Table of contents

The work of a previous paper on the effect of coupling between elastically and inelastically scattered waves for the case of E0 excitations of the nucleus throughout its volume has been extended to E2 excitations. The inclusion of coupling causes inelastic cross sections to deviate even more from the distorted wave values for E2 excitations than for E0 excitations. The effect is greater for a volume interaction than for a surface interaction of equivalent strength.

Knock-on electrons and direct electron pair creation by cosmic-ray muons in thin lead plates were investigated with a multiplate cloud chamber. The muons traversing the cloud chamber were classified into two energy intervals, viz. 450 to 1700 MeV and higher than 1700 MeV. By comparing the experimental results with theoretical predictions which were derived for the knock-on process, an effective range-energy relationship was found for electrons in lead with energies up to about 200 MeV. The experimental distributions of the knock-on production as a function of the energy transfer compare well with theoretical distributions calculated from Bhabha's formula for muons in both of these energy ranges. The comparison between the theoretical and experimental results is also satisfactory for the energy distribution for direct pair production by muons with energy above 1700 MeV. The theoretical distribution was calculated from a formula deduced anew from the differential cross section given by Murota, Ueda and Tunaka. The experimental results favour a value between 1 and 2 for the indefinite constant α of the theory.

The third-order approximation is used to derive simple analytical expressions for the partial cross sections corresponding to the scattering of high-energy charged particles by a schematic model atom involving matrix elements having a r^-2 radial dependence.

An electron density map calculated by Rollett from an accurate function of James and Coolidge has been used to obtain the coherent x-ray scattering from molecular hydrogen. The results given by the Coulson self-consistent field function are also presented, and it is shown that there is excellent agreement with the James-Coolidge results and with earlier calculations of Carter, March and Vincent.

In addition a more careful study of the incoherent scattering has been made than has previously been attempted. By considering two correlated wave functions, that of Eckart and Hylleraas for helium and that of Gurnee and Magee for hydrogen, previous approximations are shown to lead to somewhat high estimates of the total scattered intensity. For hydrogen, a maximum reduction of about 4% brings theoretical results to within 1% of the experimental values of Wollan.

The momentum distribution and the corresponding shapes of the Compton lines have also been considered for hydrogen and helium. It is suggested from the results obtained for molecular hydrogen that more accurate measurements of the Compton profile might afford a useful method of studying bonding electrons.

Measurements are presented of the gamma radiation produced when lithium, beryllium, boron, carbon and nitrogen are bombarded with 150 MeV protons, and the results are discussed with reference to the information they provide about the structure of the nuclei involved. The results include measurements of the residual states produced in (p, 2p) and (p, pn) reactions, which are, to a first approximation, measurements of the parentage of the ground state of the bombarded nucleus. These results are not consistent with a simple jj-coupling model of these nuclei and in one case where calculations are available, are consistent with an intermediate coupling model. It is shown that inelastic scattering cross sections are determined by the same matrix element as radiative transitions so that inelastic scattering is a powerful tool for picking out strong radiative transitions to nuclear ground states.

Some recent results of Vainstein, using the distorted wave approximation for the 3s-3p transition in electron-sodium atom collisions are modified on the basis of a method introduced by Seaton in 1961 which ensures that the conservation condition is satisfied. The resulting cross sections lie much closer to those obtained by experiment.

In pure silver bromide emulsions containing tabular grains some are found in which one part appears to have been rotated with respect to the other about an axis perpendicular to the large (111) faces. To ascertain whether such grains were genuine, possible angles of rotation and their frequency relative to one another were deduced on a simple theoretical basis. Similarity in calculated and observed angles, and in the calculated and observed relative frequencies of occurrence, as well as the simple fact that the angles fall into well-defined groups, show without doubt that such grains are not artefacts but special forms of misorientation. It is shown that the distribution of photolytic silver is profoundly different in such grains from that observed in normal grains.

Time resolved absorption studies have been carried out through the hot gas behind reflected shock waves during their interaction with powdered barium compounds A capillary-type flash tube with a short pulse duration and high brightness temperature was used as the background source.

A new system of BaO, probably resulting from a B(?¹II)-X¹σ transition, has been found in absorption, and vibrationally analysed. The constants derived for the upper state of the system were T_e = 32866.4 cm^-1, ω_e = 488 cm^-1 and ω_ex_e=3.6cm^-1.

