• Volume 85, Issue 3

September 2015,   pages  379-566

• Fission fragment mass distributions via prompt 𝛾-ray spectroscopy

The distribution of fragment masses formed in nuclear fission is one of the most striking features of the process. Such measurements are very important to understand the shape evolution of the nucleus from ground state to scission through intermediate saddle points. The fission fragment mass distributions, generally obtained via conventional methods (i.e., by measuring the energy and/or the velocity of the correlated fission fragments) are limited to a mass resolution of 4–5 units. On the other hand, by employing the 𝛾-ray spectroscopy, it is possible to estimate the yield of individual fission fragments. In this work, determination of the fission fragment mass distribution by employing prompt 𝛾-ray spectroscopy is described along with the recent results on 238U(18O, f) and 238U(32S, f) systems.

• Spectroscopy of heavy fissionable nuclei

Structural studies of heavy nuclei are quite challenging due to increased competition from fission, particularly at high spins. Nuclei in the actinide region exhibit a variety of interesting phenomena. Recent advances in instrumentation and analysis techniques have made feasible sensitive measurements of nuclei populated with quite low cross-sections. These include 𝐾 isomers and rotational band structures in isotopes of Pu (𝑍 = 94) to Rf (𝑍 = 104), and octupole correlations in the Th (𝑍 = 90) region. The obtained experimental data have provided insights on various aspects like moments of inertia and nucleon alignments at high spins, quasiparticle energies and evolution of quadrupole and octupole collectivity, among others. An overview of some of these results is presented.

• Spectroscopy of fission fragments using prompt-delayed coincidence technique

The time-stamp structure of the digital data acquisition system of the Indian National Gamma Array (INGA) has been utilized to carry out prompt-delayed coincidence technique for the spectroscopic study of fission fragments. This technique was found to be useful to determine the states above the long-lived isomer (with half-life up to $\sim$5 𝜇s), present in the fission fragments. The angular correlation of 𝛾-rays, emitted by the fission fragments, has also been used in the present INGA geometry to determine the spins of the de-exciting states.

• 136Sn and three-body forces

New experimental data on 2+ energies of 136,138Sn confirm the trend of lower 2+ excitation energies of even–even tin isotopes with 𝑁 &gt; 82 compared to those with N &lt; 82. However, none of the theoretical predictions using both realistic and empirical interactions can reproduce experimental data on excitation energies as well as the transition probabilities (𝐵(𝐸2; 6+ $\to$ 4+)) of these nuclei, simultaneously, apart from the ones whose matrix elements have been changed empirically to produce mixed seniority states by weakening the pairing. We have shown that the experimental result also shows good agreement with the theory in which three-body forces have been included in a realistic interaction. The new theoretical results on transition probabilities are discussed to identify the experimental quantities which will clearly distinguish between different views.

• Fission approach to cluster radioactivity

Fission theory is used to explain 𝛼 decay. Also, the analytical superasymmetric fission (ASAF) model is successfully employed to make a systematic search and to predict, with other models, cluster radioactivity. The macroscopic–microscopic method is illustrated for the superheavy nucleus 286Fl. Then a few results of the theoretical approach of 𝛼 decay (ASAF, UNIV and semFIS models), cluster decay (ASAF and UNIV) and spontaneous fission dynamics are described with Werner–Wheeler and cranking inertia. UNIV denotes universal curve and semFIS the fission-based semiempirical formula.

• Ternary fission

We present the ternary fission of 252Cf and 236U within a three-cluster model as well as in a level density approach. The competition between collinear and equatorial geometry is studied by calculating the ternary fragmentation potential as a function of the angle between the lines joining the stationary middle fragment and the two end fragments. The obtained results for the 16O accompanying ternary fission indicate that collinear configuration is preferred to equatorial configuration. Further, for all the possible third fragments, the potential energy surface (PES) is calculated corresponding to an arrangement in which the heaviest and the lightest fragments are considered at the end in a collinear configuration. The PES reveals several possible ternary modes including true ternary modes where the three fragments are of similar size. The complete mass distributions of Si and Ca which accompanied ternary fission of 236U is studied within a level density picture. The obtained results favour several possible ternary combinations.

