Anlagerungskoeffizienten und Driftgeschwindigkeiten von Elektronen in Luft

Electron drift velocities and attachment coefficients were measured in dry air (E/p=0.1–30 V/cm Torr) and in a 9∶1 nitrogen/oxygen mixture (E/p=0.2–3 V/cm Torr) in the pressure range from 50 to 200 Torr, using a modified spark chamber technique. The primary electrons were released by anα-particle. The temporal development of the electron density in the gap was determined from the amplitude of the current due to the avalanches, which were produced by applying high voltage pulses at different delay times. — It was found that in air the dissociative attachment sets in at higherE/p (～10–15 V/cm Torr) than in oxygen. At lowerE/p three body attachment is predominant. — When the high voltage pulses were applied after the transit time of the primary electrons, electron avalanches still appeared. It was concluded that they were started by electrons which were detached from negative ions. The estimated detachment rates indicate the formation of O ₂ ^? ions at lowE/p and of O^? ions at higherE/p.     

Über den zeitlichen Verlauf der Lichtemission einer Gasentladung

By means of a photomultiplier the temporal course of the light emission due to electron impact is observed in vapours as methane and methylal. It is thus possible to observe single avalanches, their successors and the transition to breakdown. In methylal the rise time constant of light emission of single avalanches is compared with the rise time constant of the electron production measured by the electrical method. These rise time constants are found to be equal. By this way a quantum efficiency of 3.5 · 10^?4 photons per avalanche electron could be derived in methylal. The growing rate of single avalanches of 10⁸ electrons is shown to be decreased by space charge. —In methane series of avalanches were investigated: The electron drift velocity (v_=bp · E/p;bp=2.3 · 10⁵ Torr cm²/Vsec,E/p:40 bis 110 V/Torrcm) and the average life time of excited states (～7 · 10^?9 sec) were measured. In methane the breakdown is produced by series of avalanches, whereas in methylal with its smallerγ the rapid increase of light emission without successors indicates a breakdown mechanism by streamer formation.     

Über Elektronenlawinen in KBr-Kristallen

In KBr-crystals electron avalanches were started by electrons released in the crystal by very short light pulses. From the temporal current growth we determined the electron drift mobilityμ=15±2 cm²/Vsec, the electron life timeτ=2±0.5 · 10^?7 sec and the multiplication factor yielding the impact ionization coefficient for electrons.     

Der Einfluß metastabiler Atome auf den statischen Durchschlag in Argon

A gas discharge in argon is initiated by an ultraviolet light pulse which releases 10⁵ electrons at the cathode within 20 nsec. The time of built-up for the static breakdown is found to be in the order of msec. The oscillogram of the current near breakdown shows a fast component consisting of several successive avalanches caused by secondary electrons liberated by ions at the cathode and a slow component which is due to the electrons liberated by metastable atoms at the cathode. The second Townsend coefficient for argon ions and a nickel cathode is determined to beγ ₊= 2.5·10^?3. The amount of electrons liberated by metastables at static breakdown is 25% of the total number of secondary electrons. The mean life-time of the metastables at 1,1 Torr and an electrode separation of 10 mm is found to be 2.2 msec, which is mainly due to de-exciting collisions at the electrodes.     

Die Entwicklung der Elektronenlawine in den Plasmakanal,untersucht mit Bildverstärker und Wischverschluß

The application of high gain image intensifier and image converter streak shutter techniques to investigations of faintly luminous early stages of gaseous discharges discloses in some detail the development of electron avalanches into anode- and cathode-directed streamers as soon as the critical amplification (n≈10⁸) is attained in the gap. The velocities of both of the streamer branches were measured quantitatively as to the dependence on pressure (p), field (E/p) and gas (Nitrogen and various mixtures), typical values being 0.5 ... 1 · 10⁸ cm/sec. We find that streamer propagation is suitably described in a coordinate-system drifting with the electrons (driftvelocityv _? ≈ 1 · 10⁷ cm/sec) and that the way it depends on the discharge parameters suggests that ionization by electron impact (in the eigen-spacecharge distorted field) and photoionization are the determining effects. — The cathode-directed streamers show a considerable intensification and acceleration as soon as they are encountered by cathodic secondary avalanches, which however are obviously not essential for the initiation of the process in itself. — The streak images give also an impression of persisting excitation and ionization processes in the region of the streamer-paths and of the transition into the beginning of the bright spark, correlated with several ionizing fronts observed moving towards anode or cathode with velocities even >10⁸ cm/sec. (The paper includes also details of the gaseous near UV-radiation which was utilized for registration with image intensifier or converter, especially with respect to the decaytime of the excited states.) The results obtained are essentially a verification and to some extend completion of predictions made on the basis of early cloud chamber investigations.     

