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“All truths are easy to understand once they are discovered; the point is to discover them” - Galileo
I work in the field of experimental nuclear physics. My research interests include studying nuclear reactions with rare isotope beams, especially in the context of astrophysical scenarios. Here you can get a glimpse of some of the research topics that I work on.
I work in the field of experimental nuclear physics. My research interests include studying nuclear reactions with rare isotope beams, especially in the context of astrophysical scenarios. Here you can get a glimpse of some of the research topics that I work on.
Cosmological Lithium Problem
Cosmological Lithium Problem
Variation of light element abundance as a function of the baryon-to-photon ratio. The green bands are from observations and blue curves are the estimations of the BBN theory. The Figure is taken from Ref.[2].
The standard Big-Bang theory has been the dominant theory that describes the origin and evolution of our universe. It rests on three pillars of observation:
the cosmic microwave background radiation (CMB)
the expansion of the universe
primordial nucleosynthesis of light elements such as hydrogen, helium and some trace amount of lithium, is also know as the Big-Bang Nucleosynthesis (BBN)
The BBN theory describes the production and abundance of these light elements. This theory was developed during the late 1940s by Ralph Alpher, Hans Bethe and George Gamow (You can read a very interesting article about their seminal work here). The primordial nucleosynthesis began when the universe was only a few minutes old and continued up to ~20 minutes. All other elements, such as the carbon in our skin, calcium in our bones and iron in our blood, were produced inside stars that formed much later in the life of the universe. The BBN took place much before the other two observational pillars. However, not everything seems to be consistent with the BBN theory. Although the observed primordial abundances of hydrogen and helium are in excellent agreement with the BBN theory. But, for lithium-7 (the dominant isotope of lithium) the BBN theory overestimates the observed abundances by 3-4 times. The figure shows the comparison of primordial abundances from observations (green bands) with the predictions of the BBN theory (blue curves). This serious discrepancy is popularly termed as the ''cosmological lithium problem''. This problem has remained unsolved for the past several decades.
References:
The Primordial Lithium Problem by Brian D. Fields
Annual Review of Nuclear and Particle Science, Volume 61, 2011, pp 47-68
Primordial nucleosynthesis by A. Coc, E. Vangioni
International Journal of Modern Physics E, Vol. 26, No. 08, 1741002 (2017)
Charge-Exchange Reactions
Charge-Exchange Reactions
The atomic nucleus is a finite many-body quantum system consisting of protons and neutrons (also called nucleons). In quantum mechanical terms, the nucleus can be considered as a system having spin and isospin degrees of freedom. The properties of the nucleus can be studied by carrying out nuclear reactions in the laboratory.
Charge-exchange reactions are a type of nuclear reaction that involves an exchange of a charge (exchange of a neutron with a proton, or vice-versa) between the projectile and the target nuclei. They are characterized by the transfer of the isospin quantum number. These reactions are an excellent probe to study the isospin response of nuclei. Although charge-exchange reactions and beta-decays are governed by completely different forces, the former is mediated by the strong interaction and the latter by the weak interaction, they can populate the same final and initial state in the nucleus. However, charge-exchange reactions offer distinct advantages over beta-decay studies. While beta-decay experiments are constrained by the reaction Q-value, charge-exchange reactions can access higher excitations of the nucleus.
Charge-exchange experiments at intermediate beam energies (~80-300 MeV/u) are of special interest. At these energies, the charge-exchange reactions are predominantly a single-step direct process. This allows a unique scenario where transition strengths associated with weak interaction can be extracted from charge-exchange reactions, mediated by the strong interaction. As a result, data extracted from the charge-exchange experiments can be utilized to study various astrophysics and neutrino physics problems. In particular, charge-exchange experiments are also useful to test theoretical models investigating the exotic neutrinoless double beta decay, a nuclear decay mode that remains yet to be observed. If this process is discovered then it would prove that neutrinos are their own antiparticles. You can read a nice article about this here.
Example of charge-exchange reaction and its comparison with beta decay. The protons are shown in red and the neutrons in blue. Figure courtesy: Charge-Exchange Group @FRIB.
A wide variety of charge-exchange probes have been utilized in experiments worldwide. Among them, the (n,p) or (p,n) reactions are the simplest. Nowadays, more complex composite probes such as (d,2He), (t,3He), (7Li,7Be), and (10Be,10B) have been successfully employed to investigate a diverse range of problems relevant to neutron skin thickness, giant resonances, neutron stars, supernova explosions etc. For further details, please refer to the references below.
References:
Excitation of Isovector Giant Resonances Through Charge-Exchange Reactions by Remco Zegers
Handbook of Nuclear Physics, pp 739–773
Charge-exchange reactions and the quest for resolution by D. Frekers, and M. Alanssari
Giant Resonances
Giant Resonances
Fig.1: Schematic representation of the isoscalar giant monopole resonance (ISGMR), also known as the "breathing mode". The proton (in red) and neutron (in blue) fluids are oscillating in phase inside the nucleus. Figure is taken from H.J. Wollersheim.
Giant resonances are among the most fascinating responses of the nucleus that we can study in the lab. Inside the nucleus, when a large fraction of the nucleons (neutrons and protons) undergo collective motion, it gives rise to the phenomena of giant resonances. They appear as broad structures at high excitation energies of the nucleus. The study of giant resonances sheds light on the bulk properties of nucleonic matter.
Macroscopically, giant resonances can be thought of as oscillations of neutron and proton fluids inside the nucleus. These collective motions can be broadly put into two groups:
Isoscalar giant resonances: In this case, the neutrons and protons oscillate in phase as shown in Fig.1. By studying this type of resonance, we gain knowledge about the compressibility of nuclear matter which improves our understanding of exotic astrophysical objects such as neutron stars and supernovae remnants.
Isovector giant resonances: Contrary to the isoscalar mode, the protons and neutrons in the nucleus oscillate out of phase in the case of isovector giant resonances. The study of isovector giant resonances has added to our understanding of the nuclear force. They have been used to measure the neutron skin thickness, which constrains the nuclear equation of state. They are essential in improving and benchmarking theoretical models of the nucleus.
These giant resonances can be further classified according to three quantum numbers describing the transition between the initial and final states of the nucleus: orbital angular momentum transfer (∆L), spin transfer (∆S), and isospin transfer (∆T). They are categorized based on their ∆L values, specifically as monopole (∆L=0), dipole (∆L=1), and quadrupole (∆L=2). My research focuses on the isovector giant resonances. Figures 2, 3 and 4 show the animations of the isovector giant monopole resonance (IVGMR), isovector giant dipole resonance (IVGDR) and isovector giant quadrupole resonance (IVGQR), respectively.
Fig.2: Schematic representation of isovector giant monopole resonance (IVGMR). Figure is taken from H.J. Wollersheim.
Fig.3: Schematic representation of isovector giant dipole resonance (IVGDR). Figure is taken from H.J. Wollersheim.
Fig.4: Schematic representation of isovector giant quadrupole resonance (IVGQR). Figure is taken from H.J. Wollersheim.
References:
Excitation of Isovector Giant Resonances Through Charge-Exchange Reactions by Remco Zegers
Handbook of Nuclear Physics, pp 739–773
Giant resonances by M N Harakeh and A van der Woude
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