Lecture # 10 Dozimetry of Ionizing Radiations

1. Lecture # 10 Dozimetry of Ionizing Radiations

2. Plan of the lecture • Interaction of ionizing radiation (α;βˉ;β+; ɣ;n) with substance • The affect of ionizing radiation on biologic objects • Dozimetry • Medical applications of ionizing radiation

3. Interaction of ionizing radiation with matter • There are different ionizing mechanisms for various types of the ionizing radiation. • Main mechanism of ionization of matter by charged corpuscular radiation, in particular, α- and β ˉ-radiation is ionization braking. • These charge particles electrically interact with the electron shells of the atoms and lose their energy on ionizing atoms.

4. Interaction ionizing radiation (IR) with substances leads to ionization of the atoms of the material. • Ionizationis a process of transforming the neutral atom into ions – positive (cations) and negative (anions). ___ Ionizing radiation __ ↓ ↓ Corpuscular : Electromagnetic: • alpha-radiation • gamma-radiation • beta-radiation • X-radiation • neutron flux • proton flux • In medicine X-radiation is used the most often.

5. Corpuscular IR (fluxes of neutrons and protons, alpha, beta radiation) Neutron (n) is an electrically neutral elementary particle with a mass close to 1 a.m.u. (Atomic mass unit is in 1840 times more than the mass of the electron) Proton (p) is a positively charged elementary particle, the nucleus of the hydrogen atom. The mass of the proton is approximately equal to the mass of the neutron (1a.m.u.) The number of protons in the nucleus of chemical element determines its serial number in the periodic table.

6. The atomic nucleus consists of protons (p) and neutrons (n). The common name for protons and neutrons is nucleons. • The number of nucleons in the nucleus A is a mass number. • The number of protons Z is the charge number, or proton number, or atomic number. • Number of neutrons is N   A = Z + N • Chemical element X:

7. Alpha-radiation (α-radiation) is a flux of α-particles. α-particle is the nucleus of a helium atom containing 2 protons (Z = 2) and 2 neutrons (N = 2), the mass number is A = 4. • Beta-radiation (β-radiation) is a flux of β-particles: • - particle is an electron, (e-) • - particle is a positron, (e +) • Therefore, we distinguish beta-positron and beta-electron radiation.

8. The main mechanism of ionization of substances under the action of charged corpuscular radiation is ionization braking.

9. ●Interaction of α-particles with matter concludes that, taking away two electrons from an atom, it becomes a neutral helium atom, and the substance is ionized.●The primary processes of interaction of α-particles with matter are ionization and excitation.●Secondary processes: increasing the speed of molecular thermal motion, characteristic X-rays, radioluminescence.

10. β ˉ-particle decelerates to thermal velocities, typically is captured by any atom. In this atom is converted into a negative ion. • Furthermore, β ˉ- particles can cause other processes: the deceleration of electrons occurs bremsstrahlung; • βˉ-particles can be scattered by of electrons substance. Negative ion particle Atom

11. ♦β+-particles (positrons) interact with the electrons of atoms. The reaction of interaction of positrons and electrons is known as the annihilation reaction.♦During the annihilation reaction the positron and electron disappear to be replaced two ɣ-photons. Atom is modified to an ion. Atom β¯+β+→2hνTransformation of one form of matter (particles of matter) to another - field (photons) photon electron Ion β+ photon

12. Three basic mechanisms of -radiation interaction with a substance: • photoeffect, • incoherent scattering (Compton effect) and • formation of electron-positron pairs The first two mechanisms are considered early.

13. Formation of electron–positron pairs is observed only at sufficiently high energies of ɣ -quanta (more than 1.2 MeV). In this case the ɣ-photon disappears to be replaced by an electron and positron. These charge particles can ionize neutral atoms. ɣ -quanta positron electron Atom

14. Besides the three basic mechanisms of ɣ-radiation interaction with a substance should be mentioned photonuclear reactions. Photonuclear reactions are radioactive transformations of atom nuclei after they have absorbed ɣ -photons. nucleus New atom ɣ-photon ɣ-photon ɣ-photon ɣ-photon Atom

15. Neutrons by themselves do not cause ionization of a substance, but in interacting with the nuclei of atoms they generate ionized radiation. Interaction of neutrons with nuclei causes either scattering of neutrons or their capture by the nuclei. • During scattering part of the neutron’s kinetic energy is transferred to the nucleus. If the nucleus is a light one, it can acquire a high velocity. Such Compton nuclei have a sufficient kinetic energy for ionizing. • When capturinga neutron the nucleus goes to an excited state, which is often an unstable one. In this case it can be observed βˉ-decay or nuclear fission, though reactions with emission of protons or α-particles can also occur.

