A model for photon detection and dosimetry with superheated emulsions

A model is presented for the analysis and prediction of the photon response of detectors based on superheated emulsions of light halocarbons in tissue equivalent gels. It is shown that on the basis of a nondimensional thermodynamic quantity, called reduced superheat, it is possible to identify the degree of superheat, or the operating temperature, corresponding to the photon sensitization of the emulsions. Moreover, on the basis of the mass energy absorption coefficients, it is possible to determine the energy dependence of the photon response. The vaporization energy necessary for bubble nucleation is estimated by means of the thermal spike theory developed for bubble chambers. The energy deposition requirements are consistent with the energy transferred by secondary electrons at the end of their range in the halocarbons. These findings provide design criteria for photon detectors based on superheated emulsions. It is shown that light halocarbons of low effective atomic numbers present the best dosimetric properties. In particular, by manufacturing superheated emulsions with octafluoropropane, or halocarbon R-218, photon sensitivity is achieved at room temperature along with a fairly constant air-kerma response.