Department of Nuclear and Atomic Physics

Tata Institute of Fundamental Research
Homi Bhabha Road, Mumbai, 400005, India

Scientific programs

Nuclear structure studies with discrete gamma ray spectroscopy

High resolution gamma spectroscopy of finite nuclei provides insight on diverse phenomena related to symmetries of nuclear structure and correlations among the nucleons. Our objective is to investigate nuclear behaviour under extreme conditions of angular momentum and isospin using different types of nuclear reactions. We are developing a DSP (Digital Signal Processing) based DAQ for the Indian National Gamma Array consisting of 24 Clover (HPGe) detectors, and its associated ancillary detectors, e.g., fast scintillators, charged particle detectors and neutron detectors. This facility moves between the different accelerators within India. We are collaborating with NUSTAR-FAIR for the HISPEC/DESPEC project at Darmstadt, Germany for the development of a position sensitive HPGe detectors with Compton imaging capabilities.

You can read more about this program here.

Study of giant resonances

The properties of hot rotating nuclei like the nuclear shape evolution and its fluctuation can be probed through the study of Giant Dipole Resonance (GDR) gamma rays emitted by compound nuclei. We have systematically studied different mass regions of nuclear mass number A~80 to 200. In the A~200 region the deformation as well as the triaxiality was seen to increase with angular momentum, whereas in the A~80 region the effect of temperature was relatively more important than that of the angular momentum. We have also investigated GDRs in the reaction 28Si+124Sn at E(28Si)~150 MeV in coincidence with low energy g-multiplicities. The measured GDR widths in mass A~150 region show different trends in the J –dependence for different nuclei. The nuclei are expected to be near the liquid drop regime and our experimental results are consistent with liquid drop behaviour. Further, we have made studies of GDR built on the 15.1 MeV state in 12C over a wide excitation energy, in order to provide complementary information on the GDR built on the ground states of unstable nuclei. Around the region A~190, we have carried out exclusive measurements of GDR decay gamma rays in 194Au, 188Os and 192Pt, backed up with detailed calculations of potential energy surfaces (PES) over a wide range of temperature and angular momentum for all these nuclei.

Study of Weakly bound nuclei

Nuclear reactions involving weakly bound (unstable) nuclei that have low breakup thresholds and exotic structures display features remarkably different from those of well-bound stable nuclei. The availability of low energy radioactive ion beams, apart from opening new avenues in this field, has also caused a revival in the study of stable weakly bound nuclei. Measurements with these nuclei are relatively easier due to large available beam intensities.In an exclusive measurement for the 7Li + 65Cu system at 25 MeV, we found that the cross-section for the a + d channel was found to be large compared to breakup into the a + t channel. We showed that the observed a + d events arose from a two step process, a direct transfer to the 2.186 MeV (3+) resonance in the a + d continuum of 6Li followed by breakup. In another development, angular distributions for 1n and 2n transfer were measured for the 6He + 65Cu system at Elab= 22.6 MeV. For the first time, triple coincidences between particles, neutrons and characteristic g-rays from the target-like residues were used to separate the contributions arising from 1n and 2n transfer. On the reactions front, we studied Reactions induced by radioactive 6,8He beams on 63,65Cu and 188,190,192Os targets and compared to reactions with stable 4He projectiles. The measurements of the fusion cross sections using a?beams in the energy range 18 to 35 MeV were used to constrain the statistical model parameters for the decay of the compound system, 196Pt. Our observations highlighted the need to distinguish between various reaction mechanisms leading to same product prior to deriving any conclusions about the effect of weak binding on other reaction process.

Hot and compressed nuclear systems and the quark-gluon plasma

Our theoretical physics program concentrates on developing comprehensive theory to study: (i) The structure and dynamics of neutron-rich asymmetric nuclei formed at intermediate energy heavy ion collisions and (ii) The properties of quark-gluon plasma formed at ultra-relativistic energy heavy ion collisions. Statistical models formulated have proven to be very useful to study nuclear multifragmentation at intermediate energies and hadron yield at high energies. Sophisticated transport calculations/models have been also developed that provide detail space-time information of the collision dynamics leading finally to the observed particles. At intermediate energies, our main effort is to constrain the poorly known density dependence of nuclear symmetry energy. Near saturation densities this is detrimental to the understanding of the structure of stable nuclei and radioactive isotopes. While its high density behavior, apart from heavy ion physics, is crucial for nuclear astrophysics. At ultra-relativistic energies, understanding the thermodynamic and transport properties of the quark-gluon plasma is a tremendous challenge. In this context, one of our approaches has been to develop transport models that encompasses the relevant or the entire stages of evolution of the system -- initial parton production from perturbative QCD, subsequent parton scattering, parton to hadron formation and hadron scattering till freeze-out. Characterizing the properties of nuclear matter under extreme conditions of temperature and density and therefrom identification of various novel phases is under active investigation.

