Introduction to Radiation

The classical atom

• All radiation is a byproduct of the decay of the atom … specifically the nucleus of the atom

Terminology

• Radiation : energy in transit– High energy results in ionization– Low energy results in non-ionization

• Radioactivity : the characteristic to emit energy capable of ionization

• Ionization : the removal of electrons from an atom (changing the overall charge on the atom) by high energy radiation

3 types of radiation

• Alpha radiation• Beta radiation• Gamma radiation

Alpha Radiation• Occurs when a particle consisting of 2

neutrons and 2 protons is ejected from a nucleus– The ejected particle is essentially the nucleus of a

helium atom

• Alpha decay occurs only in heavy, neutron rich atoms such as Uranium, Thorium and Radium– Neutron rich means that there are many more

neutrons than protons in the nucleus

• 235U92 231Th90 + 4a2

Properties of alpha radiation

• Positively charged• Very heavy• Most energetic of all of the radiation• Can cause many ionizations in a short distance– Travels a few centimeters in air– Stopped by a sheet of paper– Only dangerous if ingested

Applications of alpha radiation

• Smoke detectors– Alpha source ionizes the air to allow for the

passage of small currents through an air gap. Smoke obstructs this flow … setting the alarm off

• Power source– Used in space probes and pace makers

• Ion engines

Beta Radiation

• Ejection of an electron from the nucleus of an atom– How can a nucleus eject an electron?

• The electron is created out of the nucleus for beta decay– The nuclear weak force causes a neutron to decay

into an electron, a proton and an anti-neutrino– 1n0 1p1 + 0e-1 + v-

• Beta decay occurs in neutron rich atoms– A common radiation type in all nuclear reactors– Decay of rhenium into osmium– 187Rh75 187Os76 + 0e-1 + v-

Properties of Beta radiation

• Negatively charged• Nearly massless• React less readily compared to alpha radiation• Can travel several meters in air– Stopped by thin sheets of plastic or metal

• Travels faster than light in certain materials (water) resulting in Cherenkov radiation

Applications of Beta radiation

• Radiation therapy used to destroy cancer cells• Radioactive tracers• Used to check thickness of materials such as

paper

Gamma Radiation• Present in most of the other forms of decay

processes– After a radioactive decay the daughter nuclei is

fairly energetic and will release some of this excess energy as a gamma ray pulse / burst

Properties of gamma radiation

• Similar in nature to light or microwaves– Only higher in energy

• No mass• No charge• Travels at / near the speed of light• Interact through collisions with electrons– Lose energy slowly

• Can travel hundreds of meters in air

Application of Gamma Radiation

• Used in cancer surgery – Gamma ray knife

• Sterilization of food products– Irradiation

• Scanning– CT scans– Container scans at airports

• Molecular changes in materials– Turns white topaz into blue topaz

summary

• Charge– Alpha = positive– Beta = negative– Gamma = neutral

• Mass– Alpha = heavy– Beta = nearly massless– Gamma = massless

Half life

• Describes the time required for nuclear material to be reduced by half

• Each radioactive isotope has different half lives– Carbon-11 = 20 minutes– Uranium-238 = 4.5 x 109 years

Half life game

• Everyone tosses one coin

• Heads leave the game• Tails continue to toss

until no one is left in the game

• Questions– What does your graph

resemble?– Why is this?

Predicting the quantity remainingNumber of elapsed half-lives

Fraction remaining Percentage remaining

0 1/1 100%

1 ½ 50%

2 ¼ 25%

3 1/8 12.5%

4 1/16 6.25%

n 1/2n 100% / 2n

Applications of half life

• Dating– Carbon-14 decays to carbon-12. The ratio can be

used to date archeological artifacts• Toxicology– Predict the effects of certain toxins in the human

body over time