Chpt. 8: Radioacti

vity

Trefoil Symbol – symbol for ionising radiation

The Discovery of RadioactivityHenri Becquerel (1896):

• Investigated the effect of sunlight on crystals of uranium salts• By chance he left sample resting on a photographic plate wrapped in black paper in a dark drawer and accidentally discovered it gave off some sort of radiation

• Investigated the radiation emitted - concluded emission of radiation was spontaneous i.e. no heat, light needed

Marie & Pierre Curie (1898):

• worked with a Uranium ore (pitchblende) and purified it.• after thousands of re- crystallisations they isolated two new elements from this ore:

- polonium - radium

• 1903 – Nobel Prize for physics• 1911 – Nobel prize for chemistry

What is radioactivity?

Radioactivity is the spontaneous breaking up of unstable nuclei with the emission of one or more types of radiation.Radiation consists of :

Alpha particles α

Beta particles ß

Gamma radiation γ

Note:

For a nucleus to be stable it must contain a suitable ratio of protons to neutrons. If this ratio is not suitable the nucleus disintegrates throwing out fragments in an effort to achieve a stable ratio. This is radioactivity/radiation

Alpha ParticlesWhat are alpha particles???

• 2 protons and 2 neutrons stuck together.

*He24 same as Helium nucleus

• may be represented as He24

Properties of alpha particles:

• Low penetrating power – stopped by few cm of air or sheet of paper

• Positively charged

• Strongly Ionising – it knocks electrons of matter it passes through producing positive ions

• Slowest moving radiation

• Are deflected by magnetic and electric fields

Example

A example of a radioactive element that emits alpha particles is americium-241

Use of americiun-241 – smoke detectors

Alpha Source

Detector

Alpha particles reaching the detector - no alarm

Smoke present

Alpha Source

DetectorAlpha particles do not reach the detector – alarm goes off

No Smoke present

Beta ParticlesWhat are beta particles???

• Electrons• Formed when a neutron in an unstable nucleus is changed into a proton and an electron.

• The electron is ejected from the nucleus

Properties of Beta Particles:

• More penetrating than alpha particles – can penetrate up to about 5 mm of aluminium

• Negatively charged

• Less ionising than alpha particles

• Deflected by magnetic and electric fields

Example

An example of an element that emits beta particles is carbon-14

- carbon-14 is used to date archaeological artefacts (carbon dating)

Gamma Radiation

What is gamma radiation???

• High energy electromagnetic radiation – radiant energy not a particle (similar to X-Rays)

• An unstable nucleus emits gamma radiation in order to lose surplus energy

Properties of Gamma Rays:

• Not deflected in magnetic/electric fields• Has no charge or mass – can be emitted from a nucleus that has emitted an α or β particle• Poor ionising ability• It is fast moving similar to the speed of light• High penetrating ability – only stopped by thick slab of lead• Very harmful to human tissue – absorbed = cancer

Example

An example of an element that gives off gamma rays is cobalt – 60

- cobalt-60 used in cancer treatment (cancerous cells more susceptible to damage by ionising radiation than

normal cells)

- cobalt-60 used in food irradiation

Absorption of Radiation

Demonstration: To detect the presence of α particles, β particles and gamma radiation, and to investigate the penetrating power of each of these three types of radiation

Note:To detect radiation we can use a Geiger - Müller tube connected to a ratemeter.

Distinction between Chemical Reactions and Nuclear Reactions

• Chemical reactions involve transfer of electrons:

(a) Elements combine to form compounds(b) Compounds break up to form elements(c) Elements in compounds rearrange to

form new compounds• Nuclear reactions involve the emission of radioactive radiation from the nuclei of unstable atoms:

- Nuclear reactions cause changes in nucleus- Nuclear reactions cause

elements to change into other elements

Nuclear Reactions

Nuclear Equations – Alpha Emissions:When an alpha particle is emitted from a radioactive element the new element remaining is found by:

- subtracting 4 from the given atomic mass no. - subtracting 2 from the given atomic no.

(i.e. Move back two places on the periodic table to identify element)

Alpha Particles Emission:

When an atom looses an alpha particle it changes into an atom of the element two places before it in the Periodic Table. The mass number of the parent element decreases by four

Example: If an atom of radium-226 looses an alpha particle what is the new element formed???

22688Ra ? + 4

2He + energy

1) Complete calculations:

A: 226 – 4 = 222 - New atomic mass no.Z: 88 – 2 = 86 - New atomic no.

2) Use periodic table to identify new element:226

88Ra 22286Rn + 4

2He + energy

Complete the following equations:

a) 21484Po ? + 4

2He + energy

b) 238 92U ? + + energy

42He

Nuclear Equations – Beta Emissions:When a beta particle is emitted one of the neutrons is changed into an electron and a proton. (One neutron less and one more proton). The proton remains in the nucleus. There is:

- no change in the atomic mass number (A)

- the atomic number (Z) increases by 1

Beta Particles Emission:

When an atom loses a beta particle, it changes into an atom of the element one place after it in the Periodic Table. The mass number of the atom stays the same.

Example:If an atom of carbon-14 looses a beta particle what is the new element formed???

146C ? + 0

-1e + energy

1) Complete calculations A: no change – atomic mass numberZ: 6 + 1 = 7 - new atomic number

2) Use periodic table to identify element

146C 14

7N + 0-1e + energy

Complete the following equations:

a) 21482Pb ? + ? +

energy

b) 19879Au ? + ? +

energy

Gamma Radiation:The loss of gamma radiation does not give rise to any new atoms – energy (hf) is simply lost from the nucleus

Transmutation:The changing of one element into another is called transmutation

Half-Life

The half life of an element is the time taken for half of the nuclei in any given sample to decay.

Example: Carbon 14 half life = 5,700 years Cobalt 60 half life = 5 years Polonium 234 half life = 0.15 milliseconds

Significance of long half-life as an argument against nuclear power

Radioisotopes

• Unstable isotopes that are radioactive

Most elements of atomic number above 83 do not have stable isotopes

Many useful isotopes are made artificially. In nuclear reactors e.g. Cobalt-60

Uses of Radioisotopes(Must know 3)

1. Medical Uses:Cobalt 60 (gamma radiation) can be used to treat cancerous cells as cancerous cells are more susceptible to damage by ionising radiation than normal cells.

Gamma radiation also used to sterilise medical instruments

2. Archaeological:Carbon-14 (carbon dating) /beta emitter

• Used to indicate age of carbon containing objects(wood, fabric….)• Determination Living organism contains C-12 and C-14 in same proportions as in the air On dying, the ratio of C-12/C-14 changes with time due to radioactive decay of C-14 The extent of the changes in the ratio allows the age to be calculated

3. Americium 241 (discussed previously):

• Alpha particle emitter

• These ionise the air molecules and conduct electric current between two terminals smoke cling to the ionised molecules and slows them down

• The current decreases and a transistor switch activates the alarm

Background RadiationRadiation can be detected all around us. We call this background radiation. There are two types of sources: - natural (first 4) - artificial

Source Percentage

Radon gas 60%

Rocks+ soil 10%

Food+ drink

10%

Cosmic rays

10%

Medical 9%

weapons 0.2%

Nuclear waste

<0.1%

*Note: Radon is a radioactive gas that escapes from rocks and increases the risk of cancer