The contribution by Domenico Pacini to the cosmic ray physics N. Giglietto Dipartimento di Fisica Università di Bari CRIS Catania

Abstract Between 1909 and 1911, several researches started to investigate the unknown radiation that later identified as cosmic rays. Several experimentalists tried to identify the origin of this radiation in particular Victor Franz Hess, Theodor Wulf and Domenico Pacini. Among them the important work by Domenico Pacini, almost forgotten now but perfectly known at that time, had a crucial role to address the answer to the origin of the radiation in the right way. The most surprising and interesting thing moreover is that these important pioneers practically defined at that time the experimental ways to study comic rays: from space and air, on ground and underground using only electroscopies! Domenico Pacini in particular may be considered as the pioneer of underground cosmic ray experiments, and Hess proved with his systematic balloon studies the extra-terrestrial origin of CRs originating the air-space studies on CRs

Acknowledgments This talk is based on the work started (and going to be published) by F. Guerra(Rome) and N.Robotti (GE) that have studied the original documentation and the work by A. De Angelis (Udine University), S. Stramaglia, A. Garuccio (Bari University) and myself to translate original papers and discover documents inside the Bari University

Timeline Bequerel discovered the radioactivity 1901 – two groups J.Elster and H. Geitel (Germany) and C.T.R. Wilson(England) concluded that some unknown source of ionizing radiation exists and Ernst Rutherford and other Canadian groups confirmed 1903 Ernst, Mc Lennan, Burton, Rutherford Cooke: confirmed the radiation was penetrating using a lead absorber (In those years the radiation was thinked to be originated by the soil) 1907 T.Wulf realized a new electroscope Pacini measure the ionization rate on ground and on sea : radiation intensity doesn’t decrease with the height as calculated taking into account the air absorprtion (and still considering the soil as the origin) 1910 Theodor Wulf developed an electrometer and used it on top of Eiffel tower and in a cave in Valkenburg(Holland), the rate in the cave is less then that on surface, the rate on top of Eiffel tower doesn’t decrease and his paper on Physikalische Zeitschrift was not widely accepted

Timeline-2 Gockel (balloon), 4500 m (1910) 1910 Pacini performed measurements on ground and sea and underwater Hess performed the balloon flights (5530m) and during a solar eclipse Hess and Kolhorster ballon flights up to 9Km measurements up to 16km doesn’t evidentiate changes with the height (terrestrial origin?) 1925-new measurements (less precise than Hess) by Millikan proved the extraterrestral origin (thinking to gamma-rays) and conied the term "cosmic rays“, and excluding any effect due to the atmospheric electricity as Hess had thought. Starting the debate about neutral (gamma-rays) or charged nature of the radiation During the decade from 1927 to 1937, a wide variety of experimental investigations demonstrated that the primary cosmic rays are mostly positively charged particles

Introduction At the beginning of the XX century the italian Domenico Pacini and the austrian Victor Hess reasearchers involved in studies to understand the reason of the spontaneous ioniziation properties of the air, and developed at the same time two experimental reasearch lines that led later to the discovery of Cosmic Rays. D. Pacini, “La radiazione penetrante alla superficie ed in seno alle acque”, Il Nuovo Cimento 3 (1912) 93 now translated V. F. Hess, ¨Uber Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten“, Physik. Zeitschr. XIII, (1912) There is no doubt about the relevant role of these studies in the community at that time

Pacini’s short curriculum Born in Marino (Rome) on february, 20th 1878 Physics degree on 1902, at Rome University assistant at the Physics Institute in Rome assistant at the Main Royal Bureau of Meteorology and Geodynamics (Regio Ufficio Centrale di Meteorologia e Geodinamica), in this role was born his interest on the electrical properties of the atmosphere Agosto 1927: he become main geophysics role 1928 he becomes full professor in physics at the Bari University – Medicine Faculty 1934 died for a pulmunite

Review by Wigand, Phys. Z. XXV, 445 (1924), “About the radiation coming from above the Earth” “...Pacini was the first to try to measure the amount of external penetrating radiation, indipendently from the radioactive elements inside the soil and the air, kwnown to emit radiation.” This sentence confirm the well known Pacini contribute to the scientific community around 1910 But more recently only few reviews mention Pacini: (1)AW Wolfendale, Rep. Prog. Phys.,(1984) (2)Castagnoli e Morra, Giornale di Fisica, (1994). In this talk I'll shortly explain the experimental work done by Pacini

