The nuclear physics is the core subject for all the science and technology branches. All the resources of energy are somehow connected to the phenomena of nuclear reactions. Even the solar energy is the outcome of nuclear fusion reactions of hydrogen nuclei in sun. The nuclear physics is not only benefiting the human race with ingrowing vast technological machines on the earth, rather, it is able to explain various phenomena occurring outside the earth like how do stars shine? What holds the star’s matter? How do supernova explosions occur? How do planets, moon and new stars born? What are the elements that have been synthesized in stars?
We are living in the era of nuclear physics. Almost everyday life of a person is somehow connected to the benefits of nuclear scienceand technology. Some of these are the medical related benefits in the hospitals through nuclear radiations, security purposes at the borders through nuclear weapons or in-house benefits through the electricity produced by controlled nuclear fission reactions. The applications of nuclear physicsare not only restricted to the mother earth rather man on earth is equally interested in knowing the construction and destruction of the massive bodieslike tiny glowing balls suspended in thesky during night. Also their motion and uniqueness in their pattern is as fascinating as the light coming from them. All the events occurring in the Universe are based on the nuclear reactions occurring in the stars. Moreover, the light coming from the sun (which is the only star of our solar system) is also the outcome of nuclear reaction. Therefore, life on earth somehow depends upon the nuclear processes going on deep inside the sun. The brightest light that man has seen from the universe is from supernova explosion and has put number of questions like how do stars shine?What holds the star’s matter?How do supernova explosions occur? How do planets, moon and new stars born? What are the elements that have been synthesized in stars? These questions are tickling the human mind continuously. To unveil the knowledge related to these questions, nuclear physics is the way.
Black hole and Nebula:
The major source of energy in a star is nuclear fusion reaction of two hydrogen nuclei1. The two hydrogen nuclei fusetogether,to form one helium nucleusand releases excess amount of energy. The energy released while forming one mole of helium is around 2.7 × 106 Mega Joules. Because of this released energy, the star appears to glow. This process is also called hydrogen burning and iscontinued from millions and trillions years.As the hydrogen fuel burns out, the core of star becomes more dense and hot. As a result, the two opposite forces acting on the star play an active role.The gravitational force from the core of star tries to contract the stellar material towards its core and the other force is the outward pressure built by energy released in nuclear reactions. The balance of inward pressure through star’s own gravity and outward pressure through star’s nuclear burning process hold the stellar material stable.However,the amount of hydrogen present in star is finite. But what happen when all the hydrogen nuclei get fused and not a single hydrogen nucleus is left? In this condition, the helium nuclei start undergoing the multiple nuclear fusion reaction and produce number of elements up to the mass of iron.This element synthesis through nuclear fusion reactions stops at iron because to fuse one extra neutron or proton into iron needs large amount of energy as input. Therefore, the iron is subjected to the core of star. When a massive body runs out of its fuel, it cools off. The outward pressure on star drops out and gravity wins. The contraction of such a massive body in few seconds produces enormous shock waves which cause the outer part of the star i.e. crustto explode into the space. This explosion mechanism is called supernova explosion. The sudden explosion releases very large amount of energy and the star appears to be brightest for few seconds. The two out-products of supernova explosion areBlack hole and Nebula.
After exploding the crust, the core of star contracts and forms an incredible dense ball with very strong gravity. No further nuclear fusion reaction continues, which mean zero energy release and the star appears to be black. Because of strong gravitational force, anything which comes closer to its range gets into it and disappears. This is called black dwarfs, black hole or dead stars in the universe.On the other hand, nebula is a mixture of gases and fragments of star’s crust containing elements heavier than the mass of iron such as zinc, silver, tin, gold, mercury, lead and uranium etc. These heavier elements are formed due to further nuclear fusion of elements like carbon, oxygen and iron using the explosion energy2. This process is called nucleosynthesis. The nebula of one star travel freely in space and when comes into contact of other gases and material present in Universe, the matter combinesto give birth to a new star and planets forming a solar system. The formation of our solar system containing sun and other eight planets including earth is also a result of such process. So life of a star depends on its mass. A very high-mass star ends its life in the form of a black hole and the elements produced inside the star may be upto iron. A low mass star ends its life in the form of a white dwarf and the elements produced are up to helium only. A star having intermediate mass ends its life in the form of a neutron star.
The occurrence of such big events,like supernova explosion, is rare and natural in the space. During supernova explosion, the star appears to shine brightest in the universe because of enormous amount of energy release. Basically, the appearance of such bright light from space occurs for very short interval of time and is the only signature for us about the occurrence of supernova explosion. Many advanced techniques have been used in telescopes till present day to observe light coming from such events. From the information collected through telescopes, one can estimate the time of supernova explosion, distance of other stars and age of a star from its rate of increase or decrease in luminosity.But some of the events are beyond capturing power of telescopes. Moreover, the life span of a star is billions of years and is much larger than the life of a person on earth. (Average age of a person may be about 80 years or so.)So, recording the origin and extinction of star is not possible for a single human life on earth.Also, the supernova events that are being observed in telescopes on present day might have occurred many years back in past and the supernova events that are occurring in Universe at present day cannot be seen by us because that light has not reached to us yet. However, the exploding of matter from highly dense and hot star in the space and the direction of the emitted particles is not accessible from observatory stations. So waiting for these events to occur is boring and time wasting. We need substitution to this so that the information can be collected very easily and very quickly.To understand the mechanism of various Astro physical phenomena, there is a need to perform and analyse the nuclear reactions and simulationsin highly sophisticated laboratories.
