radioactivity w richards the weald school structure of the atom a hundred years ago people thought...
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Structure of the atomA hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it…
I did an experiment (with my colleagues Geiger and Marsden)
that proved this idea was wrong. I called it the “Scattering
Experiment”
Ernest Rutherford, British scientist:
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The Rutherford Scattering ExperimentAlpha
particles (positive
charge, part of helium
atom)
Thin gold foil
Most particles passed through, 1/8000 were
deflected by more than 900
Conclusion – atom is made up of a small, positively charged nucleus surrounded by
electrons orbiting in a “cloud”.
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The structure of the atomELECTRON –
negative, mass nearly
nothing
PROTON – positive,
same mass as neutron
(“1”)
NEUTRON – neutral,
same mass as proton
(“1”)
Atoms are roughly 10-10m in diameter, while the nucleus is 10-15 – 10-14m
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The structure of the atomParticle Relative Mass Relative Charge
Proton 1u (1.7x10-27kg) +1.6x10-19C
Neutron 1u (1.7x10-27kg) 0
Electron 0 -1.6x10-19C
MASS NUMBER (A) = number of protons + number of neutrons
SYMBOL
No. of neutrons N = A - Z
PROTON NUMBER (Z) = number of protons (obviously)
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IsotopesAn isotope is an atom with a different number of neutrons:
Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.
Notice that the mass number is different. How many neutrons does each isotope have?
A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.
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Quarks
Pe-
Low energy scattering
We can investigate the structure of protons by bombarding them with electrons:
High energy scattering
Elastic collision. Electrons and protons behave as expected.
Pe-
Inelastic collision. Energy is “absorbed” by the proton and increases its internal energy. This is Deep Inelastic Scattering and suggests that the proton is made of smaller particles called quarks.
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Introduction to RadioactivitySome substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation:
Radiation
The nucleus is more stable after emitting some radiation – this is called “radioactive decay”.
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IonisationRadiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by giving them enough energy to “knock off” electrons:
Alpha radiation is the most ionising (although short range). Ionisation causes cells in living tissue to mutate, usually causing cancer.
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The Geiger-Muller Tube
Metallic case (cathode)
Mixture of argon and halogen
Central anode
Mica end window
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Types of radiation1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom)
2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle.
3) Gamma – after or decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed.
Unstable nucleus
Unstable nucleus
Unstable nucleus
New nucleus
New nucleus
New nucleus
Alpha particle
Beta particle
Gamma radiation
Words – frequency, proton, energy, neutrons, helium, beta
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Changes in Mass and Proton NumberAlpha decay:
Am241
95Np
237
93α
4
2+
11
5
0
+1C
11
6B β+
90
39Sr
90
38Y β
0
-1+
Beta - decay:
Beta + decay:
“positron”
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Blocking RadiationEach type of radiation can be blocked by different materials:
Sheet of paper (or 6cm of air
will do)
Few mm of aluminium
Few cm of lead
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SummaryProperty Alpha Beta - Beta + Gamma
Charge
Rest mass
Penetration
What is it?
Ionising ability
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Deflection by Magnetic FieldsAlpha and beta particles have a charge:
++
-
2 protons, 2 neutrons, therefore charge =
+21 electron, therefore
charge = -1
Because of this charge, they will be deflected by electric and magnetic fields:
+
-
+
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Background Radiation
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power
13% are man-made
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Nuclear fission
Uranium nucleus
Unstable
nucleus New nuclei (e.g. barium
and krypton)
More neutron
s
Neutron
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Chain reactions
Each fission reaction releases neutrons that are used in further reactions.
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Radioactive DecayRadioactivity is a random process. The number of radioisotopes that will decay clearly depends on the number of radioisotopes present at that point in time:
Activity (in Bq) = λN
λ = “The decay constant” and has units of s-1. It is constant for a particular radioisotope.
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Half 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
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A radioactive decay graph
Time
Count
1 half life
1 half life
1 half life
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Half LifeTo calculate half life there are a few methods:
1) Read from a graph
2) Calculate using an equation
t½ = ln2
λ
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Half Life questions
100s
1) The graph shows the activity of a radioisotope. Determine the half life and decay constant.
2) If there are 106 atoms present right now calculate how many will decay over the next second.3) What percentage of a sample of radioactive material will exist after 200 years if the half life is 50 years?
4) Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.