Barn
Definition and meaning of Barn in chemistry.
A barn is a unit of area used in nuclear physics to measure cross-sections, the effective area for particle or nuclear interactions, equal to 10⁻²⁸ square meters or 10⁻²⁴ square centimeters. It quantifies the probability that a nuclear reaction will occur.
In more detail
In nuclear chemistry and particle physics, a cross-section describes how likely an interaction is between particles or nuclei. The barn is humorously named: physicists joked that nuclear cross-sections seemed "as big as a barn" compared to what theory predicted, despite being extraordinarily small. The unit was established in 1942 during Manhattan Project research. Barns remain essential in nuclear research for calculating neutron absorption rates, fission probabilities, and other reaction dynamics.
Key facts
| Field | Physical Chemistry |
|---|---|
| Symbol | b |
| Size | 10⁻²⁸ m² (or 10⁻²⁴ cm²) |
| Named | Humorously in 1942; physicists said the cross-section was 'as big as a barn' |
Boron-10 has a large neutron-absorption cross-section of over 3,800 barns, which is why boron compounds are used in control rods to absorb neutrons and regulate nuclear reactions.
Frequently asked questions
Why is such a tiny unit called a 'barn'?
Physicists chose the ironic name because the measured cross-sections were surprisingly large compared to theoretical predictions, humorously 'as big as a barn' despite being 10⁻²⁸ square meters.
How is a barn used in nuclear chemistry?
Scientists use barns to express interaction probabilities for nuclear reactions, including neutron absorption and fission, which are critical for reactor design, safety analysis, and nuclear research.