|High Energy Physics Libraries Webzine|
HEP Libraries Webzine
Issue 7 / April 2003
Museo Storico della Fisica e Centro Studi e Ricerche "Enrico Fermi", Rome, Italy.
We trace Fermi's working years in Rome at the Istituto Fisico
("Physics Institute") on Via Panisperna and describe the new Centro
Studi e Ricerche 'Enrico Fermi' (Enrico Fermi Centre for Study and
Research) established to honour his memory and achievements.
The first Theoretical Physics chair in Italy was created in 1926. This was the fundamental step in the process of modernizing Italian physics, and taking it out of a long period of torpor.
During his scientific training, Fermi was able to learn the foundations of theoretical physics from the books of Scuola Normale and Pisa University Libraries. At that time, Italian academic teaching did not include theoretical physics in its modern meaning. In addition, Fermi was able to bring himself up to date on recent research through the most important international scientific journals, in particular those from Germany, the Netherlands and Denmark where developments were exceptionally lively. Fermi's first works are mainly on relativity. Of particular importance is the paper "Sopra i fenomeni che avvengono in vicinanza di una linea oraria" (On the phenomena occurring near a world line) (1). Discussing in a general and systematic way problems which involve gravitational and electromagnetic fields, he introduces a system of spatio-temporal coordinates (the so-called "Fermi coordinates"), effective in describing the temporal course of phenomena which happen in a small area of space. However, it was clear to him that quantum theory had, at least in the short term, many more possibilities of application than relativity. Two grants allowed Fermi to be in touch with the circles of active physicists: he stayed for a while in 1923 in Gottingen, at Max Born's Institute, and in 1924 in Leiden, at Paul Ehrenfest's Institute. During that period, Fermi approached the problems which led to the discovery of quantum statistics, named after him. Back from the Netherlands in December 1924, Fermi taught Mathematical Physics at Florence University.
In January 1925, Pauli formulated the exclusion principle. This, the exclusion of the presence of two electrons with the same quantum number (that is in the same state), would be the key for Fermi to overcome difficulties in the quantization of identical particle systems. Fermi applied the idea of generalizing the exclusion principle to any identical particle system, like a perfect gas, and managed in the autumn of 1925 to formulate quantum statistics following particles (such as electrons and protons) which obey the Pauli principle, and then called "fermions".
Fermi's statistics were presented at the Accademia dei Lincei in February 1926, "Sulla quantizzazione del gas perfetto monoatomico" (On the quantization of the perfect monoatomic gas) (2): his first fundamental contribution to physics.
Orso Mario Corbino, Director of the Physics Institute of Rome University, who believed in making it a modern research centre, strongly supported Fermi and, in 1926, made it possible to create a Chair of Theoretical Physics in Italy. At the end of the year, Fermi won the competition, and became a Full Professor at the age of twenty-five.
Fermi was already known as being versatile and capable in many different fields of physics, both theoretical and experimental. Corbino asked Franco Rasetti, who had been in Pisa with Fermi, to join them to help with the activity at the Institute. Then, Emilio Segrè, Edoardo Amaldi, Ettore Majorana and later Bruno Pontecorvo started studying with Fermi.
Fermi devoted much of his time to teaching, both in regular courses
and seminars, in which he talked about problems he was himself working
on. The Institute was equipped with good spectroscopic instrumentation.
Until the end of the 1920s, Fermi and his group focused their research
on atomic and molecular spectroscopy, which at that time was the most
effective method to investigate the structure of matter. Various spectrographs
from Hilger of London were used in the experiments. This instrumentation
is now at the Physics Museum of Rome University, where some photographic
plates with the most significant spectra are also kept
In the meantime, Fermi concentrated his attention on the development of quantum mechanics. During 1926 and 1927, he published some papers which show how fully he had mastered the new theory. The first important work by Fermi in Rome was an application of the new quantum statistics to the atom. On 4 December 1927 he published the paper "Un metodo statistico per la determinazione di alcune proprietà dell'atomo" (A statistical method for the determination of some atomic properties) (3): the model's basic assumption (called Thomas-Fermi, because independently formulated by L. H. Thomas in the "Calculation of Atomic Fields" (4)) considers the electron system of the atom as a fermion gas to absolute zero, bonded to the nucleus by Coulomb attraction.
