Fast Photomultiplier Research Articles

Laser-Induced Coal Fluorescence Microscopy: ABSTRACT

A new laser-equipped fluorescence microscopy system has been developed to significantly widen the analytical scope of coal characterization. The system uses a pulsed tuneable dye laser interfaced to a state-of-the-art Leitz MPV3 fluorescence microscope. The fluorescence of the coal macerals is excited with ultraviolet radiation in the range of 260 to 450 nm and analyzed between 300 and 800 nm. The temporal decay of the fluorescence induced by the pulsed laser is studied. The anode pulses from a fast photomultiplier detecting the fluorescence are digitized by a fast waveform digitizer, and the information is then processed by a desk-top computer to obtain the decay curves and the corresponding decay times. The anode pulses which contain the time signatures of the fluoresce ce are corrected for temporal instrument response by deconvolution. The decay curves can also be spectrally resolved and, with further data manipulation, time-resolved spectra can be obtained. The decay times are believed to be as characteristic of the fluorescing macerals as the excitation and emission spectra, which are also being studied more extensively using monochromatized radiation from the conventional xenon and mercury arc lamps in the wavelength ranges mentioned above. The photomultiplier is cooled to reduce noise and to improve signal acquisition, and, with the red-sensitive spectral response of the photomultiplier, the fluorescence of other macerals such as vitrinites are being investigated. The technique of pulse counting is employed for greater sensitivity in detecting weak y fluorescing macerals. Current spectral studies involve observing statistical variations of fluorescence spectra of a given maceral and obtaining its averaged spectrum. The spectra are parameterized by such values as Q (red/green quotient), ^lgrmax and Qmax (intensity at ^lgrmax/intensity at 500 nm). Excitation and emission spectra are fed directly into the computer through an analog-to-digital converter as the corresponding monochromator scans the wavelengths. The computer signal averages each wavelength interval while sampling, and then corrects the averaged raw spectrum for spectral instrument response. This multiparameter analysis of the optical properties of coal is expected to enhance coal characterization. End_of_Article - Last_Page 1453------------

Recent progress in fast timing with CsF scintillators in application to time-of-flight positron tomography in medicine

An extensive study of the timing properties of a number of CsF scintillators 2.3 cm in diameter and 4 cm in height and of 2.4 cm diameter fast photomultipliers was undertaken in order to optimize scintillation counters for 511 keV annihilation quanta fast timing applied to the time-of-flight positron tomography system. Three types of photomultipliers, PM 1992, PM 2972 and R 1398, were tested in comparison to an XP 2020 photomultiplier. The photoelectron number for the CsF crystals was measured and discussed in terms of photocathode sensitivity and the efficiency of the photoelectron collection. A time resolution study was carried out for timing both with the anode pulse and with the dynode signal. It suggested that the Hammamatsu R 1398 photomultipliers are superior in terms of photoelectron yield and time resolution. The best time resolution, equal to 390 ps, was obtained for 511 keV γ-rays with the energy thrreshold set at 150 keV, in timing with the signal taken from the last dynode. For 10 CsF scintillators the time resolution and the light yield expressed by the photoelectron number were measured with the XP 2020 and the R 1398 photomultipliers. Because of the large spread in timing properties observed for different crystals, and in particular a significant discrepancy between the measured time resolution and the photoelectron number, a study of the light pulse shape from some CsF scintillators was carried out using the single photon method. It showed that the decay time constant of the light pulse varies between 2 ns and 3 ns, whilst the fwhm varied between 1.5 ns and 3.8 ns, which suggests that CsF scintillator production technology needs perfecting. Compared to our previous study a marked improvement in the quality of the CsF scintillators was noted, which was reflected in better energy resolution, below 20% for 662 keV γ-rays, and in a light yield at least 1.5 times higher, close, at present, to 10% of that obtained with NaI(Tl) crystals. The time resolution, measured with 2.3 cm diameter and 4 cm high CsF crystals coupled to the XP 2020 photomultipliers with a 60Co source, was equal to 300 ps for the energy thresholds set at 1 Mev and to 440 ps for the energy thresholds set at 100 keV.

