Radiation's Interaction with Matter

Radiation's Interaction with Matter

OBJ 1 Radioactivity & Radioactive Decay 1 Chart of the Nuclides General Layout Each nuclide occupies a square in a grid where Atomic number (Z) is plotted vertically Number of neutrons (N) is plotted horizontally Heavily bordered square at left side of each row gives

Name Chemical symbol Elemental Mass Thermal neutron absorptions cross section Resonance integral 2 Chart of the Nuclides Nuclides on diagonal running from upper left to lower right have same mass numbers, called isobars

Colors and shading used to indicate in chart squares used to indicate relative magnitude of Half-lives Neutron absorption properties Four different colors used Blue Green Yellow Orange

3 Chart of the Nuclides Background color of upper half of square represents T1/2 Background color of lower half of square represents greater of the thermal neutron cross section or resonance integral When nuclide is stable and thermal neutron cross section is small or unknown, entire square is shaded grey Gray shading also used for unstable nuclides having T1/2 sufficiently long (>5E8 yrs) to have survived from the time they were formed 4

Chart of the Nuclides Some squares, such as 60Co, 115In, and 116In are divided Occurs when nuclide has one or more isomeric or metastable states Has same A and Z, but different nuclear and radioactive properties due to different energy states of the same nucleus 5 Chart of the Nuclides Nuclide Properties Displayed on the Chart

Chemical Element Names and Symbols Same element names and symbols as used on the Periodic Table of the Elements Atomic Weights and Abundances Isotopic masses in AMUs are given for Stable isotopes Certain long-lived, naturally occurring radioactive isotopes Nuclides particle decay becomes a prominent mode (>10%) 6 Chart of the Nuclides Isotopic Abundance

Values on chart given in atom percent Specified for 288 nuclides (266 stable and 22 radioactive) Half-lives Half-life listed below nuclide symbol and mass number Units used

ps ns s ms s m h d a picoseconds (1E-12 s) nanoseconds (1E-9 s) microseconds (1E-6 s) milliseconds (1E-3 s) seconds

minutes hours days years 7 Chart of the Nuclides Background Color of Chart Square Upper Half 1 day to 10 day orange >10 days to 100 days yellow

>100 days to 10 years green >10 years to 5E8 years blue Background Color of Chart Square Lower Half Refers to thermal neutron cross section or resonance integral 8 Chart of the Nuclides Major Modes of Decay and Decay Energies

alpha particle - beta minus (negatron) + beta plus (positron) gamma ray n neutron

p proton d deuteron t triton electron capture IT isomeric transition e- conversion electron -- double beta decay cluster decay D delayed radiation 9 Chart of the Nuclides To understand decay schemes and energies, look at chart square for 38Cl - energies listed on 1st line in

order of abundance energies listed on 2nd line in order of abundance Particle energies always given in MeV energies always given in keV 10 Chart of the Nuclides When more than one decay mode possible, modes listed on chart in order of abundance or intensity Different modes of beta decay

(, +, -) appear on separate lines if intensity of one of the decay modes is <10% absolute intensity Conversely, appear on same line if intensities of both >10% absolute intensity with most abundant listed first. 11 Chart of the Nuclides When branching decay occurs by both - and + and/or , and each decay is accompanied by emission, format shown in 146Pm square is used

Metastable (or isomeric) state frequently decays to ground by IT emission, followed by one or more in cascade Internal conversion is process resulting from interaction between nucleus and extranuclear electrons. Nuclear excitation energy xfrd to orbital electron (usually K shell) and is indicated by e12 Chart of the Nuclides Delayed emission indicated by symbol D. When daughter product has too short of a half-life to have its own spot on the chart or half-life is much shorter than that of the parent nuclide, energy is listed with the parent

13 Chart of the Nuclides 14 Chart of the Nuclides 15 Radiation Classifications Introduction All radiation possesses energy Inherent electromagnetic Kinetic particulate

Interaction results in some or all of energy being transferred to surrounding medium Scattering Absorption 16 Radiation Classifications Ionizing or Non-Ionizing Non-Ionizing Visible light Radio and TV Ionizing

Particulate or Photonic Particulate n Electromagnetic x 17 Radiation Classifications Directly or Indirectly Ionizing Directly Ionizing Possesses charge