Der Einfluß der Eigenraumladung auf die Trägervermehrung einer Lawine in N2, CH4 und N2+CH4

In gases with highγ, e.g. N₂, in which the carrier amplification is max. 10⁴ at the static breakdown, electron avalanches with a carrier amplification up to 10⁹ are produced by overvoltage. These avalanches allow to investigate the influence of the own space charge on the carrier amplification above 10⁶. In the homogeneous field of a pulsed 3 cm discharge gap avalanches were produced in N₂, CH₄ and N₂+CH₄. The investigation of avalanches shows an exponential growth according to\(e^{\alpha _0 \nu _ - \cdot t} \) up to 10⁶. Between 10⁶ and ?5·10⁸ the electron avalanches increase underexponentially owing to the own space charge. These results are in accordance with former measurements in vapours. A comparison of the lowering ofα/α ₀ by the space charge with the theory allows the calculation of avalanche diameters and of the thermal energies of the electrons. The calculated values of the electron temperature vary from 2·0 to 2·5 eV for N₂, and from 1·0 to 2·0 eV for CH₄ and N₂+CH₄ atn<10⁷. Above ?·10⁷ the avalanche diameter increases considerably.     

Eine Untersuchung der Elektronenkomponente von Elektronenlawinen

The oscilloscopic study of pulses due to the electron component of avalanches in homogeneous fields confirms that the number of electrons increases exponentially with time. Measuring drift velocityv _? and applying recent measurements of the first Townsend coefficient α, we find the time constant τ to be equal to 1/α ·v _? as was expected. -We get the axial diffusion radius of the electron swarm and thus the mean energy of agitation (0.4 to 0.7 eV) and the cross section for electron impact. -Space charge influence becomes apparent with carrier numbers of about 10⁷ electrons, attenuating the temporal rate of further growth of the avalanche. This is in accordance with earlier work on avalanches done in our institute. The investigation was made with methane in the range of E/p from 28 to 58 V/cm · mm Hg, with pd-values from 50 to 1500 cm · mm Hg, with mean gas amplification from e¹² to e¹⁷, and with gap widths of 2, 3, and 6 cm.     

Die Eigenschaften von Elektronenlawinen bei hohen Verstärkungen in Äther

The oscilloscopic study of avalanches of high carrier numbersn allows to investigate the influence of the space charge on the carrier amplification aboven≈10⁶. The investigation was made with ether in the range ofE/p from 60 to 200 V/Torr·cm, withpd-values from 6 to 500 Torr·cm and with gap widths of 0,3; 0,9 and 2,7 cm. Betweenn=10⁶ and 10⁸ the number of carriers increases less than exp (α₀ x). The space charge field of the positive ions reduces the ionisation effect of the electrons. By comparing the lowering of α(n)/α₀ with the theory one can calculate the diffusion radius of avalanches and thus the mean energy of agitation in ether, which had a nearly constant value of 3 to 4 eV. Aboven≈5·10⁶ to 10⁷ the avalanche diameter increases considerably especially at high pressures. This effect is produced by the electrostatic repulsion of the electrons in the head of the avalanche. If the carrier number reachesn≈10₈ α(n)/α₀ increases. This is apparently due to processes (“Nachstrom”) following the development of an avalanche just after its transit time.     

确定SF6-CO2预放电参数的激光脉冲方法

在均匀电场中,用高能激光脉冲释放初始电子以研究负电性气体的电子崩的发展,决定预放电过程的基本参数(游离系数α,吸附系数η和漂移速度v等)是一个有用的方法,本文对此方法做了详细的分析。采用这种方法对SF₆-CO₂混合气体做了研究,获得了10⁸个以上的初始电子及其分布,并给出了α/P,η/P和ν与E/P(E=电场强度,P=气体压力)的关系。本文还对所用的测量系统做了讨论,提出了改进办法。 关键词：     

Die Trägervermehrung einer Lawine mit Eigenraumladung

From oscillograms of avalanches of high amplification (ether,p=370 Torr,d=0,3 cm,E/p=77) one can deduce that the number of carriers (n) increases less thane ^αx , ifn overpasses 10⁶. It is the space charge field of the positive ions which reduces the ionisation effect of electrons.     