• Cluster decay analysis and related structure effects of fissionable heavy and superheavy nuclei

Collective clusterization approach of dynamical cluster decay model (DCM) has been applied to study the attributes of hot ($T \neq 0$) and rotating ($\ell = 0$) nuclei lying in heavy and super-heavy mass regimes. We present here an overview of the characteristic fission decay properties such as shell effect, role of entrance channel, quadrupole (𝛽2) deformations and impact of hot (equatorial) compact orientation degree of freedom in comparison to cold (polar) elongated configuration. The presence of non-compound nucleus process, i.e., quasifission, is also investigated. Apart from studying the decay of excited state nuclei, the dynamics of heavy particle cluster emission is also addressed using the preformed cluster model (PCM).

• Spontaneous fission of superheavy nuclei

The macroscopic–microscopic method is extended to calculate the deformation energy and penetrability for binary nuclear configurations typical for fission processes. The deformed two-centre shell model is used to obtain single-particle energy levels for the transition region of two partially overlapped daughter and emitted fragment nuclei. The macroscopic part is obtained using the Yukawa-plus-exponential potential. The microscopic shell and pairing corrections are obtained using the Strutinsky and BCS approaches and the cranking formulae yield the inertia tensor. Finally, the WKB method is used to calculate penetrabilities and spontaneous fission half-lives. Calculations are performed for the decay of 282,292120 nuclei.

• Superheavy elements and decay properties

The 𝛼 decay properties of the isotopes of 𝑍 = 115, 117, 118 and 119 have been extensively investigated, focussing on the newly synthesized isotopes within the Coulomb and proximity potential model for deformed nuclei (CPPMDN). The 𝛼 half-lives have also been evaluated using the Viola–Seaborg systematic (VSS) and the analytical formulae of Royer and it can be seen that our calculated values match well with these theoretical values. The mode of decay of these isotopes has also been studied by calculating the spontaneous fission half-lives. Thus, we have predicted 4𝛼 chains from 287115, 3𝛼 chains from 288115, 3𝛼 chains from 293117, 4𝛼 chains from 294117 and 3𝛼 chains from 294118 and, it can be seen that our predictions on the 𝛼 decay chains also match well with the experimental observations. The study on 𝑍 = 119 has predicted six consistent 𝛼 chains from 292−295119, 5𝛼 chains from 296119, 4𝛼 chains from 297119 and 3𝛼 chains from 298,299119. Thus, through our study on isotopes of 𝑍 = 115, 117, 118 and 119 superheavy nuclei, we could predict the range of isotopes that may be detectable using 𝛼 decay and we hope that the findings on the isotopes of 𝑍 = 119 will provide a new guide for future experiments.

• Discoveries of isotopes by fission

Of the about 3000 isotopes presently known, about 20% have been discovered in fission. The history of fission as it relates to the discovery of isotopes as well as the various reaction mechanisms leading to isotope discoveries involving fission are presented.

• The EXILL campaign

A combination of germanium detectors was installed at the PF1B neutron guide of the ILL to perform prompt spectroscopy of neutron-rich nuclei produced in the neutron-capture induced-fission of 235U and 241Pu. In addition, LaBr3 detectors from the FATIMA Collaboration were installed to complement with the EXOGAM clovers to measure lifetimes of low-lying excited states. The measured characteristics and online spectra indicate very good performances of the overall set-up.

