M.R. Maier

The SISWICH, a detector telescope with intrinsic calibration

Phoswich counters provide adequate energy resolution and particle identification up to atomic numbers Z=20, but calibration has proved difficult. To overcome this difficulty, a silicon-scintillator sandwich (SISWICH) has been built. This detector consists of a Si PIN photodiode glued to the front of a CsI(Tl) scintillator crystal. The signal coming from the PIN diode contains the energy deposited by the particle by direct ionization in the silicon, which is very fast, and the light output of the scintillator, which is slow. The direct component can be calibrated against a pulser. Since the energy loss of a particular particle has been measured, its energy is known, and this can be used to calibrate the signal from the scintillator. >

GAMMASPHERE-elimination of ballistic deficit by using a quasi-trapezoidal pulse shaper

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. >

A sixteen channel peak sensing ADC for singles spectra in the FERA format

To read out multi-element small X-ray detectors for X-ray fluorescence applications with synchrotron radiation one needs the capability to record multiple singles spectra for each detector element at high rates. We have developed a sixteen channel 11 bit peak sensing ADC in a CAMAC module. We use the FERA readout bus to place the data into a commercially available histogramming module developed to generate multiple histograms from FERA ADCs. The sixteen channels digitize shaped pulses from the detectors without external gating. The digitizing time is 8 /spl mu/sec, the peak acquisition time is /spl ges/2 /spl mu/sec. The module contains a LIFO to permit block transfers in order to minimize dead times associated with the readout. There is a common CAMAC controlled analog threshold for noise suppression and a 16 bit mask to enable or disable individual ADCs. Differential non linearity is less than +8%/-4%. A /spl gamma/-ray spectrum collected using this ADC is presented.

Silicon array detector system for high-rate, low-noise X-ray spectroscopy

A silicon array detector system is being developed for X-ray fluorescence applications at synchrotron light sources. The detector is wire-bonded to integrated circuits which features 32 channels of charge-sensitive preamplifiers followed by variable-gain pulse shaping amplifiers. The ICs directly drive CAMAC-based A/D boards designed for this application. The data are transferred from the custom designed 16-channel ADC modules via FERABUS readout to commercially available histogramming modules and memory lookup units. The system features fully parallel signal processing to maintain high count rate capability and to preserve the position information. Special LabVIEW-based software has been developed for data acquisition and analysis. The system, currently being assembled for 64-channels, can easily be expanded by increasing the number of detection channels and hardware modules.

A high resolution 14-bit ADC for Gammasphere Project

A system of 110 high-performance ADCs (analog-to-digital converters), required for the Gammasphere Project at the Lawrence Berkeley Laboratory, is described. The ADCs are compact and linear (+or-0.006% integral, +or-0.5% differential), with a conversion deadtime of 6.5 mu s and a channel profile flatness of 50%. The converter is based on a new low-power (0.25 W), low-cost, monolithic high-speed sampling ADC, exhibiting a very high stability and no missing codes over the entire 16-bit range. Differential linearity was achieved by applying the Gatti method, covering a 6-b range. Measured data and methods of testing the ADC are also presented.<<ETX>>

Low cost pulse shaper for use with silicon PIN diodes

A flexible, small, and low-cost analog pulse shaper and amplifier circuit has been developed for silicon PIN photodiode detectors. The shaping amplifier produces both a fast negative output signal with less than 10-ns rise time for timing measurements and a slow positive output signal with peaking times from 500 ns to 10 mu s for energy measurements. The gain for the two signals can be set independently. The maximum output amplitude is -1.5 V for the fast signal and +10 V for the slow signal. The resolution for the energy channel was >

The sixteen channel CAMAC constant fraction discriminator for APEX

The authors report on the construction and the performance of a sixteen-channel constant fraction discriminator (CFD) for the Atlas Positron Experiment (APEX). An integrated circuit which contains all the electronic building blocks needed to construct a CFD has been used. Sixteen channels of CFD have been placed into a CAMAC module. An important feature is the time-to-charge converter (TQC) included for every CFD channel. Its calibration constant is controlled via CAMAC. The TQC allows the use of charge-sensitive analog-to-digital converters for timing measurements. Results for CFD walk, resolution, and crosstalk as well as for TQC linearity are presented.<<ETX>>

Adaptive readout technique for a sixteen channel peak sensing ADC in the FERA format

An adaptive variable block size readout technique for use with multiple sixteen channel CAMAC ADCs with a FERA-bus readout has been developed and designed. It can be used to read data from experiments with or without coincidence, i.e. singles, without having to change the readout protocol. Details of the implementation are discussed and initial results are presented. Further applications of the adaptive readout are also discussed.

GAMMASPHERE. Overview of detector and signal processing system

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.

GAMMASPHERE-timing and signal processing aspects of the BGO Compton shield

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. >