A band of zinc oxide possibly due to a polyatomic emitter has been recorded in emission from the luminosity produced during the shock excitation of powdered zinc compounds.

It is pointed out that the rate of capture of electrons and holes by charged centres should depend markedly on electric field strength when this is high enough to produce hot electrons. In particular the capture rate for an attractive centre should decrease while the rate for a repulsive centre should increase as the field rises. Each effect will alter the steady state density of carriers. For the case of repulsive centres a decrease in carrier density will occur and the possibility of using this phenomenon to produce a negative resistance state in the semiconductor is discussed. Suitably compensated n-type germanium crystals containing copper, nickel or gold appear to have the requisite properties for exhibiting negative resistance at temperatures low enough for the thermal ionization rate of the centre to be small. It is pointed out that the long time constants involved in the process preclude the use of short pulse fields.

The existing theories of change in activation energy with impurity concentration are discussed and their limitations pointed out. The Ionization energy of a pair of impurities in a dielectric medium is calculated as a function of impurity separation Assuming a random distribution of impurities, an averaging process is employed to calculate the mean change in activation energy with concentration. This approximation should be poor for high impurity densities but good for low densities. Specific calculations have been made for the case of phosphorus impurities in silicon and satisfactory agreement is obtained with experimental results

Electrical resistivity and magnetic susceptibility measurements are reported for U-70 and 74at.% Zr in both the γ and δ phases. Negative temperature coefficients of resistivity were found in the metastable γ phase of U-70at.% Zr, and also in the δ phase of both alloys between 90° and 870°K. The Pauli weak spin paramagnetism, exhibited by both specimens, always increased with temperature.

The results support the view that the δ phase is primitive hexagonal and not an ordered form of the body-centred cubic γ phase. The anomalous resistivity behaviour may be a consequence of the band structure of the alloy.

Measurements of magnetostriction on a single crystal of antiferro-magnetic nickel oxide were made in fields up to 21 kilo-oersteds. The order of magnitude of the magnetostriction is the same as that for ferromagnetic materials, viz about -20 × 10^-6 and a square law relation, with field, is obeyed up to several thousand oersteds before a gradual saturation tends to set in. The dependence of strain on field and measuring directions is described with respect to axes that constitute cylindrical symmetry.

Experiments are described which establish that the electric strength of sodium chloride decreases slowly with increase in specimen thickness. Flat topped impulses were used and the time lag to breakdown of each specimen was measured. The mean statistical time lag depended on the nature of the cathode, and it rapidly decreased with increase in the size of voltage increment. The characteristics of breakdown are similar to those predicted by the single avalanche theory of Seitz according to which disruption results from the passage of an electron avalanche exceeding a critical size.

An accurate and convenient method for measuring photofluorescence decay times in the nanosecond region is described. Results are given for a number of organic compounds and reasons for differences with previous results are discussed. For anthracene, a value of 24.2 nsec has been measured for a 1 cm cube crystal. Some previous results for quantum efficiencies are corrected on the basis of these measurements and apparent inconsistences examined with regard to the mean free path of excitons in very thin crystals.

The four crystalline field parameters of the Dy³⁺ ion in the ethyl sulphate, V₂⁰, V₄⁰, V₆⁰ and V₆⁶ are fitted at 4.2°K and 58°K to give the known level splittings and g values. It is found that V₂⁰ similar 125 cm^-1, V₄⁰ similar -26 cm^-1, V₆⁰ similar -31 cm^-1, and V₆⁶ similar 490 cm^-1, and there exists a slight variation with temperature between 4°K and 58°K. These values are, however, in serious disagreement with those obtained by the extrapolation procedure of Elliott and Stevens. In particular, V₂⁰ is extraordinarily large, in agreement with the previous work of Judd on europium ethyl sulphate. An empirical variation of V_n^m with atomic number is given which is in marked contrast to that predicted by Elliott and Stevens.

It is shown that the space correlation function of any space-dependent atomic property may be used to estimate the degree of order in a solid. The correlation function is evaluated for an Einstein solid and is shown to have both long-range order and short-range order components. The thermal scattering and band structure of an Einstein solid is treated by means of time-dependent perturbation theory on the loose-binding approximation, and it is shown that the scattering matrix is related to the spectrum of the potential, the short-range and long-range components giving rise to thermal scattering and energy gaps respectively.

