J.T. Walton

An opto-electronic feedback preamplifier for high-resolution nuclear spectroscopy

Abstract A new type of preamplifier is described that promises a significant improvement in the energy resolution of semiconductor detector spectrometers. While other applications of the principles are anticipated, this paper deals only with their use for X-ray fluorescence spectroscopy. We have achieved a total electronic contribution to the resolution of about 150 eV (fwhm-silicon) with further small improvements considered likely in the near future. This includes the detector leakage noise contribution. A conventional system using components of the same inherent quality exhibits a resolution of nearly 300 eV.

300-Element Silicon-Lithium Position-Sensitive Imaging Detector for Angiography

Silicon lithium-drifted [Si(Li)] detectors 150 mm long, 10 mm wide and 5 mm thick with 300 individual elements have been fabricated as imaging detectors for noninvasive studies of human coronary arteries using 33 keV x-rays from a synchrotron radiation beamline. This detector is an extension of earlier work on 30 mm long devices with initially 30 and later 60 elements. The detector fabrication details are discussed highlighting problems in uniform lithium-ion compensation. The device structure is examined and measurements on the interelement impedances presented. Finally an angiograph of the coronary arteries in an excised pigs heart obtained with this 300-element detector is presented.

CHARACTERIZATION OF LARGE DIAMETER SILICON BY LOW-BIAS CHARGE COLLECTION ANALYSIS IN Si (Li) PIN DIODES

Abstract Five silicon single crystals, ∼5 cm in diameter, grown under different conditions were examined for their suitability as Si(Li) detector grade material. A correlation was found between a crystal growth process parameter and incomplete and nonhomogeneous charge collection in the finished Si(Li) detectors when biased at voltages much lower than normally applied in detector operation. Data are presented and a model based on the presence of interstitial defects is proposed to explain the observed results.

Silicon detectors: New challenges

Abstract An introduction is given to the areas of current interest in silicon radiation detector design and fabrication. Problems associated with detector grade silicon are reviewed. Passivation of detector surfaces with silicon dioxide or amorphous silicon is described. Two examples of silicon detector applications in X-ray detection and X-ray angiography are presented.

Determination of surface recombination velocity and bulk lifetime in detector grade silicon and germanium crystals

The utility of a noncontact photoconductive decay (PCD) technique is demonstrated in measuring the bulk lifetime, /spl tau//sub B/, and surface recombination velocity, S, in detector grade silicon and germanium crystals. We show that the simple analytical equations which relate the observed effective lifetimes in PCD transients to /spl tau//sub B/ and S have a limited range of applicability. The noncontact PCD technique is used to determine the effect of several surface treatments on the observed effective lifetimes in Si and Ge. A degradation of the effective lifetime in Si is reported as a result of the growth of a thin layer of native oxide at room temperature under atmospheric conditions. >

Silicon Radiation Detectors with Oxide Charge State Compensation

This paper discusses the use of boron implantation on high resistivity P type silicon before oxide growth to compensate for the presence of charge states in the oxide and oxide/silicon interface. The presence of these charge states on high resistivity P type silicon produces an inversion layer which causes high leakage currents on N+P junctions and high surface conductance. Compensating the surface region by boron implantation is shown to result in oxide passivated N+P junctions with very low leakage currents and with low surface conductance.

Lithium‐ion mobility improvement in floating‐zone silicon by external gettering

Phosphorous diffusion for approximately 1 h at 950u2009°C is shown to be an effective gettering procedure to remove lithium‐ion mobility reducing defects in floating‐zone p‐type silicon wafers. The removal of these defects, which can severely impede the movement of lithium ions in silicon wafers during lithium‐ion compensation process, is crucial in the fabrication of silicon lithium‐drifted radiation detectors.

Lithium drifted silicon detector fabrication on gettered floating-zone silicon

A gettering procedure using phosphorus doped glass is shown to remove lithium-ion precipitation sites from p-type floating-zone (FZ) silicon. A model involving interaction between grown-in vacancies and oxidation-injected silicon interstitials is proposed to explain the gettering procedure. Examples of silicon lithium-drifted detectors fabricated on ungettered and gettered FZ silicon are presented. >

Proper interpretation of photoconductive decay transients in semiconductors having finite surface recombination velocity

The influence of finite surface recombination velocity on the proper interpretation of photoconductive decay (PCD) transients in semiconductors is discussed. The limitations of simple analytical equations which relate the observed effective lifetime to the material parameters are considered. It is shown that, under most circumstances, the correct application of the appropriate analytical expression requires some prior knowledge of the material parameters under investigation. Several methods are proposed to extract useful information from PCD experiments. Finally, the practicality of these methods is investigated by measuring the effective lifetimes of high‐purity germanium and float‐zone silicon using a noncontact PCD technique.

THE DEVELOPMENT OF AN ARRAY OF COOLED LARGE AREA Si (Li) DETECTORS

Abstract A system containing six cooled, 34 mm diameter by 7 mm thick, high-resolution Si(Li) detectors designed to maximize the sensitivity for counting X-rays in the 10–30 keV range to measure trace radionuclides in soil samples has been successfully fabricated. The detectors were mounted in a paddle-shaped cryostat with a single large beryllium window on each side. This configuration provides for efficient anticoincidence background suppression and effectively doubles the sensitive detector area because X-rays can impinge on the detectors from both sides. To maximize detection efficiency, the thickness of the cryostat was held to a bare minimum (25 mm); this caused severe difficulties during fabrication of the system. Cutting down the rim of the detectors reduced to an acceptable level the microphony caused by movement of the beryllium window that faces the lithium-diffused contact of the detectors. Since this system will be used for low level counting, careful testing was performed to select materials having the lowest radioactivity.