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