Dynode Materials Research Articles

Secondary electron emission of Al<sub>2</sub>O<sub>3</sub> and MgO nanofilms fabricated by atomic layer deposition

<p indent="0mm">Electron multiplier devices are widely applied in many electronic instruments like mass spectrometers and atomic clocks. It is considerably crucial for a multiplier to possess a high electron gain, and this index can be directly determined by secondary electron yield (SEY) of the dynodes. Al₂O₃ and MgO possess a relatively high SEY level among majority of dynode materials, and their film products are excellent dynode candidates. Whereas, for some multipliers like microchannel plate (MCP), only an ultrathin film of several nanometers is allowed to be coated onto the inner wall of the micro channels to avoid the variation of the channel diameter. Therefore, SEY characteristics of the ultrathin films are necessary to be figured out. Here, by using the technology of atomic layer deposition, 7 groups of ultrathin Al₂O₃ and MgO nanofilms with increase thickness (1, 3, 5, 7, 10, 30, and <sc>50 nm)</sc> are fabricated on silicon (Si) substrates. As well as, 5 groups of Al₂O₃ nanofilms (1, 2, 3, 4, and <sc>20 nm)</sc> are deposited on MgO film <sc>(20 nm)</sc> substrate. Surface composition, morphology, film thickness, and SEY have been characterized in detail. Via the experiments, it is found that SEY of the Al₂O₃/Si and MgO/Si samples largely depends on the film thickness, namely, SEY increases obviously as the film thickness rises, meanwhile, the increment of SEY decreases gradually. The SEY tendency indicates that the effect of top film on SEY becomes enhanced, and the influence of bottom substrate on SEY becomes weakened. When the film thickness increases beyond <sc>30 nm,</sc> SEY increment approaches to 0, and SEY tends to be saturated. This phenomenon demonstrates that the penetration depth of incident electrons is less than the film thickness under the circumstances. To interpret the experimental results, the SEE semi-physical theory developed for double layer structures is utilized. The calculation results indicate that the film thickness has a remarkable impact on SEY, especially when the incident energy becomes lower and the film becomes thicker, the results also reveal that the dielectric surface film possesses a great ability to modulate the surface SEY. However, SEY becomes less dependent on film thickness as the incident energy increases, and it results from the increase of penetration depth for the incident electrons. This work reveals the mechanism of the SEE characteristics for ultrathin Al₂O₃ and MgO nanofilms, which is of great significance for the subsequent research on the use of nanoscale high SEY dielectric films as the SEE functional layer in electron multipliers.

A new electron-multiplier-tube-based beam monitor for muon monitoring at the T2K experiment

Abstract Muon beam monitoring is indispensable for indirectly monitoring accelerator-produced neutrino beams in real time. Though Si photodiodes and ionization chambers have been successfully used as muon monitors at the T2K experiment, sensors that are more radiation tolerant are desired for future operation. We have investigated the electron-multiplier tube (EMT) as a new sensor for muon monitoring. Secondary electrons produced by the passage of muons at dynodes are multiplied in the tube and produce signal. Two prototype detectors were installed at the T2K muon monitor location, and various EMT properties were studied based on in situ data taken with the T2K muon beam. The signal size is as expected based on calculation, and the EMTs show a sufficiently fast time response for bunch-by-bunch beam monitoring. The spill-by-spill intensity resolution is 0.4%, better than the required value (1%). Signal linearity within $\pm$1% is achieved at proton beam powers up to 460 kW (with +250 kA focusing horn operation). A gradual signal decrease was observed during the initial exposure, due to the stabilization of dynode materials, before the response became stable within $\pm$1%. This work demonstrates that EMTs are a good candidate for future muon monitoring at T2K, and may also have other more general applications.

