View source for Meter water equivalent

{{Short description|Unit of nuclear and particle physics}}
In [[physics]], the '''meter water equivalent''' (often '''m.w.e.''' or '''mwe''') is a standard measure of [[cosmic ray]] attenuation in underground [[laboratory|laboratories]]. A laboratory at a depth of 1000 m.w.e is shielded from cosmic rays equivalently to a lab {{cvt|1000|m}} below the surface of a body of water. Because laboratories at the same depth (in meters) can have greatly varied levels of cosmic ray penetration, the m.w.e. provides a convenient and consistent way of comparing cosmic ray levels in different underground locations.<ref>{{cite web |url=http://www.deepscience.org/contents/underground_universe.shtml |title=Deep Science |publisher=[[National Science Foundation]] |access-date=2014-10-03 |url-status=dead |archiveurl=https://web.archive.org/web/20150223150019/http://www.deepscience.org/contents/underground_universe.shtml |archivedate=2015-02-23}}</ref>

Cosmic ray attenuation is dependent on the density of the material of the [[overburden]], so the m.w.e. is defined as the product of depth and density (also known as an interaction depth). Because the density of water is {{val|1|u=g|up=cm3}}, {{cvt|1|m|cm}} of water gives an interaction depth of {{convert|1|hg/cm2|g/cm2}}. Some publications use hg/cm² instead of m.w.e., although the two units are equivalent.<ref name="Grieder2001">{{cite book|first=Peter K. F. |last=Grieder |title=Cosmic Rays at Earth: Researcher's Reference Manual and Data Book |url=https://books.google.com/books?id=MwwRhQ7qEAsC&pg=PA482 |via=books.google.com |year=2001 |publisher=Gulf Professional Publishing |isbn=978-0-444-50710-5 |page=482 |access-date=2023-10-01}}</ref>

For example, the [[Waste Isolation Pilot Plant]], located {{cvt|660|m}} deep in a [[salt]] formation, achieves 1585&nbsp;m.w.e. shielding.  [[Soudan Mine]], at {{cvt|713|m}} depth is only 8% deeper, but because it is in denser iron-rich rock it achieves 2100&nbsp;m.w.e. shielding, 32% more.

Another factor that must be accounted for is the ''shape'' of the overburden.  While some laboratories are located beneath a flat ground surface, many are located in tunnels in mountains.  Thus, the distance to the surface in directions other than straight up is less than it would be assuming a flat surface.  This can increase the muon flux by a factor of {{val|4|2}}.<ref>{{cite journal
 |arxiv=2007.15925 |title=Muon flux measurement at China Jinping Underground Laboratory
 |first1=Ziyi |last1=Guo |journal=Chinese Physics C
 |year=2021
 |volume=45
 |issue=2
 |page=025001
 |doi=10.1088/1674-1137/abccae
 |s2cid=244399721
 |display-authors=etal |collaboration=JNE Collaboration
 |quote=A survey of muon fluxes at different laboratory locations situated under mountains and below mine shafts indicated that the former is generally a factor of (4±2) larger than the latter with the same vertical over-burden.
}}</ref>

The usual conversion between m.w.e. and total muon flux is given by Mei and Hime:<ref name=Mei06>{{cite journal
 |title=Muon-induced background study for underground laboratories
 |first1=D.-M. |last1=Mei  |first2=A. |last2=Hime
 |journal=Physical Review D |volume=73 |issue=5 |page=053004 |date=6 March 2006
 |doi=10.1103/PhysRevD.73.053004 |arxiv=astro-ph/0512125 |bibcode=2006PhRvD..73e3004M 
|s2cid=119446070 }}</ref>
:<math>
I_\mu(h_0) = 67.97\times 10^{-6} e^{-h_0/285} + 2.071\times 10^{-6} e^{-h_0/698}
</math>
where <math>h_0</math> is the depth in m.w.e. and <math>I_\mu</math> is the total muon flux per cm<sup>2</sup>⋅s.  (The first term dominates for depths up to 1681.5&nbsp;m.w.e.; below that, the second term dominates.  Thus, for great depths, the factor of 4 mentioned above corresponds to a difference of 698 ln&nbsp;4 ≈ 968&nbsp;m.w.e.)

== Standard rock ==
In addition to m.w.e., underground laboratory depth can also be measured in meters of standard rock. Standard rock is defined to have [[mass number]] A&nbsp;=&nbsp;22, [[atomic number]] Z&nbsp;=&nbsp;[[sodium|11]], and density {{cvt|2.65|g/cm3|g/cuin||}}.<ref name="PDG 28">{{Cite journal | doi = 10.1088/1674-1137/38/9/090001| title = Review of Particle Physics| url = https://pdg.lbl.gov/2014/AtomicNuclearProperties/HTML/standard_rock.html | journal = Chinese Physics C| volume = 38| issue = 9| pages = 1–708| year = 2014| author =  K. A. Olive | pmid = 10020536|display-authors=etal |collaboration=Particle Data Group |bibcode = 2014ChPhC..38i0001O | arxiv = 1412.1408}}</ref> Because most laboratories are under earth and not underwater, the depth in standard rock is often closer to the actual underground depth of the laboratory.

== Existing underground laboratories ==
Underground laboratories exist at depths ranging from just below ground level to approximately 6000 m.w.e. at [[SNOLAB]]{{r|Mei06}} and 6700 m.w.e. at the [[China Jinping Underground Laboratory|Jinping Underground Laboratory]] in China.<ref>{{Cite journal |doi=10.1088/1674-1137/37/8/086001 |arxiv=1305.0899 |bibcode=2013ChPhC..37h6001W |title=Measurement of cosmic ray flux in the China JinPing underground laboratory |journal=Chinese Physics C |volume=37 |issue=8 |pages=086001 |date=August 2013 |last1=Wu |first1=Yu-Cheng |last2=Hao |first2=Xi-Qing |last3=Yue |first3=Qian |last4=Li |first4=Yuan-Jing |last5=Cheng |first5=Jian-Ping |last6=Kang |first6=Ke-Jun |last7=Chen |first7=Yun-Hua |last8=Li |first8=Jin |last9=Li |first9=Jian-Min |last10=Li |first10=Yu-Lan |last11=Liu |first11=Shu-Kui |last12=Ma |first12=Hao |last13=Ren |first13=Jin-Bao |last14=Shen |first14=Man-Bin |last15=Wang |first15=Ji-Min |last16=Wu |first16=Shi-Yong |last17=Xue |first17=Tao |last18=Yi |first18=Nan |last19=Zeng |first19=Xiong-Hui |last20=Zeng |first20=Zhi |last21=Zhu |first21=Zhong-Hua |s2cid=199686625 }}</ref>

== References ==
<references/>

{{Underground laboratories}}

[[Category:Equivalent units]]
[[Category:Underground laboratories]]
[[Category:Physical quantities]]