5. Interaction of Ionizing Radiation with Matter

Transcription

1 5. Interaction of Ionizing Radiation with Matter Type of radiation charged particles photonen neutronen Uncharged particles Charged particles electrons (b - ) neg. He 2+ (a), H + (p) D + (d) Recoil nuclides Fission fragments Wir können die Wechselwirkung dieser Strahlen als Elementarprozesse betrachten (Einzelprozesse) oder als makroskopische Effekte (Abschwächung, Absorption, Streuung etc.) CHE-711-Teil2-FS17-1

2 Interaction of Ionizing Radiation with Matter Praktische Auswirkungen der Strahlung Strahlung: Bremsung, Energieabnahme Materie: Physikalische, chemische, biologische Wirkung Parameter, welche bei der Wechselwirkung eine Rolle spielen Teilchen Masse, Ladung Geschwindigkeit, kinetische Energie Spin Materie Atommasse M, I Kernladungszahl Z Anzahl e - pro Volumen Dichte Ionisationspotentiale CHE-711-Teil2-FS17-2

3 Interaction of Ionizing Radiation with Matter Synopsis of interactions with the electronshell Ungeladene Teilchen, Photonen Photoeffekt Comptoneffekt (Paar Erzeugung) Mit den Atomkernen geladene Teilchen Photonen: Neutronen: Kernreaktionen Bremsstrahlung Paarbildung Kernreaktionen Kernreaktionen CHE-711-Teil2-FS17-3

4 Ionizing Radiation Wir unterscheiden zwischen direkt ionisierend: a, b -, b +, Energie reicht aus, durch Stoss Ionen zu erzeugen und indirekt ionisierend: n + g setzen erst im Material Ionen frei In the context of radiation absorption, two definitions are important linear stopping power and linear energy transfer If no Bremsstrahlung (see later) S I and L I are equal, otherwise there will be a substantial difference also important CHE-711-Teil2-FS17-4

5 Interaction of Ionizing Radiation with Matter Übersicht der Wechselwirkung (von Materie) mit Elektronen Geladene Teilchen: - Bremsung durch unelastische Streuung - Ionisation und Anregung CHE-711-Teil2-FS17-5

6 Ionizing Radiation by collision with electrons, the incident particle ionizes matter the mean energy to remove an electron is called the W-factor W-factor for air is 33.85eV/IP When the charged particle travels through matter, it makes an energy dependent number of ionization / length this is the specific ionization SI we can determine the mean energy loss per path length Linear Energy Transfer LET = SI W CHE-711-Teil2-FS17-6

7 Ionizing Radiation The lower the energy, the higher the SI since probability of interaction with shell electron increases Bragg Peak CHE-711-Teil2-FS17-7

8 Ionizing Radiation Let s make an example 241 Am was in smoke detectors E a =5.48 MeV specific ionization (SI) = IP/cm LET = = 1.2 MeV/cm Range = = = 4.8 cm This is the maximum range since the SI increases dramatically at the end of the path CHE-711-Teil2-FS17-8

9 Ionizing Radiation Interaction with other materials SI will change since e - -density changes one measure is the relative stopping power (see before) RSP = R air /R abs (R = Range) RSP values for some materials and particles CHE-711-Teil2-FS17-9

10 Ionizing Radiation Ranges in air for different particles and energies CHE-711-Teil2-FS17-10

11 Ionizing Radiation: Electrons -The most important interaction of electrons with matter is inelastic scattering with electrons from the shells thereby ions are generated Since not every collision leads to ionisation, the average energy loss for ionisation is larger than the minimal I e of the atoms Bethe and Bloch proposed a simple formula for energy loss along a track, considering the nature of the absorber CHE-711-Teil2-FS17-11

12 Ionizing Radiation: Electrons mit m e = Ruhemasse Elektron e 0 = Dielektrizitätskonst. Vakuum an = Anzahldichte des Materials v = Geschw. Elektrons T = mittlere Ionisierungsdichte des Materials please note: since e - are light particles, relativistic effects have to be considered E = 100 kev E = 1000 kev v = 0.55 c v = 0.94 c m = 1.2 m o m = 3 m o for lower energies, the relativistic effects can be neglected CHE-711-Teil2-FS17-12

