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The Doppler-broadening of the positron annihilation line in copper has been studied with a Ge(Li) detector as a function of temperature. The proportion of annihilations occurring with free electrons has beem measured by analysing the shapes. Various hypotheses have been assessed by parameter optimization. The best value of the vacancy formation energy is estimated to be 1.26+or-0.07 eV.

The preparation of lithium fluoride with thermoluminescence properties suitable for use in radiation dosimetry has been investigated. Variants based on magnesium and titanium additions have been prepared and examined from the viewpoint of activator concentration, role of secondary impurity and purity of the lithium fluoride starting material. The following properties were examined, impurity distribution and luminescence distribution within the crystal, emission spectrum, dose response characteristics and optical absorption. A sensitive thermoluminescent preparation may be obtained by the addition of magnesium alone to a readily available source of lithium fluoride.

The generation of a plasma by fission‐fragment ionization of noble gases is reported. The experimental data consist of steady‐state current‐voltage (I‐V) characteristics obtained with ceramic‐metal ionization tubes which were operated in the high neutron flux region of a nuclear reactor. The observed dependence of I on V½ for applied potentials

Determinations of primary ? and secondary ? ionization coefficients in helium at pressures p0 near to atmospheric and low values of E/p0 (E is the electric field) show that ?/p0 = f(E/p0) and ?/? = (E/p0) for 30 < p0d < 730 cm torr. The results obtained for ?/? and its dependence on E/p0 are shown to be adequately accounted for on the basis of the assumption that the predominant secondary ionization process in helium at high pressures is the destruction of metastable states in the gas with the consequent production of non-resonance photons which liberate secondary electrons at the cathode.

Studies of electron density are reported for a quiescent neon‐argon gas plasma generated by fission fragments in the core of a nuclear reactor. Reaction kinetic equations for the various ionic and excited species are solved self‐consistently with electron‐energy balance equations to yield values of electron density and temperature. It is shown that for containers of diffusion length of about 1.6 mm or greater, there exists a maximum value of electron density at a total gas pressure p≈90 Torr and [Ar]/[Ne]≈10−4; the dominant ion in this case is the atomic Ar+ ion. At low values of neutron flux (∼1010 cm−2 sec−1) and electron density (∼1010 cm−3), the electron temperature is computed to be at or near the gas temperature, but at high values of neutron flux (∼1013 cm−2 sec−1) and electron density (∼1012 cm−3), the electron temperature is higher than the gas temperature by an important amount (∼600°K). Inpile measurements of electron density using microwave techniques are in very good agreement with the theoretical predictions.

Single crystals have been grown based on optical quality lithium fluoride with the addition of magnesium and titanium. The content and distribution of titanium has been determined by x-ray fluorescence analysis and related to the intensity of an optical absorption band near 200 nm. The variation of thermoluminescence intensity within each crystal has been studied and related to titanium distribution.

The possible use of manganese as co-activator in the thermoluminescent system LiF: Mg has been investigated. The activators were added as fluorides during single-crystal growth by the Stockbarger-Bridgman method. Enhancement of thermoluminescence sensitivity was achieved with one sample of MnF2, but it has been shown that the agent responsible was titanium impurity in the MnF2.

An apparatus for the measurement of steady-state pre-breakdown ionization currents in helium at high pressures is described, and the sampling procedure necessary to obtain with it reproducible results discussed. The primary and secondary ionization coefficients determined from these measurements are given for 3 3 < E/p0 < 5.0 v cm-1 mmHg-1 and for 150 < p0 < 560 mmHg. The values of ? are discussed in relation to previous theoretically computed values and the experimental values obtained at lower gas pressures but with similar values of E/p0. The values of the generalized secondary ionization coefficient are found to be relatively high (similar 0.1) and to increase as E/p0 decreases.

IN recent years there has been considerable effort devoted to the study of the ionization processes which give rise to ionization growth in different gases. Of particular interest from the fundamental point of view is the growth of ionization in helium, because in this case quantum mechanical methods may be more easily applied than in the more complex case of diatomic gases. Consequently, direct comparison may be made between experimental data on ionization coefficients, for example, and the quantum mechanical computations. Despite its simple atomic structure, however, the existence of high-energy metastable states of the helium atom, as well as of the helium molecule, results in the possibility of highly complex ionization processes occurring in this gas1–3. Moreover, experimentally, helium is a difficult gas on which to make observations because it has the highest ionization potential (24.6 eV) of any known gas. Most common impurities have ionization potentials ( 20 eV) to the ground-state. The effects of impurities, even when present in quantities as low as 1 part in 106, become increasingly important as the parameter E/p (E, the electric field, p, the gas pressure) decreases; because, as E/p decreases, the mean electron energy decreases and the ratio of ionized impurity atoms to ionized helium atoms then increases. These considerable experimental difficulties probably explain why there are so far no published data on the measurement of ionization growth in helium at high gas pressures and low values of E/p.

The secondary electron yield from fission fragments has been measured directly using an ultrahigh‐vacuum diode containing uranium which was operated in a nuclear reactor. The current measured in this diode consisted of contributions from the positively charged fission fragments, secondary electron emission induced by fission fragments, and photoelectrons generated by gamma radiation. The secondary electron yield Δ was determined from the secondary electron current and the computed fission fragment emission rate. For a thick (>one range) uranium‐nickel (5.7 wt. % nickel) alloy the value of Δ is 207±10.

The note gives both theoretical cause and experimental verification for the manner in which electron densities ≈1012 cm−3 vary with total gas pressure and the percentage of trace gas in a Penning mixture ionized by fission fragments.

Distribution of Newly Formed Soluble Sulphate During The Firing of a Radioactive-Labelled Brick