Transcript Chapter 25 Radioactivity

 The nucleus of the atom is composed of
protons and neutrons
 Some nuclei are stable, some are unstable
 Larger nucleus = more unstable
 Smaller nucleus = more stable
 The nucleus of an atom is like a marble in the
center of a football arena. The atom is
composed of mostly space
diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society)
 Strong force: is one of the strongest
forces in nature
 The strong force holds protons and
neutrons together
 Very strong at close distances. Very weak
as distance increases
 http://www.valdosta.edu/phy/astro/pl_sh
ows/bh_2001/bh/page5.html
 Calculate the number of protons and
neutrons for the following:
 Uranium, plutonium, oxygen, sodium,
krypton, carbon
 Isotopes: atoms of the same element with
different numbers of neutrons
 i.e 1214C and 1213C each has 12 protons
which is the Atomic number. The mass
number varies. It is the sum of the protons
and neutrons.
 Large nuclei have weaker strong force
between proton and neutrons
 Larger nuclei are unstable (radioactive)
 Force of repulsion pushes protons apart
 Radioactivity: is the process in which atomic
nuclei decay, in other words, particles are
being emitted from the nuclei.
 A nucleus with too many or too few
neutrons to protons is radioactive.
 All elements with more than 83 protons are
radioactive (Above 92 = not found in nature)
 Some elements with fewer are radioactive
also Carbon-14
Discovery of Radioactivity
 1896 Henri Becquerel Uranium salts darkened
photographic film.
 Marie and Pierre Curie discovered two new elements
polonium and radium.
 As atomic nuclei decay (radioactive),
energy is released
 Nuclear reactions/radioactivity produces
energy!!!!
 Transmutation is the process of changing
one element to another through nuclear
decay.
Type of Radioactive
Particles
 Alpha Particle: made of 2 protons and 2
neutrons
 Charge =
+2
 Mass =
4
 Least amount of energy of radioactive
particles, largest radioactive particle
 Stopped by a piece of paper
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society)
 Beta Particle: is an emitted electron
 Charge =
-1
 Mass =
.ooo5
 Beta particle is smaller
than an alpha particles
and move faster.
 Beta particles have more penetrating power
than alpha particle
 Sheet of aluminum foil will stop beta part.
(diagram courtesy of the University of Michigan
Student Chapter of the Health Physics Society)
 Gamma Rays: the most penetrating and
destructive form of radiation
 Charge =
0
 Mass =
0
 They travel of speed of light. However, they
can cause less damage to living tissues than
alphas and beta particles
 Thick concrete and lead will stop gamma rays
(diagram courtesy of the University of Michigan
Student Chapter of the Health Physics Society)
 Page 295 Math problems 1-4
 Half-Life: the amount of time it takes for a
half of a radioactive sample to decay
 Hydrogen 3 = 12.3 years
 Carbon 14 = 5,730 years
 Polonium 211 = .5 seconds
 Uranium 235 = 700 million years
(diagram courtesy of the University of Michigan Student Chapter
of the Health Physics Society )
 Carbon Dating: is used to tell the age of
plant and animals (organic compounds)
 Scientists look at the ratio of carbon 14
compared to carbon 12 in dead plants and
animals
 Only useful up to 50,000 years
Detecting Radioactivity
 Cloud Chamber: filled with water and
ethanol (alcohol) vapor. Shows alpha
and beta particle paths. Similar to a vapor
trail of an airplane
 link
Bubble chamber: is similar to a cloud chamber.
“vapor trails” left behind alpha and beta particles
 Link
 Bubble Chamber.The bubble chamber, invented in 1952 by the
American physicist Donald Glaser (1926- ), is similar in operation to
the cloud chamber. In a bubble chamber a liquid is momentarily
superheated to a temperature just above its boiling point. For an
instant the liquid will not boil unless some impurity or disturbance
is introduced. High-energy particles provide such a disturbance.
Tiny bubbles form along the tracks as these particles pass through
the liquid. If a photograph is taken just after the particles have
crossed the chamber, these bubbles will make visible the paths of
the particles. As with the cloud chamber, a bubble chamber placed
between the poles of a magnet can be used to measure the
energies of the particles. Many bubble chambers are equipped
with superconducting magnets instead of conventional magnets
(see SUPERCONDUCTIVITY,). Bubble chambers filled with liquid
hydrogen allow the study of interactions between the accelerated
particles and the hydrogen nuclei.
Electroscopes: detect charged particles. It will detect alpha (+) and
beta (-) particles. Also detect static electricity. Thin aluminum strips
will attract and repel each other.
 link
Measuring Radioactivity
 Geiger Counter: is a device that counts
charged particles by producing an electric
current
 Gives off “click” sounds when radioactive
particles are detected
Background Radiation
 Radon Gas:
55%
 Inside the body:
11%
 X-rays:
11%
 Cosmic Rays:
8%
 Rocks and Soil: 8%
 Other:
7%
Nuclear Reactions
 Nuclear Fission: is process of splitting atomic
nuclei into smaller nuclei
 “Fission” sounds like “division”
 Causes a Nuclear Chain Reaction where “stray”
neutrons hit nuclei and split them apart. Energy
is released when nuclei are split
 Atomic Bomb
 Extraordinary amount energy is released
 Critical Mass is the amount of fissionable
material needed to continue a reaction.
 link
http://lgfl.skoool.co.uk/uploadedImages/nucleur%20fission.jpg
 Nuclear Fusion: this when smaller atomic
nuclei “fuse” (join) to form larger nuclei.
Hydrogen nuclei join to form helium nuclei.
They have to be moving fast.
 A nuclear fusion reactions is taking place
within stars (the Sun). Only 1% of our Sun’s
mass has been converted into energy. (5
billion years left before the sun burns out)
 These nuclear reactions give off millions of
times more energy than chemical reactions
 link