View source for Per cent mille

{{Short description|One-thousandth of a percent}}
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A '''per cent mille''' or '''pcm''' is one one-thousandth of a percent.<ref>SCALE: A Comprehensive Modelling and Simulation Suite for Nuclear Safety Analysis and Design.  Available from Radiation Safety Information Computational Center at Oak Ridge National Laboratory as CCC-785.  Oak Ridge, TN: Oak Ridge National Laboratory, June, 2001.  Version 6.1. ORNL/TM-2005/39</ref> It can be thought of as a "milli-percent". It is commonly used in [[epidemiology]], and in [[nuclear reactor]] engineering as a unit of reactivity.

==Epidemiology==
[[Statistics]] of [[Crime statistics|crime rates]], [[Mortality rate|mortality]] and [[Prevalence|disease prevalence]] in a population are often given in {{nowrap|"per 100&thinsp;000".}}<ref>{{Cite web|url=http://cdc.gov/foodnet/reports/data/infections.html|title=Number of Infections and Incidence* per 100,000 Persons &#124; FoodNet &#124; CDC|date=28 September 2018}}</ref><ref>{{Cite web|url=http://apps.who.int/gho/data/node.main.VIOLENCEHOMICIDE|title = GHO &#124; by category &#124; Homicide - Estimates by country}}</ref>

==Nuclear Reactivity==
In nuclear reactor engineering, a per cent mille is equal to one-thousandth of a percent of the [[Nuclear reactor physics#criticality|reactivity]], denoted by Greek lowercase letter [[rho]]. Reactivity is a dimensionless unit representing a departure from [[Criticality (status)|criticality]], calculated by:<ref>{{cite web |url=http://mafija.fmf.uni-lj.si/seminar/files/2012_2013/Reactivity_Measurements_(Vid_Merljak).pdf|title=Reactivity measurements|last=Merljak|first=Vid|publisher=University of Ljublj|access-date=September 17, 2017}}ana</ref>
:<math>\rho=(k_{\text{eff}}-1)/k_{\text{eff}}</math> 
where k<sub>eff</sub> denotes the [[effective multiplication factor]] for the reaction. Therefore, one pcm is equal to:<ref>{{cite web|url=http://www.nuclear-power.net/nuclear-power/reactor-physics/nuclear-fission-chain-reaction/reactivity/|title=Reactivity|website=nuclear-power.net|access-date=September 17, 2017}}</ref>
:<math>1~\text{pcm} = \rho \cdot 10^5</math> 
This unit is commonly used in the operation of [[light-water reactor]] sites because reactivity values tend to be small, so measuring in pcm allows reactivity to be expressed using whole numbers.<ref>{{cite web |url=http://www.nuclear-power.net/nuclear-power/reactor-physics/nuclear-fission-chain-reaction/reactivity/percent-mille-pcm-unit-of-reactivity/|title=percent mille – pcm – unit of reactivity|website=nuclear-power.net|access-date=September 17, 2017}}</ref>

== Related units ==
* [[Percentage point]] difference of 1 part in 100
* [[Percentage]] (%) 1 part in 100
* [[permille|Per mille]] (‰) 1 part in 1,000
* [[Basis point]] (bp) difference of 1 part in 10,000
* [[Permyriad]] (‱) 1 part in 10,000
* [[Parts-per notation]] including parts-per million, parts-per billion etc

== See also ==
* [[InHour]] (another unit of reactivity)
* [[Dollar (reactivity)]]
* [[Parts-per notation]]
* [[Per-unit system]]
* [[Percent point function]]

==Notes==
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[[Category:Units of measurement]]


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