Monday, April 7, 2008
Announcements:
- I will be out of town the rest of this week due to a family
emergency. I will try to find someone else to teach class on
Wednesday and Friday, but these classes may be cancelled. I will
at least post notes, and you are expected to keep up with the reading
and suggested homework problems.
- There is no lab this week so that you can meet with your group members to finish your web projects.
Assignments:
- Read Chapter 16 and answer the recommended homework questions.
- Continue
working on your Web Project, and send me the URL for
your group's website once it is posted (preferably by next Monday so
that I can give feedback before the projects are due on April 21).
- For 2 days worth of class participation credit, you are
encouraged to create your own personal website in your UNC server space
(www.unc.edu/~onyen). This website does not have to be anything fancy,
just something other than the current default message that appears for
a blank webpage. Once you have completed this assignmment, send
me your URL.
Chapter 15 - Nuclear Physics
Nuclear physics is the study of the nuclei (center) of atoms and
associated energy levels that are similar to those of atomic physics
(which deals with electron energy levels), but the forces and energies
associated with nuclear reactions are about 1000 times greater!
The Constituents and
Structure of Nuclei
Nuclei consist of protons
and neutrons, collectively
called nucleons.
Nuclear mass (nearly same as atomic mass since me
<< mp): A = N + Z
Z = Atomic number
N = Neutron number
A = Nuclear (~atomic) mass number
Isotopes
are nuclei with the same atomic number (Z) but different neutron number
(N).
Ponderable: Why are atoms defined by Z instead
of N or E?
Most atoms are stable when N ~ Z or slightly greater, as can be seen in the diagram for half life of various isotopes.
Atomic mass unit:
1 u = 1/12 mass of one atom
of C-12 (common element in all organic matter)
Energy
(E=mc^2): 1 u = 931.5 MeV/c^2 (which is much greater than
atomic energies: ~ eV)
Neutrons and protons have masses slightly greater
than 1 u, with mn > mp
Nuclei are held together by the strong nuclear force, which is
attractive between all nulceons within range of few fermis (fm).
Nuclear
size: radius of a nucleus is proportional to the cubed
root of the number of nucleons: r = (1.2 fm)A^1/3
Note: The atomic diameter of the
hydrogen atom (~1 angstrom = 0.1 nm), is about 50 000 times bigger than
the nucleus
(~2 fm).
This means that if the
nucleus of the hydrogen atom were the size of a baseball, the electron
would typically be ~4 km away.
Note: It just so happens that 1
fermi (fm) = 10-15 m = 1 femtometer = 1 fm.
Radioactivity - one of three types of nuclear reactions (fission and fusion are the other two)
Radioactivity from uranium was first discovered by Henri Becquerel, and Marie and Pierre Curie discovered radium.
Every isotope with atomic number greater than 83 (bismuth) is
radioactive, and many isotopes of lighter elements are as well.
Radioactivity
refers to emissions observed when a nucleus decays to a lower energy
state or changes its composition.
Alpha
decay
- radioactivity with lowest energy. Alpha particle is He
nucleus (2 protons + 2 neutrons). half-thickness ~ paper sheet
Beta
decay
- Emission of an electron (B-) or positron (B+). Can occur
when neutron decays into proton, electron, and antineutrino.
half-thickness ~
cm Al
Gamma
decay
- Occurs when an excited nucleus drops to a lower energy state and
emits a photon (Z and N remain same). half-thickness ~ cm Pb
Ponderable: Why is E(alpha) < E(beta) <
E(gamma)?
Activity of
a radioactive sample is the number of decays per second: 1
becquerel = 1 Bq = 1 decay/s
1 curie (Ci) = 3.7e10
decays/s (~activity of 1 g of radium, which is what Marie Curie
studied)
Example: What are the daughter elements for: alpha decay of
U-238, beta decay of Th-234, and beta decay of C-14?
What are some practical uses of radioactive isotopes?
Half-life and Radioactive
Dating
Decay constant
(lambda): N = Noexp(-lambda*t)
Half life:
T1/2 = ln2/lamda
Decay rate or
activity: R = dN/dt = lambda*N
Carbon-14 dating
can be used to date organic materials up to ~15,000 y (Why?)
t =
1/lamda*ln(Ro/R) where Ro = 0.231 Bq, lambda = 1.21e-4
/y, T1/2 of C-14 is 5730 y
Nuclear Binding Energy -
energy needed to separate a nucleus into its component nucleons
Nuclear Fission is
the process where a large nucleus captures a neutron and then divides
into two smaller "daughter" nuclei. When this happens, two or
three neutrons are typically released, and this can result in other
fission reactions (chain reaction). This process is used in
nuclear power plants and nuclear bombs.