Does not need physical contact Indirectly Ionizing Does not have charge Needs physical contact 18 Radiation Characteristics Alpha () Charge +2 Range 2-4 in. (5 10 cm.) Shielding Paper Dead skin

Hazard Internal Target Organ Anything internal (living tissue) 19 Radiation Characteristics Beta () Charge Negatron (-) -1 Positron (+) +1 Range Average 10 ft. Energy Specific 10 12 ft./MeV

Shielding Plastic Wood Al, Cu Low Z Hazard Internal Target Organ External eye (lens) Living tissue 20 Radiation Characteristics

Gamma () and X-Ray (x) Charge 0 Range Infinite Shielding High Z Pb DU Hazard Internal Target Organ Living tissue 21 Radiation Characteristics Neutron (n) Charge 0

Range Infinite Shielding Hydrogenous H2 0 Concrete Plastic Hazard Internal Target Organ Living tissue 22 Energy Transfer Mechanisms Ionization Removing bound e- from electrically neutral atom or molecule by adding sufficient energy

to allow it to overcome its BE Atom has net positive charge Creates ion pair consisting of negatively charged electron and positively charged atom or molecule 23 Energy Transfer Mechanisms Ionizing Particle ee-Negative Ion N N PP++ PP

Positive Ion ++ N N ee-- 24 Energy Transfer Mechanisms Excitation Process that adds sufficient energy to e- such that it occupies higher energy state than lowest bound energy state

Electron remains bound to atom No ions produced, atom remains neutral After excitation, excited atom eventually loses excess energy when e- in higher energy shell falls into lower energy vacancy Excess energy liberated as X-ray, which may escape from the material, but usually undergoes other absorptive processes 25 Energy Transfer Mechanisms PP++ N N +N N PP+

ee-- ee-+ N N N + N ee-- +

N + ee-- ee-- 26 Energy Transfer Mechanisms Bremsstrahlung Radiative energy loss of moving charged particle as it interacts with matter through which it is moving Results from interaction of high-speed, charged particle with nucleus of atom via electric force field

With negatively charged electron, attractive force slows it down, deflecting from original path KE particle loses emitted as x-ray Production enhanced with high-Z materials (larger coulomb forces) and high-energy e- (more interactions occur before all energy is lost) 27 Energy Transfer Mechanisms ee-- ee-ee-- ee

-- + N N N + N + N +

ee-- ee-- 28 Directly Ionizing Radiation Charged particles dont need physical contact with atom to interact Coulombic forces act over a distance to cause ionization and excitation Strength of these forces depends on: Particle energy (speed) Particle charge Absorber density and atomic number

Coulombic forces significant over distances > atomic dimensions For all but very low physical density materials, KE loss for e- continuous because of Coulombic force 29 Directly Ionizing Radiation Alpha Interactions Mass approximately 8K times > electron Travels approximately 1/20th speed of light Because of mass, charge, and speed, has high probability of interaction Does not require particles touchingjust sufficiently close for Coulombic forces to interact Energy gradually dissipated until captures two e- and becomes a He atom

from given nuclide emitted with same energy, consequently will have approximately same range in a given material 30 Directly Ionizing Radiation Beta Interactions Interaction between - or + and an orbital e- is interaction between 2 charged particles of similar mass s of either charge lose energy in large number of ionization and/or excitation events, similar to Due to smaller size/charge, lower probability of interaction in given medium; consequently, range is >> of comparable energy Because s mass is small compared with that of nucleus Large deflections can occur, particularly when low-energy s

scattered by high-Z elements (high positive charge on the nucleus) Consequently, usually travels tortuous, winding path in an absorbing medium may have Bremsstrahlung interaction resulting in X-rays 31 Indirectly Ionizing Radiation No charge and n

No Coulomb force field Must come sufficiently close for physical dimensions to contact particles to interact 32 Indirectly Ionizing Radiation Small probability of interacting with matter Why? Doesnt continuously lose energy by constantly interacting with absorber May move through many atoms or molecules before contacting electron or nucleus Probability of interaction depends on its energy and absorbers density and atomic number

