1. Ch9 Radioactivity Plum Pudding or Currant Bun model
JJ Thompson thought that the atom contained electrons scattered in a dough of positive charge

2. Rutherford’s Alpha Particle Scattering Experiment
Rutherford expected the alpha particle to enter the positive “dough” and only be slightly scattered
animation
Comedy animation
Excellent Video

4. Most passed straight through foil with little or no deflection
Only 1 in 2000 were deflected
1 in were deflected by more than 90o

5. Like “firing a 15inch naval shell at tissue paper and it came back”
Excellent Video

6. Conclusions
Atom has a very small hard centre where most of the mass is concentrated
Nucleus has a positive charge as it repels alpha particle

7. The probability of deflection by more than 90o is 1 in 10 000
For n layers of atoms, the probability of deflection by any one atom is 1 in n

8. The size of a nucleus relative to an atom is about the same as a football to a football stadium.
(but not this football)

9. True, False, Don’t Know
S1: Radioactive substances make everything near to them radioactive.
S2: Some radioactive substances are more dangerous than others.
S3: Radioactive means giving off radio-waves.
S4: Saying that a radioactive substance has a half life of three days means any produced now will all be gone in six days.
S5 if something is radioactive you can reduce its rate of decay

10. 9.2 Properties of Alpha, Beta & Gamma radiation
What does this tell you about their charge and mass?

11. Can you complete the table?
Name
Symbol
Nature
Elec charge
Stopped by
Ionising 'power'
What is it?
Alpha α
Beta β
Gamme γ
Can you complete the table?

13. Can you see any patterns in the table?
Name
Symbol
Nature
Elec charge
Stopped by
Ionising 'power'
What is it?
Alpha α
Particle +2
mm air; paper
Very good
He nucleus
Beta β -1
mm Al
Medium
Very fast electron
Gamme γ
Wave
cm Pb *
Relatively poor
Electromagnetic radiation
Can you see any patterns in the table?

14. Gamma
Gamma radiation is never completely absorbed (unlike alpha and beta) it just gets weaker and weaker until it cannot be distinguished from the background.
It obeys an inverse square law in air as absorption is negligible.
Radiation spreads out over an increasing sphere. Area of a sphere = 4 π r 2, so as r gets larger, intensity will decrease as 1/r 2.
The effect of absorption by the air will be relatively small.

15. Sealed g sources do not radiate in all directions, so do not expect perfect 1/r 2 behaviour
(ii) you do not know exactly where in the Geiger tube the detection is taking place, so plotting  I -1/2 against r gives an intercept, the systematic error in the measurement.

16. Background Radiation
How might your exposure vary with occupation, location etc?

17. Spark Counter

18. Smoke alarms

19. GM Tube
Research and write up how a GM tube works

20. Geiger Tube Thin metal tube containing argon gas
Thin mica window at end
γ can enter side or window
Metal rod at centre is anode, wall is cathode
Ionised gas atoms attracted to electrodes, accelerate and produce more ions in their path
Many ions discharged in short time sending a pulse of charge through resistor
Voltage pulse across R = one count
Dead time approx 0.2ms for gas to regain non-conducting state, so max count rate = 5000/s

21. Cloud Chamber It works on the principle of ionization.
A felt ring in upper part of cylinder is soaked with alcohol, at room temperature. The bottom part is placed with dry ice.
Alcohol evaporates from the felt and forms a saturated vapor. When the vapor drifts to the cooler lower region, the alcohol forms a super-saturated vapor and is ready to condense. Each time a particle is emitted from the source it produces ions along its path and the alcohol vapor then condenses around these ions which then reflect light and can be photographed.
Cloud Chamber

22. Cloud Chamber α particles produce straight bright tracks.
Those from a given source all have the same length (range) as α particle and nucleus move apart with equal momentum and KE
β particles produce wispy feint tracks of unequal length. Why?
Uranite in a Wilson Cloud Chamber full of Ethanol vapor. The lines are the alpha and beta particle ejected due to radioactive decay.