Secondary Emission Ratio Research Articles

A high-efficiency collector for a high-voltage electron cooler

A high-perveance collector with a small secondary emission ratio has been designed for a high-voltage electron cooler. The distinctive feature of this collector is the fact that it consists of the cooled collector itself, which receives the electron flow, and a Wien filter installed ahead of it, which substantially reduces the secondary electron flux emerging from the collector. The main advantage of this design is the combination of the small secondary emission ratio and the high perveance with the small size and ease in manufacturing and adjustment. The results of calculations and experimental investigations of the combined collector are presented, and its different operating modes are investigated. The minimum attained secondary emission ratio is 5 × 10−6. The methods for further reducing this ratio are proposed.

Improvement of Image Quality in Multizone Scintillation Cameras Using Fiber-Optic Coupling to Reduce Effective PMT Size

Earlier papers by the author showed that the resolution and sensitivity of a multizone scintillation detector depend not only on the thickness of the scintillator but also on the number of detector zones, and hence the photomultiplier tube (PMT) array size. Standard design practice is to couple the PMTs directly to the scintillator surface, which means that the maximum array size is determined by the smallest PMT size available. An increase in array size, however, leads to reduction in the PMT packing fraction due to the glass walls. This paper shows that effective PMT size can be reduced as much as desired by coupling a given scintillator area to a relatively larger PMT surface area via optical fibers of suitable bandwidth for the scintillation light. Fiber-optic coupling not only increases the packing fraction to 100% but also enables the PMTs to be chosen based entirely on performance and/or cost without regard to shape and size. While low spatial resolution and image contrast is characteristic of nuclear imaging systems in general, rapid high-count-rate quantitative studies are particularly subject to severe distortion due to dead-time losses despite efforts to minimize and/or correct for these losses. An increase in the number of detector zones via fiber-optic coupling enables significantly higher resolution and sensitivity to be achieved, more accurate sampling of the light distribution to be obtained, and detector operation to be virtually linear so that dead-time losses are practically insignificant. Statistical noise in nuclear images arises from variability in the number of light photons emitted per scintillation event, differences in the quantum efficiencies of the PMTs, and PMT gain fluctuations due to variations in the secondary emission ratios of dynodes. This paper also shows that an increase in array size using fiber-optic coupling reduces the effect of PMT and preamplifier gain nonlinearity, thereby leading to substantial improvement in image quality.

Ion surface modification for space TWT multistage depressed collectors

An ion treatment system for space traveling wave tube (TWT) multistage depressed collectors (MDCs) has been built. Processes were investigated using oxygen-free, high-conductivity (OFHC) copper plates and MDCs in terms of the measurement of secondary electron emission ratio ( δ) and scanning electron microscope (SEM) analysis of surface texture. Measured δ for ion treated OFHC Cu plates, was found to be closely related to Mo deposition rate and spike texture observed on the SEM micrographs. For the optimized process, maximal δ from OFHC Cu plates was reduced to 0.6, less than a half of that of the untreated, while measured maximal δ from inner wall of MDCs varies from 0.64 to ∼1.0 depending on the geometry of the collector. With the treated four staged-MDCs, Ka-band TWTs have demonstrated a few percent of improvement in total efficiency.

Influence of the angular anisotropy of secondary emission on the characteristics of a two-sided multipactor

We study the influence of the thermal spread in ejection velocities of secondary electrons on the initial stage of development of a two-sided multipactor. The analysis employs a statistical method based on an exact analytical solution for the electron distribution function over times of transit through the vacuum gap. The obtained theoretical relations are used as the basis for a numerical algorithm which takes into account both the angular and energy distributions of secondary electrons and makes it possible to calculate quantitatively the main multipactor characteristics for the actual dependence of the secondary-emission ratio of the walls on the energy of bombarding electrons. Detailed numerical calculations of the multipactor zones are presented. It is shown that the presence of angular anisotropy can change significantly the conditions of production of the secondary-emission discharge as compared with the case of isotropic emission.

Design and Development of a Demountable Electrostatic Module for Measuring Secondary Electron Emission Ratio

In this paper, the design and development of a simple demountable electrostatic module for measuring Secondary Electron Emission (SEE) ratio have been given. SEE ratios of metals and thin film metal oxides have been successfully measured. The module consists of a narrow angle electron gun, which is the source of primary electron beam of wide energy range, and a cylindrical collector to collect emitted secondaries. Design procedure for the electron gun in particular and module in general have been described. Computer plots of equipotentials and electron trajectories are shown. The validity of SEE measurements using the module has been established using platinum as standard. It has been observed that the module reliably measures SEE ratios for beam voltages ranging between 100 V and 3.6 kV and with little modification its range can be extended down to voltages less than 100 V.

