• Demos: cloud chamber? Needs dry ice, ethanol.
• Notes
  • Do background measurement during schpiel.
  • Inverse square law
    • Any small steady source of rays which travel in straight lines, at same constant speed
    • All rays travel outward from source
    • Rate of detection = rate of emission * fraction of total amount of rays arriving at detector
    • fraction = area of detector / area of sphere of radius r (distance from source to detector)
    • Detection rate = L / r²
    • Rule works for light as well as radioactive particles
  • Alpha
    • sources - radon, heavy elements (Ex., uranium)
    • He nucleus (2+)
    • Easily blocked.
  • Beta
    • Sources (C14, K40)
    • electron or positron
    • Ex: C14 → N14 + e + anti-neutrino
    • Shielding is energy dependent - usually low energy is blocked by foil
  • Gamma
    • Sources - radioactive byproduct (alpha and beta leaves excited nucleus...).
    • High energy, short wavelength light.
    • Hard to block (thick lead)
  • Sievert
    • Measure of hazard, not of counts per second.
    • Energy radiated is weighted by type (Ex., alpha has high weighting, because a 1 MeV alpha particle does more damage than a 1 MeV beta), and where it is absorbed (if available).
  • Lab instructions:
    • Course evals
    • Skip setup by having students simply set Geiger voltage to an acceptable value.
    • Shorten shielding
  • Safety
    • β and γ sources are harmless.
    • α source - WASH HANDS after handling, before eating / drinking, and it will be harmless too.

Background Radiation

Count time	Count	Background rate (Bq) = count/time

How many background radiation events are you exposed to in a second?

The Inverse Square Law

Alpha Source
To compute the sample rate, divide the count by the time you counted for to get the total measured rate. Then, subtract the background rate, to get the rate of just the sample.

Set the count time to 100 s

Distance (cm)	Count	Sample rate (Bq)
2
3
4
5
6
7

Plot the sample rate (y variable) versus 1/r² (x variable). Is the plot a straight line? If not, what shape does the plot have?

Beta Source

Set the count time to 50 s

Distance (cm)	Count	Sample rate (Bq)
2
3
4
5
6
7

Plot the sample rate (y variable) versus 1/r² (x variable). Is the plot a straight line? If not, what shape does the plot have?







Gamma Source

Count time t = ________________

Distance (cm)	Count	Sample rate (Bq)
2
3
4
5
6
7

Plot the sample rate (y variable) versus 1/r² (x variable). Is the plot a straight line? If not, what shape does the plot have?

Shielding

Alpha Source

Shield	Time	Count	Sample Rate
Thick lead
Thick Aluminum
Thick Plastic

Which of the shield materials were able to block at least 50% of the alpha rays?





Beta Source

Shield	Time	Count	Sample Rate
Thick lead
Thick Aluminum
Thick Plastic

Which of the shield materials were able to block at least 50% of the beta rays?





Gamma Source

Shield	Time	Count	Sample Rate
Thick lead
Thick Aluminum
Thick Plastic

Which of the shield materials were able to block at least 50% of the beta rays?





One of your plots should have been non-linear. Using the shielding results as a hint, explain why the particle failed to obey the inverse square law.