After a brief introduction on the important role played by Bruno Pontecorvo in my scientific life, mainly involved in neutrino physics, I will shortly report the subjects where I could particularly appreciate his greatness which particularly impressed me. I will therefore consider in more details double beta decay especially in its neutrinoless channel were I was and am particularly involved, weak neutral currents, solar neutrinos, neutrino oscillation, determination of the neutrino mass and one unusual research of mine to which Bruno was particularly interested

Fiorini, E.

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Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2014-11763-9Colloquia: Pontecorvo100IL NUOVO CIMENTO Vol. 37 C, N. 3 Maggio-Giugno 2014Neutrino physics and my debts to BrunoE. FioriniDipartimento di Fisica G.P.S. Occhialini, Universita` di Milano-Bicocca,and INFN, Sezione di Milano-Bicocca - Piazza della Scienza 3, 20126 Milan, ItalySummary. — After a brief introduction on the important role played by BrunoPontecorvo in my scientific life, mainly involved in neutrino physics, I will shortlyreport the subjects where I could particularly appreciate his greatness which par-ticularly impressed me. I will therefore consider in more details double beta decayespecially in its neutrinoless channel were I was and am particularly involved, weakneutral currents, solar neutrinos, neutrino oscillation, determination of the neutrinomass and one unusual research of mine to which Bruno was particularly interested.1. – IntroductionI first met Bruno Pontecorvo in an international conference in Kiev were I attendedwith considerable anxiety because I was very young. I had then proposed an experimenton double beta decay with a Germanium diode and carried out first measurements withthis technique. I was incredibly flattered and honoured when the famous professor Pon-tecorvo asked of me to discuss the relationship of my results with the prediction of anew brilliant idea which he had just formulated. We started a discussion which becameimmediately quite animated because Bruno did the best he could to overcome my shynesswhich is normally unusual, but very strong in that case for the presence of such an im-portant physicist. This was always the same for the rest of his life where I had the greathonour of his friendship and the continuous theoretical and not only theoretical supportof my activity in neutrino physics. Bruno could not leave then the eastern countries,but we met many times in the Soviet Union and on the Balaton lake in Hungary foran international neutrino workshops. I apologize if in this talk I give some relevance tosubjects where I was involved in connection with Bruno. He was in all of them a sourceof inspiration and of indispensable advise.c© Societa` Italiana di Fisica 4546 E. FIORINIFig. 1. – The ββ transition of 76Ge Fig. 2. – Double beta decay2. – Double beta decayThis fascinating process (fig.1), suggested by the great physicist Maria GoeppertMaier in 1934, consists in the transition of the nucleus [A,Z] to its isobar [A,Z+2]. andcan be detected when the single beta decay to [A+1] is energetically forbidden or at leaststrongly hindered .This transition can occur to ground or to excited states of the granddaughter nucleus(fig. 2)At the time of the Kiev conference there were already many experiments on doublebeta decay , but only two were based on the principle of using a detector made of thesame material which was also a source for this process. The one by der Mateosian andGoldhaber was using a CaF2 scintillator to detect the decay 48Ca→48 Ti+2e−+(2ν¯e).The one by Fiorini et al was based on the decay 76Ge→76 Xe+2e−+(2ν¯e) on whichmany experiments are being carried out or planned today. Double beta decay was notfashionable then , but Bruno was particularly interested in my experiment because hehad a brilliant idea: that neutrinoless double beta decay was a direct ΔL=2 transitionsimilar to ΔS =2. Unfortunately this suggestion was not enforced by experimental resultsand I would like to report a comment by another great physicist Madam Wu who said:“There are many experiments on double beta decay of poor quality , the only exceptionbeing that by Fiorini et al (wait a second!). The best however is the one by Wu andcoworkers. As far as the suggestion by prof. Pontecorvo that neutrinoless double betadecay is a direct ΔL=2 transition, after the recent experimental results it lost part of itsbeauty”. Bruno said to me later: She was this time even too good with such a poor paperof mine! . I disagree because his idea was brilliant, but definitely not the best amongthe great ones of Bruno and not supported experimentally.As it is well known double beta decay can occur in three channels:1. (A,Z)→ (A,Z + 2) + 2e− + 2ν¯e2. (A,Z)→ (A,Z + 2) + 2e− + 1, 2...