Celestial Messengers: Cosmic Rays: The Story of a Scientific Adventure

337 Pages·2013·8.102 MB·English

by  Bertolotti, Mario

Astronomers’ Universe
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•
Mario Bertolotti
Celestial Messengers:
Cosmic Rays
The Story of a Scientific Adventure
123
MarioBertolotti
UniversityofRoma“LaSapienza”
DipartimentodiScienzediBasee
Applicateperl’Ingegneria(SBAI)
Rome
Italy
ISSN1614-659X
ISBN978-3-642-28370-3 ISBN978-3-642-28371-0(eBook)
DOI10.1007/978-3-642-28371-0
SpringerHeidelbergNewYorkDordrechtLondon
LibraryofCongressControlNumber:2012943470
c Springer-VerlagBerlinHeidelberg2013
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Introduction
The history of the research on cosmic rays is that of a scientific adventure.
For just one century,researchersclimbed mountains,flied in balloonsand planes,
andtravelledallaroundtheglobe,tryingtounderstandthenatureofthisradiation
which comes from outer space. At present the research has extended out of our
planet,intheopenspacewherespacecraftsaresenttoexploreoursolarsystem,and
inthefuture,likelybeyond.
Butwhatarecosmicrays?Theyareacontinuousrainofchargedparticleswhich
move at nearly the speed of light and invest our planet at every moment from all
directions. These particles are nuclei of common atoms—for the most hydrogen
nuclei—strippedbytheirelectrons,togetherwithgammarays,neutrinos,electrons,
a few antiparticles, and may be something else. The term ray is therefore strictly
speakinganoxymoron,thecosmicraysbeingparticlesandnotradiation,butithas
survivedforhistoricalreasons.
What makes them different from any other kind of radiation is their huge
individual energy, which has allowed them to play a fundamental role in the
developmentofmodernphysics.Beforetheirdiscovery,themostenergeticknown
particlesweretheonesemittedinthespontaneousdecayofradioactivenuclei.
Sometimelater,inanefforttoduplicatetheeffectsofcosmicraysincontrolled
conditionsinthelaboratory,physicistsbegantodevelopacceleratorsthatwereable
togetmoreandmorehigh-energyparticles,alsoifthehighestenergiesofthecosmic
rayparticleswillprobablyneverbereached.
The story started when, at the end of the nineteenth century, people observed
thatanelectricallychargedbodyinairlosesitschargeslowly.Tryingtounderstand
thisapparentlyuninterestingfact,whichafterallwithoutanin-depthinterpretation,
couldbeattributedtothelossesofthebodysupports,ledtothediscoverythatpairs
of ions of unknown origin always exist in the air, and later led to the conclusive
proofthataradiationofextraterrestrialoriginwasresponsible,togetherwithnatural
radioactivity,fortheirexistence.
However 20 years had to pass before the existence of an extraterrestrial
radiation—thecosmicradiation—wasuniversallyacceptedandanewandexciting
v
vi Introduction
field of research started which brought to the discovery of new constituents of
matterandtowhattodayconstituteshigh-energyphysics.
Physicists discovered in cosmic rays particles of subatomic dimensions with
energiesthousands,million,andbilliontimeslargerthanthoseofparticlesemitted
by radioactive materials existing on the Earth. For the first time they witnessed
processesinwhichenergymaterializesintoparticleswhichthensuddenlydisappear,
originatingotherstillunknownparticles.Thestudyofthenewmysteriousradiation
evolvedtogetherwith the fabricationofsuitableinstrumentsthatareable to study
itsproperties.Wemaysaythatourknowledgeinthisfieldhasprogressedchecking
the road, often starting from wrong hypotheses which were then corrected as the
instrumentationandmeasurementtechniqueswereimplemented.
People initially believed the rays to be the bare product of the Earth’s natural
radioactivity, and only when Victor Hess performed measurements with sealed
instruments at several altitudes in the atmosphere, it was proved that there was a
componentcoming from outside the Earth. People believed this componentto be
made of ultrapenetrative gamma rays, but today we know that most of it is made
ofparticles.When thepositiveelectronwas discovered,onetriedto interpretitas
a proton. When the muon was found, it was initially named meson and confused
withtheparticlethatYukawahadintroducedtoexplaintheforcesamongnucleons
in an atomic nucleus, and only later, in the pursuit of cosmic rays studies, it was
discovered that the Yukawa’s particle was something else, that is, the !-meson
whichoriginatesthemuonwhendecaying.
Quantummechanics—whichwasdevelopedintheyearsofthesediscoveries—
foundtheexplanationforthebehaviourofcosmicraysathighenergiesandaproof