The width of the gaps is found to decrease exponentially with temperature; for a one-dimensional model of sodium the temperature coefficient is found to be about 10^-3 per degree. On a Mott-type one-dimensional model of liquid sodium this implies a 20% contraction of all energy gaps on melting.

Range-energy relations for 8.78 MeV alpha particles in ethyl alcohol and carbon tetrachloride in the liquid and vapour states have been determined experimentally using nuclear emulsion and scintillation counting techniques. The range in liquid alcohol was found to be greater than the value previously published. The stopping power for both liquids was found to be different from that of the corresponding vapour, and it is suggested that polarization of the absorbing medium is mainly responsible.

Correction to Proc. Phys. Soc.77, 103

The time distribution of spurious pulses in counters with treated and untreated cathodes was studied by pulse interval analysis, and plateau characteristics, charge per pulse and Geiger threshold potential are plotted as functions of the xenon concentration. In all the counters investigated, the presence of a minimum threshold indicates an optimum xenon concentration of 3.5%. Measurements of the corona threshold with argon and argon-xenon filling mixtures show a pressure dependent increase in gas multiplication with addition of xenon. This increase in ionization is tentatively explained by the hypothesis of two-body collisions between xenon atoms and argon excited molecules leading to the formation of the molecular ion AXe⁺. Mobilities obtained from transit time measurements identify the ions involved as A₂⁺ (1 82 cm sec^-1 (v cm^-1)^-1), possibly NO⁺ (2 45 cm sec^-1 (v cm^-1)^-1) this ion appearing after the counters have been in continuous discharge, and an ion of mobility 1 64 cm sec^-1 (v cm^-1)^-1, which is not inconsistent with the postulated AXe⁺ molecular ion.

The cross section of the reaction ²³⁶U(n, γ) ²³⁷U has been measured for neutrons in the energy range 0.3 to 4 0 MeV using an activation method. Fission products in the irradiated samples were removed chemically and the ²³⁷U produced was determined by counting the γ-ray activity with a sodium iodide scintillation spectrometer. The cross section was found to increase from about 300 mbn at 0 3 MeV to about 400 mbn at 1 1 MeV. It then fell sharply to some 30 mbn at 4 MeV.

The shape of the excitation function for this reaction is typical of that found for other heavy even-even nuclides far from a closed shell. It is due to the competition between the radiative capture process and inelastic scattering from the similar collective level structures of these nuclides.

In measuring T₂ by the Carr-Purcell method the number of 180° pulses which can be used in a given sequence is limited by the accumulation of errors in pulse length, which causes a progressive departure of the freely precessing magnetic moments from the equatorial plane. Several methods have been devised to overcome this limitation, but in all cases it is assumed that phase coherence exists between the precessing nuclei and the pulsing radio-frequency field. Under such conditions the errors will accumulate linearly in the orthodox Carr-Purcell experiment, but in fact the order of stability implied in the assumption is unlikely to exist unless special care is taken to obtain it. It has been found experimentally, however, that good Carr-Purcell trains can be obtained in an incoherent system and in the present paper the reasons for this are analysed. It is shown that in such a system the errors add as if they were coplanar vectors of random direction, so that the statistical r.m.s. value of the error after n pulses is proportional to radicaln. Hence, one would expect to obtain a satisfactory train with up to 10⁴ pulses, and it is shown that this is in agreement with observations.

The splitting of the ground level of Fe³⁺ in (CH₃NH₃)Al(SO₄)₂. 12H₂O in the absence of a magnetic field has been measured directly. At 90 °K the separation of the S_Z = ± 3/2 states from the ±1/2 is equivalent to 12 227 ±5 Mc/s, and that of the ±5/2 from the ±3/2 to 22 043 ±10 Mc/s. The deduced values of the coefficients in the usual spin-Hamiltonian for the ferric alums are D = +0.1893 ±0.0002 and a - F = +0 0171 ±0.0001 cm^-1, and it is probable that a similar, equals +0.013, F similar, equals -0.004 cm^-1. The material is promising for a zero-field maser, with predicted amplifying and pumping frequencies of 12 290 ± 15 and 34 460 ± 45 Mc/s respectively at liquid-helium temperatures.