16. In passing through a material, the ɣ-radiation flux attenuates. The Bouguer law describes the attenuation of the flux of a monochromatic ɣ-radiation: whereμph, μcи μp - are the components of the overall absorption coefficient due to photoeffect, the Compton effect and formation of electron-positron pairs respectively. The value of μand its components vs. radiation energy graph are shown below: cm-1 μp μc μph MeV

17. Passage through the substance a charged particle flow is characterised by:1. linear ionization density 2. linear braking capacity3. mean free path1. Linear ionisation density (i) is the ratio of the number (n) of ions of one sign formed by the particle in travelling over path l, to the length of this path:

18. 2. Linear braking capacity (S) is the ratio of energy (E), lost by the particle in travelling over path l , to the length of this path The mean free path is the mean distance passed by the particle in a substance until its velocity decreases to the particle’s thermal velocity. Heavy charged particles have a characteristic dependence of the linear ionization density on the path travelled in a substance i 106, m-1 x10-2, m

19. At high velocities, the particles weakly ionize atoms of substance. After braking, the particles start interacting more effectively with the electron shells of the atoms to ionize them. The area of high values of i on the graph is known as the Bragg peak. Bragg peak

20. Flux attenuation of -radiation by the substance: • N0 is the number of -particles incident on a substance over some time; • Nis the number of particles penetrating to depth l ; •  is the linear attenuation constant. • Deviation from the exponential character of the N vs. dependence is observed only at small values of N.

21. Penetrating capacity is the capability of radiation to penetrate through the substance. • Neutron, ɣ- and X-radiation possess a high penetrating capacity. Charged particles possess a significantly less penetrating capacity. • Usually the greater the penetrating capacity of the radiation the less is its ionizing capacity, and vice versa.

22. Paperprotect against alfa-radiation only

23. Glassprotect against alfa- and beta- radiation

24. Steel sheetprotect against alfa-, beta-, X- and gamma- radiation

25. Concrete plateprotect against alfa-, beta-, X-, gamma- and neutron radiation

27. The affect of IR on cells, biological tissue and the body is negative and has a number of features: • a significant biological effect is created by negligible doses of IR; • the affect of IR on biological objects has a latent period, i.e. the effect of radiation takes place in some time after effect; • the tissues of a body, the cells, and parts of cells differ as to sensitivity to IR. The most sensitive to radiation are those tissues where cells divide actively.

28. DOZIMETRY • Dosimetry is an area of science and technology, studying quantitative characteristics effects of ionizing radiation on biological objects (doses), as well as methods and apparatus that allows these characteristics to define. • The radiobiological effect of ionizing radiation is the greater the greater the radiation energy absorbed by the material. Hence, irrespective of the radiation source, its impact on objects can be characterised by the radiation dose.

29. Types of doses: • Radiation dose (D) • Exposure dose (X) • Equivalent dose (H)

30. Radiation dose (D)is the ratio of radiation energy (E) absorbed by the object’s material to the mass (m) of this material. • Earlier this characteristic was known as the absorbed dose. • SI unit is grey (Gy). • At the radiation dose of 1 Gy, 1 kg of material absorbs 1 J of energy of ionizing radiation. • Off-system unit is rad (1 Gy = 100 rad). • Ratio of the radiation dose to the time of exposure is known as the dose rate (Ḋ): Unit is grey per second (Gy/s) or rad per second (rad/s).

31. In practice, it is difficult to measure the dose rate. Hence, the radiation dose absorbed by a body is estimated by measuring the degree of ionisation of air ambient to the body. → the charge of one-sign ions (q) X-radiation or ɣ-radiation A characteristic of air ionisation by X-radiation or ɣ-radiation is the exposure dose (X).

32. The exposure dose is equal to the ratio of the charge of one-sign ions (q), formed in air under the affect of X-radiation or ɣ-radiation, to the mass (m) of this air, i.e. • SI unit is coulomb per kilogram (C/kg). • In practice, a widely used off-system unit is roentgen (R). • One roentgen is the dose of X-radiation or ɣ -radiation, under the affect of which, at total ionization in 1 cm3 of air under normal conditions (temperature 0 °C and pressure 760 mm Hg), 2.08109 pairs of ions of each sign are formed. • 1 R = 2.5810-4Coulomb per kg (C/kg).

33. Exposure dose rate (Ẋ) SI unit isampere per kilogram (A/kg), off-system unit is roentgen per second (R/s).

34. The radiation dose depends on the exposure dose and a factor (f). This factor depends on the properties of the material radiated and the photons’ energy. • If the radiation dose is expressed in rads and the exposure dose in roentgens, the value of factor for water and soft tissues of the body can be taken with good accuracy to be equal to unit. • For bone tissue the value of decreases with growth of photons’ energy from 4.5 to 1.

