Lecture 6

Lecture 6 - Professor Stewart before coffee before School...

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Unformatted text preview: Professor Stewart before coffee before School of Health Sciences http://healthsciences.purdue.edu Associate Professor Director, Radiological Health Science Program http://rh.healthsciences.purdue.edu/faculty/rds.html [email protected] Robert D. Stewart, Ph.D. Tuesday, September 9, 2008 Radiological Health Science I Introduction to the Health Sciences Professions HSCI 101 HSCI 101 Introduction to the Health Sciences Professions • Often responsible for ensuring radiation safety in the clinic too! Therapy Physics and Diagnostic Imaging • Professionals who specializes in the application of the concepts and methods of physics to the diagnosis and treatment of human disease Medical Physics • Professionals who specialize in who specialize in protecting workers and the public from potential hazards of radioactive materials and radiation generating devices (e.g., linear accelerators) Health Physics • Medical Physics and Health Physics Umbrella term used for two closely related fields Radiological Health Science Slide 2/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Nuclear Medicine Health Physics (B.S., M.S., Ph.D.) Sometimes seek certification (A.A or B.S. + clinical training or residency program) Radiation Therapists, Dosimetrist, Radiology Technicians Usually seek board certification (anywhere radioactive sources found) Radiation Safety pre-medicine (B.S.) pre- Radiological Health Science Medical Physics (M.S. and Ph.D.) pre-medicine pre- Research and physics support in clinic Radiation Oncology Radiology Medical Doctor (MD) Degree Paths in Radiological Health Science Slide 3/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Source: 2006 Health Physics Society (HPS) Salary Survey http://www.hps.org/publicinformation/hpcareers.html Careers in Health Physics http://healthsciences.purdue.edu/academics/undergraduate/rhs/ Plans of Study and Additional Information Health Physics Salary Range Slide 4/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Source: 2007 American Associate of Physicist in Medicine (AAPM) Salary Survey Medical Physicist Salary Range Slide 5/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions • • • • Absorbed dose and absorbed dose rate Radioactivity, inverse square law, attenuation Time, distance and shielding As low as reasonably achievable (ALARA) Atoms, isotopes and sub-atomic particles Major types of ionizing radiation Health effects of exposure to radiation Radiation Protection Principles Lecture Outline Slide 6/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Greek Letters α = alpha β = beta γ = gamma Atom, isotope, ionization neutron, proton, electron, nucleon X-rays, α particles, β particles, γ-rays Absorbed dose, absorbed dose rate Radioactive decay, half-life, time time Attenuation, half-value (HV) thickness, shielding shielding Inverse square law, distance distance ALARA ALARA Key Words Slide 7/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Dime ~ 1013 fm = 10-02 m ⎛ 10−5 ⎞ 15 ⎜ −10 ⎟ = 10 atoms per cell ⎝ 10 ⎠ 3 Cell ~ 1010 fm = 10-05 m Cell Nucleus ~ 1 fm = 10-15 m Atom ~ 105 fm = 10-10 m Electron (-) Neutron (neutral) Proton (+) • Number of electrons equals number of protons • Number of neutrons may or may not equal number of protons A small, very dense nucleus, composed of nucleons (protons and neutrons) Surrounded by a “swarm” of negatively charged electrons The Atom Slide 8/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions N = (A-Z) = neutron number Z = atomic number (number of protons) A = mass number (total number of nucleons) A = 12, Z = 6 and N = 6 14C: A = 14, Z = 6 and N = 8 12C: and 14C are isotopes of the element carbon (Z = 6) X Number of orbital electrons = Z (atom is electrically neutral) 12C A Z An element is defined by the number of protons (Z) in the nucleus Atoms with same number of protons but different numbers of neutrons are called isotopes or nuclides Elements and Isotopes Slide 9/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Bound electron shoots away… Energy imparted to bound electron Usually capable of ionizing other nearby atoms Ejected electron 12 Radiation + 12C → 12C+ + e- ionized atom Atom becomes positively charged (because eis removed) • Ionization ≠ nuclear reaction Number neutrons and protons stays same Ionization is a process whereby one or more electrons are liberated from an atom Ionization Slide 10/33 10/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions 250 keV β in