A proposed modification of Burlin's general cavity theory for photons

Reconsideration of the backscatter components in the intermediate cavity theory for photon beams

Cavity theories are used to relate the dose in a medium to the dose in a cavity. No satisfactory analytical approach exists for the case of intermediate cavities, such as TLDs, when interface effects are important. Burlin proposed a general cavity theory that accounts for the cavity size but which ignores interface effects between cavity material and the surrounding materials. Horowitz modified Burlin's theory to include a more systematic account for the contribution of electrons generated in the cavity. Both Kearsley and Haider proposed new models that consider backscattering effects at cavity interfaces. However, these last models introduce additional parameters whose values are not easily available. By using a pure analytical approach, higher orders of backscatter terms were considered and a photon cavity expression was derived. The proposed formula depends on backscattering and attenuation coefficients of the cavity material and the surrounding media as well as the size of the cavity and angular effects. This expression has the Burlin and Bragg-Gray formulae as limiting cases and follows better the pattern of Ogunleye's experimental results.

Application of cavity theory to the response of various TLDs to 60Co gammas degraded in water

Several types of thermoluminescent dosimeters (LiF:Mg, Ti, Al2O3:Mg,Y and CaF2:Mn) were irradiated at different depths in a water phantom placed at a distance of 2.5 m from a panoramic 60Co source. Detectors were encapsulated in Plexiglas holders with a wall thickness of 0.5 cm. Reference dosimetry was carried out using a Fricke dosimeter and an ionization chamber. The experimental data were compared with the predictions of the general cavity theory for gamma ray spectra at different depths of water. The suitability of parameters of the cavity theory proposed by different authors was evaluated in the analysis of the experimental results. The results show that there is no need for any modification to the original and simple Burlin expression which gives very good agreement with the experimental values.

An EGS4 Monte Carlo examination of general cavity theory

EGS4 Monte Carlo simulations have been performed to examine general cavity theory for a number of TLD cavity materials irradiated in megavoltage photon and electron beams. The TLD materials were LiF, Li2B4O7, CaF2 and CaSO4 irradiated in Perspex, water, Al, Cu and Pb phantoms. For megavoltage photon beams, this has been done by determining the dose component (1-d) resulting from photon interactions in the cavity compared with the dose component resulting from photon interactions in the phantom material (d) by Monte Carlo simulations and analytical techniques. The results indicate that the Burlin exponential attenuation technique can overestimate the dose contribution from photon interactions in a 1 mm thick LiF cavity by up to 100% compared with the Monte Carlo results for LiF TLDs irradiated in a water or Perspex phantom. However, there is agreement to within 1% in the quality dependence factor, determined from Burlin's cavity theory, Monte Carlo simulations and experimental measurements for LiF and Li2B4O7 TLDs irradiated in a Perspex or a water phantom. The agreement was within 3% for CaF2 TLDs. However there was disagreement between Monte Carlo simulations and Burlin's theory of 6 and 12% for LiF TLDs irradiated in copper and lead phantoms respectively. The adaptation of Burlin's photon cavity theory and other modifications to his photon general cavity theory for electrons have been shown to be seriously flawed.