During the summer of the same year, he wrote a textbook of modern physics "Introduzione alla Fisica Atomica"(Introduction to Atomic Physics) (5), which filled a gap in Italian academic teaching. In September, he was amongst the protagonists at the Physics International Conference in Como together with Bohr, Born, Heisenberg, Lorentz, Pauli, Planck, Rutherford, and Sommerfeld. In his talk, Arnold Sommerfeld showed the importance of Fermi statistics for the understanding of electron behaviour in metals, inexplicable according to classical theories. During the winter of 1928 and the beginning of 1929, on investigating Dirac's papers, in which in 1927 he extended quantization's rules of mechanical systems to electromagnetic fields (6), Fermi decided to work out the theory again "Sopra l'elettrodinamica quantistica"(On quantum electrodynamics) (7). The most relevant aspect of Fermi's paper on electrodynamics is the reissuing of the theory in a simpler mathematical form, which better allowed the understanding of the physics concepts. Here is something profound that is peculiar to Fermi's style, a very distinctive way of his conception of physics: in Hans A. Bethe's opinion, the 1932 paper is "an example of simplicity in a difficult field which I think is unsurpassed" (8).
At the end of the 1920s, Fermi was convinced that atomic physics was going to end its tasks; physics should now explore the nucleus. The new trend was confirmed in 1931 by the decision to organize in Rome an International Congress of Nuclear Physics, with Guglielmo Marcon as chairman (9). In the transition phase from the study of the atom to the study of the nucleus in the Rome laboratory, spectroscopic activity was going on, together with the new research field and the acquisition of the necessary experimental techniques. Nuclear physics was progressing rapidly. In 1932, James Chadwick discovered the neutron, and soon after Heisenberg and Majorana developed a theory of forces binding neutrons and protons in the nucleus. Among the unresolved problems which emerged at the Solvay Congress in 1933 (10), the scientific community had to face a hard one: beta radioactivity, a phenomenon consisting of an electron's emission at high energy from some nuclei. Two months after the Congress, Fermi elaborated a complete and coherent theory of beta decay and published a paper in December 1933, "Tentativo di una teoria dell'emissione dei raggi beta" (Tentative theory of beta ray emission), in La Ricerca Scientifica (11). In this work, he interprets beta decay as the transformation of a neutron into a proton and the simultaneous emission of an electron and a neutrino, a neutrino which escapes observation and that carries out with it momentum and energy. Fermi adopted the nuclear model with protons and neutrons only, and, because he did not want to renounce the conservation of energy, accepted Pauli's proposal, of the existence of a new particle, the neutrino, with no electrical charge and with a mass as high as the electron mass or less. He admitted therefore, that a fraction of the energy liberated from beta decay, cannot be observed. The emission theory of the electron and neutrino was built by Fermi by analogy to the emission of a photon from an excited atom in the irradiation process, according to which photons are created during their emission and are destroyed in the absorption.
According to quantum electrodynamics, in the emission phenomenon of a photon from an atom, the photon does not exist before the emission, but is created in the transition of an electron from one state to another, to which corresponds a lower energy level (in the reverse process, an electron absorbs a photon, switching from a certain energy level to a higher level). The neutron and proton could be considered as two different quantum states of the same particle; the electron and the neutrino emitted in beta decay of a nucleus are not pre-existent to decay, but are created simultaneously with the transition of a neutron into a proton (12).
In the middle of January 1934 Irène Curie and Frédéric Joliot communicated the discovery of artificial radioactivity, by alpha particles (13). Until then, various nuclear transmutations had been observed, but all with final products represented by stable nuclides. Joliot and Curie for the first time succeeded in making evident the transformation of a stable nucleus into a radioactive nucleus. Curie and Joliot were able to activate light elements only: it was evident that alpha particles were not fit for activating heavy elements, because, having a positive electric charge equal to two times the electronic one, they suffer the repulsive action of the Coulomb field surrounding nuclei against which they are thrown.
After hearing the news of the discovery of artificial radioactivity by Joliot and Curie, Fermi was convinced that, after a wait of several years in Rome, it was finally possible to start promising experimental research in nuclear physics.