χ’s3. (A,Z)→ (A,Z + 2) + 2e−Where χ is a massless Goldston boson. The first process is the only one whichpreserves the lepton number and it has been detected in eleven nuclei, in some case alsoto excited levels. The most experimentally appealing is process 3, called neutrinolessdouble beta decay, rather improperly since also process 2 is neutrinoless. In neutrinolessdouble beta decay, since the recoiling energy of the nucleus is negligible, the two electronsNEUTRINO PHYSICS AND MY DEBTS TO BRUNO 47Table I. – Present results on neutrinoless DBD.share the total energy and in their sum energy spectrum a peak appears correspondingto the total transition energy. Other possible ΔL=2 lepton violating processes like twopositron decay, positron decay plus electron capture and two electron capture will not beconsidered here. The present results are shown in Table I together with the correspondinglimits on the effective neutrino mass, where the uncertainties on transition nuclear matrixelements are considered . It can be seen that the only evidence for neutrinoless doubledecay by of Klapdor et al is in contrast with Gerda I results on the same nucleus bothwith the first paper and, being at 90 % confidence level, also with the second one. In anindirect way this is also true for the latest results on 136Xe.3. – Discovery of weak neutral currentsAround the end of the sixties I was involved in searches on neutrino physics with bub-ble chambers. Our friend, the late Carlo Franzinetti, suggested to the Italian Institute forNuclear Physics (INFN) the construction of a massive bubble chamber with heavy liquidas a competitor to hydrogen bubble chambers for the easier construction and lower price.The Institute was very favourable, but the project could not be realized in Italy due tolack of funds. Fortunately another great physicist and friend, Andre Lagarrigue, was ableto construct the famous chamber Gargamelle and generously accepted the participationof my country under my direction already during construction.The participation of the Italian group included also Carlo, also an admired of Andre.Lagarrigue. Gargamelle gave fundamental results in neutrino physics, but in the firstcollaboration meeting which took place in Milan weak nuclear currents were not the firstin the list.Even Franzinetti after an animated discussion wrote on my blackboard le correntineutre le lascio a Fiorini (I leave neutral currents to Fiorini) (fig. 3). It was Brunowho , in the neutrino-72 meeting in Balatonfured supported me together among otherscollegues, like Antonino Pullia, on their importance.48 E. FIORINIFig. 3. – Arguments between Carlo and BrunoFig. 4. – A leptonic neutral current Fig. 5. – A hadronic neutral currentAs usual Bruno was right and in 1973 we proved the existence of neutral currentsboth in leptonic and hadronic neutrino interactions (fig. 4 and 5)4. – Solar neutrinosThis is definitely the subject where the extraordinary genius of Bruno Pontecorvoreached the maximum accomplishment for the future of neutrino physics. As an exper-imentalist I particularly liked that in the famous Canadian internal report on neutrino,while suggesting the best detector, he was even considering the price which is still nowone of the more serious problem due to the large needed mass. My activity was closelyconnected with his suggestions inducing me and my group to join the European GALLEXcollaboration for a radiochemical experiment with Gallium in the Laboratori Nazionalidel Gran Sasso.Figures 6 to 9 show the spectrum of solar neutrinos, an animated discussion withNino Zichichi and Till Kirsten, GALLEX and the GALLEX Collaboration.NEUTRINO PHYSICS AND MY DEBTS TO BRUNO 49In the Neutrino Conference in Spain Bruno was there sitting in the front row andI was chairing the session. After the report on the detection by GALLEX of the ppneutrinos by Till Kirsten, Bruno wanted to say something , but was prevented by theParkinson disease which severely affected him. I translated his congratulation to theaudience and to Till who bowed to him simply saying : ”Professor Pontecorvo we wereindebted of this to you”.Fig. 6. – Spectrum of solar neutrinosFig. 7. – Discussion with Nino ZichichiFig. 8. – The GALLEX detectorFig. 