In the mid-20th century, two units used in radiobiology to express quantities of fast neutrons. They were based on a particular device, the Victoreen R meter.
The n-unit was the quantity of fast neutrons that produced the same reading in a Victoreen R meter equipped with a 100-r chamber as 1 roentgen of x-rays would.
The N-unit was the quantity of fast neutrons that produced the same reading in a Victoreen R meter equipped with a 25-r chamber as 1 roentgen of x-rays would.
“Note: These are not actually proposed units, but have been employed as convenient interim methods of specifying neutron doses until satisfactory units and methods of measurement can be devised.”1
1. National Research Council.
A Glossary of Terms in Nuclear Science and Technology.
New York: American Society of Mechanical Engineers, 1955.
For a picture and description of an R meter:
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