During those years Enrico Fermi and his collaborators succeeded in making the laboratory in Via Panisperna, a well equipped one for nuclear research. Very important for the new trend of the Institute was the period from autumn 1932 to the end of 1933. In particular, Franco Rasetti, back from Berlin where he worked in the Kaiser Wilhelm Institut für Chemie with Lise Meitner, built Geiger-Müller counters, ionization chambers, and a Wilson cloud chamber, and, using a radium ampoule, built a polonium-beryllium neutron source (14).
Fermi soon recognized the possible advantages offered by neutrons to induce artificial radioactivity with no electric charge, neutrons seemed to be an efficient instrument for producing radioactive nuclides. The uncertainty in the realization of Fermi's project was in the intensity of the neutron source availability. Neutrons are in fact emitted as nuclear reaction products only, with a low performance.
At the beginning of 1934 in Via Panisperna, new research was starting (15). Various substances were irradiated by neutrons emitted by the polonium-beryllium source. First attempts were negative: none of the bombarded samples, examined by a Geiger-Muller counter, showed any appreciable activity. Fermi was sure this initial failure was due to an insufficient source intensity. He also realized that to induce artificial radioactivity with neutrons he could use a more intense radon-beryllium source, as well, because gamma radiation emitted by this type of source does not interfere with the observation of a delayed effect. He thus realized a radon-beryllium source and started irradiating elements systematically. A significant part of the instrumentation of the Istituto Fisico di Via Panisperna is kept in the Physics Museum of "La Sapienza" University, in Rome (http://www.phys.uniroma1.it/DOCS/MUSEO/home.htm ); twelve of the radon-beryllium sources used by Fermi's group are at the Domus Galilaeana in Pisa (http://www.domusgalilaeana.it/ ) (16). First attempts were negative: the elements irradiated by neutrons did not show any radioactivity. Nevertheless, Fermi was not discouraged and continued the research. When he arrived at aluminium and fluorine, he finally observed in the irradiated samples a small but significant beta activity. On 25 March 1934, Fermi announced his discovery with a paper published in La Ricerca Scientifica, "Radioattività indotta da bombardamento di neutroni" (Radioactivity induced by neutron bombardment) (17).
The laboratory in Rome was in a state of frantic activity: Fermi coordinated accurate and systematic research, participating in every stage of the work, from measurements to chemical manipulations. Between April and July 1934, 62 elements were bombarded by neutrons. This series of experiments identified, through the different half-lives, 50 new radioactive nuclides. Of these, 16 were identified by radiochemistry techniques (18). From these results, Fermi was able to identify which nuclear reactions were induced by neutrons.
Beginning in September 1934, something anomalous happened in the laboratory: the reproduction of results seemed to go through a crisis. Around the middle of the month, the group, joined by the newly graduated Bruno Pontecorvo, started a systematic study in order to set a quantitative range of activity induced by neutrons in different elements.
Until summer 1934, in fact, Fermi's group distinguished activities in an exclusively qualitative way, in classifying them as strong, medium and weak. In order to fix irradiation conditions in the most favourable way for obtaining reproducible results, they started studying the activity of silver with a half-life of 2.3 minutes. Difficulties soon arose: silver's activity fluctuated noticeably as the experiment was repeated, apparently under the same conditions, with the same procedures. Pontecorvo noticed that induced activity in the silver cylinder depended on the bench used for executing the irradiation. A wooden bench seemed to have "magic properties": the sample irradiated on this bench acquired more radioactivity than when activated on a marble table placed in the same room. In order to understand this situation, a systematic search was carried out in the laboratory. Because the activation seemed to depend on materials close to the source and the silver cylinder during irradiation, a neutron source and the sample were placed inside a "castelletto", a box built from small lead bricks. With an increase in the distance between the source and the sample, inside the box, activation decreased more slowly with respect to what happened outside. The effect was attributed to the scattering of neutrons from the lead of the box's walls. To verify this conclusion, it was decided to irradiate the silver sample by inserting a lead wedge of the same thickness as the walls of the box. The lead wedge necessary to the measurements was ready on 20 October. Two days after, Fermi started tests personally. With him there was only Enrico Persico, because others were busy with faculty duties. That morning Fermi, before putting the wedge between the neutron source and the silver sample to be activated, "suddenly" decided to test the effect of a light element and put in a piece of paraffin 4 cm thick (19).
