Grandeurs dosimétriques

Présentation au sujet: "Grandeurs dosimétriques"— Transcription de la présentation:

1 Grandeurs dosimétriques
Grandeurs et mesures- 2 Grandeurs dosimétriques Kérma, Dose, TLE et plus Jour 2 – Leçon 8

2 Objectif Connaitre les grandeurs dosimétriques, la terminologie associée et les concepts sous-jacents Apprendre davantage sur le kerma (débit Kerma), l'exposition (débit d’exposition), la dose absorbée (débit de dose), le transfert linéique d'énergie (TLE), et la dose aux organes Additional underlying terms and concepts will be discussed, as shown on the next slide.

3 Contenu Kerma (débit) Coefficient massique d’absorption d’énergie
Kerma dans l’air Exposition (débit) Dose absorbée (débit) Energie communiquée Transfert Linéique d’Energie (TLE) Dose à l’Organe To fully develop the concepts of kerma, exposure, and absorbed dose, we need to discuss and understand concepts such as mass energy absorption coefficient and energy imparted.

4 Kerma Kerma (Kinetic Energy Released per unit Mass)
Kerma: énergie cinétique transférée à la matière par unité de masse Le Kerma est définit par: K = Où Le Kerma Indique la somme de toutes les énergies cinétiques Etr transférées à des particules secondaires de première génération dans un élément de masse dm du milieu. dEtr dm Kerma is a concept useful primarily for photons and neutrons in any medium. It includes all of the energy transferred to charged particles by the incoming radiation, even if some of that energy is not deposited locally. Kerma represents the transference of energy from photons or other primary incident radiation (e.g. neutrons) to directly ionizing particles (e.g. electrons).

5 Kerma L’unité de kerma est le J kg-1
Le nom spécial de l’unité de kerma est le Gray (Gy) One of the main values of the concept of kerma is in clarifying the steps involved in energy deposition.

6 Débit de Kerma . Le débit de kerma, K, est le quotient de dK par dt, où dK est l’incrément du kerma pendant le temps dt: K = Le débit de Kerma s’exprime en J kg-1 s-1 et le nom spécial qui lui est attribué est le Gray par seconde (Gy.s-1 ou Gy.h-1 ) . dK dt Kerma rate is simply energy transferred to charged particles per unit time.

7 Exposition L’exposition est:
Une quantité utilisée pour indiquer la quantité d'ionisations produites dans l'air par un rayonnement x ou gamma L’unité de l’exposition dans le SI est le Coulomb par kilogramme (C/kg) One R = 2.58 x 10-4 C/kg. R stands for the unit of roentgen, an old unit which is still used in some applications. Note that exposure applies only to electromagnetic radiation and only in air. Thus, it has definite limitations but we still need to understand the concept since many portable health physics instruments still read out in units of roentgens or multiples thereof. Also, for example, in the United States the roentgen is still allowed to be used by licensees of the USNRC on radiation survey records (but not on official dose records).

8 Exposition La relation entre l’exposition et le Kerma dans l’air est la suivante: X = où “W” est l'énergie moyenne dépensée par un électron pour produire une paire d'ions et «e» est la charge de l’électron W Ka (1 – g) e For fast electrons in dry air, W/e has the average value of ± 0.05 J C-1. Note that the old unit for exposure, which was the roentgen, R, is related to the SI unit as follows: 1 R = x C kg-1 Many radiation protection portable survey instruments are still calibrated in units of roentgen and multiples thereof, so it is still necessary to understand and use the roentgen. g : est la fraction de l'énergie des électrons secondaires initiale qui est rayonnée en tant que rayonnement de freinage

9 Exposition L'exposition est mesurée dans des conditions d'équilibre électronique Electronic equilibrium is discussed in more detail in another session, but basically, when the same number of electrons are set in motion in a given volume by the primary radiation as come to rest in that same volume, we say that “electronic equilibrium” has been attained. For electronic equilibrium to exist, the attenuation of the primary radiation beam must be negligible in a distance equal to the mean range of the electrons.

10 Débit d’Exposition x = L’unité est C. kg-1 .s-1 ou (C.kg.h-1.h-1)
Le débit d’exposition, x, est le quotient de dx par dt, où dx est l’incrément de l’exposition dans le temps dt: x = L’unité est C. kg-1 .s-1 ou (C.kg.h-1.h-1) dX dt . Exposure rate is simply the production of ionization in air per unit time.

