Editing Cosmic ray (section)

===Identification===
[[Bruno Rossi]] wrote in 1964:
<blockquote>In the late 1920s and early 1930s the technique of self-recording electroscopes carried by balloons into the highest layers of the atmosphere or sunk to great depths under water was brought to an unprecedented degree of perfection by the German physicist [[Erich Regener]] and his group. To these scientists we owe some of the most accurate measurements ever made of cosmic-ray ionization as a function of altitude and depth.<ref>{{cite book|last=Rossi|first=Bruno Benedetto|title=Cosmic Rays|location=New York|publisher=McGraw-Hill|year=1964|isbn=978-0-07-053890-0}}</ref></blockquote>

[[Ernest Rutherford]] stated in 1931 that "thanks to the fine experiments of Professor Millikan and the even more far-reaching experiments of Professor Regener, we have now got for the first time, a curve of absorption of these radiations in water which we may safely rely upon".<ref>{{cite journal|last1=Geiger|first1=H.|last2=Rutherford|first2=Lord|last3=Regener|first3=E.|last4=Lindemann|first4=F.A.|last5=Wilson|first5=C.T.R.|last6=Chadwick|first6=J.|last7=Gray|first7=L.H.|last8=Tarrant|first8=G.T.P.|last9=Dobson|first9=G.M.B.|display-authors=6|year=1931|title=Discussion on Ultra-Penetrating Rays|journal=Proceedings of the Royal Society of London A|volume=132|issue=819|page=331|bibcode=1931RSPSA.132..331G|doi=10.1098/rspa.1931.0104|doi-access=free}}</ref>

In the 1920s, the term ''cosmic ray'' was coined by [[Robert Millikan]] who made measurements of ionization due to cosmic rays from deep under water to high altitudes and around the globe. Millikan believed that his measurements proved that the primary cosmic rays were gamma rays; i.e., energetic photons. And he proposed a theory that they were produced in interstellar space as by-products of the fusion of hydrogen atoms into the heavier elements, and that secondary [[electron]]s were produced in the atmosphere by [[Compton scattering]] of gamma rays. In 1927, while sailing from [[Java]] to the Netherlands, [[Jacob Clay]] found evidence,<ref>{{cite journal|author=Clay, J.|year=1927|title=Penetrating Radiation|journal=Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam| volume=30|issue=9–10|pages=1115–1127|url=http://www.dwc.knaw.nl/DL/publications/PU00011919.pdf |archive-url=https://web.archive.org/web/20160206055736/http://www.dwc.knaw.nl/DL/publications/PU00011919.pdf |archive-date=2016-02-06 |url-status=live}}</ref> later confirmed in many experiments, that cosmic ray intensity increases from the tropics to mid-latitudes, which indicated that the primary cosmic rays are deflected by the geomagnetic field and must therefore be charged particles, not photons. In 1929, [[Walther Bothe|Bothe]] and [[Werner Kolhörster|Kolhörster]] discovered charged cosmic-ray particles that could penetrate 4.1&nbsp;cm of gold.<ref>{{cite journal|author1=Bothe, Walther| author2=Werner Kolhörster|date=November 1929|title=Das Wesen der Höhenstrahlung|journal=Zeitschrift für Physik|volume=56|issue=11–12|pages=751–777|doi=10.1007/BF01340137|bibcode=1929ZPhy...56..751B|s2cid=123901197}}</ref> Charged particles of such high energy could not possibly be produced by photons from Millikan's proposed interstellar fusion process.{{citation needed|date=April 2015}}

In 1930, [[Bruno Rossi]] predicted a difference between the intensities of cosmic rays arriving from the east and the west that depends upon the charge of the primary particles—the so-called "east–west effect".<ref>{{cite journal|title=On the Magnetic Deflection of Cosmic Rays|author=Rossi, Bruno|journal=Physical Review|date=August 1930|volume=36|issue=3|page=606|doi=10.1103/PhysRev.36.606|bibcode=1930PhRv...36..606R}}</ref> Three independent experiments<ref>{{cite journal|title=The Azimuthal Asymmetry of the Cosmic Radiation|author=Johnson, Thomas H.|journal=Physical Review|date=May 1933|volume=43|issue=10|pages=834–835|doi=10.1103/PhysRev.43.834|bibcode=1933PhRv...43..834J}}</ref><ref>{{cite journal|title=A Positively Charged Component of Cosmic Rays|author=Alvarez, Luis|author2=Compton, Arthur Holly|journal=Physical Review|date=May 1933|volume=43|issue=10|pages=835–836|doi=10.1103/PhysRev.43.835|bibcode=1933PhRv...43..835A}}</ref><ref>{{cite journal|title=Directional Measurements on the Cosmic Rays Near the Geomagnetic Equator|author=Rossi, Bruno| journal=Physical Review|date=May 1934|volume=45|issue=3|pages=212–214|doi=10.1103/PhysRev.45.212|bibcode=1934PhRv...45..212R}}</ref> found that the intensity is, in fact, greater from the west, proving that most primaries are positive. During the years from 1930 to 1945, a wide variety of investigations confirmed that the primary cosmic rays are mostly protons, and the secondary radiation produced in the atmosphere is primarily electrons, photons and [[muon]]s. In 1948, observations with [[nuclear emulsion]]s carried by balloons to near the top of the atmosphere showed that approximately 10% of the primaries are helium nuclei (alpha particles) and 1% are nuclei of heavier elements such as carbon, iron, and lead.<ref>{{cite journal|title=Evidence for Heavy Nuclei in the Primary Cosmic radiation|author=Freier, Phyllis|journal=Physical Review|date=July 1948|volume=74|issue=2|pages=213–217|doi=10.1103/PhysRev.74.213|last2=Lofgren|first2=E.|last3=Ney|first3=E.|last4=Oppenheimer|first4=F.|last5=Bradt|first5=H.|last6=Peters|first6=B.|bibcode=1948PhRv...74..213F|display-authors=etal}}</ref><ref>{{cite journal|title=Investigation of the Primary Cosmic Radiation with Nuclear Photographic Emulsions|author=Freier, Phyllis|journal=Physical Review|date=December 1948|volume=74|issue=12|pages=1828–1837|doi=10.1103/PhysRev.74.1828|last2=Peters|first2=B.|bibcode=1948PhRv...74.1828B|display-authors=etal}}</ref>

