Editing Cosmic ray (section)

==Detection methods==
[[File:VERITAS array.jpg|thumb|600px|The [[VERITAS]] array of air Cherenkov telescopes.]]
There are two main classes of detection methods. First, the direct detection of the primary cosmic rays in space or at high altitude by balloon-borne instruments. Second, the indirect detection of secondary particle, i.e., extensive air showers at higher energies. While there have been proposals and prototypes for space and balloon-borne detection of air showers, currently operating experiments for high-energy cosmic rays are ground based. Generally direct detection is more accurate than indirect detection. However the flux of cosmic rays decreases with energy, which hampers direct detection for the energy range above 1 PeV. Both direct and indirect detection are realized by several techniques.

===Direct detection===
Direct detection is possible by all kinds of particle detectors at the [[ISS]], on satellites, or high-altitude balloons. However, there are constraints in weight and size limiting the choices of detectors.

An example for the direct detection technique is a method based on [[nuclear track]]s developed by Robert Fleischer, [[P. Buford Price|P.&nbsp;Buford Price]], and [[Robert M. Walker (physicist)|Robert&nbsp;M. Walker]] for use in high-altitude balloons.<ref>{{cite book|author1=R.L. Fleischer|author2=P.B. Price|author3=R.M. Walker|date=1975|title=Nuclear tracks in solids: Principles and applications|publisher=[[University of California Press]]|bibcode=1975ucb..book.....F }}</ref> In this method, sheets of clear plastic, like 0.25&nbsp;[[millimetre|mm]] [[Lexan]] polycarbonate, are stacked together and exposed directly to cosmic rays in space or high altitude. The nuclear charge causes chemical bond breaking or [[ionization]] in the plastic. At the top of the plastic stack the ionization is less, due to the high cosmic ray speed. As the cosmic ray speed decreases due to deceleration in the stack, the ionization increases along the path. The resulting plastic sheets are "etched" or slowly dissolved in warm caustic [[sodium hydroxide]] solution, that removes the surface material at a slow, known rate. The caustic sodium hydroxide dissolves the plastic at a faster rate along the path of the ionized plastic. The net result is a conical etch pit in the plastic. The etch pits are measured under a high-power microscope (typically 1600× oil-immersion), and the etch rate is plotted as a function of the depth in the stacked plastic.

This technique yields a unique curve for each atomic nucleus from 1 to&nbsp;92, allowing identification of both the charge and energy of the cosmic ray that traverses the plastic stack. The more extensive the ionization along the path, the higher the charge. In addition to its uses for cosmic-ray detection, the technique is also used to detect nuclei created as products of [[nuclear fission]].

===Indirect detection===
There are several ground-based methods of detecting cosmic rays currently in use, which can be divided in two main categories: the detection of secondary particles forming extensive air showers (EAS) by various types of particle detectors, and the detection of electromagnetic radiation emitted by EAS in the atmosphere.

Extensive air shower arrays made of particle detectors measure the charged particles which pass through them. EAS arrays can observe a broad area of the sky and can be active more than 90% of the time. However, they are less able to segregate background effects from cosmic rays than can [[IACT|air Cherenkov telescopes]]. Most state-of-the-art EAS arrays employ plastic [[scintillator]]s. Also water (liquid or frozen) is used as a detection medium through which particles pass and produce [[Cherenkov radiation]] to make them detectable.<ref>{{cite web|url=http://www.nscl.msu.edu/files/PAN%20cosmic%20ray%20articles.pdf|title=What are cosmic rays?|publisher=Michigan State University National Superconducting Cyclotron Laboratory|access-date=23 February 2013|url-status=dead|archive-url=https://web.archive.org/web/20120712052716/http://www.nscl.msu.edu/files/PAN%20cosmic%20ray%20articles.pdf|archive-date=12 July 2012}}</ref> Therefore, several arrays use water/ice-Cherenkov detectors as alternative or in addition to scintillators.
By the combination of several detectors, some EAS arrays have the capability to distinguish muons from lighter secondary particles (photons, electrons, positrons). The fraction of muons among the secondary particles is one traditional way to estimate the mass composition of the primary cosmic rays.

