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Dr. Anil Kakodkar, Chairman, AEC, Mr. B. Bhattacharjee, Director, BARC, Distinguished invitees, members of TC/TSC, fellow scientists and engineers, student invitees, ladies and gentlemen,
First of all, I would like to thank Director, BARC for this privilege of delivering the Founder's Day lecture and speak to this very distinguished audience. I have chosen to speak on the topic of "Frontiers in Nuclear Research". Nuclear research has many dimensions and several frontiers. In this lecture, I shall focus on some selected aspects of basic nuclear physics research and accelerator applications close to my own research interests which I have been pursuing at this centre for many years.
Atomic Nucleus, Fission and Transuranic Nuclei
The tiny atomic nucleus with a diameter as small as 10-12 located at the centre of the atom is a quantum-mechanical system, inside which, its constituents - neutrons and protons - move with velocities as large as few tenths the velocity of light. In addition, the protons also experience a strong disruptive Coulomb repulsion. Yet, the nucleus is a strongly bound system. This is so because there exists a strong attractive short range force between the nucleons, what we call Nuclear Force. In the thirties, nuclear physics research was being pursued by many pioneers to understand the characteristics and origin of the nuclear force and to probe the details of the nuclear structure. In the thirties, Enrico Fermi started irradiation of uranium and thorium with neutrons, with the idea of extending the periodic table beyond uranium. It is indeed a fascinating story how these experiments led to the Transuranic puzzle which culminated in the epoch-making discovery of Nuclear Fission in 1939 by Hahn and Strassman. From the events which followed thereafter, there is no doubt that the discovery of nuclear fission is one of the most important discoveries of the
20th century. Consequent large scale governmental funding to scientific research is an important factor which contributed to the exponential growth of not only nuclear physics or physics but the whole of science in the subsequent years. While the discovery of fission appeared as an unexpected event, pioneers like Niels Bohr realized immediately that it could have been anticipated on the basis of the already developed liquid drop model of the nucleus. Bohr and Wheeler soon came out with their classic paper on the theory of nuclear fission involving the concept of fission barrier which was based on the liquid drop model. In the years following the discovery of fission, the unfinished task of extending the periodic table of elements beyond uranium was pursued with renewed vigour using nuclear reactions with the charged particle beams from cyclotron and neutrons from reactor and many transuranic elements extending upto atomic number Z~104, were discovered and synthesized in the period upto the sixties. As predicted by the liquid drop model, the spontaneous fission half lives of these very heavy nuclei were found to decrease exponentially with increasing fissility parameter
Z2/A, with the result that the heavier nuclei beyond Z~100 were not expected to survive even for a fraction of a second.
Superheavy Nuclei
However, it was also well established in the sixties that the atomic nucleus has a shell structure similar to that of the atom, and consequently the nuclei show significant increase in stability at the shell closures corresponding to the so-called magic numbers at Z or N = 2, 8, 20, 28, 50, 82 and also N = 126. It was therefore realized that the liquid drop model picture cannot be quite valid for accurately determining nuclear stability, particularly in the vicinity of the magic numbers. During 1967-1972, it became possible to determine nuclear binding energies as a function of proton number Z, neutron number N and nuclear deformation to within an accuracy of about one MeV using a new theoretical approach initially proposed by Myers and Swaitecki and thereafter generalized by Strutinski. This approach synthesized the nuclear macroscopic behaviour as given by the liquid drop model with the nuclear microscopic behaviour as predicted by the nuclear shell model. These theoretical studies led to the prediction that the fission barriers of the actinide nuclei are double-humped in nature with a secondary minima, and this prediction was confirmed experimentally with the discovery of a large number of spontaneously fissioning shape isomers. The double-humped nature of the fission barrier also led to significant revisions in the evaluated nuclear data and consequently had an effect on the design of fast reactors. The other theoretical prediction as a result of incorporating shell effects is that the fission barriers of superheavy nuclei can have sizeable barrier heights to permit significant stability against spontaneous fission at the next doubly-closed shells (magic numbers), which are theoretically expected at around Z=114, N = 184. Additional regions of deformed shell stabilized nuclei in the superheavy region are also expected. The superheavy nuclei around the above magic numbers are calculated to also have enhanced stability against alpha decay. Taking into account all modes of decay, theoretical calculations predict long half lives ranging from few years to thousands of years for nuclei with Z ~ 110 - 114 and N = 184. Thus, due to shell effects one expects an island like domain of higher stability in the superheavy region, although it is difficult to make a more accurate theoretical prediction on the precise location of this domain. One important nuclear property of these superheavy nuclei is predicted to be that fission (or spontaneous fission) of such nuclei will result in the emission of about 10 or more neutrons as compared to 2.5 neutrons for the fission of uranium, making these superheavy elements very interesting nuclear material. In general, materials with such superheavy atoms can also turn out to be of much interest to chemistry and material science.
