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Copyright owned by the author(s) owned of the Copyrightby terms under In celebration of the centennial anniversary of In theof celebration theof centennial anniversary

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XXXIV editionof the Brazilian Workshop on Nuclear Foz de Iguaçu, Parana Brasilstate, 5-102011 June Odilon A. P. Tavares P. Tavares Odilon A. Centro Brasileiro de Pesquisas Físicas – CBPF/MCT 22290-180 Riode Janeiro - RJ, Brazil E-mail: Discovering the Atomic Nucleus the Atomic Discovering

PoS(XXXIV BWNP)024 Ernest sir Odilon Tavares Joseph John sir echnological echnological applications nown? nown? Who identified them as he spirit of celebration of the y y of the discovery of the atomic iscoveries iscoveries in science, even taken as hich hich initiated a major scientific, s honored in 1895 with a student- English physicist wska wska Curie (1867-1934), devoted pported pported at the expense of the energy gain about two years later, especially especially later, two years about gain ears have passed since the public 1911 by its discoverer enjoy better health, comfort and well- sicist Pierre Curie (1859-1906) and his that have caused to scientists of that time Christchurch Christchurch () to work at g g them to discover two new chemical presented presented the beginning of a new science, hic hic films, and were also found in thorium 1896 the French physicist Antoine Henri ation emitted by uranium saltsation emitted by uranium that showed usion usion of hydrogen nuclei), and, to a much e nuclei such as uranium, thorium, radium, ger ger scale, in reactions between light atomic the way for understanding the real structure ble, ble, spontaneous, and of unknown origin, ncluding, of course, ourselves. ourselves. of course, ncluding, osophical Society on March 7, 1911. 7, on March Society osophical h concentrates h all theconcentrates electric charge positive s provided us with better days. days. better us with s provided ration from nuclear power plants and also in eaking eaking of heavy nuclei such as uranium) which

Ernestthe Rutherford (1871-1937), discoverer of 2 ir

Investigations on these new radiations were taken a taken radiations were new on these Investigations But how long ago and how did atomic nuclei become k In In the late nineteenth century, new and important d Nuclear Nuclear physicists celebrate this year the centenar The present article has been thus prepared within t Humanity Humanity and, indeed, all life on our planet are su Knowledge Knowledge on atomic nuclei and the huge amount of t

Cambridge Cambridge University () under advice of the by by a young New Zealander, Ernest researcher Rutherford, scholarship, who and, wa therefore, moved from crossed bodies opaque to light, blackened photograp salts. and its Becquerel (1852-1908) discovered Becquerel (1852-1908) a new type of radi at least peculiar features. Such radiations, invisi nucleus, announced in (England) in early revolutionary, were made on entirely new phenomena some perplexity, surprise, and even skepticism. In Rutherford. Rutherford. This remarkable achievement not only re the , but was the basis and opened i us, is around that everything which make of atoms centenary centenary of the that ha revolution and technological philosophical, discovery of the atomic nucleus, w nuclei that are present in the Sun (thermonuclear f that was and has primarily been generated, to a lar smaller scale, in the so-called nuclear fission (br occurs in the nuclear reactors for electricity gene the natural radioactivity exhibited by some unstabl others. and many samarium, 2. Radiations and Radioactivity 1. Prologue 1. Prologue Discovering the Atomic Nucleus Thomson Thomson (1856-1940). Also, in Paris, the French phy wife, the Polish themselves physicist to the and study of chemist these Marie radiations, leadin Sklodo the ultra-minuscule central region theof central whic the atom ultra-minuscule and virtually the entire by New announcement Zealander physicist s atomic mass? One hundred y being. being. the atomic nucleus, in Manchester Literary and Phil Literary Manchester in nucleus, the atomic resulting from that has made possible to modern man PoS(XXXIV BWNP)024 Odilon Tavares t t cathode rays were nto nto another one due JJ Thomson was able to sir ontreal ontreal (Canada), where he to some physicists and chemists that beta (inclined arrows) disintegrations, disintegrations, arrows) beta (inclined nium, nium, both strong radiation emitters, for but but having much more energy, to the positive electric charge because of the ) discovered in 1902 the transmutation of enjoy better working conditions, and in d d (stable element), after having generated , later named Radon. Rutherford together s s that the total mass of the electrons in any ed them as alpha and beta rays. In Paris, 1]. Rutherford concluded further that the one of the first conclusions reached by m m uranium salts were of two very different at at they had in their structure electrons and ews ews in the field was of radioactivity, to able tomic tomic mass and, therefore, the mass of the regions of intense electric fields. fields. electric intense of regions calling them electrons. them calling electric charge emitted from the cathode) of nium, bismuth). nium, charge. charge. atoms. In addition,