An historic method of secondary particle detection still used for demonstration purposes involves the use of [[cloud chamber]]s<ref>{{cite web|url=http://www.chess.cornell.edu/Outreach/document/cloudchamber.pdf |archive-url=https://web.archive.org/web/20130606193736/http://www.chess.cornell.edu/Outreach/document/cloudchamber.pdf |archive-date=2013-06-06 |url-status=live|title=Cloud Chambers and Cosmic Rays: A Lesson Plan and Laboratory Activity for the High School Science Classroom|publisher=[[Cornell University]] [[Laboratory for Elementary-Particle Physics]]|date=2006|access-date=23 February 2013}}</ref> to detect the secondary muons created when a pion decays. Cloud chambers in particular can be built from widely available materials and can be constructed even in a high-school laboratory. A fifth method, involving [[bubble chamber]]s, can be used to detect cosmic ray particles.<ref>{{Cite journal|last1=Chu|first1=W.|last2=Kim|first2=Y.|last3=Beam|first3=W.|last4=Kwak|first4=N.|title=Evidence of a Quark in a High-Energy Cosmic-Ray Bubble-Chamber Picture|doi=10.1103/PhysRevLett.24.917|journal=Physical Review Letters|volume=24|issue=16|pages=917–923|year=1970|bibcode=1970PhRvL..24..917C}}</ref>

More recently, the [[CMOS]] devices in pervasive [[smartphone]] cameras have been proposed as a practical distributed network to detect air showers from ultra-high-energy cosmic rays.<ref>{{cite web|url=https://arstechnica.com/science/2014/10/cosmic-ray-particle-shower-theres-an-app-for-that/|title=Cosmic ray particle shower? There's an app for that.|work=Ars Technica|last=Timmer|first=John|date=13 October 2014}}</ref> The first [[mobile app|app]] to exploit this proposition was the CRAYFIS (Cosmic RAYs Found in Smartphones) experiment.<ref>[http://crayfis.ps.uci.edu/ Collaboration website] {{webarchive|url=https://web.archive.org/web/20141014162644/http://crayfis.ps.uci.edu/|date=14 October 2014}}</ref><ref>[http://crayfis.ps.uci.edu/paper.pdf CRAYFIS detector array paper. ] {{webarchive|url=https://web.archive.org/web/20141014162639/http://crayfis.ps.uci.edu/paper.pdf|date=14 October 2014}}</ref> In 2017, the CREDO ([[Cosmic-Ray Extremely Distributed Observatory]]) Collaboration<ref>{{Cite web|url=https://credo.science/|title=CREDO|website=credo.science}}</ref> released the first version of its completely open source app for Android devices. Since then the collaboration has attracted the interest and support of many scientific institutions, educational institutions, and members of the public around the world.<ref>{{Cite web|url=https://www.eurekalert.org/pub_releases/2018-10/thni-cfl100418.php|title=CREDO's first light: The global particle detector begins its collection of scientific data|website=EurekAlert!}}</ref> Future research has to show in what aspects this new technique can compete with dedicated EAS arrays.

The first detection method in the second category is called the [[IACT|air Cherenkov telescope]], designed to detect low-energy (<200&nbsp;GeV) cosmic rays by means of analyzing their [[Cherenkov radiation]], which for cosmic rays are gamma rays emitted as they travel faster than the [[speed of light]] in their medium, the atmosphere.<ref>{{cite web|url=http://www.lanl.gov/milagro/detecting.shtml|title=The Detection of Cosmic Rays|publisher=Los Alamos National Laboratory|work=Milagro Gamma-Ray Observatory|access-date=22 February 2013|date=3 April 2002|url-status=dead|archive-url=https://web.archive.org/web/20130305045916/http://www.lanl.gov/milagro/detecting.shtml|archive-date=5 March 2013}}</ref> While these telescopes are extremely good at distinguishing between background radiation and that of cosmic-ray origin, they can only function well on clear nights without the Moon shining, have very small fields of view, and are only active for a few percent of the time.

A second method detects the light from nitrogen fluorescence caused by the excitation of nitrogen in the atmosphere by particles moving through the atmosphere. This method is the most accurate for cosmic rays at highest energies, in particular when combined with EAS arrays of particle detectors.<ref>{{cite journal|last1=Letessier-Selvon|first1=Antoine|last2=Stanev|first2=Todor|title=Ultrahigh energy cosmic rays|journal=Reviews of Modern Physics|volume=83|issue=3|pages=907–942|doi=10.1103/RevModPhys.83.907|bibcode=2011RvMP...83..907L|arxiv=1103.0031|year=2011|s2cid=119237295}}</ref> Similar to the detection of Cherenkov-light, this method is restricted to clear nights.

Another method detects radio waves emitted by air showers. This technique has a high duty cycle similar to that of particle detectors. The accuracy of this technique was improved in the last years as shown by various prototype experiments, and may become an alternative to the detection of atmospheric Cherenkov-light and fluorescence light, at least at high energies.