# Basics of Atomic Weapons

## Friday, February 03, 2006

### The Use And History Of The "Barn" Unit

From Wikipedia
http://en.wikipedia.org/wiki/Barn_(unit)

Barn (unit)

A barn (symbol b) is a unit of area. While the barn is not an SI unit, it is accepted (although discouraged) for use with the SI. It is used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions. A barn is approximately equal to the area of a uranium nucleus.

Definition
1 barn (b) = 10−28 square meters (mÂ²)

Commonly used prefixed versions
The picobarn (pb) = 10−40 mÂ² is frequently used.

Origin
The etymology is clearly whimsical - the unit is said to be "as big as a barn" compared to the typical cross sections for nuclear reactions. It may have been thought as beneficial to use the term to obscure discussions of weapons research during World War 2.

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The concept of cross section is the crucial key that opens the communication
between the real world of experiment and the abstract, idealized world of
theoretical models. In a high- energy physics experiment, we specify
interactions of elementary particles quantitatively in terms of cross sections.
The cross section is the probability that an interaction will occur between a
projectile particle-say, a proton that has been accelerated in the Tevatron-and
a target particle, which could be an antiproton, or perhaps a proton or neutron
in a piece of metal foil.

We can measure the probability that two
particles will interact in experiments. We can also calculate this quantity in a
model that incorporates our understanding of the forces acting on a subatomic
level. In the famous experiment in which Rutherford studied the scattering of
alpha particles off a foil target, the cross section gives the probability that
the alpha particle is deflected from its path straight through the target. The
cross section for large-angle scattering is the fraction of alpha particles that
bounce back from the target, divided by the density of nuclei in the target and
the target thickness. The comparison of the measured cross section with the
calculated one verified the model of the atom with a minute, massive center,
carrying an electrical charge.

We can picture the cross section as the
effective area that a target presents to the projected particle. If an
interaction is highly probable, it's as if the target particle is large compared
to the whole target area, while if the interaction is very rare, it's as if the
target is small. The cross section for an interaction to occur does not
necessarily depend on the geometric area of a particle. It's possible for two
particles to have the same geometric area (sometimes known as geometric cross
section) and yet have very different interaction cross section or probability
for interacting with a projectile particle.

During wartime research on
the atomic bomb, American physicists who were bouncing neutrons off uranium
nuclei described the uranium nucleus as "big as a barn." Physicists working on
the project adopted the name barn for a unit equal to 10-24 square centimeters,
about the size of a uranium nucleus. Initially they hoped the American slang
name would obscure any reference to the study of nuclear structure; eventually,
the word became a standard unit in particle physics.

Today, although
experimental techniques and theoretical calculations have considerably increased
in complexity compared to the early days of scattering experiments, the concept
which links theory and experiment has not changed. In the Tevatron, for
instance, we measure the probability of producing a pair of top quarks in a
proton-antiproton collision. We measure this production cross section by
counting the number of top quark events observed in the detector and by knowing
the time-integrated luminosity, the product of the number of particles per unit
time in the proton and antiproton beam, per area of the beam. By comparing the
top quark production cross section with predictions, which are based on a model
of elementary particles and their interactions, we probe our understanding of
the strongest known force between elementary particles.

Next time, I will get more data on the definition of the "Shake"