Fred S. Goulding

Pulse-shaping in low-noise nuclear amplifiers: A physical approach to noise analysis☆

Analysis of the relative noise performance of many types of pulse-shaping systems is discussed in terms of recent work on noise analysis. The simplicity of the method, and its direct connection to physical processes, is stressed. Applications of the technique to time-variant as well as time-invariant systems are illustrated.

Accurate determination of the ionization energy in semiconductor detectors

Abstract The average energy g3 expended for electron-hole pair generation in silicon and germanium lithium-drifted detectors by gamma rays, electrons, and alpha particles has been measured as a function of temperature. These data indicate that the difference between eα and ee− in silicon is considerably less than previously reported, and in germanium eα ∼ ee−.

LEAKAGE CURRENT IN SEMICONDUCTOR JUNCTION RADIATION DETECTORS AND ITS INFLUENCE ON ENERGY-RESOLUTION CHARACTERISTICS

Abstract The theoretical limits of noise in detector-amplifier combinations are discussed and related to the bulk properties of the semiconductor (lifetime and resistivity). A new detector structure which includes a guard ring as an integral part of the detector is described and its effect in eliminating surface leakage is discussed. Experimental results in good agreement with theory for detector leakage and noise resolution are presented. The residual surface effects not eliminated by the guard ring are shown to be important in very-low-leakage detectors, and the results of surface treatments to reduce these effects are briefly mentioned. Observations of the energy resolution of these detectors using various types of particles indicate that factors other than noise are important. Surface imperfections such as are encountered in polished surfaces may cause poor resolution when heavily ionizing particles are detected. Multiple peaks have also been observed in α spectra obtained with diffused junctions, possibly due to other surface effects. A brief description is given of the special electronic circuits used in these measurements. Techniques used in producing the guard-ring counter are described in detail. These employ gaseous diffusion to produce a thin surface junction, followed by etching through a photo-resistant mask to produce the desired geometry.

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.

A .beta.-gage method applied to aerosol samples.

An instrument for the routine measurement of aerosol mass using the beta-particle attenuation method is described. Factors affecting the precision and accuracy of the measurement are discussed in detail. Results of intercomparison studies between the beta gauge method and conventional gravimetric are presented. The design of the present instrument is particularly well suited for the automatic analysis of membrane filter obtained from modern dichotomous samplers.

High Rate X-Ray Fluorescence Analysis by Pulsed Excitation

We describe the application of pulsed X-ray excitation to X-ray fluorescence spectrometry as a method for increasing the output counting rate of the system by a substantial factor. Using a pulsed X-ray tube that is immediately turned off when a signal is detected, and held off during the pulse processing time, it is possible to eliminate the need for a pulse pile-up rejection. We have achieved output counting rates significantly greater than with conventional operation for equivalent shaping networks. No significant degradation in spectrometer resolution was observed at the increased counting rates.

High-Purity Germanium: Detector Fabrication and Performance

The availability of germanium with less than 1010 acceptors/cm3 has enabled us to fabricate and study the performance of germanium detectors of substantially larger volume than has hitherto been possible except by using lithium-drifting techniques. Detectors having a sensitive volume of up to 25 cm3 have been made. These detectors have a lithium diffused n+ contact and a metal barrier non-injecting back contact, and are totally depleted. The energy resolution of these detectors is similar to that of equivalent sized lithium-drifted detectors, but the high-purity detectors are easier to fabricate, and present considerably fewer handling problems.

A Survey of the Applications and Limitations of Various Types of Detectors in Radiation Energy Measurement

This paper provides a collection of data relevant to the use of various types of semiconductor detector in nuclear spectroscopy and gives examples of the use of detectors in specific experimental applications. Basic data on absorption of various kinds of radiation in germanium and silicon are given, and these are related to the characteristics of different types of semiconductor detectors. A brief outline of the optimization procedure for the detector-amplifier system to obtain good energy resolution follows. Included in a brief review of applications of detectors are examples in the fields of a-particle spectrometry, high-energy particle reactions, and low-temperature nuclear alignment studies.

Large-scale measurement of airborne particulate sulfur☆

Abstract We describe an aerosol sampling and analysis system which represents an integral approach to large-scale monitoring of airborne particulate matter. During our two-year participation in the St. Louis, Missouri, Regional Air Pollution Study (RAPS), 34,000 size-fractionated samples were collected by automated dichotomous samplers characterized by a particle size cutpoint of 2.4 μm. The total mass of the particulate matter was measured by beta-particle attenuation and the elemental composition, including sulfur, was determined by photon-excited X-ray fluorescence. The long-term performance of the system will be reported. Potential systematic effects related to the sampling and analysis of sulfur particles are treated here in detail. Both the accuracy and precision of sulfur measurement are estimated to be 2%. While the X-ray attenuation correction required is typically only a few per cent, a larger correction is required for a small fraction of the samples due to the migration of the sulfur into the filter. This correction is derived from the ratio of sulfur determinations made on the front and back surfaces of the membrane filter. Laboratory and field experiments have shown insignificant gaseous SO2 conversion on the type of filters employed in the study. Preliminary data on the composition and the temporal and spatial distribution of the St. Louis aerosol are presented. About 90% of the sulfur was found in the fine-particle fraction. Sulfur variations were significantly slower than those of the trace elements. Sulfates usually constitute about 35% of the total fine-particle mass, but may rise to 41% during an ‘episode’. The long-term (4 month average) sulfur data indicate that the background air masses arriving at St. Louis from the west and north were about 30% lower in particulate sulfur than those from the east and south. Also, an urban station may experience local increases in sulfur level up to a factor of two greater than the general background. Short-term (6 h average) data indicate that the effects of stationary SO2 sources extend for long distances, (at least 40 km) and are highly directional in character.

SEMICONDUCTOR RADIATION DETECTORS.

The past decade has seen the rapid development and exploitation of one of the most significant tools of nuclear physics, the semiconductor radiation detector. Applications of the device to the analysis of materials promises to be one of the major contributions of nuclear research to technology, and may even assist in some aspects of our environmental problems. In parallel with the development of these applications, further developments in detectors for nuclear research are taking place: the use of very thin detectors for heavyion identification, position-sensitive detectors for nuclear-reaction studies, and very pure germanium for making more satisfactory detectors for many applications suggest major future contributions to physics.