9. – the GALLEX collaborationEven if indicated by the first solar neutrino experiments Bruno Pontecorvo could notenjoy the success of his prediction of oscillations in the repeated experiments on solar andatmospheric neutrinos, followed by beautiful experiments on neutrinos with acceleratorsand reactors (fig.10).In the correponding equation only one of the mixing angles (θ13) was missing up to oneyear ago. Recently however, after indications by Double Chooz and accelerator in Japan,two measurements in Reno (South Corea) and Daya Bay (China) reported respectively:sin2θ13 = 0.092± 0.016(stat)± 0.005(syst)sin2θ13 = 0.103± 0.013(stat)± 0.011(syst)What a pity that Bruno is not with us to celebrate this extraordinary result of theoscillation theory! Oscillation experiments have proved unambiguously the non zero50 E. FIORINIFig. 10. – Bruno and neutrino oscillationsvalue of the difference between the squares of the mass of two neutrinos of differentflavours.. As a consequence we know that the neutrino mass is finite even if we areunable to determine its value. This is today one the most challenging adventure evenoutside elementary particle physics. Neutrino mass can be determined in three ways :cosmology and single and double beta decay (fig 11).Cosmological methods (fig. 12) have reached a sensitivity on the sum of the neutrinomasses of the order of a tenth of eV. This is however in some way model dependent .Fig. 11. – The three ways to determine the neutrino massNEUTRINO PHYSICS AND MY DEBTS TO BRUNO 51Fig. 12. – Cosmology5. – The second mystery of Ettore MajoranaA very important event in 1979 for all of us and also for Italian physics was theunexpected first visit to our country by Bruno (fig.15 and 16).Single beta decay (figure 13) is in principle the only model independent method sinceit is based on the deformation of the beta spectrum due to the mass of the neutrino(figure 14). The present limit of 2 eV is going to be improved by an order of magnitudeby the international KATRIN experiment.Only one year after I had the great pleasure to have Bruno in Neutrino-1980 which Ihad organized in Erice under the generous hospitality of Nino Zichichi. Starting from thenand in the numerous following visits of Bruno in Italy and even in Milan the presenceand advice of Bruno was fundamental for me in the main search on which I was andam involved: neutrinoless double beta decay. My present activity , strongly related toBruno, aims to understand the neutrino nature: is it a lepton conserving Dirac particleor a lepton violating Majorana particle as suggested by another great Italian physicistEttore Majorana just one year before his mysterious disappearance. The existence ofFig. 13. – Beta decay of tritium Fig. 14. – The beta spectrum52 E. FIORINIFig. 15. – With Bruno in Rome Fig. 16. – With Gilberto Bernardinineutrinoless double beta decay would be the only way to solve this mystery since itwould indicate the violation of the lepton number. If existing neutrinoless double betadecay could also allow to determine, even if indirectly, the neutrino mass. Stimulatedby this exciting question many second generation double beta decay are planned , asindicated in table II.At present CUORE (for Cryogenic Underground Observatory for Rare events) whichis near completion in the Laboratori Nazionali del Sasso is the only Cryogenic massivesecond generation. It is based on the search for the decay130Te→130 Xe + 2e− + (2ν¯e)Where 130Te has an isotopic abundance of 34 % and a transition energy of 2347 keV.Four massive arrays were or are being constructed:• MIBETA (Milan) an array of 20 bolometers of TeO2 → 6.8 kg• CUORICINO (CUORICINO Coll.) 62 Bolometers of TeO2 → 40.7 kg• CUORE0 a test array practically identical to one column of CUORE, in operationTable II. – Second generation experiments and plans on neutrinoless DBD.NEUTRINO PHYSICS AND MY DEBTS TO BRUNO 53Fig. 17. – CUORICINO, CUORE0, CUORE• CUORE (CUORE collaboration) 988 crystals of TeO2 → 741 kg, proposed in 1998and presently in constructionThe development the last three of these arrays is shown in fig.17. CUORE0 is presentlysuccessfully taking data and CUORE is near to completion .6. – The problem of the Roman leadBruno was a great physicist with multidisciplinary interests. For this reason I dareto report here on an unusual activity of mine on which he was much interested at thelevel to assist to conferences of mine at the LNGS and at Accademia Nazionale dei Linceiof which he was a foreign member. Lead is in principle an excellent shielding materialfor experiments on rare events due to his