Streuung von 25 keV-Elektronen an Gasen

The angular distribution of 25 kev electrons scattered elastically and inelastically by molecular hydrogen has been measured in the angular range 7·10^?4≦?≦4,3·10^?2. By the separation of the inelastically scattered electrons observation of deviations from the Debye Ehrenfest theory of the electron diffraction by molecules at small angles is possible. These deviations are due to the alteration of the electron density distribution of the hydrogen atoms induced by the bonding. The energy loss spectra at different scattering angles (energy resolution ≈1 ev) shows a strong peak atΔE=12,6 ev. At larger angles forΔE>15 ev a continuum appears. That part of the inelastic processes which leads to ionization of the molecule is raising with increasing scattering angle. Normalization of the experimental values to the theoretical elastic differential cross section enables comparison of the angular distribution of the 12,6 ev energy loss with the distribution calculated byRoscoe. The shape of the experimental curve is in fairly good agreement with the calculated one but the experimental values at small angles are 20–30% higher. For zero angle the energy loss spectrum is taken with better resolution (≈0,04 ev). It shows vibrational states of the Lyman and Werner band and higher terms. The probability of the excitation of some vibrational states of the Werner band (square of the overlapp integral) calculated here is inspite of the required approximations in excellent agreement with the measurement, while Hutchisson's result fails for the Lyman band.     

Inelastische Positronenstreuung am Atomkern

The ratio of the total and differential cross section for the inelastic positron-nucleus scattering (ē, N)-process to the total (γ, N) -cross section is derived in Born approximation for electric and magnetic dipole transitions. The result agrees with that obtained for the (e, N)-processes. Using the relativistic Coulomb Eigenfunctions for the continuous spectrum of the positrons, the Coulomb correction, the effect of screening and that of finite nuclear size agree with the (e, N)-process, when the annihilation of positrons with atomic electrons is neglected, and for positron energiesE _1,2 ⁺ >10 MeV. The effect of finite nuclear size is only calculated in Born approximation. ForE _1,2 ⁺ ≦2 MeV only the Coulomb correction differs from that obtained for the (e, N)-process. In the angular distribution for the (ē, N)-process there should be no interference of positron waves scattered by different multipoles, where the inelastic scattered positrons are detected. Numerical calculations have been carried out for nuclei withZ=6.29 and 82 and scattering angles ?=1°, 132°, 160° and 180° of the positron. This theory can be compared with the experiments in progress by W.C.Barber et al. using positrons for the inelastic scattering process at nuclei. The two-and three-virtual quanta-exchange effect in the (ē, N)-cross section is below 1.3% for positron energies between 10≦E ₁ ⁺ ≦300 MeV, and decreases rapidly for higher energies. This theory is also valied for inelastic scattering processes with positiveμ-mesons at nuclei; one has only to change the mass in the following equations.     

Winkelkorrelationsmessungen an >30y Holmium 166 und Bestimmung desg R -Faktors des 4+-Rotationsniveaus von Erbium 166

The spins of several excited states of Er¹⁶⁶ have been investigated byγγ-angular correlation measurements. The spin sequence 0+, 2+, 4+, 6+ for the ground state rotational band was presumed to be correct. Unique assignments were derived for the states of 1076 keV, 1377 keV and 1785 keV asI=5, 7 and 6 respectively. These results are in agreement with the spins proposed byGallagher jr. andSoloviev. The multipolarities of theγ-transitions of 408 keV, 709 keV, 811 keV and 831 keV were derived as 95%E1+(5±1)%M2, 99·6%E1+(0·4±0·5)%M2, 99·1%E2+(0·9±0·3)%M1, and 96·1%E2+(3·9±1)%M1 respectively. The unusual mixing ratios of the transitions of 811 keV and 831 keV can be understood as a consequence of theK-selection rule. Eachγ-transition from the 1785 keV state should be stronglyK-forbidden and one expects a half-life ofT _1/2≈3·10^?9s. A measurement of the time spectrum of the coincidences between theβ-radiation and the high energyγ-lines gave however:T _1/2(1785 keV state)≦3·10^?10s. The rotation of the angular correlation between the 184 keV line and theγ-group at 820 keV has been measured in an external magnetic field of 53000 gauss as:ω·τ(4+)=0·083±0·006. This value contains small corrections for an additional rotation of the angular correlation of the 831 keV–184 keV triple cascade in the 6+state and for a small attenuation by internal fields. WithT _1/2(4+state)=1·23·10^?10s, andβ=7·08 one gets for theg-factorg _R=+0·266±0·024 in good agreement with recent results for the 2+ state.     

