New values of exposure to dose conversion factors: Csub(lambda), for photons, and Csub(E) for electrons

New formulae for the exposure-to-dose conversion factors Csub(lambda), for photons, and Csub(E), for electrons, are derived from first principles: the formulae are consistent both for ionisation chambers of different constructions (thimble and flat) and for photon and electron beams. The individual terms present in the Csub(lambda) and Csub(E) factors also permit the derivation of values for these factors for ionisation chambers constructed of different wall materials. Tabulated values of Csub(lambda) and Csub(E) in water and Perspex, for 60Co γ ray calibration and 2 MV X ray calibration, are listed for thimble ionisation chambers with graphite, graphite layered nylon and A-150 plastic walls and a flat ionisation chamber with a Mylar wall. Measurements to confirm the validity of the new values are described and, within the experimental errors present, are found to agree with the theoretically derived values for Csub(lambda) and Csub(E) for 60Co γ rays, 8 MV X rays, 18 MV X rays and 3 to 30 MeV electrons. Since the HPA, NACP and AAPM codes of practice produce Csub(lambda) and Csub(E) values which differ from these tabulated ones, reservations are expressed about the introduction of these new codes of practice. (author)