Needle Growth Induced by Corona Discharge

It is shown that the growth of a dense array of chromium microneedles is induced on a field cathode tip interacting with a negative corona discharge in ambient Cr(CO)₆ of 1 – 5 × 10^?2 Torr. The growth begins with the initiation of a corona discharge surrounding the tip, and it proceeds as long as the discharge is sustained. The needles emit field electrons during the growth, thereby sustaining the discharge, while the discharge lengthens the needles by supplying metallic particles to their tips. The morphological structure of the needles is strongly affected by an external magnetic field, leading us to hypothesize that the needle grow axially as the electron avalanches, originating at the cathode tip, propagate outward from tip.     

Über Elektronendriftbeweglichkeit und Stoßionisierung in KBr-Kristallen bei hohen elektrischen Feldstärken

A swarm of electrons passing a KBr-crystal has been investigated by fast oscilloscopic techniques at high electric fields (3×10⁴ to 4×10⁵ V/cm). From the temporal current shape we determined the electron drift mobilityμ=15±2 cm²/Vsec, the electron life timeτ=2±0.3 × 10^?7 sec and the multiplication factor. From this measurement the ionization coefficientα for electrons was calculated as a function of the applied electric field.     

Über Zusammenhänge zwischen Elektronenspinresonanz,elektrischer Leitfähigkeit und Photoleitung bei reinem und dotiertem Bor

The conductivity mechanism in pure and doped, β-rhombohedral, polycrystalline boron between 1.5 °K and 900 °K is clarified by measurements of electrical conductivity, photoconductivity, electron paramagnetic resonance and thermoelectric effect. The semiconductor behaviour of boron between 1.5 and 900 °K is similar to that of doped and compensated germanium and silicon at helium temperatures concerning the temperature-independent number of carriers and the thermally activated conduction process at low and high carrier concentrations. The paramagnetic centres are nearly localized electrons at 1.5 °K and nearly free electrons at 900 °K with a continuous transition between these two extreme kinds of behaviour. Mobilities of charge carriers in carbon doped boron over a range from 10¹⁶ cm^?3 to 10²⁰ cm^?3 and 77 °K to 900 °K were measured for the first time and were found to obey an exponential law.     

The leak microstructure

The capabilities of a new microstructure, anode point based, for the detection of gas ionizing radiations are presented. For every single detected ionizing radiation it gives a pair of ‘induced’ charges (anodic and cathodic) of the same amount (pulses of the same amplitudes), of opposite sign, with the same collection time and essentially in time coincidence, that are proportional to the primary ionization collected. Each pulse of a pair gives the same energy and timing information, thus one can be used for these information and the other for the position. The complete lack of insulating materials in the active volume of this microstructure avoids problems of charging-up and makes stable and repeatable its behavior. It is possible to observe primary avalanches with a size of more than 2.5 × 10⁷ electrons (4 pC), which give current pulses with a peak of more than 0.26 mA on 100 Ohm and about 30 ns duration, with 5.9 KeV X-rays of ⁵⁵Fe working in proportional region in 760 Torr of isobutane gas. Single electrons emitted by a heated filament (Ec<1 eV) can also be detected in 760 Torr of isobutane; with an estimated gas gain of 1.2 × 10⁶ and a counting rate up to 800 Kpulses/sec per single microstructure. Some new features and three different types of sensitive-position two-dimensional read-out detectors based on these microstructures, which are in developmental stage, are presented.     

Untersuchungen über die Entwicklung des Kanaldurchschlags in Gasen

The temporal development of the current of a discharge leading to breakdown was investigated under static and homogeneous field conditions with wide band oscilloscopic techniques. The discharges were started by 10³ to 10⁵ electrons released within some 10^?9 sec along traces of single α-particles parallel to the electric field. Measurements have shown that streamer mechanism occurs in gases as CH₄ and CO₂ at static breakdown, if a sufficient density of space charge is produced by a great number of overlapping avalanches. The gas amplification of thesingle avalanche only has a value of about 10⁵. Also in O₂ and dry air at highpd-values breakdown develops with streamer mechanism. — In electronegative gases as freon 12 and O₂ a prolongation of the time necessary for streamer formation is caused by the reduction of the effective total space charge by the negative ions. The results of a detailed study of the time necessary for streamer development and the time constant of the increase of the current leading to breakdown confirm the model of streamer mechanism.     

