Download - P1b:Radioactivity SJT
Radiation and the Universeb.Radioactivity
Mr S Thompson
The structure of the atomELECTRON –
negative, mass nearly
nothing
PROTON – positive,
same mass as neutron
(“1”)
NEUTRON – neutral,
same mass as proton
(“1”)
The structure of the atom
Particle Relative Mass Relative Charge
Proton 1 +1
Neutron 1 0
Electron 0 -1
MASS NUMBER = number of protons + number of neutrons
SYMBOL
PROTON NUMBER = number of protons (obviously)
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.
Introduction to Radioactivity
Some 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 “radioactice decay”.
Types of radiation1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 neutrons – the nucleus of a Helium atom)
2) Beta () – an atom decays into a new atom by changing a neutron into a proton and electron. The fast moving, high energy electron is called a beta particle.
3) Gamma – after or decay surplus energy is sometimes emitted. This is called gamma radiation and has a very high frequency 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
IonisationRadiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons:
Alpha radiation is the most ionising (basically, because it’s the biggest). Ionisation causes cells in living tissue to mutate, usually causing cancer.
Blocking RadiationEach type of radiation can be blocked by different materials:
Sheet of paper
Few mm of aluminium
Few cm of lead
Deflection by Magnetic Fields
Alpha 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:
+
-
+
Background Radiation
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power
13% are man-made
Uses of radioactivity1) Medical uses – gamma rays can be used to destroy cancerous cells or to sterilise medical instruments
2) Tracers – a tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe:
Gamma source
Tracers can also be used to develop better plant fertilisers and in medicine to detect tumours:
The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.
Uses of radioactivity 2
Rollers
Beta emitter
Beta detector
Paper
Dangers of radioactivity
OUTSIDE the body and are more dangerous as radiation is blocked by the skin.
INSIDE the body an source causes the most damage because it is the most ionising.
Alpha
Beta
Radiation will ionise atoms in living cells – this can damage them and cause cancer or leukaemia.
Gamma
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
A radioactive decay graph
Time
Count
1 half life
Dating materials using half-lives
Question: 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.
8
8
Answer: The sample was originally completely uranium…
…of the sample
was uranium
4
8
2
8
1
8Now only 4/8 of
the uranium remains – the
other 4/8 is lead
Now only 2/8 of uranium
remains – the other 6/8 is
lead
Now only 1/8 of uranium
remains – the other 7/8 is
leadSo it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old.
1 half life later…
1 half life later…
1 half life later…
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
Evidence about the origins of the universe…
Source of light “Spectra
”
If you pass the light through a gas something different is seen…
helium
Some wavelengths of light are absorbed by
the gas – an “absorption spectrum”.
After
helium
If the light source is moving away the absorption spectra look a little different…
helium
Before
The absorption lines have all been “shifted” towards the longer wavelength end (red end)…
After
Before
A similar effect happens with sound – this is called “The Doppler Effect”
Hear Doppler Effect
This is called red shift. The faster the light source moves the further its light will be “shifted”
Light from different stars and from the edge of the universe also shows this “red-shift”. This suggests that everything in the universe is moving away from a single point.
This is the BIG BANG theory
Red shift summaryLight from other galaxies has a longer wavelength than expected. This shows that these galaxies are moving away from us very quickly. This effect is seen to a greater extent in galaxies that are further away from us. This indicates that the further away the galaxy is, the faster it is moving.
This evidence seems to suggest that everything in the universe is moving away from a single point, and that this process started around 15 billion years ago. This is the big bang Theory.
Observing the UniverseConsider different types of telescope:
Ground-based telescopes
Space-based telescopes
What are the advantages and disadvantages of each?