6.
How long does material
remain radioactive? halflife, some uses and implications
6a.
The halflife of a radioisotope
 Some atomic nuclei are very unstable and only exist for a few
microseconds, seconds, minutes, hours or days.
 Others are very stable and take millions of years to decay away to form another atom.
 Some isotopes are completely stable and do not undergo
radioactive decay at all.
 The radioactivity (emissions) of any radioactive material
always decreases with time.
 A measure of the stability of a radioisotope is given by its halflife
(defined below).
 Unstable nuclei disintegrate at random, you cannot
predict which one decay to emit alpha, beta, gamma or other nuclear/ionising radiations.
 What you can say is the radioactivity must always decay
over time but never quite reaches zero, except after a very long period of time
(infinity?).
 The decay follows are particular pattern, illustrated by
the graph below, known as a decay curve.
 The graph will drop steeply for very unstable nuclei but
show a very small gradient if more stable.
 Every graph shows the same mathematical feature which is
that for a particular time interval the amount of
 The halflife of a radioisotope is the
average time it takes for half of the remaining undecayed radioactive nuclei
(atoms) to decay to a different nucleus (atom).

It means in one halflife of time, on average, half of the undecayed unstable
nuclei of a particular isotope disintegrate.
 Within a halflife, half of the remaining
unstable nuclei decay (disintegrate), equivalent to a 50% reduction in
the radioactivity.
 A short halflife means the activity
(radioactivity) will fall quickly e.g. falls by 50% in a few minutes
with lots of unstable nuclei decaying.
 so, a short halflife means relatively rapid
decay, a long halflife means a relatively slow decay and measurable
radioactivity lasts much longer.
 See the decay curve graph below
representing the behaviour of relatively unstable radioactiveisotope with a
halflife of 5 days.
 This means from radioactivity measurements we can analyse
the data and calculate from the graph the halflife of a radioactiveisotope or
some calculation based on an initial level of activity and a later measurement
of the decreased activity. Whatever method, you need accurate activity data
linked to time.
 This also means that we can make predictions of activity
 The radioactivity of any sample will
decrease with time as the unstable atoms decay to more stable atoms,
though sometimes by complex decay series routes e.g. _{92}U isotopes
eventually decay to _{82}Pb
isotopes.


The older a sample of a radioactive material, the less
radioactive it is.

The decrease in
radioactivity follows a characteristic pattern shown in the graph or decay
curve.

The y axis can represent the %
radioisotope left OR the measured radioactivity.

After every
halflife, in
this case 5 days, working out from the graph, the % radioisotope (or radioactivity) is halved,
producing the initially steeply declining curve which then levels out
towards zero after infinite time!

6b.
Four Uses of decay data and halflife
values
(1) Determination of the halflife of a
Radioisotope
 The radioactivity from a radioisotope is measured over a period of time.
 Graphical or mathematical analysis is performed to calculate the time it takes for the radioactivity of the isotope to halve.
 For shortlived radioactive isotopes, the radioactivity is likely to be measured in terms of the count rate.
 Therefore the halflife will be the time it takes for the count rate to halve.

An example of what this means is shown in the diagram below.


