F. S. Goulding

A new particle identifier technique for Z = 1 and Z = 2 particles in the energy range > 10 MeV

Abstract Protons, deuterons, tritons, helium-3 and α particles produced in nuclear reactions have previously been identified by use of ΔE and E counters to determine dE/dx and E. Multiplying these together produces an output that is dependent on the type of particle. This technique is based on the theoretical relationship between dE/dx, E and the mass and charge of the particle. Unfortunately there is an obvious restriction on the technique, since dE/dx changes as the particle passes through the ΔE counter. For the E(dE/dx) product to have any real meaning, the ΔE counter must be thin and absorb only a small part of the total energy. This limits the use of this technique in a given experiment to small energy ranges and to selected types of particle. The new identifier also uses a ΔE (thickness T) and E counter, but employs the empirical relationship: Range = aE1.73, where a depends on the type of particle. Using the relationship, one can show T/a = (E+ΔE) 1.73 −E 1.73 . The identifier employs logarithmic elements to calculate this quantity and to produce an output that has a fixed value for each type of particle. The thickness of the ΔE counter is not limited to a very small value, and the identifier can cope with mixtures of all five types of particle, each covering a fairly wide energy range. Experimental results using this identifier and lithium drifted silicon detectors are presented and illustrate the clear separation between He3 and He4 particles (difficult to achieve with the multiplier type of identifier).

Semiconductor detectors for nuclear spectrometry, I☆

Abstract These papers were prepared primarily as background information for attendees at the meeting who have not been directly concerned with the development of detectors. The first chapter contains information on semi-conductor materials and the theory of junctions necessary for an understanding of detectors. In the second chapter the methods of making detectors by diffusion and also by lithium drifting are detailed and the relative merits of different types of detectors are discussed. We then consider the problems associated with obtaining good energy resolution in detector systems dealing primarily with electrical noise and statistical fluctuations of the charge produced in the detector. In the final chapter, a semi-theoretical treatment of the detector pulse shape considerations is given and the subject of radiation damage is discussed. While the treatment is generally applicable to detector systems the author has made no effort to write a review of the voluminous literature on this subject. The methods of detector manufacture, for example, are representative of only a portion of our own groups work in this area and are only to be regarded as indicative of a general technique. Other papers presented at the conference describe methods used by other groups.

PHYSICS OF ULTRA-PURE GERMANIUM

Abstract A broad review of the physics of point defects, i.e. electrically active and neutral impurities and impurity complexes is given. Basic material science which is crucial for understanding the physics is summarized in an introductory section. It is followed by the detailed description of the novel measurement techniques—photothermal ionization spectroscopy, high-Q electron paramagnetic resonance and deep level spectroscopy. These spectroscopic techniques have had a profound impact on the investigations of ultra-pure germanium. The major part of this work deals with the physics of point defects in ultra-pure germanium. The high purity of this semiconductor allows the undisturbed observation of highly excited bound states of shallow donors and acceptors. Many new previously unknown acceptor and donor centres have been discovered in ultra-pure germanium. The nature of several of these centres can be understood in terms of a complex consisting of two impurities—a light interstitial atom (for example, h...

Signal Processing for Semiconductor Detectors

This is a tutorial paper designed to provide a balanced perspective on the processing of signals produced by semiconductor detectors. The general problems of pulse shaping to optimize resolution with constraints imposed by noise, counting rate and rise time fluctuations are discussed.

SOME ASPECTS OF DETECTORS AND ELECTRONICS FOR X-RAY FLUORESCENCE ANALYSIS

Abstract This paper present some of the less recognized and potentially important parameters of the electronics and detectors used in X-ray fluorescence spectrometers. Detector factors include window (dead-layer) effects, time-dependent background and excess background. Noise parameters of field-effect transistors and time-variant pulse shaping are also discussed.

Pulsed Feedback Tecniques for Semicondctor Detector Radiation Spectrometers

Methods of applying pulsed-charge feedback to the charge-sensitive preamplifiers used with semiconductor detectors are discussed. All have in commn the accumulation of radiation-induced charge pulses on a feedback capacitor to produce a voltage ramp at the output of the feedback stage, which is reset at an appropriate point by pulsing a charge feedback path. Advantages of pulsed feedback over the conventional dc feedback techniques are discussed, together with the precautions required to reduce the effect of the large reset pulse on the later electronics. The application of pulsed-light feedback to low energy X-ray spectrometers is discussed and results are presented. We also discuss sane aspects of this system that tend to limit its high-rate performance. A brief account of the use of a transistor current-switch feedback system to reduce overload problems in high-energy ?-ray spectrometers is also given.

XRF ANALYSIS - SOME SENSITIVITY COMPARISONS BETWEEN CHARGED-PARTICLE AND PHOTON EXCITATION

A comparison is made between the limits of detection for trace elements when charged-particle and photon excited X-ray fluorescence analysis are performed on a specific type of sample (5 mg/cm/sup 2/ organic based). Large-scale analysis (approximately 30,000 samples per year) at levels of 1 ppM or lower is shown to be practical with either technique when well executed. Determining the physical reason for unexplained detector background is shown to be very important particularly for the potential improvement that might be realized in photon-excited analysis applications.

Detection of Low Energy X Rays with Si(Li) Detectors

The continuing improvement in energy resolution of semiconductor detector X-ray spectrometers has led to interest in the use of these devices at energies less than 2 keV. This is an energy region of potential analytical interest since the K X rays of several elements of biological and chemical importance occur at these energies. However, the low X-ray fluorescence yield in low-Z elements, combined with the absorption of low-energy X rays due to the entry window of the vacuum chanber and that of the detector, have made work in this energy region impossible with the standard semiconductor detector X-ray spectrometers. In the present work, the X-ray absorption path has been reduced to a minimum to permit low-energy X ray studies of excitation, entry window thickness, detector linearity and resolution. Using electron beam excitation on low-Z targets, we have performed measurements of characteristic K X rays of elements down to and including carbon (277 eV).

Design Philosophy for High-Resolution Rate and Throughput Spectroscopy Systems

The paper describes the philosophy behind the design of a pulse processing system used in a semiconductor detector x-ray spectrometer to be used for plasma diagnostics at the Princeton TFTR facility. This application presents the unusual problems of very high counting rates and a high-energy neutron background while still requiring excellent resolution. To meet these requirements three specific new advances are included in the design: (i) A symmetrical triangular pulse shape is employed in the main pulse-processing channel. A new simple method of generating a close approximation to the symmetrical triangle has been developed. (ii) To cope with the very wide dynamic range of signals while maintaining a constant fast resolving time, approximately symmetrical triangular pulse shaping is also used in the fast pulse pile-up inspection channel. (iii) The demand for high throughput has resulted in a re-examination of the operation of pile-up rejectors and pulse stretchers. As a result a technique has been developed that, for a given total pulse shaping time, permits approximately a 40% increase in throughput in the system. Performance results obtained using the new techniques are presented.

A SPECTROMETER FOCAL PLANE DETECTOR FOR HEAVY IONS

Abstract A resistive-wire position-sensitive proportional transmission counter has been built for the detection of heavy ions in the focal plane of a magnetic spectrometer. The 45 × 6 cm2 counter measures position and energy loss with a resolution of 0.7 mm and 8–10% respectively. Time-of-flight is measured with a plastic scintillator behind the proportional counter. The position, time and energy loss signals are used to identify heavy ions with unit mass and atomic number resolution up to about A = 20, Z = 10.