For over 25 years, scientists have sought to synthesize superheavy nuclei at or near the above regions of closed proton and neutron shells. Presently active research is in progress to synthesize these doubly-closed shell superheavy nuclei, through fusion of two nuclei by suitably chosen heavy-ion reactions. In order to choose suitable reactions, one needs to acquire a good understanding of the role of (i) nuclear dynamics (ii) entrance channel parameters (mass-asymmetry, neutron richness of projectile-target combination and bombarding energy) and (iii) washing out of the stabililizing effects of the closed nuclear shells with nuclear excitation energy, on the mechanism of fusion, compound nucleus formation, fission and the de-excitation process.
Research on Nuclear Dynamics with our Pelletron Accelerator at TIFR
For production of these superheavy nuclei, the two suitably chosen colliding heavy nuclei should fuse and form a compound nucleus. But it was discovered that during collision, there is a dissipation of the kinetic energy of the colliding nuclei as well as there is a nucleon exchange between the two. As a result, there is a new process of quasifission in which the colliding nuclei reseperate after some mass and energy exchange, and this process competes with fusion. Nucleon exchange between two nuclei in proximity was postulated by the Trombay team almost a decade earlier than its discovery to explain the mass distributions in fission. Recently, to explain fragment angular distributions, the Trombay team postulated presence of another process called pre-equilibrium fission, which is dependent on the entrance channel mass-asymmetry and which is again a competing process to fusion. Soon after the commissioning of our pelletron Accelerator at TIFR in 1989, the experiments carried out by our team confirmed the presence of pre-equilibrium fission. Several other experiments carried out by the Trombay team utilizing the pelletron accelerator have provided useful information on the nuclear dynamics, an understanding of which is important for selecting a proper window of entrance channel parameters for the synthesis of superheavy nuclei. For example, the Trombay team have obtained interesting results on the entrance channel dependence of nuclear dynamical times which are extremely small and in the range of about
10-20 s, using what can be called "neutron clock". Our theoretical work carried out in 1971 also showed that shell effects disappear with the excitation energy and, therefore, entrance channel energies should be optimized to give not only large fusion cross-section but also a low excitation energy of the compound nucleus.
Synthesis of Superheavy Nuclei
The efforts to synthesize the doubly magic superheavy nuclei with the medium energy heavy ion accelerators are being pursued at several laboratories in the world, particularly at GSI, Darmstadt, Germany, LBL, Berkeley, USA and JINR, Dubna, Russia. Apart from the dynamical aspects discussed earlier, the other most significant hurdle in the path of synthesis of these superheavy nuclei is that by the heavy-ion fusion process the presently available projectile-target combinations cannot land in the doubly closed shell domain around Z=114, N=184 which is highly neutron rich. To achieve landing at the centre of the domain of these magic superheavy nuclei, what is needed is a beam of highly neutron rich projectiles, as well as targets comprising neutron rich nuclei. Currently, there is considerable interest in the development of such Rare Ion Beam (RIB)(also called Radioactive Ion Beam) accelerator facilities, which can provide neutron rich unstable heavy ion beams and search for these exotic superheavy nuclei will be an important application of such upcoming facilities. Building these facilities require high current particle accelerators to produce these neutron rich ion species and hence, synthesis of superheavy nuclei is an important motivation for the development of high current particle accelerators. As discussed later, the high current and high energy accelerators are also needed for a new type of nuclear energy generation system called Accelerator Driven Sub-critical Reactor Systems (ADS), which are also a subject of much current interest.
Quark-Gluon Plasma (QGP)
In the recent years, studies involving collisions between two energetic nuclei have taken the nuclear physics research to many new dimensions; and one of these dimensions involve search for the expected phase transition between hadronic matter and quark-gluon plasma in the relativistic heavy ion collisions. We recall that it is now firmly established that the nucleons have an internal structure comprising of three quarks bound together with the carriers of force called gluons. According to the Quantum ChromoDynamics (QCD), the nature of strong forces between the quarks do not allow a quark to move outside a nucleon, and become free. The interest in the study of heavy ion collisions at ultra-relativistic energies (E/A - few hundred GeV) comes from the expectation that these collisions are expected to lead to the formation of nuclear systems of extremely high temperatures T (T-few hundred MeV) and densities (5 to 10 times normal nuclear density), where a phase transition is expected to occur from normal hadronic matter to the Quark-Gluon Plasma (QGP) where the quarks and gluons are free to move in the nuclear dimensions. Such a phase transition has been predicted theoretically by the calculations based on QCD, which represents the most accurate theory of the strong interactions.
According to the Big Bang theory, less than trillionth of a second after its creation the early universe, about the size of a marble, underwent a transition to a new state of matter, as steam does in cooling to form water. It is this quark-gluon plasma that physicists are now trying to make and study in the laboratory by colliding relativistic heavy-ion beams. Previous studies with lower energy Pb ion collisions at the CERN laboratory hinted at the existence of QGP. Now the Relativistic Heavy Ion Collider (RHIC) at BNL, USA is aiming to recreate the conditions of the early universe by colliding gold ions at much higher collision energies hoping that the high temperatures and densities achieved in the collisions should for a fleeting moment, allow the quarks and gluons to exist "freely" in a soup-like plasma. Our Trombay team has been collaborating in these studies being carried out with the PHENIX detector of RHIC, in which Trombay scientists have contributed to the software efforts as well as in fabricating part of the muon tracking system of the hardware of the large PHENIX detector. From the results obtained and analyzed so far and a number of papers recently published, it appears that the scientists are quite close to confirming the discovery of this new QGP state of matter.
