4.2 radioactive decay

Dr Pusey

www.puseyscience.com

Syllabus points “Some nuclides are unstable and spontaneously decay,

emitting alpha, beta (+/-) and/or gamma radiation over time until they become stable nuclides”

“Alpha, beta and gamma radiation have different natures, properties and effects”

Learning goals Define:

Alpha particle

Beta particle

Gamma radiation

Ionisation

Penetration

Compare and contrast the structure of alpha, beta and gamma radiation

Compare and contrast the properties of alpha, beta and gamma radiation (including ionising ability and penetration rate)

Describe how to shield alpha, beta and gamma radiation

Identify that in a nuclear reaction both atomic and mass numbers are conserved

Construct nuclear decay equations

Balance nuclear decay equations

Interpret nuclear decay equations

Recall which decays can occur simultaneously and which cannot

Writing nuclear equations

Mass is always conserved

Charge is always conserved

Use the chemical symbols for the elements

E.g.

Parent nucleus Daughter nucleus

Types of radiation

In a radioisotope the nucleus is unstable and may spontaneously emit radiation to become more stable

There are three different types of radiation: Alpha - α

Beta - β

Gamma - γ

The energy of radiation can be measured in electron-volts(eV)

where 1 eV = 1.6 × 10-19 J

Alpha radiation

Represented by the lowercase Greek letter ‘alpha’ - α

Consists of 2 protons and 2 neutrons, it is a helium nucleus

Positively charged (+2)

Usually emitted by heavier elements such as uranium and radium

Can be absorbed by a sheet of paper

Range in air is only a few centimetres

Emitted at a velocity of 5-7% the speed of light

α

Alpha decay - Example

atomic number decreases by 2

mass number decreases by 4

238

92

234

90 2

4

U Th + α

Alpha decay - General

A

Z

A-4

Z-2 2

4

X Y + α

Your Turn Complete the following nuclear equations:

Your Turn Complete the following nuclear equations:

Beta radiation

Represented by the lowercase Greek letter ‘beta’ - β

Consists of 1 electron

Negatively charged (-1)

Can be absorbed by 3.5 cm of lead or a sheet of aluminium

Range in air is a few metres

Emitted with a velocity of 30-90% speed of light

β

Beta decay - Example

atomic number increases by 1

mass number remains the same

14

6

14

7C N + β

-1

0

Beta decay - General

A

Z

A

Z+1X Y + β

-1

0

Your Turn

Complete the following nuclear equations:

Your Turn Complete the following nuclear equations:

Gamma radiation

Gamma radiation electromagnetic wave (not a particle)

No charge, it’s a type of energy, not a particle

Absorbed by several cm of lead

Range in air is almost unlimited

Electromagnetic wave, so it travels a the speed of light

γ

From: http://en.wikipedia.org/wiki/Electromagnetic_spectrum#mediaviewer/File:EM_Spectrum_Properties_edit.svg

Gamma decay

atomic number remains the same

mass number remains the same

60

28

60

28Ni Ni + γ

Your TurnWrite the following nuclear equations,

make sure they are balanced:a)Plutonium-244 decays by alpha emission

b)The decay of nitrogen-16 by beta emission

c)Uranium-235 loses an alpha particle

d)Phosphorus emits a beta particle

e)Thallium decays through emitting gamma rays

Your Turn

Combinations

When an atom decays it can emit more than one type of radiation at the same time

Gamma radiation is not normally emitted alone, it usually accompanies alpha or beta radiation

α γ β γ α βE.g.

Penetration & ionisation Penetration = how deep radiation can travel

into a material

Ionising ability = ability of radiation to create ions

Alp

ha • Slow speed

• Relatively large • +2 charge• Can pick up loose

electrons to become a He atom

• Not much energy• Easy to stop• High ionising

ability• Low penetrating

ability

Bet

a • Faster than alpha• Small particle• -1 charge• Repelled by

electron cloud of other atoms

• More energy than alpha

• Do not ionise as readily as alpha

• More penetrating than alpha

Gam

ma • Moves at the

speed of light• No charge• Only interacts

when it directly hits a nucleus/electron, rare occurrence

• Low ionising ability

• High penetrating ability

Resources

AV Bozeman Science - Radiation and Radioactive Decay

(10:55) Tyler DeWitt – Alpha Decay (13:07) Tyler DeWitt – Beta Decay (9:58) Khan Academy – Alpha, beta and gamma decay (8:06)Further Reading BBC Bitesize Science – simple recap of important info ARPANSA website – info on alpha, beta and gamma

radiation pHet simulation - Alpha Decay pHet simulation - Beta Decay

How did you go? Define:

Alpha particle

Beta particle

Gamma radiation

Ionisation

Penetration

Compare and contrast the structure of alpha, beta and gamma radiation

Compare and contrast the properties of alpha, beta and gamma radiation (including ionising ability and penetration rate)

Describe how to shield alpha, beta and gamma radiation

Identify that in a nuclear reaction both atomic and mass numbers are conserved

Construct nuclear decay equations

Balance nuclear decay equations

Interpret nuclear decay equations

Recall which decays can occur simultaneously and which cannot