Continuous Dynode Electron Multiplier Research Articles

Development of an analysis method of minor uranium isotope ratio measurements using electron multipliers in Thermal Ionization Mass Spectrometry

A simple analytical procedure was developed to measure with high accuracy the isotope ratio of minor isotope of natural uranium present in small quantities using a thermal ionization mass spectrometer (TIMS). The reduction of quantities used for analysis and the measurement of non-abundant isotopes are of prime interest in the nuclear industry. Indeed it is necessary to reduce the analyst received dose and the effluent released, as well as realizing measurement at trace level. The new generation of TIMS is equipped with a multicollection system of electron multipliers: discrete dynode electron multiplier (SEM) and continuous dynode electron multiplier (MIC), that improve the sensitivity compared to faraday cups. The procedure developed was verified using Certified Reference Material IRMM 052. Results were evaluated relying on NF T 90-210 norm regarding method validation. First, the isotope ratio 234U/238U was examined by total evaporation using the SEM and MIC to measure 234U and the faraday cup to measure 238U. In a second approach, the isotope ratio 235U/238U was studied by total evaporation using the SEM to measure 235U and the faraday cup to measure 238U. The classical method with peak-jumping SEM measurement was also used. Total evaporation method employing only the faraday cup was used to confront the results obtained. The analyzable quantity was reduced from 250ng to 50ng for the 235U/238U isotope ratio and from 1270ng to 50ng for the 234U/238U isotope ratio with acceptable uncertainties thanks to the use of electron multipliers. For all experiments were the accuracy was achieved, the calculated uncertainties were below to 0.28% for the 235U/238U isotope ratio and 5% for the 234U/238U isotope ratio.

Multiplication mechanism in open-type continuous-dynode electron multipliers

The multiplication process in open-type continuous-dynode electron multipliers (channel electron multipliers, microchannel plates) is considered as a Markov process that is continuous in time (channel length) and discrete in space (number of electrons in the channel). As a result of this consideration, the relationships for the mean gain and the probability distributions of the number of electrons at the channel output in the saturated and nonsaturated operating modes were obtained. The relationships are in good agreement with the measured dependences of the gain on the power-supply voltage and the distributions of the number of electrons at the channel output.

Dependence of detector dead time on analyte mass number in inductively coupled plasma mass spectrometry

ICP mass spectrometry is being increasingly used for isotopic analysis and for element determination by means of isotope dilution. For accurate isotope ratio determination, both the mass discrimination and the detector dead time have to be appropriately corrected for. For a Finnigan-MAT Element and a Perkin-Elmer SCIEX Elan 5000 ICP mass spectrometer, the constancy of the detector dead time across the mass range was systematically investigated. For the Element, which is equipped with a conversion dynode and a secondary electron multiplier with discrete dynodes, the dead time was observed to be independent of the analyte mass number. For the Elan 5000, however, which is equipped with a Channeltron—continuous dynode—electron multiplier, the detector dead time was observed to be strongly mass-dependent. The decreasing gain of Channeltron electron multipliers with increasing ion mass may be at the origin of the behaviour established. This observation has to be taken into account when highly accurate isotope ratio measurements are required and hence the detector dead time has to be properly determined and corrected for.

Development of Conventional Quadrupole Mass Analyzer for RI Target Analysis and Its Performance

A Knudsen-cell-based quadrupole mass analyzer has been developed for the analysis of a small amount of RI sample. To achieve a high signal/noise ratio, a continuous dynode electron multiplier was installed at the end of the mass filter and was operated in a pulse counting mode. Furthermore, the cylindrical shape molecular trap was set around the mass analyzer to reach high degree of vacuum and to reduce the RI contamination area in the apparatus.The isotope ratio of 135Cs/137Cs samples has been measured to demonstrate the performance of the mass analyzer. Taking the advantage of the Knudsen-cell's precise controllability over temperature, the contribution of the isobar 137Ba was successfully removed in the mass spectrum of cesium. With samples containing about 0.3μg 137Cs and almost the same amount of 135Cs, the isotope ratio measurement with the total error of 3% has been achieved. The molecular trap was also shown to be effective to reduce the contamination area in the apparatus.

Development of Conventional Quadrupole Mass Analyzer for RI Target Analysis and Its Performance.

