P. G. Coleman

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.

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.

An accurate measurement of the natural decay rate of orthopositronium

The natural decay rate of orthopositronium has been measured to an accuracy of 0.20% using a fast counting technique and modified method of analysis. The quenching coefficients for orthopositronium and Zeff for free positron annihilation in freon, argon, nitrogen and krypton have also been measured.

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.

Measurement of total scattering cross-sections for positrons of energies 2–400 eV on molecular gases: H2, D2, N2, CO

The results of a time-of-flight experiment are presented, in which the total cross-sections for 2–400 eV positrons on H2, D2, N2, and CO were measured.

A method of improving the statistical accuracy in lifetime measurements on positrons annihilating in gases

A description is given of a method of improving the statistical accuracy in the measurement of the life time of positrons and positronium in gases. This is achieved by using a thin scintillator for direct detection of the positrons before they enter the gas. A new system of electronics involving the vetoing of prompt events is also described and some preliminary results for lifetime measurements are quoted.