Photomultiplier Tube Gain Research Articles

Design and study of photomultiplier pulse-shaping amplifier powered by the current flowing through a voltage divider

A new version of Photomultiplier Tube (PMT) pulse amplifier, entirely powered by the current flowing through the base voltage divider, was designed and tested. This amplifier was designed for application in the JLAB G0 Experiment E00-006 as a part of high voltage base for XP2262 Photonis PMT. According to JLAB G0 experiment requirement, these PMT's operate with plastic scintillators at high counting rate (about MHz). Tests in JLAB experimental Hall C indicate that low energy gamma background cause up to 0.1mA of PMT average anode current (without amplifier). At this radiation condition, PMT gain decreases by 50% within about 1 month of operation. The amplifier needs to reduce PMT anode current and to shape PMT anode pulse prior to sending it through a long cable line (more then 400ft of RG-213 and RG-58 coax cables). Shaping of the PMT output pulse helps to reduce attenuation effect of the long cable line without significant reduction of timing accuracy. The results of this study of designed amplifier and PMT plus amplifier system are presented.

An iterative energy-centroid method for recalibration of PMT gain in PET or gamma camera

In gamma and positron emission tomography cameras, photomultiplier tubes (PMT) are still the most popular photosensors. Gain uniformity of the PMTs in these instruments is very important for preventing degradation in image resolution and distortion. When an Anger decoding detector head is exposed to a uniform flood source without lead collimation, the energy spectrum acquired by any single PMT has no photopeak for conducting recalibration. A method using the centroid of a wide energy spectrum even without photopeak as a gain-tuning index is presented in this paper. With a uniform flood illumination, without collimation, the gain equalization of PMTs within an array can be tuned to its initial level by an iterative centroid match. If an electrically adjustable gain control circuitry such as tunable PMT high-voltage or variable-gain amplifier is applied, the recalibration can be done automatically with this method. Our experiments demonstrated a 2.5%-3% gain accuracy that is enough to eliminate distortion in total energy and two-dimensional decoding. Counting efficiency increases significantly, and the calibration or system-tuning time will be shortened because of efficiency improvement.

NaI(Tl) electron energy resolution

NaI(Tl) electron energy resolution η e was measured using the Modified Compton Coincidence Technique (MCCT). The MCCT allowed detection of nearly monoenergetic internal electrons resulting from the scattering of incident 662 keV gamma rays within a primary NaI(Tl) detector. Scattered gamma rays were detected using a secondary HPGe detector in a coincidence mode. Measurements were carried out for electron energies ranging from 16 to 438 keV, by varying the scattering angle. Measured HPGe coincidence spectra were deconvolved to determine the scattered energy spectra from the NaI(Tl) detector. Subsequently, the NaI(Tl) electron energy spectra were determined by subtracting the energy of scattered spectra from the incident source energy (662 keV). Using chi-squared minimization, iterative deconvolution of the internal electron energy spectra from the measured NaI(Tl) spectra was then used to determine η e at the electron energy of interest. η e values determined using this technique represent variations in light production from monoenergetic electrons, light collection at the photomultiplier tube (PMT) photocathode, photoelectron production, photoelectron collection at the first dynode, and PMT gain, as well as noise from the accompanying electronics. It has been found that the electron energy resolution varies from 24.8% at 16 keV to 6.7% at 438 keV. Results from this study can be used to verify the significance of the different contributions to intrinsic energy resolution.

A LASER-BASED CALIBRATION SYSTEM, FOR THE CDF PLUG UPGRADE CALORIMETERS

A laser-based calibration and monitoring system for the plug upgrade calorimeter is described. A pulsed nitrogen ultraviolet laser excites all of the about 1000 photomultiplier tubes (PMTs) in each plug calorimeter. The light is focused into a quartz fiber and routed first to a primary distributor (PD) and from there to 12 secondary distributors (COWs), which feed the individual PMTs. The system will be used as a debugging tool during calorimeter rigging and will help the wire source calibration system in initially equalizing the PMT gains. In addition, an LED imbedded in each COW can set a continuous DC level on each PMT, allowing for PMT burn-in and gain conditioning.

Studies of the optical properties of new hemispherical photomultiplier tubes

The main optical and electrical properties of the new, fast, and low-gain six-stage hemispherical photomultiplier tubes (PMTs) 9116 A and 9117 A from Electron Tubes Inc. have been studied in detail for their use in the air Cherenkov telescope MAGIC, a 17-m O telescope to be installed in 2001 on the Canary Island of La Palma. Special emphasis is put on the discussion of the detection efficiency and the factors contributing to its absolute value and its uniformity over the photocathode. In particular, the photoelectron collection efficiency and its dependence on the voltage between the photocathode and first dynode have been measured. Also, the influence of the first dynode multiplication uniformity on the overall uniformity of the PMT response is discussed. The hemispherical cathode together with a specially shaped light concentrator allows increase of the effective quantum efficiency. This improvement has been measured as a function of wavelength. Aging and afterpulsing rate studies have been carried out at a PMT gain of 2/spl times/10/sup 4/.