high atomic number, reasonable mechanicalproperties and accessible cost. Unfortunately this metal contains 210Pb which decayswith an half lifetime of 22.6 years. As a consequence only ancient Lead is free from 210Pbradioactivity. The discovery of a Roman ship transporting lead sunk near the coast ofSardinia prompted me to investigate the properties of this Lead and to try to obtain alarge quantity of it to shield our experiment on rare events in the Laboratori Nazionalidel Gran Sasso. This navis oneraria magna was transporting about two thousand Leadingots of 32 kg each. INFN funded the recovery of one thousand of them (the other werespread out inside the sand) (fig.18). About three hundred of these ingots where given tous to be used for our experiments under condition of saving the beautiful inscriptions onthe top of each ingot (fig.19)The radioactivity spectrum of Roman Lead is compared in fig. 20 with those of modernLead and of industrial Lead specially produced to ensure low radioactivity. The spectrumof Roman Lead cannot be distinguished from the abscissa line. With high sensitivitymeasurements carried out with bolometers we were able to set a 90% c.l. limit of 4mBq/kg for Roman Lead, the lowest existing for this metal.54 E. FIORINIFig. 18. – Recovery of the Lead ingotsFig. 19. – Inscription on one of the ingotsFig. 20. – Spectra from various sample of Lead: 1. Modern Lead; 2. Com-mercial Low Radioactivity Lead; 3 Roman Lead7. – ConclusionsThis report presented as an inadequate tribute to Bruno Pontecorvo cannot end witha standard conclusion. While writing it I could realize how much I owe to this manwho was so great not only from the scientific, but also from the human point of view.His contributions to our science and in particular to neutrino physics all over his life isunforgettable. Due to our different ages I could not follow directly his great impact inphysics before, during and immediately after the war. Since however I met him fequentlyin Russia and later in Italy and in other countries I had the great privilege of learningso much from him. Since he also honoured me with his friendship in so many occasionsI dare to report two episodes that show the humanity in the country where he chooseto spend the last part of his life. The first episode where he was not present, but whichhe much liked when I reported to him, took place when I was In Protvino (Serpukov)to investigate on behalf of INFN a possible bubble chamber experiment there. Tired bythe work I was walking in the country and passed near an hut where a farmer said halloto me. We started a totally useless conversation since we did not know a single world ofthe other, but the farmer entered in his hut and gave me as a gift an enormous cabbage.I did not refuse it, worry to offend him but one can imaging my difficulty to offer it andNEUTRINO PHYSICS AND MY DEBTS TO BRUNO 55have it cooked in the canteen of our hostel!I have a particular unforgettable recollection of one time I was invited by the Academythe Soviet Union for a neutrino conference which took place in a village in the beautifulElbrus mountain. While the foreign members of the Conference were located in a beau-tiful Hotel where also the Conference took place, I was housed in a much more modestHouse of the Pioneers . Life there was very pleasant and strange: I was sleeping alonein a big room with six beds and the food was good, but arrived in a somewhat irregularway. We received steaks and beer for breakfast and fruit for lunch. Once I asked themaid a coffee, one of the few Russian words I knew, and she disappeared for a quarterof an hour. When she returned with the coffee I realized that she had prepared that inher house! All people there knew only Russian, but that was no problem for me becauseBruno spoke with me in Italian introducing me as tovarich Fiorini . I remember thateach morning Bruno and myself had a rather long walk to the Conference in a beautifullandscape talking in Italian on physics. As I told once to Gillo, Bruno’s brother , thiswas the best week of my life.I still remember when in Milano I received the following mail by Samoil Bilenki:“Dear Ettore, Bruno died yesterday night by pneumonia in the Dubna Hospital. It isa great loss for our science and his friends. I worked with him for 20 years and thankGod for that”.Since Bruno was born in Italy, but died in Russia I would like to repeat a sentenceby the teacher of Puskin:Of companions of our life, when they leave us, we should not say with sorrow “Theyleft us”, but with gratitude “They were”.

