M. G. Strauss

Simple technique for precise determinations of counting losses in nuclear pulse processing systems

Abstract A new technique has been developed to determine precisely the fraction of all counting losses suffered in counting and pulse height analysis systems. Of particular significance is the applicability of this method to time-dependent counting rates. The method is based on a technique in which the rate of radiation incident upon the detector is sampled continually. Upon the detection of a preset number of counts, a pulser signal is injected into the preamplifier. At the end of the experiment, the total number of injected pulses is compared with the number of these pulses that are recorded in the multichannel analyzer or scaler. Since the pulser peak suffers the same fractional losses as the spectral components of the detected nuclear events, such a comparison provides an accurate representation of the counting losses due to all loss-producing effects. These include losses incurred in ADCs, computers, pileup rejectors, etc. Losses suffered due to pileup may not be negligible compared with those due to the ADC and memory of fast multichannel analyzers. The application of this technique to constant counting rates, and to varying counting rates encountered in charged-particle-induced reactions and in radioactive-decay studies are treated in detail.

Ultra Stable Reference Pulser for High Resolution Spectrometers

A solid state, double pulse generator with a pulse amplitude temperature coefficient of 0 to -8 ppm/°C was developed. It is shown that when a servo-stabilized spectrometer is locked on the pulser peaks, the gain and zero-intercept of the system drift only ±20 ppm/day or less than 1/10 of the optimum resolution of Ge(Li) detectors. The pulser generates signals which rise in 15 nsec, remain flat-topped for 40 ?sec and then fall exponentially to the baseline in 250 ?sec. The pulse rate can be varied from 0.1 - 1000 pulse-pairs/sec in 9 steps. The amplitude of the two pulses can be set independently from 5.5-700 mV, using a 7-bit precision R-2R attenuator. When used in conjunction with a preamplifier test capacitor of 1 pF, this amplitude range corresponds to 0.1 - 12.7 MeV (Ge). A tag pulse, generated in coincidence with each reference pulse, can be used for gating a multichannel analyzer, a coincidence system or a stabilizer. The performance of the pulser was measured with a computer based, servo-stabilized multichannel analyzer in which the two 60Co lines from a Ge(Li) detector were used as the stabilizer reference peaks. The pulser signal was injected into the preamplifier via a 1 pF air-dielectric capacitor mounted in a temperature regulated oven. The pulser peak was set between the 60Co peaks, and its location was measured as a function of temperature, supply voltage and repetition rate.

Pulse height defect due to electron interaction in the dead layers of Ge(li) γ-Ray detectors☆

Abstract Gamma rays giving rise to ionizing electrons in the dead layer of Ge(Li) detectors result in slow pulses of reduced amplitude which degrade energy spectra and coincidence resolution. It is shown that when internal conversion electrons from 207Bi (∼ 1 MeV) impinge on a detector with a thin dead layer of 10 μm, the peaks characteristic of the K and L electrons are well defined; but when they impinge on a detector with a dead layer of 0.3 mm, the electron spectrum shows no features except a continuum which rises at the low energy end. The distribution of risetimes, resulting from the interaction of conversion electrons in the latter detector, exhibits a peak which indicates an abundance of slow pulses. A similar peak is apparent in the risetime distribution from γ-rays in the continuum of the energy spectrum but not from γ-rays in the full energy peak. This suggests that a significant fraction of the pulses in the continuum is due to electron and/or positron interaction in the dead layers. A pulse shape discrimination technique was developed whereby defective pulses are identified and rejected. Gamma-ray spectra from a single Ge(Li) detector and from a Ge(Li)-NaI(TI) 3-crystal pair spectrometer were studied. A comparison of these spectra with and without rejection of slow pulses shows that in an 11 mm thick detector the fraction of the continuum due to defective pulses approaches 50%. The discrimination technique is described and results of several experiments are presented and discussed.

