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Radiation Physics Lecture

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Nuclear Physics and Society Physics Department University of RichmondNuclear Basics1Motivation: Educate the Public and University communities about basic nuclear physics ideas and issuesU.S. Department of Energy Workshop July 2002, Washington D.C. Role of the Nuclear Physics Research Community (universities and national laboratories) in Combating Terrorism Education and Outreach CommunityLocal PD and FD2DOE Workshop Cargo Containers 10,000,000 per year 10,000 per ship! 5 / minute @ L.A. < 3% inspectedBorder Control/ US Customs1,000,000 visas/year 422 ports of entry 1700 flights / day290 ships / day60 trains / day 1200 busses / day540,000,000 border entries / yearTime per primary inspection8 seconds => 1 hour delay3What the Course is/is notThis is not a radiation workers courseThis is not a course that will certify you for anythingWe hope that we can introduce you to some basic facts about nuclear physics, about radiation, about detectors etc., which may be useful for you to know.4Who are WeCon BeausangChairman & Associate Professor Physics DepartmentJerry GilfoyleProfessor, Physics DepartmentPaddy ReganProfessor Physics Department, University of Surrey, U.K.5Nuclear Physics and SocietyMonday April 13th Lecture 1: The types of radiation, their properties and how these can be used to detect them. Some basic definitions. Introduction to radiation detectors.Tuesday April 14th Laboratory Session: 12:15 3:30 pm Environmental Radiation Laboratory experience Measurement of half-life Demonstration of shielding Find the source Lecture 2: The creation of the elements. Nuclear physics in the cosmos. Wednesday April 15th Laboratory Session 2: 12:15 3:30 Repeat of Tuesdays experience Lecture 3: Applications of Nuclear Physics: Nuclear weapons, nuclear power and nuclear medicine. Thursday April 16th Lecture 4 Some of the frontiers of modern nuclear physics research 6The Cookie QuizAlpha cookieBeta cookieNeutron cookie Gamma cookie7The Cookie QuizAlpha cookie Beta cookieGamma cookieThrow awayPut in pocket Hold in clenched fist Eat one Neutron cookieGOAL: Minimize your radiation exposure8The Cookie Quiz: Answer 1Neutron cookieThrow awayPut in pocketBeta cookieHold in clenched fistAlpha cookieEat one Gamma cookie9The Cookie Quiz: Correct AnswerAlpha cookie Gamma cookie Beta cookie Neutron cookieThrow awayGOAL: Minimize your radiation exposurePut in pocket Hold in clenched fist Eat one Mutiny at once Retire from the navy and Toss ALL cookies away10What are we made of ? when was sugar andIspice and all things nice young(er), I waswhat little girls are made of thats curious snips and snails puppy dogs tails and thats what little boys are made of. ok mum, so what are sugar, spice and snails etc. made of? cells molecules atoms nucleiThe Uncertainty PrincipleHeisenberg (Quantum Mechanics) D(position) D(momentum) > Constant Beausang (Teaching)D(truth) D(clarity) > ConstantAtoms are made of Electrons very light, but occupy most of the volume inside an atom Nuclei lie at the Core of Atoms very heavy, very small, very compact occupies almost none of the volume inside the atomHow do we know?How to see the invisible? size of your probe scatteringDetector Zinc-sulfide screenAlpha-particle beamThe eyes of Geiger and MarsdenDiscovery of the nucleus ~191016-inch Battleship shells and tissue paperThink of atoms as being like a mini solar system The sun at the center is the nucleus, the electrons orbit the nucleus, like the planets orbit around the sun Bohr ModelElectronsVery small Point-like particles (i.e.nothing inside an electron) Very light ~ 1/2000th of proton massNegatively charged (-1 elementary charge) Electrons occupy almost all the space in the atom (orbiting the nucleus like the earth and other planets orbit the sun) Have almost none of the mass of the atom All of chemistry has to do with electrons from different atoms interacting with each otherThe