13 Ionizing Radiation: Electrons both formulas predict a minimum value de dx at a certain energy......depending only on the mass of the particle thus, the slower the particle the more ionization per length will appear CHE-711-Teil2-FS17-13

14 Ionizing Radiation: Electrons absorption of electrons (b - radiation) Note: typical b - decay shows a continuous energy distribution, hence it has many low energy electrons inspecting the Bethe-Bloch formula, it is obviously an exponential formula empirically,it translates into: Y(x) = Y(0) e -µ x with µ = konst or N(x) = N 0 e -µ x with µ = linear absorption coefficient (see x-ray crystallography) CHE-711-Teil2-FS17-14

15 Ionizing Radiation: Electrons absorption of electrons (b - radiation) thus, the absorption of electrons decreases linearly often, instead of path x one takes mass-equivalent range d = d x then with µ/d = mass absorption coefficient note that µ is a function of the electron energy and the material it allows to calculate the maximum range of electrons in a material it allows to calculate the thickness of materials for shielding CHE-711-Teil2-FS17-15

16 Ionizing Radiation: Electrons Example: equivalent range of e - in Al one can easily calculate the path for reducing the e - -flux to 50% x 1/2 = ln2 µ and d 1/2 = (ln2)/(µ/8) x 1/2 can be determined experimentally and µ be calculated for a particular material CHE-711-Teil2-FS17-16

17 Ionizing Radiation: Electrons There exists also a semiempirical relationship between µ, d and Emax and there are semiempirical relationships for connecting range with electron energy (0.15 < Eβ < 0.8 MeV) CHE-711-Teil2-FS17-17

18 Ionizing Radiation: Electrons Calculate the maximum range of different β-emitters CHE-711-Teil2-FS17-18

19 Ionizing Radiation: Electrons How much energy can be lost in a single collision? of particular interest: collision with a shell electron maximum energy transfer incoming particle : mass M i, speed V i1 electron : mass m e speed 0 after collision : M i, v 2, m e, v e Energy: ½ M i v 12 = ½ M i v 22 + ½ m e v e 2 momentum: M i v 1 = M i v 2 + m e v e (non-relativistic) CHE-711-Teil2-FS17-19

20 Ionizing Radiation: Electrons maximum energy transfer speed of reflected particle MET Q max = nicht relativistisch If M i = m e (electron on electron) then Q max = E explains why light particles have a zigzag pass in matter take an a-particle colliding with an e - m e = kg m a = kg u u Q max /E = = = 0.05 %!! That s why heavy particles travel straight CHE-711-Teil2-FS17-20

21 Ionizing Radiation: Electrons before going further: what is the speed of a particle of a given energy E and rest mass m 0 easy E = ½ m o v 2 true for energies with speed away from c relativistic equation: aufgelöst: for an electron with E = 100keV 1 ev = J m o = kg c = m/sec E = 100 kev v = m/sec (rel) m/sec (nicht rel) CHE-711-Teil2-FS17-21

22 Ionizing Radiation: Electrons % of speed of light c: this can be calculated for all particles back to maximum energy transfer Q max = with reduces to Q max = 2g 2 m e v i since <<<1, usually CHE-711-Teil2-FS17-22

23 Ionizing Radiation: Protons Examples for protons H Proton Kinetic Energy E (MeV) Q max (MeV) x x x 10 6 Maximum percentage energy transfer 100Q max /E CHE-711-Teil2-FS17-23

24 Ionizing Radiation Specific ionizations for e.g. elecrons in air can be calculated if their velocity is known SI = (keep in mind that v changes upon absorption) example: 32 P (E max (b) = MeV) calculate v = m/s b = 0.87 SI = 6000 IP/m LET = 0.2 MeV/cm keep in mind that not only ionization takes place but also scattering of electrons at the nucleus CHE-711-Teil2-FS17-24

25 Ionizing Radiation: Bremsstrahlung The most important interaction is inelastic scattering Which results in the emission of Bremsstrahlung CHE-711-Teil2-FS17-25