In 1952, talking to Subrahmanyan Chandrasekhar, Fermi remembered those days in October 1934: "I will tell you how I came to make the discovery which I suppose is the most important one I have made. We were working very hard on the neutron induced radioactivity and the results we were obtaining made no sense. One day, as I came to the laboratory, it occurred to me that I should examine the effect of placing a piece of lead before the incident neutrons. And instead of my usual custom, I took great pains to have the piece of lead precisely machined. I was clearly dissatisfied with something: I tried every "excuse" to postpone putting the piece of lead in its place. When finally, with some reluctance, I was going to put it in its place, I said to myself: "No! I do not want this piece of lead here; what I want is a piece of paraffin". It was just like that: with no advanced warning, no conscious, prior, reasoning. I immediately took some odd piece of paraffin I could put my hands on and placed it where the piece of lead was to have been". (20)
The effect of paraffin was stupefying: induced activity in silver
rose beyond 50%. During lunch time, Fermi found the explanation of
the strange behavior of the filtered neutrons: the neutrons were slowed
down by a large number of elastic collisions against the protons present
in the paraffin, and in this way became more effective. In the afternoon
of the same day, 22 October, the experiment was repeated in the garden
fountain of the Institute (Fig. 2). The source
and the sample were immersed in water to test the effect of a large quantity
of hydrogenous substance, different to paraffin. Results were in perfect
accordance with the morning's experiments, confirming Fermi's hypothesis.
Later in the evening of this long, exciting day, Fermi, together with
his collaborators, wrote the announcement of the discovery: "Azione di
sostanze idrogenate sulla radioattività provocata da neutroni"
(Influence of hydrogenous substances on the radioactivity produced
by neutrons) (21).
In the Rome Institute the importance of the discovery was quickly understood. Corbino suggested filing a patent request which was deposited by Fermi on 26 October 1934: "Metodo per accrescere il rendimento dei procedimenti per la produzione di radioattività artificiali mediante il bombardamento con neutroni" (A method for increasing performance in the process of producing artificial radioactivity, by neutron bombardment).
This discovery opened a new phase in the Institute's research in Via Panisperna, completely devoted to the study of slow neutrons and to neutron behaviour in motion through hydrogenous substances. After formulating the slow neutron theory, in the autumn of 1935, Fermi started a systematic study of absorption and diffusion of slow neutrons in different elements which continued until May 1936, and which led to the identification of the existence of absorption bands; that is, many nuclides for which neutrons with kinetic energy falling in some characteristic bands of target nuclei, are strongly absorbed (22). Research on slow neutrons would be a fundamental step in the use of nuclear energy through chain reactions.
At the end of 1936, the Physics Institute moved from Via Panisperna to the new University City; Corbino died soon after. Fermi was conscious that to maintain the level of excellence of neutron physics acquired, it was necessary to build a particle accelerator to replace traditional radon-beryllium sources; targets made from weak elements and irradiated by accelerated deuterons or protons could in fact produce a neutron source, stronger than traditional ones. Just to practise with accelerators and whilst waiting for the necessary funds to build a powerful machine, a small 200 keV accelerator was realized, in Rome in June 1937 (23). During the summer, Ernest O. Lawrence gave Fermi directions for building "an inexpensive cyclotron". In the same period, Marconi, who as President of the Consiglio Nazionale delle Ricerche (Italian National Research Council) always supported Fermi's scientific activity, died. This made Fermi's isolation worse. In May 1938, the request to finance a nuclear physics institute was rejected due to lack of funds (24). In the meantime, the political situation was deteriorating, so in September the family decided to leave Italy. Fermi got a leave of absence to teach a course at Columbia University in New York. On 10 November he received the official announcement of the Nobel Prize by the Royal Swedish Academy. When the family reached Stockholm in December for the ceremony (December 10, 1938), they were determined not to return to Italy; from Sweden they left directly for the United States.
On 2 January 1939, with his arrival in New York, Fermi's extraordinary Italian season finally ended.