NucleusMade up of protons and neutronsAlmost all of the mass of the atom is concentrated in the nucleus. >99.9% of the known mass in the universe.Occupies almost none of the volume of the atom. Radius < 1/10,000 Volume < 1/1,000,000,000,000The nucleus is the source of almost all the things we commonly think of as being radioactive.The NucleusProtonsPositively charged (+1 elementary charge) Size ~ 1 fm (10-15 m) Mass 938 MeV/c2 =1NeutronsNeutral (0 charge) Size ~ 1 fm (10-15 m) Mass 939 MeV/c2~1Neutrons are slightly more massive than the protons!!! This has huge consequences for us!Delicate BalancesLaws of Physics1) If it can happen it will happen 2) If some law forbids it to happen it will happen more slowly 3) If a process is really REALLY forbidden to happen it just takes a long time Standard Model: Neutron and proton are very close relativesquark structure proton (uud) neutron (udd)Many laws allow neutrons to `change into into protons change a d-quark into a uquark (or vice versa) beta-decayThe half life of a free neutron (i.e., one not inside a nucleus) is only about 12 minutes!!! Mass Neutron = 939.565330 MeV/c2 Mass Proton = 938.271998 MeV/c2 But Inside a nucleus neutrons are stableImagine if they were not! Then in ~ 1-2 hours the entire universe would be made of HydrogenE = mc2The half life of a free proton is > 1031 years Inside some nuclei protons can decay into neutronsThe NucleusAtoms are electrically neutral The number of protons in a nucleus is equal to and determines the number of orbiting electronsthe chemistry the element nameHydrogen (11H)1 proton, 0 neutrons Mass = 1Helium (42He)Uranium (23892U)(Alpha-particle)2 protons, 2 neutrons Mass = 492 protons, 146 neutrons Mass = 238The NucleusMany elements have several stable nuclei with the same number of protons but different numbers of neutrons same name same chemistry different massIsotopesThe Periodic Table of the ElementsChart of the Nuclei6 Z = No. of Protons 5 4 3 2 1 0 01H8C 7B 6Be 5Li 3He 4He 2D 3T9C 8B 7Be 6Li 5He10C 9B 8Be 7Li 6He11C 10B12C 11B13C 12B14C 13B15C 14B16C 15B 14Be17C9Be 10Be 11Be 12Be 8Li 7He 9Li 8He 10Li 9He11Lin1 2 3 4 5 6 7 8 9N = No. of NeutronsChart of the NucleiThe Landscape~300 stable ~ 7000 unstable radioactive.Half LifeTime taken for half of the substance to decay awayExample:If you have 1000 radioactive nucleiand If their half life is 30 minutes After 30 minutes 500 nuclei remain After 60 minutes 250 remainThere is a huge variation in half lives of different isotopes . From a tiny fraction of a second to roughly the age of the universe.After 90 minutes 125 remainAfter 120 minutes 62 remain28Some Isotopes & Their Half LivesISOTOPEHALFLIFEAPPLICATIONSUraniumbillions of years 5730 y30.2 y 12.3 y 74 d 66 h 6hNatural uranium is comprised of several different isotopes. When enriched in the isotope of U-235, its used to power nuclear reactor or nuclear weapons. Found in nature from cosmic interactions, used to carbon date items and as radiolabel for detection of tumors.Blood irradiators, tumor treatment through external exposure. Also used for industrial radiography. Labeling biological tracers. Implants or "seeds" for treatment of cancer. Also used for industrial radiography. Parent for Tc-99m generator. Brain, heart, liver (gastoenterology), lungs, bones, thyroid, and kidney imaging, regional cerebral blood flow, etc.29Carbon-14Cesium-137 Hydrogen-3 Irridium-192 Molybdenum-99 Technicium-99mThe Amount of Radioactivity is NOT Necessarily Related to Size Specific activity is the amount of radioactivity found in a gram of material. Radioactive material with long halflives have low specific activity. 