Nuclear Physics and SocietyPhysics DepartmentUniversity of RichmondNuclear Basics
Motivation: Educate the Public and University communities about basic nuclear physics ideas and issues 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 … • 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 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 • Con Beausang • Chairman & Associate Professor Physics Department • Jerry Gilfoyle • Professor, Physics Department • Paddy Regan • Professor Physics Department, University of Surrey, U.K.
Nuclear Physics and Society • Monday 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
The Cookie Quiz Alpha cookie Beta cookie Neutron cookie Gamma cookie
The Cookie Quiz Beta cookie Alpha cookie Gamma cookie Neutron cookie Throw away Put in pocket Hold in clenched fist GOAL: Minimize your radiation exposure Eat one …
The Cookie Quiz: Answer 1 Neutron cookie Throw away Beta cookie Put in pocket Gamma cookie Eat one … Alpha cookie Hold in clenched fist
The Cookie Quiz: Correct Answer Alpha cookie Beta cookie Throw away Gamma cookie Neutron cookie GOAL: Minimize your radiation exposure Put in pocket Mutiny at once Retire from the navy and Toss ALL cookies away Hold in clenched fist Eat one …
What are we made of ? … when I was young(er), I was curious … • … 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
The Uncertainty Principle • Heisenberg (Quantum Mechanics) • D(position) D(momentum) > Constant • Beausang (Teaching) • D(truth) D(clarity) > Constant
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? Detector Zinc-sulfide screen How to see the invisible? … size of your probe … scattering The eyes of Geiger and Marsden Alpha-particle beam 16-inch Battleship shells and tissue paper Discovery of the nucleus ~1910
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 ~ 1/2000th 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/c2 = 1 • Neutrons • Neutral • (0 charge) • Size ~ 1 fm (10-15 m) • Mass 939 MeV/c2 ~ 1 Neutrons are slightly more massive than the protons!!! This has huge consequences for us!
Delicate Balances • Laws of Physics • If it can happen … it will happen … • If some law forbids it to happen … it will happen more slowly … • If a process is really REALLY forbidden to happen … it just takes a long time …
quark structure • … proton (uud) • … neutron (udd) Standard Model: Neutron and proton are very close relatives 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/c2 Mass Proton = 938.271998 MeV/c2 But … Inside a nucleus … neutrons are stable The half life of a free proton is > 1031 years Inside some nuclei protons can ‘decay’ into neutrons E = mc2 Imagine … if they were not! Then in ~ 1-2 hours the entire universe would be made of Hydrogen
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 (11H) • 1 proton, 0 neutrons • Mass = 1 • Helium (42He) (Alpha-particle) • 2 protons, 2 neutrons • Mass = 4 • Uranium (23892U) • 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 6 5 4 Z = No. of Protons 3 2 11B 12C 13C 10B 9Be 1 0 6Li 7Li 6 7 8 9 8He 9He 11Be 12Be 3He 4He 9Li 8C 7B 17C 13B 15B 16C 14B 15C 9C 14C 8Li 10C 9B 8Be 8B 12B 10Li 11Li 7He 7Be 10Be 11C 5He 6He 5Li 6Be 14Be 1H 2D 3T n 0 1 2 3 4 5 N = No. of Neutrons
Chart of the Nuclei The Landscape ~300 stable ~ 7000 unstable … radioactive.
Half Life 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.
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-60has the sameactivity as1800 tons of natural Uranium
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
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 Five Common Types Alpha Decay Beta Decay Gamma Decay Fission Neutron Emission Each type of radiation has different properties which affect the hazards they pose, the detection mechanism and the shielding required to stop them. 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 An alpha particle () is an energetic, He nucleus (42He2) Alpha decay mostly occurs for heavy nuclei Example 23894Pu 23492U + 42He Half-life: 88 years Energy =5.56 MeV
Alpha Decay • 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 23894Pu144 23492U142 + a 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).
Decay Chains 23894Pu 23492U + t1/2 = 88 yrs 23492U 23090Th + t1/2 = 2.5 105 yrs 23090Th 22688Ra + t1/2 = 8.0 104 yrs 22688Ra 22286Rn + t1/2 = 1.6 103 yrs 22286Rn 21884Po + t1/2 = 3.8 days 21884Po 21482Pb + t1/2 = 3.1 min 21482Pb 21483Bi + t1/2 = 27 min 21483Bi 21484Po + t1/2 = 20 min 21484Po 21082Pb + t1/2 = 160 s
Decay Chains 21082Pb 21083Bi + t1/2 = 22 yrs 21083Bi 21084Po + t1/2 = 5 days 21084Po 20682Pb + t1/2 = 138 days 20682Pb is STABLE
Decay Chains Pu U Th Ra Rn Po Pb Hg Au
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 A beta-particle is an electron (e) or its anti-particle the positron (e+) n p + e- + p n + e+ +
Beta Decay In terms of nuclei beta-decay looks like 13755Cs82 13756Ba81 + 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 it takes more shielding to stopbeta-particles (electrons or positrons) than alpha particles • ~ few mm or 1 cm of lead • ~ few feet of air
Beta-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 stopbeta-particles (electrons or positrons) than alpha particles • ~ few mm or 1 cm of lead • ~ few feet of air
Gamma-Decay • 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 • 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 • 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 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 The heavy parent nucleus fissions … into two lighter fission fragment nuclei … Sometimes this process happens spontaneously … sometimes you can ‘poke’ at the nucleus and induce it to fission Plus some left over bits … energetic neutrons Example: 252Cf is a spontaneous fission source …
Fission …Fission Fragments 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 Light fragment Heavy fragment They have a broad range of masses Probability Mass
Induced Fission Some nuclei can be made to fission when struck by something … Usually the something is a neutron Example:235U + n fission Remember … in the fission process extra neutrons are released If some of these strike other 235U nuclei … they can induce another fission
Induced Fission Chain Reaction Controlled … nuclear power plant … exactlyone neutron per fission induces another fission. Uncontrolled … nuclear bomb … more than one neutron per reaction induces another fission