Origin and nature of penetrating radiation, studies around 1910 Study of discharge rates at different weather conditions and during the day (electrical properties and atmosphere studies) (hyphothesis of an electrical property of the atmosphere) Study of discharge rates at sea level, study of variation with distance from the coast Testing the hyphothesis of soil radioactivity Underground and space (balloon) measurements - Conclusive tests of extraterrestral nature of CRs - Need a developement of new instruments for the different experimental conditions studies on the air discharge and construction of new kind of electroscopes

The detector evolution (all contributions) The main initial effort was the realization of instruments to detect the radiation: Modification of the original electroscope to be protected by some shielding Studies of the shielding effects on the detected radiation Defining the way to protect the instrument for the different exp. conditions (balloon, sea,underwater)

Pacini ( ) considered the experimental problem For each cm3 of air. Near the soil, about 13 ions every second are generated.[1] Are the known sources of this ionization sufficient to explain the production of this ion rate? An appropriate measurement is necessary to understand this point. [1] well known average value see: C.T.R. Wilson Proc.Roy.Soc. 68 (1901) and Elster and Geiter Ann. d. Physics, 2 (1900) 425

Using the Ebert instrument (later the Wulf instrument for underwater measurements), Pacini measured the discarge rate of the electroscopy and therefore the radiation intensity, in 3 experimental situations: 1)Electroscopy at ground level (on soil and on sea) )Electroscopy on the sea surface (Livorno and Genova gulf) few miles from the coast 1907 and later 3)Undersea 1911, june-july (3m depth on Livorno gulf and Bracciano lake in Italy) The Beginning of UNDERGROUND Physics !!!

The Italian Navy Cacciatorpediniere “Fulmine” used by Pacini in

Pacini measurements On “La radiazione penetrante alla superficie ed in seno alle acque”, Il Nuovo Cimento 3 (1912) 93 now translated “The penetrating radiation on surface and undersea” 1810v1.pdf D. Pacini reported that “the measurements on sea in 1910 let me to conclude that a not negligible fraction of the penetrating radiation is not dependent by the radioactive elements in the soil” since on the sea surface the level of radiation was practically the same as over the soil (about 2/3 respect ground). So the missing prove to conclude for an extraterrestrial origin is to verify the ionization rate vs the depth. In the same paper he explain how proceeded: “using the same electroscope and inserting it in a copper box” he performed new measurements from 24 to 31 june 1911, 300m far from the coast and 3m depth in the sea in the (Livorno) Leghorn sea and later underwater in the Bracciano lake (Rome-Italy)

Pacini measurements-2 For both set of measurements the rate resulted attenuated according to an exponential law I/I0=exp(- ld) The same paper report “The measurements by Hess and Goeckel have repeated and obtained the same conclusion previously reported by me i.e. that exists in the atmosphere a ionization source, producing penetrating radiation, and not depending from the radioactive elements in the soil” So both Pacini and Hess reached the conclusion that something in the atmosphere produce the air ionization Some fragments of some letter in 1920 between Pacini and Hess seem to confirm that Hess was well informed about Pacini results. [there is room for more detailed historical searches]

Surface measurements: 13,2−12,2−12,1−12,6−12,5−13,5−12,1−12,7 Average: 11 ions per cm3 per second 10,2−10,3−10,3−10,1−10,0−10,6−10,6 Average: 8.9 ions per cm3 per second Undersea measurements: Pacini concluded that the measurements on sea, undersea and under lake let to exclude the soil as the only responsable of the radiation therefore he was the first to think to exclude a terrestrial contribution to the penetrating radiations

In the same years Hess, was performing his balloon experiments fully supported by austrian imperial Academy of Sciences The flight 7 was the good one (5200 m), on August, , few weeks after the undersea measurements by Pacini The beginning of the space research of CRs He reached the same conclusion of Pacini: The penetrating radiations has a non-terrestrial origin And his measurements were definitively excluding a relevant contribution from soil radiactive elements

After Hess measurements and Gockel confirms, however the physics community was reluctant to exclude the terrestrial origin[*] of CRs and the studies on the radiation take different paths (and the Ist world war in the middle): people mainly interested to the atmosphere electricity studies and people studying the composition of the radiation, that later let to discover new particles. The dibate about the origin restarted only about 10 years later Pacini and Hess measurements [*] see for example the interpretation as radiactive elements in the air by G. Ho ff mann, Zeit. f. Phys. 7 (1921) 254; Phys. Zeit. 24 (1923) 475; Phys. Zeit. 25 (1924) 177; Phys. Zeit. 26 (1925) 40 and 669 F. Behounek, Phys. Zeit. 27 (1926) 8 [**] G. Homann, Zeit. f. Phys. 7 (1921) 254; Phys. Zeit. 24 (1923) 475; Phys. Zeit. 25 (1924) 177; Phys. Zeit. 26 (1925) 40 and 669 Kolhoerster balloon Measurements[**]