Spurenanalyse von Cu in ZnS-Phosphoren mittels Neutronenaktivierung

The half life of the 206 kev state of Re¹⁸⁷ was determined with delayed coincidence technic to be:T _1/2=(5·70±0·05)·10^?7 s. The rotation of the spin of the 206 kev state in a magnetic field has been observed in a differential angular correlation measurement with the 480 kev?72 kevγ?γ cascade. From the observed Larmor frequency a magnetic moment ofμ=(5·02±0·06)μ _K for the 206 kev level has been derived. The 480 kev?72 kevγ?γ angular correlation was found to be undisturbed in an aqueous solution of ammoniumtungstate.     

Derg R -Faktor des 2+-Rotationsniveaus von Er168

The rotation of the angular correlation between theγ-group at about 820 keV and the 80 keV radiation in the decay of Tm¹⁶⁸ has been observed in an external magnetic field of 20300 gauss. The result:ω _R·τ=0,485±0,051 yields for theg _R -factor of the 80 keV state of Er¹⁶⁸:g _R =+0,25±0,03. The evaluation includes a paramagnetic correction factor of:β=7,26, (B _eff=β·B _ext). The angular correlation is slightly attenuated by internal fields. For a liquid source of Tm (NO₃)₃ solved in 3 n HNO₃, a measurement of the differential angular correlation as a function of the delay time gave:λ ₂=(5,8±2,9)·10⁷sec^?1, assumingA ₂(t)=A ₂(0)·e ^?λ ₂·t. The half life of the 80 keV state was found to be:T _1/2=(1,92±0,04)·10^?9sec in agreement with earlier measurements.     

Streuung von 25 keV-Elektronen an Gasen

The differential cross section of neon, argon, krypton, and xenon for the elastic scattering and for the excitation of optical transitions has been measured. For neon and argon the elastic angular distribution can be described by the cross section calculated according toWentzel andLenz. For krypton and xenon there are considerable deviations from the Wentzel-Lenz cross section. In these cases it is better to calculate the elastic differential cross section from Slater eigenfunctions, from Thomas-Fermi-Dirac or from Hartree-Fock electron density distribution. The differential cross sections for the excitation of optical transitions, so far as measured here, obey for?<? _c the dipol approximation, i.e. for angles not to small the inverse square angular dependence is valid.? _c is higher for the heavier rare gases. The oscillator strengths for some transitions have been determined from the scattering measurements in satisfactory agreement with theoretical values.     

Das magnetische Moment des 2,083 MeV-Zwei-Protonen-Niveaus des gerade-gerade-Kerns Ce140

The nuclearg-factor of the 2·083 MeV-state of Ce¹⁴⁰ has been determined as:g=1·11±0·04 This value suggests the interpretation of the level as the (g7/2,d5/2)₄ two proton shell model configuration. The investigation implies an integral measurement as well as a time dependent differential measurement (spin rotation method) of the rotation of the 329 keV-487 keVγγ-angular correlation in strong external magnetic fields. Liquid sources of lanthanum nitrate solved in 3n HNO₃ were used in these experiments. Some cerium (IV)nitrate was added to make sure that the 4f;-electronic shell remained empty after theβ ^?-decay. This prevented paramagnetic effects. Additional runs were performed without adding cerium (IV)nitrate and finally by using liquid sources of lanthanum chloride solved in 2n HCl with some iron (II)-chloride added. It was expected that the Fe⁺⁺ ions would reduce the primarily formed Ce⁺⁺⁺⁺ ions into the Ce⁺⁺⁺ state. The Ce⁺⁺⁺ shell is paramagnetic and enlarges the effective magnetic field at the position of the nucleus by about 36%. As no definitely faster rotation could be observed, one has to conclude that the ion reaction Ce⁺⁺⁺⁺+Fe⁺⁺?Ce⁺⁺⁺+Fe⁺⁺⁺ takes more than about 10^?8s. The half life of the 2·083 MeV state has been redeterminded asT _1/2=(3·41±0·04)·10^?9s. Differential measurements of the angular correlation of the 329 keV-487 keV cascade as a function of the time delay showed that perturbations by internal fields were very small. The coefficientsA ₂ andA ₄ of the angular correlation and the attenuation parameterλ ₂ were determined from measurements with sources of the first type as:A ₂=?0·105±0·003A ₄=?0·001±0·002 andλ ₂=(0·0065±0·0040)·10⁹S^?1=(0·032±0·020)·λ A comparison with the theoretical coefficients of a (3-4-2)-cascade shows that the multipolarity of the 329 keV radiation is a mixture of 99·83%M1 and (0·17±0·03) %E2.     

