Transcript Radioactivity - Clydebank High School

The Atom
Particle
Proton
Neutron
Electron
Symbol
Location
1p
1
Nucleus
1n
0
Nucleus
Charge
1+
0
0e
-1
Electron
cloud
1-
The Atomic
Number = number of protons
( = number of electrons in neutral atom)
The Mass Number = Number of protons +
number of neutrons.
Isotope = atoms with same atomic number but a
different mass – different number of neutrons.
Radioactivity
 The
stability of atom depends on how many protons and
neutrons it has.
 If the numbers of protons are approximately equal to the
number of neutrons then the atom will be stable.
Example – the lighter elements.
 However – if the is a large difference between the
number of protons and neutrons the atom is unstable.
 As the atoms get heavier, the neutron:proton ratio
increases. The neutrons help keep the nucleus from
flying apart since the + protons repulse each other.
What is radioactivity?
The unstable nuclei start to
release energy
spontaneously – we call this decay.
The nuclei will keep doing this until the
neutron:proton ratio lie in the band of stability.
Radioisotopes are isotopes that are radioactive.
Radioactive decay is unlike normal chemical
reaction because it is the nucleus that changes
and not the electrons.
The nature of Radiation
When a
nucleus undergoes decay it emits small
radioactive particles.
There are 3 types: alpha (α), beta (β)and
gamma (γ).
Radiation
Symbol
Mass
Charge
Alpha
α
4
2+
Beta
β
V light
1-
Gamma
γ
0
0
Alpha
and beta are particles.
Gamma is electromagnetic radiation.
Alpha 4 He 2+
2
Beta are high energy electrons o
e–
-1
Each type of
radiation has unique properties.
Alpha is the heaviest and least penetrative – it
travels for a very short distance and can be
stopped by paper.
Beta – very light will travel almost 0.6 cm and be
stopped by Aluminium foil.
Gamma is the most penetrative – it is stopped by
lead or thick concrete.
Nuclear Reactions
When a radioactive atom starts to decay the
original nuclei will change.
We can balance the change by making sure the
total mass on the LHS and RHS are the same
and the total atomic number on the LHS and RHS
are the same.

