PH 103

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# PH 103 - PowerPoint PPT Presentation

PH 103. Dr. Cecilia Vogel Lecture 23. Review. Nuclei a, b ,g decays Radiation damage. Outline. Nuclear physics exponential decay decay constant half-life. How Quickly Decay Occurs. As with all quantum processes it’s all probability.

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Presentation Transcript
PH 103

Dr. Cecilia Vogel

Lecture 23

Review
• Nuclei
• a, b ,g decays

Outline

• Nuclear physics
• exponential decay
• decay constant
• half-life
How Quickly Decay Occurs
• As with all quantum processes it’s all probability.
• Cannot determine when a particular nucleus will decay,
• just probability that it will have decayed in a certain time.
• Can predict how many will decay in certain time, but not which ones.
• for example can predict how long before half have decayed = half-life
Half-Life
• Half-life is
• the time it takes for the number of radioactive nuclei to decrease by half.
• After another half-life, half of what’s left will be gone.
• Now only 1/4 left.
• After another half-life, half of what’s left will be gone.
• Now only 1/8 left.
• etc
• This kind of behavior is
• exponential decay.
Half-Life
• This kind of behavior is exponential decay:
• N = # nuclei left at time t
• No = initial # nuclei
• l = decay constant
• (not wavelength!)
Decay Constant
• Decay Constant, l
• how fast it decays
• higher l, faster decay
• inversely proportional to half-life
Where to Find the Info
• Appendix B, of course!
• Find the isotope you’re interested in,
• rightmost column holds half-lives.
• If there’s nothing in that column,
• BEWAREof units – they vary!
• From the half-life, you can find the decay constant,
• from the decay constant, you get decay as function of time.
Working Backward
• If you know the decay constant and the time, you can plug into exponential.
• What if you need to calculate the decay constant or the time from N’s?
• Solving decay eqn yields:
Decay Rate
• Another quantity of interest is
• The number of nuclei that decay per unit time, called the decay rate
• For each nucleus that decays
• one emitted particle will be counted.
• Decay rate = activity
• Decay rate is also an exponential function of time.
Units
• Units of t and t½ are time units:
• seconds, minutes, years, etc.
• Units of l are inverse time
• Be consistent with your time units!!
• N and No are unitless numbers
• or moles
• Units of R and Ro
• 1 decay/s = 1 Bq = 1 Bequerel (sometimes written as s-1)
• 1 Ci = 1 Curie = 3.7X1010 Bq