Signal linearity, gain stability, and gating in photomultipliers: application to differential absorption lidars

We present the results of a study that identifies a photomultiplier tube (PMT), divider networks, and gating circuitry for use in the current detection mode, in which the specific objectives were to hold variations in both signal gain over a 25-μs gate period and signal linearity up to 20 mA to less than ±0.1%. The study, aimed at optimizing the performance in a nadir-looking airborne UV differential absorption lidar, is sufficiently general to apply to other critical gated or pulsed PMT applications in which performance at the 0.1% level is required. Signal-induced gain increases peculiar to pulsed or gated signals from PMT's with BeCu dynodes that can have values between 1 and 10% over 25 μs were reduced to less than 0.1% by the use of a 2-in. (5.08-cm)diameter PMT (EMI 9214) with CsSb dynodes. Compliance with the linearity requirement was achieved for gated signals up to 8 mA at a current gain of ~10(7) with the EMI 9214 PMT controlled by a resistive divider network with an inverted taper, in which the linearity data showed no tendency toward overlinearity caused by either space charge effects or induced divider-network voltage changes.     

Optical feedback in photomultipliers

Conclusions The measurements showed that, along with ordinary optical feedback, there occurs an optical feedback caused by ionic feedback; the contribution of the latter, however, to the overall background level is extremely small.As expected, the FÉU-42 was more suitable for operation in the coincidence detectors. It should also be noted that the ratio of the number of pulses due to the ordinary optical feedback to the number of pulses due to the optical feedback caused by ions was significantly greater for the FÉU-42.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 88–89, June, 1968.     

Investigation of prototype gas-filled photomultipliers

Parallel-plate gas gain of more than 7000 has been observed in a methane-filled low pressure hexode with a semitransparent photocathode. The photocathode lifetime has been studied in a similarly constructed phototetrode. Conclusions are drawn for the development of future higher gain devices for use in particle physics experiments in high magnetic fields.     

On the composition of a multialkali photocathode

During our technological experiments concerning the standard S-20 process, we observed appreciable deviations from the accepted Na₂KSb composition of the multialkali photocathode. First results which were achieved by the method of the double wedge developed earlier by the authors for multialkali composition, which are rather closer to NaK₂Sb, are presented in a short form.     

The performance of photomultipliers exposed to helium

We report results of a study to determine how the performance of photomultipliers is affected by exposure to He. In our tests we monitor two 5 in. diameter EMI hemispherical photomultipliers while they are operated in He environments. Initially we observed He⁺ afterpulses, at an approximately constant delay relative to the primary anode pulse. As the He gas pressure in the tubes increases however, strings of pulses typical of Townsend discharges occur. For the glass composition and geometry of the photomultipliers used in our tests, the internal gas concentration as a function of exposure to He is calculated using Fick's law for the permeation of solids by gases. The He permeation constant for the photomultiplier glass is obtained from a semiempirical formula developed by Altemose. We calculate the internal He concentration resulting from the He exposure which is observed to cause the regular occurrence of discharges and find that it is consistent with that required for production of discharges in the Townsend model. Guidelines are presented for using our results to estimate lifetimes of photomultiplier's of different geometries and glass types when operated in He environments.     

Signal-induced fluorescence in photomultipliers in differential absorption lidar systems

Signal-induced bias (SIB) superimposed on the tail of lidar signals detected by photomultiplier tubes (PMT's) has long been recognized as significant interference in lidar data retrieval. Analysis of the data from a lidar transmitting in the horizontal direction revealed that SIB is a well-shaped pulse and is thus called a signal-induced pulse (SIP). The SIP starts near the end of the signal and has an exponentially decaying tail. The decay time constant is negatively correlated with the pulse rise time. The origins of afterpulses discussed in the literature could not explain the cause of the SIP. This research leads to the hypothesis that the SIP is the fluorescence emitted from the PMT wall that is impinged by ions in the PMT. A model was developed, and the results show good agreement with the data. This model is applied to remove SIP's during processing of data from an ozone-profiling lidar.     

Silicon photomultipliers characterization for the EMR prototype of the MICE experiment

The SiPMs are excellent candidates for the replacement of PMTs in many experimental situations. In this article we describe the performances of different types of SiPMs from Hamamatsu and FBK-IRST before and after irradiation with photons and neutrons in terms of signal to noise ratio, time resolution and efficiency. The SiPMs are connected to a scintillation tracker/calorimeter, composed of eight layers (4x and 4y) of 10 scintillating bars each and have been tested at the CERN PS T9 beamline. The tracker/calorimeter is a small-size prototype of a bigger detector called EMR (Electron Muon Ranger), a particle identification system developed for the MICE experiment.     