water 50 cell diameters 500 μm Electron tracks generated using EGS4 Monte Carlo computer code (tracks shown to scale) 50 keV β in water 50 μm “track” Energetic electrons (β- radiation) in water Slide 11/33 11/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions 1.8 to 2.3 nm + Secondary ionization (δ-rays) 500 eV electron = Ionization at Molecular Level (zoomed 1000x) Slide 12/33 12/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Does not require input of external energy or mass No known way of stopping radioactive decay of an unstable atom • Excess mass or energy (electromagnetic radiation) emitted from nucleus mass energy • Happens spontaneously Radioactive decay converts an unstable atom into a stable (or more stable) atom • Too much energy or mass Configuration of neutrons and protons with the nucleus of some atoms is unstable Nuclear Transformation (“radioactive decay”) Slide 13/33 13/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions (Z = 56, N = 81, A = 137) 137Ba* + e- (β- radiation) Nucleus of barium still has too much energy to be stable (Z = 55, N = 82, A = 137) 137Cs (n → p + β-) In β- radioactive decay, a neutron is converted to a proton + an energetic electron (β- radiation). 137 Nuclear transformation of 137Cs Slide 14/33 14/33 Purdue University School of Health Sciences nucleus in low-energy (ground) state nucleus in high-energy (excited) state Excess energy in the “excited” 137Ba nucleus radiated away as electromagnetic radiation (“γ-rays”) (Z = 56, N = 81, A = 137) 137Ba (γ-ray) + electromagnetic radiation HSCI 101 Introduction to the Health Sciences Professions (Z = 56, N = 81, A = 137) 137Ba* 137 Transformation of 137Ba* Slide 15/33 15/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions • Number of neutrons and protons may increase, decrease or stay the same (nucleus in “excited” state verses “ground” state) • Orbital electrons are (with one exception) not involved in nuclear transformations not • If the number of protons increases or decrease, the atom may become positively or negatively charged Nuclear transformation is a physical process that occurs Nuclear within the nucleus of an atom nucleus • Atom becomes a positively charged ion • Number of neutrons and protons does not change Ionization is a physical process that produces an alternate Ionization configuration of the orbital electrons orbital Ionization vs Nuclear Transformation Slide 16/33 16/33 Purdue University School of Health Sciences UV-A and UV-B are usually not considered ionizing radiation UV-C radiation with a wavelength shorter than about 124 nm (E = 10 eV) is often considered ionizing. Non-ionizing EM Ionizing EM HSCI 101 Introduction to the Health Sciences Professions Electromagnetic (EM) Radiation Slide 17/33 17/33 Purdue University School of Health Sciences Image courtesy Joey Silvers Sometimes charged particles are deflected by the positively charged nucleus. Transfer of energy results in emission of photons (bremsstrahlung radiation) 25,000 μm HSCI 101 Introduction to the Health Sciences Professions 5 MeV β in water 2,500 cell diameters X-rays (bremsstrahlung radiation) Slide 18/33 18/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions • Short wavelength, high energy Electromagnetic Radiation (x-rays and γ-rays) • Electrons, positrons, neutrons, protons, α (4He2+) particles • In outer space, heavy particles (carbon, iron, …) Atomic or sub-atomic particles with sufficient kinetic energy to ionize matter. Major Types of Ionizing Radiation Slide 19/33 19/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions 500,000 μm (50 cm) 1 MeV e- in water NOTE: 50 cm = 19.685 inches = 50,000 cell diameters (100 times more penetrating) 1 MeV photons in water 500 cell diameters 5,000 μm Why do we care about the type of radiation? Slide 20/33 20/33 Purdue University School of Health Sciences (hours, days or weeks) Death – Acute Radiation Sickness Large doses delivered quickly Ionization x Unwanted chemical reactions among cellular constituents Protracted exposures (small and large doses) Dr. Robert Bruce Banner turns into the Hulk because of exposure to γ-rays* (several years later) Cancer HSCI 101 Introduction to the Health Sciences Professions radiation passes through the human body? What happens when … Slide 21/33 21/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions • Time, distance and shielding • As low as reasonably achievable (ALARA) How do we protect people from radiation? Radiation Protection Slide 22/33 22/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Time (days) 0 10 20 30 40 50 60 70 80 90 100 1 2 