The possibility of observable recombination lines below 200 MHz is shown to be greatest for the case of negative absorption in low-density gas in front of strong radio sources. The amplification of background radiation can conceivably exceed 100 per cent, especially when the electron temperature is low; on the other hand if the temperature is sufficiently high, dielectronic recombination may give rise to detectable recombination lines from elements other than hydrogen. The optimum frequencies are in the range 50–200 MHz for the interstellar medium, and 1–100 MHz for such intergalactic matter as may exist. The effects of external radiation fields, thermal and nonthermal, are explored quantitatively and the relevantb_n-factors computed for a 1000-level atom; it is found that even the general galactic radiation field will significantly alter the atomic level populations. Observations of these masing low-frequency recombination lines would be useful in sorting out the hot and cold components of the interstellar medium, and in investigating their properties.

The improvement terms in the generalised energy-momentum tensor of Callan, Coleman and Jackiw can be derived from a variational principle if the Lagrangian is generalised to describe coupling between ‘matter’ fields and a spin-2 boson field. The required Lorentz-invariant theory is a linearised version of Kibble-Sciama theory with an additional (generally-covariant) coupling term in the Lagrangian. The improved energy-momentum tensor appears as the source of the spin-2 field, if terms of second order in the coupling constant are neglected.

The angle integrated exact expression for the projected energy is derived from two different expansions of the rotation operator. In one, the spin matrix polynomial expansion method is used while in the other the disentangling theorem for angular momentum operator is used.

Information on the low-lying levels up to ∼1.9 MeV excitation of the doubly odd nucleus⁵⁰V has been obtained through the Ge (Li)-Ge (Li) coincidence study with the⁵⁰Ti(p, nγ)⁵⁰V reaction. Branching ratios have been measured and tentative spin-parity assignments have been made. A detailed comparison with other measurements reported recently has also been made. Using the lowest seniority wave functions with (f_7/2)_p³ (f_7/2)_n⁻¹ configuration, energy levels and electromagnetic properties have been calculated. These have been compared with the present and earlier experimental data.