Neutrino physics and my debts to Bruno

DOI 10.1393/ncc/i2014-11822-3Colloquia: The Legacy of Bruno PontecorvoIL NUOVO CIMENTO Vol. 37 C, N. 5 Settembre-Ottobre 2014Neutrino physics and my debts to BrunoE. FioriniDipartimento di Fisica G.P.S. Occhialini, Universita` di Milano-Bicocca,and INFN, Sezione di Milano-Bicocca - Piazza della Scienza 3, 20126 Milan, ItalySummary. — After a brief introduction on the important role played by BrunoPontecorvo in my scientific life, mainly involved in neutrino physics, I will shortlyreport the subjects where I could particularly appreciate his greatness which par-ticularly impressed me. I will therefore consider in more details double beta decayespecially in its neutrinoless channel were I was and am particularly involved, weakneutral currents, solar neutrinos, neutrino oscillation, determination of the neutrinomass and one unusual research of mine to which Bruno was particularly interested.1. – IntroductionI first met Bruno Pontecorvo in an international conference in Kiev were I attendedwith considerable anxiety because I was very young. A the time I had proposed an exper-iment on double beta decay with a Germanium diode and carried out first measurementswith this technique. I was incredibly flattered and honoured when the famous professorPontecorvo asked of me to discuss the relationship of my results with the prediction of anew brilliant idea which he had just formulated. We started a discussion which becameimmediately quite animated because Bruno did the best he could to overcome my shynesswhich is normally unusual, but very strong in that case because of the presence of suchan important physicist. This was always the same for the rest of his life during whichI had the great honour of his friendship and the continuous theoretical —and not onlytheoretical— support of my activity in neutrino physics.Bruno could not leave then the eastern countries, but we met many times in the SovietUnion and on the Balaton lake in Hungary for an international neutrino Workshops.I apologize for giving in this talk some relevance to subjects in which I was involved inconnection with Bruno. He was in all of them a source of inspiration and of indispensableadvise.c© Societa` Italiana di Fisica 1718 E. FIORINIFig. 1. – The ββ transition of 76Ge. Fig. 2. – Double beta decay.2. – Double beta decayThis fascinating process (fig. 1), suggested by the great physicist Maria GoeppertMaier in 1934, consists in the transition of the nucleus [A,Z] to its isobar [A,Z+2] andcan be detected when the single beta decay to [A+1] is energetically forbidden or at leaststrongly hindered.This transition can occur to ground or to excited states of the granddaughter nucleus(fig. 2)At the time of the Kiev conference there were already many experiments on doublebeta decay, but only two were based on the principle of using a detector made of thesame material which was also a source for this process.The one by der Mateosian and Goldhaber was using a CaF2 scintillator to detect thedecay 48Ca→ 48Ti + 2e− + (2ν¯e).The one by Fiorini and collaborators was based on the decay 76Ge→ 76Xe+2e−+(2ν¯e)on which many experiments are being carried out or planned today.Double beta decay was not fashionable then, but Bruno was particularly interestedin my experiment because he had a brilliant idea: that neutrinoless double beta decaywas a direct ΔL = 2 transition similar to ΔS = 2.Unfortunately this suggestion was not enforced by experimental results and I wouldlike to report a comment by another great physicist Madam Wu who said:“There are many experiments on double beta decay of poor quality, the only exceptionbeing that by Fiorini et al (wait a second!). The best however is the one by Wu andcoworkers. As far as the suggestion by prof. Pontecorvo that neutrinoless double betadecay is a direct ΔL = 2 transition, after the recent experimental results it lost part ofits beauty”.Bruno said to me later: “She was this time even too good with such a poor paper ofmine!”. I disagree because his idea was brilliant, but definitely not the best among thegreat ones of Bruno and not supported experimentally.As it is well known double beta decay can occur in three channels:1. (A,Z)→ (A,Z + 2) + 2e− + 2ν¯e,2. (A,Z)→ (A,Z + 2) + 2e− + 1, 2...