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This paper reports recent advances in our microcalorimeter spectrometer that result in an energy resolution of 3.1 eV FWHM for 6 keV X-rays. We have developed a detailed model that indicates that further minor improvements will realize 2 eV FWHM at 6 keV and well under 2 eV at 1 keV. The results demonstrate the value of the microcalorimeter for many applications where a premium exists for very fine resolution in the 0.2–10 keV energy band.

This paper presents an integrated time domain approach to the optimization of the signal-to-noise ratio in all spectrometer systems that contain a detector that converts incoming quanta of radiation into electrical pulse signals that are amplified and shaped by an electronic pulse shaper. It allows analysis of normal passive pulse shapers as well as time-variant systems where switching of shaping elements occurs in synchronism with the signal. It also deals comfortably with microcalorimeters (sometimes referred to as bolometers), where noise-determining elements, such as the temperature-sensing element's resistance and temperature, change with time in the presence of a signal. As part of the purely time-domain approach, a new method of calculating the Johnson noise in resistors using only the statistics of electron motion is presented. The result is a time-domain analog of the Nyquist formula.

A simple technique to extract the location and value of parasitic series resistance in junction devices is discussed and an instrument which performs these measurements is described. Measurements of gate series resistance on several high figure of merit junction field-effect transistors are presented. The limitations in noise behavior due to this resistance are discussed.

GAMMASPHERE uses 110 very large germanium detectors. Such detectors exhibit charge trapping effects on energy resolution initially due to a native electron trap that is present in virtually all germanium. Furthermore, radiation damage is a serious problem in GAMMASPHERE experiments, producing hole traps that degrade resolution and eventually require annealing to restore the original performance. The technique discussed here uses the current pulse shape from a detector to develop a parameter related to the radius of the largest interaction in the "track" of a gamma ray in the detector. Since the charge trapping loss in a signal can be related to the distance carriers travel, the "radius" parameter can be used by software to apply a trap correction to the signal. >

Gammasphere uses an spherical array of very large (7.2 cm dia.) germanium detectors and only high-multiplicity events are studied. To achieve a reasonable coincidence rate, the individual detector channels must handle high rates with minimum pile-up losses. Ten microseconds was chosen as the total processing time for a signal which means that the shaped signal peaks in about 4 us. The combination of short pulse shaping and the fluctuating long charge collection times (up to 400 ns) in the detectors exaggerates the energy resolution degradation due to ballistic deficit effects. We describe a method of producing a flat-topped pulse with a simple time-invariant network that satisfies GAMMASPHERE requirements and eliminates ballistic deficit effects. >

We describe the design of the signal processing system and considerations involved in the timing performance of the BGO scintillator Compton shield that surrounds each of the 110 large germanium detectors used in a geodesic array covering the complete sphere surrounding the target in GAMMASPHERE. It is shown that the main timing limitation results from the statistics of photoelectron emission from the photocathode of the photomultiplier tubes (PMTs) that observe the light from the scintillators and that achieving the required timing demands triggering on the first photoelectron. The circuits to do this for a single detector assembly must be able to deal with signals from 14 PMTs associated with the 7 elements of the scintillator shield surrounding each germanium detector. >

A large area microcalorimeter with 95% quantum efficiency at 6 keV has been developed. When fully illuminated, it produces X-ray spectra between 0.5 and 7 keV with an energy resolution of 23 eV. On a smaller, but similar detector, we measure 18 eV. The resolution is obtained at input count rates of 10–50 Hz in real-time with analog pulse processing and thermal pile-up rejection. The detector is incorporated into a compact and portable cryogenic refrigerator system that is ready for use as a tool for analytical spectroscopy.

We are developing a cryogenic dielectric microcalorimeter for X‐ray spectroscopy. This type of detector takes advantage of the temperature dependence of the dielectric permittivity in ferroelectric and pyroelectric materials. It should have many advantages over traditional resistive devices, for example, minimal self heating and the reduction of Johnson noise. Several properties of dielectrics are important to their operation as a microcalorimeter. Here we present the measurements of the dielectric permittivity, ϵ = (δD/δE)T/ ϵ0 and the spontaneous polarization, Ps , for the mixed‐crystal quantum ferroelectric KTa1−xNbxO3 (KTN) with a doping of x = 0.012, as a function of temperature and electric field across the device. Prototype dielectric microcalorimeters have been developed and operated between 4.2 K and 1.3 K. We also show the signal response from infrared LED radiation and a spectrum from an α‐particle source.

