Quiz, NSC 109
April 13, 2007
Each question is worth 5 points. You may turn in your answers on Monday.
1. What does the radioactive decay of uranium to lead have to do with the gas in a party
balloon?
Uranium decays by alpha-radiation. Alpha radiation is helium nuclei. The helium nuclei
pick up electrons, and become helium gas. Helium gas collects in natural spaces inside
the earth (like natural gas wells).
Helium, once released into the atmosphere, escapes to outer space within a century or
less; it’s too light for Earth’s gravity to hold.
2. Suppose you’re given three radioactive cookies – an alpha emitter, a beta emitter and a
gamma emitter. Your sadistic captors require you to eat one, hold one in your hand, and put
the third in your pocket. How do you minimize your radiation exposure?
You eat the gamma emitter. Gamma radiation is highly penetrating and most of it will
pass right through your body.
You put the beta emitter in your pocket. Your clothes are probably thick enough to stop
beta radiation.
You hold the alpha emitter in your hand. Your skin will stop alpha rays, and since the
first layer of skin cells are dead anyhow, they won’t even do any damage!
3. An archeologist extracts a sample of carbon from an ancient ax handle and finds that it emits
an average of 10 counts per minute. She finds that the same mass of carbon from a living tree
emits 40 counts per minute. If the half-life of carbon-14 is 5730 years, what is the age of the
ax handle?
The intensity has been reduced to ¼ the reference value, so we expect that the ax handle
is two half-lives old (4 = 22). Two half-lives is 11,460 years, which we round to 11,500
years.
4. Is the decay 16O → 12C + 4He possible?
If so, would this reaction yield energy or
absorb energy? Use the chart at right to
help you answer (Figure 16.31, p. 402)
The decay is possible, because the 8
protons and 8 neutrons in 16O are
accounted for. However, the mass
per nucleon goes up during the
decay – which means that the
reaction will absorb energy.