Electronic and magnetic properties of solids using nuclear techniques

Investigation of solid state phenomena at short length and time scales via hyperfine techniques with the application of heavy-ion accelerator and radioactive nuclei, has been rigorously pursued in the department. The electric and magnetic hyperfine fields of “probe” nuclei inside solids provide microscopic structural, electronic and magnetic details regarding materials. In the past, we have undertaken the study of different facets of condensed matter physics in different type of materials including: · Formation and stability of 3d, 4d and 4f impurities in metals and alloys · Physics of metallic spin glasses · Structural transformation in rare-earth intermetallic alloys etc. · High-Tc superconductors · Nuclear structure of high spin isomeric states. Our current focus is the application TDPAC and TDPAD to study various problems like local magnetism of isolated 4d impurities in metallic hosts, strongly correlated electron systems including heavy-fermion compounds, coherent Kondo-lattice alloys, spin-liquid alloys, dynamic lattice distortions in the unconventional superconductor Sr2RuO4, and so on. Various other physical scenarios are being investigated to shed light on condensed matter problems using nuclear techniques.

Search for Neutrinoless Double Beta Decay

The mass and nature of neutrinos play an important role in theories beyond the standard model. The nuclear β decay and double β decay can provide the information on absolute effective mass of the neutrinos, which would represent a major advance in our understanding of particle physics. At present, neutrinoless double β decay is perhaps the only experiment that can tell us whether the neutrino is a Dirac or a Majorana particle. Given the significance of the 0νββ, there are nearly 30 planned and proposed experiments worldwide, and these proposals show a rich diversity of approaches with many novel techniques involving different areas of research. In our plans, we decided to focus on the Feasibility Study of Neutrinoless Double Beta Decay in 124Sn, as one of the sister projects at the India-based Neutrino Observatory. Our goal is to make a prototype bolometric detector of 124Sn (~ 1 Kg equivalent) and install it in a low background area. Based on preliminary calculations for 124Sn and assuming 0.5% energy resolution with a background of ~ 0.2 cts/kev/yr, we envisage a large detector of ~1 ton at INO underground laboratory to achieve a sensitivity of mn~ 200 meV in 1 year observation time. The detector will be subdivided into several segments, size and number of which will be optimized after detailed simulation studies.

Heavy Ion Accelerator facility and related developments

The 14 MV Pelletron accelerator facility at TIFR, set up as a collaborative project between BARC and TIFR, has been a major centre for heavy ion accelerator based research in India since its commissioning in 1988. Several major experimental facilities have been established at this centre to pursue research in nuclear physics, atomic physics and interdisciplinary areas. A Superconducting Linear Accelerator has been indigenously developed as a booster to the existing Pelletron accelerator. This is the first superconducting accelerator in India, with all critical components of LINAC booster such as the resonators, liquid helium cryostats, RF control electronics, beam line components, cryogenics etc being designed and developed indigenously. Currently, we harbour plans to develop a low energy, injector system for the existing superconducting LINAC. To utilize the full capabilities of LINAC, we propose to set up a 400 kV ion source deck using a high frequency ECR source of a commercially proven design. The proposed injector system would vastly extend the capability of the LINAC as a research tool for nuclear physics. One of the main advantages of the injector facility is that it will facilitate experiments in inverse kinematics, i.e. heavy beams on light targets. In addition to the nuclear physics interests, high charge state ion beams will open up new windows in interdisciplinary areas (e.g. atomic physics, condensed matter physics, radiation biology).

Intense laser matter studies

This area of research at TIFR was born out of a simple expedition from ion molecule collisions, starting in early nineties with acquisition of a picoseconds laser that could deliver focused intensity up to 1014 Wcm-2 with further expansion with the setting up of a 20 TW laser facility in 2006. With these facilities, we have carried out several experimental investigations into the fundamentals of atomic physics. For instance, we have used polarization of light as a parameter to control to a certain degree the dynamics of ionization, fragmentation and alignment of molecules in intense laser fields. We have investigated bond formation in super intense laser fields, with particular focus on features like Intensity selective ionization and dissociation, Chirp dependence in molecular ionization/fragmentation. and Electromagnetically induced transparency in intense field ionization. In our studies of intense laser interactions with solids, we have investigated X-ray emission, gamma ray emission and giant magnetic fields produced in such interactions. Apart from the modification of matter under intense fields, we have also carried out substantial work on understanding the propagation of intense femtosecond laser pulses though bulk media, some notable results of which include Efficient white light generation in a Barium Fluoride crystal, Few cycle pulse generation, characterization and application to molecular ionization, and Femtosecond beam lithography. On the physics-biology interface front, several new experiments have been developed and many novel system have been studied with the use of femtosecond lasers. Some features of this work include: Development of an optical Tweezers and its application to study and control rotation of red blood cells, Observation of novel features of DNA Protein interaction using an AFM, and deciphering the stress levels in human saliva using nonlinear effects induced by femtosecond lasers.

