UNIT 1: RADIOACTIVITY AND NUCLEAR PHYSICS

Traditional atomic model Not correct but it is a good enough

approximation. Nucleus contains 2 types of nucleons:

Protons and Neutrons. Orbiting electrons. Atomic number is number of protons. Mass number is number of nucleons. Elements have the same atomic number

but can have different mass numbers = isotopes.

Chemical symbols for isotopes

Properties of subatomic particles

Property

Proton Neutron Electron

Mass 1.673 x 10-27 kg

1.675 x 10-27 kg

9.109 x 10-31 kg

Charge +1 0 -1

Location Nucleus Nucleus Orbiting

Hydrogen Isotopes

Radioactive isotopes

Every element has some unstable isotopes called radioisotopes.

These are rare. The most common isotope is usually stable (eg. C-12 not C-13 or C-14).

Radioisotopes become stable by decaying into a stable isotope.

They decay by emitting radiation.

Alpha decay The nucleus ejects an α particle (2

protons and 2 neutrons) to become smaller and more stable.

Uranium-238: 92 protons, 146 neutrons.

Emits an α particle: 2 protons, 2 neutrons.

Thorium-234: 90 protons, 144 neutrons.

Th-234 is the daughter nucleus of the alpha decay

Beta decay

A neutron can decay into a proton and an electron (after about 10 minutes).

The electron (β particle) is ejected from the nucleus at close to the speed of light.

The atomic number increases by 1. The mass number stays the same.

Thorium-234: 90 protons, 144 neutrons

One neutron decays into a proton and an electron

Protactinium-234: 91 protons, 143 neutrons

Gamma decay

After an alpha or beta decay the nucleus can be momentarily “excited”.

The excited nucleus emits a high energy gamma ray (which is a photon).

If a nucleus is in an excited state we put an asterisk next to the symbol.

Protactinium-234*: 91 protons, 143 neutrons

Emits a γ particle Protactinium-234: 91 protons, 143

neutrons

Decay of Uranium-238

http://en.wikipedia.org/wiki/Uranium-238

http://en.wikipedia.org/wiki/Decay_chain

Penetration power

Smoke detectors Americium-241 emits α particles. These charged particles allow a

current to flow between two electrodes. If smoke blocks the α particles then the current is reduced and an alarm sounds.

Ionising power Radiation can ionise atoms by removing one or

more electron from it’s orbit. α particles interact strongly with atoms because

of their slow speed and +2 charge. This slows them down quickly (low penetration power).

β particles also with atoms but are moving faster and only interact with the outside electrons. They take longer to slow down and have higher penetration power than α particles.

γ particles interact with atoms very weakly because they have no charge and most of an atom is empty space. This means they pass through most materials.

Properties of α,β,γ particlesProperty α

particleβ particle

γ particle

Mass 6.644 x 10-27 kg

9.109 x 10-31 kg

0 kg

Charge +2 -1 0

Effect of electric or magnetic field

Small deflection Large deflection No deflection

Speed (c%) 5-7% 30-99% 100%

Penetration power

A few cm in airA Piece of paper

1-2m of airA few cm of Al

Goes through airMany cm of lead absorbs most

Ionising power Strong Weak Very weak