D. Alexiev

An instrumental solution to the phenomenon of negative capacitances in semiconductors

Abstract A number of authors make reference to “negative capacitances” observed during impedance measurements of metal-semiconductor and other semiconductor device structures at sufficiently low frequencies for parasitic inductances to be assumed negligible. Often, these negative capacitances are attributed to physical phenomena associated with the devices being measured. It is demonstrated in this paper that many such interpretations incorrectly neglect the importance of parasitic series inductances at low frequencies when device conductance is large, as in a forward biased Schottky barrier, or when large device leakage currents are present. Simulations of experimental data for a Schottky diode show that typical values of probe lead and other instrumental inductance may be sufficient to provide an instrumental explanation for the apparent effect.

High purity liquid phase epitaxial gallium arsenide nuclear radiation detector

Abstract Surface barrier radiation detector made from high purity liquid phase epitaxial gallium arsenide wafers have been operated as X- and γ-ray detectors at various operating temperatures. Low energy isotopes are resolved including 241 Am at 40°C and the higher gamma energies of 235 U at −80°C.

Calculation of the Response of a Miller Exponential Signal Correlator to ODLTS Signals

A theoretical derivation of the response function of a Miller exponential signal correlator to a optical deep level transient signal is presented for the calculation of trap energy and capture cross-section in high resistivity semiconductors.

Analysis of inelastic interactions for therapeutic proton beams using Monte Carlo simulation

The irradiation of various tissue-like materials by therapeutic proton beams was simulated using Monte Carlo. The contribution of inelastic reaction products to the depth-dose distribution was determined. The use of silicon microdosimeters for verifying Monte Carlo calculations was also investigated. The importance of these studies to Monte Carlo-based treatment planning systems is emphasized.

LPE GaAs as an X-ray detector for astronomy

Abstract Gallium arsenide detectors offer an attractive alternative to silicon for high energy X-ray (> 10 keV) astrophysics. Thick (130 μm) detectors fabricated from GaAs grown using the liquid phase epitaxy (LPE) process are now achieving an energy resolution which is approaching that needed in astrophysics. At the same time progress continues to be made in the field of true imaging optics at these high energies using multi-layer reflective surfaces and it is now not unreasonable to imagine a hard X-ray astrophysics mission with grazing incidence optics and a GaAs solid state position sensitive spectrometer in the focal plane. A brief introduction to hard X-ray astrophysics and optics is given. Results obtained with an LPE GaAs device are presented including both detector performance and the electrical characteristics. At room temperature a resolution of ∼ 2.8 keV HWHM was achieved.

Comparison of

Energy resolution and detection efficiency were compared between new scintillators, lanthanum bromide (LaBr₃:Ce) and lanthanum chloride (LaCl₃:Ce) with conventional detectors, sodium iodide NaI(Tl) and cadmium zinc telluride (CdZnTe or CZT). The study has shown that LaBr₃:Ce and LaCl₃:Ce scintillator detectors provided by Saint-Gobain offer better resolution than NaI(Tl) detector. LaBr₃:Ce and LaCl₃:Ce detectors could resolve some closely spaced peaks from ¹³³Ba and ¹⁵²Eu, which Na(I) could not. LaBr₃:Ce has slightly better resolution and higher efficiency than LaCl₃:Ce. An overall advantage of LaBr₃:Ce and LaCl₃:Ce detectors over NaI(Tl) and CZT has been discussed. The intrinsic activity of LaBr₃:Ce is also demonstrated in this study.

{\hbox {LaBr}}_{3}:{\hbox {Ce}}

A deep-level transient-conductance spectrometer for high-resistivity semiconductors, using a radiofrequency (~40 MHz) marginal oscillator as a conductance detector, is described. Spectra are generated by periodically filling deep-level trapping centres with carriers stimulated by a pulsed GaAs laser, and processing the trap-emptying conductance signal through an exponential Miller correlator as the sample temperature is slowly ramped. Simple capacitive coupling of samples to the oscillator tank circuit eliminates problems such as unwanted defect annealing and other material changes often associated with the high-temperature techniques necessary for ohmic contact formation. Representative deep-level spectra are given for semi-insulating Bridgman-grown CdTe.

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Several adhesive tapes have been studied in terms of their suitability for securing gold wires into positions for neutron flux measurements in the reactor core and irradiation facilities surrounding the core of the Open Pool Australian Light water (OPAL) reactor. Gamma ray spectrometry has been performed on each irradiated tape in order to identify and quantify activated components. Numerous metallic impurities have been identified in all tapes. Calculations relating to both the effective neutron shielding properties of the tapes and the error in measurement of the activity caused by superfluous activity due to residual tape have been made. The most important identified effects were the prolonged cooling times required before safe enough levels of radioactivity to allow handling were reached, and extra activity caused by residual tape when measured with an ionisation chamber. Knowledge of the most suitable tape can allow a minimal contribution due to these effects, and the use of gamma spectrometry in preference to ionisation chamber measurements of the flux wires is shown to make all systematic errors due to the tape completely negligible.

{\hbox {LaCl}}_{3}:{\hbox {Ce}}

Abstract P-type silicon was doped by neutron transmutation (NTD-Si) to produce high-resistivity n-type silicon suitable for the production of surface barrier radiation detectors. Deep level transient spectroscopy (DLTS) analysis showed no remnant traps following annealing (850°C) of the NTD-Si in the presence of a phosphosilicate glass getter. Surface barrier radiation detectors constructed from this material showed no significant charge trapping and compare faavourably with those constructed of float-zone (FZ)Si.

With NaI(Tl) and Cadmium Zinc Telluride (CZT) Detectors

Radiation transport simulations of the most probable gamma- and X-ray emissions of (123)I and (54)Mn in a three photomultiplier tube liquid scintillation detector have been carried out. A Geant4 simulation was used to acquire energy deposition spectra and interaction probabilities with the scintillant, as required for absolute activity measurement using the triple to double coincidence ratio (TDCR) method. A sensitivity and uncertainty analysis of the simulation model is presented here. The uncertainty in the Monte Carlo simulation results due to the input parameter uncertainties was found to be more significant than the statistical uncertainty component for a typical number of simulated decay events. The model was most sensitive to changes in the volume of the scintillant. Estimates of the relative uncertainty associated with the simulation outputs due to the combined stochastic and input uncertainties are provided. A Monte Carlo uncertainty analysis of an (123)I TDCR measurement indicated that accounting for the simulation uncertainties increases the uncertainty of efficiency of the logical sum of double coincidence by 5.1%.