During a test of his equipment for measuring the east–west effect, Rossi observed that the rate of near-simultaneous discharges of two widely separated [[Geiger counter]]s was larger than the expected accidental rate. In his report on the experiment, Rossi wrote "...&nbsp;it seems that once in a while the recording equipment is struck by very extensive showers of particles, which causes coincidences between the counters, even placed at large distances from one another."<ref>{{cite journal|title=Misure sulla distribuzione angolare di intensita della radiazione penetrante all'Asmara|author=Rossi, Bruno|journal=Ricerca Scientifica|date=1934|volume=5|issue=1|pages=579–589}}</ref> In 1937, [[Pierre Victor Auger|Pierre Auger]], unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that high-energy primary cosmic-ray particles interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, and photons that reach ground level.<ref>{{citation|display-authors=1|last1=Auger|first1=P.|last2=Ehrenfest|first2=P.|last3=Maze|first3=R.|last4=Daudin|first4=J.|last5=Fréon|first5=R. A.|title=Extensive Cosmic-Ray Showers| journal=Reviews of Modern Physics|volume=11|issue=3–4|pages=288–291|date=July 1939|doi=10.1103/RevModPhys.11.288|bibcode=1939RvMP...11..288A| postscript=.}}</ref>

Soviet physicist [[Sergei Vernov]] was the first to use [[radiosonde]]s to perform cosmic ray readings with an instrument carried to high altitude by a balloon. On 1 April 1935, he took measurements at heights up to 13.6 kilometres using a pair of [[Geiger counter]]s in an anti-coincidence circuit to avoid counting secondary ray showers.<ref>{{cite book|author1=J.L. DuBois|author2=R.P. Multhauf|author3=C.A. Ziegler|date=2002|title=The Invention and Development of the Radiosonde|url=http://www.sil.si.edu/smithsoniancontributions/HistoryTechnology/pdf_lo/SSHT-0053.pdf |archive-url=https://web.archive.org/web/20110605132224/http://www.sil.si.edu/smithsoniancontributions/HistoryTechnology/pdf_lo/SSHT-0053.pdf |archive-date=2011-06-05 |url-status=live|series=Smithsonian Studies in History and Technology|volume=53|publisher=[[Smithsonian Institution Press]]}}</ref><ref>{{cite journal|author=S. Vernoff|date=1935|title=Radio-Transmission of Cosmic Ray Data from the Stratosphere|journal=Nature|volume=135|issue=3426|pages=1072–1073|doi=10.1038/1351072c0|bibcode=1935Natur.135.1072V|s2cid=4132258}}</ref>

[[Homi J. Bhabha]] derived an expression for the probability of scattering positrons by electrons, a process now known as [[Bhabha scattering]]. His classic paper, jointly with [[Walter Heitler]], published in 1937 described how primary cosmic rays from space interact with the upper atmosphere to produce particles observed at the ground level. Bhabha and Heitler explained the cosmic ray shower formation by the cascade production of gamma rays and positive and negative electron pairs.<ref>{{cite journal|last1=Bhabha|first1=H. J.|last2=Heitler|first2=W.|title=The Passage of Fast Electrons and the Theory of Cosmic Showers|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=159|issue=898|year=1937|pages=432–458|issn=1364-5021|doi=10.1098/rspa.1937.0082|url=http://repository.ias.ac.in/2364/1/2364.pdf |archive-url=https://web.archive.org/web/20160102023556/http://repository.ias.ac.in/2364/1/2364.pdf |archive-date=2016-01-02 |url-status=live|bibcode=1937RSPSA.159..432B|doi-access=free}}</ref><ref>{{cite journal|last=Braunschweig|first=W.|display-authors=etal|title=A study of Bhabha scattering at PETRA energies|journal=Zeitschrift für Physik C|volume=37|issue=2|date=1988|pages=171–177|doi=10.1007/BF01579904|s2cid=121904361}}</ref>