John E. Eberhardt

Reflectivity of natural and powdered minerals at CO2 laser wavelengths

The diffuse reflectance and depolarization properties of natural and powdered minerals were examined at 128 CO2 laser wavelengths. Powder reflectivity was classified into three regimes: (1) surface (reststrahlen); (2) low intermediate; and (3) bulk (Kubelka-Munk) remission from subsurface grains. Data are presented on NaCl, Al2O3, MgO, BaCO3, CaCO3, BaSO4, feldspar (NaAlSi3O8), and apatite [Ca5F(PO4)3]. Reduction of feldspar rocks to 210-μm grain size had little effect on their reflectance spectra. Kubelka-Munk-type behavior seems unlikely to dominate the reflectance spectra of natural surfaces. Albedos were measured for NaCl, sulfur, gold-plated sandpaper, graphite, and sandblasted aluminum.

Development of a Fast-Neutron Detector With Silicon Photomultiplier Readout

A robust neutron detector was developed to monitor the neutron flux in fast-neutron/gamma radiography (FNGR) cargo scanners. The main features of the new neutron detector were an ability to handle high count rate for statistical precision and a high damage threshold for longevity when used close to intense fast neutron sources (up to 1010 n/s). The detector comprises a fast scintillator coupled to a shielded 100 pixel Hamamatsu 1 mm times 1 mm silicon photomultiplier (SiPM) via a 220 cm length of wavelength shifting (WLS) plastic fibre. The WLS fibre was glued onto two long sides of a 10 times 10 times 50 mm scintillator wrapped in Tyvek reflector. Scintillation pulses of up to ~40 photons reached the SiPM. With the detector mounted 60 mm from a Thermo A-325 DT neutron generator (up to 6 times 107 n/s) the count rate was over 4 times 104 counts per second. The SiPM gain was effectively stabilised against temperature variations by automatically trimming the bias voltage with SiPM temperature. Comparison data is provided for a photodiode-scintillator neutron detector placed 134 cm from the A-325 neutron generator and for a PMT-WLS-scintillator neutron detector.

Neutron generator burst timing measured using a pulse shape discrimination plastic scintillator with silicon photomultiplier readout

An EJ-299-34 plastic scintillator with silicon photomultiplier (SiPM) readout was used to measure the fast neutron output of a pulsed Thermo-Fisher A-325 Deuterium-Tritium sealed tube neutron generator (STNG). The SiPM signals were handled by a prototype digital pulse processing system, based on a free-running analogue to digital converter feeding a digital signal processor (DSP). Pulse shape discrimination was used to distinguish between detected fast-neutrons and gammas. Pulse detection, timing, energy and shape were all processed by the DSP in real-time. The time-dependency of the neutron output of the STNG was measured for various pulsing schemes. The switch-on characteristics of the tube strongly depended on the operating settings, with the delay between pulse turn-on and the production of neutrons ranging between 13 μs to 74 μs for the tested pulse rates and duty cycles. This work will facilitate the optimization and modeling of apparatus that use the neutron generators pulsing abilities.

Macroscopic laser-induced fluorescence of coal: rank determination

Laser-induced fluorescence of coal with an ultraviolet nitrogen laser has been used to examine coal on a macroscopic scale. Fluorescence was excited in an area more than four orders of magnitude greater than that of individual macerals. Fourteen Australian coals, ranging in rank from low to high volatile bituminous coal, were studied. The fluorescence intensity correlated well with vitrinite reflectance (r2 = 0.79), an accepted rank parameter.

Neutron-Gamma Pulse Shape Discrimination Using Organic Scintillators With Silicon Photomultiplier Readout

Neutron/Gamma pulse shape discrimination (PSD) was measured using stilbene and EJ-299-34 plastic scintillators with readout by silicon photomultipliers (SiPMs). The SiPM pulses were digitized and processed for energy and pulse shape information using a digital technique performing numerical weighted integrations on each pulse. A genetic algorithm (GA) was developed to optimize the weighting vectors used for the pulse shape discrimination. Efficient PSD was obtained down to an electron-equivalent energy of 127 keV with stilbene and 391 keV with the EJ-299-34 PSD plastic. Separation at lower energies was possible at reduced detection efficiency, down to 78 keV and 186 keV at 50% efficiency. The detectors were used to measure separated gamma and neutron spectra from an Americium-Beryllium neutron source and from a Na-22 gamma source. The GA-optimized weighted integration was compared with digital charge comparison (DCC). The GA exhibited slightly improved performance with a 400 MSps digitization rate and showed a significant advantage at sample rates below 100 MSps.