χ’s,3. (A,Z)→ (A,Z + 2) + 2e−.Where χ is a massless Goldston boson.NEUTRINO PHYSICS AND MY DEBTS TO BRUNO 19Table I. – Present results on neutrinoless DBD.The first process is the only one which preserves the lepton number and it has beendetected in eleven nuclei, in some case also to excited levels. The most experimentallyappealing is process 3, called neutrinoless double beta decay, rather improperly since alsoprocess 2 is neutrinoless. In neutrinoless double beta decay, since the recoiling energy ofthe nucleus is negligible, the two electrons share the total energy and in their sum energyspectrum a peak appears corresponding to the total transition energy.Other possible ΔL = 2 lepton violating processes like two positron decay, positrondecay plus electron capture and two electron capture will not be considered here.The present results are shown in table I together with the corresponding limits on theeffective neutrino mass, where the uncertainties on transition nuclear matrix elementsare considered.It can be seen that the only evidence for neutrinoless double decay by of Klapdoret al. is in contrast with the results of Gerda I on the same nucleus both with the firstpaper and, being at 90 % confidence level, also with the second one. In an indirect waythis is also true for the latest results on 136Xe.3. – Discovery of weak neutral currentsAround the end of the sixties I was involved in searches on neutrino physics with bub-ble chambers. Our friend, the late Carlo Franzinetti, suggested to the Italian Institute forNuclear Physics (INFN) the construction of a massive bubble chamber with heavy liquidas a competitor to hydrogen bubble chambers for the easier construction and lower price.The Institute was very favourable, but the project could not be realized in Italy due tolack of funds. Fortunately another great physicist and friend, Andre Lagarrigue, was ableto construct the famous chamber Gargamelle and generously accepted the participationof my country under my direction already during construction.The participation of the Italian group included also Carlo, also an admired of Andre.Lagarrigue. Gargamelle gave fundamental results in neutrino physics, but in the firstcollaboration meeting which took place in Milan weak nuclear currents were not the firstin the list.20 E. FIORINIEven Franzinetti after an animated discussion wrote on my blackboard “. . . le correntineutre le lascio a Fiorini” (I leave neutral currents to Fiorini) (fig. 3).It was Bruno who, convinced of their importance, supported me in the neutrino-72meeting in Balatonfured, together with other collegues, like Antonino Pullia.Fig. 3. – Arguments between Carlo and Bruno (credit: JINR, Dubna).Fig. 4. – July 1973: Gargamelle first neutralcurrent (credit: CERN Archives).Fig. 5. – A hadronic neutral current.As usual Bruno was right and in 1973 we proved the existence of neutral currentsboth in leptonic and hadronic neutrino interactions (fig. 4 and 5)4. – Solar neutrinosThis is definitely the subject where the extraordinary genius of Bruno Pontecorvoreached the maximum accomplishment for the future of neutrino physics. As an ex-NEUTRINO PHYSICS AND MY DEBTS TO BRUNO 21perimentalist I particularly liked that in the famous Canadian internal report aboutrevealing the neutrino, in suggesting the best detector, he was even considering the costof the target material which is still now one of the more serious problem due to the largeneeded mass. My activity was closely connected with his suggestions inducing me andmy group to join the European GALLEX collaboration for a radiochemical experimentwith Gallium in the Laboratori Nazionali del Gran Sasso.Figures 6 to 9 show the spectrum of solar neutrinos, an animated discussion withNino Zichichi and Till Kirsten, GALLEX and the GALLEX Collaboration.In the Neutrino Conference in Spain Bruno was there sitting in the front row and I waschairing the session. After the report on the detection by GALLEX of the pp neutrinos byTill Kirsten, Bruno wanted to say something, but was prevented by the Parkinson diseasewhich severely affected him. I translated his congratulation to the audience and to Tillwho bowed to him simply saying: “Professor Pontecorvo we were indebted of this to you”.Fig. 6. – Spectrum of solar neutrinos.Fig. 7. – Discussion with Nino Zichichi(credit: E. Fiorini and Collaborators).Fig. 8. – The GALLEX detector (credit:GALLEX Collaboration).Fig. 9. – The GALLEX Collaboration.Even if there were hints of oscillations by the first solar neutrino experiments, BrunoPontecorvo could not enjoy the success of his prediction of oscillations in the repeatedexperiments on solar and atmospheric neutrinos, followed by beautiful experiments onneutrinos with accelerators and reactors (fig. 10).22 E. FIORINIFig. 