topic 7. 2 radioactive decay

These notes were typed in association with Physics for use with the IB Diploma Programme by Michael

Dickinson

For further reading and explanation see:Physics, Tsokos (purple): Ch 6.2

Physics, Giancoli (mountain): Ch 30

Topic 7. 2 Radioactive Decay

Certain isotope’s nuclides are said to be unstable – that is, the strong nuclear force does not bind the nucleus together indefinitely and it eventually breaks apart.

Radioactive decay – the process of these unstable nuclides breaking apart.

These unstable nuclides are said to be radioactive.

Natural radioactive decay (natural transmutation) – the break down and change of radioactive elements to change into a different element.

7.2.1 Describe the phenomenon of natural radioactive decay.

Three types of decayAlpha decayBeta decayGamma Radiation

During alpha and beta decay, the radioactive nuclide changes into one of a different atomic number. This is therefore a different element. This change is called natural transmutation.


Alpha Decay (α)An unstable nuclide emits an alpha particle to try and

become more stable.Alpha particle – (2protons and 2neutrons)Nucleus of a helium atom

Alpha decay generally occurs in very large nuclides. (lead)

Larger nucleus means more repulsive Coulomb force acting between the protons, spans the entire nucleus

Strong force only acts between neighboring nucleons. If coulomb force becomes too big then the strong force

is no longer able to hold nucleus together.


Let’s look at radium – 222 (atomic number 88)Ra -222 – Parent nucleusZ = 88N = 134

α – alpha particleZ = 2N = 2

Rn – Daughter NucleusZ = 86N = 134


HeXX NAZN

AZ

422

42

Beta Decay (β) – is an electronAn unstable nuclide emits and beta particle to

try and become more stable.Emits an electron from the nucleus. Electrons have a mass number of 0 and

atomic number -1. Symbol - ****Draw on board****


Beta Decay (β) – is an electronA neutron in the nucleus changes into a

proton, and electron, β, and an almost undetectable particle called an antineutrino. v (with a line)

The elctron and the antineutrino are emitted from the nucleus at very high speed.

Since the number of nucleons remains the same, the mass number does not change.

The number of protons and therefore the atomic number Z, increases by 1


Let’s look at Iodine – 131 (atomic number 53)I – 131 – Parent nucleusZ = 53N = 78

Xe – 131 – Daughter NucleusZ = 54N = 77


11 N

AZN

AZ YX

Gamma Radiation (γ)After alpha or beta emission, the daughter nucleus

is left in an “excited state”. The protons and neutrons reorganize themselves in an attempt to become more stable.

As they do this they lose energy. This energy is emitted from the nucleus as a pulse of gamma radiation

This energy emission is a little like the energy emitted from an atom as an electron drops to a lower energy state.

However, the energy levels oa a nucleus are much wider than that.


Gamma Radiation (γ)Where an atom emits radiation in the order of

a few eV, the nucleus emits radiation in the range of a few keV, or a few MeV


Alpha (α)Helium nucleus+2 chargeMass: 4u(7350 x meStrong ionizing ablilityStopped by a sheet of thick paper or card or

by the skin.Travels a few cm in airEffect in an electric field ***See board***Effect in an magnetic field ***See board***

7.2.2 Describe the properties of alpha (α) and beta (β) particles and gamma (γ) radiation.

Beta (β)High speed electron-1 chargeMass: 1/1800uWeak ionizing abilityStopped by a few mm of aluminum or other

metals. Travels up to a meter in air.Effect in an electric field ***See board***Effect in an magnetic field ***See board***


Gamma (γ)Electromagnetic Wave0Mass: 0uVery weak ionizing abilityNever completely stopped although reduced

by thick concrete or lead.Effect in an electric field ***See board***Effect in an magnetic field ***See board***


Decay SeriesParent nuclide don’t always decay into a

stable daughter nucleus. The daughter nucleus then needs become the parent and decay again, producing another daughter.

This will continue until there is a stable nuclide.


Ion is a charged atom. Atoms become ionized by gaining or losing electrons.

Ionizing radiations – radiation that has the ability to strip away electrons.

Can damage or destroy cells.Can alter the chemical information in cells.Can cause mutations and/or cancer.The ionization effects of radiation can be used

to detect and identify the different forms of radiation.

7.2.3 Describe the ionizing properties of alpha (α) and beta (β) particles and gamma (γ) radiation. 7.2.4 – Outline the biological effects of ionizing radiation.

7.2.5 Explain why some nuclei are stable while others are unstable.

Two interactions inside the nucleus.Coulomb, electrostatic force and Strong Nuclear Force.

Very fine balance must be maintained to stay stable.

Neutrons help to increase the nucleus size and keep protons further apart to reduce the coulomb repulsive forces.

There has to be just the right ratio of neutrons to protons.

If it’s not just right the strong force is reduced the nucleus becomes unstable


This graph shows the relationship between neutrons and protons.

Stable nuclides – Z < 20Protons and neutrons are about equal

Above 20 the nuclides have more neutrons than protons.


Nuclides above the stability line have too many neutronsBeta minus (electron emission)

Nuclides below the stability line have too few neutronsBeta plus (positron emission)Positron is same mass and size charge as electron

only positive.Larger unstable nuclides decay by alpha

emission.Above Z = 83


topic 7. 2 radioactive decay

Documents

radioactive elements

helium atom alpha decay

number of protons

radioactive nuclide

different atomic number

nucleus changes

mass number

number of nucleons