benchandasinkofdatawhoseunderstandingcontributedtoitsformulationinthe
finalformofnowadays.
Since the 1950s, in an attempt to develop a general theory of elementary
processes,largeacceleratorsthatareabletoproducealargenumberofhigh-energy
particles suitable for experiments have been built; still, even today particles with
super-energiesmaybefoundonlyincosmicrays,afewonesbutextremelyuseful
since they help to discover secrets of the Universe that no accelerator may ever
unveil.
The Hess discovery has in fact uncovered new scenarios in astrophysics and
cosmology. The mysterious radiation carries important messages concerning the
physical conditions of regions far away in the space and, in an effort to explain
theoriginofit,physicistshavedevelopedagreatnumberofnewideasonthenature
oftheeventsthattakeplaceinstarsandinthemassesofdilutedgaswhichfillthe
interstellarspace.Thelastchallengehasbeenthestudyofneutrino,andcosmicrays
havegivenus,asweshallsee,agoodexperimentaldatatoworkwith.
However what is the impact on our everyday life? The particles which hit
us continuously from space, interacting with living matter, contribute to genetic
mutations which may stand at the origin of life itself and of its evolution; they
influence the telecommunicationsystems, both on Earth and among satellites and
spacecrafts,whilethelessenergeticpart,consistingofelectronsemittedbyourSun
during storms in its atmosphere, originates the marvellous phenomenonof boreal
auroras.
Introduction vii
The study of cosmic rays has contributed to the understandingof geophysical,
solar,andplanetaryphenomena:theyarethesolesampleofmattercomingdirectly
tousfromspace.AllotherinformationwehaveconcerningourUniverseisindirect
information reaching us through the study of the light emitted by stars. There
are also surprising effects. It is well known that one of the methods used to date
archaeologicalobjects, old downto 100,000years, employsthe carbonisotope of
mass14.Thisisotope14Cisradioactiveanddecaysintonitrogen,withalifetimeof
5,568years.Withinsuchashorttime,itisobviousthatthisisotopeiscontinuously
regeneratedbysomenaturalprocesses:thebombardment,bytheneutronsgenerated
in the high atmosphere by cosmic rays, of atmospheric nitrogen atoms which, by
transmutation, produce it. The radioactive carbon so generated rapidly oxidizes
into radioactive carbon dioxide which mixes with the normal carbon dioxide
already existing in the atmosphere. The decaying and production processes are at
equilibrium, so that the proportion of the radioactive isotope does not change in
time. The age is derived from the abundance of carbon 14 in the sample under
examination, assuming that the carbon is incorporatedinto the living sample (for
examplea tree) until there is an exchangewith the atmosphere.When the sample
dies, the exchange ceases and the decaying carbon 14 is no more replaced, so its
contentstartstodecrease.1
In the following pages I will try to retrace the basic steps of the discovery of
cosmic radiation up to the ideas of its mysterious origin, which has not yet been
completelyclarifiedandwhichthereforemaystilldeservesurprisingfindings.
Acompleteexpositionwhichtakesintoaccountallthecontributionsandallthe
subtle arguments that were made during the 100 years of research from the Hess
discoverywouldbetoolongandcomplex;Iwillgiveasyntheticdescriptioncom-
prehensiveenough,however,toallowtounveilthefascinationoftheseresearches.
Bibliographic references pertaining to all described facts are provided. Short
biographicnotesof some of the moreimportantpeopleinvolvedin the researches
have also been included,in an attempt to outline the personalities of the involved
scientists.
Iapologizeforanyomission.Commentswillbewelcome.FinallyIwanttogive
a special thank to my wife Romana who allowed me to spend all my free time
on this work, to my friends Bruno and Viera Crosignani who went through the
text suggesting improvements,to Claus Ascheronand Deepak Ganesh for help in
publicationandeditingthemanuscript.
Rome,Italy MarioBertolotti
1ThisprocesswasproposedbyW.F.Libby[Phys.Rev.69,671(1946)]whosuggestedthatcosmic
neutronsreactwithatmosphericnitrogenofmass14producingcarbon,accordingtothereaction
N14 n p C14.Theproduced carbon thendecays with“-emission C14 “ N14
7 6 6 7
C ! C ! C
turningbacktonitrogenandtheprocessmaystartagain.
•
Acknowledgements
Y. Suzuki, to allow me the use of Fig. 11.4,Itaru Shimizu to allow me the use of
Fig.11.5,A.M.HillastoallowmetheuseofFig.13.2,F.Halzentoallowmetouse
Fig.11.6,T.GaissertoallowmetheuseofFig.12.10.
ix