It is shown that the first order magnetocrystalline anisotropy constant K₁ of a `square-loop' ferrite material can be determined from measurements of the approach to saturation of magnetization in a polycrystalline specimen. The energy (γ = 0.20 erg cm^-2) and the thickness (δ similar 1.6 × 10^-5 cm) of 180° domain boundary walls in a commercial manganese magnesium ferrite, Ferroxcube D.1, are estimated from results obtained in this way. The temperature variation of coercivity in the same material is also discussed in the light of these results, and it is found that the Goodenough theory of domain creation is inapplicable. Pores were observed in the material. Although these have a diameter greater than the estimated domain wall width δ, the experimental results presented are not consistent with the Néel disperse field theory.

Measurements of the initial permeability of a commercial square-loop ferrite (Ferroxcube D.1) as a function of frequency and temperature are reported. These show the presence of a relaxation, which is due to the damping of domain boundary walls in the material by a process of electron diffusion. It is found that all the domain walls in the material are not affected by this process. The presence of the relaxation does not allow the dynamic characteristics of the domain walls, important in conventional pulsed-reversal experiments; to be determined by the method employed here.

Formulae are obtained for the distribution function f(r₁₂) of the interelectronic distance r₁₂ in the ground state of helium, and for the mean value of any positive integral power of r₁₂. Five different wave functions are considered, showing varying degrees of electron correlation. The greater the degree of correlation the larger is the mean electronic separation. Curves of f(r₁₂) are plotted, and from them the size and depth of the Coulomb hole are evaluated. There is a close correlation between the mean value of 1/r₁₂ and the error in the energy as calculated with these five wave functions.

The anisotropy of the electrical conductivity of Bi₂Te₃ at room temperature has been determined by a four-probe method. The anisotropy ratio for undoped p-type material is close to 3.0; for iodine-doped n-type material it is appreciably higher and increases with the impurity concentration. Measurements of the galvanomagnetic effects in highly doped n-type Bi₂Te₃ have shown that the observed variation of the anisotropy ratio can be interpreted in terms of a change of shape of the equi-energy surfaces in momentum space with increasing density of charge carriers.

The electron and photovoltaic effects in silicon and selenium elements are studied experimentally. It is shown that the response to cathode rays is closely related to their penetration into the cells, and indicates data such as the depth of the barrier and the diffusion length of minority carriers. In particular, it is shown that electrons in grey selenium are mobile, with a diffusion length of about 8 microns.

A brief description is given of some fatigue effects encountered.

A new phenomenon is reported which is concerned with the excitation of fluorescent solutes in organic liquid dielectrics when high electric fields are applied. The characteristics of the emitted light have been studied and related to the pre-breakdown conduction current and the electric strength. Dielectric liquids showing this effect have been synthesized by dissolving known concentrations of fluorescent materials in simple organic liquids. These have confirmed the exeprimental results obtained with more complex transformer oil. Also, experiments are described which show that even highly degassed liquids exhibit a pressure dependence of electric strength if brought into a state of super-saturation by applying tension to the liquid.

Electron shadow patterns have been used to study the configuration of domains present in single crystals of haematite (α-Fe₂O₃) in the temperature range 0 to 700 °C. The observations were made using a furnace designed to fit into the specimen chamber of the electron diffraction camera.

It has been found that once a crystal has been heated to above its Curie temperature the configuration of domains present in the specimen at room temperature is partially stable. Repeated heat treatment of the crystal to above its Curie point leads to a partial `memory effect' similar to that described by Blackman, Haigh and Lisgarten in 1957 for the low temperature transition in haematite.

A study of the variation with temperature of the sizes of features in the shadow patterns provides a method of determining the Curie temperature of haematite crystals with an accuracy to 2 to 3 degrees. In this way Curie temperatures between 643 and 688 °C have been observed for crystals from different origins.

Finally an attempt has been made to correlate the Curie temperature of various crystals with their lattice parameters as determined by x-ray powder photographs.

The small heat changes accompanying the magnetization of 99.9% pure gadolinium were measured on a rod specimen. The results are analysed, and the principal magnetization processes are identified. Domain wall movement which occurs in low fields is discussed in the light of a theory by Goodenough, and the coercivity which the theory predicts is found to agree fairly well with experiment.

