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1 Nuclear Physics and Society Physics Department University of Richmond Nuclear Basics
Motivation: Educate the Public and University communities about basic nuclear physics ideas and issues 2 U.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 Community Local PD and FD
DOE Workshop … 3 Border Control/ US Customs 1,000,000 visas/year 422 ports of entry 1700 flights / day 290 ships / day 60 trains / day 1200 busses / day 540,000,000 border entries / year Time per primary inspection 8 seconds => 1 hour delay Cargo Containers 10,000,000 per year … 10,000 per ship! 5 / minute @ L.A. < 3% inspected
What the Course is/is not 4 This is not a radiation workers course This is not a course that will certify you for anything We 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.
Who are We 5 Con Beausang Chairman & Associate Professor Physics Department Jerry Gilfoyle Professor, Physics Department Paddy Regan Professor Physics Department, University of Surrey, U.K.
6 Monday April 13 th 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 14 th 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 15 th 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 16 th Lecture 4 Some of the frontiers of modern nuclear physics research Nuclear Physics and Society
8 The Cookie Quiz Alpha cookie Beta cookie Gamma cookie Neutron cookie Throw away Put in pocket Hold in clenched fist Eat one … GOAL: Minimize your radiation exposure
9 The Cookie Quiz: Answer 1 Alpha cookie Beta cookie Gamma cookie Neutron cookie Throw away Put in pocket Hold in clenched fist Eat one …
10 The Cookie Quiz: Correct Answer Alpha cookie Beta cookie Gamma cookie Neutron cookie Throw away Put in pocket Hold in clenched fist Eat one … GOAL: Minimize your radiation exposure Mutiny at once Retire from the navy and Toss ALL cookies away
… when I was young(er), I was curious … What are we made of ? … sugar and spice and all things nice … that’s what little girls are made of … snips and snails and puppy dogs tails … that’s what little boys are made of. … ok mum, … so what are sugar, spice and snails etc. made of? … cells … molecules … atoms … nuclei
Atoms … 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 atom
How do we know? How to see the invisible? … size of your probe … scattering Alpha-particle beam Detector Zinc-sulfide screen Discovery of the nucleus ~1910 The eyes of Geiger and Marsden 16-inch Battleship shells and tissue paper
Think 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 Model
Electrons Very small Point-like particles (i.e.nothing inside an electron) Very light~ 1/2000 th of proton mass Negatively 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 other
The Nucleus Made up of protons and neutrons Almost 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 < 1/1,000,000,000,000
The nucleus is the source of almost all the things we commonly think of as being radioactive.
The Nucleus Protons Positively charged (+1 elementary charge) Size ~ 1 fm (10 -15 m) Mass 938 MeV/c 2 = 1 Neutrons Neutral (0 charge) Size ~ 1 fm (10 -15 m) Mass 939 MeV/c 2 ~ 1 Neutrons are slightly more massive than the protons!!! This has huge consequences for us!
Delicate Balances Laws of Physics 1)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 relatives quark structure … proton (uud) … neutron (udd) Many laws allow neutrons to `change into’ into protons … change a d-quark into a u- quark (or vice versa) … beta-decay
The half life of a free neutron (i.e., one not inside a nucleus) is only about 12 minutes!!! Mass Neutron = 939.565330 MeV/c 2 Mass Proton = 938.271998 MeV/c 2 But … Inside a nucleus … neutrons are stable The half life of a free proton is > 10 31 years Inside some nuclei protons can ‘decay’ into neutrons Imagine … if they were not! Then in ~ 1-2 hours the entire universe would be made of Hydrogen E = mc 2
The Nucleus Atoms are electrically neutral The number of protons in a nucleus is equal to and determines the number of orbiting electrons the chemistry the element name Hydrogen ( 1 1 H) 1 proton, 0 neutrons Mass = 1 Helium ( 4 2 He) (Alpha-particle) 2 protons, 2 neutrons Mass = 4 Uranium ( 238 92 U) 92 protons, 146 neutrons Mass = 238
The Nucleus Many elements have several stable nuclei with the same number of protons but different numbers of neutrons … same name same chemistry different mass Isotopes
Chart of the Nuclei 1H1H 2D2D 3 He 4 He 6 Li 7 Li n 9 Be 3T3T 5 He 6 He 5 Li 6 Be 7 Be 8 Be 8 Li 7 He 9 Li 10 Be 10 Li 11 Li 8 He 9 He 11 Be 12 Be 10 B 11 B 9B9B 14 Be 12 B 13 B 14 B 15 B 8B8B 7B7B 12 C 13 C 14 C 15 C 16 C 17 C 11 C 10 C 9C9C 8C8C Z = No. of Protons 0 1 2 3 4 5 6 N = No. of Neutrons 012345 6789
Chart of the Nuclei The Landscape ~300 stable ~ 7000 unstable … radioactive.