Study of silicon photomultipliers fast amplifier and thermoregulation

The silicon photomultipliers (SiPM) are adopted in various physical applications, from medical physics to astrophysics, for their advantages in terms of cost and weight with respect to traditional photo detectors. Their low bias voltage supply (about 30 V), hardiness and resistance to magnetic field are ideal characteristics for space application. In the frame of INFN-Irst collaboration, some of them have been developed and produced at FBK (Trento-Italy), and have been characterized in the INFN laboratories of Bologna (DaSiPM2 collaboration).The SiPM can be used in conjunction with fibres and counters in high energy physics experiments. To exploit the SiPM time resolution, a fast amplifier has been studied. The SiPM gain depends critically on temperature and a thermal stabilization is also necessary. The use of a thermoelectric cooler module based on a Peltier cell has been investigated, and the results are shown.     

Extensive electrical model of large area silicon photomultipliers

In this paper the full electrical model of silicon photomultipliers fabricated at STMicroelectronics Catania R&D clean room facilities is presented. An accurate investigation on both SiPM single microcell and entire SiPM structure to extrapolate all the electrical elements has been executed by means of dedicated characterizations carried out on appropriate layout structure. The electrical simulations results are compared to the experimental data showing a good fit and therefore verifying the accuracy of the proposed model. This model can be used to describe all the SiPMs with different sizes manufactured using the same technology. Moreover, starting from this extensive electrical model, custom application software was developed in order to predict the possibility to profitably use the SiPM technology for different applications.     

Modulated laser source for monitoring the stability of detectors employing photomultipliers

Light from an argon high stability laser is used together with an acoustic-optic modulator to provide constant energy calibration pulses to the photomultiplier tubes in an experimental setup. This solution is proposed as the most appropriate when many (up to one/two hundred) of PMTs must be monitored and controlled for gain stability. In our case, gain changes should be limited to 1% which means a still better reproducibility for both the intensity and the width of the calibrator light pulses. In this paper we motivate our choice, present a simple crystal controlled pulse generator which assures the required long-term width stability and give the results obtained in several experimental runs.     

Development of femtosecond photocathode RF gun

Femtosecond electron bunches are essential for the observation of ultrafast reactions and phenomena in materials. To improve the time resolution of pulse radiolysis and ultrafast electron diffraction (UED), which involve the use of ultrashort electron bunches and ultrashort light, a femtosecond photocathode RF gun driven by a femtosecond laser was investigated experimentally. Bunch charge generated in the RF gun was studied based on the Schottky effect. Thermal emittance was estimated as 0.26 mm mrad at an rms laser spot size of 0.21 mm. Bunch length and longitudinal emittance were obtained as 180 fs and 0.87 deg keV, respectively, at a bunch charge of 3.5 pC and rms laser spot size of 0.38 mm.     

Spectral response variation of a negative-electron-affinity photocathode in the preparation process

In order to research the spectral response variation of a negative electron affinity (NEA) photocathode in the preparation process, we have done two experiments on a transmission-type GaAs photocathode. First, an automatic spectral response recording system is described, which is used to take spectral response curves during the activation procedure of the photocathode. By this system, the spectral response curves of a GaAs:Cs-O photocathode measured in situ are presented. Then, after the cathode is sealed with a microchannel plate and a fluorescence screen into the image tube, we measure the spectral response of the tube by another measurement instrument. By way of comparing and analyzing these curves, we can find the typical variation in spectral-responses. The reasons for the variation are discussed. Based on these curves, spectral matching factors of a GaAs cathode for green vegetation and rough concrete are calculated. The visual ranges of night-vision goggles under specific circumstances are estimated. The results show that the spectral response of the NEA photocathode degraded in the sealing process, especially at long wavelengths. The variation has also influenced the whole performance of the intensifier tube.     

NEA光电阴极的性能参数评估

利用自行研制的动态光谱响应测试系统直接实现NEA光电阴极的光谱响应在线测试,并对光谱响应曲线进行曲线拟合,间接实现了电子表面逸出几率、扩散长度、后界面复合速率及积分灵敏度的评估。对(Cs,O)激活的GaAs反射式光电阴极进行了性能参数评估,给出了测试结果。     

On the surface morphology of thin alkali halide photocathode films

Thin alkali halide films are currently used as transmissive UV-photocathodes and as protecting layers for visible photocathodes. The surface morphology of 20 and 75 nm thick evaporated CsI, NaI and CsBr films was investigated by means of a scanning electron microscope, to which the samples were transferred, under vacuum, with practically no contact with air. It is shown that the film continuity, in particular that of NaI, is strongly affected by short exposure to moisture. CsI, which is the less hygroscopic material among the three, exhibits the most continuous structure.     

Performance of transmission dynode photomultipliers in magnetic fields up to 1.5 T

The performance of commercially available photomultiplier tubes with 1, 2, 8 and 16 parallel transmission dynodes is examined in homogeneous magnetic fields B with strengths up to 1.5 T and axial as well as nonaxial orientations. The samples with 16 dynodes allow an operation with gains G ≥ 10³ and time resolutions σ ≤ 230 ps in such strong fields and for tilting angles θ ≤ 50°. The dependence of relative gain G/G(B = 0) and time resolution σ on B and θ is less expressed for the PMs with the smaller numbers of dynodes and a more compact design. The influence of B(θ) on the transit time is discussed qualitatively.