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an element changes (or disintegrates, or decays) i JJ Thomson in 1897 . These led him to conclude tha i.e. sir In In 1898, Rutherford moved to McGill University in M By By the beginning of last century it became evident By of and alpha successive arrows) means By (vertical

alpha particles emitted from radon and radium had in passed they when observed were that deflections a to spontaneous emission of alpha or beta rays [Fig. remained remained for a period of nine years. There he could 1899 Rutherford discovered a noble gas, radioactive with the English chemist Frederick Soddy (1877-1956 radioactive elements, the fundamental particles that atoms were made of, were made that atoms particles fundamental the Fig. 1

Discovering the Atomic Nucleus elements in 1898, which they named Radium and Polo this reason called radioactive. In Rutherford in Cambridge was thatthat the radiations fro same year, types, types, both corpuscular in their nature, and he nam Henri aboutfull ofnow Becquerel, the enthusiasm n demonstrate demonstrate in 1900 that beta corpuscles of cathode rays rays (particles with negative were identical, the experiments by conclude from his experiments with tube cathode-ray atom should be a very small fraction of the total a electric positive theto be should associated atom radioactive elements in series (radium, radon, polo (radium, in series elements radioactive as time goes uranium and thorium transform into lea positive positive charges, so as to ensure the of neutrality atoms atoms were the basic constituents of matter, and th PoS(XXXIV BWNP)024 Odilon Tavares t. ester, ester, Rutherford was able to e a vacuum chamber (box from ts ts used the zinc sulfide screen they g g observations, in 1909 Rutherford ped ped by Rutherford and the German of alpha rays passed through a small -1955), -1955), alpha rays were definitively 1908 that it was possible to see light a particles as being positively charged e main tool of work in the experiments t t three hundredths of a millimeter) for a hysicist hysicist Ernest Marsden (1889-1970), the lightly deflected their from lightly initial direction hic film. film. hic uminescence). uminescence). This simple and successful ng ng year, with the help of another qualified became even greater, with edges not well ple ple who assisted withand cooperated in him r electrons, therefore ions of double positive never never they hit a screen coated with a layer of cles cles were deflected from its original direction mica was mica interposed the between hole and the a well-defined circular area of diameter equal

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Deflection of alpha particles by a thin mica shee a mica thin by particles of alpha Deflection Fig. 2 A A technique for counting alpha particles was develo Back Back to England in 1907, now at University of Manch Without Without a convincing explanation for these puzzlin In In Manchester, whenever Rutherford and his assistan While in Montreal Rutherford had noticed that insid gather around brilliant him and qualified peo young a series of experiments that could confirm the alph corpuscles. corpuscles. assigned a young student, the New Zealand-British p

[Fig 2].[Fig to that of the hole. However, when a thin sheet of screen, the region where the scintillation occurred defined, showing again that part of the alpha parti observed inside a vacuum chamber that, when a circular beam hole, the scintillations were observed in physicist (1882-1945) when they knew in signals signals (scintillations) produced by alpha rays whe zinc zinc sulfide (salt which exhibits the method property of of counting alpha l particles had become in th involving detection of alpha particles. The followi assistant, assistant, the English physicist identified as Thomas helium atoms that have lost Royds both thei (1884 charge. electric beam of them produced a diffuse spot on a photograp on a spot diffuse a produced of them beam where air had rays been alpha removed) were s being when passing through very thin sheets of mica (abou Discovering the Atomic Nucleus Rutherford3. Ernest alpha the and particles PoS(XXXIV BWNP)024 cm the -10 Odilon Tavares n gold foil. n gold if the current Thomson’s atomic e meaning of e meaning of the backscattering nucleus of the atom, to explain . The electrons, in turn, would be cm). Rutherford also assumed that the d as the correct model for the atom, the -8 variation with the inverse square of the ould not experience any recoil because ngle ngle backscattering over 20,000 alphas!). , , which implied multiple scattering, could possible to estimate as ~ 6x10 tly tly against the nucleus, with the resulting the the atomic nucleus is entirely compatible rds, rds, hitting the scintillation screen when it tion tion in which they practically kick in the by by the incident alpha particles. This meant nd nd the gold target. With the aid of a small ted ted there (for gold, the mass of the nucleus ticles ticles to directions very different from the however, not null, in agreement with what dence dence the greater the deflection angle, and ticle). Finally, the repulsive force resulting 10 a nucleus (and thus suffer a deflection) was er er than ~10° would become zero, which was gold sheets, gold alpha and the to particles detect o including backscattering, by means of a sidered essentially that sidered theessentially was deflection the sheets. And then he formulated his theory of l positions, namely, in front of, to both sides, if the sheet was transparent to them. In fact, ident ident alpha particles passed through the gold observe observe in a darkened room designed for that rticle rticle (of double positive electric charge) and r being supposed to be concentrated in a tiny ticle ticle and the atomic nucleus, both considered m, m, in a spherical region of radius comparable le to explain those particles backscattered. backscattered. particles those explain to le number number of alpha particles deflected at a given e. To the ofastonishment Marsden everybody, angles angles much greater than about 10°. Marsden,

