Measurement of the specific primary excitation of helium and neon by fast charged particles

The specific primary excitation of helium and of neon has been measured. Momentum-separated particles from an accelerator, identified by Cerenkov counters, were directed through a series of 69 spark chambers. The specific primary excitation was derived from the measured spark-chamber efficiency as a function of particle momentum. The numbers of primary electrons at the minimum of ionization N/sub min/ and at the Fermi plateau N/sub pl/, per centimeter of gas at normal pressure and temperature, are the following: in helium N/sub min/=3.5 +- 0.2; N/sub pl/=5.1 +- 0.4; in neon N/sub min/=11.6 +- 0.3; N/sub pl/=18.1 +- 0.7. For a particle with momentum p and mass m the Fermi plateau starts at (p/m) /sub pl/approx.80 for helium and at (p/m)/sub pl/approx.70 for neon. Significant discrepancies exist between the various experimental and theoretical results. The main cause of the experimental discrepancies can be explained by the different energy sensitivities of the detectors