Comparison is made with earlier results for cobalt.

The paramagnetic resonance spectrum of divalent manganese has been investigated in the crystals CdF₂, KMgF₃ and CdTe which have cubic symmetry, and in the crystals ZnO, MgCl₂, and CdCl₂ which have axial symmetry. The measured cubic field splitting parameter a is compared with values predicted from recent theoretical work.

The matrix element is calculated (1) for orthogonalized plane waves assuming that the ion moves rigidly and (ii) for the equivalent plane waves on the assumption that the net effective potential (Hartree-Fock potential plus pseudo-potential) moves rigidly with the ion. It is shown that these two results are equivalent if the pseudo-potential operator can be treated as a localized potential.

The distortion approximation is applied to the processes H⁺(or He²⁺)+He(1s²)¹S->H⁺(or He²⁺)+He(1s,2p)^1P H⁺(or He²⁺)+He(1s²)¹S->H⁺(or He²⁺)+He(1s,3p)^1P cross sections being obtained for incident proton energies in the range 10 keV to 1000 keV and for incident alpha particle energies in the range 40 keV to 4000 keV. For proton impact the maxima cross sections are found to be 0.18πa0² and 0.035πa0², at 88 keV and 124 keV respectively; and for alpha particle impact they are found to be 0 61 πa0² and 0.101πa0² at 511 keV and 935 keV respectively.

By using an internal lithium deuteride target in the Harwell cyclotron, and `hardening' the resulting scattered neutrons with polythene, a low intensity beam of neutrons with an energy peak at 50 MeV was produced. This beam passed through a 9 cm hydrogen bubble chamber and produced in it reactions of the type n+p->d+γ which were distinguished by the range-angle distribution of the deuterons from the background of elastic n-p scatters. A cross section of 110±60 μbn was found for the process.

The Manchester spectrum of muons arriving vertically at sea level is discussed in terms of the processes leading to the formation of the sea-level hard component of cosmic radiation. Evidence is presented to show that above a momentum of 20 GeV/c there are particles, probably muons, at sea level which cannot be explained in terms of π-μ decay only.

It is shown that a definite energy gap exists in a one-dimensional liquid provided that the maximum deviation of the distance between atoms is less than a critical value. Higher gaps successively appear as the disorder is further reduced.

The absorption coefficient of gaseous ozone has been measured for the mercury lines at 2537, 2894, 2967, 3021, 3342, and 5770 Å, by measuring the optical absorption and the pressure as the ozone decays in a sealed quartz absorption tube. This avoids the chemical determination of the ozone. The determination gives independent and absolute values of the absorption coefficients, and their standard deviations are 2½% or less. Previous measurements of the absorption coefficients disagree by as much as 25%, but the present results generally agree within the limits of error with those of Inn and Tanaka.

It is shown that the methods proposed by Montroll and Kroll for deriving the lattice frequency spectrum from the experimental specific heat curve will in practice yield only a `smoothed' spectrum, averaged over a range of 1.6: 1 or more in frequency. Any practicable improvement in experimental accuracy will reduce this smoothing only slightly. The same smoothed spectrum can be derived more simply by a procedure involving only one quadrature, and a still simpler trial-and-error method is also described and illustrated.

A method is described for determining the inter-electrode gas temperature at any chosen time following the current interruption of an arc discharge, using a rotating mirror camera to measure the velocity of a shock wave passing through the gas. From a series of such measurements the rate of temperature decay may be found, and results are given for times between 100 μsec and 1 sec after interrupting arcs in air between carbon electrodes. The average rates of decay of the mean core temperature during 1 msec following current interruption were 1.5 deg μsec^-1 for an 8 mm gap (10 or 40 A) and 2.8 deg μsec^-1 for a 2 mm gap (40 A). The temperature decay subsequent to a 40 A arc has also been calculated from a knowledge of the gas properties and with the assumptions of cylindrical symmetry and energy loss by thermal conduction only.

The apparatus and method are described by which the separation of electrostatic charge between similar polyethylene spheres, initially at different temperatures, during an impulsive contact has been observed.

The amount of such a charge separation varies during a series of observations both progressively and randomly. A component of this charge separation is shown to depend upon the direction of the thermal gradient across the interface. The progressive variation in charge separation with number of impacts is shown to be consistent with a transfer of `active centres' from one surface to the other. The quantity of charge which separates between two surfaces is also shown to vary with different contact areas on one surface.