1 gram of Cobalt-60 has the same activity as 1800 tons of natural Uranium30For Example: Suppose we have 1,000,000,000 atoms of material A with a half life of 1 second and1,000,000,000 atoms of material B with a half life of 1 year(real sources have many more atoms in them) Suppose they both decay by alpha emission.In the First Second Substance A: Half the nuclei will decay 500,000,000 alpha particles will come zipping out at you.1 year = 365 days * 24 hours * 60 minutes * 60 seconds = 31,536,000 secondsIn the First Second for substance B Only ~ 500,000,000 / 31,536,000 = 16 nuclei will decay only 16 alpha particles will come zipping at you31On the other hand In 10 seconds almost all of the radioactivity in substance A is gone awayBut it takes years for the activity of substance B to go away!Nuclear Bombs The fissile material (U or Pu) has a long half-life. Low specific activity. Not much activity on the outside. Dirty Bombs The radioactive material wrapped around the explosive would probably have a much shorter half-life. Perhaps significant activity on the outside.32Types of RadioactivityFive Common Types Alpha Decay Each type of radiation has different properties which affect the hazards they pose, the detection mechanism and the shielding required to stop them.Beta DecayGamma DecayFissionNeutron EmissionEach of the particles emitted in the decay carries a lot of kinetic energy. Damage can be caused when this energy is absorbed in a human cell.33Alpha DecayAn alpha particle () is an energetic, He nucleus(42He2)Alpha decay mostly occurs for heavy nucleiExample238Pu 23492U + 42He 94Half-life: 88 yearsEnergy =5.56 MeV34Alpha DecayVery easy to shieldA sheet of paper, skin, or a few cm (~inch) of air will stop an alpha particleExternal Hazard: Low Internal Hazard: High35Alpha Decay238Pu144 23492U142 + 94 Parent nucleus 23894Pu144 Daughter Nucleus 23492U142 Often the daughter nucleus is also radioactive and will itself subsequently decay. Decay chains or families (e.g. uranium, thorium decay chains).36238Pu 23492U + 94 U 23090Th + 9290Tht1/2 = 88 yrsDecay Chains234t1/2 = 2.5 105 yrs t1/2 = 8.0 104 yrs t1/2 = 1.6 103 yrs t1/2 = 3.8 days t1/2 = 3.1 min t1/2 = 27 min t1/2 = 20 min t1/2 = 160 s37230 22688Ra + 21884Po + 226Ra 22286Rn + 88 Po 21482Pb + 84214222 Rn 86 218Pb 21483Bi + 82 21484Po + 214 Bi 83 214Po 21082Pb + 84Decay ChainsPb 21083Bi + 82 21084Po + 210t1/2 = 22 yrs t1/2 = 5 days t1/2 = 138 days210 Bi 83 210Po 20682Pb + 84206 82Pbis STABLE38Decay ChainsPu UThRa RnPoPb Hg Au39Beta DecayA beta-particle is an electron (e) or its anti-particle the positron (e+) The neutron and the proton are very similar to each other (very closely related). A neutron can change into a proton, or vice versa. When this happens, an energetic electron (or positron) is emitted. -+ np+e This is called beta-decaypn++ e+40Beta DecayIn terms of nuclei beta-decay looks like137 Cs 137 Ba 55 82 56 81+ e- + As in the case of alpha decay the daughter nuclei are usually radioactive and will themselves decay. Beta-particles are HARDER to stop Since the electron is lighter than an alpha-particle and carries less charge. Therefore, the range of a beta-particle is greater and it41Beta-DecayBeta-particles are HARDER to stop Since the electron is lighter than an alphaparticle and carries less charge. Therefore, the range of a beta-particle is greater and it takes more shielding to stop beta-particles (electrons or positrons) than alpha particles ~ few mm or 1 cm of lead ~ few feet of air42Gamma-DecayA beta-decay or alpha-decay typically leaves the daughter nucleus in a highly excited state. To get to the ground state the nucleus (rapidly almost instantly) emits one or more gamma-rays Gamma-rays are a very energetic form of light. More energy and more penetrating than x-rays. No charge Much more penetrating than either alpha or beta. Few inches of Pb, many feet of air43Gamma-DecayGamma-ray energies are characteristic of the nucleus.Measure the energies identify the nucleus.