When interactions occur, produces directly ionizing particles that cause secondary ionizations 33 Indirectly Ionizing Radiation Gamma absorption and x-rays differ only in origin Name used to indicate different source s originate in nucleus X-rays are extra-nuclear (electron cloud) Both have 0 rest mass, 0 net electrical charge, and travel at speed of light Both lose energy by interacting with matter

via one of three major mechanisms 34 Indirectly Ionizing Radiation Photoelectric Effect All energy is lost happens or doesnt Photon imparts all its energy to orbital e Because pure energy, photon vanishes Probable only for photon energies < 1 MeV Energy imparted to orbital e- in form of KE, overcoming attractive force of nucleus, usually causing e- to leave orbit with great velocity Most photoelectrons are inner-shell e35

Indirectly Ionizing Radiation High-velocity e-, called photoelectron Directly ionizing particle Typically has sufficient energy to cause secondary ionizations Most photoelectrons are inner-shell electrons 36 Indirectly Ionizing Radiation ee-Gamma Photon (< 1 MeV) Photoelectron

ee-- ee-+ N N N + N + N +

ee-- ee-- 37 Indirectly Ionizing Radiation Compton Scattering Partial energy loss for incoming photon Dominant interaction for most materials for photon energies 200 keV 5 MeV Photon continues with less energy in different direction to conserve momentum Probability of Compton interaction with distance from nucleus most Compton electrons are valence electrons Beam of photons may be randomized in direction and

energy, so that scattered radiation may appear around corners and behind shields where there is no direct line of sight to source Probability of Compton interaction with distance from nucleus most Compton electrons are valence electrons 38 Indirectly Ionizing Radiation Pair Production Occurs when all photon energy is converted to mass (occurs only in presence of strong electric field, which can be viewed as catalyst) Strong electric fields found near nucleus and are stronger for high-Z materials disappears in vicinity of nucleus and -- + pair appears

Will not occur unless > 1.022 MeV Any energy > 1.022 MeV shared between the --+ pair as KE Probability < photoelectric and Compton interactions because photon must be close to the nucleus 39 Indirectly Ionizing Radiation Electron ee-Gamma Photon (E > 1.022 MeV) ee-ee++ ee-+

N N N + N ee-- + Positron N

+ ee-- ee-0.511 MeV Photons ee -- ee-- 40 Indirectly Ionizing Radiation

Neutron Interactions Free, unbound n unstable and disintegrates by - emission with half-life of 10.6 minutes Resultant decay product is p+, which eventually combines with free e- to become Energy Range H atom Category Thermal 0.025 eV (< 0.5 classified eV) n interactions energy~ dependent Intermediate 0.5 eV10 keV based on KE

Fast 10 keV20 MeV Relativistic > 20 MeV 41 Indirectly Ionizing Radiation Classifying according to KE important from two standpoints: Interaction with the nucleus differs with n energy Method of detecting and shielding against various classes are different

n detection relatively difficult due to: Lack of ionization along their paths Negligible response to externally applied electric, magnetic, or gravitational fields Interact primarily with atomic nuclei, which are extremely small 42 Indirectly Ionizing Radiation Slow Neutron Interactions Radiative Capture Radiative capture with emission most common for A slow1n

A 1 * A1 results Z P 0 n ZD Z D nuclei Reaction often in radioactive Process is called neutron activation 43 Indirectly Ionizing Radiation Charged Particle Emission

Target atom absorbs a slow n, which its mass and internal energy Charged particle then emitted to release excess mass and energy Typical examples include (n,p), (n,d), and (n,). For example A Z 1 0 X n A 1 Z

* D A 4 Z 2 4 2 D 44 Indirectly Ionizing Radiation

Fission Typically occurs following slow n absorption by several of the very heavy elements Nucleus splits into two smaller nuclei, called primary fission products or fission fragments Fission fragments usually undergo radioactive decay to form secondary fission product nuclei There are some 30 different ways fission may take place with the production of about 60 primary fission fragments 45 Indirectly Ionizing Radiation Fast Neutron Interactions Scattering

Free n continues to be free n following interaction Dominant process for fast n Elastic Scattering Occurs when n strikes nucleus of approx. same mass Neutron can xfer much of its KE to that, which recoils off with energy lost by n No emitted by nucleus Recoil nucleus can be knocked away from its e- and, being (+) charged, can cause ionization and excitation 46 Indirectly Ionizing Radiation ee--

N N PP N N++ 47 Indirectly Ionizing Radiation Inelastic Scattering Occurs when n strikes large nucleus n penetrates nucleus for short period of time Xfers energy to nucleon in nucleus Exits with small decrease in energy