Accelerators in Nuclear Energy Development
Over the years, a variety of charged particle accelerators have been invented, built and used for the exploration of structure of nuclei delivering primary beams of protons, heavy ions and electrons over a wide energy range and secondary beams of other particles such as neutrons, pions, muons, neutrinos and radioactive ions. Also higher and higher energy particle accelerators have been built to probe structure of nucleons and study particle physics at a deeper level. These developments in accelerators and ion beam technologies have also found important applications in several areas such as medicine, material science, environmental sciences and also in industry. In the recent years, there is a growing interest in the possibily of use of particle accelerators for harnessing nuclear energy. Possible generation of fusion energy through inertial confinement fusion employing heavy ion pulse beam drivers and also through muon-catalysed fusion and the emerging applications of the Accelerator Driven sub-critical reactor Systems(ADS) are good examples of the spin-offs of the accelerator based research to practical applications in nuclear energy development. Muon catalysed fusion is an area of active research aimed to achieve fusion between the D and T at room temperature by quantum-mechanical tunneling of the Coulomb barrier between D and T, made possible by bringing the two nuclei sufficiently close by forming a muonic molecule of D and T. Following fusion reaction, the products alpha particle and neutron are released and the muon is freed to catalyse another reaction. The muons needed to form DT molecule have to be generated by the decay of pions which are produced by a high energy (~GeV) proton accelerator. Since energy is needed to produce a pion, for an energy break-even, one muon should catalyse at least a few hundred D-T reactions, before it is lost through capture by alpha particle. While this is not yet achieved, muon catalysed fusion continues to be a promising area of research for fusion energy generation.
Accelerator Driven sub-critical reactor System (ADS)
ADS is a new type of fission energy producing reactor in which nuclear power (say, 200-1000 MWe) can be generated in a multiplying reactor core (k<1) without the need to take the reactor to criticality (k=1). But ADS has to be driven by an external neutron source. Fission energy released by 1 neutron in a multiplying medium of k is about
(k/1-k).(180/v) MeV, where v is the average number of neutrons released per fission. Thus to have a thermal fission power of 500-1000 MW, one would require a supply of about
1018 n/s. With a 1 GeV proton beam, about 30 neutrons per proton can be generated by spallation reaction in a high Z material, like lead. Hence a proton beam of about 1 GeV energy and 10-20 mA current will be required for the ADS. Among the advantages of ADS, one can see the following: (i) no restriction on the fuel-type as the system is sub-critical and therefore, ADS is ideally suited to transmute long-lived minor actinides (ii) no restrictions due to neutron economy consideration, as the neutrons can be supplied externally and therefore, long-lived fission products can also be transmuted together with the generation of fission energy (iii) effective
h
of a sub-critical system (~h/k) is increased which can be exploited to achieve higher burn-ups of the fissile fuel before discharge and increased breeding ratios in a breeder reactor and (iv) has a new degree of freedom in running the reactor due to presence of an external neutron source. Our analysis show that the features of ADS can lead to more effective utilization of our naturally available uranium and thorium fuels, and, in particular, can provide an additional route to the utilization of thorium with the use of
232Th-233U fuel cycle. ADS in conjunction with AHWR can therefore, speed up our efforts towards optimum utilization of our thorium resources for nuclear electricity generation.
While Accelerator-driven sub-critical reactor systems have many attractive features, one of the most challenging component in the development of ADS is the high-energy and high-current accelerator capable of delivering average proton beam power of 10MW or more. There is worldwide interest to develop an accelerator system of such a high beam power, which is at least one order of magnitude larger than the beam power of presently operating high energy and high current accelerators. Realizing the vast potential of high energy and high intensity proton accelerators for ADS, a programme has been evolved for stage-wise development of systems and technologies in our department. This perspective plan of action expected to be taken up for stage-wise implementation in the coming years will be a challenging task, with new opportunities for further advanced technology developments in the country.
Concluding Remarks
In conclusion, medium energy nuclear physics research, studies on nuclear dynamics, superheavy nuclei and nuclear reaction with Rare Ion Beams (RIB) are important emerging areas of contemporary interest. In the relativistic energy regime, creation and study of the QGP will continue to be an area of great interest and promise for unfolding new physics. As regards applications, while the use of heavy ion beams for internal confinement fusion and high energy proton beams for muon catalysed fusion appear promising for harnessing fusion energy, we can expect dramatic progress internationally in the area of ADS and some prototype ADS systems may come in operation in the coming years in the world. We hope that our own programmes in the country will also make significant contributions in these emerging areas on the frontiers and will also make necessary advances in the development of required accelerator technologies.
Thank you.
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