Bojan T. Turko

A Picosecond Resolution Time Digitizer for Laser Ranging

The Time Digitizer capable of covering a range of 0.34 sec in 9.76 psec increments is described. The time interval between a pair of start-stop pulses is digitized coarsely in 20 nsec periods by a very accurate 50 MHz reference clock. The residual fractions of a clock period at the start and the stop end of the measured interval are stretched in two interpolators and digitized in 9.76 psec increments. An equivalent digitizing frequency of 102.4 GHz is thus achieved. The digitizer is built in a CAMAC minicrate and communicates via a standard crate controller. It is intended for use in the laser ranging between ground stations and the Laser Geodetic Satellite (LAGEOS). It is shown that the distribution in any two adjacent 9.76 psec channels of a small number of identical test time intervals is essentially binomial. The performance of the digitizer and test results are given.

Single photon timing system for picosecond fluorescence lifetime measurementsa)

A single‐photon timing system is described which is capable of extracting fluorescence lifetimes as short as 25 ps. The system is an improved version of an earlier apparatus. The new system uses a synchronously pumped, mode‐locked dye laser with 10‐ps pulses operating at 82‐MHz repetition rate. A fast photodetector and a leading‐edge discriminator were developed to use with this light source. Also, a special rate reduction circuit was built to eliminate large oscillations in fluorescence decay spectra due to the excessive stop rates that overload commercial time‐to‐amplitude converters.

Multichannel Interval Timer

A CAMAC based modular multichannel interval timer is described. The timer comprises twelve high resolution time digitizers with a common start enabling twelve independent stop inputs. Ten time ranges from 2.5 ¿s to 1.3 ¿s can be preset. Time can be read out in twelve 24-bit words either via CAMAC Crate Controller or an external FIFO register. LSB time calibration is 78.125 ps. An additional word reads out the operational status of twelve stop channels. The system consists of two modules. The analog module contains a reference clock and 13 analog time stretchers. The digital module contains counters, logic and interface circuits. The timer has an excellent differential linearity, thermal stability and crosstalk free performance.

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.

The Electronics for the Donner 600-Crystal Positron Tomograph

The data acquisition system, designed for the Donner 600-Crystal Positron Tomograph, is described. Coincidence timing resolution of less than five nanoseconds full width at half maximum and data rates in excess of one million events per second are achieved by using high-speed emitter coupled logic circuits, first-in first-out memory to derandomize data flow, and parallel architecture to increase throughput. These data rates allow the acquisition of adequate transmission data in a reasonable amount of time. Good timing resolution minimizes accidental coincidences and permits data rates greater than 100,000 image-forming events per second for high-speed dynamic emission tomography. Additional scatter and accidental rejection are accomplished for transmission data by using an orbiting source and a look-up table for valid events. Calibration of this complex electronic system is performed automatically under computer control.

Space Borne Event Timer

The Space Borne Event Timer is a part of the NASA laser ranging system that is intended to operate aboard the Space Shuttle orbiting over California. The object is to measure, by laser ranging, the earth movement along the San Andreas fault and possibly forecast future earthquakes. A number of cube reflector targets will be placed along both sides of the fault. The ranging system aboard the Space Shuttle will fire a burst of laser pulses at each target and detect the reflected light. Time differences between pulses from the two sides of the fault will indicate earth displacements. The Space Borne Event Timer is a CAMAC compatible system that provides extremely accurate timing data and controls the operation of the ranging system. For each event the time is given in 19.53 increments from the instant of firing the laser to the instant the reflected light is received back, within a range of 130 days.

A Modular 125 PS Resolution Time Interval Digitizer for 10 MHz Stop Burst Rates and 33 MS Range

A high resolution multiple stop time interval digitizer is described. It is capable of resolving stop burst rates of up to 10 MHz with an incremental resolution of 125 ps within a range of 33 ms. The digitizer consists of five CAMAC modules and uses a standard CAMAC crate and controller. All the functions and ranges are completely computer controlled. Any two subsequent stop pulses in a burst can be resolved within 100 ns due to a new dual interpolation technique employed. The accuracy is maintained by a high stability 125 MHz reference clock. Up to 131 stop events can be stored in a 48-bit, 10 MHz derandomizing storage register before the digitizer overflows. The experimental data are also given.

Scintillation Photon Detection and Event Selection in High Resolution Positron Emission Tomography

The scintillation photon detection and event selection subsystem for the high spatial resolution Donner 600-channel positron emission tomograph is described. The tomograph spatial resolution of better than 3.0 mm, FWHM, is obtained by using 600 closely-packed bismuth germanate 3 mm thick scintillation detectors. Each detector has its own 13.5 mm diameter photomultiplier and event selection channel, consisting of two fast pulse amplifiers, a charge integrator, a differential pulse height discriminator and precision time delay for event strobing. The event selection channel is capable of starting a timing cycle from the first photoelectron emitted from the photocathode. It is also capable of discriminating between true events and noise and of eliminating events occurring simultaneously in two adjacent channels. Adjustments of the event strobing time and two discriminator levels are performed by a computer.

Development of Electro-Optical Instrumentation for Reactor Safety Studies

The development of new electro-optical instrumentation for reactor safety studies is described. The system measures the thickness of the water film and droplet size and velocity distributions which would be encountered in the annular two-phase flow in a reactor cooling system. The water film thickness is measured by a specially designed capacitance system with a short time constant. Water droplet size and velocity are measured by a subsystem consisting of a continuously pulsed laser light source, a vidicon camera, a video recorder, and an automatic image analyzer. An endoscope system attached to the video camera is used to image the droplets. Each frame is strobed with two accurately spaced UV light pulses, from two sequentially fired nitrogen lasers. The images are stored in the video disk recorder. The modified automatic image analyzer is programmed to digitize the droplet size and velocity distributions. Many special optical, mechanical and electronic system components were designed and fabricated. They are described in detail, together with calibration charts and experimental results.

Measurement of Thickness of Thin Water Film in Two-Phase Flow by Capacitance Method

A technique has been developed for measuring water film thickness in a two-phase annular flow system by the capacitance method. An experimental model of the flow system with two types of electrodes mounted on the inner wall of a cylindrical tube has been constructed and evaluated. The apparatus and its ability to observe fluctuations and wave motions of the water film passing over the electrodes is described in some detail.