26 Bremsstrahlung The stopping power of atoms or materials does notonly depend on ionization but also on direct electron-target nucleus interactions This energy loss generates photons, so called Bremsstrahlung thus: total stopping power From Bethe-Formula, the ratio between collision and radiation is thus: the higher the energy, the more Bremsstrahlung and the higher the atomic number, the more Bremsstrahlung CHE-711-Teil2-FS17-26

27 Bremsstrahlung since the stopping efficiency by Bremsstrahlung increases by z 2, but the stopping by ionization only by z, the formation of Bremsstrahlung increases with E The following formula gives this ratio example: Pb shielded source of 90 Y(E max = 2.28MeV) produces 10% Bremsstrahlung CHE-711-Teil2-FS17-27

28 Bremsstrahlung CHE-711-Teil2-FS17-28

29 Bremsstrahlung The Bremsstrahlung is used to produce Synchrotronradiation CHE-711-Teil2-FS17-29

30 Bremsstrahlung this means: don t shield b-emitters with lead!! let s make an example: How much energy does a 2.2 MeV electron loose by passing through 5mm Lucite (Acrylglas)? r = We calculate the maximum range of 2.2 MeV using the formula for low Z materials R = E ( lne) = 1.06 g/cm 2 CHE-711-Teil2-FS17-30

31 Bremsstrahlung The same result can be received from the range vs energy graph CHE-711-Teil2-FS17-31

32 Bremsstrahlung 2. now we relate the stopping power and the energy by the formula for low Z materials InE after = (3.29-lnE before ) ½ = E after = 1.11 MeV and 1.09 MeV is absorbed with the formula from p % are converted to Bremsstrahlung (4.23 kev) CHE-711-Teil2-FS17-32

33 Ionizing Radiation: High Energy Photons Interaction of g-radiation and x-rays with matter - Photons do not steadily lose energy as they penetrate matter - The distance the photons can travel before they interact with an atom is governed statistically by a probability, which depends on the specific medium and on the photon energy Three principle modes of interaction Photo Effect Compton Effect Pair Formation CHE-711-Teil2-FS17-33

34 Ionizing Radiation: High Energy Photons The Photo Effect In coming g-quant - interaction between g - quanta and electrons of the inner shells - emission of a photoelectron (ionization) - dominates with low photon energies - absorption of the g -quant Photo electron Electron of the shell L- shell Higher energy levell radiation - electron gap filled by an outer-sphere electron (X-ray fluorescence, secondary radiation) K- shell Lower energy levell g-quant Photon CHE-711-Teil2-FS17-34 CHE-611-FS10-Teil2-34

35 Ionizing Radiation: High Energy Photons The Photo Effect The photoelectron contains the complete energy of the g quant minus an energy j that the electron expends in escaping the atom T = hn -j Every g-rays emitting nucleus emits g-quanta with a distinct energies (fingerprint) g spectroscopy CHE-711-Teil2-FS17-35

36 Ionizing Radiation: High Energy Photons The Photo Effect The photo effect depends strongly on the atomic number Z and the energy hn of the photons probabilit y = Z4 ( hn ) 3 CHE-711-Teil2-FS17-36

37 Ionizing Radiation: High Energy Photons The Compton effect incoming g-quant scattered g-quant Compton electron - interaction between g -quanta and e - of the outer electron shells (Compton electrons) - emission of a Compton electron (ionization) - g -quant loses energy (shift to longer wavelengths, Compton shift) - the Compton shift only depends on the scattered angle, not on the wave length of the incident-photon - resulting quant can undergo more Compton reaction or finally photo reactions CHE-711-Teil2-FS17-37

38 Ionizing Radiation: High Energy Photons The Compton effect Compton continuum - The emitted Compton electrons have no defined energy (Compton continuum) CHE-711-Teil2-FS17-38

39 Ionizing Radiation: High Energy Photons The mystical Pair Formation E = m c Never forget: 2 Þ A photon with an energy of at least MeV can be converted into an e + / e - pair in the field of an atomic nucleus h n ³ 2 c 2 m e - Excess energy is kinetic energy of the products In coming g-quant - The distribution of the excess energy is continuous CHE-711-Teil2-FS17-39

40 Ionizing Radiation: High Energy Photons The mystical Pair Formation - Pair production becomes more likely with increasing photon energy - The probability also increases with the atomic number probabilit y» Z 2 CHE-711-Teil2-FS17-40

