Ch9 Radioactivity Plum Pudding or Currant Bun model JJ Thompson thought that the atom contained electrons scattered in a dough of positive charge http://www-outreach.phy.cam.ac.uk/camphy/nucleus/nucleus1_1.htm

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

Most passed straight through foil with little or no deflection Only 1 in 2000 were deflected 1 in 10 000 were deflected by more than 90o

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

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

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 10 000n

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

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

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

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?

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?

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.

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.

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

Spark Counter

Smoke alarms

GM Tube Research and write up how a GM tube works

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

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. http://www.sfcc.edu.hk/klaw/project/Radioactivity/detection/cloud.htm Cloud Chamber

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.