3 4 5 6 7 8 9 10 Half-Life 10000.0 5000.0 2500.0 1250.0 625.0 312.5 156.3 78.1 39.1 19.5 9.8 Atoms Number of atoms at time t Initial number of atoms (= 10,000) N (t ) = N 0 ⋅ ( 2 −T ) Number of half-lives After each half-life, the number of untransformed atoms is reduced by a factor of two Definition: average amount of time required for half of the initial number of atoms to transform (decay) Half-life Slide 23/33 23/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions radiation passing through person ∝ 1/r2 Number of particles passing through a person distance r away from the source decreases approximately as the square of distance (inverse square law) For the same number of radioactive atoms, particle emission rate increases as the half-life decreases r Number of ionizing particles emitted by a source increases as the nuclear transformation (decay) rate increases Radioactive source Particle emission and the inverse square law Slide 24/33 24/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Mass of the matter Biological damage ∝ dose ∝ number of particles Absorbed dose = Energy deposited in a region • 1 Gray (Gy) = 1 Joule per kilogram (J/kg) Absorbed dose is the amount of energy deposited in a region of matter per unit mass Absorbed dose Slide 25/33 25/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Time to deliver the dose (h) A 1 Gy dose of radiation will not cause acute not radiation sickness (ARS) 1 Gy in 100 hours (0.01 Gy/h) = small effect 1 Gy in 0.01 hour (100 Gy/h) = bigger effect Biological effects of radiation tend to increase as dose rate increases Absorbed dose rate = Absorbed dose (Gy) • 1 Gy per hour (Gy/h) = 1 Joule per kilogram per hour (J/kg-h) Amount of energy deposited in a region of matter per unit mass per unit time Absorbed dose rate Slide 26/33 26/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions r Radioactive source Keep the amount of time spent next to a radiation source to the absolute minimum • Biological damage ∝ absorbed dose ∝ number of particles The longer you stand near a radiation source, the more radiation that will pass through you 1st radiation protection principle (time) Slide 27/33 27/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions r Increase distance by a factor of 2 Decrease dose and dose rate by a factor of 4 Radioactive source Stays as far away from a radiation source as possible • inverse square law Amount of radiation passing through a person decreases rapidly as distance increases 2nd radiation protection principle (distance) Slide 28/33 28/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions 500,000 μm (50 cm) Number of particles at depth x I(x) = I0e-μx x Radiation is attenuated exponentially as it penetrates attenuated through matter Initial number of particles 10 MeV photons in water Penetration of radiation through matter Slide 29/33 29/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions 16 particles 8 particles 1 HV thickness HV thicknesses 0 1 2 3 16 particles 8 particles 4 particles 2 particles On average, how many particles would make it through two additional HV thicknesses? Half-value thickness is the amount of a substance required to reduce the amount of radiation by a factor of two Half-value (HV) thickness Slide 30/33 30/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Use a radiation “shield” to reduce the amount of radiation reaching you Any additional matter between you and a radiation source will reduce the amount of radiation passing through you 3rd radiation protection principle (shielding) Slide 31/33 31/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions ALARA is intended to balance the many useful applications of radiation against the potential negative human health effects of exposure • Radioactive elements in the Earth’s crust, radiation from our sun, extrasolar and extra-galactic radiation (cosmic-rays) As low as reasonable achievable (ALARA) Everyone exposed to low levels of radiation from environment ALARA Principle Slide 32/33 32/33 Purdue University School of Health Sciences HSCI 101 Introduction to the Health Sciences Professions Reduce overall dose by factor ~ 16 1 HV thickness = decrease dose by factor of 2 Increase distance by 2 = decrease dose by factor of 4 ½ exposure time = ½ the absorbed dose • Minimize time near the source • Stay as far away as possible • Use radiation shields One or more of the three principles can be applied at the same time Summary and Practical Application Slide 33/33 33/33 Purdue University School of Health Sciences ...
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