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.
The methods of steady-state and pulsed millimeter-wave rotational spectroscopy were used to probe the vibrationally excited products, CS and SO produced in the laser photolysis of CS2 and SO2 at 193 nm. The time dependence of the vibrational populations was measured for CS up to v=8 and for SO up to v=5. For CS, a Boltzmann-like population distribution was found with a vibrational temperature of 5795 K corresponding to a vibrational energy of 9.8 kcal mol−1. In the case of SO, significant excitation was observed only for the states v=0, 1 and 2 with v=2 having the largest initial population. A small percentage of the SO molecules were found in v=5. The precursor gases were diluted in Ar to enhance the ratio of rotational to vibrational relaxation times. Because rotational transitions were probed, it was necessary to wait for rotational equilibrium to be established before making the measurements. As a result, the vibrational populations may have been partially relaxed at the time of measurement. In order to evaluate the extent of relaxation, a discussion of the various relaxation processes is given.
Microchannel plates (MCPs) are compact electron multipliers of high gain. They have been used in a wider range of particle and photon detection systems perhaps more than any other kind of detector. A typical MCP consists of about 10,000,000 closely packed channels of common diameter which are formed by drawing, etching, or firing in hydrogen, a lead glass matrix. Typically, the diameter of each channel is ~ 10 microns. Each channel acts as an independent, continuous dinode photomultiplier. In astronomy, and in the many other fields that use MCPs, the detectors are generally used for distortionless imaging with very high spatial resolution. Physical principles of MCP operation The fundamental physical principals of MCP detectors are gain, efficiency, energy resolution, spatial resolution, time resolution, and dark noise. X-rays interact with the channel plate glass and electrodes (and with the associated photocathode material) via the photoelectric effect. For X-ray energies below about 5 keV, detection proceeds in a 'single channel mode'. That is, no significant fraction of the X-ray beam entering a given channel penetrates the channel wall to illuminate the neighboring channels. At higher energies, this 'channel crossing' phenomena becomes important. Another important property of MCPs is their relative immunity to magnetic fields. A single plate with typical operating parameters is completely unaffected by being immersed in a 0.5 Tesla magnetic field. Stacks of plates in certain orientations are immune to much higher fields. This property has only just begun to be exploited in the space astronomy world.
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.
Abstract.The characteristics of a high gain type ITT F41Z9 photomultiplier having three microchannel plates in cascade for electron multiplications have been investi gated. These plates are in the Z-configuration. Meas urements 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 freqjency have also been investigated and discussed.
This report was prepared as an iccount of work sponsored by the United Stale, Government. Neither ihe United Statee nor the United States Atomic Eneriy Commission. nor any or their employee,, nor any of their contractors subcontractora, or their err.ployeea, make, any warraniy. express or implied, or assumes any legal liability or responsibility Toi the accuracy, com pleteness or usefulness or anv information, apparatus, product or process disclosed, or represents that ila usr would not infringe privately owned rights.JUAST discriminator which has a time walk less than ±35 psec over a range of input-pulse amplitudes from 3SmV to 8V. The optimum operating conditions of the photomultipliers have been determined for minimum time spread and relative collec tion 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.
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.
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