Timing improved by the use of dynode signals studied with different scintillators and photomultipliers

The first part reports a study of the plastic scintillators NE 111 and Pilot U, and the liquid scintillators NE238A and BIBUQ dissolved in toluene. The scintillators were coupled to XP 2020 and RCA 8850 photomultipliers. The shape of the light pulse, the photoelectron yield, and the time resolution for γ-rays from 60Co were measured. The results show that BIBUQ in toluene is the fastest scintillator currently available. The second part deals with a comparison of time resolution obtained with the BIBUQ scintillator coupled to the photomultipliers XP 2020, RCA 8850, and C31024, respectively. Besides standard work with the anode pulse, the dynode signal was used for timing with XP 2020 and RCA 8850 to avoid influence of space-charge effects. Since the use of dynode signals greatly improved the time resolution, the study was extended to an analysis of the mechanism of the output-pulse generation process at both the anode and the dynode. It is concluded that the fastest scintillation counter at present is BIBUQ coupled to the XP 2020 photomultiplier working with the dynode pulse. A time resolution of 80 ps was measured for two counters with a 60Co source. It is shown that timing with the dynode signal is superior compared with the anode signal for typical fast photomultipliers, independent of the shape and intensity of the light pulse supplied by the scintillator. Improvement of the time resolutions by factors of 1.2–1.4 was observed. Time resolution based on the anode pulse is apparently distorted by the fact that a parasitic component is induced at the anode by the electrons travelling towards the last dynode from the preceding dynode. As a result, the optimum fraction of the pulse used for timing becomes different from that calculated from the statistical properties of the scintillation counter. An additional important limitation of the time resolution is observed in timing with the anode signal when more than 1 MeV energy is absorbed from a γ-ray in the organic scintillator. This limitation is ascribed to space-charge effects.

A fast neutron spectrometer for fusion experiments

A fast neutron spectrometer (2 × 10 5 events s −1) has been developed for the TFR Tokamak. It consists of a NE 213 scintillator coupled to a fast photomultiplier, a fast γ-neutron discriminator of a new type, and a fast data acquisition system. Preliminary experiments carried out either on an accelerator or on the TFR are satisfactory: a spectrum could be measured within 60 ms on the accelerator and within a single shot (500 ms) on the TFR.

Prebreakdown light emission in KCl (100) subjected to a nanosecond pulse in a divergent field

Prebreakdown phenomena of KCl (100) in a point-plane electrode system on the application of a high-voltage pulse of 1 ns rise time and 100 ns duration, were investigated by optical methods. Two spike light pulses were detected using a fast photomultiplier just at the instants of application and removal of high voltage for both point polarities. When successive positive pulses were applied to a point electrode the light output decreased monotonically. The light intensity obtained at the second pulse returned to the value of the first one when the time interval between pulses succeeded 3 h. These results can be explained by the effect of space charge associated with the holes, which is formed by positive streamer growth on the application of the preceding pulse.

Timing coincidence studies with fast photomultipliers

Abstract The time response of RCA C70045D photomultipliers was studied using a subnanosecond light flasher. The tubes, which have an output rise time of approximately 0.5 ns, were used in coincidence to study the variations in the fwhm of the time spectrum over a certain dynamic range of pulse amplitudes for both leading edge and constant fraction discrimination. A comparison has also been made for the measured time resolutions with some of the other fast photomultipliers.

Time resolution studies of fast photomultipliers

The timing performances of two high speed RCA-C70045D photomultipliers were studied and compared with that of an optimised RCA-8850 photomultiplier. The C70045D is a 14 stage, side-on type tube employing a semi-transparent bi-alkali photocathode (cesium-potassium-antimony) of spherical section with beryllium oxide as the secondary emitting surface. This tube is not in the current RCA photomultipliers catalogue PIT-700C. In the earlier catalogue PIT-700B, it was listed as the fastest conventional photomultiplier. The test was carried out using a measuring system incorporating a subnanosecond light flasher. This had less than 200 ps rise time, with full width at half maximum of less than 300 ps and a repitition frequency capability of up to 1 MHz. The minimum overall time resolution of the measuring system was 132 ps.