Messung der Zirkularpolarisation der 478 keV-Gammaquanten aus der Reaktion Li6(d,p)Li7 mit 2 MeV-Deuteronen

The circular polarizationP _γ of 478 keVγ quanta from the reaction Li⁶(d, pγ) Li⁷ with 2 MeV deuterons was measured by forward Compton scattering from magnetized iron. The protons were observed at 25° lab angle. The fast neutron background in theγ detector was suppressed by pulse shape discrimination. The resultP _γ=+ 0.022 ± 0.17 with the Basel sign convention confines the ratio of the reduced widthsΘ _± for the capture of the neutron withj=1±1/2 to the regions ?2.36≦Θ ₊/Θ _?≦?0.74 or ?0.06≦Θ ₊/Θ _?≦+0.61.     

Numerische Berechnungen zum Resonanzverhalten der Streuquerschnitte bei atomaren Stößen. I

The semiclassical approximation can be used to calculate the extrema in the energy dependence of the total cross section for collisions between neutral atoms and molecules. For this treatment, the classical deflection functionΘ(β) must be known in the region nearβ ₀, where it passes smoothly through 0° (glory scattering). Numerical values forβ ₀,Θ′ (β ₀) andΘ′' (β ₀), which determine the amplitude of the extrema, are presented for aKihara (12,6) potential for various reduced energiesK (0.01≦K≦100) and potential parameters α (?0,3≦α≦0.5).     

Anregung von UV-Strahlung in Stickstoff und Wasserstoff durch einen Elektronenschwarm

The radiation excited in nitrogen and hydrogen by an electron swarm moving in a uniform electric field is investigated. In nitrogen the bands of the 2 ^nd positive group in the region from 3400 to 3800 Å are the dominating part of the radiation. In hydrogen the main part of the radiation is lying in the far ultraviolet (1000 to 1200 Å) due to the excitation of the state 2¹ II _u . The coefficientδ describing the intensity of the radiation is measured with a photomultiplier device as a function of the electric fieldE and the pressurep in the range from about 10 to 100 Torr. Besides the dependence ofδ onE/p a pressure dependence is found which is explained as a quenching process of the excited molecules by collisions. The pressurep ₀ for equal probability of radiating and non-radiating decay of the states involved is 60 Torr in N₂ and 20 Torr in H₂. The theoretical interpretation of the dependence onE/p supports the excitation energy of the state C³ II _u of N₂ for electron impact to be equal to the spectroscopical value of 11·04 eV and yields 0·9·10^?16 cm² for the maximum of the excitation cross section. The results of the measurements in H₂ can be described assuming a cross section of 0·55·10^?16 cm² for the excitation of the state 2¹ II _u by electron impact.     

