The Efficency of Radiation Detection by Photographic Films: State-of-the-Art and Methods of Improvement

The performance of photographic films as radiation detectors has rested at a state-of-the-art platean for some time. In terms of detective quantum efficiency (abbreviated DQE) this plateau is at about 1% for the best films, which 1% should be compared with 100% for an ideal photon counter. The physical causes for the 100-fold discrepancy between reality and ideal performance are examined in detail: incomplete light absorption; a small inefficiency in convertihg photons into photoelectrons; a large loss due to recombination. of electrons with holes; a small loss dictated by a four-atom threshold;. a small loss dictated by random grain position; a large loss from the use of a wide dispersion of grain sizes. These factors account quantitatively for substantially all of the discrepancy between reality and ideal performance. We show that the recombination loss is a necessity that must ordinarily be tolerated in order to secure a good shelf life for films. Part of the loss may be recouped by the well-known method of latensification. Films with a wide dispersion of grain sizes will have a higher DQE if the grains operate at a threshold of two processed photons instead of four. Latensification permits one to approach the two-atom threshold condition.