Discrepancy between Cλ and CE (ionisation chamber dosimetry)

It has become established that ionization chambers can be used for high energy photon and electron dosimetry. The equations relating ionization chamber readings to absorbed dose in water contain the factors C lambda and C E , the conversion factors for measurements made in phantoms irradiated with photons and electrons, respectively. Nahum and Greening (see ibid., vol.21, p.862, 1976) have presented calculations of C lambda which differ from values quoted by the ICRU (1969). They also point out that photon and electron beams which generate the same mean primary electron energies should have the same conversion factors but the recommendations differ by 4%. Some explanations of the discrepancies are given.