Elastic scattering of 0.5 MeV gamma rays by heavy elements

The absolute differential cross sections for the elastic scattering of 0.511 MeV gamma rays were measured in the angular range 0.5? to 10? for the elements withZ=72 toZ=92. The experimental method, employing annihilation radiation in coincidence, permits the investigation at very small momentum transfer. In general the results obtained are in very good agreement with the predictionZ ² r ₀ ² for the differential Rayleigh cross section atθ=0? and with the form factor theory, using Hartree-Fock-Slater wave functions, for the investigated angular range. However, for the elements lead, bismuth and thorium we found marked deviations from the form factor predictions, occurring at very small scattering angles. This is attributed to an outer atomic charge distribution different from the other investigated elements, which is also indicated by considerably greater metallic radii. Also effects due to the crystalline structure cannot be excluded. — Further, an elastic, resonance-like andZ-dependent scattering effect by bound electrons is discussed which is due to the formation of an intermediate, bound positronium state by electron-positron pairs.     

Die Intensität der Bremsstrahlung und der charakteristischenK-Röntgenstrahlung dünner Anoden

The intensities of bremsstrahlung and characteristicK X-rays of thin Al, Mn, Cu, Se, Ag and Sn targets were measured with a scintillation counter. The angle between the 50 keV electrons and the direction of emission was 90°. — The spectral intensity of bremsstrahlung for 20 keV quantum energy was determined in agreement with the theory ofKirkpatrick andWiedmann. The result obtained isi _v=(2.7) · 10^?53.Z ^{2, 0} ergs per steradian, unit frequency interval, bombarding electron and atomper-cm². The cross section ofK-ionisation by electron impact has been found out by measuring the characteristicKX-ray intensity in agreement with known theoretical and experimental values. The result of this work isσ _K=(7.0) · 10^?16 ·Z ^?4,3 cm².     

Messung der Lebensdauer angeregter Kernniveaus von Al28, Cs133, Pr141, W182 und Pt195

The halflife of excited states in some nuclei has been measured by the method of delayed coincidences. Two different experimental arrangements have been used: a conventional scintillation counter equipment and a fast gasfilled parallelplate avalanche-counter. The results of these measurements are: 31 keV-level in Al²⁸:T _1/2=(1.91±0.08) · 10^?9 sec, 81 keV-level in Cs¹³³:T _1/2=(6.25±0.05) · 10^?9 sec, 145 keV-level in Pr¹⁴¹:T _1/2=(1.85±0.03) · 10^?9 sec, 100keV-level in W¹⁸²:T _1/2=(1.45±0.04) · 10^?9 sec, 1290 keV-level in W¹⁸² T _1/2=(1.05±0.03) · 10^?9 sec, 99 keV-level in Pt¹⁹⁵:T _1/2≦1.6 · 10^?10 sec, 129 keV-level in Pt¹⁹⁵:T _1/2=(6.2±0.7) · 10^?10 sec. These experimental values are discussed and compared with theoretical model predictions.     

Atom-Atom-Streuexperimente: Geschwindigkeitsabhängigkeit einiger totaler Streuquerschnitte

The velocity dependence of total cross sections for the collision partners Cs-Hg, K-Hg, Xe, Kr, Ar has been measured in crossed beams. Except for Cs-Hg glory scattering oscillations were resolved. Invariably good agreement with theoretical predictions is found for the extrema velocities assumingn-6 potentials with 8≦n≦16 or exp-6 potentials with 10≦α≦18, for the amplitudes of the oscillations no fit is obtained. K-Hg, probably also K-Xe and K-Kr appear to deviate from theν ^?2/5 average velocity dependence expected for dipole-dipole interaction at large distances.     

Relaxationszeiten für die Umorientierung von Quecksilberatomen im metastabilen 63 P 2-Zustand durch Stöße

From the linewidth of the radiofrequency resonance between the Zeeman levels of the 6³ P ₂ state the lifetimeT _m of the metastable Hg atoms in a certain magnetic sublevel was derived as a function of the Hg vapour pressure. The lifetime of the metastable stateT(6³ P ₂) was determined under the same experimental conditions by optical absorption measurements. In the pressure range from 2·10^?3 to 2.4·10^?2 mm HgT(6³ P ₂) was longer thanT _m by one to two orders of magnitude. This result means thatT _m is only dependent on disorienting collisions and is practically not affected by the collisional destruction of the metastable state. ThusT _m is the relaxation time for the disorientation of the metastable atoms by collisions. The pressure dependence of the relaxation time indicates that the disorientation is achieved by two-body collisions with ground state Hg atoms. The disorientation cross section was found to be (2.1±0.2)·10^?14 cm².     