You would use a GeigerMarsden counter, or similar
scintillation counter to make measurements of the radioactivity of a
radioisotope.
 The graph shows the rapid decay of a very
unstable radioactive isotope in terms of count rate per minute (cpm) versus
minutes.
 Although not shown, before plotting the graph, you should
do a blank test for the background radiation and subtract this
from ALL the readings.
 You would do a blank test by taking several readings
without the presence of the radioisotope and use the average to correct the
readings.
 An alternative to this is to use heavy lead shielding to
protect the Geiger counter from background radiation, but should still do a
blank test with the identical experiment setup.
 From the graph you can work out the time
(halflife) it takes for half of the radioactive atoms to decay from the
decrease in count rate.
 e.g. in terms of time elapsed, count rate ==> we
get
 0min, 400cpm ==> 10min, 200cpm ==> 20min, 100 cpm
etc.
 clearly showing the
halflife is 10
minutes.
 2nd example not using a graph, but 'simple fraction'
reasoning.
 Suppose a sample of a radioisotope gives an initial
activity of 1200 counts per minute (cpm).
 If the activity has fallen to 150 cpm after 180 days,
calculate the halflife of the radioisotope.
 The simple method just involves involving halving from
the initial value of activity until you reach the final value.
 In terms of activity: 1200 == ÷2 ==> 600 == ÷2 ==> 300 ==
÷2 ==> 150
 so, to get from 1200 to 150 required 3 halvings.
 From the definition of halflife, this means 3 halflives
elapsed for the activity to drop from 1200 cpm to 150 cpm.
 Therefore the halflife is 180÷3
= 60 days
 You need to practice these sort of calculations
of halflife
determination, radioactive residue left, and dating calculations (see below) using the
multiple
choice QUIZ (higher GCSE = AS GCE)
 You can do a class experiment to illustrate the random
nature of radioactive decay and halflife e.g.
 Use say 50 normal dice numbered 16 shaken in a
container.
 Make zero time that before the first 'throw' (tip the lot
out of the box), so t = 0, d = 50
 Throw the dice and remove all sixes, pretending they were
the ones to disintegrate (decay).
 Make this t = 1, d = dice left.
 Just repeat a few times, removing all the sixes each time
and counting the dice left.
 After, say six goes, plot the number of dice left versus
the time interval 0, 1, 2, 3 etc. and a decay curve graph will emerge.
 You can then estimate the 'halflife' in time intervals.
 The time interval could represents, seconds, minutes,
hours ... millions of years, it doesn't matter!
(2) Using halflife data in hazard analysis
or prediction of radioisotope residue
 From the halflife you can calculate how
much of the radioactive atoms are left e.g. after
one halflife, ^{1}/_{2} is left, after two
halflives, ^{1}/_{4} is left, after three
halflives, ^{1}/_{8} is left in other words its a
'halving pattern' etc.
 Example Q: The halflife of a radioisotope is 10
hours. Starting with 2.5g, how much is left after 30 hours?
 2.5g =10h=> 1.25g =10h=> 0.625g
=10h=> 0.3125g (after
total time of 30h)
 Another way to think  if the time
elapsed is equal to a whole number of halflives you can just divide the
30 h by 10 h, giving 3 halflives.
 Therefore you just have to halve the
amount three times!
 e.g. 2.5 ==> 1.25 ==> 0.625 ==>
0.3125g
 The halflife of a radioisotope has implications about its use and storage and
disposal.
 If the halflife is known then the radioactivity of a source can be predicted in the
future (see (1) above).
 Plutonium244 produced in the nuclear power industry has a halflife of 40 000 years!
 Even after 80 000 years there is still a 1/4 of
the dangerously radioactive material left.
 Quite simply, the storage of high level
nuclear reactor radioactive waste is going to be quite a costly problem for
many (thousands?) of years!
 Storage of waste containing
these harmful substances must be stable for hundreds of thousands of years! So
we have quite a storage problem for the 'geological time' future! see also
dangers
and background radiation.
 Radioisotopes used as tracers must have short halflives, particularly those used in medicine to avoid the patient being dangerously over exposed to the harmful radiation,
but a long enough halflife to enable accurate measurement and monitoring of the
tracer.
(3)
Archaeological dating with the isotope carbon14

 Most carbon atoms are of the stable isotope carbon12.
 A very small % of them are radioactive due to
carbon14 with a halflife of 5700 years.
 It decays by beta emission to stable nitrogen14.

 Archaeologists can use any material
containing carbon of 'organic living' origin to
determine its age.
 This can be bone, wood, leather etc. and the technique is
sometimes called radiocarbon14 dating.
 When the 'carbon containing' material is in a living organism there is a constant interchange of carbon with the environment as food or carbon dioxide.
 This means the carbon14 % remains constant as
long as the organism is alive!
 When the organism is dead the exchange stops and the carbon14 content of the material begins to fall as it radioactively decays.
 Compared to when it was 'alive' ...
 if an object has ^{1}/_{2} (^{1}/_{2}
of 1, 50%) of the expected carbon14 it must be 5700 years old,
 if it only has ^{1}/_{4 } (^{1}/_{2} of a
^{1}/_{2}, 25%) of the expected ^{14}C left, the object it must be
11400 years old (5 700 + 5 700),
 and if only ^{1}/_{8
} (^{1}/_{2 } of_{ }
^{1}/_{4},
12.5%) of the ^{14}C left it is 17100 years old (11 400 + 5700)
etc. etc.

