Half-lifeHalf-life

L/O;- To understand radioactive half-L/O;- To understand radioactive half-lifelife

Half life is the time taken for a radioactive substance to reach half its radioactivity

In other words the time it takes for half of the atoms to have decayed!!

Half lifeHalf lifeThe decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay…

At start there are 16 radioisotope

s

After 1 half life half have

decayed (that’s 8)

After 3 half lives another

2 have decayed (14 altogether)

After 2 half lives another

half have decayed (12 altogether)

= radioisotope = new atom formed

Modelling Half-lifeModelling Half-lifeCollect 20 cubes in a tray. Collect 20 cubes in a tray. Shake all 20 cubes in your hand and Shake all 20 cubes in your hand and

let them fall into the tray – remove let them fall into the tray – remove all the cubes that land black side up.all the cubes that land black side up.

Count the number of cubes left – Count the number of cubes left – record this number record this number

Continue until there are no dice left Continue until there are no dice left Draw a graph of your results Draw a graph of your results Work out the half-life Work out the half-life

Throw Throw Number of Number of Dice Left Dice Left

StartStart

11

22

33

44

55

66

77

88

99

1010

Number of Throws

Num

ber

of

dic

e

Half the number of

Dice

HALF LIFE

Compare your results to other members of the class

Were the results of the class reliable?

Were your own results accurate?

Half LifeHalf Life AverageAverage

Swap your graph with the Swap your graph with the person next to you and person next to you and

mark it using these criteriamark it using these criteria The X axis starts from The X axis starts from

00 The X axis has a The X axis has a

scale scale The X axis is labelledThe X axis is labelled The Y axis starts from The Y axis starts from

00 The Y axis has a The Y axis has a

scale scale The Y axis is labelledThe Y axis is labelled

The points are plotted The points are plotted correctly correctly

They have drawn a They have drawn a line/curve of best fitline/curve of best fit

The have drawn an The have drawn an accurate line of best accurate line of best fitfit

They have given the They have given the graph a title graph a title

Give your partner a mark out of 10 and set them a

target for next time

Can you think of anything which is

usually really important in a

graph but is not important for this

graph?

Hospitals use Barium ‘meals’ and Thallium as tracers to look at the digestive system and blood flow.

Why do you think the radioactive material Barium and Thallium are used instead of other radioactive

substances?

Hydrogen-3 (tritium)12 yearsHydrogen-3 (tritium)12 years Beryllium-101 million 600 thousand yearsBeryllium-101 million 600 thousand years Carbon-145 thousand 700 yearsCarbon-145 thousand 700 years Silicon-32500 yearsSilicon-32500 years Potassium-401 thousand million yearsPotassium-401 thousand million years Calcium-4214 thousand yearsIron-553 Calcium-4214 thousand yearsIron-553

yearsyears Cobalt-605 yearsNickel-5975 thousand Cobalt-605 yearsNickel-5975 thousand

yearsyears Nickel-6310 yearsNickel-6310 years Krypton-81200 thousand yearsKrypton-81200 thousand years Krypton-8310 yearsKrypton-8310 years Strontium-9029 yearsYttrium-902 daysStrontium-9029 yearsYttrium-902 days Zirconium-931 million 500 thousand yearsZirconium-931 million 500 thousand years Niobium-93m14 yearsNiobium-93m14 years Niobium-9420 thousand yearsNiobium-9420 thousand years Molybdenum-933 thousand 500 yearsMolybdenum-933 thousand 500 years Technetium-99200 thousand yearsTechnetium-99200 thousand years Ruthenium-1061 yearRuthenium-1061 year Palladium-1076 million 500 thousand yearsPalladium-1076 million 500 thousand years Cadmium-113m14 yearsCadmium-113m14 years Tin-126100 thousand yearsTin-126100 thousand years Antimony-1253 yearsAntimony-1253 years Antimony-12612 daysAntimony-12612 days Tellurium-125m58 daysTellurium-125m58 days Iodine-12915 million 700 thousand yearsIodine-12915 million 700 thousand years Cesium-1342 yearsCesium-1342 years Cesium-1352 million 300 thousand yearsCesium-1352 million 300 thousand years Cesium-13730 yearsCesium-13730 years Cerium-1443 hundred daysCerium-1443 hundred days Promethium-1473 yearsPromethium-1473 years

Hafnium-1829 million yearsHafnium-1829 million years Rhenium-18750 thousand million yearsRhenium-18750 thousand million years Lead-20514 million 300 thousand yearsLead-20514 million 300 thousand years Lead-21022 yearsLead-21022 years Bismuth-208400 thousand yearsBismuth-208400 thousand years Bismuth-2105 daysBismuth-2105 days Bismuth-210m3 million yearsBismuth-210m3 million years Polonium-210138 daysRadon-2224 daysPolonium-210138 daysRadon-2224 days Radon-22311 daysRadon-22311 days Actinium-22510 daysActinium-22510 days Actinium-22722 yearsActinium-22722 years Thorium-22719 daysThorium-2282 yearsThorium-22719 daysThorium-2282 years Thorium-2297 thousand 300 yearsThorium-2297 thousand 300 years Thorium-23077 thousand yearsThorium-23077 thousand years Thorium-2311 dayThorium-2311 day Thorium-23214 thousand million yearsThorium-23214 thousand million years Thorium-23424 daysThorium-23424 days Uranium-23272 yearsUranium-23272 years Uranium-233200 thousand yearsUranium-233200 thousand years Uranium-234200 thousand yearsUranium-234200 thousand years Uranium-235700 million yearsUranium-235700 million years Uranium-23623 million yearsUranium-23623 million years Uranium-2384 thousand million yearsUranium-2384 thousand million years Neptunium-2372 million yearsNeptunium-2372 million years Plutonium-23888 yearsPlutonium-23888 years Plutonium-23924 thousand 100 years Plutonium-23924 thousand 100 years Plutonium-2406 thousand 500 years Plutonium-2406 thousand 500 years Plutonium-24114 years Plutonium-24114 years Plutonium-242400 thousand yearsPlutonium-242400 thousand years

What is the half-life of Sodium-24?What is the half-life of Sodium-24? What would you expect the count rate of this What would you expect the count rate of this

sample to be after 20hours?sample to be after 20hours? How long should it take for this sample to fall to How long should it take for this sample to fall to

50cps50cps

What is the half-life What is the half-life of this sample?of this sample?

How long will it take How long will it take for this sample to fall for this sample to fall to 25 cps?to 25 cps?

What will the cps be What will the cps be after 3 half-lives?after 3 half-lives?

What will the cps be What will the cps be after 10 days?after 10 days?

What will the cps be What will the cps be after 3 half-lives?after 3 half-lives?

Strontium - 90 has a half-life of 28.1 years. A sample has an initial activity of 400 MBq . 1. What will the activity be after one half life?2. How long will it take the sample to fall to 100 MBq?3. What will be the activity after 92 years?

What is the half-life What is the half-life of Caesium 137?of Caesium 137?

How long will it take How long will it take this sample to fall this sample to fall below 10 cpsbelow 10 cps

What will the cps be What will the cps be after 3 half-lives?after 3 half-lives?

What will the cps be What will the cps be after 100 years? after 100 years?

A radioactive decay graphA radioactive decay graph

Time

Count

1 half life

An exam question…An exam question…

Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock?

(3 marks)

The rock must be 2 half lives old – 2.6 billion years

Hospitals use Barium ‘meals’ and Thallium as tracers to look at the digestive system and blood flow.

Why do you think the radioactive material Barium and Thallium are used instead of other radioactive

substances?