Following Fermi's legacy, the Centre's activity is concentrated on projects, which are already in action, with special attention to "interdisciplinary" studies:
We are waiting for the Museum to be ready and the Library of the Istituto Fisico on Via Panisperna to be
restored. Emilio Segrè, who was a collaborator of Fermi's in
Rome, remembers in his biography (25) that the Istituto
had an excellent library. At the end of the Twenties, he remarks, "Fermi's
knowledge and interests embraced all of physics, and he diligently
read several journals […] From as early 1928 Fermi made little use of
books; Laska's collection of mathematical formulas and Landolt Börnstein's tables
of physical constants were almost the only reference books he had in
his office. When he needed a complicated equation from a book in the
library, Fermi would often propose a wager, saying he would derive the
equation before we could find it in a book; and usually he won. The only
treatise that I know he read after he came to Rome was
Weyl's Gruppentheorie und Quantenmechanik" (26). At Fermi's disposal on Via Panisperna's Library
were the most important international physics journals such as Proceedings
of the Royal Society, Nature, Physical Review, Comptes
Rendus de l'Académie des Sciences, Journal de Physique,
Zeitschrift für Physik.
(1) Sopra i fenomeni che avvengono in vicinanza di una linea oraria, Rend. Lincei 31 (I), (1922), pp. 21-23, 51-52, 101-103. See also: E. Fermi, Note e Memorie (F. N. M.), 2 vol., E. Segrè et al. Editors, Accademia Nazionale dei Lincei and University of Chicago Press, Rome and Chicago, vol. I, 1962, vol. II, 1965 (vol. I, pp. 17-23).
(7) E. Fermi, Sopra l'elettrodinamica quantistica, Rend. Lincei 5 (1929), 881-887; F.N.M. vol. I, 305-310. Fermi resumed all his work on electrodynamics (Sopra l'elettrodinamica quantistica, cit.; Sulla teoria quantistica delle frange di interferenza, Rend. Lincei 9 (1929), 72-77 - Il Nuovo Cimento 7 (1930), 153-158 - F.N.M. vol. I, 314-318; Sopra l'elettrodinamica quantistica, Rend. Lincei 12 (1930), 431-435 - F.N.M. vol. I, 386-390; Le masse elettromagnetiche nella elettrodinamica quantistica, Il Nuovo Cimento 8 (1931), 121-132 - F.N.M. vol. I, 391-400) first in April 1929, at the Institut Henri Poincaré in Paris, then, in 1930, at a summer school at Ann Arbor, Michigan University. Contents of these lectures are in: "La théorie du rayonnement", Ann. de l'Inst. H. Poincaré 1 (1931), 53-74 and "Quantum theory of radiation", Rev. Mod. Phys. 4 (1932), 87-132 (F. N. M. vol. I, 401-445).
(10) Structure et propriétés des noyaux atomiques. Rapports et discussions du 7me Conseil de Physique tenu a Bruxelles du 22 au 29 octobre 1933 sous les auspices de l'Institut International de Physique Solvay, edited by Commission administrative de l'Institut, Gauthier-Villars, Paris, 1934.
(12) Fermi, in applying quantum field theory to beta radioactivity, calculates the mean life of a decaying nucleus, linking it to the kinematic process and to the intensity of the interaction responsible for the disintegration. In comparing the value of the mean life, theoretically calculated, with the measured one, he finds that such an interaction is much less strong than the electromagnetic interaction responsible for atomic transitions; for this reason it will be later called "weak interaction" or "fermian" (the weak interaction will later prove itself to be responsible not only for neutron decay, but also for the most elementary particles). Excluding the presence of electrons in the atomic nucleus, Fermi with his work put an end to the debate on the nature of the neutron, which could be accepted as an elementary particle (and which was valid until the quark model, many years later).
(15) Fermi's neutron research is analysed in: L. Carbonari, Enrico Fermi e la scoperta della radioattività indotta dai neutroni, Thesis, Department of Physics, Università "La Sapienza", Rome, 2001 (unpublished).
(16) See: M. Leone, N. Robotti and C. A. Segnini, Fermi Archives at the Domus Galilaeana in Pisa, Physis 37 (2000), 501-533; since 1956 the Domus Galileana has preserved original documents relative to the scientific activity carried out by Fermi during his life in Italy.
(17) E. Fermi, Radioattività indotta da bombardamento di neutroni, La Ricerca scientifica 5 (1) (1934), 283 (F. N. M. vol. I, 645-646); Radioactivity Induced by Neutron Bombardment, F. N. M. vol. I, 674-675.
(19) See: L. Carbonari and F. Sebastiani, La scoperta dell'azione di sostanze idrogenate sulla radioattività provocata da neutroni: nota in margine al ritrovamento di un cimelio fermiano, 2002, to be published on Physis.