Die Reichweite von Elektronen und Positronen kleiner Energie in Kupfer,Silber und Gold

In continuation of previous work the range of 40 to 160 keV monoenergetic electrons and positrons in copper, silver, and gold is determined and the corresponding range-energy-relations are stated. In copper the positrons have a shorter range than electrons of the same energy, but in silver and gold they have a longer range, in disagreement with the values calculated byNelms 2. For the three absorption materials all the ranges of electrons can be described by the formula E^?=14,3 · (Z^4/3/A) · R^{0, 61}, and all the ranges of positrons by E⁺=29,7 · (Z/A) · R^{0, 60} (E in keV, R in mg/cm²). The ratio of the range of positrons to that of electrons for copper agrees rather well with the calculated one, but for silver and gold it differs considerably. This might be due to the increasing influence of the elastic scattering of the particles on nuclei.     

Wechselwirkung von Elektronen mit Gitterschwingungen in Ammoniumchlorid und Ammoniumbromid

Energy loss spectra of polycrystalline NH₄Cl and NH₄Br films have been studied between 10 and 400 meV using 30-keV electrons. The resolution of the spectrometer was between 4 and 10 meV, the scattering angle smaller than 1.2×10^?4 radian. Strong energy losses and gains were found near 20 meV corresponding to excitation of translational lattice vibrations wellknown from reststrahl measurements. Weaker energy losses above 100 meV are due to excitation of ammonium ion fundamentals and their interaction with torsional vibrations and, presumably, with translational lattice vibrations. In this energy loss range agreement is stated concerning the general behaviour between energy loss and infra-red absorption spectra. In the finer details, however, striking deviations occur, which must be ascribed to the different interaction mechanisms of lattice vibrations with electrons and photons, respectively.     

Elastische und unelastische Streuung von Elektronen an Gasen

The theoretical differential cross section for the elastic scattering and for the excitation of optical transitions in helium by electron impact has been refined in Born approximation by use of the two parameter Eckart eigenfunction for the ground state and for the excited states. The angular distributions of 25 kev electrons scattered elastically and inelastically by helium were measured in the angular range 2·3·10^?4≦?≦4·10^?2. The intensity distribution of the elastically scattered electrons is in accordance with the theoretical curve for?>7·10^?3 and is disturbed at smaller angles by the primary beam. Normalization of the experimental values to the theoretical elastic differential cross section leads to agreement between the experimental differential cross section for the excitation of the 2¹ P and 3¹ P state and the scattering formulae given in this paper. There are small systematic deviations (<20%) for the 2¹ P differential cross section in the angular range 3·10^?3<?<1·10^?2 only. The oscillator strength of these two transitions has been determined from the scattering measurements:f ₂₁=0·312±0·04 andf ₃₁=0·0898±0·006.     

Messung der Energie- und Winkelabhängigkeit der Asymmetrie der äußeren Bremsstrahlung

In order to obtain an azimuthal dependence of the external bremsstrahlung produced by electrons, the electron beam has to be transversely polarized. In first-order Born approximation the differential cross section does not depend on the azimuthal angle φ¹, but the second-order approximation includes the spin vector of the electrons^2,3 and yields, consequently, a φ-dependence. This paper deals with a measurement of the bremsstrahlung asymmetry as function of the photon energy and the emission angle Θ. The transversely polarized electron beam was produced by momentum deflection of longitudinally polarizedβ-decay electrons emitted from a⁹⁰Sr source. With a kinetic energy of 300±10 keV and a degree of transverse polarization of 74% the electrons hit a Pb target. Because of the high γ-background it was necessary to detect the emitted bremsstrahlung in coincidence with the incident electrons. We found effects of about 3% in contrast to considerably larger values (13%) published in an earlier paper. The asymmetry was numerically calculated according to Johnson and Rozics. Since at an electron energy of 300 keV the Born approximation gives not yet good results, theory and measurement agree only in order of magnitude, particularly at highγ-energies.     

Elektronenbeschleunigung in einem Plasmabetatron mit und ohne hohem Leitungsstrom

A previous paper by Drees and Paul reported measurements on a plasma betatron. The study was continued using a betatron field with a vector potential taking into account the self magnetic field of a plasma current. The plasma was produced by a high frequency electric quadrupole field without an azimuthal magnetic field. The bremsstrahlung intensity of the accelerated electrons was observed as a function of gas pressure and accelerating field. The maximum energy of the electrons was 1.3 MeV compared to 1.5 MeV given by the field parameters. The maximum number in this energy range was about 10¹⁰ per pulse corresponding to a circulating runaway current of ～ 1 A. The conduction current was drasticly reduced by coating the inner wall of the quartz glass torus with a thin layer of graphite. This change in the plasma current did not influence the γ radiation intensity.