• Photofission experiments at the ELI-NP facility

At ELI-NP, high-power lasers together with a very brilliant 𝛾 beam are the main research tools. The high-power laser system (HPLS) and the 𝛾 beam system (GBS) of ELI-NP are presented. The expected performance of the electron accelerator and production lasers of the GBS, and the targeted operational parameters of the 𝛾 beam are described. Possible laser-induced fission and 𝛾 beam photofission experiments which are under preparation at ELI-NP, and the different set-ups and instrumentation, are designed for these experiments, are presented.

• Detector instrumentation for nuclear fission studies

The study of heavy-ion-induced fusion–fission reactions require nuclear instrumentation that include particle detectors such as proportional counters, ionization chambers, silicon detectors, scintillation detectors, etc., and the front-end electronics for these detectors. Using the detectors mentioned above, experimental facilities have been developed for carrying out fusion–fission experiments. This paper reviews the development of detector instrumentation at IUAC.

• Nuclear fission as a tool to contrast the contraband of special nuclear material

An integrated mobile system for port security is presented. The system was designed to perform passive measurements of neutrons and 𝛾-rays to search and identify radioactive and special nuclear materials as well as for the active investigations by using the tagged neutron inspection technique of suspect dangerous materials. The discrimination between difficult-to-detect uranium samples and high 𝑍 materials as lead was specifically studied. The system has been employed in laboratory detection tests and in a seaport field test.

• ANURIB – Advanced National facility for Unstable and Rare Ion Beams

An ISOL post-accelerator type of RIB facility is being developed at Variable Energy Cyclotron Centre (VECC), Kolkata, India. In this scheme, Rare Ion Beams (RIBs) will be produced using light ion beams (𝑝, 𝛼) from the 𝐾 = 130 cyclotron, the RIB of interest will be separated from the other reaction products and accelerated up to about 2 MeV/u using a number of linear accelerators. Recently, a few RIBs have been produced and accelerated using this facility. As an extention of this effort, another RIB facility – ANURIB will be developed in a new campus as a green-field project. ANURIB will have two driver accelerators – a superconducting electron LINAC to produce n-rich RIBs using photofission route and a 50 MeV proton cyclotron for producing p-rich RIBs. In this paper, the status of the RIB facility in the present campus and future plans with the ANURIB facility will be discussed.

• Nuclear transmutation strategies for management of long-lived fission products

Management of long-lived nuclear waste produced in a reactor is essential for long-term sustenance of nuclear energy programme. A number of strategies are being explored for the effective transmutation of long-lived nuclear waste in general, and long-lived fission products (LLFP), in particular. Some of the options available for the transmutation of LLFP are discussed.

• Fast reactor programme in India

Role of fast breeder reactor (FBR) in the Indian context has been discussed with appropriate justification. The FBR programme since 1985 till 2030 is highlighted focussing on the current status and future direction of fast breeder test reactor (FBTR), prototype fast breeder reactor (PFBR) and FBR-1 and 2. Design and technological challenges of PFBR and design and safety targets with means to achieve the same are the major highlights of this paper.

• Conceptual design of Indian molten salt breeder reactor

The third stage of Indian nuclear power programme envisages the use of thorium as the fertile material with 233U, which would be obtained from the operation of Pu/Th-based fast reactors in the later part of the second stage. Thorium-based reactors have been designed in many configurations, from light water-cooled designs to high-temperature liquid metal-cooled options. Another option, which holds promise, is the molten salt-fuelled reactor, which can be configured to give significant breeding ratios. A crucial part for achieving reasonable breeding in such reactors is the need to reprocess the salt continuously, either online or in batch mode. India has recently started carrying out fundamental studies so as to arrive at a conceptual design of Indian molten salt breeder reactor (IMSBR). Presently, various design options and possibilities are being studied from the point of view of reactor physics and thermal hydraulic design. In parallel, fundamental studies on natural circulation and corrosion behaviour of various molten salts have also been initiated.

• Summary: 75 years of nuclear fission – present status and perspectives

• List of Participants

• Pramana – Journal of Physics

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• Editorial Note on Continuous Article Publication

Posted on July 25, 2019