Solid‐State Pulse‐Height Encoding System with Pileup Reduction for Counting at High Input Rates

The system described selects undistorted pulses and digitizes them for multichannel pulse‐height analysis. The system consists of a double RC differentiating linear amplifier, a low jitter gated single‐channel analyzer, a linear gate, and an analog‐to‐digital converter (ADC). The ADC is comprised of a simple discharge‐type pulse‐height to time converter, gating a temperature compensated 2‐Mc astable multivibrator. A temperature change of 25°C results in an over‐all gain change of 1%. Baseline shift is minimized by direct coupling. At an input rate of 100 000 pulses/sec the spectrum shifts by less than 1%. Pileup events are rejected by a unique baseline crossing discriminator. The rejection efficiency is about 80%. The apparent pulse width, for pileup considerations, is thereby reduced from 1.5 to 0.25 μsec. The circuits and their performance are discussed.

GENERAL PURPOSE ANALOG PULSE HEIGHT COMPUTER

Experimental techniques in nuclear physics often call for arithmetic operations on pulses whose amplitudes convey the experimental data. An instrument capable of pulse height multiplication, division, exponentiation, as well as pulse height addition and subtraction is described. These operations are performed in about 0.5 μsec with a precision of ±0.1% of full scale over a pulse height range of 20:1. At room temperature the computer exhibits a drift of about 0.05%/C° or ±0.3%/day. With an input rate of 10 000 counts/sec the pulse height distribution shifts 0.2%, and with 25 000 counts/sec it shifts 1%. Multiplication, division, and exponentiation are performed by using logarithms and antilogarithms. The log and antilog function generators are based on the fact that the emitter‐base voltage of a silicon planar transistor is proportional to the logarithm of its collector current. Addition and subtraction are performed by using linear operational amplifiers. Results of experiments are shown in which the comp...

Pulse Shape Distributions from Gamma-Rays in Lithium Drifted Germanium Detectors

Pulse shape distributions resulting from the interaction of ?-rays in germanium detectors were studied as a function of field intensity, ?-ray energy, and detector width. It was observed that at 2300 V/cm the initial rate of charge collection approached 10 nsec/mm. Due to a slow component the total charge collection was about 30% longer. At 1000 V/cm the collection time was somewhat longer, and at 250 V/cm it was considerably longer. These observations appeared to be independent of ?-ray energy in the range of 0.6 - 3 MeV, and detector widths of 3 - 11 mm. Events interacting near the center of the detector result in fast rising pulses and those interacting near one of the electrodes have a risetime which is approximately twice as long, thus giving rise to a distribution of pulse shapes. With low energy ?-rays all shapes occur with equal probability, but at higher energy the pulses tend to have a more uniform shape. Due to the spread in risetimes, signals from monoenergetic ?-rays do not cross a given pulse height level at the same time and therefore produce timing uncertainties. With a 1 cm detector timing uncertainties of more than 30 nsec have been observed at the 50% level, and less than 5 nsec at the 5% level. It therefore appears that for accurate timing of all signals, the timing discriminator should be set at the lowest level consistent with the system noise. The variations in risetime can also affect the energy resolution.

CCD-based detector for protein crystallography with synchrotron X-rays☆

A detector with a 114 mm aperture, based on a charge-coupled device (CCD), has been designed for X-ray diffraction studies in protein crystallography. The detector was tested at the National Synchrotron Light Source with a beam intensity, through a 0.3 mm collimator, of greater than 10(9) X-ray photons/s. A fiberoptic taper, an image intensifier, and a lens demagnify, intensify, and focus the image onto a CCD having 512 x 512 pixels. The statistical uncertainty in the detector output was evaluated as a function of conversion gain. From this, a detective quantum efficiency (DQE) of 0.36 was derived. The dynamic range of a 4 x 4 pixel resolution element, comparable in size to a diffraction peak, was 10(4). The point-spread function shows FWHM resolution of approximately 1 pixel, where a pixel is 160-mu-m on the detector face. A data set collected from a chicken egg-white lysozyme crystal, consisting of 495 0.1-degrees frames, was processed by the MADNES data reduction program. The symmetry R-factors for the data were 3.2-3.5%. In a separate experiment a complete lysozyme data set consisting of 45 1-degrees frames was obtained in just 36 s of X-ray exposure. Diffraction images from crystals of the myosin S1 head (a = 275 angstrom) were also recorded; the Bragg spots, only 5 pixels apart, were separated but not fully resolved. Changes in the detector design that will improve the DQE and spatial resolution are outlined. The overall performance showed that this type of detector is well suited for X-ray scattering investigations with synchrotron sources.

