Inverse picobarns to you

The impact of some spectacular colliding beam records this summer has been muted by the cryptic terminology which scientists invariably use when describing their best achievements. Early this century, nuclear physicists calculated the collision rate of a particle beam with a sample of stationary atoms by multiplying the intensity of the beam (particles per unit area per second), the total number of targets, and the area of each target. In searching for a yardstick to measure this effective 'cross-section', they first thought of a unit called a 'barn', equal to 10^-24 square centimetres, the approximate surface area of a nucleus. A beam crossing such an area would be almost certain to produce a nuclear collision - 'as easy as hitting a barn door'