Barn
Definition and meaning of Barn in chemistry.
A barn is a unit of area used to measure nuclear cross-sections. This value represents the target area for particle or nuclear interactions. One barn is equal to 10⁻²⁸ square meters.
In more detail
In nuclear chemistry, a cross-section describes how likely a reaction is to happen between particles. It does not measure the actual physical size of the nucleus. Instead, it describes the likelihood of a collision.
Scientists use the barn because atomic nuclei are extremely small. If they used standard square meters, the numbers would have huge negative exponents. The barn makes these tiny numbers much easier to write and discuss.
The name itself comes from a joke made by American physicists in 1942. During the Manhattan Project, they found that some nuclear targets were much larger than expected. They said hitting these nuclei was like hitting the broad side of a barn.
Since then, the term has remained a standard scientific unit. Students often mistakenly assume that a larger cross-section means a larger physical nucleus. In reality, the cross-section depends on the type of reaction and the energy of the particles.
Different isotopes of the same element can have completely different cross-sections. For example, some isotopes absorb neutrons easily, while others let neutrons pass right through. This variation is crucial when choosing materials for building nuclear power plants.
Key facts
| Field | Physical Chemistry |
|---|---|
| Symbol | b |
| Size in m² | 10⁻²⁸ m² |
| Size in cm² | 10⁻²⁴ cm² |
| Named | Humorously in 1942 during the Manhattan Project |
| Purpose | Quantifying probabilities of nuclear reactions |
Boron-10 has an exceptionally large neutron-absorption cross-section of over 3,800 barns. This property makes boron ideal for controlling nuclear fission reactions. Engineers make control rods containing boron compounds to place inside nuclear reactors. When the rods are lowered, they absorb free neutrons and slow down the nuclear reaction. This action keeps the reactor safe and prevents dangerous runaway reactions.
Frequently asked questions
Why is such a tiny unit called a 'barn'?
Physicists in 1942 joked that some nuclei were very easy to hit. They described the targets as being as big as the broad side of a barn.
Does the barn measure actual physical area?
No, it measures the likelihood of a collision. Some reactions happen easily, giving them a larger cross-section than their physical size.
What is a millibarn?
A millibarn is one-thousandth of a barn. Scientists use this smaller unit for nuclear reactions that have a very low probability of occurring.