., alpha particles were being deflected at angles 5 atomic atomic nucleus i.e

they were actually being "reflected" by a very thi a very by "reflected" being actually werethey i.e., Rutherford Rutherford and Geiger were able to demonstrate that At the end of to1910 managed thRutherford glimpse Rutherford’s Rutherford’s hypothesis of a scattering center, the average average distance between the atomic nuclei as seen that the chance for an alpha particle to pass near quite small, even rarer would be an incidence direc backscattering. Although small, these chances were, was observed (recall that Marsden had recorded a si The nearer the nucleus was to the direction of inci these events would be rarer until the extreme situa original direction [Fig.3]. direction original alpha particles when they struck on very thin gold alpha according to particle scattering which he con result of a single interaction between the alpha pa the positive charge of the atom, but now this latte central region of the atom which he called 4. Rutherford and the atomic nucleus4. Rutherford and atomic the instead instead ofmica using sheets, usemade of thin very Discovering the Atomic Nucleus task of searching for alpha particles deflected at he used a zinc sulfide screen that could move in al and even in the region near the incident beam, behi telescope focused on the screen it was possible to the individual points of light which indicated the angle of from thescattering direction of incidenc found that a few alpha particles were thrown backwa was near the region of the incident beam, for the gold leaf used by Rutherford’s group it was with the observation that the great majority of inc sheet without suffering appreciable deflections as deflections deflections of alpha particles at large angles, unique and als rare impact between these particles and orbiting orbiting around the nucleus, like a planetary syste to the size of the atom (estimated at the time as ~ atomic atomic nucleus, considered the scattering almost all center, the mass of the atom w was to be concentra amounts to nearly fifty times that of the alpha par from the positive electric charges of the alpha par centers. charge the between distance in the model as point charges, followed the law of much greater than 90°, than greater 90°, much model model (the one known as "plum pudding") was accepte number of alpha particles deflected at angles great in contradiction with observations. Thomson's model not explain conclusively the deviation of alpha par one of the incident beam, so little would be possib be so little would beam, incident the one of PoS(XXXIV BWNP)024 (the matter ver ver those of Odilon Tavares atomic atomic nucleus ok ok a series of experiments on electric charge of the atom and 1911, in Literary and Philosophical solutely solutely correct until present days. ealized ealized that as a matter of fact the atom o explain quantitatively the observations of different velocities (from sources of ar ar atom to explain the scattering of alpha llic llic foils of silver, tin, copper, gold, and grader), and scattering angles that covered to lose velocity gradually thus following a hich hich he named the incident alpha particle and the target n the experimental uncertainties. Only small small Only uncertainties. experimental n the rits of the conclusion regarding the presence the presence rits regarding of the conclusion fact that actually does not occur. These ery close to the nucleus, and, especially iii) (<2°), (<2°), ii) it did not take into account other for it was in complete agreement with the etail etail in the following May in volume 21 of ) the rarer the large deflection angles angles deflection large the rarerthe ) great as 100 million tons per cm³). cm³). per tons million as 100 great positive electric charge and mass should be odel contained serious limitations such as i) it such asodel it i) contained limitations serious r etal The foils. of theconception nucleus of the lt of the emission of radiation by the electrons d’s d’s theory of alpha particle scattering with the

6 (backscattering). (backscattering).