You can read more about this program here and here.

Heavy-ion atomic collisions with atoms, molecules and clusters

Fast-ion atomic collision physics was started with the installation of the Pelletron accelerator. The main interest in this area of research is to understand the collisional ionization dynamics of atoms (in solid targets or isolated gas), molecules and clusters including fullerenes under highly charged ion (HCI) impact. The initial focus was to explore the influence of solid-state medium on the collision processes such as, inner shell-ionization and electron transfer, radiative capture, wake-field induced effects and the strong transient magnetic fields. The collision studies are now expanded with the introduction of electron spectroscopy, recoil-ion ToF spectroscopy and high resolution x-ray spectroscopic techniques. Recently our group has been actively involved to investigate coherence-driven Young type nano-scale electron-interference in ionization of homo-nuclear diatomic molecules. Our group was one of the first to report the fully measured the oscillation in electron DDCS spectrum; influence of interference on forward-backward asymmetry; fingerprint of interference on angular distributionas in fast electron collisions. On the fullerene front, we have made extensive studies of colective excitation and solid-like wake field induced effect on electron capture, single and multiple ionization. The low energy electron spectroscopy have been used to explore the model-independent evidence of the giant-dipole resonance peak, for the first time. In the inner shell front our group has played a major role by investigating: large relativistic wave function effect on the K-ionization of high Z elements, dramatic enhancement of the selected L and M subshell fluorescence yields, and first ever measurements of the subshell resolved L-K electron transfer. Recently in a major development we have introduced a low energy highly charged ion-accelerator, based on 14.5 GHz ECR-ion source mounted on a 400keV deck. Initial installation has been completed and has been tested by obtaining charge states as high as 16+ for Ar. It has been planned to increase the number of beam-lines in order to start several collision experiments using: C60 and other noblegas clusters; insulators, biomolecules, gas-atoms etc. This facility is expected to play a major role in atomic and interdisciplinery physics which will also compliment our present atomic physics research with Pelletron/LINAC.

You can read more about this program here.

Low energy electron collisions

Setting up of a new technique to measure absolute partial cross sections for the formation of ions in low energy electron – molecule collisions has launched TIFR as a notable centre for low energy electron-molecule collisions in the world. This set up has been used to study dissociative electron attachment (DEA) on a variety of poly atomic molecules which are important for various applications and basic science. We have also studied a variety of molecules important in earth’s atmosphere, plasma processing industry, radiation damage and astrochemistry. We have shown that the presence of functional group dependence in electron attachment gives rise to site/bond selective dissociation of molecules and interpreted this behavior due to formation of valance excited Feshbach resonances. We developed a technique to measure absolute cross sections from excited molecules, and showed that dissociation channels in the DEA process could be controlled by electronic excitation. On a different note, we have dabbled into biological studies and investigated low-energy electron induced damage onto biomolecules in the condensed phase. Our experiments on double stranded supercoiled DNA in the energy range 2 to 30eV have shown that resonant strand breakages are not restricted to just a single type of DNA but applicable to others. Similar experiments have been carried out on full proteins and analyzed using mass spectrometry techniques, observing that changes in proteins may have a strong bearing on programmed cell death (PCD).

You can read more about this program here and here.

Nano-Optics and Mesoscopic Optics

Our effort is directed at studying the transport of optical waves through media which have a variation in the refractive index over length scales comparable to the wavelength. These experiments mostly deal with radiation at or near visible wavelengths. The structure can be ordered, or disordered, or even a combination of both. While some parallels can be drawn with the propagation of electrons in crystals, there also exist significant differences. For example, light can experience amplification which leads to fascinating phenomena hitherto unpredicted by theoretical studies. The existence of sophisticated laser sources, light detectors, and nanofabrication techniques makes it possible to experimentally study even the most elusive of phenomena. We aim to study light propagation through such nanostructured media in a passive, active, dielectric or a metallic environment.

You can read more about this program here.

Copyright: TIFR; Administrator: S. Mujumdar ; Last modified on 10 Apr, 2013.