PULSE SHAPE DISTRIBUTIONS FROM GAMMA-RAYS IN LITHIUM DRIFTED GERMANIUM DETECTORS.

Abstract Pulse shape distributions resulting from the interaction of γ-rays in planar germanium detectors were studied as a function of field intensity, γ-ray energy, and detector width. It was observed that at 2300 V/cm the initial rate of charge collection approached 10 nsec/mm. Due to a slow component the total charge collection was about 30% longer. At 1000 V/cm the collection time was somewhat longer, and at 250 V/cm it was considerably longer. These observations appeared to be independent of γ-ray energy in the range of 0.6–3 MeV and detector widths of 3–11 mm. Events interacting near the center of the detector result in fast rising pulses and those interacting near one of the electrodes have a risetime which is approximately twice as long, thus giving rise to a distribution of pulse shapes. With low energy γ-rays all shapes occur with equal probability, but at higher energy the pulses tend to have a more uniform shape. Due to the spread in risetimes, signals from monoenergetic γ-rays do not cross a given pulse height level at the same time and therefore produce timing uncertainties. With a 1 cm detector timing uncertainties of more than 30 nsec have been observed at the 50% level, and less than 5 nsec at the 5% level. It therefore appears that for accurate timing of all signals, the timing discriminator should be set at the lowest level consistent with the system noise. The variations in risetime can also affect the energy resolution. If a 1 cm detector is used in conjunction with a pulse shaping network consisting of a 1 μsec RC differentiator and integrator, the variations in risetimes may cause an amplitude spread on the order of 0.03%. At γ-ray energies of 10 MeV this is comparable to the spread due to statistics.

CCD-based parallel detection system for electron energy-loss spectroscopy and imaging

A generic, 2-dimensional integrating detector based on a charge-coupled device (CCD) is being developed for applications in physical and biomedical research. A detector system, operating in a 1-dimensional mode, is currently being designed for electron energy-loss spectroscopy (EELS); when operated in a 2-dimensional mode it is suitable for electron imaging studies. An energy-loss spectrometer (Gatan 607) is being equipped with a magnifying quadrupole electron lens to produce an electron dispersion of approx.15 mm on a YAG:Ce scintillator. The scintillation light is coupled via an optical lens system to a 512 pixel x 512 pixel CCD. Energy-loss spectra are recorded successively in 5 pixel x 512 pixel frames. With a microscope beam current of 0.5 nAmp and electron energy of 120 keV, the integration (exposure) time for 100 frames is 40 msec and the readout and digitization time for 100 frames, using a 14-bit ADC, is 0.5 sec. The statistical precision expected by summing the 100 frames in the computer is 10/sup -4/ for the zero-loss peak (DQE = 0.4). For a peak of 1/1000 the intensity of the zero-loss peak, the precision is 10/sup -2/ (DQE = 0.2). To obtain the same precision (10/sup -2/) with a 512-pixel linear photodiodemorexa0» array would require data from at least 280 frames and an acquisition time 35 to 70 times longer than the 40 msec required for the exposure of 100 frames in the CCD.«xa0less

2-D Position-Sensitive Scintillation Detector for Neutrons

A 2-di mensional neutron-position scintillation detector, based on the well-proven principle of the Anger ¿-ray camera, has been developed. A 6Li-loaded, Ce-activated glass scintilator is coupled via a unique 44 mm thick light guide to a closely spaced hexagonal array of nineteen 5 cm diameter photomultipliers. The 1 mm thick x 22 cm diameter glass scintillator is made up of four optically cemented quadrants. The location of a scintillation is determined by calculating the X and Y centroids using a resistor weighting scheme. A spatial FWHM resolution of 2-3 mm was obtained. At 0.025 eV (1.8 A) the efficiency of the glass is 80% and the FWHM pulse height resolution is 14%. These characteristics in combination with low ¿-ray sensitivity in the thin scintillator provide for excellent background rejection. Drift in the photomultipliers gain results in output change of <0.05 mm/°C. The detector was evaluated in experiments at the pulsed spallation neutron source ZING-P. In an experiment with a single crystal diffractometer a Laue pattern from a NaCl crystal was obtained. In a resonance radiography experiment Au and in, contained in a test object, were graphically resolved. For these applications the scintillation detector has several inherent advantages over a gas proportional counter.