Surface Characterization and Secondary Electron Emission Properties of Alumina Containing MgO Film on Ag-Mg-Al Alloy

Ag-Mg alloy is used as a dynode material in electron multiplier tubes due to the high secondary electron yields (δ) of the surface of MgO film. However, MgO film is readily degraded under strong electron or ion bombardment, which results in a decrease in the lifetime of devices. In this study, alumina-containing MgO films of ~50–150 nm were developed on a Ag-2Mg-2Al alloy (silver alloy containing 2 wt % Mg and 2 wt % Al) after a thermal activation process performed at 500–600 °C under low oxygen pressures of 5.0–20.0 Pa. Auger electron spectroscopy and X-ray photoelectron spectroscopy analyses reveal that the film consists of a thin layer of pure MgO and a relatively thicker layer of alumina-containing MgO located beneath the top MgO layer. The alumina-containing MgO film exhibits high δ value of 7.7 at a primary electron energy of 580 eV and a much better stability under energetic electron bombardment than pure MgO film on Ag-Mg alloy. Alumina has higher bond dissociation energy than MgO, and the presence of alumina in the film contributes to mitigating the dissociation of the MgO film under electron bombardment. The Ag-2Mg-2Al alloy with alumina-containing MgO film is a promising candidate as a dynode material for electron multiplier tubes.

Investigation of the secondary electron emission characteristics of alternative dynode materials for imaging photomultipliers

The requirement to accurately detect and image very high speed photon events is a necessity for many applications across a range of disciplines. Photon imaging detectors often make use of the planar geometry of MCPs for high resolution imaging. However, the inherent rate limitations of MCPs provide opportunities for discrete dynode devices where rapid recharge times and high throughput are required.Novel dynode materials and designs offer the possibility to further improve event time resolution and pulse height distribution. Significant performance advantages can be achieved by increasing the gain of each dynode stage by utilizing materials such as CVD diamond. Secondary electron emission (SEE) coefficients for polycrystalline CVD diamond of 84 have been reported for Hydrogen terminated surfaces and similar values for Cs terminated surfaces [1, 2]. These values are far in excess of currently used materials such as AgMgO(Cs) and CuBeO(Cs) which have SEE coefficients < 15 [3].Polycrystalline diamond is a widely available, relatively inexpensive material that can be deposited over large areas, making the manufacture of large format imaging detectors a practical possibility. The application of new dynode designs for imaging photomultipliers requires knowledge of the secondary electron emission characteristics of the material. We describe a facility to measure the relevant characteristics of novel dynode materials and designs with 2D imaging capability, and discuss the challenges and complexities involved with SEE coefficient measurements. We present secondary electron emission measurements for a variety of dynode materials and designs and discuss their application in imaging photon-counting devices.

Photoelectron backscattering in vacuum phototubes

In this article we describe results of studies of a photoelectron backscattering effect in vacuum phototubes: classical photomultipliers (PMT) and hybrid phototubes (PH). Late pulses occurring in PMTs are attributed to the photoelectron backscattering and distinguished from pulses due to an anode glow effect. The late pulses are measured in a number of PMTs and HPs with various photocathode sizes covering 1–50 cm range and different types of the first dynode materials and construction designs. It is shown that the late pulses are a generic feature of all vacuum photodetectors—PMTs and PHs—and they do not deteriorate dramatically amplitude and timing responses of vacuum phototubes.

1/f noise study in vacuum photodiodes

We report here measurements of 1/f noise in vacuum photodiodes. The theoretical expression for the Hooge parameter αH is formally equivalent to the one for secondary emission 1/f noise. There is also 1/fγ noise due to fluctuations in the cathode work function; here γ∼3/2 is expected. The spectrum SIa( f ) for the latter varies as I2aV0a and for the former it varies as IaV3/2a, where Va is the anode voltage, and Ia the anode current. Enhanced shot noise was observed at higher frequencies; at lower frequencies the noise spectrum was of the 1/f type. The latter could be separated into a V0a component and a V3/2a component by measuring SIa( f ) vs Va. The V3/2a component of the 1/f spectrum could also be deduced from the current dependence of SIa( f ); both measurements mutually agreed well. The measured V3/2a component also agreed well with Handel’s predictions; by proper choice of the anode-cathode distance dca (dca=0.6 cm) almost exact agreement could be obtained. In view of the fact that the overall accuracy of the data is not better than 25%–30%, any value of dca between 0.4 cm&amp;lt;dca&amp;lt;0.8 cm should be considered as giving good agreement; this is a very plausible range of values. We observed some poisoning effects in older devices. They are attributed to the decomposition of the dynode material under electron bombardment and give rise to long-term instability of the device noise.

