G R Heyland

Measurement of total cross sections for low energy positron-helium collisions. (Positron backscattering from metal surface)

An efficient method for the conversion of the high energy positrons emitted by Na22 into slow positrons of energy about 1 eV has been developed using the backscattering of positrons from a gold surface coated with magnesium oxide powder. These positrons have been used for the measurement of the total cross section for positron-helium collisions at energies between 2 eV and 20 eV. A preliminary value for the positron-argon cross section at 8 eV is also given.

Total scattering cross sections for low-energy positrons in the molecular gases H2, N2, CO2, O2 and CH4

The method described by Charlton et al. (1980) to measure total cross sections for the scattering of electrons from helium and argon has been used for similar measurements with positrons in the molecular gases H2, N2, CO2, O2 and CH4 over an energy range from 2 to 20 eV. The apparatus used is, with minor modifications, the same as that of Charlton et al. and incorporates a localised scattering cell close to the slow positron source which requires the measured attenuations to be normalised to previously known absolute cross sections. The apparatus used in this study, together with the normalisation procedure and the method of data analysis, is described in detail.

Positronium formation cross sections in the inert gases

New measurements of positronium formation cross sections are reported for the inert gases at energies up to 150 eV. These data show pronounced structure in the cross sections of all gases in the form of secondary peaks and plateaus above the ionisation potentials.

The analysis of data obtained with time to amplitude converter and multichannel analyser systems

An exact general formula is derived relating the data obtained from t.a.c-m.c.a. systems to the input signal distribution; it is shown how the latter is modified by the measuring process. A general method of data correction is given, together with important approximations, which evaluates the input signal distribution from the data without any loss of signal. A simple and accurate method of system calibration based only on counting and not requiring the use of calibrated delay lines is also given.

A time of flight method of investigating the emission of low energy positrons from metal surfaces

A time of flight technique with a relatively weak radioactive source of positrons has been used to observe the enhanced emission of positrons with energy less than 1 eV from metal surfaces. Careful design of the source assembly has resulted in an appreciably higher yield of low energy positrons than has been observed by other methods.

The measurement of total cross sections for positrons of energies 2-400 eV in He, Ne, Ar, and Kr

The total cross sections for positrons in He, Ne, Ar and Kr have been measured in the energy range 2-400 eV, using a time-of-flight technique, and compared with those of other authors.

Measurement of positron-rare gas total cross-sections at intermediate energies

Total scattering cross-sections for positrons in the rare gases in the energy range of 2–960 eV have been measured using a time-of-flight system for which the uncertainty arising from forward scattering is substantially lower than in the 1974 experiment of Canter et al. A comparison of these cross-sections with current theoretical estimates is made.

Total cross-section measurements for 2–400 eV positrons in the rare gases

Experimentally determined values of the total scattering cross-sections for positrons in the energy range 2–400 eV on He, Ne, Ar, Kr and Xe are presented.

Measurements of low-energy e+or--Ne and e+-Ar total scattering cross sections

Measurements of positron and electron-Ne total scattering cross sections are reported in the energy range 2-50 eV. e+-Ar total cross sections are presented in the energy range 2-20 eV. In both the e+-Ne and e+-Ar cases marked discrepancies are found with other recent measurements and these are discussed along with possible systematic errors which may affect the data. The e--Ne measurements are found to be in excellent agreement with recent experiments.

Positron lifetime spectra for the noble gases

Lifetime spectra for positrons annihilating in helium gas have been measured at room temperature for gas densities up to 60 amagat. The equilibrium annihilation rate for the free positron component is proportional to the gas density D, in amagats, given by lambda 1=(0.793+or-0.004 D mu s-1 which is equivalent to Zeff=3.94+or-0.02 and is in good agreement with recent calculations. The variation of the equilibrium decay rate with gas density for the ortho-positron component is also linear, given by lambda 2=7.24+(0.1006+or-0.0006) D mu s-, which is consistent with pick-off quenching. No other long-lived component has been observed. For neon at room temperature, a shoulder region of width 1700+or-200 ns amagat has been established. Some new measurements for argon, krypton and xenon are also reported. A comparison is made with other recent measurements.