C. P. Cork

Development of High Efficiency Virtual Impactors

The authors briefly review the development of the virtual impactor which, as an inertial particle separator according to aerodynamic sizes, has played a unique role in particle sampling, concentration, classification, and generation. Its performance characteristics in size separation are predictable by theoretical model calculations. However, its behavior in terms of internal wall losses has thus far defied quantitative analysis, and its ultimate control has eluded most practitioners in virtual impactor design. Through experimentation, the authors identify the relevant parameters in a virtual impactor and indicate their relative sensitivity and acceptable ranges of variability. With the detailed illustration of specific high-efficiency virtual impactor design, which has a cutpoint of 2.5 μm and wall losses of under 1%, it is demonstrated the underlying principles cited are crucial to minimizing losses and may be generally applicable to future developments.

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.

A 2D smart pixel detector for time resolved protein crystallography

A smart pixel detector is being developed for Time Resolved Crystallography for biological and material science applications. Using the Pixel Detector presented here, the Laue method will enable the study of the evolution of structural changes that occur within the protein as a function of time. The X-ray pixellated detector is assembled to the integrated circuit through a bump bonding process. Within a pixel size of 150/spl times/150 /spl mu/m/sup 2/, a low noise preamplifier-shaper, a discriminator, a 3 bit counter and the readout logic are integrated. The read out, based on the Column Architecture principle, will accept hit rates above 5/spl times/10/sup 8//cm/sup 2//s with a maximum hit rate per pixel of 1 MHz. This detector will allow time resolved Laue crystallography to be performed in a frameless operation mode, without dead time. Target specifications, architecture and preliminary results on the 8/spl times/8 front-end prototype and the column readout are presented.

Si(Li) X-Ray Detectors with Amorphous Silicon Passivation

Lithium-drifted silicon [Si(Li)] detectors with thin lithium n+ contacts and amorphous silicon (¿-Si) junction passivation are described. These detectors (7 mm thick, 9 cm2 area) are intended for use in a sixelement detector array which is designed to measure trace amounts of plutonium in soil samples. Results are given showing a spectral resolution of ~ 400 eV (FWHM) for the 17.8 keV Np L x-rays entering through either the lithium n+ contact or the gold barrier contact on these detectors. Measurements on the effects of the fractional H2 concentration on the electrical behavior of the ¿-Si/Si interface are reported. The increase with time in the lithium window thickness when the detectors are stored at room temperature is discussed.

The advanced light source control system

Abstract The criteria for the design of a modern control system are discussed in the context of the special requirements for the control of the Advanced Light Source (ALS). This is followed by the description of the system being built, which has a number of special features. The emphasis has been placed on providing an extremely wide I/O and CPU bandwidth (greater than 120 Mbits/s and 1200 Mips, respectively) and on using commercially available hardware and software. The only major custom design item is the Intelligent Local Controller (ILC), of which about 600 will be used to interface to the hardware. The use of personal computers for the operator stations allows the use of a wealth of industrial standards for windows, languages and tools.

A LASER-BASED MONODISPERSE CARBON FIBER GENERATOR

Abstract A generator for delivering a controlled number of carbon fibers of prescribed uniform lengths for the purpose of on-line detector calibration is described. Results showed that it is feasible to generate carbon fibers from about 5 mm long down to 30 μm or less with an accuracy of approximately 5 μm.

An in vitro electrophysiological assessment of the direct cellular toxicity of carbon monoxide

Abstract The spontaneous electrical activity of roller-tube cultured cerebellar Purkinje cells of rats was monitored to assess the relative effects of gas- and liquid-phase hypoxic and/or carbon monoxide insults. It was shown that both hypoxia and CO insults directly affect functional cellular activity. A tentative dose-response function was determined for gas-phase CO, in which threshold effects were seen at concentrations of 500 to 1000 ppm. Severe disruptions of electrical activity were always seen with CO concentrations of 5000 ppm or greater. In certain cases, CO appeared to have a cellular toxicity over and above that due to hypoxia alone, as evidenced by direct, sequential comparisons on the same cell. The inhibitory effects of CO are partially reversed by exposure of the nerve cells to white light. This effect presumably comes from the photodisassociation of CO from iron heme groups in the cells cytochrome system.

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.

Cryostat and Electronic Development Associated with Multi-Detector Spectrometer Systems

To overcome the problems of fabricating a practical cryostat that contains a large number of semiconductor detectors and the electronics associated with a high resolution spectrometer system has required a number of significant technological developments. These problems and our corresponding solutions are presented.

THE DEVELOPMENT OF AN ARRAY OF COOLED LARGE AREA Si (Li) DETECTORS

Abstract A system containing six cooled, 34 mm diameter by 7 mm thick, high-resolution Si(Li) detectors designed to maximize the sensitivity for counting X-rays in the 10–30 keV range to measure trace radionuclides in soil samples has been successfully fabricated. The detectors were mounted in a paddle-shaped cryostat with a single large beryllium window on each side. This configuration provides for efficient anticoincidence background suppression and effectively doubles the sensitive detector area because X-rays can impinge on the detectors from both sides. To maximize detection efficiency, the thickness of the cryostat was held to a bare minimum (25 mm); this caused severe difficulties during fabrication of the system. Cutting down the rim of the detectors reduced to an acceptable level the microphony caused by movement of the beryllium window that faces the lithium-diffused contact of the detectors. Since this system will be used for low level counting, careful testing was performed to select materials having the lowest radioactivity.