Half Life 28 Time taken for half of the substance to decay away Example: If you have 1000 radioactive nuclei and If their half life is 30 minutes After 30 minutes 500 nuclei remain After 60 minutes 250 remain After 90 minutes 125 remain After 120 minutes 62 remain There is a huge variation in half lives of different isotopes …. From a tiny fraction of a second to roughly the age of the universe.
Some Isotopes & Their Half Lives ISOTOPE HALF- LIFE APPLICATIONS Uraniumbillions of years Natural uranium is comprised of several different isotopes. When enriched in the isotope of U-235, it’s used to power nuclear reactor or nuclear weapons. Carbon-14 5730 y Found in nature from cosmic interactions, used to “carbon date” items and as radiolabel for detection of tumors. Cesium-137 30.2 y Blood irradiators, tumor treatment through external exposure. Also used for industrial radiography. Hydrogen-3 12.3 yLabeling biological tracers. Irridium-19274 dImplants or "seeds" for treatment of cancer. Also used for industrial radiography. Molybdenum-9966 hParent for Tc-99m generator. Technicium-99m 6 hBrain, heart, liver (gastoenterology), lungs, bones, thyroid, and kidney imaging, regional cerebral blood flow, etc. 29
The 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 half- lives have low specific activity. 1 gram of Cobalt-60 has the same activity as 1800 tons of natural Uranium 30
31 For Example: Suppose we have 1,000,000,000 atoms of material A with a half life of 1 second and 1,000,000,000 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 seconds In 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 you
32 On the other hand … In 10 seconds … almost all of the radioactivity in substance A is gone away But 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.
Types of Radioactivity 33 Each type of radiation has different properties which affect the hazards they pose, the detection mechanism and the shielding required to stop them. Five Common Types Alpha Decay Beta Decay Gamma Decay Fission Neutron Emission Each 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.
Alpha Decay 34 An alpha particle ( ) is an energetic, He nucleus ( 4 2 He 2 ) Alpha decay mostly occurs for heavy nuclei Example 238 94 Pu 234 92 U + 4 2 He Half-life: 88 years Energy =5.56 MeV
Alpha Decay 35 Very easy to shield A sheet of paper, skin, or a few cm (~inch) of air will stop an alpha particle External Hazard: Low Internal Hazard: High
Alpha Decay 238 94 Pu 144 234 92 U 142 + Parent nucleus 238 94 Pu 144 Daughter Nucleus 234 92 U 142 –Often the daughter nucleus is also radioactive and will itself subsequently decay. –Decay chains or families (e.g. uranium, thorium decay chains). 36
Decay Chains 37 238 94 Pu 234 92 U + t 1/2 = 88 yrs 234 92 U 230 90 Th + t 1/2 = 2.5 10 5 yrs 230 90 Th 226 88 Ra + t 1/2 = 8.0 10 4 yrs 226 88 Ra 222 86 Rn + t 1/2 = 1.6 10 3 yrs 222 86 Rn 218 84 Po + t 1/2 = 3.8 days 218 84 Po 214 82 Pb + t 1/2 = 3.1 min 214 82 Pb 214 83 Bi + t 1/2 = 27 min 214 83 Bi 214 84 Po + t 1/2 = 20 min 214 84 Po 210 82 Pb + t 1/2 = 160 s
Decay Chains 38 210 82 Pb 210 83 Bi + t 1/2 = 22 yrs 210 83 Bi 210 84 Po + t 1/2 = 5 days 210 84 Po 206 82 Pb + t 1/2 = 138 days 206 82 Pb is STABLE
Beta Decay 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. This is called beta-decay 40 A beta-particle is an electron (e) or its anti-particle the positron (e + ) n p + e - + p n + e + +
Beta Decay 41 In terms of nuclei beta-decay looks like 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 it takes more shielding to stop beta-particles (electrons or positrons) than alpha particles ~ few mm or 1 cm of lead ~ few feet of air 137 55 Cs 82 137 56 Ba 81 + e - +
Beta-Decay 42 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 it takes more shielding to stop beta-particles (electrons or positrons) than alpha particles ~ few mm or 1 cm of lead ~ few feet of air
Gamma-Decay 43 A 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 air
Gamma-Decay 44 Gamma-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.
Fission 45 What holds nuclei together? Protons repel each other (opposites attract, like repel) Coulomb Force Some other force must hold nuclei together The STRONG FORCE Attractive and Stronger than the Coulomb Force But short range
Fission 46 What 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. FISSION An enormous amount of energy is released. This energy is utilized in power plants and in fission bombs.