Potential and limitations of caesium iodide as a dynode material for use in electron multipliers

High and reproducible gains were obtained from thin films of CsI deposited on tantalum substrates. The most probable energy of emission of the secondary electrons emitted from CsI dynodes was determined to be <or approximately=1 eV. The dynodes were found to be stable under electron bombardment up to output current densities <or=10-8 A cm-2. Owing to the high vapour pressure of CsI at elevated temperatures, the dynodes cannot withstand vacuum-bake at temperatures >or=300 degrees C. Exposure to dry air did not harm the dynodes: however, the gain of the dynodes decreased when they were exposed to humid air. A demountable electron multiplier incorporating CsI dynodes was assembled under normal laboratory ambient. The stage-gain was found to be higher than that of conventional alloy dynodes. A great advantage of this multiplier lies in the fact that it can be easily rejuvenated to give reproducible overall gain.

A model of secondary electron yields from atomic and polyatomic ion impacts on copper and tungsten surfaces based upon stopping-power calculations

The calculation of the velocity dependence and magnitude of the kinetic secondary electron yield for atomic and polyatomic ions, denoted by γ̄a and γ̄m, respectively, is modeled on stopping-power equations and compared to experimental results from the dynode materials copper and tungsten. From the experimental data, the relationship γ̄m=Σγ̄a is verified, which is necessary to the model calculation of γ̄m. The fundamental assumption of this model is that γ̄a is proportional (even at low ion velocities) to the electronic stopping power (dE/dx)e of that projectile in the target. This fundamental assumption of γ̄a∝ (dE/dx)e has been previously used by Sternglass for calculation of γ̄a’s at ion velocities above 4×108 cm/sec. Example calculations are performed for the velocity dependence of γ̄a for hydrogen, carbon, and fluorine striking copper, and from these results the predicted γ̄m curves for polyatomic ions are in close agreement with experiment. Analogous calculations for a tungsten surface are outlined and the final results are compared with experiment. The model’s utility is shown by illustrating that the measurement of γ̄ can provide not only a relative measurement of (dE/dx)e for different ion atomic numbers on the same target, but can also provide a means of determining the nuclear stopping power (dE/dx)n at low ion velocities where range measurements are difficult to perform. Values of (dE/dx)n derived from measurements for fluorine and carbon striking copper are the same as values predicted by Lindhard and Scharff; for a tungsten target the experimental (dE/dx)n values are a factor of 2 larger than theoretical. The relationship of the stopping power values derived from γ̄ measurements with this model are compared to the values derived from ion range measurements. In addition, the model’s relationship to the secondary electron emission model of Parilis and Kishinevskii is discussed.

Variance of ion-electron coefficients with atomic number of impacting ions

The relation between ion-electron coefficients γ and atomic number Z 1 of impacting ions on the electron multiplier of a mass spectrometer was investigated. Ions of 22 elements ranging from Li to Pb were measured on three dynode materials (CuBe, Al and Ni) for an ion energy of 8.0 keV. These results were extrapolated to ion-electron coefficients for constant ion velocity. The ion-electron coefficients obtained for constant ion velocity were combined with results given by Lao et al. [2] to form a set of 44 coefficients for the CuBe dynode. A periodic increase of γ( Z 1) at constant velocity was found for the dynode materials used. The trend of this increase agrees well with theory. Maxima in γ( Z 1) appeared for the noble gases and for group IVb, minima for groups IIa and IIb. The dynode material influenced the amplitude of the oscillations in γ( Z 1), but did not affect the location of maxima and minima in γ( Z 1). The comparison of the results obtained for γ( Z 1) showed good agreement with the periodicity found for electronic stopping.

A high gain aluminium electron multiplier

A twenty stage electron multiplier using aluminium as dynode material is described. When operated in DC mode, very stable gains approaching 106 were obtained with input currents of the order of 10-12 A, even after repeated exposures to the atmospheres.