Nuclear Fusion occurs
when two small nuclei merge to form a larger nucleus. Energy is
needed to overcome the Coulomb repulsion of the two positive nuclei,
but the final product results
in a net release of energy (an exothermic reaction). This is the
process that fuels our Sun.
Practical Applications of
Nuclear Physics - Nuclear radiation can be useful, but it can
have harmful effects as well.
Radiation dosage can be measured in different ways:
roentgen:
1 R = 2.58e-4 C/kg (ionization charge produced by 200-keV X-rays
in 1 kg of dry air at STP)
radiation absorbed
dose (rad): 1 rad = 0.01 J/kg (energy absorbed by
any type of radiation)
Relative
Biological Effectiveness (RBE) = (dose of 200-keV X-rays)/(does
of particular radiation)
roentgen
equivalent in man (rem) = (dose in rad)*RBE
A dose of 1 rem causes same
biological damage, regardless of radiation type.
Four sensible ways to reduce radiation exposure:
1) Use smallest amount of radioactive material
necessary
2) Use appropriate shielding to block radiation
3) Increase distance from radioactive source
(exposure decreases as inverse square of distance)
4) Minimize exposure time (more damage results from
long-term exposure)
Elementary Particles
- fundamental building blocks of matter
There are four fundamental forces in nature.
From strongest to weakest they are:
1) strong nuclear
force - relative strength: 1, range of ~ 1 fm
2) electromagnetic
force - relative strength: 10-2, range: infinite
(proportional to 1/r2)
3) weak nuclear
force - relative strength: 10-6, range: ~0.001 fm
4) gravitational
force - relative strength: 10-43, range: infinite
(proportional to 1/r2)
Leptons are
elementary particles that experience the weak nuclear force but not the
strong nuclear force.
Hadrons are
composite particles that experience both the weak and strong nuclear
force.
Quarks are
elementary particles that combine to form hadrons. Mesons are formed from
quark-antiquark pairs; baryons are
formed from combinations of three quarks.
Chapter 16 - Fusion and Fission
Nuclear Fusion occurs
when two small nuclei (like H and He) are brought close together
(within ~fermi) so that the strong nuclear
force can fuse the nuclei together to form a single larger
nucleus. Energy is
needed to overcome the Coulomb repulsion of the two positive nuclei,
but the final product results
in a net release of energy (an exothermic reaction). This is the
process that fuels our Sun.
Nuclear Fission is
the process where a large nucleus (like that of uranium or plutonium)
captures a neutron and then divides
into two smaller "daughter" nuclei. When this happens, two or
three neutrons are typically released, and this can result in other
fission reactions (chain reaction). This process is used in
nuclear power plants and nuclear bombs.
Sample nuclear reaction
equations - mass and energy must balance
Nuclear binding energy - see Fig. 16.4
Does a nucleus have more mass than its constituent parts?
Why is more thermal energy needed to fuse He atoms than H atoms?
What is special about the radioactivity of U-235?
What kind of radiation is associated with the emission of neutrons?
How are the control rods in a nuclear reactor used?
Is it possible for a nuclear reactor to explode?
Is plutonium radioactive?
What kind of nuclear bomb would terrorists be likely to build?
What is the most likely kind of nuclear terrorism?
What is meant by "weapons grade nuclear material"?
The critical mass of enriched U-235 is 25 kg (about the size of a
cantelope), while only 8 kg of Pu-239 is needed (about the size of a
large orange).
What is meant by the term "critical mass"? How is this term used
in other contexts relating to people?
Video: Fat Man and Little Boy
- Scene
15 (Not Responsible), ~4:00 min
At the beginning of this scene, the post-doc
from the University of Chicago, Michael Murrayman, (John Cusack)
questions Robert Oppenheimer about the ethics of their project, to
which Oppenheimer responds that "we are responsible for solving a
technical problem, to build this device. We are not responsible
for its use." What do you think about this? Who is
responsible for the application of scientific knowledge? As
graduation approaches, you might consider joining the Graduation Pledge Alliance
which was initiated at my alma mater, Manchester College.
In the second part of this scene, there is a
radiation lab accident where a radioactive source goes
"critical". Michael tells everyone in the lab to mark their
locations. Why? Michael's radiation exposure was more than
1000 rad. This is about three times higher than the LD50 limit of
300 rad, which means that 50% of the people exposed at this level will
die within 60 days. The LD100 limit (Lethal Dose for 100% of the
exposed population even with the best available treatment) is 800 rem,
so this is why Michael states that everyone in the room will live
except for him.
Science provides us with knowledge, but each individual person has a
responsibility to evaluate the merits of this information and decide
how this information will be used. This challenge is inherent in
all real-world problems (not just those related to physics), and this
is why evaluation is the highest level of cognitive reasoning (per Bloom's
taxonomy). Hopefully this
course has improved your critical reasoning skills to make you a
well-functioning member of society.