Post on 13-Jan-2016
Nuclear Reactions
Dr. G. MaynesIllustrations from Brown, LeMay and
Bursten
Radioactivity
• Atoms can not be created nor destroyed by chemical means….
• But since Marie Curie and others in the early 1900’s, we know one atom can change into another– Process is called radioactive decay
Three major forms of radioactivity
• Alpha decay– Particle given off is a helium nucleus –
• Beta decay– Particle given off is an electron – – But not from the electron cloud!• Essentially, a neutron splits into a proton and an
electron
Radioactive Decay Con’t
• Gamma decay– No particle given off – energy only - • “Gamma ray”
• Ability to penetrate increases inversely to mass:– Gamma greatest, then Beta, finally Alpha
Nuclear Equations
• Demonstrate the changes in the nucleus
• Alpha decay of U-238:
• Beta decay of I-131
Nuclear Equations, Con’t
• Gamma are generally not shown – does not change the isotope
• Positron emission– Like an electron (“no” mass) but with a positive
charge– Converts a proton to a neutron– C-11
Nuclear Equations, Con’t
• Electron capture• Nucleus captures an orbiting electron• Electron is shown on the left (reactant) side• Rb-81
Why are nucleii stable, anyhow?
• If like charges repel, protons should want to separate
• “Strong Nuclear Force” accounts for real behavior
• Short range force• Associated with neutrons– “Nuclear glue”, so to speak
Ends at Bismuth(At. # 83)
All heavier elementsare radioactive!Many favor alphaemission
Primarily betaemission
Primarily Positron emission
Primarily electron capture
Nuclear Decay Series
• Heavy isotopes frequently undergo multiple decay reactions before they achieve stability
• These series of changes can be mapped out, one particle at a time
How fast do nucleii decay?
• Measured in half life– Different for each isotope– Range from seconds to millions of years or more
• Defined as time for half the original mass to become something else– C-14: 5715 years• Used to date formerly living matter• C-12 content remains constant, C-14 decreases• “Life” incorporates C-12/C-14 at standard ratio; after “death” ratio changes
Calculating Mass Changes
• If the half life is 5.3 years, how much a one gram sample of Co-60 is left after 15.9 years?
• Note: formulas in the book let you calculate any interval; we’ll stick with whole multiples
Utilizing Nuclear Energy
• We modified the Law of Conservation of Mass after 1945:– Matter and energy can be neither created nor
destroyed, only interconverted
• If you add up the masses of all particles left after a nuclear reaction, some has been lost
• The lost mass becomes energy
Einstein’s Equation
• From physics, Force = mass x acceleration Work = force x distance• Work is energy; measured in joules (kgm2/s2)• Einstein gave us E = mc2
• E is the energy of a nuclear reaction in joules• M is the amount of mass “lost”• C is the speed of light, 3 x 108 m/s
Nuclear Fission
• Fission is to break up• One isotope absorbs a neutron• The unstable result breaks into 2 smaller
isotopes and releases 2 – 3 neutrons• A CHAIN REACTION RESULTS• U-235 becomes Ba-142, Kr-91 and 3 neutrons
Uses of Nuclear Fission
• Atomic bomb• Nuclear reactors• Difference is the degree of control of the
emitted neutrons
Nuclear Fusion
• Fusion is to stick together• Basically, hydrogen atoms combining in series
to finally become helium– Several positrons are emitted to reduce atomic
number
Practical Fusion
• The energy of the sun• No radioactive waste; would be great source
of energy – can not yet contain plasma• “Hydrogen” bomb– Takes an A-bomb to initiate– Deuterium plus tritium
Uses of Nuclear Energy
• “The bomb”, of course• Nuclear power– Ships– Power plants
• Radiotherapy– Kill cancer cells
• Incidental dosages– Radon, “background”, and medical x-rays