Nuclear Reactions Dr. G. Maynes Illustrations 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 elements are radioactive! Many favor alpha emission Primarily beta emission 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 (kgm 2 /s 2 ) Einstein gave us E = mc 2 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 10 8 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