The barn

Figure 15.2 of the 4th edition of Intermediate Physics for Medicine and Biology shows the cross section for the interaction of photons with carbon versus photon energy. The caption of the figure says “The cross section is given in barns: 1 b = 10⁻²⁸ m².” Where did this strange unit come from?

The July 1972 issue of Physics Today published a letter by M. G. Holloway and C. P Baker, explaining “How the Barn was Born.”

Some time in December of 1942, the authors, being hungry and deprived temporarily of domestic cooking, were eating dinner in the cafeteria of the Union Building of Purdue University. With cigarettes and coffee the conversation turned to the topic uppermost in their minds, namely cross sections. In the course of the conversation, it was lamented that there was no name for the unit of cross section of 10⁻²⁴ cm². It was natural to try to remedy this situation.

The tradition of naming a unit after some great man closely associated with the field ran into difficulties since no such person could be brought to mind. Failing in this, the names Oppenheimer and Bethe were tried, since these men had suggested and made possible the work on the problem with which the Purdue project was concerned. The “Oppenheimer” was discarded because of its length, although in retrospect an “Oppy” or “Oppie” would seem to be short enough. The “Bethe” was thought to lend itself to confusion because of the widespread use of the Greek letter. Since John Manley was directing the work at Purdue, his name was tried, but the “Manley” was through to be too long. The “John” was considered, but was discarded because of the use of the term for purposes other than as the name of a person. The rural background of one of the authors then led to the bridging of the gap between the “John” and the “barn.” This immediately seemed good and further it was pointed out that a cross section of 10⁻²⁴ cm² for nuclear processes was really as big as a barn. Such was the birth of the “barn.”

To the best knowledge of the authors, the first public (if it may be called that) use of the barn was in Report LAMS-2 (28 June, 1943) in which the barn was defined as a cross section of 1 × 10⁻²⁴ cm².

The authors would like to insist that the “barn” is spelled just that way, that no capital “b” is needed, and that the plural is “barns” with no letter “e” involved, and that the symbol be a small “b.” The meanings of “millibarn” and “kilobarn” are obvious.