used in photon and charged particle detection with high spatial (/spl sim/10 /spl mu/m) and temporal (<0.5 ns) resolution. All the advances in MCP detection technologies can be successfully implemented for the detection of thermal neutrons by using microchannel plates manufactured from a modified glass mixture doped with neutron absorbing atoms. In this paper we compare the efficiency of thermal neutron detection for two standard MCP geometries: circular-pore and square-pore microchannel plates doped with the /sup 10/B isotope. The results of our modeling indicate that the detection of thermal neutrons with a square-pore MCP is 11-23% more efficient than for the circular geometry, and can be as high as /spl sim/80% for the existing MCP technology. The same microchannel plates can be used as very efficient and compact thermal neutron collimators. In this paper we compare the efficiency of circular- and square-pore MCP collimators with the help of our model, the validity of which has already been verified by our experimental measurements reported last year. The rocking curve of 5 mm and 2.5 mm thick MCPs doped with 3 mole% of /sup nat/Gd/sub 2/O/sub 3/ is predicted to be only /spl plusmn/0.1/spl deg/ and /spl plusmn/0.3/spl deg/ wide, respectively, for both geometries. A very compact device with high thermal neutron detection efficiency and angular sensitivity can be built by combining an MCP neutron detector with an MCP collimator.

Very compact high performance microchannel plate thermal neutron collimators

In most neutron scattering experiments and in boron neutron capture therapy, the angular spread of the neutron beam is defined by the quality of the neutron collimator. A typical collimator consists of a large number of parallel plates coated with neutron absorbing material, and at present these plates are at least few centimeters in length. In order to obtain collimation in both vertical and horizontal planes, two orthogonally aligned collimators are installed in the neutron beam. We present a new type of high performance neutron collimator made with Gd-doped microchannel plates (MCPs). Such collimators are only few millimeters thick and the rocking curve is expected to be even sharper than that of conventional 0.5/spl deg/ collimators. While collimation is performed in two perpendicular planes simultaneously, the geometry of these new collimators can be changed so that the degree of collimation in each direction is controlled independently. The modeling of the proposed collimator indicates that for the existing MCP technology the rocking curve can be made as sharp as 0.2/spl deg/ FWHM, which can be further improved by current developments in the MCP technology. The preliminary experimental evaluation of our first very thin (only 0.6 mm) MCP collimators confirms the accuracy of our numerical model.

A comparison of experiment and simulation for neutron guidance through glass polycapillary fibers

The transmission efficiency and exit divergence of cold neutrons guided through straight and bent polycapillary fibers made of borosilicate glass have been measured. Experimental results have been compared with a ray‐tracing simulation. Good agreement between the two has been obtained by considering in the simulation only reflectivity losses due to absorption. Therefore, we conclude that the influence of other loss mechanisms on reflectivity, such as surface roughness and waviness, are not significant compared with absorption loss for the borosilicate polycapillary fibers we have measured. Using the same simulation program, we have characterized the performance of a neutron focusing lens placed at the end of a 58Ni guide tube and optimized for a neutron spectrum from a cold source at a temperature of 65 K. An order of magnitude increase in neutron intensity within a submillimeter focal spot is predicted.

Analysis of cubic boron nitride thin films by neutron depth profiling

Abstract Cubic boron nitride (c-BN) thin films are of interest in such diverse areas as semiconducting devices, wear resistant coatings, and low friction coatings, owing to the exceptional electrical and mechanical properties of that material. In this work, the boron to nitrogen ratio of thin films produced by physical vapor deposition (PVD) was determined and compared with the crystal phase of BN thin films. Thin films (approximately 0.1 μm) of c-BN were deposited onto heated silicon substrates by electron beam evaporation of boron with concurrent nitrogen and argon ion bombardment. The films were characterized by IR spectroscopy. The stoichiometry of the BN layer was determined by neutron depth profiling (NDP) using the nuclear reactions 10 B(n,α) 7 Li and 14 N (n,p) 14 C. Boron-to-nitrogen ratios were determined with uncertainties of 2%–3%. Shifts in IR spectra were observed over a narrow range of boron-to-nitrogen ratios near unity. Stoichiometric results are compared with the crystal phase. Details of the NDP analysis are presented.