Fluorescence alteration of coal and polycyclic aromatic hydrocarbons

Abstract Coal and polycyclic aromatic hydrocarbon (PAH) fluorescence was excited by 442 nm laser light at flux density of ≈ 27 kW/m2. Illuminating various coals in vacuum at ≈ 1 Pa caused a small decrease in their fluorescence. In air or oxygen a very rapid decrease in fluorescence was followed by slow fluorescence growth. This growth was faster for lower rank coals. Physically reversible oxygen attachment enhanced the fluorescence. Similar results were obtained with anthracene, phenanthrene and pentacene layers. A photo-reaction mechanism is proposed to explain features of the positive alteration of coal and PAH fluorescence in oxygen.

Laser-induced macrofluorescence of coal: oxidation and spectral effects

Abstract Exposure of Goonyella and Bulli seam coal to air at room temperature for 3 days reduced the 800 nm macrofluorescence by 5%. The macrofluorescence was reduced by 40% after exposure to air for 4 days at 105 °C. A 64% reduction in the 800 nm macrofluorescence of Goonyella coal resulted from treatment with 30% hydrogen peroxide solution for 1 h at 100 °C. This decrease in 800 nm macrofluorescence is associated with an increase in oxidation. A similar spectral change was noted across an oxidation line in the Goonyella seam by measuring the ratio of infrared to green intensity at 825 and 510 nm and correlating the ratio with core depth. Spectral measurements were made while illuminating the samples at 325 and 442 nm over relatively large surface areas (0.1–22 cm 2 ) with pressed fine coal samples and with lump coal. The method may be suitable for the on-line characterization of coal oxidation.

Carbon dioxide laser tuning through 110 lines in 3 ms for airborne remote sensing

Rapid tuning of a 3-m long glow discharge excited CO(2) laser is achieved by a rotating polygonal mirror scanning the intracavity beam across a diffraction grating. The system can tune through 110 different rotational lines spanning wavelengths from 9.2 to 11.1 microm in 3 ms with typical peak powers of 100-500 W at burst repetition rates up to 360 Hz and is suitable for airborne remote sensing. The effects of laser output coupler reflectance, burst rate, scan direction, and current modulation are investigated, and the performance of an optimized system for mineral reflectance measurements is presented. A reflection spectrum of a shale obtained with the rapidly tuned laser at a range of 5 m is shown to compare well to data gathered for the same sample using a slowly tuned cw CO(2) laser.

Laser-induced macrofluorescence of coal : oxidation and macroalteration

Abstract Measuring the fluorescence decay of bulk coal samples with prolonged illumination may be a useful method for the on-line grading of coal. When large areas (≈0.02-0.12 cm2) of Goonyella and Upper Hunter coal samples were illuminated with a 442 nm laser the resulting macrofluorescence emission decreased during prolonged exposure (macroalteration). With a fresh Goonyella coal the 800 nm macroalteration in 5 min with a 1.5 mm diameter spot and 8 mW incident power was 30% in nitrogen and 55% in oxygen. Oxidized coals macroaltered less than fresh coals. Macroalteration was faster at near infrared emission wavelengths than at visible emission wavelengths.

Software-based digital pulse processing for Silicon Photomultiplier radiation detectors

A Silicon Photomultiplier (SiPM) was used for the optical readout of a stilbene single-crystal scintillator for pulse shape discrimination (PSD) of fast-neutrons and gammas. We are developing digital pulse processing (DPP) techniques for the optimal handling of SiPM signals in the presence of noise intrinsic to these devices. The DPP is based on a weighted integration, with the optimal processing parameters found using genetic algorithm heuristics. Efficient PSD was measured at energies of 165 keVee and above using the optimized DPP, a significant improvement over the digital charge comparison method, which provided efficient PSO only above 395 keVee. The successful application of PSO using SiPM and OPP opens up the PSO technique for a new range of applications where the SiPMs small size, low power and cost are beneficial.