 Hello. So, this is beginning of the course titled Neutrons as Prove of Condensed Matter. This course will cover various aspects of a technique called Neutron Scattering for understanding structure and dynamics in condensed matter. This course will on one hand deal with the basics of Neutron Scattering on the other hand will also familiarize you with the various techniques and various experimental facilities that are available in the country and outside the country for use in Neutron Scattering techniques. So, before I start the topic it will be interesting to give you a very brief introduction to Neutron which is a nuclear particle. Neutron was predicted or rather discovered in a short letter to the editor by Chadwick where he found that matter should resemble that of a quantum of high energy because these experiments were done earlier also where one tried to understand radiation coming from radium alpha beryllium sources and they found that they knock out or give very large momentum to hydrogenous materials and earlier people thought like people like great scientist like Julliard Curie that this is a possibly a gamma ray, but gamma ray because it could knock off knock hydrogenous materials very efficiently that to include very large energies to the radiation. It was Chadwick who very clearly stated that if quantum if up to the present evidence is in favor of the neutron and if the conservation of energy and momentum is there then it is a mass which is close to the mass of proton and this is the elusive neutron and if he stated very politely if the conservation of energy and momentum can be relinquished then possibly you can talk about quantum hypothesis means it is a gamma radiation, but we know that it was his assumption or his understanding was correct and neutron was discovered in 1932 by Chadwick in an experiment this is a spin half particle with a magnetic moment of minus 1.91 nuclear magneton and it is a nuclear particle so existing in almost every nucleus in every nucleus almost in every nucleus and but then after that many things happened very rapidly after discovery in 1932 in quick succession people tried to bombard things with neutron and in the process fission was discovered to which we know Liza Meitner, Otto Frisch, Otto Hahn who got the Nobel Prize for discovery fission they worked a lot and it was discovered that when you bombard uranium with neutron you get instead of having transuranic elements which you do get but you also get elements which are of much lower atomic number and they called it fission because the uranium nuclear nucleus giving rise to this fission giving rise to much lower z value materials and then very quickly the first nuclear reactor is built under a baseball under a basketball court at Chicago University known as Chicago pile and then all of us are familiar or we know the history of first nuclear explosion atom bomb and the drop of atom bombs and Hiroshima and Nagasaki but we will also point out that after the discovery very quickly nuclear power plants came and also came the research reactors so people realize quite early that neutrons can be an excellent probe why I'll complete shortly of matter and research reactors like Daito in UK, NRX in Canada, Brookhaven National Laboratory they started coming up this is 1956 we had a reactor in 1956 which was highly enriched uranium core reactor we had a Syras reactor in we in 1962 which was similar to the NRX reactor Canada presently we have a reactor completely indigenously built and also instruments built by the scientist in India known as Dhruva there's a new reactor it's called new observe which has come up in 2018 and you also have plans for a high flux research reactor or HFRR in future for use in research now this research is mostly done using thermal neutrons so thermal neutrons are neutrons which are available in the core of a reactor so in a in a sequoene sexual neutrons are produced in a fission in a reactor energy of the rough few million electron volts but to keep a reactor critical critical means the reactor will keep operating using neutrons generated in in it's called a critical reactor so it's called a chain reaction which keeps the reactor operating without an external aid but this chain reaction is so controlled that from generation to generation number of neutrons remain exactly same and neither they decrease nor they increase and this is known as a critical reactor to maintain this criticality the reactor that the neutron energy has to be brought down brought down to thermal range thermal range thermal range and this is done by using something called a moderator and the thermal energies in the range of milli electron volts the moderators are nothing but low Z materials mostly h2o can be d2o can be carbon which through ping pong ball collision with neutron of high energy brings the neutron energy down in this inelastic collision so neutron gives its energy to the moderator nucleus and its own energy comes down and this process goes on till the neutron temperature thermalizes at the moderator temperature and at that time we can call the neutrons as thermal neutrons and usually a moderator used our h2o d2o graphite because this is a ping pong ball collision and we are familiar that when the mass of the two I mean the projectile and the nucleus are same then we can have maximum momentum and energy transfer so neutrons thermalize at moderated temperature and these are the neutrons known as thermal neutrons so to show you the energy distribution this is typical maxolian distribution of the moderator at a of the neutron at a moderator temperature of typically say 320 Kelvin so typically around 50 degree centigrade that is a typical 40 to 50 degree centigrade is the temperature of the moderator which is present in the reactor and once it thermalizes it is not that you have a neutrons of one energy but you have an energy distribution which is maxolian so this is the maxolian distribution in this you can see there is a peak which is typically around 30 millilectron volts and we have also in the spectrum we have low energy neutrons which are actually called cold neutrons broadly below 500 millilectron volt energy we also have hot neutrons where typically energies are more than half a electron volt or 500 millilectron volt so these are typical distribution of thermal neutrons and we can choose the energy of an incident neutron beam from this maxolian spectrum by choosing a particular energy band with the use of something called