Influence of the PMMA and the ISO Slab Phantom for Calibrating Personal Dosemeters

INTRODUCTION Wide agreement has been reached among the main Bodies and Organizations involved in standardization of radiation protection to define as operational quantity for individual monitoring of external exposure, the personal dose equivalent, Hp(d) (1,2,3). However, as Hp(d) is defined in the human body, it is not a measurable quantity. In practice, it has to be estimated from the indication of an individual dosemeter, which is worn on the trunk and calibrated on a suitable phantom. The calibration phantom should provide a backscatter contribution, similar to that of the part of the body where the dosemeter is worn. Since its definition in 1985 (4), several reports have been published to clarify and define the experimental set up to be used for the calibration of dosemeters in terms of the above-mentioned quantity. Although the ICRU sphere was first considered to be a good phantom, its practical use presented so many difficulties that it was rejected. On the other hand, other phantoms such as the PMMA slab phantom of different sizes or the IAEA cubic water phantom were adopted. In 1992, ICRU, in its report 47 (5), gave a list of five different phantoms that were being used by several laboratories and were considered to be accurate enough within an accepted overall uncertainty in most radiation protection measurements of 30 %. ICRU 47 also included a new quantity to be used for calibration, which is defined as Hp(d) in a phantom which has the composition of ICRU tissue and the same size and shape as the calibration phantom. In the following text, this phantom will be named as the ICRU phantom. Nevertheless, to achieve uniformity in calibration procedures, ICRU 47 recommended the 30 cm x 30 cm x 15 cm PMMA slab phantom. However, recently, the International Organization for Standardization (ISO), in ISO 4037-3 (3), has proposed another phantom, which will be named, in this text, the ISO phantom, which consists of a 30 cm x 30 cm x 15 cm water phantom with PMMA walls (front wall 2.5 mm thick). Many studies, such as the works from Grosswendt (6) and Gualdrini and Morelli (7), have analysed the characteristics of several practical calibration phantoms by means of Monte Carlo codes for photon transport. Calculations confirm that the new ISO phantom is a better substitute of the ICRU phantom than the PMMA phantom, mainly at low photon energies. The values for the personal dose equivalent for the ISO phantom are found to be very close to those calculated for the theoretical ICRU phantom, thus indicating that, in practice, backscatter correction factors are not needed in this new phantom. In 1995, the secondary calibration Laboratory from the Institute of Energy Technology (INTE) at the Technical University of Catalonia in Barcelona, Spain, agreed with the Spanish National Laboratory to adopt the ICRU recommended PMMA phantom, and to improve calibration procedure accuracy by introducing a correction factor for backscatter differences in PMMA and ICRU tissue (8). Such corrections were of the order of 8 % for the low-energy X-ray qualities. In 1999, before implementing the new ISO standard the INTE laboratory carried out the present work, whose aim was to estimate the influence of both phantoms when calibrating personal dosemeters with photons in the range 33 keV, 1.5 MeV.