Superconducting tunnel junction devices are being developed for use as high-resolution, high-efficiency X-ray spectrometers. A device with niobium X-ray absorbing layers coupled to two aluminum layers on either side of the tunneling barrier which serve as quasi-particle traps has been tested. These devices were fabricated photolithographically using a modified niobium/aluminum/niobium trilayer fabrication process. The first devices have a very thin barrier with specific normal state resistance of 1.5*10/sup -6/ Omega -cm/sup 2/, and also exhibit very low leakage current ( >

We have measured the ionization produced in a Si(Li) detector by silicon nuclei of kinetic energies in the range 3.2--21 keV. The results are in rather good agreement with the calculations of Lindhard, Scharff, and Schiott and demonstrate the interest of Si(Li) detectors for dark-matter searches. We have also measured the intrinsic fluctuations in the ionization energy due to nucleon recoils.

A new mode of operation for ionization detectors is described. The amount of ionization produced in a detector is determined by measuring the amount of heat generated during the carrier collection process. Very high detection sensitivities, including single carrier detection, may be achieved at cryogenic temperatures. Results from an experimental device operated at T = 0.3 K are presented.

Gammasphere consists of a close-packed spherical array of 110 coaxial germanium detectors about 65 mm in diameter and 80 mm long each of which is in the middle of a 7-element bismuth germanate scintillator acting as a Compton suppression shield to result in a very large peak to Compton ratio in gamma-ray spectra. The need to maintain excellent energy resolution in such a complex system and to cope with high counting rates (up to 20 kHz in each detector) imposes major challenges in system design and its implementation is the main theme here. A significant new departure in detector technology is the use of longitudinal segmentation (essentially splitting a coaxial detector into two D-shaped elements) to reduce the angle subtended by a detector element at the target. This reduces the signal spread produced by the Doppler effect in gamma rays emitted from nuclei which are generally recoiling at high velocity after the impact of a high-energy incident beam particle. This feature is also covered.

A new and different time variant pulse processing system has been developed based on a simple CR-RC filter and two analog switches. The new pulse processing technique combines both ballistic deficit immunity and resilience to parallel noise without a significant compromise to the low energy resolution, generally considered a mutually exclusive requirement. The filter is realized by combining two different pulse-shaping techniques. One of the techniques creates a low rate of curvature at the pulse peak, which reduces ballistic deficit, while the second technique increases the tolerance to low frequency noise by modifying the noise history. Several experimental measurements are presented, including tests on a co-planar grid CdZnTe detector. Improvements on both the resolution and line shape are shown for the 662 keV line of /sup 137/Cs.

We report on the performance of our NTD-Ge microcalorimeters. To date, the spectral resolution for x-ray and gamma-ray lines from radioactive sources and laboratory plasmas is 4.8 eV in the entire 1 - 6 keV band and 52 eV at 60 keV. Technical details responsible for this performance are presented as well as an innovative electro-thermal approach for enhancing count-rate capability.

A new type of charge restoration is described for use particularly in germanium gamma-ray spectrometers for accelerator and space physics applications. A bipolar pulsed reset technique is applied to these applications for the first time. This technique overcomes the problems introduced by the need here to AC couple detectors and the fact that very large energy depositions occur due to charged particles present in substantial fluxes, particularly in space. The circuit is described and experimental results are presented and discussed.

The use of a ballistic deficit corrector circuit that calculates and applies a correction on a pulse-by-pulse basis has been shown to improve the energy resolution of spectrometers using large-diameter coaxial detectors at high energies and high counting rates. The potential importance of this circuit in correcting for the loss in signal amplitude due to majority carrier trapping has also been recognized. This work has led to further approaches to improving performance. Experimental results are described and demonstrate the complex nature of the problem of simultaneously correcting for both types of loss. The proposed design of a signal processor that should achieve good simultaneous correction for both types of loss is discussed. >

A new technique has been developed for restoring the charge to the feedback capacitor in charge sensitive preamplifiers. The method, termed transient-reset,'' was developed for use in very low noise Si(Li) X-ray spectrometers. It has proved valuable as a diagnostic tool for evaluating the noise components of the input in these systems. The technique is described and compared with the pulsed- light and transistor-reset methods. To complement the charge restoration technique, a new configuration of Si(Li) X-ray detector with a very thin aluminum entrance window has been developed. Measurements of the effective window thickness are compared with similar results obtained on gold, nickel, and ion-implanted boron contacts. 10 refs., 8 figs., 1 tab.

