The time-of-flight (TOF) method for the determination of electron-diffusion coefficients D and drift velocities w described earlier could not be used to investigate a number of gases because a Gieger—Müller counter was used for an electron detector. A new experimental apparatus using differential pumping and an electron-multiplier detector was developed to remove this restriction. Using this new apparatus, D and w have been measured for He, Ar, H2, N2, CO, CO2, CH4, and C2H4. Considerable divergence of D values from published values by the Townsend method have been obtained, indicating a fundamental difference in the methods.

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The ionization of helium atoms by antiprotons is considered in an independent-particle model (IPM) with a model potential that accounts for polarization and binding effects of the electrons in the two-centre Coulomb potential. The model is designed to reproduce the exact ionization potential of the quasimolecular system and to allow for the existence of one bound state in the united-atom limit. We find a single-ionization cross section that is in good agreement with the results of various correlated two-electron calculations, and a double-ionization cross section that is considerably improved compared to a standard IPM calculation with a fixed atomic target potential.

It has been shown that a commercially available halogen type Geiger-Mueller counter when properly shielded with Sn and Pb provides a very practical and satisfactory device for measuring the dose due to y-radiation in the presence of neutrons. The device measures the dose in roentgens with nearly uniform response for effective X-ray energies above about 200 keV and up to at least 1.25 MeV y-energies. Calculations show that the fast neutron response is less than 0.15 per cent, and experiments indicate that it is less than 0.5 per cent.Experiments show that about 5 x lo9 thermal neutrons per om2 give a response equivalent to 1 r of y-radiation. When necessary, the thermal neutron response can be decreased by a factor of 300 by using a Li shield described in the text.

We have developed a photoionization method for complete conversion of a quantumselected population to ionization, making possible sensitive and absolute measurement of the selected populations in a gas. Each photoionization involves the absorption of two photons (from a pulsed dye laser), one of which is resonant with an intermediate state.In this demonstration we measured the absolute number of He(2 i S) states per ion pair following interaction of pulses of 2-MeV protons with He.