214
.REFERENCES
1, Vogt, E. and Wannier, G. H. Phys. Rev. 95, 1190 (1954).
2. Dalgarno, A. Adv. in Phys. 11, 281 (1962).
3. Pauling L. The Nature of Chemical Bond, Cornell Univ.,
Press, 1960, 3rd. ed., Ch. 13; also Hirschfelder, J. O.,
Curtiss, C, F., and Bird, R. B. Wolecudar Theory of Gases
and Liquids. Wiley, New York, 1964.
4, O'Malley, T. F., Spruch, L., and Rosenberg, L. J. Math.
Phys. 2, 491 (1961).
5. Newton, R. Scattering Theory of Particles and Waves.
McGraw-Hill, New York, 1966.
6. Weber, G. G. and Bernstein, R. B. J. Chem. Phys. 42, 2166
(1965).
7. Waech, T. G. and Bernstein, R. B. J. Chem. Phys. 46, 4905
(1967).
8. Fermi, E. Nuove cimentoe 11, 157 (1934); for further references, see Breene, R. G., Jr. The Shift and Shape of Spectraf Lines. Pergamon Press, New York, 1961, Ch. 7.
GENERALIZED OSCILLATOR STRENGTHS OF THE HELIUM ATOM.
Hit. TRANSITIONS FROM THE GROUND STATE TO THE 3'D AND 4'P STATES*
Y.-K. Kim and Mitio Inokutz
The generalized oscillator strengths of He for the
LS — 3D and 4!P transitions have been calculated
with correlated wave functions, according to both the
length and velocity formulas. The agreement between
* Abstract of an article published in Phys. Rev. 184, 38-43
(August 5, 1969).
the two alternative results is within 3%or less for
moderate values of the momentum transfer. The resulting Born cross sections for charged-particle impact
ure also given. Compared with our values, available
experimental data on the 3\D excitation are substantially larger, while for the 4'P excitation they agree
within +50 %.
SPECIFIC PRIMARY IONIZATION
FF. Rieke and William Prepejchal
Primary ionization cross sections have been measured for 16
additional gases. Present and previous results are summarized
in graphical form, New measurements show that high-energy
positrons and electrons have primaryionization cross sections
that are equal within one percent.
Primaryionization cross sections have been measured
for 16 newgases, supplementing work reported earlier™.
The results are given in the first 16 lines of Table 77.
Early in the investigation, measurements were made
on several gases with positrons as primaries; these measurements were in the nature of absolute determinations.
The results indicated that the cross sections for positrons might be shghtly greater than for electrons, but
the excess was well within the uncertainty of the absolute
determinations. We have recently made somerelative
measurements that afford a much more accurate comparison. For these measurements, a source was prepared
to contain suitable activities of both Co** for positrons
and Ce!4-Pr™4 for electrons. With such a mixed source,
it was possible by simply reversing the current in the
magnetic analyzer to measure counter efficiencies for
positrons and for electrons alternately while maintain-
ing exactly constant counting conditions. To avoid uncertainties in analyzer calibration with field reversed,
the field strength was monitored throughout with a
gaussmeter. Comparisons were made with argon at
primaryenergies where the positrons and electrons gave
comparable counting rates. The results are given in
Table 78; they indicate that at high energies the ionization cross sections for positrons and for electrons differ
byless than a percent. It seems unprofitable to carry
the comparison further.
In line with our aim of determining cross sections as
accurately as is feasible, we have reviewed our older
results and made adjustments where they appeared to
be indicated.
The procedure for obtaining the constants 17? and C
from the observed counter efficiencies by the method of
least squares has been simplified in that wall effects are
represented by one adjustable constant instead of two.
The solution now used amounts to assuming that the
observed efficiencies 7 can be expressed bytherelation
—In (1 — 94) = NEo({E)(P — Po),
where Py) represents wall effects and is independent of
primary energy /; o(#) follows the Bethe formula. A
small systematic error has been eliminated by taking
into account scattering of the primary electrons by the