Phase-Doppler interferometry with probe-to-droplet size ratios less than unity II Application of the technique

Practical limitations associated with the use of small probe volumes with respect to the droplet size that is being measured by the phase-Doppler interferometry technique are discussed. An intensity-validation scheme and corresponding probe volume correction factor have been developed that reject trajectory errors and account for the rejections in calculation of the probe cross-sectional area. The intensity-validation scheme also provides a tractable method of setting the photomultiplier tube gain and laser power. Volume flux measurements in dilute sprays have shown a significant improvement over those made by standard phase-Doppler interferometry techniques at small beam waist/droplet size ratios.

Fast photomultiplier tube gating system for photon counting applications

A normally “on” linear-focused 14-stage end-window photomultiplier tube (PMT) (Electron Tubes Limited 9893Q/100B), designed for fast photon counting, has been gated through control of the voltage applied to the first dynode. The gating circuit reduces the gain of the PMT during a laser pulse, in order to discriminate against the detection of scattered light, and then increases the gain promptly to observe extremely low levels of laser-induced fluorescence (LIF). An extinction factor for the laser scattered photons of >105 was observed, and has enabled count rates for photons due to LIF as low as 1 Hz to be measured for a laser pulse-repetition frequency of 7 kHz. The rise of the PMT gain is monitored directly by observation of the fluorescence using time-resolved photon counting, and the PMT turn-on time is 30 ns. No significant distortion of the temporal profile of the fluorescence was observed during PMT turn-on. The system, which can also be operated in ungated mode, is rugged and reliable, and has been employed outdoors during atmospheric field measurements of the hydroxyl radical using LIF. The gating circuit is suitable for the rejection of laser scattered light in applications with extremely low levels of fluorescence on the submicrosecond timescale.

A flow cytometer designed for fluorescence calibration.

In the development of suitable standards and calibration materials for fluorescence measurement, it becomes necessary to make accurate fluorescence measurements of these materials on flow cytometers. The results of such measurements may be affected by numerous sources of error, prominent among which are deviations of logarithmic amplifiers (log amps) from ideal response. To minimize the deleterious effects of log amps and multicolor fluorescence compensation circuitry on measurements, we built a flow cytometer with electronics incorporating high-precision peak detectors usable over a range from below 2 mV to 10 V, and we developed data acquisition software that transfers held peak values to a commercial 16-bit data acquisition system mounted in a personal computer running Windows 95. Fluorescence compensation is done in software, and transformation of the compensated data from a 16-bit linear to an 8-bit, 4-decade logarithmic scale is accomplished using a look-up table. Although dynamic range may be restricted by noise in the data acquisition system, high sensitivity can be achieved by photomultiplier tube gain adjustment, and it is likely that the use of a lower noise data acquisition system and/or digital processing of pulse information will enable operation over the full 4-decade dynamic range. Even at its current performance level, our instrument provides substantially better linearity over most of the scale than can be obtained using conventional electronics incorporating log amps; we believe this characteristic is critical for use in standards development.

The pulsed light calibration system of the ZEUS calorimeter

The ZEUS calorimeter is a compensating calorimeter consisting of uranium and scintillator plates. Light is transported via light guides to photomultiplier tubes (PMTs). The design goal is an energy calibration good to within 1%. Stability is measured and PMT gains are set using the signal from uranium radioactivity. Another important component of the calibration is the pulsed light system, which distributes light from a central laser or from distributed LEDs to the photomultiplier tubes via optical fibers. The light pulse gives a similar PMT response as a signal generated by a particle in the calorimeter. This has allowed the monitoring of the following properties of the readout chain: • • Number of photoelectrons/GeV/PMT. This enables us to differentiate between changes in the photomultiplier tubes and changes in the calorimeter (such as radiation damage). • • Linearity of the PMT readout chain between 0 and 400 GeV. • • Time delays from the PMTs and from the electronics are known to within 1 nsec. It is necessary to know these delays in order to precisely calculate the reconstructed charge and to eliminate background to e-p interactions from beam gas events. • • Short-term monitoring of PMT gain. This has been used to measure PMT gain changes under varying magnetic fields to better than 1% and will be used to measure the PMT gain under varying HERA background conditions.

Photomultiplier tube gain suppression: a new method for the sequential measurement of fluorescence and phosphorescence

A technique for the sequential measurement of fluorescence and phosphorescence depolarization is reported. The technique is based on a newly developed photomultiplier gain-suppression technique. The gain-suppression characteristics of the circuit are studied by applying a constant level of incident light. To demonstrate this new technique, a solid sample ofthetriplet probe eosin is excited by a pulsed, frequency-doubled Nd:YAG laser; the resulting orthogonally polarized emissions are collected by two balanced photomultipliers (PMT5). The gains of the PMTs are suppressed for the duration of the fluorescence emission, and subsequently returned to normal, to maintain the fluorescence and phosphorescence signals at the same level. The suppression of the gain is controlled by a specially designed dual-channel PMT gain-suppression circuit. The signals from the PMTs are recorded by two digital oscilloscopes, one set at a fast sampling rate to measure the static fluorescence anisotropy, the other set at a slow sampling rate to measure the slow phosphorescence depolarization. The anisotropy values can then be deduced from the two sets of recordings.