An approximation formula for the secondary emission yield

An algorithm for obtaining analytical expressions for the normalized secondary emission ratio delta / delta /sub max/ as a function of the normalized impact voltage is presented. These expressions are well suited for implementation on personal or mainframe computers with a very high accuracy over the whole range of the normalized impact voltage. >

A new formula for secondary emission yield

Mathematical expressions for the secondary emission yield as a function of impact voltage and direction are required by computer programs for ray tracing, which are also intended to follow secondary trajectories. It is shown that the accuracy requirements are quite severe for lower voltage impacts, up to about three times V/sub max/ (where V/sub max/ is the voltage at which the normal secondary emission ratio reaches its maximum value, delta /sub max/), but are less stringent above this range. A new formula for the low-voltage region, which agrees with experimental data better than the formula of R.G. Lye and A.J. Dekker (1957), is proposed. For the voltage region above 4 V/sub max/, the Lye and Dekker formula is retained, with a smooth transition centered between 3 and 4 V/sub max/. The directional effect is modeled by adjusting the values of delta /sub max/ and V/sub max/ before applying either formula, rather than applying either Bruining's or Muller's correction factor afterward. This results in an automatic modeling of the known absence of any appreciable directional effect below about half V/sub max/. Some experimental verification of the formula used to modify delta /sub max/ is presented. >

Discharge at the pillbox window for an LHRF launcher

The discharge at a pillowbox-type waveguide vacuum window for a lower hybrid resonant frequency (LHRF) launcher at 2 GHz is studied experimentally. A discharge is observed near the window at a pressure below 10/sup -2/ Pa. The discharge has little or no effect on the RF-transmission waveform; however, the temperature rise of the ceramic due to the discharge reaches about 100 times higher than that in the no-discharge case. Moreover, the breakdown of gas in the waveguide is enhanced by the discharge. The discharge can be suppressed by sufficient RF conditioning and/or by coating the window with materials having a low secondary electron emission ratio such as carbon or TiN. These results are accurately explained by the theory of single-surface multipactoring discharge induced by an electric field perpendicular to the ceramic and the space charge.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An Operational Model for Microchannel Plate Devices

The behavior of optical detectors using microchannel plates (MCPs) as electron multipliers is described in terms of the physical parameters of the MCP (channel resistance, channel capacitance, channel wall secondary emission ratio, etc.). Three operating modes are included: (1) DC operation at low light levels, (2) pulse detection operation at high peak currents, and (3) single electron counting with cascaded V-plate and Z-plate MCPs.

Measurement of the statistics of transmission secondary emission from negative-electron-affinity silicon

The statistical variation in the transmission secondary emission from negative-electron-affinity (NEA) silicon has been studied using a scintillation counting technique. The results of measurements on boron-doped p-type silicon of between 5 and 10 mu m thickness show that distribution is Gaussian and characterized by a Fano factor of about 11 for a primary electron energy of 20 keV and a secondary emission ratio of 143.

Secondary electron emission from 0.5–2.5-MeV protons and deuterons

Measurement of the secondary electron currents leaving Al, V, Fe, 316 stainless steel, Nb, and Mo foils undergoing 0.5–2.5-MeV proton and deuteron bombardment were made to determine the secondary electron emission ratios for these ions. The measured secondary electron yields were of the order of 1.0, with the deuterons producing generally higher yields than the protons.

Secondary electron emission from the GaN:Cs–O surface

High secondary emission ratios have been obtained for the GaN:CsO surface. A peak ratio of 51 at 3 keV primary electron energy has been observed. At 20 keV the ratio is 24, compared with 2 when the GaN is uncesiated. Analysis of the data indicates that the diffusion length in the material is between 300 and 800 Å, and the surface escape probability for secondary electrons is 0.36. These results indicate that the GaN:CsO surface has been activated to negative electron affinity (NEA).

Secondary emission and photoemission from negative electron affinity GaP : Cs

Secondary emission and photoemission measurements have been made on a negative electron affinity (NEA) GaP : Cs sample. An analysis of the photoemission data gives a surface escape factor B =0.33 and an electron diffusion length L =0.2 μm. The secondary emission ratio data extended over the primary energies Ep of 0–14 keV. If the range of the primary electrons R is assumed to be of the form R=a Epb μm (Ep in keV), then analysis of the secondary emission data gives a =0.0095 μm and b =2.0.

Characteristics of the Deposited Oxide Films for the Continuous Dynode Electron Multiplier

The mixture of TiO2with Al2O3or MgO can readily be deposited. And the values of the deposited films'electric resistances can readily be set to the necessary resistance value for the dynode of the continuous type electron multiplier with heat treatment in atmosphere. The films are stable, especially the deposited one of the mixture with Al2O3, and the change of the resistance with time in atmosphere is negligible.The muximum secondary electron emission ratio is from 2.7 to 2.8 and the corresponding accelerating voltage of the primary electrons is from 300to 330V. The films can be used as the dynode of the continuous type electron multiplier.