Hyperfine structure and lifetime measurements of the 8p 2P [formula omitted] levels of indium 115

The natural lifetime and hyperfine structure of the 8p 2P 1 2 and 8p 2P 3 2 levels of 115In have been determined. The levels have been populated in an atomic beam by pulsed stepwise laser excitation. The fluorescence from the excited states was observed by a fast photomultiplier. The lifetimes, deduced from the fluorescence decay are 316 ± 20 ns for the 2P 3 2 level and 312 ± 20 ns for the 2P 1 2 level. The hyperfine structure splittings were determined by means of quantum beats. The A-factor found for the 2P 1 2 level is 44.2 ± 0.4 Mhz. The A and B factor deduced from the hyperfine splittings of the 2P 3 2 level are 16.3 ± 0.1 MHz and 11.4 ± 0.3 MHz, respectively.

Computer simulations of Čerenkov light from large cosmic-ray showers

Detailed computer simulations have been made for the air Cerenkov radiation produced in large cosmic-ray showers. The hadron cascade employed in the air shower simulation incorporates Feynman scaling; primary particles ranging in mass from protons to iron nuclei are considered. The simulation results compare well with recent measurements of the average characteristics of Cerenkov light from large cosmicray showers made with a synchronized array of fast photomultipliers at the Haverah Park EAS array; satisfactory overall agreement is found between the observations and the predictions for showers of primary energy (≈2·1017÷≈2·1018) eV having their electron cascade maximum at depths in the atmosphere of (≈680÷≈770) g cm−2. For the cascade model employed here these depths of maximum arise for a heavy primary mass composition.

Status of timing with plastic scintillation detectors

Timing properties of scintillators and photomultipliers as well as theoretical and experimental studies of time resolution of scintillation counters are reviewed. Predictions of the theory of the scintillation pulse generation process are compared with the data on the light pulse shape from small samples, in which the light pulse shape depends only on the composition of the scintillator. For larger samples the influence of the light collection process and the self-absorption process on the light pulse shape are discussed. The data on rise times, fwhm's, decay times and light yield of several commercial scintillators used in timing are collected. The next part of the paper deals with the properties of photomultipliers. The sources of time uncertainties in photomultipliers as a spread of the initial velocity of photoelectrons, emission of photoelectrons under different angles and from different points at the photocathode, the time spread and the gain dispersion introduced by electron multiplier are reviewed. The experimental data on the time jitter, single electron response and photoelectron yield of some fast photomultipliers are collected. As the time resolution of the timing systems with scintillation counters depends also on time pick-off units, a short presentation of the timing methods is given. The discussion of timing theories is followed by a review of experimental studies of the time resolution of scintillation counters. The paper is ended by an analysis of prospects on further progress of the subnanosecond timing with scintillation counters.

Ĉerenkov radiation in large cosmic-ray air showers

A synchronized array of fast photomultipliers has been used to reconstruct the image in Ĉerenkov light of individual large cosmicray showers high in the atmosphere. The details of the images are in agreement with simulations based upon conservative models for cascade development. Observation of the fluctuations of the image between showers may enable the mass of the energetic cosmic rays to be determined.

Formative time and structure of martensite from amorphous FeNi films

Using a fast detector the applicability of CTEM was extended to follow phase transformations in the nanoseconds range. Iron-rich FeNi films vapour-quenched at 4 K grow in an amorphous state with peculiar crystallographic and magnetic properties (1). On exceeding a critical thickness or on heating amorphous films with inferior thickness up to a critical temperature, the films martensiticly crystallized. This diffusionless transformation occurred in such films where the amorphous phase was stabilized by stress and not by reacting gases.The present paper reports investigations on both the structure of martensite and on the propagation of martensitic crystallization of amorphous 70 at % FeNi films. They were grown at 4 K on carbon foils in the EM as previously described (1). The propagation of the phase transformation was investigated by following the change of the bright-field image intensity with fast scintillator, photomultiplier and oscilloscope. Crystallization processes occurring in areas of 1.5 μm ∅ and within 10 ns were resolved in first experiments (fig. 3).

Influence of incident light wavelength on time jitter of fast photomultipliers

The study of the single photoelectron time resolution as a function of the wavelength of the incident light was performed for a 56 CVP photomultiplier having an S-1 photocathode. The light flash from the XP22 light emitting diode generator was passed through passband filters and illuminated the 5 mm diameter central part of the photocathode. A significant increase of the time resolution above 30% was observed when the wavelength of the incident light was changed from 790 nm to 580 nm. This gives experimental evidence that the time jitter resulting from the spread of the initial velocity of photoelectrons is proportional to the square root of the maximal initial energy of photoelectrons. Based on this conclusion the measured time jitter of C31024, RCA8850 and XP2020 photomultipliers with the use of the XP22 light emitting diode at 560 nm light wavelength was recalculated to estimate the time jitter at 400 nm near the maximum of the photocathode sensitivity. It shows an almost twice larger time spread at 400 nm for the C31024 and RCA8850 with a high gain first dynode and an about 1.5 times larger time spread for the XP2020 photomultiplier, than those measured at 560 nm.