Millikan in the studies around 1925 considered the hypothesis of neutral radiation from outside and conied the term “Cosmic Rays” starting the debate about the nature of the radiation. The measurements in showing geomagnetic effects on the intensity let to conclude in favour of charged particles against gamma-rays and new detectors confirm the corpuscolar nature of radiation[*] [*] A. Compton, Phys. Rev. 41 (1932) 681; Phys. Rev. 43 (1933) 387 M. Schein et al., Phys. Rev. 59 (1941) 615

In 1936 Hess got the Nobel prize for the discovery of cosmic rays (together Anderson for the positron discovery) The Nobel prize Pacini, died before (1934) but in the report on the cosmic rays, commissioned by the Nobel Committee, prepared by E. Hulthén, one of the members of the Committee sorbers, Hulthén points out that considering the experiments by Eve, Pacini, Mache and others it became clear that only a small fraction of the ionisation could be attributed to radioactive substances. Moreover Hulthén notes that the Gockel measurements, in agreement with measurements of Pacini, “show that a not insignificant part of the radiation is independent of direct action of substances in the crust Earth“ but the careful works by Hess confirm the results of Eve but he was the first to prove a clear increase of the radiation with the height

Pacini studies were well known, appreciated and inserted in an international framework, and the CR discoveries in every case are due to the joint efforts of many people In Pacini notes and papers is well evident the logical path followed, based on the idea, reported in his first paper, that the radioactivity in the soil and in the air was not sufficient to explain the observed measurements of the air ionization However Pacini studies were scarsely supported in Italy by the scientific academy (i.e. balloon flights were performed in Italy for other studies) and after the war the Pacini role was forgotten Conclusion about Pacini work

Conclusions-1 Hess, Pacini and Wulf and other researchers defined around 1910 the basic experimental lines to study CRs and contributed to the instrument developements to detect this penetrating radiation. Pacini was probably among the first to think to exclude a terrestrial origin of CRs, and was the first to study CRs underground and on the sea (I’m particular proud to say this here near the site of one undersea experiment) Hess systematic measurements however revelead to be more complete, systematic and conclusive about the origin, were better known and probably better supported by his national agency Pacini and Wulf were practically forgotten for several reasons but mainly due to the different support of science agencies and academy

Conclusions-2 CR discovery was the result of a world effort with many contributions despite the slow and hard communication problems at that time (Pacini and Hess published in their native language and the others in english) and the war problems, but finally the CR nature problem was understood about 20 years after the first measurements Let us start the celebration of the centenary of CRs discoveries (2012) starting to remember Pacini underground measurements(1911) and ending with Hess air measurements(1912) and extablishing the link with the actual underground and space researches on CRs

Electroscopies

Citations to Pacini studies Nature note at page 107 del vol. LXX (1904) G.A.Cline phys. Rev. 30 (1910) 35 Wigand, Phys. Z. XXV, 445 (1924)

Pacini bibliography D. Pacini, Intorno ad un fenomeno di polarità di scarica, Il Nuovo Cimento, 13 (1907), D. Pacini, Misure di ionizzazione dell'aria su terraferma ed in mare, Il Nuovo Cimento, 15 (1908), 5-23 D. Pacini, Sulla radioattività indotta dell'atmosfera nel golfo ligure, Il Nuovo Cimento, 15 (1908), D. Pacini, Intorno ad un fenomeno di polarità di scarica, Il Nuovo Cimento, 13 (1907), D. Pacini, Misure di ionizzazione dell'aria su terraferma ed in mare, Il Nuovo Cimento, 15 (1908), 5-23 D. Pacini, Sulla radioattività indotta dell'atmosfera nel golfo ligure, Il Nuovo Cimento, 15 (1908), D. Pacini, Questioni di elettricità atmosferica, Il Nuovo Cimento, 19 (1910), D. Pacini, Sui prodotti del radio e del torio nell'atmosfera, Il Nuovo Cimento, 19 (1910), D. Pacini, La radiazione penetrante alla superficie ed in seno alle acque, Il Nuovo Cimento, 3 (1912), D. Pacini, Sulle emissioni di cariche elettriche per parte di sali non radiattivi alla temperatura ordinaria, Il Nuovo Cimento, 7 (1914),

Nuovo Cimento 15, In this paper systematic measurements of ionization at ground level and on the sea at different daily times

Nuovo Cimento 15,

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Nuovo Cimento 1912,3,93-100