Calculation of perturbation correction factors for some reference dosimeters in high-energy photon beams with the Monte Carlo code PENELOPE

The BNM-LNHB (formerly BNM-LPRI, the French national standard laboratory fo r ionizing radiation) is equipped with a SATURNE 43 linear accelerator (GE Medical Systems) dedicated to establishing national references of absorbed dose to water for high-energy photon and electron beams. These standards are derived from a dose measurement with a graphite calorim eter and a transfer procedure to water using Fricke dosimeters. This method has already been used to obtain the reference of absorbed dose to water fo r cobalt-60 beams. The correction factors rising from the perturbations gen erated by the dosimeters were determined by Monte Carlo calculations. To me et these applications, the Monte Carlo code PENELOPE was used and user code s were specially developed. The first step consisted of simulating the electron and photon showers prod uced by primary electrons within the accelerator head to determine the char acteristics of the resulting photon beams acid absorbed dose distributions in a water phantom. These preliminary computations were described in a prev ious paper. The second step, described in this paper, deals with the calcul ation of the perturbation correction factors of the graphite calorimeter an d of Fricke dosimeters. To point out possible systematic biases, these corr ection factors were calculated with another Monte Carlo code, EGS4, widely used for years in the field of dose metrology applications. Comparison of t he results showed no significant bias. When they were possible, experimenta l verifications confirmed the calculated values.