# The Naked Scientists Forum

### Author Topic: What does the roentgen measure?  (Read 4401 times)

#### dines1

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##### What does the roentgen measure?
« on: 06/02/2005 23:10:00 »
« Last Edit: 07/05/2014 22:52:15 by chris »

#### DrPhil

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##### Re: roentgen
« Reply #1 on: 06/02/2005 23:46:35 »
A unit of radiation exposure equal to the quantity of ionizing radiation that will produce one electrostatic unit of electricity in one cubic centimeter of dry air at 0°C and standard atmospheric pressure.

Wilhelm Konrad Roentgen   German physicist who discovered x-rays and developed x-ray photography, revolutionizing medical diagnosis. He won a 1901 Nobel Prize.
« Last Edit: 07/02/2005 00:33:36 by DrPhil »

#### gsmollin

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##### Re: roentgen
« Reply #2 on: 09/02/2005 05:34:00 »
Which electrostatic unit of electricity? and how many Coulombs? Coming from 1901, it should be cgs, and may be an abacouloumb, or a statacouloumb(?). I could look it up, but I think you should, since you answered the post.

#### DrPhil

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##### Re: roentgen
« Reply #3 on: 09/02/2005 14:46:05 »
It's about 2.08x10^9 ion pairs per cubic centimeter.
Conversion to coulombs/cc, coulombs/g or coulombs/kg is left as an exercise for the student.

#### gsmollin

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##### Re: roentgen
« Reply #4 on: 09/02/2005 16:02:36 »
I looked it up. It's the cgs unit, statcoulomb. What's really interesting about this is that the SI units for a roentgen are coulombs/kg. So the roentgen is a measure of the ionization in a mass of air produced by the incident radiation. There is no direct information about the intensity or the energy of the radiation, just its ability to produce ionization in air. This was convenient, since the gold-leaf electroscope was a common instrument of the 19th century. One could put a known amount of charge into the electroscope, with the gold leaves sealed inside the glass envelope. When the ionizing radiation discharged the electroscope, one could infer that much ionization was produced in the volume of the air-filled chamber.

#### BechtelEngineer

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##### Re: roentgen
« Reply #5 on: 05/05/2014 22:05:53 »
None of the above, R is a unit of charge.

#### alancalverd

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##### Re: roentgen
« Reply #6 on: 05/05/2014 22:24:28 »
'Fraid not. It was defined as charge per unit volume of air. Always has been, even though the value has changed from time to time and place to place, which is why it has been displaced by coulomb/kilogram as the SI unit of ionisation with a "reverse definition" of the roentgen as 2.58 x 10^-4 C/kg, because there is an awful lot of measuring equipment out there scaled in R or its derivatives.

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##### Re: roentgen
« Reply #6 on: 05/05/2014 22:24:28 »