A description is given of apparatus designed to measure the crack propagation energy of plastics under conditions where they are brittle. This includes a brief history of the work leading up to the present studies and details of a novel type of instrument suitable for measurements over a range of temperatures. Use of these techniques is illustrated by typical results on Perspex, polystyrene and polythene.

In addition, some of the interesting features found on the surfaces of cracks are described. The patterns found on cracks in polystyrene are illustrated and their possible origins are briefly discussed. Finally the colours found on freshly formed cracks in Perspex are described.

The friction, wear and surface deformation in abrasion of typical very plastic non-metallic materials, AgCl and AgBr, are investigated, on emery papers at 200-500 g load. They are strikingly similar in form and magnitude to those of metals of comparable hardness. The wear rate M is related linearly to the friction coefficient μ when the emery particle diameter D is greater than 20 microns, but on finer abrasive the (M,μ) locus curves to lower μ, as for soft metals.

Optical microscopic study of single abrasion grooves shows the wear to be mainly by a cutting action like that of a lathe tool, but that much plastic flow occurs; and it indicates a high adhesion component of the friction.

Electron diffraction shows a backwardly tilted <110> orientation in the abraded surface region. Its form, and its preferred azimuthal range, shows its close relation to the rolling texture and to the compression texture when extensive lateral flow is permitted. The correlation between the tilt δ of the orientation axis from the specimen normal and μ is discussed.

This paper describes some recent measurements on the angular scattering properties of the lunar surface at the wavelength of 10 cm. The scattering law of the surface is shown to be P(θ) = σ₀ exp(-10.2θ) over the range of θ from 4 to 13° and a comparison with other results shows that it is independent of wavelength up to λ = 1.5 metres. For angles up to 30° the scattering law deviates from an exponential but is still independent of wavelength up to at least λ = 75 cm. Evidence is given to show that the surface may be considered as rough at radio wavelengths.

A method of synthesizing electrical circuit filters to transmit prescribed functions of frequency in a specified pass-band is applied to dielectric interference filters containing up to four half-wave layers. The method relies on an approximation that the half-wave layers are the only parts of the filter which are sensitive to frequency. The reflecting quarter-wave multilayers are described by a single parameter whose values are deduced in the synthesis. The transmission functions are either maximally flat in the pass-band or contain equal ripples. The rate of transition from the pass-band to the stop-band is increased rapidly by increasing the number of half-wave layers.

The synthesis is applied to filters specified to transmit an 8% bandwidth within which the transmissivity is not less than 0.91. The exact transmission characteristics of these examples show excellent agreement in the pass-band with the simple functions.

The method can be applied to the design of filters of moderate bandwidth for any spectral region.

Polycrystalline specimens of InAs-In₂Se₃ and InSb-In₂Se₃ alloys have been prepared by suitable annealing of ingots. Measurements have been made of the room temperature values of Hall effect, conductivity, thermoelectric power and infra-red absorption over the whole of the available range of solid solution, and hence values of carrier density n, mobility μ and optical energy gap E_g determined. It is found that the behaviour is similar to that of the corresponding In₂Te₃ alloys in that the alloys rich in A^IIIB^V are n-type and highly degenerate. The observed values of n for the four systems concerned (A^IIIB^V-A₂^IIIB₃^VI) are compared, and possible explanations of the results in terms of solubility of tellurium and selenium in the A^IIIB^V compounds, or the band structure of the A^IIIB^V compounds are discussed. The use of thermoelectric power data to give values of the Fermi level is considered, and it is shown that to give consistent results in the composition range where ionized impurity scattering occurs, a scattering relation of the form τ is proportional to E^+1/2 must be used. The results suggest that the InAs-In₂Se₃ system may show a change in conduction band minimum at a composition of low In₂Se₃ content.

Expressions for the growth and decay of cathodoluminescence intensity are derived for phosphors which show exponential decay schemes under ultra-violet excitation. A model is used in which excitation of luminescence centres can take place either by direct interaction with primary electrons or indirectly by transfer of energy from excited lattice electrons. Two systems are considered: (i) with monomolecular recombination in the lattice and monomolecular recombination in the centres, (ii) with biomolecular recombination in the lattice and monomolecular recombination in the centres. Comparison between the theory and published experimental data indicates that such a model with bimolecular recombination in the lattice is compatible with many of the available experimental results.

