N. W. Madden

Radiation Damage Resistance of Reverse Electrode GE Coaxial Detectors

Two high-purity germanium coaxial detectors, having opposite electrode configurations from one another, but fabricated from the same germanium crystal, were irradiated simultaneously with fast neutrons from an unmoderated 252Cf source. Both detectors were 42 mm diam. The detector having the conventional electrode configuration was about 28 times more sensitive to radiation damage than was the detector having the p+ contact on the coaxial periphery. These results prove that germanium coaxial detectors having the conventional electrode configuration should not be used in any situation subject to significant radiation damage. This conclusion was anticipated because the defects produced by neutron and proton irradiation of germanium act predominantly as hole traps.

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.

Transistor Reset Preamplifier for High Rate High Resolution Spectroscopy

Pulsed transistor reset of high resolution charge sensitive preamplifiers used in cooled semiconductor spectrometers can sometimes have an advantage over pulsed light reset systems. Several versions of transistor reset spectrometers using both silicon and germanium detectors have been built. This paper discusses the advantages of the transistor reset system and illustrates several configurations of the packages used for the FET and reset transistor. It also describes the preamplifer circuit and shows the performance of the spectrometer at high rates.

Germanium Detectors with a Built-In Transverse Drift Field

A new kind of position-sensitive germanium detector based on a novel mode of charge transport is proposed and experimental devices have been fabricated. In these germanium drift-chambers, position information is obtained by measuring carrier drift times. One-dimensional spatial resolutions of ¿ 0.5 mm over an active area of ~ 2.5 × 2.5 cm2 have been achieved in one detector for 60 keV photons. The simplicity of such detectors makes them attractive as position-sensitive gamma-ray spectrometers compared to discrete element detectors. The problems and limitations of the present detectors are discussed.

High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission for the next (2000) solar maximum

The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum.

Low Noise Preamplifiers/Amplifiers for the Time Projection Chamber

The special problems associated with the Time Projection Chamber resulted in the development of a new preamplifier/amplifier system which is discussed here. The need for 16,000 channels of low noise electronics imposed serious constraints on the cost, size, and power consumption of the Circuits. Also the preamplifiers are designed to work inside the detector chamber. This chamber operates at 10 atmosphere pressure and is sensitive both to the presence of magnetic materials that distort the drift field, and to the effects of gas contamination. These problems resulted in the use of new components and fabrication techniques as described in the paper. The large number of units involved requires automatic testing as discussed in the paper.

A high-resolution Si(Li) spectrometer with thermoelectric cooling

Abstract A Si(Li) spectrometer cooled by a thermoelectric refrigerator exhibited a peak width of 258 eV fwhm for X-rays of 5.9 keV. The measured electronic noise was equivalent to 224 eV fwhm.

Large area, low capacitance Si(Li) detectors for high rate X-ray applications

Large area, single-element Si(Li) detectors were fabricated using a novel geometry which yields detectors with reduced capacitance and hence reduced noise at short amplifier pulse-processing times. A typical device employing the new geometry with a thickness of 6 mm and an active area of 175 mm/sup 2/ has a capacitance of only 0.5 pF, compared to 2.9 pF for a conventional planar device with equivalent dimensions. These low capacitance detectors, used in conjunction with low capacitance field effect transistors, will result in X-ray spectrometers capable of operating at very high count rates while still maintaining excellent energy resolution. The spectral response of the low capacitance detectors to a wide range of X-ray energies at 80 K is comparable to typical state-of-the-art conventional Si(Li) devices. In addition to their low capacitance, the new devices offer other advantages over conventional detectors. Detector fabrication procedures, I-V and C-V characteristics, noise performance, and spectral response to 2-60 keV X-rays are described. >

Pulse Shape Discrimination For Background Rejection In Germanium Gamma-Ray Detectors

We report on the development of a pulse shape discrimination (PSD) technique to reject the B-decay background resulting from activation of germanium (Ge) gamma-ray detectors by cosmic ray secondaries. These B-decays are a major source of background at 0.2-2 MeV energies in well shielded Ge detector systems. The technique exploits the difference between the detected current pulse shapes of single- and multiple-site energy depositions within the detector: B-decays are primarily single-site events while photons at these energies typically Compton scatter before being photoelectrically absorbed to produce multiple-site events. Algorithms have been developed to distinguish between single- and multiple-site pulse shapes. Depending upon the amount of background due to sources other than B-decay, PSD can more than double the detector sensitivity. In addition, we report on tests of PSD by laboratory activation of a detector with a fast neutron source, and on the first direct measurement of the B-decay background at balloon float altitude using a Ge detector with PSD.

The LBL/UCSB 76Ge Double Beta Decay Experiment: First Results

A paper given at the IEEE Nuclear Science Symposium last year presented the scientific justification for this experiment and discussed the design of the detector system. At the present time two of the dual detector systems (i.e., four out of a final total of eight detectors) are operating in the complete active/ passive shield in the low background laboratory at LBL. Early results (1620 hrs) of an experiment using two detectors yield a limit of 4 × 1022 years (68% confidence) for the half life of the neutrinoless double beta decay (ßßo¿) of 76Ge. Although this experiment was carried out above ground, the result approaches those achieved by other groups in deep underground laboratories. Based on studies of the origins of background in our system, we hope to reach a limit of 3 × 1023 years (or more) in a two month/ four detector experiment to be carried out soon in an underground facility.