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Zack Subin
Physics 10
2/7/07
Section 4 Handout
I. Warmup Questions
A. Where does the sun get its energy from?
B. What is an isotope?
C. Does a relatively stable nucleus have a longer or a shorter
half life?
D. Where do we get most of our helium?
E. What is the relative mass of a proton to an electron?
1.
2.
3.
4.
5.
2:1
20:1
200:1
2000:1
20,000:1
F. About how much more energetic are nuclear reactions than
chemical reactions?
1.
2.
3.
4.
10 times
1000 times
1 million times
1 trillion times
G. What causes most cancers?
1.
2.
3.
4.
5.
6.
Natural radioactivity
Artificial radioactivity
Exposure to both natural and artificial toxic chemicals
Smoking
Sunlight
We don’t know.
H. What is a Sievert? A rem? How much will cause cancer?
How much will kill you from radiation sickness?
I. What are the different types of radiation, and how do they
rank in terms of penetrating ability?
J. What killed most of the people who died at Hiroshima?
K. Which releases more radioactivity, on average, into the local
environment: a coal plant or a nuclear plant?
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II. Atom & Element Review
A. What is an isotope?...
III.Nuclear Stability
A. Competition between “Strong” force interactions and electric
repulsion.
1. The strong nuclear force acts at short distances.
2. The electric force of repulsion between protons acts at larger
distances.
B. “Band of Stability”
1. The most stable atoms are not too big (e.g. no larger than Lead), and
they have roughly equal numbers of protons and neutrons.
2. In large nuclei, slightly more neutrons than protons will be optimal.
3. See Figure 1.
C. Decay
1. If the nucleus gets too large, it will jettison an alpha particle – two
protons and two neutrons.
2. If the nucleus has too many neutrons, it will emit a beta particle – an
electron – (and an antineutrino, generally unobserved), in the process
converting one of its neutrons to a proton.
3. If the nucleus has too many protons, it can emit a “positron” (an antielectron), converting a proton to a neutron.
4. Gamma rays are often released along with the above decays.
D. Potential Energy
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1. Some nuclei are more energetically favorable than others. In other
words, they have less potential energy to get rid of. This is analogous to
the fact that some chemicals are more energetically favorable than others.
Water is more favorable than hydrogen and oxygen — that’s why a little
spark will induce hydrogen and oxygen to spontaneously combine and
release a lot of energy. The only difference is that the energies involved in
the nucleus are much much bigger.
2. When we do relativity and E  mc 2 , we’ll see that the extra potential
energy in less favorable nuclei shows up as extra mass.
a) This is actually true for chemical potential energy too, but the effect is so small that
we don’t usually notice it.
b) The products of a fusion reaction can be as much as a few percent less massive than
the reactants.
3. We can look at a chart of potential energy vs. size of the nucleus. See
Figure 2.
4. Light nuclei can release energy via fusion, and heavy nuclei via decay
or fission.
5. Iron-56 is the peak of stability.
E. Chain of nuclear fusion in a star ending in a supernovae.
1. More and more energy can be created in heavy stars after the
hydrogen fuel runs out by continuing to fuse heavier and heavier elements
– up to Iron-56.
2. Heavier elements than iron-56 are created in the explosion of a
supernova – the violent death of a large star. (This is similar to how
nitrogen oxides are created in a car engine— it actually is less
energetically favorable to form these oxides, but it so hot that some forms
anyway, and after it cools, it takes a while for this pollutant to go away.)
3. Some of these elements ended up in the gas that formed the solar
system, the earth, and ultimately us.
F. Fission and Fusion
1. Fission & fusion generally do not occur spontaneously.
2. A small number of certain isotopes will spontaneously split into
fragments, often releasing neutrons. These same neutrons can trigger the
process of fission itself: next week we will talk more about that.
3. Fusion generally requires high temperatures to overcome the electrical
repulsion of the nuclei.
a) This is a form of activation energy: it’s very similar to the reason that gasoline
doesn’t burn at room temperature.
IV.
Background Radiation
A. See Figure 3.
B. About 300 mrem in the U.S.
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1. Half is from radon.
2. Smaller portions are from K-40 and C-14 in the body, and cosmic rays.
3. About a tenth is from medical procedures (on average – this is likely
concentrated on a subset of the population getting lots of X-rays or
undergoing radiation therapy).
4. Less than 1% is from nuclear fallout, nuclear reactors, etc.
C. Assuming the linear hypothesis, about 1/40 of cancers come
from background radiation.
V. Elementary Particles
A. If we have time and you are curious...
VI.
Discussion Questions
A. Explain the “linear hypothesis.” Do you think it is a good
basis for policy?
B. How does radiation cause cancer?
C. Is it fair to expect that a microwave or a cell phone antenna
could cause cancer?
Figure 1: Band of Stability
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Figure 2: “Binding Energy” per Nucleon (Think of this as a measure of stability; the opposite of
the “potential energy” mentioned earlier.)
Figure 3: Background Radiation
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