Die charakteristische Thermokraft der Grenzflächenatome

For a single-band conductor where two or more scattering mechanisms are present, each giving rise to a characteristic thermoelectric powerS _n and a electrical resistivity? _n the resultant thermoelectric powerS is given, as a first approximation, by\(S = \sum\limits_n {\varrho _n S_n /\varrho } \). Denoting withS ₀ the characteristic thermoelectric power due to the scattering of the conduction electrons by the boundary atoms, and withS _i and? _i the resultant thermoelectric power and electrical resistivity arising from all other scattering mechanisms, one may writeS=S ₀+? _i(S _i?S ₀)/?. The thermoelectric powerS and the electrical resistivity? of thin layers of potassium, evaporated in a vacuum ～5·10^?9 Torr on a glass substrate at 90° K temperature, were measured at different thicknesses. The variation ofS as a function of 1/? verifies the above mentioned relation. Thus, the thermoelectric power, characteristic for the scattering by potassium boundary atoms can be determined.     

Delbrück-Streuung von 17 MeV-Photonen

Differential cross sections have been measured for the small angle scattering ofγ-rays by iron, silver, tantalum, lead and uranium with 17 MeV photons from Li⁷ (p, γ) Be⁸ at mean four momentum transfersq of 0·5mc and 1·3mc and with 7 MeV photons from F¹⁹(p, αγ)O¹⁶ at mean four momentum transfer of 0·5mc. Under these experimental conditions only Compton, Rayleigh and possibly Delbrück scattering are of importance. Extrapolation of known theoretical results to higher energies shows, that Rayleigh and Compton scattering from bound electrons should depend only onq for small angles, smallq and fixedZ. Using this it follows, that at 17 MeV andq≈0·5mc an additional scattering process must be present, which increases with growingZ and which is negligible in the measurements at 17 MeV withq≈1·3mc and at 7 MeV withq≈0·5mc. These results are in qualitative agreement with the approximate theory for Delbrück scattering ofBethe andRohrlich, however experimental cross sections at 17 MeV andq≈0·5mc are about a factor of 1·6 lower than those predicted by this theory. This discrepancy is not unexpected, since exact calculations of Delbrück scattering amplitude fromKessler andZernik at 2·62 MeV and 6·14 MeV show even greater deviations in the same sense.     

Elektronenpolarisation im Energiebereich unterhalb 50 eV durch Streuung an freien Hg-Atomen

The angular dependence of spin polarizationP(Θ) of electrons elastically scattered by a beam of mercury atoms is measured in a double scattering experiment for electron energies of 45±1 eV; 23±1 eV; 7±1 eV; and 3,5±1 eV. Maximum degree of polarization obtained isP(100°)=0,39±0,07; electron energy 7±1 eV; electron current 10^?12–10^?11 A.     

Paramagnetische Relaxation von CeCl3

The paramagnetic relaxation in CeCl₃ was investigated in the temperature interval between 1.07°K and 4.21°K using a mutual inductance bridge at frequencies between 3 Hz and 3200 Hz. The dependence of the complex susceptibility on temperature below theλ point is given by a Debye function. Above this temperature, however, deviations occur. The temperature dependence of the relaxation time forT<T _λ can be described byτ～T ^?n where 1.82≦n≦2.35 for 470 Oe≦H≦3360 Oe. At the highest temperatures Orbach Processes occur over the first excited crystal field component which according to these measurements lies atE _II=k(56±10)°K. In the entire temperature range the relaxation processes are determined by further relaxation mechanisms in addition to the spin lattice relaxation. The nature of these could not, however, be determined.     

Über die Abhängigkeit des Polarisationsgrades vom Streuwinkel bei Mott-Streuung langsamer Elektronen

The angular dependence of polarizationP(Θ) of electrons elastically scattered by a beam of mercury atoms is measured in a double scattering experiment for electron energies of 0,9; 1,2; and 1,5 keV. The results are compared with theory and found to be in good agreement. Maximum degree of polarization measured isP=0,53±0,10 (electron current 10^?12A).