monochromators all these I will be describing to you later but you can take out a slice of energy from this maxolian for the experimental purposes how we do it that I will be describing later so thermal and epithermal neutrons typically around one millilectron volt to 500 millilectron volt and this scattering is an ideal tool and possibly unique probe of microscopic structural dynamics and magnetic properties of condensed matter the reason being there are many reasons for choosing thermal neutrons for study of condensed matter one is that the wavelength is very commensurate with the interatomic distances which is typically around 0.1 to 10 nanometers which is one angstrom 200 angstroms and there are various kinds of structures we can find at this length scale so crystallographic structure crystallographic arrangement inner crystal they are typically around one angstrom level and also various inhomogeneities in a bulk medium like pores in a rock or a chemical precipitate inside another medium or a colloidal sample you have structures at around 10 nanometer 100 angstrom length scale interestingly in this respect they are quite similar to what you have for x-rays and we have to accept that x-rays are the by far the most used tool for understanding structure in condensed matter but one advantage of neutron is that they are just as energetic as atoms and molecules in condensed matter so their energy is also as you saw just now I said in millilectron volt ring and this is commensurate with various dynamical processes in condensed matter not only solids but in liquids so that's why I say I use the term condensed matter because like phonomes they are typically around 10 to 100 millilectron volts energy range there are inelastic processes like vibrations they are typically around say 500 millilectron volts there are also very slow dynamics like diffusion and there the energy will be typically around say hundreds of microelectron volts so one two orders of magnitude lower and for these all these kind of dynamics and their study neutrons are preferred x-rays generally are not used for dynamic because x-ray you know that typically one angstrom x-ray will have energy of around 12 kiloelectron volt so this energy is much above the energy ranges in the condensed matter and x-rays are the basic tool for structural analysis another big advantage of neutrons is they get very deep into the sample neutron is neutral particle and it can get very deep of the tens of centimeters or even more now this is a property which is much better than most of the tools that we can use x-rays it can go to 10 to 100 microns if you talk about electrons they are absorbed by 30 to 40 angstrom depth if you talk about protons it will be even less so light it will enter a medium provided the medium is transparent to light otherwise it cannot enter a media so in these respects neutron can get deep into most of the samples except a few strong absorbers like cadmium gadolinium there are some strong neutron absorbers but apart from them in most of the materials neutrons can penetrate very very deep and so that's why we can get bulk information from use of neutrons in our experiments another very interesting property is that there is an extremely good contrast between isotopes and neighboring atoms in periodic table we know in case of x-rays the cross section increases in a power now called modulus law z minus mu to the power three if i remember it correct so neighboring neighboring atoms in the periodic table don't have much contrast with respect to x-rays because their charge cloud is almost the same size and x-rays are scattered by the charge cloud around the atom so nickel and copper an example they are the neighboring atoms in the periodic table and there will be poor contrast whereas in case of neutron the scattering takes place from the nucleus so neutron nuclear interaction is strong interaction and it depends on the first on the isotope like hydrogen or deuterium like hydrogen coherence has a cross section negative deuterium has a cross section which is positive there's a huge contrast between hydrogen and deuterium and there are many such examples also they don't vary systematically across the periodic table because the nuclear interactions dictate what should be the scattering cross section for that particular element so one is there is very good contrast between isotopes in that respect the physics or chemistry remains same if we change one isotope with another but the structurally to neutron they provide a completely different kind of contrast factors also neutron is magnetic because it has got a spin of minus 1.91 nuclear magneton and that's why for understanding magnetic structure as well as dynamics possibly neutron is the only tool which can give us microscopic view of the magnetic material also it is non-destructive characterization so unlike sample preparation in case of transmission electron microscope so this probe is non-destructive in nature so that's all these things make neutron a very desirable tool for understanding structure and dynamics in condensed matter and countries build nuclear reactors and presently accelerator waste spallation neutron sources for such studies so briefly I just show you that this is how extract scattering cross section changes with atomic weight but here I show the same for neutrons and you can see this is a zigzag line where first one is hydrogen and deuterium they have different values and actually hydrogen has negative scattering amplitude whereas deuterium has positive scattering amplitude so the use of hydrogen deuterium contrast is white because we know that most of our organic and biological systems have hydrogen as a very very large component and that's why we can use the contrast between hydrogen and deuterium which I will come to later to use them to highlight the contrast between various parts of let us say it can be a protein it can be a polymer molecule and use the study to understand not only the whole material but a part of the molecule or a part of the protein similarly there is also negative scattering cross section from nickel which is nickel 62 which is different from nickel 58 so this is typically a nature of scattering cross section of or scattering amplitude for neutrons as a function of atomic weight this is for thermal neutrons and this is for x-rays so compared to x-rays this is also a desirable property for neutrons I will stop and then I will take you for the neutron sources in the next part