Nucleus left in excited state, emitting radiation, which can cause ionization and/or excitation 48 Indirectly Ionizing Radiation ee-- ee-- PP++ N

N N N N N N PP++ PP++ N N ee--

ee-- 49 Indirectly Ionizing Radiation Reactions in Biological Systems Fast n lose energy in soft tissue largely by repeated scattering interactions with H nuclei Slow 0n1 captured in soft tissue and release energy in one of two principal mechanisms: and 1 0 1

0 14 7 1 1 2 1 * 2 1

(2.2 MeV) 1 1 (0.66 MeV) n H H H 15 7 * 14 6

n N N C p 50 Radioactivity and Radioactive Decay Following a transformation, nucleus is usually more stable than it was, but not necessarily stable Another transformation will take place by nucleus emitting radiation Amount of energy given off and emission type depends on nucleus configuration immediately before transformation As nucleus energy , nucleus disintegrates or decays Called radioactive decay Atom before decayparent Atom after decaydaughter

Steps from parent to daughter traced to stability called decay chain 51 Radioactivity and Radioactive Decay Parent-Daughter Relationships and Equilibrium Produces daughter product and radiation is emitted Daughter also produces radioactivity when it decays, as does each successive daughter until stability is reached Activity contributed by the parent vs. daughters varies based on half-life of both parent and

daughters When activity production rate is same as product decay rate, equilibrium is said to exist 52 Radioactivity and Radioactive Decay Secular Equilibrium 1/2,P >> 1/2,D (Parent half-life infinitely > daughter) As parent activity , daughter proportionately During 10 half-lives of the daughter, essentially no parent decay takes place during secular equilibrium Two conditions necessary Parent must have 1/2 much longer than any other nuclide in the series Rule of Thumb Sufficiently long period of time must have elapsed to

allow for in-growth of the decay Secular equilibrium is reached in 6products daughter half-lives. 53 Radioactivity and Radioactive Decay 54 Radioactivity and Radioactive Decay 1/2 = 53 m

1/2 = 15.3 m 1/2 = 6.57 h 55 Radioactivity and Radioactive Decay Transient Equilibrium 1/2,P > 1/2,D (Parent half-life > daughter, but not infinitely) Daughter activity decays at same approx. rate as parent Different way of saying daughter atom formation rate = daughter atom decay rate Same fractional decrease in parent and daughter

Rule of Thumb activities Transient equilibrium is reached in 4 daughter half-lives. 56 Radioactivity and Radioactive Decay 57 Radioactivity and Radioactive Decay Transient1/2,P > 1/2,D , but not very long 1/2 = 1.68 d 1/2 = 12.75 d

58 Radioactivity and Radioactive Decay No Equilibrium 1/2,P < 1/2,D Parent activity decays at faster rate than daughter Equilibrium is never reached 59 Radioactivity and Radioactive Decay 60 Radioactivity and Radioactive Decay

1/2 = 3.1 m 1/2 = 27 m 1/2 = 19.9 m 1/2 = 23.3 y 61 Decay Modes and Emissions Alpha Decay () With few exceptions, only relatively heavy nuclides decay by emission Essentially a helium nucleus (2 p+, 2 n) Charge of +2

62 Decay Modes and Emissions p n 63 Decay Modes and Emissions Beta (Negatron) Decay (-) High n:p ratio usually - decays n changed into p n:p ratio, results in - emission

Have same mass as e Because n has been replaced by p, Z 1, but A remains unchanged 64 Decay Modes and Emissions Because n has been replaced by p, Z 1, but A remains unchanged Standard notation for - decay is: A A Z X Z 1 X

210 210 210 For example, Pb decays to produce 210Bi as 82 83 follows: Pb Bi 65 Decay Modes and Emissions

p n 66 Decay Modes and Emissions Neutrinos and anti-neutrinosneutral particles with negligible rest mass Travel at speed of light and are noninteracting Account for energy distribution among + (positrons) and - (negatrons) 67

Decay Modes and Emissions Nuclide having low n:p ratio) tends to decay by positron emission Positron often mistakenly thought of as positive electron In reality, positron is anti-particle of electron (has charge of +1) + used to designate positrons With positron emitters, parent nucleus changes p+ into n and emits a + Because p+ replaced by n, Z 1 and A remains unchanged Neutrino also emitted during + emission 68 Decay Modes and Emissions

