Today we're going to talk about the half-lives of nuclear reactions and radioactive decay processes.
We'll also discuss the Geiger counter and how it's used to detect radioactivity.
Half-Life (t½) = the time required for the number of nuclides to reach half the original value.
Assuming first-order kinetics (see Section 12), the equation for the half-life is as follows:
t½ = ln 2 / k = 0.693 / k , where k is the rate constant.
The half-lives of radioactive isotopes (nuclides) can vary tremendously, ranging from 109 years to 10-4 seconds.
Um... that's a range of 1,000,000,000 years to only 0.0001 seconds!
ex: Consider technetium-99, or 99Tc. The rate constant for its radioactive decay is 1.16 x 10-1 hr-1. Calculate the half-life of this nuclide.
➞ we'll need the half-life equation:
t½ = 0.693 / k
t½ = 0.693 / 0.116 hr-1
t½ = 5.98 hr
So, it takes almost 6 hours for a sample of Tc-99 to decrease to half the number of original nuclides.
Detection of Radioactivity
Radioactive decay processes generate high energy particles such as alpha-particles, beta-particles, and gamma-rays.
These particles can produce ions when traveling through matter.
A geiger counter takes advantage of this...
How Does a Geiger Counter Work?
Geiger counter = measures radioactivity by "capturing" particles through a window opening in the counter.
Once inside, these radioactive particles ionize Ar(g) to Ar+(g), and it's the amount of Ar+(g) that gets measured.
Here's the 7 steps involved...
1. Radiation moves randomly outside.
2. Some radiation enters the window.
3. When radiation (α-particle, etc.) collides with Ar(g) in the tube, it causes:
Ar(g) → Ar+(g) + e-
4. The positively-charged Ar+ ions are attracted to the outside of the tube.
5. The electrons (e- 's) are attracted to a wire running down the middle of the tube.
6. Many electrons travel down the wire, making a "burst" of current in a circuit connected to it.
7. Electrons make the meter's needle deflect, and the loudspeaker makes a "click" every time particles are detected.
Next up in SECTION 19 - The Nucleus and Nuclear Chemistry,
We'll talk about Carbon-14 dating, followed by Nuclear Fission and Nuclear Fusion.
Hope you stick around!...