(21) E. Fermi, E. Amaldi, B. Pontecorvo, F. Rasetti, E. Segrè, Azione di sostanze idrogenate sulla radioattività provocata da neutroni, La Ricerca Scientifica 5 (2) (1934), 282-283 (F. N. M. vol. I., 757-758); Influence of Hydrogenous Substances on the Radioactivity Produced by Neutrons, F. N. M. vol. I, 761-762.
(22) E. Amaldi and E. Fermi, Sopra l'assorbimento e la diffusione dei neutroni lenti, La Ricerca Scientifica 7 (1) (1936), 454-503 (F. N. M. vol. I, 841-891); On the Absorption and the Diffusion of Slow Neutrons, Phys. Rev. 50 (1936), 899-928 (F. N. M. vol. I, 892-942). E. Fermi, Sul moto dei neutroni nelle sostanze idrogenate, La Ricerca Scientifica 7 (2) (1936), 13-52 (F. N. M. vol. I, 943-979).
Memorial Symposium in Honor of Enrico Fermi, Reviews of Modern Physics 27 (1955), 249-275.
Symposium Dedicated to Enrico Fermi, Accademia Nazionale dei Lincei, Rome, 1993.
E. Amaldi, From the Discovery of the Neutron to the Discovery of Nuclear Fission, Physics Reports 111 (1984), 1-322.
Structure et propriétés des noyaux atomiques. Rapports et discussions du 7me Conseil de Physique tenu a Bruxelles du 22 au 29 octobre 1933 sous les auspices de l'Institut International de Physique Solvay, Commission administrative de l'Institut Editor, Gauthier-Villars, Paris, 1934.
Proceedings of theTenth Congress of History of Science (Ithaca, N. Y.), Hermann, Paris, 1964.
L. Carbonari, Enrico Fermi e la scoperta della radioattività indotta dai neutroni, Thesis, Department of Physics, Università "La Sapienza", Rome, 2001 (unpublished).
D. Cooper, Enrico Fermi and the Revolutions of Modern Physics, Oxford University Press, New York, 1999.
E. Fermi, Note e Memorie (Collected Papers) 2 vol., E. Segrè et al. Editors, Accademia Nazionale dei Lincei and University of Chicago Press, Rome and Chicago, 1962 (vol. I) and 1965 (vol. II).
E. Fermi, Introduzione alla fisica atomica, Zanichelli, Bologna, 1928.
E. Fermi, Molecole e cristalli, Zanichelli, Bologna, 1934; Molecules, Crystals and Quantum Statistics, transl., Benjamin, New York, 1966.
L. Fermi, Atoms in the Family, University of Chicago Press, Chicago, 1954.
O. R. Frisch, The Discovery of Fission, Physics Today 20, no. 11 (1967), 43-48.
G. Holton, The Scientific Imagination. Case studies, Cambridge University Press, Cambridge, 1978.
A. Pais, Inward Bound, Oxford University Press, New York, 1986.
E. Persico, Souvenir de Enrico Fermi, Scientia 90 (1955), 316-324.
B. M. Pontecorvo and V. N. Pokrovskij, Enriko Fermi v vospominanijakh ucenikov i druzej [Enrico Fermi remembered by his students and friends], Nauka, Moscow, 1972.
F. Rasetti, Biographical Notes and Scientific Work of Franco Rasetti, typed copy unpublished kept in the Amaldi Archive at Dipartimento di fisica dell'Università "La Sapienza" in Rome (pos. 1E/22), edited in 1958 and revised 1968.
B. J. Reeves, Italian Physicistst and TheirInstitutions,1861-1911, PhD Thesis, Harvard University, 1980 (unpublished) (A copy is kept in the Amaldi Archive at the Dipartimento di Fisica dell'Università "La Sapienza", in Rome.).
E. Segrè, Enrico Fermi Physicist, The University of Chicago Press, Chicago, 1970.
http://www.centrofermi.it/ The Museo Storico della Fisica e Centro Studi e Ricerche "Enrico Fermi"
http://www.osti.gov/accomplishments/fermi.html (commemorating the centennial of his birth)
Museo Storico della Fisica e Centro Studi e Ricerche "Enrico Fermi"
Museo Storico della Fisica e Centro Studi e Ricerche "Enrico Fermi"
Istituto Nazionale di Fisica Nucleare
Laboratori Nazionali di Frascati (Rome)