Chapter 25 Radioactivity/

Chapter 10

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_shows/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 12

14C 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