Photon counting x‐ray spectroscopy using microcalorimeters combines the high resolution power of a Bragg crystal with the broad bandwidth capability and high efficiency of charge sensitive detectors. To date, the developers of microcalorimeters take advantage of the strong temperature dependence of resistance in doped semiconductor crystals for converting the calorimeter temperature changes into useful voltage signals. Resistive sensors limit the ultimate sensitivity of microcalorimeters because they dissipate heat and add Johnson noise to the electronic conversion circuit. Potentially, these detrimental properties can be eliminated by exploiting the temperature dependence of the dielectric constant in ferroelectric materials. A mixed crystal quantum ferroelectric shows great promise for a dielectric thermal sensor. We present results from our most recent measurements which include the first detection of individual 6 MeV alpha particles with a dielectric calorimeter operating at 1.2 K.

Factors that limit the capabilities of semiconductor detectors at photon energies below 5 keV include energy resolution, detector efficiency, and detector-related continuum background. These properties can be controlled to a certain extent by optimal detector design and fabrication techniques. We describe measurements on the low energy response of Si(Li) detectors obtained using a tunable monochromatic source. Window thicknesses have been measured for a number of devices using different materials for the entry contact. The interpretation of these results in terms of existing detector window models is discussed. Results obtained using a new contact structure demonstrate that a dramatic reduction in window-related absorption in Si(Li) detectors can be achieved.

The merits of microcalorimetry below 1°K for high resolution spectroscopy has become widely recognized on theoretical grounds. By combining the high efficiency, broadband spectral sensitivity of traditional photoelectric detectors with the high resolution capabilities characteristic of dispersive spectrometers, the microcalorimeter could potentially revolutionize spectroscopic measurements of astrophysical and laboratory plasmas. In actuality, however, the performance of prototype instruments has fallen short of theoretical predictions and practical detectors are still unavailable for use as laboratory and space-based instruments. These issues are currently being addressed by the new collaborative initiative between LLNL, LBL, U.C.I., U.C.B., and U.C.D.. Microcalorimeters of various types are being developed and tested at temperatures of 1.4, 0.3, and 0.1°K. These include monolithic devices made from NTD Germanium and composite configurations using sapphire substrates with temperature sensors fabricated from NTD Germanium, evaporative films of Germanium-Gold alloy, or material with superconducting transition edges. A new approache to low noise pulse counting electronics has been developed that allows the ultimate speed of the device to be determined solely by the detector thermal response and geometry. Our laboratory studies of the thermal and resistive properties of these and other candidate materials should enable us to characterize the pulse shape and subsequently predict the ultimate performance. We are building a compact adiabatic demagnetization refrigerator for conveniently reaching 0.1°K in the laboratory and for use in future satellite-borne missions. A description of this instrument together with results from our most recent experiments will be presented.

A field instrument is being developed for the noninvasive determination of absolute lung density using unique Compton backscattering techniques. A system consisting of a monoenergetic gamma-ray beam and a shielded high-resolution, high-purity-germanium (HPGe) detector in a close-coupled geometry is designed to minimize errors due to multiple scattering and uncontrollable attenuation in the chest wall. Results of studies on system performance with phantoms, the optimization of detectors, and the fabrication of a practical gamma-ray source are presented. It is concluded that the compact one-source one-detector system that has been developed has the potential of becoming a clinical instrument for measuring pulmonary edema. >

At high energies the energy resolution of gamma -ray spectrometers using large-diameter coaxial germanium detectors can be dominated by ballistic-deficit effects, particularly when short processing times are used to permit high-rate operation. This results because that rise-time variations in the detector signal are reflected in amplitude fluctuations after the signals pass through pulse shapes. A method is described for compensating for these effects on a pulse-by-pulse basis, using the observation that the loss of amplitude for slowly rising signals is accompanied by a delay in the peak time of the shaped output signal. The simple analog corrector circuit uses a relationship whereby the amplitude deficit is roughly proportional to the square of the time delay in the peak. Results show considerable improvements in the energy resolution of germanium gamma -ray spectrometers at high energies. >

Like other methods devised to measure quantities of interest in basic physics, X-ray fluorescence spectroscopy is now becoming a tool of the analytical chemist. Semiconductor detectors that have contributed much of the experimental data on nuclear energy levels are proving equally valuable in measuring fluorescent X rays, and are so providing a versatile tool for rapid multielement analysis of many types of sample.