41 Ionizing Radiation: High Energy Photons The Annihilation of Positrons The produced positron immediately reacts with an electron e + + e- = hn Since the total momentum before the decay is zero, two photons must be produced in order to conserve momentum The produced photons going off in opposite directions Due to 2m e c 2 = hn the photon energy is 511 kev (1.022 MeV/2) CHE-711-Teil2-FS17-41

42 Ionizing Radiation: High Energy Photons Advantages and Disadvantages of the Pair Formation Disadvantage: - The presents of 511 kev annihilation photons around any positron source is always a potential radiation hazard Advantages: - Pair Formation helps to convert high energy photons (> MeV) into photons with less energy (511 kev) Þ easier to shield Question: How would you shield a g-emitter? CHE-711-Teil2-FS17-42

43 Ionizing Radiation: High Energy Photons Occurrence of the three mechanisms of interaction Atomic number of absorber CHE-711-Teil2-FS17-43

44 Ionizing Radiation: High Energy Photons Interaction of Neutrons with Matter - Neutrons have no charge and don t interact with the shell electron (no direct ionization) - Interactions between neutrons and matter are interactions with nuclei (only secondary ionization processes) Classification of Neutrons Thermal Neutrons: Energy distribution according to the Maxwell Boltzmann equation Energy ev (most probable energy in the distribution at 20 C) Slow Neutrons: Also called intermediate of resonance neutrons. Energy 0.1 MeV Fast Neutrons: Energy 20 MeV Relativistic Neurtons: Energy > 20 MeV CHE-711-Teil2-FS17-44

45 Ionizing Radiation: High Energy Photons Interaction of Neutrons with Matter - main mechanisms: elastic and inelastic impacts, neutron capturing Elastic and inelastic impacts slow neutron, W 2 slow neutron, W 2 Fast neutron, W 1 Backscattered nucleus, W 3 Fast neutron, W 1 W 3 W 1 = W 2 + W 3 W 1 > W 2 + W 3 Energy range: 10 kev - 1 MeV Energy range: 1-10 MeV - emission of excess energy as g -quants CHE-711-Teil2-FS17-45

46 Ionizing Radiation: High Energy Photons Interaction of Neutrons with Matter Elastic Scattering Qmax 4mME = n ( M + m) 2 M = Mass of a neutron m = Mass of the recoil nucleus E n = Kinetic energy of the neutron Fast neutron, W 1 slow neutron, W 2 Backscattered nucleus, W 3 Maximum Fraction of Energy Lost, Q max / E n W 1 = W 2 + W 3 Energy range: 10 kev - 1 MeV CHE-711-Teil2-FS17-46

47 Ionizing Radiation: High Energy Photons Interaction of Neutrons with Matter Slowing-down neutrons is called neutron moderation If a neutron reaches thermal energies, it will move about randomly by elastic scattering until absorbed by a nucleus Nuclear reaction: (n,p), (n, 2n), (n, a), (n, g) Neutron Activation Analysis CHE-711-Teil2-FS17-47

48 6. Biological action of ionizing radiation Interaction of radiation with a biological system leads to an energy transfer The biological impact depends on: type of radiation type of irradiated biological material How to quantify the amount of transferred energy? How to quantify the biological impact? CHE-711-Teil2-FS17-48

49 6. Biological action of ionizing radiation Dose and Dose Rate Ion Dose (Exposure) I = produced charges mass of irradiated air I = DQ Dm Radiation source SI Unit: I = C(As) kg = 6, Ion pairs kg air Ionisation chamber Old unit: R (Roentgen) - Measurement of the ionisation in an ionisation chamber - gasfilled container with a window of thin material - electric current is produced by ions which are produced by the influence of radiation -4 2,58 10 C 1R = kg air C 3 1 = 3,88 10 R kg air Strahlenmenge, die nötig ist, um positive und negative Ionen von einer elektrostatischen Einheit im Volumen von einem Kubikzentimeter (1 cm³) Luft bei Normalbedingungen freizusetzen Eine Dosis von 1 Röntgen pro Kubikzentimeter Luft 2 Milliarden Ionenpaare CHE-711-Teil2-FS17-49