10. – Bruno and neutrino oscillations.In the correponding equation only one of the mixing angles (θ13) was missing up to oneyear ago. Recently however, after indications by Double Chooz and accelerator in Japan,two measurements in Reno (South Corea) and Daya Bay (China) reported respectively:sin2 θ13 = 0.092± 0.016(stat)± 0.005(syst),sin2 θ13 = 0.103± 0.013(stat)± 0.011(syst).What a pity that Bruno is not with us to celebrate this extraordinary result of theoscillation theory!Oscillation experiments have proved unambiguously the non zero value of the dif-ference between the squares of the mass of two neutrinos of different flavours. . . As aconsequence we know that the neutrino mass is finite even if we are unable to determineits value. This is today one the most challenging adventure even outside elementaryparticle physics.Neutrino mass can be determined in three ways: cosmology and single and doublebeta decay (fig. 11).Cosmological methods (fig. 12) have reached a sensitivity on the sum of the neutrinomasses of the order of a tenth of eV. This is however in some way model dependent.Single beta decay (fig. 13) is in principle the only model independent method sinceit is based on the deformation of the beta spectrum due to the mass of the neutrino(fig. 14). The present limit of ∼ 2 eV is going to be improved by an order of magnitudeby the international KATRIN experiment.5. – The second mystery of Ettore MajoranaA very important event in 1979 for all of us and also for Italian physics was theunexpected first visit to our country by Bruno (figs. 15 and 16).NEUTRINO PHYSICS AND MY DEBTS TO BRUNO 23Fig. 11. – The three ways to determine the neutrino mass.Fig. 12. – Cosmology (credit: NASA Archives).Fig. 13. – Beta decay of tritium. Fig. 14. – The beta spectrum.24 E. FIORINIFig. 15. – With Bruno in Rome in 1978,during his first visit to Italy on the oc-casion of Edoardo Amaldi’s 70th birthday(credit: Amaldi Archive, Department ofPhysics, Spienza University, Rome).Fig. 16. – With Gilberto Bernardini (credit:Amaldi Archive, Department of Physics,Sapienza University, Rome).Only one year after I had the great pleasure to have Bruno in Neutrino-1980 which Ihad organized in Erice under the generous hospitality of Nino Zichichi.Starting from then and in the numerous following visits of Bruno in Italy and evenin Milan the presence and advice of Bruno was fundamental for me in the main searchon which I was and am involved: neutrinoless double beta decay.My present activity, strongly related to Bruno, aims to understand the neutrino na-ture: is it a lepton conserving Dirac particle or a lepton violating Majorana particle assuggested by another great Italian physicist Ettore Majorana just one year before hismysterious disappearance?The existence of neutrinoless double beta decay would be the only way to solve thismystery since it would indicate the violation of the lepton number. If existing neutrinolessdouble beta decay could also allow to determine, even if indirectly, the neutrino mass.Stimulated by this exciting question many second generation double beta decay areplanned, as indicated in table II.Table II. – Second generation experiments and plans on neutrinoless DBD.NEUTRINO PHYSICS AND MY DEBTS TO BRUNO 25Fig. 17. – CUORICINO, CUORE0, CUORE.At present CUORE (for Cryogenic Underground Observatory for Rare events) whichis near completion in the Laboratori Nazionali del Sasso is the only Cryogenic massivesecond generation. It is based on the search for the decay130Te→ 130Xe + 2e− + (2ν¯e),where 130Te has an isotopic abundance of 34% and a transition energy of 2347 keV. Fourmassive arrays were or are being constructed:• MIBETA (Milan) an array of 20 bolometers of TeO2 → 6.8 kg;• CUORICINO (CUORICINO Coll.) 62 Bolometers of TeO2 → 40.7 kg;• CUORE0 a test array practically identical to one column of CUORE, in operation;• CUORE (CUORE collaboration) 988 crystals of TeO2 → 741 kg, proposed in 1998and presently in construction.The development the last three of these arrays is shown in fig. 17. CUORE0 ispresently successfully taking data and CUORE is near to completion.6. – The problem of the Roman leadBruno was a great physicist with multidisciplinary interests. For this reason I dare toreport here on an unusual activity of mine on which he was much interested at the levelto assist to conferences of mine at the LNGS and at Accademia Nazionale dei Lincei ofwhich he was a foreign member.Lead is in principle an excellent shielding material for experiments on rare eventsdue to his high atomic number, reasonable mechanical properties and accessible cost.Unfortunately this metal contains 210Pb which decays with an half lifetime of 22.6 years.As a consequence only ancient lead is free from 