A relatively inexpensive computer-controlled system for performing the drift process used in fabricating Si(Li) detectors is described. The system uses a small computer to monitor the leakage current, applied voltage, and temperature on eight individual drift stations. The associated computer program initializes the drift process, monitors the drift progress, and then terminates the drift when an operator set drift time has elapsed. The improved control of the drift with this system has been well demonstrated in the fabrication of a variety of Si(Li) detectors. A few representative system responses to detector behavior during the drift process are described. >

The authors consider the problem of ballistic deficit, which is the loss of output signal amplitude due to the interplay between the finite charge-collection times in a detector and the characteristic time constants of the amplifier. Quantitative estimates of ballistic deficits have been developed by using numerical integration techniques on a microcomputer and by exact calculations using Laplace transforms. A practical procedure is developed in which an actual amplifier with even an unspecified complex-pole network can be accurately related to an equivalent semi-Gaussian shaping network the exact solution of which is known. Simple formulas for estimating ballistic deficits and resulting degradation in spectral energy resolutions are given. >

The critical problem in high-resolution cosmic gamma-ray spectroscopy in the energy range from 0.02 to 10 MeV is the limited spectral sensitivity of the detectors used. This results from the small effective area of the detectors and the high background noise due to induced radioactivity and scattering in the detectors high-energy particle environment. The effective area can be increased by increasing the number of detectors, but this becomes prohibitive because of the size and expense of the resulting instrument. We have taken a new approach: a segmented large-volume germanium gamma-ray detector which can effectively discriminate against internal background yet maintain the high spectral resolution and efficiency of conventional coaxial Ge detectors. To verify this concept, a planar detector divided into two segments has been fabricated and laboratory measurements agree well with Monte Carlo calculations. A large coaxial detector which will be divided into five segments is being built using the techniques developed for the planar detector. Monte Carlo calculations show that the sensitivity (minimum detectable flux) of the segmented coaxial detector is a factor of 2-3 better than conventional detectors because of the reduction in the internal background.

A large-volume (150 cm/sup 3/) germanium detector with a full-depletion capacitance of only approximately=1 pf has been fabricated. The effect of impurity space charge was utilized to obtain an appropriate electric field distribution in the detector so that carriers are collected on a small-area electrode. Detectors based on this principle are capable of very-low-noise operation and have immediate applications in experiments for the direct detection of dark matter. Detector pulse shapes and carrier-trapping effects were also examined for possible applications involving higher energy radiations. >

Field effect transistors have been fabricated on high-purity germanium substrates using low temperature technology. The aim of this work is to preserve the low density of trapping centers in high-quality starting material by low temperature (

We present an innovative approach for performing photon counting X-ray spectroscopy with cryogenic microcalorimeters. The detector concept takes advantage of the temperature dependence of the dielectric constant in ferroelectric materials. A dielectric calorimeter has many potential advantages over traditional resistive devices, particularly in the reduction of Johnson noise. This makes the energy resolution for photon counting spectroscopy limited only to the noise produced by the intrinsic temperature fluctuations of the device. The detector concept is presented and its predicted performance is compared with resistive calorimeters. Calculations have shown that practical instruments operating with an energy resolution less than 20 eV may be possible at 300 mK.

A description is given of the front-end electronics, which has been hybridized in order to accommodate high-packaging-density requirements. The hybrid package contains eight channels of amplifiers together with all the associated circuits for calibration, event recognition, and power-economy switching functions. A total of 1280 such hybrids are used. The amplifiers and associated circuits are described, and their performance is discussed. >

Semiconductor drift chambers for position and energy measurements have been invented, built and tested. A short 'Ccount is here given on the principles of the drift chamber and on the experimental results. Detailed reports have been written on the quoted bibliography and new reports will be published shortly.

A brief review is given of the types of silicon and germanium detectors used or presently being developed for nuclear experiments. Large-area silicon and germanium detector telescopes for use in long-range particle detection and identification are emphasized. Large area position-sensitive detectors are also described. Some results are presented regarding radiation damage and damage repair by annealing. Evidence is also presented for the importance of producing large area silicon crystals of adequate quality to reduce trapping problems to negligible proportions.

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.

This paper describes a system developed for fast-slow coincidence experiments in nuclear physics and discusses the design of functional units including a low level discriminator, single or dual-channel analyzer, coincidence circuit and linear gate. The system is designed to use the ± 4 V double-delay-line shaped pulses produced by the amplifier described in the previous paper or equivalent pulses from other amplifiers. Operation with single phase pulses of similar amplitude is also possible but with reduced performance. Uncertainty of the discriminator threshold and the spurious pedestal and disturbances introduced by the linear gate are less than 10 mV under all normal operating conditions. Functional units of the system are constructed on printed circuit boards which plug into a base unit where interconnections are made between them through a programming plug. This arrangement allows the experimenter to change interconnections quickly and makes the whole system extremely versatile. The system also takes advantage of the timing accuracy of the zero cross-over of double-delay-line pulses to give a coincidence resolving time of 40 nsec while using no fast amplifiers or coincidence circuits. A technique of compensating for baseline shift at high counting rates of single phase pulses is also described.

