The motion of hydrogen atoms due to the internal vibrational modes of a molecule is large and curvilinear. Because of the curvilinear motion the mean displacements of these atoms cannot be described by ellipsoids alone, but higher order terms or cumulants are necessary. Equations determining these terms from the usual riding motion are presented. By analysing a neutron diffraction study of high accuracy it is shown that the size of these higher cumulants is sufficient for them to be determinable. For deuterated samples however really high accuracy will be necessary in the diffraction experiment.

Theoretical and experimental studies have been made of the scintillation pulse shapes of organic scintillators. In a unitary crystal the pulse shape is characterised by the scintillation decay time τs. In solution scintillators additional parameters are required to describe the pulse shape: Δt, the pulse width (FWHM); tm, the time at which the intensity reaches a maximumpmax; and t½, the time at which the intensity reaches ½pmax. General relations are derived for τs, Δτ, τm, τ½, and pmax for binary liquid solutions in terms of molecular parameters, and the corresponding expression is given for the scintillation pulse shape of a binary plastic solution.

The scintillation pulse shapes of 4 binary liquid solutions and 12 binary and 15 ternary plastic solutions were measured using a single-photon sampling fluorometer. The results confirm the theoretical analysis. A new plastic scintillator (NE 111) with improved timing characteristics has thus been developed, and some of its applications in nuclear physics are briefly discussed. The time association of nuclear events can be measured to within 130 ps with this scintillator, the limiting factor being the time jitter of the photomultiplier itself.

Norman Feather was one of the first physicists to realise the power of the coincidence method for elucidating nuclear level schemes. He observed coincidences occurring during the sensitive time of a Wilson cloud chamber in the 1920s. This note reports an application of some recent advances in coincidence technique in an attempt to set upper limits to the intensities of some unconfirmed γ-rays of 144Pr, which are indicated by broken lines in text-fig. 1.

This paper gives an account of some numerical methods which have been applied to solve the equations of second order stability theory in the flat plate boundary layer.

Measurement of the frequencies, linewidths and cross-sections of optical phonons in neodymium aluminate by means of inelastic light scattering have permitted the prediction of a trigonal-to-cubic phase transition below the melting point of the crystal. This phase transition was subsequently directly measured via differential thermal analysis.

The interpretation of optical interference experiments at low light intensities and with independent light sources is discussed. A new experiment on interference between modulated light beams is described briefly, and its significance is indicated. There are two appendices, one a fuller discussion of the conditions under which interference and intensity correlation may be produced, the other a comment on one aspect of Jánossy and Nagy's discussion (1956) of the feasibility of the alternating slit interference experiment.

This paper shows that the coherence function associated with a band-limited optical spectrum is expressible as an infinite product involving its zeros. Only a finite number of zeros are physically significant and these can be determined from measurements of the visibility of interference fringes; but an ambiguity remains in the sign of the imaginary part of each zero. If the spectrum is to be recovered using the visibility measurements, an auxiliary experiment is needed to supply the necessary signs, but it need not be especially accurate. If the signs are not known there is only a limited number of different spectra that are compatible with the visibility measurements. Finally it is suggested that wavelength measurements made on an asymmetric spectral line with a Michelson interferometer may yield differing results when used with long and short delays.

The possibility of α-particle emission from fully accelerated fission fragments is considered. The results of trajectory calculations for such α-particles emitted during the fission of 252Cf are presented, and their significance in relation to the observed energy and angular distributions of the α-particles is discussed.

Mirror Gamow-Teller β-decay takes place from members of isospin multiplets to a common final state or to final states that are mirrors of each other. It is found that in these cases the intrinsic speed of the positon emission is systematically lower, by 10–15 per cent, than that of the mirror negaton emission. This difference could be due to a fundamental weak interaction effect, a second-class term in the weak hadronie current, or to a failure of exact symmetry in the nuclear structure. The influence of binding energy differences is studied in the latter context and it is concluded that they cannot be responsible for the empirical asymmetry. The effects of changes of configurational mix across the multiplets is also considered; very few estimates have ben explicitly made but it would be surprising if this effect turned out to be of constant sign and magnitude over the wide range of A involved. No present firm conclusion can be made but none of the empirical evidence is inconsistent with the reality of a largely momentum-transfer-independent second-class current.