Laurent Bourgois

Principes généraux de protection contre l’exposition externe

Si, dans les chapitres precedents, nous avons vu comment, a partir d’un champ de rayonnements, estimer les grandeurs dosimetriques utiles en radioprotection, a partir de ces valeurs il convient de definir les protections ad hoc afin d’etre compatibles avec les limites liees aux objectifs de radioprotection.

Grandeurs et unités fondamentales de la dosimétrie externe

Dans un premier temps, nous allons nous attacher a definir les grandeurs caracteristiques de la dosimetrie et des champs de rayonnements, appelees grandeurs radiometriques. Les grandeurs radiometriques et dosimetriques fondamentales sont essentiellement au nombre de trois : la fluence Φ; la dose absorbee D; le kerma K.

Protection and Operational Dosimetric Quantities and Calibration

This chapter is devoted to protection and operational dosimetric quantities. These quantities are of paramount importance for radiation protection problems: The limit exposure of workers and public are in term of protection quantity while measurement with radiation device is in term of operational quantity. In what follows, these quantities are defined, compared and calculated for major radiation field. A final part is dedicated to the calibration of radiation protection devices.

Source Evaluation of the External Exposure

This chapter is devoted to characterize main source terms of primary and secondary particles for external exposure. The assessment of neutron, photon and electron yield is detailed in usual physical process such as: fission reactions, ion interactions, bremsstrahlung on thick targets, induced activity in materials... This characterization allows the access of values of protection and operational quantities previously defined. This prior knowledge is required for designing radiation protection shieldings and, more generally, for developing appropriate radiological counter measures that will be developed in the next chapter. The end of the chapter addresses the study of “exotic facilities” such as klystrons or high intensity lasers.

Protection Against External Exposition, General Principles

This chapter is devoted to biological protection against external exposure. It introduces shieldings against α, β, γ and neutrons radiation and other secondary source term developed previously such as bremsstrahlung photons, neutrons from different reactions (nuclear reactions photoneutrons, …). This chapter also addresses protection against scattered radiation, and the mazes or skyshine. A section is devoted to the production of harmful gases (non-radioactive, e.g. ozone) due to radiation. Finally we are interested in computer codes for calculating shielding.

Quantities and Fundamental Units of External Dosimetry

In this chapter, the radiation field is characterized in terms of energy and flux at a point of space. These physical datas than make possible to obtain the radiometric and dosimetric quantities of reference for external exposure, such as: fluence, kerma for the indirectly ionizing radiations and the dose. In addition, other quantities such as linear energy transfer and relative biological effectiveness are detailed; they relate to the effect of radiation on biological tissues and to the damage that can occur at the cellular level.

Principle of the Monte-Carlo Method Applied to Dosimetry and Radiation Protection

In previous chapters, we have seen that it is possible to resolve dosimetric and radiation protection problems with fairly easy analytical approaches. By the way, some of these logics are implemented in deterministic codes (e.g. point kernel method). However, according to circumstances, these methods can lead to significant bias on results especially for complex radiological cases. In order to offset these issues, it may be necessary to switch to a Monte Carlo method. Computational algorithms based on this method rely on repeated random sampling to obtain numerical results. It allows a smooth definition of physical parameters during the particle transport for more accuracy in final results. Furthermore, this method is particularly suitable in multi-particles transport problems and for complex geometries involving multi-materials and various densities. In what follows, the Monte-Carlo method applied to particle transport is detailed in terms of implementation in codes and features for radiation protection and dosimetric problems. The use of computer codes to estimate the various radiometric and dosimetric quantities previously defined in this book has become essential. These operate using methods of numerical calculations with approximations that affect more or less the true value of desired quantities. We chose to detail a particular, which is the reference method for the simulations in the field of dosimetry and radiation protection, and whose evocation staked all previous chapters: the Monte Carlo method.

Ionizing Radiation Interaction in Tissues: Kerma and the Absorbed Dose

In previous chapter, the basic concepts of dosimetry and all elements for the characterization of a radiation field in a point in space, have been defined. This chapter attempts to detail the physical concepts to estimate the mean energy transferred to secondary particles for indirectly ionizing particles and energy locally imparted for directly ionizing particles. These estimates allow then analytical approaches, under certain conditions, for basic dosimetric quantities that are respectively: kerma and absorbed dose. These two variables and fluence are the primary quantities which are connected to protection and operational and quantities that are defined in the next chapter. The principles and techniques of measurements of these primary dosimetric quantities are discussed in this chapter.

Grandeurs de protections, grandeurs opérationnelles et étalonnage

Nous avons vu precedemment que lorsqu’un individu est expose a un rayonnement ionisant, l’ionisation des atomes peut provoquer l’alteration ou la mort des cellules touchees. Selon l’intensite de l’irradiation, l’action sur l’organisme est tres differente : aux fortes doses, l’effet est immediat et intervient de maniere certaine pour chaque individu expose a une dose superieure au seuil d’apparition de l’effet, dit deterministe ; aux faibles doses, l’effet est retarde et n’apparait pas obligatoirement pour chaque individu irradie. C’est un phenomene stochastique pour lequel il n’existe pas de seuil d’apparition.

Interaction des rayonnements ionisants dans les tissus : évaluations du kerma et de la dose absorbée

Nous avons etabli dans le chapitre 1 que l’energie transferee ou communiquee aux particules secondaires chargees aux tissus, revet un caractere fondamental quant a la determination des grandeurs dosimetriques. Dans ce chapitre, une analyse des processus physiques conduisant a ces echanges d’energie doit permettre, autant que possible, de caracteriser de fa con analytique les grandeurs de reference definies precedemment : Φ, K, D.