50 6. Biological action of ionizing radiation From Ion Dose to Energy Dose absorbed radiation energy D = D = mass ΔW Δm the formation of 1 ion pair requires 34 ev SI Unit: 1 Gy (Gray) 1 Gy = 1 J/kg Old unit: rd (rad) 1rd = 10-2 Gy with this information we have a direct information about the transfered energy CHE-711-Teil2-FS17-50

51 6. Biological action of ionizing radiation From Energy Dose to Equivalent Dose Damage of organic material (tissue) can only be expected if the energy is absorbed by the tissue (Interactions radiation -matter) The bigger the absorption is, the bigger is the impact Highly ionizing radiation has a higher impact than weakly ionizing (a > n > b, g, X) Energy dose exclusively reflects the pure energy value (not the impact) Equivalent dose H = D W W = weighting factor of the radiation SI Unit: 1 Sv (Sievert) 1 Sv = 1 J/kg old unit:1 rem 1 Sv = 100 rem (roentgen equivalent men) CHE-711-Teil2-FS17-51

52 6. Biological action of ionizing radiation Equivalent Dose Representing the stochastic health effects of ionizing radiation on the human body. Equivalent Dose enables the comparison of different types of radiation. Equivalent dose H = D W W = weighting factor of the radiation Normal cell Damaged cell Radiation types X-rays, g- and ß- radiation W Neutron radiation about 10 a - radiation 20 1 Biological sample after irradiation with Beta- Particles relative destruction: 1 Energy dose: 1 Gy Biological sample after irradiation with Alpha- Particles relative destruction: 1 Energy dose: 0.05 Gy Dose rate: dh (Sv / h) dt CHE-711-Teil2-FS17-52

53 6. Biological action of ionizing radiation instantaneous Physical Process energy transfer Radiobiological Functional Chain minutes molecular & biochemical changes hours somatic cell cellular changes gamete cell days acute direkt damage next generation weeks/ month neoplasms (cancer, leukemia) years non-malignant later damage genetic damage deterministic stochastic CHE-711-Teil2-FS17-53

54 6. Biological action of ionizing radiation Direct vs. Indirect Radiation Effect CHE-711-Teil2-FS17-54

55 6. Biological action of ionizing radiation DNA Damages Single-/ Double-strand breaks Chemical bond between Neighboring nucleotides Chemical modification of a nucleotide (mutation) / losing of one nucleobase Chemical linkage of two strands CHE-711-Teil2-FS17-55

56 6. Biological action of ionizing radiation DNA Damages spontaneous radiation-induced Event per second per hour per year per mgy Single-strand break 1.4 ca. 5 x 10 3 ca. 4.4 x Double-strand break 0.04 Depurination ca. 1.5 x 10 3 ca. 1.4 x Base damage 0.8 ca x 10 3 ca. 1.1 x Total 2.2 ca. 8 x 10 3 ca.7 x 10 7 ca. 2.0 CHE-711-Teil2-FS17-56

57 6. Biological action of ionizing radiation Radiation Damages CHE-711-Teil2-FS17-57

58 6. Biological action of ionizing radiation Stochastic vs. Deterministic Effects Bei den somatischen Strahlenwirkungen unterscheidet man zwischen stochastischen und deterministischen Strahlenwirkungen. CHE-711-Teil2-FS17-58

59 6. Biological action of ionizing radiation Deterministic Radiation Damage CHE-711-Teil2-FS17-59

60 6. Biological action of ionizing radiation Deterministic Radiation Damage CHE-711-Teil2-FS17-60

61 6. Biological action of ionizing radiation Deterministic Radiation Damage Strahlenverbrennung der Haut Strahlendermatitis und Epilation CHE-711-Teil2-FS17-61

62 Biological action: Dose and dose rate Dose Limits Equivalent Dose Limits StSG ( ) / StSV ( ) Equivalent Dose Limits (annual) Body Equivalent Dose Limits for tissues & organs (annual) Lens of eye Skin, hands, and feet 1 msv (public) 20 msv (people working with activity) max. 50 msv (exceptional with permission) 150 msv 500 msv CHE-711-Teil2-FS17-62