What Is Barn?
A barn is a very tiny unit of area that scientists use to describe how likely it is that tiny particles, like protons or neutrons, will hit something in nuclear and particle physics experiments.
Definition
A barn is a special unit for area used in nuclear science. It is written as b.
1 barn = 0.0000000000000000000000000001 square meters.
In scientific form this is:
- 1 b = 10-28 m2
- 1 b = 10-24 cm2
Scientists often talk about the cross section of a reaction in barns. Cross section means how big the target looks to the particle, or how easy it is for the particle to hit it.
History / Origin
The barn unit started during the Second World War in the United States, in secret nuclear research known as the Manhattan Project.
Physicists there needed a word for a very large nuclear cross section. As a joke, they picked the word barn because of the English saying could not hit the broad side of a barn. For them a large nuclear target was easy to hit, just like a big barn wall is easy to see.
The funny name stayed. Later the barn became a standard unit used by nuclear and particle physicists all over the world.
Symbol & Abbreviation
The barn has a short symbol and also smaller versions for very tiny areas.
- Main symbol: barn = b
- Millibarn = mb = 1 000 times smaller than a barn
- Microbarn = µb = 1 000 000 times smaller than a barn
- Nanobarn = nb
- Picobarn = pb
- Femtobarn = fb
- Attobarn = ab
These prefixes milli, micro, nano, pico, femto and atto are the same ones used in the metric system for other units.
Current Use Around the World
The barn is not used in everyday life. You will not see it in building plans or maps. It is mainly used by scientists who study very small particles.
Common places where the barn is used include:
- Nuclear physics research, to describe how likely a nuclear reaction is to happen
- Particle accelerators, such as CERN, to measure how often particles collide
- Reactor physics, to study how neutrons interact with fuel inside a nuclear reactor
- Astrophysics, to model reactions that happen inside stars
Although the word barn is funny, it is accepted in serious science papers, reports and data tables all around the world.
Example Conversions
Here are some simple examples to show how small a barn is.
- 1 barn in square meters:
1 b = 0.0000000000000000000000000001 m2 = 10-28 m2 - 1 barn in square centimeters:
1 b = 0.0000000000000000000000001 cm2 = 10-24 cm2 - 0.5 barn in square meters:
0.5 b = 5 × 10-29 m2 - 1 millibarn in barns:
1 mb = 0.001 b - 1 microbarn in barns:
1 µb = 0.000001 b
To imagine the size, think about this. A normal sheet of paper has an area of about 0.06 square meters. You would need an extremely huge number of barns to cover that same area. So a barn is useful only when we talk about things that are much smaller than atoms.
Related Units
Units related to the barn include normal area units and its own smaller versions.
- Square meter m2 basic metric unit of area
- Square centimeter cm2 smaller metric area unit often used in labs
- Square millimeter mm2
- Millibarn mb = 10-3 b
- Microbarn µb = 10-6 b
- Nanobarn nb = 10-9 b
- Picobarn pb = 10-12 b
- Femtobarn fb = 10-15 b
- Attobarn ab = 10-18 b
FAQs
Q. What is a barn used for
A. A barn is used to measure the area that a tiny particle seems to present as a target. This is called the cross section. It tells scientists how often certain nuclear or particle reactions will happen.
Q. Why is it called a barn
A. The name came from a joke by scientists working on early nuclear projects. A large cross section was like the side of a barn, something big and easy to hit. The funny name became standard.
Q. Is a barn a big or small unit
A. A barn is extremely small. It is used only for things that are smaller than atoms. For normal life, like rooms, fields or cities, a barn is far too tiny to be useful.
Q. Can I use barns to measure land area
A. In theory yes, but it would be very impractical. You would need an enormous number of barns to equal even one square meter. For land we use units like square meters, square kilometers, acres or hectares.
Q. How do barns relate to square meters
A. One barn is 10-28 square meters. This means you would have to multiply a barn by 10 000 000 000 000 000 000 000 000 000 to get one square meter.
Q. Who decided that barn is an official unit
A. The unit was first used informally by physicists. Over time, because it became so common in nuclear data and research papers, it was accepted and is now widely used and understood in the physics community.
