100 years since the discovery
of Cosmic Rays. A brief history
Andrea Chiavassa
Universita` degli Studi di Torino
Carpathian Summer School of Physics 2012
June 24 – July 7, Sinaia, Romania
• Most of the material
comes from the book:
“L’enigma dei raggi
cosmici” by Alessandro
De Angelis. Ed.
Springer (in italian)
• The key instrument that led
to the discovery of cosmic
rays was the electroscope
• 1785 Coulomb observed that,
even if insulated, an
electroscope spontaneously
discharge
• 1879 Crookes measured that
the speed of discharge
decreased if the air pressure
was reduced  the cause is
the ionized air
• 1986 Becquerel discovered the spontaneous decay
of radioactive elements
• In the presence of a radioactive material, a
charged electroscope promptly discharges
• The electroscope discharge can be attributed to
charged particles emitted during radioactive
decays
• The discharge rate of an electroscope was then
used to measure the level of radioactivity
• Where does these element come from?  most
natural answer  from the ground
Experimental situation in the beginning of the XX century
•
1900: Wilson and Elster & Geitel improve the technique for a careful insulation of electroscopes in a
closed vessel, improving the sensitivity
•
1901: Wilson’s measurements in tunnels with solid rock overhead (to check if the radiation was
coming from outside) show no reduction in ionization
•
1903-06: Rutherford & Cooke and McLennan & Burton show that ionization is marginally reduced
when the electroscope was surrounded by metal shields. McL&B put also the electroscope in a box,
and they fill it with water. Mache compares the variations of the radioactivity when the electroscope
is surrounded by shields of metal with the diurnal variations; he finds no significant reduction
•
1907: Strong studies radioactivity in a variety of places including (I) his lab (2) the center of a cistern
filled with rain water and (3) the open air; results dominated by statistical & systematic errors
•
1907-08: Eve makes measurements over the Atlantic Ocean, which indicate as much radioactivity
over the centre of the ocean as he had observed in England and in Montreal
•
1908: Elster & Geitel observe a fall of 28% when the apparatus is taken from the surface down to the
bottom of a salt mine. They conclude that, in agreement with the literature, the Earth is the source of
the penetrating radiation and that certain waters, soils and salt deposits, are comparatively free from
radioactive substances, and can therefore act as efficient screens
Technological Improvements:
Father Wulf contributions
• Father Theodor Wulf,
German scientist and
Jesuit priest
• Electroscope replacing the
metallic leaves with two
thin plate made by silicon
glass
• Sensibilty  1 Volt
• He measured the radiation
level on top of the Eiffel
tower  the radiation was
much higher than expected
 anyhow he concluded
that the most plausible
explanation was “radiation
emission from the ground”
• These results were summarized by Kurz in
a review article in 1909.
• He discussed three sources of this
penetrating radiation
– Extraterrestrial radiation, probably originated
by the sun
– Earth crust radioactivity
– Radioactivity coming from the atmosphere
• Kurz concludes that the extraterrestrial
origin was disfavored
• The prevailing data interpretation was that
radiation was of terrestrial origin
• E.g. in 1911 Schrödinger (working in the Wien
group) wrote that : “the third source […] is
completely hypothetical and should be introduced
, only if suitably justified, in case the first two
hypothesis were absolutely insufficient to explain
the observations”
• Calculation of the radiation decrease with
height were performed (e.g. A.S. Eve 1907)
• Two further experimental checks were
proposed. Radiation measurements:
– During balloon flights
– Underwater
Underwater measurements
• Domenico Pacini, Bari University,
performed such studies.
• Pacini began his studies starting
from the problem of the air’s
ionization and became familiar
with electroscopes.
• From 1907 to 1911 he measured the
ionization in different places, from
sea level to mountain altitudes, and
their variations due to temperature,
pressure and humidity
• Since end 1908, Pacini can use the
destroyer “Fulmine” from the Navy
• Fluctuations appear similar, which
raises in Pacini doubts about the
Terrestrial origin
• In 1909 presenting his results at
“Accademia dei Lincei”
• “….in the hypothesis that the origin of all penetrating
radiation is in the soil, since we must admit that, at least when
it is not covered be recent waterfalls, they are emitted from
the ground at a constant rate, we cannot justify the results
obtained so far.”
These results were cited by Cline (1910) and by Marie Curie in “Traité de radioactivité
The measurement•
in 1910
•
(quoted by Hess)
First, two electroscopes (A and B) with walls of
different thickness are cross-calibrated
Simultaneous measurements are performed at
ground and on the sea’s surface, and then the
instruments are exchanged
• “The number of ions due to penetrating
radiation on the sea is estimated to be 2/3 of
that on the ground”
• ``the evolution of the phenomenon on the sea
surface and on the land reveals for both the
same trend of the penetrating radiation during
the ten days of observation […] But it is clear
that in order to show the existence of a
possible correlation […] a period of time longer
than that I dedicated to the experiment would
be needed.’’