The tuning algorithms used by the Donner 600 crystal tomograph

The authors describe the computer algorithms used to adjust the energy thresholds and timing delays in the Donner 600 Crystal tomograph. The adjustments are made using 1890 computer controlled digital-to-analog converters (DACs) and an orbiting positron source. The energy threshold for each crystal is adjusted by measuring the counting rate of each crystal-crystal coincidence as a function of the DAC settings that control a pulse height window for the crystal in question. The DAC settings that correspond to the 511-keV photopeak are noted, thus determining the conversion from DAC setting to energy and allowing the DACs to be set to any desired energy window. The DAC settings controlling the timing delay for each channel are systematically adjusted to maximize the overall event rate. The authors discuss the convergence of the tuning algorithms and report on the photomultiplier tube gain and timing variations over a period of 18 months. >

The UCR gamma ray telescope data acquisition system

A description is given of an electronics system based on the DEC Falcon SBC-11/23+, which has been designed and built to support a balloon-borne double Compton gamma-ray telescope. The system provides support for commands, data acquisition, data routing and compression, and photomultiplier tube gain control. The software consists of a number of interrupt-driven routines of differing priorities to handle each system task. This includes two circular buffers for onboard processing and bit encoding before transmission of the information to the ground computer. Acquisition of gamma-ray events at rates above the 200-Hz telemetry constraint is easily achieved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Safeguards Instrument to Monitur Spent Reactor Fuel

A hand-held instrument for monitoring irradiated nuclear fuel inventories located in water-filled storage ponds has been developed. This instrument provides sufficient precise qualitative and quantitative information to be useful as a confirmatory technique for International Atomic Energy Agency inspectors, and is believed to be of potential use to nuclear fuel managers and to operators of spent-fuel storage facilities, both at reactor and away-from-reactor, and to operators of nuclear fuel reprocessing plants. Because the Cerenkov radiation glow can barely be seen by the unaided eye under darkened conditions, a night vision device is incorporated to aid the operator in locating the fuel assembly to be measured. Beam splitting optics placed in front of the image intensifier and a preset aperture select a predetermined portion of the observed scene for measurement of the light intensity using a photomultiplier (PM) tube and digital readout. The PM tube gain is adjusted by use of an internal optical reference source, providing long terrm repeatability and instrumerit-to-instrument consistency. Interchangeable lenses accommodate various viewing and measuring conditions.

A Microprocessor Controlled, Personnel Thermoluminescence Dosimetry Reader

Using a DEC LSI-11 microprocessor, we have developed a thermoluminescence dosimetry reader for personnel dosimetry. This microprocessor provides system control, accumulates data for temporary storage on a cassette, and later outputs the data to a large computer. The microprocessor also provides a human/system interface through a computer terminal. A programmable high-voltage supply and a light-emitting diode are used in the reader svstem to automatically control photomultiplier tube gain.

Lidar for multiple backscattering and depolarization observations

A lidar system that can measure multiple scattering and depolarization in the atmosphere is being used to study the climatic effects of cirrus clouds and to perform other investigations. The lidar system and its novel aspects are described in this paper. The influence of multiple scattering on noise, signal, and SNR is considered. Special receiver field stops incorporated for multiple scattering measurements, use of low voltage to control the photomultiplier tube gain, and a precision power/energy monitor are described. A technique for aligning transmitter and receiver axes and measuring transmitter beamwidth is presented. The multiple-scattered components of backscattered light are determined by inserting a center-blocked field stop to restrict the receiver field of view to the region outside of the diverging transmitted beam. Typical returns with and without the opaque field stop indicate the amplitude of multiple scattering from cirrus clouds and prove the feasibility of this technique. The depolarization ratio delta and backscatter coefficients from an altostratus cloud illustrate the potential of these quantities for the study of cloud structure and phase.

Stabilization of photomultipliers in liquid scintillation counters

A special stabilization system has been constructed to control the overall gain of photomultiplier tubes in a liquid scintillation counter. The system uses GaAsP light emitting diodes in the measurement chamber to produce separate reference light pulses for each photomultiplier tube. The system has provided a significant improvement in both short and long term stability and in operational reliability.

Gain stability in high-current photomultipliers at high variable counting rates

Variations in the gain of 56AVP photomultiplier tubes is studied by changing the counting rates. It is observed that at high counting rates the gain of a photomultiplier tube appreciably increases with increasing counting rate. The energy resolution of the photomultiplier tube is found to deteriorate at high counting rates. It is found that this gain shift is nearly eliminated by keeping the voltages stabilized for the last two stages of the photomultiplier chain. This also helps in maintaining the energy resolution.