Electron-beam induced reduction of carbon concentration on BeO and its effects on secondary electron emission

Electron-beam stimulated reduction of C concentration on BeO, which requires an oxygen ambient ∼ 10 −7 torr, was studied using Auger spectroscopy. As the C Auger line decreases, those of Be and O increase, the secondary electron emission ratio δ increases, and the work function decreases. We have concluded that these effects stem from electron stimulated desorption of C. The desorption process is monomolecular, at least initially, with a cross-section of about 2.1 × 10 −20cm 2, from which we infer that the desorbing species is CO +. The adsorbed C, initially in the graphite form, can be converted to the carbide form by heat treatment as low as 450°C. This conversion is accompanied by a decrease in work function of 2.50 V from the value associated with the graphite C. The magnitude of the Auger electron signal from carbide C decreases under electron beam irradiation; however, in this case, the reduction in the magnitude of the C Auger line is not accompanied by any change in δ or yield curve. It is suggested that this decrease is not due to desorption of the carbide C, but rather to an electron-induced change in the C bonding state.

Reflection and transmission secondary emission from GaAs

Reflection and transmission secondary-emission ratios have been measured for a 3.5-μm-thick self-supporting GaAs film activated to negative electron affinity. A reflection-mode gain of 540 at 20 keV primary energy was observed. In transmission, the gain at 20 keV primary energy was 112. Analysis of the data gives the surface-escape probability as 0.18 and the electron-diffusion length as 3.0 μm. The data are well described by a simple diffusion-transport model.

REFLECTION AND TRANSMISSION SECONDARY EMISSION FROM SILICON

Reflection and transmission secondary emission ratios have been measured for a thin Si sample with one surface activated to a state of negative electron affinity. Reflection-mode gains of 950 were observed at 20-keV primary energy. The highest transmission secondary emission ratio was 725 at 20-keV primary energy. The high secondary emission gains result from the effective negative electron affinity at the surface of the sample together with a long escape depth for internal secondaries.

Variations in Single Photon Pulse Height Distribution with Focusing Voltage in Photomultipliers

In photomultipliers the focusing voltage which gives the most peaked distribution is also the one which results in the largest count rate. For low focusing voltage there is a significant loss in single-photon events. The results of this experiment are consistent with the idea that the pulse distribution depends upon variations in the effective secondary emission ratio.

Afterpulses in Photomultipliers

Present fabrication and activation techniques have reduced afterpulses from most photomultipliers to a point where they are rarely important. Nevertheless, with some tubes in certain applications they may present a serious problem. More important, however, is the fact that the study of afterpulsing gives valuable clues to the nature of dark current in multipliers and also reveals a surprisingly high yield of electrons from alkali antimonide (cathode) surfaces per ion incident on the surface. It has been found that afterpulses are the result of ionization of gas by electrons in the volume between the cathode and first dynode, The positive ions formed strike the cathode and cause the release of secondary electrons. The sharp pulse characteristic of afterpulses is due to the fact that the field distribution between the cathode and first dynode is such that the time required by an ion of given mass and charge to reach the cathode is nearly independent of the point of origin Observation of afterpulses produced by single photoelectrons indicates that the secondary-emission ratio for ions in a cesium-containing photocathode is in the neighborhood of 3. The time of occurrence of afterpulses can be closely correlated with the atomic mass of the residual gas in the tube. The pulse-height distribution of afterpulses produced by single ions appears to be identical with the larger dark-current pulses, and it is quite certain that this component of dark current is the result of ion bombardment of the cathode.

Performance of a Photomultiplier with a Porous Transmission Dynode

The root-mean-square deviation in number of secondary electrons from a normal secondary emitter increases with the square root of the secondary-emission ratio. The constant of proportionality is slightly greater than unity. The pulse-height resolution of a multiplier is therefore improved by the use of a high-gain first dynode. The secondary-emission ratio from a porous potassium chloride transmission dynode has a much higher value than that from conventional dynodes. Ratios of fifty to a hundred are easily obtained. If the statistics of emission obeys the same laws as for conventional secondary emission, porous dynodes would be advantageous as the first stage of a photomultiplier for scintillation counters used for energy resolution. A multiplier incorporating a porous transmission first dynode was built and tested. The pulse-height resolution was poor compared with that of a normal photomultiplier. The pulse-height resolution can be expressed in terms of FWHM as follows: FWHM =2.35 =1/?Ne?1 + M where Ne is the average number of photoelectrons per pulse and M is the degradation of resolution by the multiplier structure. A conventional photomultiplier operated at normal gain has an M value in the neighborhood of 0.5. A multiplier with a normal first dynode of gain 30 would have an M value of 0.06. The porous-dynode multiplier with a gain of 30 for the first stage had an M value of the order of 4.3.