“such results seem to indicate that a substantial
part of the penetrating radiation in the air […]
has an origin independent of the direct action of
active substances in the […] Earth’s crust.” 11
The 1911 Underwater experiment
• In June 1911 Pacini performed a series of measurements
(lasting 7 days) in the Tirrenian Sea (in front of Livorno)
• He measured the discharge time of an electroscope located
– Sea level (300 m from the shore)
• Mean decrease (8 measurements)  12.6 Volt/hour
 equivalent to 11.0±0.5 ions/(sec cm3)
– 3 m below sea level (water depth 7m)
• Mean decrease (7 measurements)  10.3 Volt/hour
 equivalent to 8.9±0.2 ions/(sec cm3)
• The difference, 2.1 ions/(sec cm3), was attributed by Pacini
to a radiation independent from that originated by the earth
crust
These results were published in:
“Il Nuovo Cimento” February 1912
• “being the absorption coefficient of the water 0.034, we can easily
deduce from the equation I/I0 = exp(-d/l), […], that, in the
experimental conditions, the activity due to both the sea bed and the
surface were negligible. The explanation being that, due to the
absorption power of the water and to the minimal quantity of
radioactive elements in the sea, an absorption of the radiation
originated from the outside occurs when the instrument is located
underwater. […..] it exists in the atmosphere an appreciable source of
ionization, with penetrating radiation, independent from the direct
action of soil radioactive substances.
• Pacini thus showed, for the first time, that the experimental results cannot be
explained with earth crust’s radioactivity.
• Pacini was not able to exclude an atmospheric origin of the radiation (even
citing Eve’s calculation showing that this contribution was negligible).
Scientific Balloon Flights
• Balloon flights had been used for
scientific purposes since their
beginning.
– Nov 30, 1784, 1st scientific flight in
London with barometer , thermometer,
hydrometer & electrometer by J.Jeffries
(USA) & Blanchard
– Aug. 24, 1804, Gay-Lussac & Biot
(France) 4000 m hydrogen balloon
– Sept. 16, 1804, Gay-Lussac 7016 m
altitude, to study air properties at
different p, T
Balloon measurements of the
penetrating radiation
• Bergwitz 1908
– 9 hour flight from Braunschweig. 1300 m.
– 25% of the radiation at ground. 0% expected
• After the flight an older university professor advised against it;
he said that Bergwitz would lose his scientific reputation if he
continues to pursue the idea of an extraterrestrial radiation
• Albert Gockel (Freiburg, Switzerland)
– 1908, 4 flights from 650 to 3000 m
– 11 December 1909
– 15 October 1910
– 2 April 1911
Ion pairs/(cm3s)
• Measures with two Wulf electrometers
2
0
1
0
1
2
3
Altitude (km)
• Gockel concluded that: penetrating radiation
in the atmosphere independent on radioactive
source
1) Gockel cites the results of Pacini
Discovery of Cosmic Rays: V. Hess
• Victor Franz Hess:
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24.06.1883: born in castle Waldstein near to Peggau/Austria
1901-1905: physics study Univ. Graz
1906: PhD
1907-1910: University Vienna at Exner
1911-1920: Assistent of S. Meyer at the Vien Academy of Science
1921-23: USA, director United States Radium Corp.
1923 Returned to Graz University
1925-1931: Professor in Graz
1931-1937: Prof. in Innsbruck
1936: Nobelprize
1937-1938: Univ. Graz
1938: emigration, USA
1938-1956: Prof. at Fordham Univ.
17.12.1964: died in Mt. Vernon, New York
• Hess began his works studying Wulf’s results.
• As the g absorption length was fundamental in
the interpretation of the Wulf and Gockel
results, Hess decided to accurately determine
its value
• Then he performed, in early 1912, careful
calibration measurements with electroscopes
improved by him using radium sources
Hess’s first balloon fligth
• 28 August 1911
• 4 hours – up to 1070 m
- 2nd flight in the night of October 12th 1911
- Both flights confirmed the Wulf and Gockel results
Six next flights around Wien
3 mm foil to measure g
Thin Zn foil to measure b
7th and conclusive flight
• 7th August 1911
• ~60 km from Aussig to Pieslow
• Maximum height 5200 m
Hess results
i) Slightly decrease just above ground level
ii) Between 1000 and 2000m, slight increase
iii) Between 3000 and 4000m, 50% increase
respect ground level
iv) Between 4000 and 5000m, radiation is more
than 100% compared to ground level
• “The results of present observations are more reliably
explained assuming that an highly penetrating
radiation enters our atmosphere from the top, and then
produces in the lower layers part of the ionization
observed in closed detectors.”
• Possible reasons for the success:
–
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Detailed knowledge of the electrometers from calibration
Improvement of the electrometers
Independent measurements with 3 electrometers: 2 g, 1b
Systematic studies at day and night
• Hess published (1913) an article on Physikalische
Zeitschrift, calling this radiation “Höhenstralung”
(radiation from above)
Confirmation by Kolhörster
• After the World War I the main center of activities
moved to US
• Millikan and Bowen built a light (~200 gr)
electrometer that flew in Texas up to 15000m,
using data transimission technologies developed
during the war.