Nuclear Equations
 Alpha
decay
 When the original atom releases an alpha particle we
can figure out the name of the new atom by taking away
the mass and atomic number of an alpha particle from
the original atom.
 Example
 Polonium with a mass of 210 emits an alpha particles.
 210 P —> 4 α 2+ + 206 Pb
84
2
82
Beta
90
38
Decay
Sr —> 0 e +
-1
90
Y
39
Gamma Decay
Gamma particles have no mass and charge – there
emission has no effect on the mass and atomic
number of the radioisotope.
Making Radioisotopes
This can be
done by bombarding the target
nucleus with small particles: alpha, neutrons or
protons.
Example
Neutron bombardment
238 U + 1 n —> 239 U
92
0
92
Alpha
bombardment
27 Al + 4 He 2+ —> 30 P + 1 n
13
2
Proton Capture
14 N + 1P —>
7
1
15
15
O
8
0
Half Life ( t ½)
 This
is the time taken for a radioisotope to to half it’s
initial activity. The time taken for half the radioactive
atoms to disintegrate.
 The rate of decay depends on the mass of the isotope.
 However – the half life of a particular isotope will always
be the same regards of the mass of the sample size – 1
g of Uranium will have the same half life as 100g of
Uranium.
 Temperature and pressure and particle size etc have no
effect on radioactive decay.
n
= number of half lives ( t ½)
 After n half lives the fraction of the original sample left is
( 1/2) n
 The length of half life various from one radioisotope to
the next – some are very short, others very long.
 The half life of an atom will always be the same if it is in
element form or in a compound.
 This is because radioactive decay is nuclear – unlike
chemical reactions. The nucleus of an atom in an
element and in a compound will be the same. Only their
electron arrangement will be different.
Calculations using half life ( t ½)
Phosphorous has
a half life of 14 days.
The sample has a mass of 80g.
Calculate the mass of the sample of the
radioisotope after 56 days.
Step 1 – Work out how many t ½ have passed.
Time
= 56 days
T ½ = 14 days
Therefore 4 t1/2 have passed.
 Step
2
 Time ( days)
Mass (g)
0
80
14 ( 1 t ½)
40
28 ( 2 t ½)
20
42 ( 3 t1/2)
10
56 ( 4 t1/2)
5
The remaining mass is 5 g.
 If
a radioisotope has a t ½ of 5.6 years. How long will it
take 50g of the sample to decay leaving 12.5g of the
original sample?
 Step 1
 How many t ½ has the sample gone through to become
12.5 g
 Time
Mass
0
50
 1 t1/2
25
 2 t1/2
12.5g
 The
sample has gone through 2 half lives to leave
12.5g.
 Therefore 2 x 5.6 years = 11.2 years.
 What is the half life of a sample if it’s radioactive count
changes from 200 counts per minute to 50 counts per
minute in 36 days?
 Time
Activity ( counts per min)
0
200
1t½
100
 2 t1/2
50
 Therefore 2 half lives have passed in 36 days – t ½ = 18 days.
Uses of radioisotopes
 Medical
 Cobalt
60 is a gamma emitter and is used to treat deep rooted
tumours.
 32 P is a beta emitter and is used to treat skin cancer.
 Radioactive Iodine is used to monitor the Thyroid gland. It is taken
orally or injected and the gland is scanned over a certain period –
plotting the concentrations of the isotope in the body. The isotope
can also be used to kill cancerous cells.
 Gold 198 – a wire of the element is placed directly into the tumour
– the isotope decays over a few days – dosing the tumour.
Industrial Uses of Radioisotopes
 Co
60 is a gamma emitter and is used to examine
castings and welds for imperfections.
 Radioisotopes can be used to measure thickness of
sheet material. Beta and gamma is used for this. 2
identical sources used – i as a reference and the other
passes through the material. The 2 are compared and
the distance can be measured.
 Smoke alarms contain an alpha emitter – not very
penetrative . 241 americium is used.
Scientific Research
 Carbon
dating
 14 C is produced in the atmosphere by the
bombardment of N by neutrons.
 14 N + 1 n —> 14 C + 1 p
7
0
6
1
The 14 C is always being taken up by plants the same as
12 C so there is a known ratio for
14 C : 12C
We measure the ratio in the substance to be dated and
then using the half life of 14C we can age the object.
Example
 Piece
of wood contains 3.75 counts per minute.
 New wood has an activity of 15 cpm, 14C has a t ½ of
5600 years.
 t1/2
Counts
0
15
1t½
7.5
2t½
3.75
2 t ½ have passed therefore the wood is 2 x 5600 =
11,200 years old.
3
H ( tritium) is used to date the age of vintage
wines. ( t ½ = 12.4 years)
32P is used to trace how plants use phosphate
fertilisers.
Background Radiation
There will always be a safe level of radiation in the air
called background radiation.
This comes from cosmic rays, igneous rocks, gamma
rays.
Nuclear Fusion
This is when to light nuclei fuse to form a
heavier nuclei.
1H + 2 H —> 3 He
1
1
2
Nuclear fusion happens in the sun releasing vast
amounts of energy.
2H + 2 H —> 3 He + 1 n
1
1
2
0

Because the T and
P in the centre stars are so
high further fusion reaction take place producing
even heavier nuclei. All natural elements where
produced in nuclear fusion in the stars!
The Hydrogen Bomb uses nuclear fusion. Very
high T allow the small nuclei to join and not
repulse one another.
Nuclear Fission
This is
when heavy nuclei split into lighter nuclei.
The energy can be harnessed – this is the type of
reaction used in nuclear power stations, generate
electricity, submarines etc.
235 U + 1n —> 90 Sr + 144 Xe + 2 1n
92
0
38
54
0
The 2 neutrons produced will bombard more 235
Uranium starting a chain reaction.
The reaction can
be controlled by adding a non
fissionable material to the reactor that will absorb
the neutrons – preventing further bombardment
of 235 U.
The Atomic Bomb was a sample of 235U above
critical size and so leading to a nuclear explosion.
239 Plutonium is used in fast breeder reactors.