 Example of a simple dating calculation.
 An archaeologist had a sample of bone from a
prehistoric skeleton analysed for its carbon14 content.
 The bone sample was found to contain 6.25% of
the original carbon14, calculate the age of the skeleton.
 Just using a simple halving calculation
technique you get ...
 100% ==> 50% ==> 25% ==> 12.5% ==> 6.25%
 So to get to 6.25% takes four halflives
 therefore the age of the skeleton is 4 x
5700 = 22800
years
 For more details
The decay curve for carbon14 is shown in an Excel file
webpage
(4) Geological dating of
igneous
rocks
 Certain elements with very long halflives can be used to
date the geological age of igneous rocks and even the age of the Earth.
has a halflife of 1.3 x 10^{9} years. It decays to form .
 If the argon gas is trapped in the rock, the ratio of potassium40 to
argon40 decreases over time and the ratio can be used to date the age of rock formation
i.e. from the time the argon gas first became trapped in the rock. The method is
more reliable for igneous rocks, rather than sedimentary rocks because the
argon will tend to diffuse out of porous sedimentary rocks but would be well
trapped in harder and denser igneous rocks.
 If the ^{40}Ar/^{40}K ratio
is 1.0 (50% of ^{40}K decayed, 50% left ) the rocks are 1.3
x 10^{9} years old
 If the ^{40}Ar/^{40}K ratio
is 3.0 (75% of ^{40}K decayed, 25% left) the rocks are 2.6 x
10^{9} years old
 If the ^{40}Ar/^{40}K ratio
is 7.0 (87.5% of ^{40}K decayed, 12.5% left) the rocks are
3.9 x 10^{9} years old
 These are worked out on the basis of
100% =halflife=> 50% =halflife=> 25% =halflife=> 12.5%
etc. etc.
 Long
lived isotopes of uranium (element 92) decay via a complicated series of relatively
shortlived radioisotopes to produce stable isotopes of lead (element 82). The
uranium isotope/lead isotope ratio decreases with time and so
the ratio can be used to
calculate the age of igneous rocks containing uranium compounds.
RADIOACTIVITY and NUCLEAR PHYSICS INDEX
1.
Atomic structure, fundamental particles and radioactivity
2. What
is radioactivity? Why does it happen? What radiations are emitted?
3. Detection of radioactivity, measurement,
dose units, ionising radiation sources, background radiation
4.
The
properties and dangers of alpha, beta & gamma radioactive emission
5. The
uses of radioactive Isotopes emitting alpha, beta or gamma radiation
6. Half–life of radioisotopes, how
long does material remain radioactive? Uses of decay data & half–life values
7. Nucleus
changes in radioactive decay? how to write nuclear
equations? Production of Radioisotopes
8.
Nuclear fusion reactions and the formation of 'heavy elements'
9. Nuclear Fission Reactions, nuclear power energy resources
RADIOACTIVITY
multiple choice QUIZZES and WORKSHEETS
EasierFoundation
Radioactivity Quiz
or
HarderHigher
Radioactivity Quiz
five
wordfills on radioactivity
*
Q2
*
Q3
*
Q4
*
Q5and
ANSWERS!
crossword
puzzle on radioactivity
and
ANSWERS!
OTHER CHEMICAL CALCULATION PAGES

What is relative atomic mass?,
relative isotopic mass and calculating relative atomic mass

Calculating relative
formula/molecular mass of a compound or element molecule

Law of Conservation of Mass and simple reacting mass calculations

Composition by percentage mass of elements
in a compound

Empirical formula and formula mass of a compound from reacting masses
(easy start, not using moles)

Reacting mass ratio calculations of reactants and products
from equations
(NOT using
moles) and brief mention of actual percent % yield and theoretical yield,
atom economy
and formula mass determination

Introducing moles: The connection between moles, mass and formula mass  the basis of reacting mole ratio calculations
(relating reacting masses and formula
mass)

Using
moles to calculate empirical formula and deduce molecular formula of a compound/molecule
(starting with reacting masses or % composition)

Moles and the molar volume of a gas, Avogadro's Law

Reacting gas volume
ratios, Avogadro's Law
and GayLussac's Law (ratio of gaseous
reactantsproducts)

Molarity, volumes and solution
concentrations (and diagrams of apparatus)

How to
do volumetric titration calculations e.g. acidalkali titrations
(and diagrams of apparatus)

Electrolysis products calculations (negative cathode and positive anode products)

Other calculations
e.g. % purity, % percentage & theoretical yield, volumetric titration
apparatus, dilution of solutions
(and diagrams of apparatus), water of crystallisation, quantity of reactants
required, atom economy

Energy transfers in physical/chemical changes,
exothermic/endothermic reactions

Gas calculations involving PVT relationships,
Boyle's and Charles Laws

Radioactivity & halflife calculations including
dating materials
(this page)
