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 thermod ynamic quantity, called reduced superheat, it is possible to identify the d egree of superheat, or the operating temperature, corresponding to the phot on sensitization of the emulsions. Moreover, on the basis of the mass energ y absorption coefficients, it is possible to determine the energy dependenc e of the photon response. The vaporization energy necessary for bubble nucl eation is estimated by means of the thermal spike theory developed for bubb le chambers. The energy deposition requirements are consistent with the ene rgy transferred by secondary electrons at the end of their range in the hal ocarbons. These findings provide design criteria for photon detectors based on superheated emulsions. It is shown that light halocarbons of low effect ive atomic numbers present the best dosimetric properties. In particular, b y manufacturing super heated emulsions with octafluoropropane, or halocarbo n R-218, photon sensitivity is achieved at room temperature along with a fa irly constant air-kerma response. (C) 2000 American Association of Physicis ts in Medicine. [S0094-2405(00)00102-4].