(just like atoms or molecules give off characteristic colors of light). Measuring the gamma-ray is by far the best and easiest way to measure what type of radioactive substance you are dealing with.44FissionWhat holds nuclei together? Protons repel each other (opposites attract, likerepel)Coulomb Force Some other force must hold nuclei together The STRONG FORCE Attractive and Stronger than the Coulomb Force But short range45FissionWhat happens if you have a lot of protons (i.e in a heavy nucleus)? Eventually the Coulomb repulsion will win and the nucleus will fall apart into two smaller (radioactive!!) nuclei.FISSIONAn enormous amount of energy is released.This energy is utilized in power plants and in fission bombs.46FissionThe heavy parent nucleus fissions into two lighter fission fragment nuclei Plus some left over bits energetic neutronsExample:252Cfis a spontaneous fission source Sometimes this process happens spontaneously sometimes you can poke at the nucleus and induce it to fission 47Fission Fission FragmentsAre emitted with a huge energy but stop very quickly (very short range).Are all radioactive nuclei and will decay usually by beta-and gamma-decayLight fragmentThey have a broadrange of massesProbability Heavy fragmentMass 48Induced FissionSome nuclei can be made to fission when struck by something Usually the something is a neutronExample:235U+ n fissionRemember in the fission process extra neutrons are releasedIf some of these strike other 235U nuclei they can induce another fission49Induced FissionChain ReactionControlled nuclear power plant exactly one neutron per fission induces another fission.Uncontrolled nuclear bomb more than one neutron per reaction induces another fission50What is a Dose of Radiation? When radiations energy is deposited into our bodys tissues, that is a dose of radiation. The more energy deposited into the body, the higher the dose. Rem is a unit of measure for radiation dose. Small doses expressed in mrem = 1/1000 rem. Rad & R (Roentgens) are similar units that are often equated to the Rem.51From Understanding Radiation, Brooke Buddemeier, LLNLTypical DosesAverage Dose to US Public from All sources Average Dose to US Public From Natural Sources Average Dose to US Public From Medical Uses 360 mrem/year 300 mrem/year 53 mrem/yearCoal Burning Power PlantAverage dose to US Public from Weapons Fallout Average Dose to US Public From Nuclear Power Occupational Dose Limit for Radiation Workers Coast to coast Airplane roundtrip Chest X ray Dental X ray0.2 mrem/year< 1 mrem/year < 0.1 mrem/year 5,000 mrem/yr 5 mrem 8 mrem 10 mremHead/neck X rayShoe Fitting Fluoroscope (not in use now) CT (head and body)20 mrem170 mrem 1,100 mrem52Therapeutic thyroid treatment (doseUnderstanding Radiation, Brooke Buddemeier, LLNL From to the wholeTypes of Exposure & Health Effects Acute Dose Large radiation dose in a short period of time Large doses may result in observable health effects Early: Nausea & vomiting Hair loss, fatigue, & medical complications Burns and wounds heal slowly Examples: medical exposures and accidental exposure to sealed sources Chronic Dose Radiation dose received over a long period of time Body more easily repairs damage from chronic doses Does not usually result in observable effects Examples: Background Radiation and Internal DepositionInhalation53 From Understanding Radiation, Brooke Buddemeier, LLNLDividing Cells are the Most Radiosensitive Rapidly dividing cells are more susceptible to radiation damage. Examples of radiosensitive cells are Blood forming cells The intestinal lining Hair follicles A fetusThis is why the fetus has a exposure limit (over gestation period) of 500 mrem (or 1/10th of the annual adult limit)From Understanding Radiation,Brooke Buddemeier, LLNL54At HIGH Doses, We KNOW Radiation Causes Harm High Dose effects seen in: Radium dial painters Early radiologists Atomic bomb survivors Populations near Chernobyl Medical treatments Criticality Accidents In addition to radiation sickness, increased cancer rates were also evident from high level exposures.55From Understanding Radiation,Brooke Buddemeier, LLNLEffects of ACUTE ExposuresDose (Rads*)25-50EffectsFirst sign of physical effects (drop in white blood cell count)100Threshold for vomiting (within a few hours of exposure) ~ 50% die within 60 days (with minimal supportive care) ~50 % die within 60 days (with supportive medical care) ~ 100% die within 30 days320 - 360480 - 540 1,000* For common external exposures 1 Rad ~ 1Rem = 1,000 mrem56 From Understanding Radiation,Brooke Buddemeier, LLNLAt LOW Doses, We PRESUME Radiation Causes Harm No physical effects have been observed Although somewhat controversial, this increased risk of cancer is presumed to be proportional to the dose (no matter how small).The Bad News: The Good News: Radiation is a carcinogen and a mutagen Radiation is a very weak carcinogen and mutagen!Very Small DOSE = Very Small RISKFrom Understanding Radiation Brooke Buddemeier, LLNL 57Radiation DetectorsRange of Radiation Alpha: Beta: Gamma: Neutron: Small. Smallish Long Very long Shield with a piece of paper Shield with a inch or so of Pb Shield with a few inches of Pb Shield with many inches of parafinTo detect the radiation it has toa) Get to and b) Get into your detector58Radiation DetectorsAlmost all work on the same general idea When an energetic charged particle passes through matter it will rapidly slow down and lose its energy by interacting with the atoms of the material (detector or body) Mostly with the atomic electrons It will kick these electrons off of the atoms leaving a trail of ionized atoms behind it (like a vapor trail of a jet plane) Radiation detectors use a high voltage and some electronics to measure these vapor trails. They measure a (small) electric current).59Radiation DetectorsLike a bullet going through something A friction force will slow it down and stop itFrictionMore Charge More friction More Massive More friction More friction Shorter Range60Radiation DetectorsIt has to get into your detector e.g. Alpha . A few inches of air or a piece of paper stops it if your detector is a few feet away, it will not detect the alpha e.g. Alpha if the sides of the detector are too thick the alpha will not get in and will not be detected61Radiation DetectorsNeutrons and gamma-rays are neutral No charge much less friction much longer rangeWhen they penetrate matter eventually they also will interact somehow (gamma-rays interact via Compton scattering, photoelectic effect or pair production, neutrons will collide with protons in the nuclei) and these interactions produce energetic charged particles.The detectors are sensitive to these secondary particles.62Types of detectorAlpha, Beta and Gamma radiationFilm BadgesGas Counters (Geiger counters)ScintillatorsSolid State Detectors63Film BadgesWill detect: beta, gamma and neutronNeed to send away and develop the film and then later will tell you what does you receivedUsed by radiation workers TLC devices similar idea but with real-time readout64Gas Counterse.g. Geiger CountersWill Detect: Alpha, Beta, some gammaNo identification just tells you something is thereWith a thin entrance window GM-tube is sensitive to alphas65ScintillatorsMake a flash of light when something interacts Sodium Iodide Cesium IodideWill Detect: Alpha (with thin window), beta (with thin window) and gamma. Gives moderate to bad energy information some information on the type of radiation66Semiconductor DetectorsGermanium SiliconWill Detect: Gamma rays (also beta and alphas in a laboratory, not in the field)Excellent energy resolution: Can measure exactly was source you are looking at.67Spare Transparencies68Radioactive DecayWhen a physical process can happen it will happen. When it is forbidden to happen it just takes a little longer!When can a nucleus decay? If a nucleus can decay it willWhen there is a lighter nucleus for it to decay intoWhen this decay is allowed by certain conservation laws .Conservation of energy Conservation of charge Certain other quantum numbers69Beta Decaynp+e+Various laws must be obeyed, including1. Conservation of Energy E = mc2 a heavy particle can decay into lighter one(s). The excess energy is turned into kinetic energy of the light particles 2. Conservation of Charge An electron is produced 3. Conservation of Lepton Number a very nebulous particle called a neutrino is also produced70


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