Relative integrated photoelectric intensity measurements have been made upon each of the ν' = 0, νdouble prime = 9-19; ν' = 1, νdouble prime = 8-12; 16-20; ν' = 2, νdouble prime = 7-9, 15, 16, 19-21, bands of the Schumann-Runge System. These measurements have been interpreted, with the aid of Franck-Condon factors q_{ν'νdouble prime} and r-centroids r_{ν' νdouble prime}, to determine the variation of electronic transition moment R_e(r) with internuclear separation r as R_e(r) = const. (1 - 1.1807r + 0.35047r²); 1.44(Å) < r < 1.76(Å)

A `smoothed' array of relative vibrational transition probabilities p_{ν'νdouble prime} = R_e²(r_{ν'νdouble prime})q_{ν'νdouble prime} and band oscillator strengths f_{ν'νdouble prime} have thereby been determined for all bands of the system between 2450 and 5000Å.

Three photon annihilations of ortho-positronium in gases as a function of pressure have been investigated using a scintillation spectrometer technique. Quenching rates per atmosphere were determined for argon (0 035λ₀), neon (0.024λ₀), helium (0.014λ₀), oxygen (3λ₀), nitric oxide (400λ₀), and carbon dioxide (0.08λ₀), the last three being only very approximate. λ₀ is the natural annihilation rate of ortho-positronium, 0.7 × 10⁷ sec^-1. The high quenching rates in oxygen and nitric oxide are attributed to ortho-to-para conversion by electron exchange. The order of magnitude of the smaller quenching rates is understandable if the positronium atom annihilates with an electron of a gas molecule during collision (pick-off quenching).

Graphite formed by cracking methane under various conditions has been exposed to neutron irradiation. Changes in electronic properties were measured: (i) after radiation damage by neutron doses of the order 10¹⁷ neutrons/cm², (ii) after `mild' annealing around 470 °C, and (iii) after `strong' annealing at 2200 °C and above. Effects of radiation damage are proportionately greatest in specimens whose properties are initially nearest to those of ideal graphite. Some discrimination can be made between the types of disorder originally present, and the disorder added by irradiation.

Changes of thermoelectric power of graphite in the direction of the c axis show `latent' increases attributed to disorder frozen in until after mild annealing.

A direct determination has been made of the Fierz interference term b for the pure Gamow-Teller decay of ⁵⁸Co. The K/β⁺ ratio has been determined by an internal source scintillation counter coincidence method. The value obtained was 4.92 ± 0.09. This may be compared with the recent predicted value of 4.87. Allowing for the variation in the theoretical value arising from β⁺ end point uncertainty this yields a value for b of (+0.6 ± 2.3)%. Two interpretations of the result are possible on the basis of two-component neutrino theory. If, as is customarily assumed, C_A = C_A', C_T = -C_T', then b should vanish identically; and the small experimental value for b confirms the theory. If alternatively, C_A = C_A', C_T = +C_T' then the experimental result requires that C_T, C_T' be very small.

The resonant scattering cross section for the 673 kev electric quadrupole transition in ¹³²Xe was directly determined for a particular arrangement of source, scatterer, and detector. The resonant scattering condition was provided by the persistence of nuclear recoil following a preceding transition when the source was in the gaseous phase. The differential cross section for a mean scattering angle of 149° was evaluated as σ(149°) = 1.75 × 10^-26 cm² sterad^-1, and the value τ = (9.7±3.0)×10^-12 sec was found for the mean-life of the level. This result is in good agreement with that obtained from a Coulomb excitation measurement. The enhancement of the E2 transition probability when represented by the ratio B(E2; 0 → 2)/B(E2)S.P., has the value 20; this is consistent with the results for even-even nuclei which have characteristic vibrational spectra.

A nuclear resonant scattering method was used to estimate the mean-life of the 638 kev level in ¹³¹Xe. The result of 4 × 10^-13 sec, when compared with the Weisskopf estimate, is consistent with an enhanced E2 transition or a retarded M1 transition. However, in reported Coulomb excitation experiments there is no evidence for an E2 transition probability consistent with the present mean-life, and it is suggested that the transition may be predominantly M1.