A re-evaluation of the I-value for graphite based on an analysis of recent work on W, Sc,a and cavity perturbation corrections

Recent developments have cast doubt on the value for the electron stopping power for graphite used in dosimetry. Information has come from a number of areas: a new measurement of the mean excitation energy, I, for graphite; experiments indicating that the crystallite density for graphite, rather than its bulk density, should be used to evaluate the stopping power; experiments in medium-energy x-rays comparing free-air and cavity ionization chambers and Monte Carlo calculations of the behaviour of graphite-walled cavity chambers in Co-60 gamma-rays and medium-energy x-rays. The stopping powers for graphite currently recommended were evaluated using an I-value of 78 eV. The use of the more recent value I = 86.9 eV reduces the graphite stopping power for Co-60 by around 1.5%, which in a simple analysis increases the W-value for air by a similar amount. However, such an increase is in conflict with the evidence that the W-value is close to the value 33.97 eV currently recommended. This paper presents the various components and constraints, analyses the uncertainties and shows that the use of a graphite I-value of 82.5 eV, a W-value of 33.97 eV and the crystallite density of graphite reasonably explain the experimental observations. The standard uncertainty of this I-value determination is 1.5 eV.