35. ♦The radiobiological effect of IR depends on the type of radiation. Slow (thermal) neutron radiation is more effective than photon (X and gamma) radiation by a factor of three, and -radiation is 20 times more effective.♦To characterise the radiobiological effect created by different kinds of radiation, the equivalent doseis used.The biological effect created by radiation, at a specified equivalent dose, does not depend on the kind of radiation. where k is thequality factor (relative biological effectiveness). • SI unit issievert (Sv), • off-system unit isrem (the man roentgen-equivalent). • 1 Sv = 100 rem.

36. Natural sources of ionizing radiation create a certain radiation level that continuously affects all the organisms on Earth. This level is known as the background. The annual equivalent dose related to the natural background is equal to 125 to 200 mrem. • In medicine the maximum permissible doseis used. The maximum permissible dose is the maximum value of an individual annual equivalent dose, which, at uniform exposure during 50 years, shall have no detrimental effect on man’s health. The maximum permissible annual equivalent dose for professional exposure is equal to 5 rem.

37. Different tissues have different radiosensitivity. Therefore the notion of an effectiveequivalent dose (HE) is introduced. The effective equivalent dose is found from the formula where HTi is the mean value of an equivalent dose for the i-th tissue (organ); Wi is the weight coefficient accounting for the contribution of the i-th tissue (organ) to the body’s total radiosensitivity.

38. The values of coefficient for different organs are given in the following table:

39. Internal radiation occurs when sources of ionizing radiation get into the body with aspiration, food or water, or with injury of the skin. Internal radiation has several features: • - the time of body tissue exposure to radiation increases since radiation continues over the entire time during which the radioactive material stays in the body; • - the radiation dose for the internal organs increases due to the small distance between the organs (tissues) and the radiation sources; • - at internal radiation, those types of radiation that are safe during external radiation become hazardous. For example, - radiation which is completely held back by the skin during external radiation.

40. The impact of ionising radiation can lead to death of the organism. To characterise organisms as per the criterion of the probability of a lethal effect, the minimal lethal doseis introduced. • The minimum lethal dose from ɣ-radiation, at radiation of the whole organism, for man is about 600 rem. LD50 is the radiation dose at which 50 % of radiated organisms die. For man exposed to ɣ-radiation, LD50 is equal to 2.5 to 3.5 Gy.

41. Dozimetry instruments Different instruments are used for detecting ionising radiation, determining their characteristics, and measuring radiation doses.These instruments can be divided into two groups: detectors of ionising radiation and dosimeters.Ionising radiation detectors are intended for registering radiation and measuring some of their characteristics (e.g., energy and velocity of particles, the ratio of their charge to mass, and others). Detectors include the Wilson chamber, the bubble chamber, the Geiger counter, thick-layer photo plates, and others. • The Geiger counterThe bubble chamberThe Wilson chamber

42. Dozimeters are devices for measuring dozes of ionizing radiation, or values related to dozes. Depending on the physical phenomenon determining the dozimeter’s principle of operation is based, the following devices are distinguished: ionization, luminescent, semiconductor and photo dozimeters.

43. Medical Applications of Ionizing Radiation • Ionizing radiation is widely used in medicine. • Radionuclide diagnostics is based on applying ionizing radiation. Let us remind that the notion of nuclides is equivalent to the notion of the atom nucleus, and radionuclides are radioactive atoms. • Dynamic and static methods of radionuclide diagnostics are distinguished.

44. Dynamic diagnostics involves the introduction to the patient (usually intravenously to the blood) of aradiopharmaceutical preparation, i.e. a substance labelled with a certain radionuclide. After this, one measures the value of the radiation flux created by the preparation in the organ being examined. • By registering the radiation flux vs. time dependence, important information is obtained on the functioning of the organ being diagnosed. Hence, dynamic methods are mainly functional diagnostics ones. Dynamic renal scintigraphy, normal state. Posterior planar sequence (1 min/image) (a). Results of the semiqunatitaitve evaluation (b).

45. During static radionuclide diagnostics, one usually examines the spatial distribution of the radiopharmaceutical preparation in the organ being examined. The features of accumulation, or distribution, of the preparation in the organ reflect, primarily, alterations in its structure. Scintigraphic anatomy of the liver. 99mTc (pertechnetate)-colloid with activity 2 MBq / kg is administered intravenously

46. heart lung lung spine tumour Radiation therapy Radiation therapy is used primarily for destroying malignant tumour cells. X-radiation and γ-radiation is widely applied for this purpose. Electrons accelerated to high velocities can also be used. The most effective to date method, however, is the application of heavy charged particles. This is due to the presence of a characteristic Bragg peak for these particles in the linear ionisation density vs. depth of particles penetration into a material dependence causes the cells to be affected at a certain depth.