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

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"

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