11 Dose Absorbée La dose absorbée , D, est donnée par: D = de/dm
où (de) est l’énergie moyenne communiquée à la matière de masse (dm) par le rayonnement _ Energy imparted is the energy incident minus the energy leaving the mass; minus the energy released in nuclear transformations ( to keep the dose from becoming negative when the mass contains a radioactive source). The medium should always be specified. Kerma represents the transference of energy from the photons (or neutrons) to the directly ionizing particles. The subsequent transference of energy from these directly ionizing particles to the medium (e.g. air or tissue) is represented by the absorbed dose. In the region of electronic equilibrium, the kerma and absorbed dose are equal.

12 Dose Absorbée L’unité de la dose absorbée est le J kg-1
Le nom spécial pour la dose absorbée est le Gray (Gy) Note that kerma and absorbed dose have the same units and in fact are equal in the region of electronic equilibrium.

13 Energie communiquée L’Energie communiquée est l’écart entre la somme des énergies de toutes les particules ionisantes ayant pénétré dans un volume, et la somme des énergies de toutes celles qui l'ont quitté, cet écart étant diminué de l'équivalent énergétique de toute augmentation de masse au repos résultant des réactions nucléaires ou des réactions entre particules élémentaires qui ont eu lieu dans ce volume. Energy imparted is basically the energy that is absorbed locally by the medium. The energy imparted determines the dose.

14 Débit de dose absorbée . Le débit de dose absorbée est la variation de la dose absorbée (dD) par unité de temps (dt) D = : L’unité est J kg-1 s-1 et le noms spécial pour l(unité de la dose absorbée estle Gray par seconde (Gy s-1) . dD dt The absorbed dose rate is simply the absorbed dose delivered per unit time period.

15 Transfert Linéique d’Energie
Le transfert linéique d’énergie est l’énergie transférée par une particule au milieu traversé par unité de longueur Le TLE s’exprime en J.M-1 On peut aussi, pour plus de commodité, exprimer le TLE en eV.m-1 ou keV.µm-1 Given the wide range of values for energy deposition for the same dose, the biological effect will vary according to the type of radiation. For example, comparing the effects of alpha and gamma rays is rather like comparing the effect on a target of a cannonball with that of buckshot. The energy transferred may be the same, but the kind of damage inflicted will be different.

16 Transfert Linéique d’Energie
Exprime le niveau d’énergie transférée à l’échelle microscopique For alpha particles, as the dose decreases the number of hit cells decreases, but not the degree of impact on those cells, whereas for gamma rays the quantity of energy deposited per cell goes down, but not the number of cells hit. This difference makes alpha particles more damaging relative to gamma rays and thus they have a higher radiation weighting factor relative to gamma rays. The lineal energy transfer is the microdosimetric analogue of the linear energy transfer (LET).

17 Transfert Linéique d’Energie
Le LET est défini généralement comme: L = [ ] où dE est l’énergie déposée en traversant la distance dl dE dl Measurement of the number of ionizations which radiation causes per unit distance as it traverses the living cell or tissue is called the linear energy transfer of the radiation. The concept involves lateral damage along the path, in contrast to path length or penetration capability. Medical X-rays and most natural background radiation are low LET radiation, while alpha particles have high LET. On the average, fission fragments have high LET. In order to have a quantitative sense of the frequency of the different cell effects caused by radiation exposure, imagine a colony of 1,000 living cells exposed to a 1 cGy X-ray (about the dose for one X-ray spinal examination). There would be two or three cell deaths, two or three mutations or irreparable changes in cell DNA and about 100,000 ionizations in the whole colony of cells -- ranging from 11 to 460 ionisations per cell.

18 Transfert Linéique de l’Energie
Une mesure de la façon dont, en fonction de la distance, l'énergie est transférée du rayonnement à la matière exposée; Un TLE élevé indique que l’énergie est déposée dans une petite distance While cells can repair some damage, no one claims that there is perfect repair even after only one such X-ray. A comparable 1 cGy exposure to neutrons which have higher linear energy transfer (LET) would be expected to cause more cell deaths and more mutations. The ionizations caused would range from 145 to 1,100 per cell. Alpha particles which occur naturally would cause roughly 10 times as many cell deaths and mutations, and 3,700 to 4,500 ionizations per cell. Alpha particles have high linear energy transfer. The average number of cell deaths and mutations caused by fresh fission particles (i.e. those present soon after detonation of a nuclear bomb) would be even greater, with the ionizations as frequent as 130,000 per cell.