• They measured a radiation level one fourth of the
one reported by Hess and Kolhörster. The
difference was attributed to turnover at very high
altitudes. In fact the effect is due to geomagnetic
cutoff.
• Millikan concluded, at the American Physical
Society in 1925, that “the whole penetrating
radiation is of local origin”
Nevertheless, in 1926, Millikan and
Cameron performed radiation
absorption measurements in lakes, at
different depths, and at high altitudes.
Concluding that the radiation was
made of high energy g rays and that
“these rays propagate uniformly
through space in all directions”
• This radiation was named Cosmic Rays
• Neither Pacini, Hess and Kolhörster results were cited
• Bergwitz, Hess and Kolhörster wrote an article emphasizing
their priority on the balloon results (Zeit. Phys. 1928)
Are cosmic rays charged or neutral?
• Due to the penetrating
power this radiation was
supposed to be made by
g rays
• Cosmic rays intensity
was measured at
different
geomagnetic
latitudes.
• Clay 1927/28.
• Boat tripes:
– Giava – Genova
– Giava – Amsterdam
– Giava - Southampton
Increasing ionization with latitudes.
Cannot be explained by g rays
• Millikan argued against these results
• In 1932 Compton conducted an experiment
coordinating more than 70 scientists that
measured the radiation independently all over
the world.
• In 1933 also Millikan accepted the dependence
of the radiation level on latitude and that
cosmic rays are charged particles.
• Now the question became: are cosmic rays
positive or negative particles?
• Bruno Rossi idea: if cosmic rays are mainly
positive particles interacting with the earth
magnetic field they should appear mainly from
west
• He performed (1930) the measurement in Firenze:
no conclusive results
• He moved to Asmara (nearer to equator) in 1933,
showing that cosmic rays are mainly composed by
positively charged particles
• He published the result in 1934
• Few months before Alvarez and Compton
obtained the same results (citing Rossi as the first
to propose the method)
• Rossi also developed a coincidence system. During
his Asmara measurements he detected a number of
coincident events, between distant geiger counters,
well above the one expected by accidentals.
• The result was not stastically significant
• These events were deeply investigated
(1937) by Pierre Auger. Concluding that
extensive particle showers are
generated in atmosphere by the
interaction of cosmic rays with air
nuclei.
• Extensive Air Showers
Early days of Particle Physics
• 1933, Anderson detects antimatter using
cloud chamber in the presence of a
magnetic field
– A cloud chamber contains a gas supersaturated
with water vapor. In the presence of a charged
particle, vapor condenses into droplets
• The band across the middle is a lead plate,
which slows down the particles. The
radius of curvature of the track above the
plate is smaller than that below => it must
be travelling upwards (having lost energy)
• From the direction in which the path
curves one can deduce that the particle is
positively charged
• Mass can be deduced from the long range
of the upper track - a proton would have
come to rest in a shorter distance
positron
1936 Nobel prize: Hess & Anderson
Hess and Anderson were
awarded with Nobel Prize in
1936. Hess was nominated by
Clay, Compton:
– The time has now arrived […]
when we can say that the socalled cosmic rays have their
origin at remote distances from
the Earth […] and that the use of
the rays has by now led to results
of such importance that they may
be considered a discovery of the
first magnitude. [...] It is, I
believe, correct to say that Hess
was the first to establish the
increase of the ionization
observed in electroscopes with
increasing altitude; and he was
certainly the first to ascribe with
confidence this increased
ionization to radiation coming
from outside the Earth
35
• 1933 – Blankett & Occhialini
– Pair production
• Yukawa predicted the existence of a particle of
mass ~200 me mediating strong interactions.
• 1935 – Anderson & Neddermayer
– Detection of a particle with penetrating power higher
than those already known and lighter than protons.
“Mesotron” identified with Yukawa’s particle.
– Rossi measures the decay time of this particle (~2ms)
and that it decays in an electron + a neutral particle (n)
• The high penetrating power was contraddicitting
the Yukawa hypothesis.
• We were in presence of two particles: p and m
• 1947. Powell,
Occhialini and Lattes
 p detection
• Nuclear emulsions
exposed at high altitudes
(Chacaltaya, 5500 m)
1950  Powell awarded with Nobel Prize
• 1947
• Rochester & Butler
• Neutral Kaons
– Mass ~500 MeV
• So called “Strange
particles”
• Particle physics moved to
accelerators
• Cosmic rays research
moved to “astrophysical”
studies.
– Spectrum
– Anisotropies
– Chemical Composition
1990-2000
n oscillations
• Solar n
– Homestake experiment
• (380 m3 Perchloroethylene)
– Super-kamiokande
• (50000 T water, 11000 pmt)
• Atmospheric n
– Super-kamiokande
• 2002 Nobel Prize to
Davies, Koshiba and
Giacconi
Conclusions
• Cosmic Rays discovery
– Victor HESS, 1912
– Conclusion of a series of studies conducted by
different physicists
• Birth of particle physics
– Antimatter
– m, p detection
– n oscillations
• Spectrum, Anisotropies, Chemical composition
• Energies up to 1020 eV