A transistor amplifier designed mainly for precise scintillation counter spectroscopy is described. Techniques such as double-delay-line pulse shaping and considerable negative feedback result in excellent linearity, gain stability and overload behaviour at high counting rates. An input charge of 2 X 10−13 Coulombs develops a ± 4V output pulse into a 100 Ω matched cable which feeds the Chalk River kicksorter. With l μsec pulse shaping lines the input equivalent R.M.S. noise is 4000 ion-pairs. Lower noise may be achieved by using shorter lines. The amplifier rise-time is 0.2 μsec (10 to 90 %). Use of plug-in printed circuit boards increases the versatility of the amplifier and simplifies the adjustment procedures.

A method of pulse-shape discrimination is described in which the slow-to-total charge ratio is measured at a photomultiplier dynode. The ratio determination is approximately independent of amplitude over at least a 10:1 operating range. The performance of the system as a particle discriminator selecting neutrons in a background of γ-rays is shown for stilbene and a liquid scintillation counter. The time required to determine the shape of a pulse is about 1 μs and could be less. Thus errors due to pulse pile-up at high counting rates are not serious. The system uses a conventional amplifying system requiring only a single dynode pulse. No special photomultiplier or very fast circuits are required since the charge in the fast component is measured and no attempt is made to follow its fast rise. These features enable the system to be tested effectively with artifical pulses from a test-pulse generator.

The activities funded under the present EPA/ERDA interagency agreement have been concentrated mainly in two areas. An elemental sulfur monitor for the measurement of fine particulates has been designed, constructed, and is currently being tested. A prototype of the device will be delivered in the near future. A second program has involved the development of the ..beta..-attenuation technique for the large-scale measurement of the masses of particulate deposits. The emphasis has been primarily concerned with the fabrication of improved hardware to perform the measurement conveniently and accurately. Other activities included consultation with EPA on questions concerning air sampling techniques and X-ray fluorescence analysis of aerosol samples. An additional long-term interest has been the development of X-ray techniques which are capable of determining the chemical form of the sulfur atoms in an environmental sample. Since these latter activities are in a more speculative period of development, the present progress report emphasizes the activities in the area of elemental sulfur monitoring and ..beta..-gauge development.

Publisher Summary One of the most important techniques in nuclear spectroscopy depends upon the measurement of the amplitude of the electrical impulse that can be produced when a quantum of radiation is absorbed in a detector. A histogram of pulse amplitudes produced by a mixture of nuclear radiations of different energies can therefore be used to determine the energy spectrum of the radiation absorbed in the detector. This chapter illustrates that to examine a spectrum fully, analyzers capable of measuring up to 1000 points in a spectrum are required. If limited ranges of a spectrum are to be examined or if the detector resolution is poor, smaller numbers of spectrum points are required from the analyzer.

A very brief statement of the problems of nuclear reactor design prefaces an account of some of the nuclear-physics measurements necessary. The time-of-fiight neutron spectrometer installed at the Atomic Energy Research Establishment is described in detail with reference to these measurements. The performance of the machine, which makes use of a 3.2-MeV linear electron accelerator as neutron source, is shown in comparison with that previously published for other spectrometers. It is shown that the energy resolution obtainable at energies over 10 eV with the A.E.R.E. spectrometer is rather better than has previously been available for comparable counting rates. This is illustrated by comparison of experimental observations of the cobalt total neutron cross-section in the neighbourhood of a scattering resonance at 120 eV.

After a brief historical overview of the electronic instrument design profession, the changes resulting from CAD/CAE (computer-aided design/computer-aided engineering) techniques are assessed. A traditional design cycle is identified consisting of design, prototyping, rework, final engineering, and documentation. It is felt that this new instrument design technology will destroy this cycle of design, development, and engineering. In its most elementary form, CAD can be regarded as a drafting tool; however, when a CAD system is surrounded by logic and analog simulators, by features such as autoplacement of parts and autorouting of connections, and even by automatic fabrication and test equipment, CAD becomes a tool that revolutionizes the instrument engineering process. Conventional prototyping is replaced by simulation and the roles of engineers and technicians change radically. It is argued that the experience of present designers is a valuable resource that should not be lost and that industry and government laboratories must provide institutional mechanisms, such as granting sabbatical time, to permit design engineers to catch up with the advances in technology and to maintain this forefront knowledge. >