19 Dose à l’organe Doses à l'organes peuvent résulter tant de l'irradiation externe que interne (cas de contamination interne par exemple) La mesure / le calcul de la dose aux organes suite à l’exposition externe aux rayonnements est généralement plus simple que pour le cas de la contamination interne En effet, les diapos suivantes focalisent sur la dose aux organes suite à une exposition interne

20 Dose à l’organe Suite à une admission dans le corps d'un matériau radioactif, il existe une période au cours de laquelle le matériau donne lieu à des doses équivalentes délivrées dans les organes ou les tissus de l'organisme à des débits différents L'intégrale dans le temps du débit d'équivalent de dose est appelée dose équivalente engagée.  is usually taken to be 50 years for adults and 70 years for children. Depending on the half lives of the radionuclides, there will be effects over time from radiation doses. To account for the continuing irradiation of organs and tissues after intake, there is the committed dose concept. The committed equivalent dose is the time integral of the equivalent dose-rate in a specific tissue following intake of a radionuclide into the body. Unless specified otherwise, an integration time of 50 y after intake is recommended for the occupational dose, and 70 y for members of the public.

21 Organes spécifiques pour lesquels les débits de doses sont calculés
gonades La moelle osseuse (rouge) vessie sein thyroïde peau reste côlon poumon estomac foie œsophage Surface de l'os There is a specific list (shown in this slide) of organs and tissues for which radiation doses are typically calculated. There are 12 specific organs/tissues plus remainder. The specific remainder organs are shown in the next slide.

22 Organes restants glandes surrénales Gros intestin supérieur
intestin grêle rein pancréas cerveau rate thymus utérus muscle The remainder consists of the tissues and organs shown in this slide. The list includes organs which are likely to be selectively irradiated. In those exceptional cases in which a single one of the remainder tissues or organs receives an equivalent dose in excess of the highest dose in any of the twelve organs for which a weighting factor is specified, a weighting factor of should be applied to that tissue or organ and a weighting factor of to the average dose in the rest of the remainder as defined in this slide.

23 Fantôme pour le calcul de dose à l’organe
To calculate organ doses over various organs and tissues in a human body, an anthropomorphic phantom, a mathematical representation of a human body, is usually employed. The particular phantom shown in this slide was developed by Cristy and Eckerman (1987). This phantom is a hermaphroditic phantom that has a trunk and most internal organs of an adult male, but also has breasts, ovaries and a uterus which are representative of an adult female. This phantom consists of three major sections: (i) the trunk and arms represented by elliptical cylinder, (ii) the legs and feet represented by two truncated circular cones, and (iii) the head and neck represented by an elliptical cylinder capped by half an ellipsoid. Within these sections, well over 150 organs and structures were modeled geometrically and assigned one of three tissue types: skeletal, lung or soft tissue. In this phantom, the organs and exterior shape of the phantom are defined by simple mathematical equations, such as cylinders, ellipsoids, cones, or their combinations. In this model, four coordinate systems were used in addition to the main coordinate system to model some organs, which cannot be easily modeled in the main coordinate system (i.e., adrenals, gall bladder and heart).

24 Dose à l’organe X-ray beam striking the duodenum during a fluoroscopy procedure. Radiation will often selectively irradiate a particular organ or part of the body, depending on beam size and orientation. Scattered radiation can also irradiate other organs and tissues, as portrayed in the slide.

25 Résumé Les grandeurs dosimétriques et les terminologies associées ont été discutées Les étudiants ont étudié le kerma, l’exposition, la dose absorbée, le Transfert Linéique d’Energie et la dose à l’organe

26 Où trouver plus d’Information
Knoll, G.T., Radiation Detection and Measurement, 3rd Edition, Wiley, New York (2000) Attix, F.H., Introduction to Radiological Physics and Radiation Dosimetry, Wiley, New York (1986) International Atomic Energy Agency, Determination of Absorbed Dose in Photon and Electron Beams, 2nd Edition, Technical Reports Series No. 277, IAEA, Vienna (1997)

27 Où trouver plus d’Information
International Commission on Radiation Units and Measurements, Quantities and Units in Radiation Protection Dosimetry, Report No. 51, ICRU, Bethesda (1993) International Commission on Radiation Units and Measurements, Fundamental Quantities and Units for Ionizing Radiation, Report No. 60, ICRU, Bethesda (1998) Hine, G. J. and Brownell, G. L., (Ed. ), Radiation Dosimetry, Academic Press (New York, 1956)

28 Où trouver plus d’Information
Bevelacqua, Joseph J., Contemporary Health Physics, John Wiley & Sons, Inc. (New York, 1995) International Commission on Radiological Protection, Data for Protection Against Ionizing Radiation from External Sources: Supplement to ICRP Publication 15. A Report of ICRP Committee 3, ICRP Publication 21, Pergamon Press (Oxford, 1973)