Fission 47 The heavy parent nucleus fissions … into two lighter fission fragment nuclei … Plus some left over bits … energetic neutrons Example: 252 Cf is a spontaneous fission source … Sometimes this process happens spontaneously … sometimes you can ‘poke’ at the nucleus and induce it to fission
Fission …Fission Fragments 48 Are emitted with a huge energy but stop very quickly (very short range). Are all radioactive nuclei and will decay usually by beta-and gamma-decay Mass Probability Heavy fragment Light fragment They have a broad range of masses
Induced Fission 49 Some nuclei can be made to fission when struck by something … Usually the something is a neutron Example: 235 U + n fission Remember … in the fission process extra neutrons are released If some of these strike other 235 U nuclei … they can induce another fission
Induced Fission 50 Chain Reaction Controlled … nuclear power plant … exactly one neutron per fission induces another fission. Uncontrolled … nuclear bomb … more than one neutron per reaction induces another fission
What is a “Dose” of Radiation? When radiation’s energy is deposited into our body’s 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. 51 From Understanding Radiation, Brooke Buddemeier, LLNL
Typical Doses Average Dose to US Public from All sources360 mrem/year Average Dose to US Public From Natural Sources300 mrem/year Average Dose to US Public From Medical Uses53 mrem/year Coal Burning Power Plant0.2 mrem/year Average dose to US Public from Weapons Fallout< 1 mrem/year Average Dose to US Public From Nuclear Power< 0.1 mrem/year Occupational Dose Limit for Radiation Workers5,000 mrem/yr Coast to coast Airplane roundtrip5 mrem Chest X ray8 mrem Dental X ray10 mrem Head/neck X ray20 mrem Shoe Fitting Fluoroscope (not in use now)170 mrem CT (head and body)1,100 mrem Therapeutic thyroid treatment (dose to the whole body)7,000 mrem 52 From Understanding Radiation, Brooke Buddemeier, LLNL
Types 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 Deposition 53 Inhalation From Understanding Radiation, Brooke Buddemeier, LLNL
Dividing 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 fetus 54 This is why the fetus has a exposure limit (over gestation period) of 500 mrem (or 1/10 th of the annual adult limit) From Understanding Radiation,Brooke Buddemeier, LLNL
At 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. 55 From Understanding Radiation,Brooke Buddemeier, LLNL
Effects of ACUTE Exposures Dose (Rads*)Effects 25-50 First sign of physical effects (drop in white blood cell count) 100 Threshold for vomiting (within a few hours of exposure) 320 - 360 ~ 50% die within 60 days (with minimal supportive care) 480 - 540 ~50 % die within 60 days (with supportive medical care) 1,000 ~ 100% die within 30 days 56 * For common external exposures 1 Rad ~ 1Rem = 1,000 mrem From Understanding Radiation,Brooke Buddemeier, LLNL
At 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:Radiation is a carcinogen and a mutagen The Good News: Radiation is a very weak carcinogen and mutagen! Very Small DOSE = Very Small RISK 57 From Understanding Radiation Brooke Buddemeier, LLNL
Radiation Detectors 58 Range of Radiation Alpha: Small.Shield with a piece of paper Beta:SmallishShield with a ½ inch or so of Pb Gamma: LongShield with a few inches of Pb Neutron:Very longShield with many inches of parafin To detect the radiation it has to a)Get to and b) Get into your detector
Radiation Detectors 59 Almost 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).
Radiation Detectors 60 Like a bullet going through something A friction force will slow it down and stop it Friction More Charge More friction More Massive More friction More friction Shorter Range
Radiation Detectors 61 It 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 detected
Radiation Detectors 62 Neutrons and gamma-rays are neutral No charge … much less friction … much longer range When 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.
Types of detector 63 Alpha, Beta and Gamma radiation Film Badges Gas Counters (Geiger counters) Scintillators Solid State Detectors
Film Badges 64 Will detect: beta, gamma and neutron Need to send away and develop the film and then later will tell you what does you received Used by radiation workers TLC devices … similar idea but with real-time readout
Gas Counters 65 e.g. Geiger Counters Will Detect:Alpha, Beta, some gamma No identification … just tells you something is there With a thin entrance window GM-tube is sensitive to alphas
Scintillators 66 Make a flash of light when something interacts Sodium Iodide Cesium Iodide Will Detect: Alpha (with thin window), beta (with thin window) and gamma. Gives moderate to bad energy information … some information on the type of radiation
Semiconductor Detectors 67 Germanium Silicon Will 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.
Radioactive Decay 69 When can a nucleus decay? … When there is a lighter nucleus for it to decay into When this decay is allowed by certain conservation laws …. Conservation of energy Conservation of charge Certain other ‘quantum numbers’ When a physical process can happen … it will happen. When it is forbidden to happen … it just takes a little longer! If a nucleus can decay … it will
Beta Decay 70 Various laws must be obeyed, including 1.Conservation of Energy E = mc 2 … 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 produced n p + e - +