Ionising Radiation
Quantum and Nuclear | Earth and Space

Unusual units

Stories from Physics for 11-14 14-16 IOP RESOURCES

Banana equivalent dose (BED)

A unit used to provide an everyday measure of radiation dose. A single banana produces an ionising radiation dose of around 0.1 μSv which has been set as the value of the banana equivalent dose. The ionising radiation dose given by a range of sources can be expressed in BEDs:

SourceBanana Equivalent DoseAverage loss in life expectancy
Dental X-ray503 minutes
Eating 135 g of Brazil nuts503 minutes
Flight from London to New York70037 minutes
Year of normal background dose27,0001 day
Exposure limit for nuclear workers200,0007 days
Acute radiation effects, including nausea and drop in white blood cell count10 million1 year
Spending 10 minutes close to Chernobyl reactor core after meltdown500 million50 years

Barn A unit used to measure the cross-sectional area of atomic nuclei. It was devised in 1942 by MG Holloway and CP Baker and arises from the expression ‘as big as a barn door’ implying that, to a subatomic particle, the nucleus of an atom is an unmissable target. A barn represents an area of 10 -28m 2 and the terms microbarn, nanobarn and femtobarn are used by particle physicists.

Crab The Crab Nebula contains a pulsar which is one of the brightest objects in the sky at X-ray and gamma-ray wavelengths. The nebula has become a standard for the measurement of the X-ray intensity of astronomical bodies. For example, an object with an intensity one thousandth of the nebula may be reported to measure 1 millicrab.

Dirac Due to his laconic style, the Cambridge colleagues of Nobel-prize winning physicist Paul Dirac are said to have defined one Dirac as a rate of speaking equal to one word per hour.

Decimal hour, second and minute In post-revolutionary France, a decimal time system was introduced. The day was divided into ten decimal hours, each of 100 minutes, leading to a decimal minute of 1.4 standard minutes and a decimal second lasting 0.86 standard seconds. The decimal time system lasted only two years from its implementation. Clocks and watches that indicated decimal time were produced. In fact, the mathematician and astronomer Pierre-Simon Laplace had his watch converted to decimal time and his work used the new time units. You can see decimal time pieces in the collection of the Musée des Artes et Métiers in Paris.

Garn A unit of space sickness jokingly used by NASA. Astronaut Senator Jack Garn was reported to have suffered one of the most extreme cases of space sickness on the Space Shuttle in 1985. The Garn represents the maximum level of sickness that it is possible to reach – most astronauts reach a level of a tenth of a Garn.

Grave The group of scientists, including French chemist Antoine-Laurent de Lavoisier, who developed the system of measures that evolved into the modern metric system, originally proposed the grave as a unit of mass equivalent to the mass of 1 litre of water at the ice point. However, during the French revolution, Lavoisier was executed, and the new commission decided on the ‘gramme’ which was defined as the mass of water with a volume of one cubic centimetre.

Inferno, eon and Hubble In a paper published shortly before his death in 1968, Soviet-American theoretical physicist and cosmologist George Gamov proposed a new set of units for cosmologists, including the inferno (10 9 K), the eon (10 9 years) and the Hubble (10 9 light-years).

Jar An obsolete unit of capacitance. The unit is believed to be one of the oldest electrical units, introduced in 1834 by Sir William Harris. A jar represents the capacitance of an early Leiden jar, with 9 × 10 8 jars being equivalent to 1 farad. The unit was used by the Royal Navy in the Admiralty Handbook of Wireless Telegraphy as late as 1938.

Kan A story, which may be apocryphal, is that a rival of American physicist Robert Millikan suggested that the unit of conceit should be the kan. The kan was defined as a large amount of conceit so for everyday measurements the millikan would be more appropriate.

Micromort A unit to quantify risk that is equivalent to a one-in-a-million chance of death. For example, travelling in a car carries a risk of 250 miles per micromort whereas walking has a greater risk per distance at 7 miles per micromort. Horse riding is rated at 0.5 micromorts compared to 8 micromorts for hang-gliding, assuming constant risk within the activity and over time. On the theme of fatalities, the Darwin has been proposed as the probability that one undergraduate student will suffer a fatal injury if left to their own devices in carrying out a practical.

Morgen A measure of area somewhere in the range of 2,168 m 2 to 6,643 m 2. It arises from the Germanic word for morning and refers to the amount of land that could be ploughed in one morning. When the Dutchman Peter Minuit bought the island of Manhattan, its area was given as 11,000 morgens.

Shake In nuclear and astrophysics contexts, the shake is equal to 10 ns and is thought to originate from the phrase ‘two shakes of a lamb’s tail.’ The unit refers to the period between one nucleus ejecting a neutron and the consequent fission of a second nucleus.

Smoot A unit of length defined as the height of MIT undergraduate Oliver R. Smoot in 1958. New entrants to a fraternity at MIT were set the task of measuring the length of Harvard Bridge. Smoot laid down on the deck of the bridge, his fellow initiates chalked his height (170 cm) on the surface and he repeated the process to measure the length of the bridge as just over 360 Smoots. A plaque on the bridge commemorates the measurement. Smoot went on to became the Chairman of the American National Standards Institute.


Ionising Radiation
is used in analyses relating to Radioactive dating
can be analysed using the quantity Half-Life Decay Constant Activity
features in Medical Physics
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