R.L. Wixted

Tests of a prototype pixel array detector for microsecond time-resolved X-ray diffraction

X-ray test results from a prototype 92 × 100 pixel array detector (PAD) for use in rapid time-resolved X-ray diffraction studies are described. This integrating detector is capable of taking up to eight full-frame images at microsecond frame times. It consists of a silicon layer, which absorbs the X-rays, bump-bonded to a layer of CMOS electronics in which each pixel has its own processing, storage and readout electronics. Tests indicate signal performance characteristics are comparable with phosphor-based CCD X-ray detectors, with greatly improved time resolution, comparable linearity and enhanced point spread. This prototype is a test module en route to a larger detector suitable for dedicated operation. Areas of needed improvement are discussed.

Characterization of a prototype pixel array detector (PAD) for use in microsecond framing time-resolved X-ray diffraction studies

A 4 by 4 pixel array device has been designed and built as a prototype for a high-speed two-dimensional X-ray imaging detector. This detector is a two-tier device with an X-ray sensitive photo-diode array bump-bonded to a 1.2 micrometer CMOS analog integrated circuit. Each 150 micrometer square pixel of the detective layer is matched to a corresponding section of the electronics. Each pixel of the electronics is capable of integrating the signal from the diode for the desired framing time, storing the result in one of eight storage capacitors and sequentially outputting the stored value to an on chip buffering op amp. Tests of the electronics have demonstrated a full-well (per pixel, per frame) of over 10.500 12 keV X-rays with an electronics noise level corresponding to less than 2.8 12 keV X-rays. Speed tests indicate the ability to integrate to the full-well within 2 microseconds (an average pixel count-rate of 5.2 GHz). We present detailed characterizations of the performance of this initial device, including an analysis of noise, stability, linearity and point spread. In addition, we will discuss plans for large scale integration towards the goal of a 1024 by 1024 pixel detector.

Development of a fast pixel array detector for use in microsecond time-resolved x-ray diffraction

A large-area pixel x-ray detector is being developed to collect eight successive frames of wide dynamic 2D images at 200kHz rates. Such a detector, to conjunction with a synchrotron radiation x-ray source, will enable time-resolved x-ray studies of proteins and other materials on time scales which have previously been inaccessible. The detector will consist of an array of fully-depleted 150 micron square diodes connected to a CMOS integrated electronics layer with solder bump-bonding. During each framing period, the current resulting from the x-rays stopped in the diodes is integrated in the electronics layer, and then strored in one of eight storage capacitors underneath the pixel. After the last frame, the capacitors are read out at standard data transmission rates. The detector has been designed for well-depth of at least 10,000 x-rays (at 12 keV), and a noise level of one x-ray. Ultimately, we intend to construct a detector with over one million pixels (1024 by 1024). We present the result of our development effort and various features of the design. The electronics design is discussed, with special attention to the performance requirements. The choice and design of the detective diodes, as they relate to x-ray stopping power and charge collection, are presented. An analysis of various methods of bump bonding is also presented. Finally, we discuss the possible need for a radiation-blocking layer, to be placed between the electronics and the detective layer, and various methods we have pursued in the construction of such a layer.

Prototype studies of a fast RICH detector with a CsI photocathode

A prototype fast RICH detector with a CsI photocathode coupled to a wire chamber filled with atmospheric pressure ethane has been studied in a 3.5 GeV/c π− beam. Using a 1 cm thick C6F14 radiator, 8.5 photoelectrons per ring were detected. Measurements were made of the CsI quantum efficiency, photoelectron position resolution, and Cherenkov ring radius resolution. UV photon feedback appears to have degraded the resolution of the detector. Techniques to improve the photon detection efficiency and reduce backgrounds are discussed.

A Pixel-Array Detector for Time-Resolved X-ray Diffraction

An integrating pixel-array detector for recording time-resolved X-ray diffraction measurements on microsecond timescales has been designed and tested as a 4 x 4 pixel prototype. Operational characteristics and radiation tolerance are discussed. A 100 x 92 array with 151.2 micro m square pixels is currently under construction.