210Pb radioactivity.26 E. FIORINIThe discovery of a Roman ship transporting lead sunk near the coast of Sardiniaprompted me to investigate the properties of this lead and to try to obtain a largequantity of it to shield our experiment on rare events in the Laboratori Nazionali delGran Sasso. This “navis oneraria magna” was transporting about two thousand leadingots of 32 kg each. INFN funded the recovery of one thousand of them (the other werespread out inside the sand) (fig. 18).About three hundred of these ingots where given to us to be used for our experimentsunder condition of saving the beautiful inscriptions on the top of each ingot (fig. 19).Fig. 18. – Recovery of the lead ingots (credit: E. Fiorini and Collaborators).Fig. 19. – Inscription on one of the ingots (credit: E. Fiorini and Collaborators).NEUTRINO PHYSICS AND MY DEBTS TO BRUNO 27Fig. 20. – Spectra from various sample of lead: 1. Modern Lead; 2. Commercial Low Radioac-tivity Lead; 3 Roman Lead.The radioactivity spectrum of Roman lead is compared in fig. 20 with those of modernlead and of industrial lead specially produced to ensure low radioactivity. The spectrumof Roman lead cannot be distinguished from the abscissa line.With high sensitivity measurements carried out with bolometers we were able to seta 90% c.l. limit of 4mBq/kg for Roman lead, the lowest existing for this metal.7. – ConclusionsThis report presented as an inadequate tribute to Bruno Pontecorvo cannot end witha standard conclusion. While writing it I could realize how much I owe to this man whowas so great not only from the scientific, but also from the human point of view.His contributions to our science and in particular to neutrino physics all over his lifeis unforgettable. Due to our different ages I could not follow directly his great impact inphysics before, during and immediately after the war. Since however I met him fequentlyin Russia and later in Italy and in other countries I had the great privilege of learningso much from him. Since he also honoured me with his friendship in so many occasionsI dare to report two episodes that show the humanity in the country where he choose tospend the last part of his life.The first episode where he was not present, but which he much liked when I reportedto him, took place when I was In Protvino (Serpukov) to investigate on behalf of INFNa possible bubble chamber experiment there. Tired by the work I was walking in thecountry and passed near an hut where a farmer said hallo to me. We started a totallyuseless conversation since we did not know a single word of one another speech, but thefarmer entered in his hut and gave me as a gift an enormous cabbage. I did not refuseit, as I was afraid of offending him but one can imaging my difficulty to offer it and haveit cooked in the canteen of our hostel!I have a particular unforgettable recollection of one time I was invited by the Academythe Soviet Union for a neutrino conference which took place in a village in the beautiful28 E. FIORINIElbrus mountain. While the foreign members of the Conference were located in a beau-tiful Hotel where also the Conference took place, I was housed in a much more modestHouse of the Pioneers. Life there was very pleasant and strange: I was sleeping alonein a big room with six beds and the food was good, but arrived in a somewhat irregularway. We received steaks and beer for breakfast and fruit for lunch. Once I asked themaid a coffee, one of the few Russian words I knew, and she disappeared for a quarterof an hour. When she returned with the coffee I realized that she had prepared that inher house!All people there knew only Russian, but that was no problem for me because Brunospoke with me in Italian introducing me as tovarich Fiorini. I remember that eachmorning Bruno and myself had a rather long walk to the Conference in a beautifullandscape talking in Italian on physics. As I told once to Gillo, Bruno’s brother, this wasthe best week of my life.I still remember when in Milano I received the following mail by Samoil Bilenki:“Dear Ettore, Bruno died yesterday night by pneumonia in the Dubna Hospital. It isa great loss for our science and his friends. I worked with him for 20 years and thankGod for that”.Since Bruno was born in Italy, but died in Russia I would like to repeat a sentenceby Puskin’s teacher:Of companions of our life, when they leave us, we should not say with sorrow “Theyleft us”, but with gratitude “They were”.

http://eprints.bice.rm.cnr.it/18691/1/ncc10836.pdf

Neutrino physics and my debts to Bruno

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