An Absolute Calibration of theLi7(p,n)Threshold Voltage

The ${\mathrm{Li}}^7\mathrm{}(p,n)\mathrm{}$ threshold has been the most generally accepted secondary voltage standard for nuclear experiments done with an electrostatic generator. Recent measurements of this threshold by electrostatic analyzer methods have given a result about 2 percent higher than the older value made by an extrapolation of a resistor measurement of the $\mathrm{Li}(p,\gamma )$ resonance at 440 kv. The ${\mathrm{Li}}^7\mathrm{}(p,n)\mathrm{}$ threshold has now been measured by a third method, the radio frequency ion speed gauge. The calibration by this method is absolute in that the kinetic energy of the particle is determined in terms of its known mass to charge ratio and two easily measurable parameters: a length and a frequency. The results of this calibration place the threshold at (1.8812±0.1 percent) Mev, in good agreement with values obtained from the electrostatic analyzer measurements.