Ballistic range model detection system

A model detection system designed for ballistic range use is described in which a fast lens and photomultiplier detect the light reflected and/or refracted by the model as it passes through an incident light beam. The system is capable of detecting transparent as well as opaque models of various compositions with diameters as small as 1/32 in. (0.70 mm) moving with speeds up to 30,000 ft/sec (9.144 m/sec) under atmospheric or vacuum conditions.

Study of light collection process from cylindrical scintillators

The time spread and the efficiency of the light collected from cylindrical samples of a fixed 2.5 cm diameter and different heights made from perspex were experimentally studied placing the XP 22 light emitting diode in a small hole at the centre of the top of the samples. The samples, coated with NE 560 reflecting paint, aluminum foil and just polished ones were optically coupled to the photocathode of a C 31024 photomultiplier. The overall time resolution of the system, measured with single photoelectrons, was 300 ps for 2.5 cm diameter of the central part of the photocathode. The performed study showed that the light collection process introduces a large time spread which limits very seriously the time resolution obtainable with scintillation counters. The standard deviation of the photons transit time in the 2.5 × 2.5 cm 2 sample, coated with NE 560 paint, was measured to be equal to 0.5 ns, thus significantly larger than the time jitter of fast photomultipliers. The obtained data on the light collection process in the samples coated with NE 560 paint confirmed Nutt's predictions that the probability density function of the transit time of photons is represented well by a single exponential function. The standard deviation σ L is proportional to the height of the samples for a fixed diameter and depends on the quality of reflecting surfaces. To interpret larger values of σ L found in the experiment compared to calculated ones, the back reflections from the boundary between the glass window of the photomultiplier and the photocathode itself are postulated. It was shown that, in the samples coated with the reflecting paint, light is mainly collected due to randomly distributed reflections from the paint. Thus the measured characteristics for the point light source may be also representative for the light generated in the whole volume of the sample. The light collection process in the sample coated with aluminum foil is more complex and the presented data show mainly the transmission properties of these samples. But it was shown that for that purpose the polished samples introduce the lowest time jitter.

A blue light-emitting diode for use in calibrating and monitoring the gains of photomultipliers

Some experimental results on the usefulness of a new silicon carbide blue-emitting diode for calibrating and monitoring fast photomultipliers with S11 cathodes are presented. Comparisons are made with commercially available red and green gallium phosphide light-emitting diodes.

Measuring the length and the time structure of the electron bunch in electron storage rings by single photon timing

We try to show in this work that the single photon timing method, using a fast photomultiplier, is the best technique at present available to measure the time properties of electron circular accelerators and competing advantageously with the use of the fastest available photodiode. This method has been applied to the Orsay e +e − collision ring (ACO), where we have measured the shape and the length of the electron bunch as a function of the machine working parameters: electron energy, circulating current and voltage in the rf cavity.

Accuracy of single photoelectron time spread measurement of fast photomultipliers

The accuracy of time spread measurements of fast photomultipliers has been investigated, using single photoelectrons. The effect of the finite light pulse width on the measurement accuracy is determined and discussed. Experimental data have been obtained on a special measuring system for light pulse widths ranging from 200 ps to 10 ns, using fast photomultipliers 8850 and C31024 with optimized operating conditions for minimum transit time spread. A modified exponential function expression and curve-fitting parameters are given, which fit closely the experimentally obtained data over a wide dynamic range of light pulse widths.

Basic Limitation of Scintillation Counters in Time Measurements

The time resolution of a scintillation detector can be calculated from the scintillator illumination function, and the response of the photomultiplier to an instantaneous pulse of light. Such calculations using the parameters of present scintillators and photomultipliers agree with measured resolutions and predict improvements which can be achieved. Parameters of commercial and some promising experimental fast scintillators are given, as well as measurements on transit time spread of some fast photomultipliers.

Single photoelectron time spread measurement of fast photomultipliers

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.