An extensive 22 GHz survey of H 2 0 masers has been made with the Parkes 64 m radio telescope. We present data here for 68 sources, of which almost one-half were discovered in the present series of observations. The telescope beamsize, 100' arc to half-power, has enabled us to improve the position measurements of most of the known sources and in a few cases has allowed us to resolve a 'source' into several spatially separated components. The wide velocity coverage of our observations (> 200 km s -1) has led to the discovery of several interesting new high-velocity features. In addition to details for each source, the general properties of the sample are discussed. In the typical interval of 8 months between successive observations, the peak intensity of many sources varied only slightly (by less than a factor of 2); a few sources, however, showed more dramatic variations.

The discovery of three new H20 sources associated with HII regions is reported. Observations of other southern H20 sources show, in several instances, considerable intensity changes over a 20-month interval.

We present details of 53 H20 masers located in the galactic plane between longitudes 260· and 326·. Twenty-one of these are new ones found at the sites of recently discovered OH masers. The properties of the individual masers are discussed, in particular the variability of sources known for several years and the high velocity emission exhibited by some masers. The association of H20 and OH masers has been investigated and out of 36 OH masers in this region of sky, a remarkably high number (34) have nearby H20 masers.

The neutron transformation of helium-3 has been applied to neutron spectroscopy in the medium energy range in order to overcome the disadvantages of previous methods which have usually been based on neutron scattering. By observing the pulse-height distribution when a proportional counter filled with helium-3, krypton, and carbon dioxide is irradiated with neutrons, the spectrum can be obtained. Details of the counter construction and vacuum equipment for filling the counter are described. In the range of energy from thermal to 1 Mev the spread (relative standard deviation of a peak) is about 3% and the wall effect agrees with theoretical predictions. The pulse height varies linearly with energy and some evidence suggesting that the mean energy required to produce an ion pair in krypton for helium-3 nuclei is about 6% greater than that for protons is presented. Since the neutron reaction cross section of helium-3, as a function of energy, is required to evaluate neutron spectra from observed distributions, it has been remeasured in the range 120 kev to 1 Mev with greater accuracy than previous measurements made by J. H. Coon [Phys. Rev. 80, 448 (1950)].

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Twenty-five new H2 0 masers in the southern galactic plane have been discovered using a 22 GHz maser receiver on the Parkes telescope. The search was conducted at the locations of all the known 1665 MHz OH masers in the longitude range 340° through the galactic centre to 2°; of the 46 type I OH masers 38 are now found to have an H20 maser nearby; three other main-line OH emitters, possibly unusual late-type stars, also show H2 0 emission. We give an up-to-date tabulation of all 46 currently known H2 0 masers in the region which includes for completeness several H 2 0 masers with no readily detectable associated type I OH emission.Although a few very active regions of star formation harbour several masers, most commonly there is one, and usually only one, site of H2 0 maser emission near each OH maser. In a few wellinvestigated instances an H 2 0 maser is found to be slightly displaced spatially from its OH maser counterpart; this fact leads us to propose a 'binary' origin for such displaced pairs (perhaps applicable to most other H 2 0/OH pairs) in which the OH maser/compact HII region/new star is one of the pair, and a dense molecular cloud of stellar mass, not necessarily undergoing transformation to a star, is the other component giving rise to the H 2 0 emission.