A Knudsen-cell-based quadrupole mass analyzer has been developed for the analysis of a small amount of RI sample. To achieve a high signal/noise ratio, a continuous dynode electron multiplier was installed at the end of the mass filter and was operated in a pulse counting mode. Furthermore, the cylindrical shape molecular trap was set around the mass analyzer to reach high degree of vacuum and to reduce the RI contamination area in the apparatus. The isotope ratio of 135Cs/137Cs samples has been measured to demonstrate the performance of the mass analyzer. Taking the advantage of the Knudsen-cell's precise controllability over temperature, the contribution of the isobar 137Ba was successfully removed in the mass spectrum of cesium. With samples containing about 0.3μg 137Cs and almost the same amount of 135Cs, the isotope ratio measurement with the total error of 3% has been achieved. The molecular trap was also shown to be effective to reduce the contamination area in the apparatus.

Secondary electron yield of SiO 2 and Si 3N 4 thin films for continuous dynode electron multipliers

We have evaluated the secondary electron yield (δ) versus primary electron energy ( E p) of native oxide (SiO x), thermal oxide (SiO 2), and LPCVD Si 3N 4 films for use as electron-emissive layers in thin-film electron multipliers. Measurements ofδ(E p) were made using the retarding potential method with the electron gun operated in a pulsed mode. The maximum yield (δ m) in the energy range of interest E p = 20−400 eV forSiO x, Si 3N 4, and SiO 2 films was 1.5, 2.9. and 3.8, respectively. For comparison, δ m for reduced lead silicate glass (RLSG), used in conventional electron multipliers, was 2.5. While δ m > 1 for each of these materials, indicating that all should support electron multiplication, both the SiO 2 and Si 3N 4 films haveδ(E p) characteristics which are superior to RLSG. Gain modeling indicates that electron-emissive layers of SiO 2 or Si 3N 4 show potential for significantly enhancing the performance of thin-film electron multipliers over conventional RLSG devices.

Continuous dynode electron multiplier using lead zirconate titanate ceramic

Continuous dynode electron multiplier using lead zirconate titanate ceramic

Detection Efficiency of a Continuous Dynode Electron Multiplier (Ceratron) for Positrons

The detection efficiency of the continuous dynode electron multiplier, Ceratron, for positrons has been obtained as a function of the incident energy Ein for a slow positron beam measurement. For comparison, the efficiency curve for a channel electron multiplier (CEM) was also obtained under the same conditions. In the use of a Ceratron, the counting rate in conventional use is low compared with that under the best conditions, obtained using a new method. The dependence of the efficiency on Ein is very large for the Ceratron, but rather small for the CEM. The dependences in the low energy region for the Ceratron and for the CEM show that calibration of the efficiency on Ein for each detector is necessary in the conventional use for electrons. The detection efficiency of a Ceratron for positrons under the best conditions is almost the same as that for a CEM.

SnO2 + Al2O3 continuous dynode electron-multiplier

SnO2 + Al2O3 continuous dynode electron-multiplier

Microchannel plate detectors

A microchannel plate (MCP) is an array of 104-107 miniature electron multipliers oriented parallel to one another (fig. 1); typical channel diameters are in the range 10-100 μm and have length to diameter ratios (α) between 40 and 100. Channel axes are typically normal to, or biased at a small angle (~8°) to the MCP input surface. The channel matrix is usually fabricated from a lead glass, treated in such a way as to optimize the secondary emission characteristics of each channel and to render the channel walls semiconducting so as to allow charge replenishment from an external voltage source. Thus each channel can be considered to be a continuous dynode structure which acts as its own dynode resistor chain. Parallel electrical contact to each channel is provided by the deposition of a metallic coating, usually Nichrome or Inconel, on the front and rear surfaces of the MCP, which then serve as input and output electrodes, respectively. The total resistance between electrodes is on the order of 10 Ω Such microchannel plates, used singly or in a cascade, allow electron multiplication factors of 10-10 coupled with ultra-high time resolution (< 100 ps) and spatial resolution limited only by the channel dimensions and spacings; 12 μm diameter channels with 15 μm center-to-center spacings are typical. Originally developed as an amplification element for image intensification devices, MCPs have direct sensitivity to charged particles and energetic photons which has extended their usefulness to such diverse fields as X-ray) and E.U.V.) astronomy, e-beam fusion studies) and of course, nuclear science, where to date most applications have capitalized on the superior MCP time resolution characteristics). The MCP is the result of a fortuitous convergence of technologies. The continuous dynode electron multiplier was suggested by Farnsworth) in 1930. Actual implementation, however, was delayed until the 1960s when experimental work by Oschepkov et al.) from the USSR, Goodrich and Wiley) at the Bendix Research Laboratories in the USA, and Adams and Manley) at the Mullard Research Laboratories in the U.K. was described in the scientific literature. These developments relied heavily on a wealth of information on secondary electron emission) and earlier work on the technique of producing resistive surfaces in lead glasses by high temperature reduction (250-450 °C) in a hydrogen atmosphere. Finally, since most of the electrical performance characteristics of channel multipliers are not a function of channel length, l, or channel diameter, d, separately, but only a function of the ratio l/d =α, an almost arbitrary size reduction is possible. Such size reduction may be achieved by glass fiber drawing techniques which form the basis of fiber op tic device fabrication). In addition to a significant dimensional reduction resulting from these methods, a logarithmic compression of repetitive manufacturing steps is also possible, i.e., one can achieve a structure with ~10 holes requiring ~2 x 10 fiber alignment steps by a draw/multidraw technique. Prior to the application of reliable fiber drawing techniques, however, the first operational MCPs were