Standard notation for + decay is: A Z X A Z1 X 57Ni + decays to produce 57Co as For example, 57 57

follows: 28 Ni 27 Co 69 Decay Modes and Emissions p n 70 Decay Modes and Emissions Electron Capture (EC)

For radionuclides with low n:p ratio, another decay mode, known as EC, can occur Nucleus captures e- (usually from K shell) Could capture L-shell electron, but K-electron capture much more probable Decay frequently referred to as K-capture Can result in formation of Auger e In lieu of characteristic X-ray being emitted Atom ejects bound e Auger e- are monoenergetic 71 Decay Modes and Emissions Transmutation resembles positron emission A Z X

A Z1 X Electron combines with p+ to form a n, followed by neutrino emission Electrons from higher energy levels fill vacancies left in inner, lower-energy shells Excess energy emitted causes cascade of characteristic X-rays 72 Decay Modes and Emissions

Gamma Emission () Decay resulting in transmutation generally leaves nucleus in excited state Nucleus can reach unexcited, or ground, state by emitting Gammas are type of electromagnetic radiationbehave as small bundles or packets of energy, called photons, and travel at speed of light 73 Decay Modes and Emissions essentially the same as X-ray usually higher energy (MeV); whereas, X-rays usually in keV range

Basic difference between and X-ray is origin originate in nucleus, Xrays originate in electron shellsrays originate in electron shells General decay equation slightly different from others A * A Z Z X X Most decay reactions have emissions associated with them Some decay by particulate emission with no emission 74

Decay Modes and Emissions Isomeric Transition (IT) Commonly occurs immediately after particle emission Nucleus may remain in excited state for measurable period of time before dropping to ground state Nucleus that remains excited known as isomer because it is in a metastable state Differs in energy and behavior from other nuclei with the same Z and A Generally achieves ground state by emitting delayed (usually > 10rays originate in electron shells9 s) 75

Decay Modes and Emissions Internal Conversion An alternative isomeric mechanism to radiative transition Excited nucleus of -emitting atom gets rid of excitation energy Tightly bound e- (K or L) interacts with nucleus by absorbing Eexcitation and is ejected Electron known as conversion electron Distinguished from - by energy Conversion e- monoenergetic - spectrum of energies 76 Decay Modes and Emissions

Each radionuclide, artificial or natural, has characteristic decay pattern Several aspects associated with pattern: Decay modes Emission types Emission energies Decay rate 77 Decay Modes and Emissions All nuclei of given radionuclide seeking stability decay in specific manner 226Ra decays by emission, accompanied by only decay mode open to 226Ra

Some nuclides may decay with branching, where a choice of decay modes exists Some nuclides may decay with branching, where a choice of decay modes exists 57Ni, mentioned previously, decays 50% by EC (K capture) and 50% by + emission Nuclides decay in constant manner by emission types, and emissions from each nuclide exhibit distinct energy picture 78 Decay Modes and Emissions Single Ra nucleus may disintegrate at once or wait 1000s of years before

emitting an All that can be predicted with certainty is 1/2 of all 226Ra nuclei present will disintegrate in 1,622 years Called the half-life Half-lives vary greatly for naturally occurring radioisotopes 79 Natural Decay Series Natural Decay Series Uranium, radium, and thorium occur in three natural decay series, headed by uranium-238, thorium-232, and uranium 235, respectively In nature, in secular equilibrium

80 Natural Decay Series Uranium-238 (Radon-222) (Radon) 81 Natural Decay Series Uranium-235 (Radon-219) (Actinon) 82 Natural Decay Series Thorium-232 (Radon-220) (Thoron)

83 Radioactive Decay Law Radioactive Decay Law Decays at a fixed rate and is not a function of temperature, pressure, etc. Half-life defined as amount of time it takes for activity to be reduced to 1/2 the original value Occurs at an exponential rate 84 Radioactive Decay Law 85

Radioactive Decay Law Can be expressed as ()n or e-tt. When calculating half-life, units of time (t) must be the same time units as the half-life ()) Radioactive Decay Formula At A0 (1 / 2or) n At A0 e t Decay Constant ())

Equivalent to natural log of 2 (ln 2) divided by halflife (1/2) 86

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