Isotope effect of ion—electron emission

Mass discrimination of isotopes was investigated with the electron multiplier of an AEI MS 5 mass spectrometer. Ions of 15 elements ranging from Mg to Pb were measured on three dynode materials (CuBe, Al and Ni) in the energy range 7.0–8.6 KeV. A nonlinear correlation between the ion—electron coefficient γ and the ion velocity ν – γ ∼ ν b — was found. The exponents b showed a decrease with increasing kinetic energy of the ions, a decrease with growing atomic number of the target material and a nonmonotonous increase with increasing atomic number of the ions.

Bialkali (K2 CsSb) Photocathode as a High-Gain Secondary Electron Emitter

It has been found that the K2CsSb photocathode material has much higher secondary emission gain than the conventional Cs3Sb dynode material. Gain factors as high as 30 have been measured.

Recent Work on Fast Photomultipliers Utilizing GaP(Cs) Dynodes

There exists an increasing demand for photomultipliers with better time resolution, higher count rate capabilities and generally faster time response. This paper discusses fundamental electron-optical considerations which determine the speed of response of electrostatically-focussed photomultipliers. The nature of time broadening is discussed with regard to geometrical and initial velocity effects at dynode surfaces. Time limitations inherent in the dynode material itself are also treated. Examples of different fast photomultiplier structures are given. A discussion of various delta-function light sources is included and the relative merits of these sources with respect to time performance, ease of use and cost is reviewed.

A Parallel Plate Continuous Dynode Electron Multiplier with Carbon as Dynode Material

A Parallel Plate Continuous Dynode Electron Multiplier with Carbon as Dynode Material

Recent Developments in GaP(Cs)-Dynode Photomultipliers

Recent developments in GaP(Cs)-dynode photomultipliers are reviewed. The physics of secondary emission is discussed and models are described which explain secondary emission from conventional dynode materials and from GaP(Cs) dynodes. The concept of negative electron affinity is explained in terms of an energy diagram. Device performance is reported with regard to both electron and time resolution. A general discussion of the characteristics of GaP(Cs)-dynode photomultipliers is included.

The use of electron multipliers in mass spectrometry

A study was made of the practical aspects of using an electron multiplier with a fast scanning 60° mass spectrometer. Two types of multiplier construction—the “Venetian Blind” and the “Box-and-grid” were used, and two types of dynode material—beryllium copper and silver magnesium. The performance of the multipliers was observed under various conditions, including baking and hydrocarbon contamination, and their subsequent effects on multiplier gain and noise. Methods of dynode reactivation were also investigated. It was found that the silver magnesium dynode, “Box-and-grid” constructed electron multiplier gave the best overall performance, and a multiplier was developed which had a stable gain with time, low background noise level, was unaffected by baking or hydrocarbon contamination under normal operating conditions and yet could be readily reactivated in situ if necessary.

Reversible Verstärkungsänderungen von photomultipliern bei unterschiedlichen zählraten

The time dependence of the gain of several RCA, DuMont and EMI photomultiplier tubes has been measured prior to, during, and after a constant illumination of the photocathode. The light source was a NaI(T1) crystal irradiated with γ-rays of Cs 137 or Co 60. A change in the counting rate results in rapid and slow variations in the gain. The rapid gain variations take place within an interval of less than 10 seconds. The amplitude is dependent upon the change in counting rate and the tube chosen. Moreover the sign may be positive or negative for individual tubes of the same type. It may be assumed that a sudden reversible sensitization or desensitization of the photocathode explains the effect. The slow gain variations after a change in counting rate are made up of two exponential components. The respective saturation value after an increase in counting rate is approached faster than after a decrease in counting rate. The amplitude is dependent upon the average anode current and the high voltage. The slow gain variations of all examined RCA tubes (6342, 6342A, 5819) were found to be negative, whereas the changes of the DuMont 6292 and EMI 6255 tubes were found to be positive. It may be assumed that reversible dynode effects explain the changes. The different behaviour of the various types of photomultiplier tubes is not correlated to the dynode material. The absolute value of the gain variations of all tubes examined were approximately of the same magnitude under comparable conditions.