Characterization of CsI photocathodes for use in a fast RICH detector

We have completed a series of measurements that provide a basic understanding of the properties of CsI photocathodes for use in ring imaging Cherenkov (RICH) detectors. The quantum efficiency of CsI has been measured relative to an NIST-calibrated photodiode and is in excellent agreement with a similar measurement by Breskin et al. A representative value of the quantum efficiency is 20% at 180 nm. The quantum efficiency of a fresh photocathode is unaffected by temperature, but heating the photocathode can be helpful if it has been exposed to water vapor, or has been aged by a large integrated photocurrent. Detailed studies of aging show a “fast” component that appears to be associated with a rise in the work function, and a “slow” component associated with conversion of the bulk CsI to Cs. We judge that a practical lifetime of a CsI photocathode is until it has lost 20% of its initial quantum efficiency, which process is dominated by the “fast” rise in the work function. This rise occurs both due to photoelectron transport with an effective lifetime of 0.1 μC/mm2 and due to positive-ion bombardment with an effective lifetime of 15 μC/mm2. When the CsI photocathode is used in a chamber with gas gain greater than 150 the latter lifetime is the relevant one. This lifetime should be sufficient for use of a RICH detector at an e+e− B factory. The reduction of quantum efficiency of a CsI photocathode in a gas-filled chamber has been studied for several gases over a wide range of reduced electric field. This effect can be minimized by use of atmospheric-pressure methane in a chamber with anode wires rather than a mesh. We have also demonstrated that excellent spatial resolution for the location of the photoelectrons can be obtained using a coarse cathode-pad readout if the anode-cathode spacing is similar to the pad width.

Detection of internally reflected Cherenkov light, results from the BELLE DIRC prototype

Abstract The DIRC (Detector of Internally Reflected Cherenkov light) is a new type of ring imaging Cherenkov detector which utilizes precisely machined quartz bars as Cherenkov radiators. Bar quality is determined by both laser and cosmic ray measurements. The Cherenkov photons are transported several meters along the bar until they reach the detector cavity where they are proximity focussed on an array of conventional photomultiplier tubes. Results from a prototype device comprising a 2 × 4 × 240 cm 3 quartz bar read by an array of 480 PMTs are presented. Sample images, which are the first observed in this type of detector, are also shown. Measurements of the light yield (approximately 20 photoelectrons per image) and the angular resolution, which are both in good agreement with Monte Carlo predictions, are shown to lie well within the expected operating tolerances of the DIRC.

A 32-CHANNEL WAVEFORM SAMPLING FASTBUS MODULE WITH LOCAL DSP PROCESSING

Abstract A 32-channel FASTBUS module that uses switched-capacitor arrays to sample and store input waveforms at rates up to 30 MHz is described. The captured data are digitized using commercial 12-bit analog-to-digital converters and are routed to an array of dual port memories. The data in the dual port memories can be readout directly over the FASTBUS backplane or can be subjected to further processing by a digital signal processor mounted on the FASTBUS auxiliary card.

Development of a pixel array detector for time resolved x-ray imaging

We are developing an integrating Pixel Array Detector (PAD) for microsecond time-resolved x-ray diffraction. The current detector prototype has a format of 92×100 pixels, each 150μm square, and covers an active area of 15×13.8 mm2. The detector was tested at the CHESS D1 beamline and imaging capabilities within the microsecond time resolution regime were successfully demonstrated. This prototype is a test module en route to a larger detector of 1000×1000 pixels suitable for dedicated operation. Areas in need of improvement (radiation-hardness, large area coverage) will be discussed.

A FAST DSP-BASED CALORIMETER HIT SCANNING SYSTEM

Abstract A custom made digital signal processor (DSP) based system has been developed to scan calorimeter hits read by a 32-channel FASTBUS waveform recorder board. The scanner system identifies hit calorimeter elements by surveying their discriminated outputs. This information is used to generate a list of addresses, which guides the read-out process. The system is described and measurements of the scan times are given.