Self-triggered channel ion source

An ion source made from a continuous dynode electron multiplier of the straight channel type and an ion reflector device is described. Its principle of operation is based on a continuous self-sustained ion and electron cascade in the multiplier channel. Operating characteristics and the analysis of the ions produced are presented. The intensity of the total ion current delivered (10(-9) to 10(-8) A) is enhanced by at least six orders of magnitude relatively to previously described similar devices. This source, as illustrated, may be used in gaseous mass spectrometry.

Single ion counting in high-resolution mass spectrometry

A modification that permits the utilization of an air-stable, continuous-dynode electron multiplier that can be used either as an ion-counting or as an ion current measuring device without jeopardizing the photoplate ion detection capability of the high-resolution mass spectrometer is described.

A 65 keV Proton Scanning Microscope

We have constructed a prototype scanning gun microscope which uses 65 keV hydrogen ions as a probe. A field ionization ion source, consisting of a tungsten tip (∼ 1000Å radius) cooled to 78° K in an H2 atmosphere, yields H+ and H2+ ions, which are then accelerated and focused by a two-electrode accelerating lens. The spot is deflected electrostatically to scan the specimen and the transmitted particles are detected by a continuous-dynode electron multiplier.

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.

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

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

Oxidic semiconducting glass continuous dynode electron multipliers

Semiconducting oxide glasses containing multiple valence elements have found limited use in electronic applications because of their low mobility and relatively low conductivity. The latter characteristic has been used to advantage in the construction of continuous dynode electron multipliers. Parallel plate and tube structures, with no special surface treatment, have operated successfully with gains greater than 10 8 for single electron inputs. The output is linear with input until saturation sets in at output currents of approximately 10% of the current flowing in the dynode.

Oxidic semiconducting glass continuous dynode electron multipliers

Oxidic semiconducting glass continuous dynode electron multipliers

Parallel Plate Dynode Electron Multiplier; the Method of Production and its Characteristic

An easy method of making a parallel plate electron multiplier, which is used for detection of x-rays, electron beams, and charged particles, is developed. The multiplier consists of two continuous dynodes, which are prepared by evaporating aluminum onto glass substrates on which carbon has been evaporatedin advance. The gain is in the range of106to 109, when the applied voltage is 3 kV. Some of thefeatures are quite good linearity of response except for the saturation region, extremely small noiseand long lifetime, and ease in handling. It also can be used for the pulse count techniques. A briefreview of the outline of the continuous dynode electron multiplier is added.

Collection Efficiency of Continuous Dynode Electron Multiple Arrays

The ratio of incident electrons to electrons which participate in the multiplication process for continuous dynode electron multiplier arrays has been experimentally determined. For an array which had a geometrical transmission of 50%, it was found that 75% of the incident electron current reached the multiplication section of the array tubes. Of this percentage, 50% were considered to be initial electrons and the remaining 25% reflected or secondary electrons from the top electrode.

Gain-degradation effects in continuous-dynode electron multipliers

The development and commercial availability of continuous dynode electron multipliers has provided a useful capability for substantial current amplification (gains of 106to 108) in a small, light, simple, reliable low-power device that can be subjected to atmospheric exposure. For space probe particle sampling and other applications, long-term gain stability (or predictable gain variations for which compensation can be provided) is highly desirable.