. . What made the nuclear model of the atom prevail o The better the “aim” (low value of (low value “aim” the The better a very tiny central region containing the positive Fig. 3 During During the years 1911-13 Geiger and Marsden underto Presented Presented to public for the first time on March 7, i.e. density of the atomic nucleus could be estimated as estimated be could nucleus the of atomic density practically practically (99.97%) its entire mass held up itself m was realized that it the Afterwards, experiment. failed to explain deflections at very small angles forces, besides the electrostatic repulsion between concentrated in an ultra-minuscule central region w nucleus, which could eventually act at distances v the inconsistency with atomic stability, as a resu should be in fact a great empty space, and that its in their movements around the nucleus, leading them spiral path until finally falling in the nucleus, a of a nucleus in the Rutherford, differently from Thomson atom, and Nagaoka, r an image which remains ab limitations, however, did in no way diminish me no the way in did however, limitations, Society Society of Manchester, Rutherford's theory of nucle particles by thin metallic foils was published in d Philosophical Magazine on theof alpha scattering particles thinby very m atom, Thomson Thomson (1903) and Nagaoka (1904) was its ability t Discovering the Atomic Nucleus the large interval 5°- 150°. In all cases Rutherfor withi confirmed resulted atom ofhypothesis nuclear scattering scattering of alpha particles. They used aluminum thin of meta different thicknesses, alpha radium and radon, and particles thin mica sheets as energy de PoS(XXXIV BWNP)024 a a , the , the N/S which hits , Rutherford E Odilon Tavares Ze from the scattering R is expressed in g/cm³, is in g/cm³, expressed ρ results proportional to proportional results ets inetic energy inetic energy N/S

to Rutherford’s formula for the the for formula to Rutherford’s was done was to count the number , and nuclear charge number is directly related to the ratio ratio the to related is directly number A tering center and the detector; detector; the center and tering ident alpha particles; particles; alpha ident it should be very thin); thin); be should very it he quantity quantity he aluminum aluminum targets or when using high speed placed at a distance r area unit of the screen, scintillation S bout 1°– 1°– 2°, bout and where

7 from the original direction of incidence by using , atomic mass θ ρ /2). θ ( 4 in MeV (million electron volt), and and volt), electron in MeV(million , density

t α E .

in g, g, in A scattering of alpha particles by thin metallic targ thin by metallic particles alpha of scattering represents the total number of alpha particles of k Q Q of the target material); material); target the of in cm², cm², in S /A ρ

Physical and geometrical quantities which enter in enter which quantities and geometrical Physical the inverse of the quantity sin theof quantity inversethe the inverse square of the kinetic energy ofinc the energy the kinetic square of inversethe the square of the electric charge of the nucleus; the of nucleus; charge electric the of the square the inverse square of the distance between the scat the between the distance square of inversethe the thickness of the metallic plate (provided that (provided plate metallic the of thickness the the number of nuclei per unit volume (because this (because volume per unit nuclei ofthe number In In the experiments to test Rutherford’s model what in cm, in cm,

R R Fig. 4 vi) iv) v) iii) ii) Rutherford’s celebrated formula (1911) shows that t shows (1911) formula celebrated Rutherford’s i) and t Discovering the Atomic Nucleus deviations from the theory were noted in cases for alpha particles. particles. alpha which is valid for scattering than angles whicha is greater scattering valid for center center [Fig.4]. If ZnS ZnS scintillation screen detector of known area a thin metallic plate of thickness of alpha particles scattered at a given angle found for the number of particles alpha pe detected result PoS(XXXIV BWNP)024 , Dover Isaac Isaac Newton Odilon Tavares sir sir Discoveries in Chemistry, later Baron .html .html e atomic nucleus in 1911 in nucleus atomic e pioneering achievements in nuclear uential uential persons upon the destinies of reme” reme” — is, according to the American Radiations Radiations from Radioactive Substances, side side the famous physicist bey in . bey hn hn Simmons, among the 20 most important ince of Nelson (New Zealand), the son the Zealand), (New ofince Nelson me. me. Without any doubt, a beautiful path for a

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From From X-Rays to Quarks: Modern Physicists and their — winner of the 1908 Nobel Prize : www.nobel-winners.com/Chemistry/ernest_rutherford

Ernest Rutherford (1871-1937), the discoverer of th of the discoverer (1871-1937), Ernest Rutherford

sir sir

Having Having a character that impressed everyone, for his Internet Rutherford Rutherford of Nelson, and considered “scientist sup astrophysicist astrophysicist Michael Hart, among the 60 most infl humanity, humanity, and to the American science journalist Jo scientists of all fields of knowledge and of all ti inrural the prov Grove, in Spring was born who boy of a wheelwright and a schoolteacher, was buried be science,

Bibliography [1] E. Rutherford, J. Chadwick, and C. D. Ellis, Cambridge University Press, Cambridge 1930. Cambridge Press, University Cambridge [2] E. Segrè, and the naturalist Charles Darwin in Westminster Ab Westminster in Darwin Charles naturalist and the

Discovering the Atomic Nucleus 5. Epilogue

Publications, 2007. 2007. Publications, [3]