Branko Leskovar

Photon counting system for subnanosecond fluorescence lifetime measurements

A new photon counting system has been developed for subnanosecond fluorescence lifetime measurements. The system incorporates a nanosecond light pulser, a dual counter unit, and a constant‐fraction discriminator. The operating conditions of the light pulser have been adjusted to minimize the spread of the light pulse waveshape. The discriminator has upper and lower level adjustments and a time walk of no more than ±35 psec over a 50‐mV to 5‐V input pulse amplitude variation. The measuring system has a total system time resolution, expressed as the FWHM of the light pulse, of 800 and 1480 psec using photomultipliers 8850 and 8852, respectively, with full photocathode illumination and optimized operating conditions. The system will measure both single and multiple decay components, and it is designed and optimized for experiments involving measurements of decay time constants as short as 90 psec.

Single photoelectron time spread measurement of fast photomultipliers

Abstract Time spread measurements, using single photoelectrons of fast photomultipliers having dynodes with cesium-activated gallium-phosphide secondary emitting surfaces have been investigated. Measurements of the time spread, made by means of a specially developed measuring system with improved time resolution capabilities, on several 8850s and C31024s are presented and discussed. Critical evaluation of the measuring system is given, particularly concerning the influences of the light pulse width and the constant fraction discriminator timing errors on the system time-resolution capabilities. The measuring system incorporates a subnanosecond light-pulse generator capable of operating at a repetition rate of 3 MHz with output light pulses having a full width at half-maximum of less than 200 ps. It also incorporates a constant fraction discriminator which has a time walk less than ±35 ps over a range of input-pulse amplitudes from 35 mV to 8 V. The optimum operating conditions of the photomultipliers have been determined for minimum time spread and relative collection efficiency. The error of the time spread measurements, because of the finite width of the light pulse, is discussed and determined for different pulse widths.

PHASE COMPENSATED ELECTROMAGNETIC DELAY LINES FOR WIRE CHAMBER READOUT.

Abstract In this paper a phase-compensated distributed parameter electromagnetic delay line suitable for the readout of the positions of ionizing tracks in multi-wire proportional chambers is described. Delay lines with delays of 40 to 200 ns/cm having a rectangular cross section have been designed. The rectangular cross section enables them to be capacitatively coupled to the wire chamber simply by clamping them on the wire chamber external leads. The coupling coefficients between the chamber and delay line are approximately 3 to 10%. The electronic readout accuracy is better than ±0.1 mm due to the delay line.

Measurement of very short fluorescence lifetimes by single-photon counting.

Measurement of very short fluorescence lifetimes by the single‐photon technique is made possible by an improved fluorescence lifetime system. Fluorescence lifetimes of 4.94±0.07 nsec for anthracene in cyclohexane, 640±30 psec for diphenyl butadiene in cyclohexane, and 90±30 psec for erythrosin in water were determined. The use of a small wavelength shift between excitation and emission minimizes the effect of the wavelength dependence of the photomultiplier response and light pulser emission. The effects of deaeration of solutions and time averaging of the excitation profile are presented. We investigated the origin of small‐amplitude early and late artifactual peaks in the light pulser and fluorescence profiles. Complications in the analysis of lifetime data introduced by intrinsic fluorescence and phosphorescence processes in commonly used absorption filters are discussed. Certain ’’blind spots’’ are found in the electronic pulse pileup rejection schemes most commonly used in photon counting.

Microwave absorption coefficients of atmospheric pollutants and constituents

Abstract Calculations of the transition frequencies and absorption coefficients of microwave rotational transitions are given for a number of atmospheric pollutants and constituents. New measurements of the absorption coefficients are made in the vicinity of 70 GHz. The apparatus used in these measurements is briefly described. The calculated absorption coefficients are compared with these measurements and with existing measurements at other frequencies where available. Transitions with frequencies up to about 200 GHz are considered for the molecules and radicals SO 2 , O 3 , H 2 O, NO 2 , H 2 S, H 2 CO, NH 3 , CO, OCS, N 2 O, NO, OH, O 2 , SO. Also discussed are criteria for the selection of appropriate transitions for the development of high sensitivity monitors to be used in air pollution and combustion research.

140 GHz pulsed fourier transform microwave spectrometer

A high frequency energy pulsing system suitable for use in a pulsed microwave spectrometer (10), including means (11, 19) for generating a high frequency carrier signal, and means (12) for generating a low frequency modulating signal. The carrier signal is continuously fed to a modulator (20) and the modulating signal is fed through a pulse switch (23) to the modulator. When the pulse switch (23) is on, the modulator (20) will produce sideband signals above and below the carrier signal frequency. A frequency-responsive device (31) is tuned to one of the sideband signals and away from the carrier frequency so that the high frequency energization of the frequency-responsive device (31) is controlled by the pulse switch (23).

Performance Studies of High Gain Photomultiplier Having Z-Configuration of Microchannel Plates

The characteristics of a high gain type ITT F4129 photomultiplier having three microchannel plates in cascade for electron multiplications have been investigated. These plates are in the Z-configuration. Measurements are given of the gain, dark current, cathode quantum efficiency, anode pulse linearity, electron transit time, single and multiphoton time spreads, fatigue, and pulse height resolution. The gain as a function of transverse magnetic field has been measured and is discussed. Photomultiplier characteristics as a function of the input pulse repetition frequency have also been investigated and discussed.

Performance Studies of Prototype Microchannel Plate Photomultipliers

The characteristics of prototype photomultipliers having high gain microchannel plates for electron multiplication have been investigated. Measurements are given of the dark current, quantum efficiency, anode pulse amplitude, electron transit time, single photoelectron time spread, and pulse height resolution of LEP HR 350 and HR 400 photomultipliers. The gain, the collection efficiency, and the single electron pulse amplitude as functions of the ambient axial and transverse magnetic fields have been measured and are discussed. Measurement techniques and descriptions of the measuring systems are given in detail.

A Measuring System for Studying the Time-Resolution Capabilities of Fast Photomultipliers

A measuring system has been developed for studying and optimizing the time-resolution capabilities of fast photoelectric devices, such as standard and microchannel type photomultipliers. The system incorporates a subnanosecond light-pulse generator capable of operating at repetition frequencies up to 3 MHz with output light pulses having a full width at half-maximum of less than 200 psec, and a zero-crossing discriminator which has a time walk of ±40 psec over a 170 mV to 8 V input-pulse amplitude variation. Averaging the data from several successive measurements, the measuring system time resolution is 20 psec.

Performance Studies of Photomutipliers Having Dynodes with GaP(Cs) Secondary Emitting Surface

Characteristics of photomultipliers having dynodes with cesium-activated gallium phosphide secondary emitting surfaces have been investigated. We present measurements of characteristics, generally not available from manufacturers data sheets, for several 8850s, C70133Bs, 8852s, and C31024s. The transit-time difference, the single-electron transit-time spread, and the collection and quantum efficiency uniformity as a function of the position of the photocathode sensing area are measured and discussed. Measurement techniques and descriptions of measuring systems are given in detail. Ephasis is put on the determination of optinum photamultiplier operating conditions, particularly concerning the collection efficiency, and minimum transit-time spread.