Connection of nuclear physics to astrophysics:
The traditional nuclear physics is based upon nuclear reactions carried out at low incident energies3. The electron-capture reaction is responsible for the nucleosynthesis of rare isotopes of elements. In this type of nuclear reaction, an energetic electron (as projectile) overcoming the coulomb repulsion, come closer to the nucleus (as target) with ‘Z’ number of protons and ‘N’ number of neutrons. The electron combines with one of the proton of nucleus and gives one neutron and an electron-neutrino (νe) as a by-product. Thus the proton number of target nucleus reduces to one and neutron number increases to one. Since, it is difficult to trace the electron-capture reaction on earth due to its low rate and instability of relevant nuclei;the neutron induced reactions are used as an alternative to access the information. These reactions are helpful in understanding the synthesis of different elements and their isotopes heavier than iron4.
The neutron capture reactions are of two types, slow neutron capture (s-process) and rapid (or fast) neutron capture (r-process)5. The weak s-process reactions the elements so produced are below strontium-88 (as in massive stars) and the main s-process terminates up to synthesis of Bismuth-209 (as in Red Giant star). Further neutron capture reaction of Bismuth-209 will undergoes beta-decay and produces Polonium-210 which further undergoes alpha-decay in short timescale and produces Lead-208. The synthesis of elements heavier than Bismuth-209 through neutron capture nuclear reaction is the result of r-process also called the explosive nucleosynthesis. The elements produced in r-process are beyond the actinides series to Uranium. Apart from these two processes, there are total 30 proton-rich isotopes between Selenium-74 to Murcury-196, which can be synthesized through p-process only. This branch of nuclear physics studying the nuclear reactions providing astrophysical information is called nuclear astrophysics.
The nuclear reactions at low incident energy are being studied for the understanding of different stellar processes occurring in the star and the synthesis of new elements. However, the information about the motion of broken parts of star’s crust out of highly dense and thermally excited core is absent from above discussed nuclear processes. For that purpose, the nuclear matter at high temperature and density can be produced in laboratory frame by colliding two heavy nuclei at incident energy ranging between 20 MeV/nucleon (energy above the binding energies of both the colliding nuclei) to 2 GeV/nucleon (energy below the required energy used to break the nucleons)6,7. First,heavy projectile and heavy target nuclei are boosted towards each other with some incident energy. In the initial state time is taken to be 0 fm/c (Here, 1fm/c ≈ 10-23 sec). Whenboth the nuclei collide with each other, the pre-equilibrium or compression state occursat t ≈ 10-20fm/c and the density of condensed matter becomes two to three times more than the normal nuclear matter density. This compression state of nuclear matter creates the pressure gradient which pushes the nuclear matter out of the compressed zone (up to t ≈ 30-50 fm/c)and results in the formation of many small nuclei called nuclear fragments8. The mechanism of breakage of highly condensed matter into cold nuclear fragments in very short interval of time can be correlated to the explosion of crust of star from highly dense and hot core into the space. The relation between the thermo-dynamical variables like temperature, pressure and density of nuclear matter further signalizes the physics corresponding to formation of early Universe,supernova explosions, formation and cooling rate of dead stars. The main advantageof studying heavy ion reactions at high energies is that by choosing different collision systems,incident energies etc. one can access different densities and asymmetriesof nuclear matter.Such type of nuclear reactions provides a plausible description about the fragmentation distribution pattern, direction of emission of small particles from the highly dense and hot nuclear matter9.
This branch of nuclear physics which is based on the study of nuclear reactions providing astrophysical information is called nuclear Astro Physics.
Particle accelerators in India and abroad:
Many particle accelerator facilities have been established world-wide to study different phases of the nuclear matter. These facilities are: the Heavy Ion Synchrotron (SIS) at Germany, Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory (BNL) USA, Super Proton Synchrotron (SPS) at CERN Switzerland, the GANIL Cyclotron inFrance, Relativistic Heavy Ion Collider in USA and Large Hadron Collider (LHC) at CERN, Switzerland. In future, the Compressed Baryonic Matter (CBM) experiment at the Facility for Antiproton and Ion Research (FAIR) in Germany, National Superconducting Cyclotron Laboratory (NSCL) in Michigan State University, USA and the Nuclotron-based Ion Collider facility (NICA) at JINR, Russia will also study nuclear matter at large baryon density. The SIS covers the beam energy of the range 0.1- 2.0 GeV/nucleon, AGS up to 33 GeV/nucleon and SPS operates up to 450 GeV.The recent developed Facility for Research in Experimental Nuclear Astrophysics (FRENA)10 facility atSaha Institute of Nuclear Physics (SINP), Kolkata, India has opened up new avenue to study the phenomena of nuclear astrophysics at low energy but very high current 3 MV Tandetron. In this nuclear accelerator various processes of nucleosynthesis will be studied in future. This new facility in India will also provide the reaction in the Hydrogen and Helium burning phases of star. Although all these facilities of nuclear physics have helped us a lot to understand the mysteries of universe yet we still need to work on the accuracy of these predictions. A mixture oflarge-scale hydrodynamical simulations, nuclear theories and experimentsand an abundance of accurateobservations are needed to achieve the goal.