 So, in this part I will familiarize you with various sources of thermal neutrons that are used in neutrons scattering. Though neutrons are discovered by using a source which is radium alpha beryllium source, but these are very weak sources, the radioactive sources. We usually go for either a nuclear reactor or something called as palatial neutrons also. Presently we have Dhruva in India which we have been using since 1985 and we have plans for a future high flux research reactor. The highest flux research reactor is called a reactor high flux at ILL Grenoble which is a highly enriched core and the flux of ILL Grenoble ILL reactor is around 10 to the power 15 neutrons per centimeter square per second. In case of our reactor Dhruva it is 1.38 into 10 to the power 14 neutrons per centimeter square per second. So, this is the flux we usually write in terms of neutrons per centimeter square per second and in case of synchrotron you may be familiar with it that they are write photons per centimeter square per second per ster radium. So, there it is a brightness and this is the flux. There is a subtle difference because the synchrotron sources are highly collimated and highly convergent beams of very very small angular width and that is why there we talk about photons per centimeter square per second per ster radium whereas here we go up to neutrons per centimeter square per second and neutrons it is difficult to focus there are some devices, but usually it is not so easy to focus neutrons and discuss these things when the time comes in our discussion. So, these are the RHSF, IRL is Grenoble is the highest flux reactor, but there are other reactors like FRM2 in Germany, FRM2 in Germany, also NCNR, Neutron Center for NCNR, NISD Center for Neutron Research at NISD in USA, these are the most famous reactors that are operating presently and there is another kind of source which is proton accelerator based spallation neutron sources. Nuclear reactors are critical assemblies, critical assemblies, critical assemblies, critical means that the chain reaction is going on continuously unless we stop it the reactor goes on operating and generating neutrons and number of neutrons from one generation to another generation remains same. So, it keeps on working indefinitely without any external aid for neutrons. Accelerator based sources are not critical assemblies, they are sub-critical assemblies, they are sub-critical assemblies, a reactor needs to work in a critical mode because otherwise we cannot get sources and neutrons continuously, but in case of accelerators first an accelerator is used to get a proton beam, proton beam beam of nearly 1 giga electron volt. Why such a large energy is required because here the operation is such that there is a, let us consider this as a nucleus and the proton beam impinges on it so that there is a huge coulomb barrier because this is positively charged and all the nuclei are positively charged so they repel each other, we have to overcome this huge potential barrier of nucleus and once the proton enters the nucleus of such a high energy then it shares the energy with the all the nuclear particles and then this there is a spellation the phenomena is called spellation or boiling of, a simple term boiling of, spellation, spellation, so in this spellation reaction we get a very large number of neutrons around typically around 30 neutrons per spellation, so you can compare this in a reactor in a fission on an average we get 2.5 neutrons per fission and out of this 2.5 neutrons we use one neutron to keep the reactor critical or alive and just 1.5 neutrons we can use either for our experiments or we can take it out and we can do some other job with the neutrons so basically typically we have around 2.5 neutrons per fission so there are very large number of neutrons in a spellation and also here the neutrons come with energy typically in mega electron volts in spellation the energy is much higher it can be 30 40 mega electron volts or even hundreds of MEVs when neutron are knocked off by the proton in the forward direction so they can be a very high energy so such a spellation neutron sources are usually pulsed in nature that means the neutrons they don't come continuously but the accelerator accelerates a bunch of protons which are impinging on a target and then there is a gap then there is a second bunch so they are actually pulse sources so accelerator based sources are pulse sources usually there is one accelerator based source at this is PSI Switzerland which is accelerator based source but it is nearly continuous and there is a very large proton current of around few milli amperes amperes and this is almost like a reactor source but otherwise all other major sources I named a few here the ICs spellation neutron source at Rutherford-Appleton laboratory spellation neutron sources Oak Ridge then there is a J park in Japan there the sources are generally the spellation neutron sources are pulsed in nature and because reactors are continuous and spellation sources are pulse the kind of spectroscopy we do use the neutrons differ we shall be describing you in my later lectures in case of reactors you have a continuous beam so use the continuous beam to do all our experiments here it's a pulse beam so I will be giving details later but what to use most is time of flight or TOF spectroscopy in spellation neutron sources details will come later so the nature is different and I will try to describe the time of flight spectroscopy or the monochromatic spectroscopy vis-a-vis the kind of instrument that you can build in my lectures and I will explain to you why one is how one is done and how the other is done so now it's a tool for radius sources so that you get familiarized with the look and the scale of things this is the core of the reactor high flux at ILL Grenoble source is here from where I got this photograph so this is a reactor done by run by UK France and Germany as a collaborative project and the most possibly the sort after new neutron source where people from all over the world converge to do experiments this is just to highlight the issue that one can do neutron scattering and how this is an important important technique that such huge setups are run by multi countries it is like for example it is like the certain setup for particle physics so these are centralized large facilities what you are seeing here is the photograph of the reactor core from top the reactor core is very small if you can look at my hands the very small it's a small enriched core enriched means it's enriched with uranium by more than 90% and it is surrounded by a large moderator tank and the beam lines I will show you later in various schematics all look at this reactor core and receive neutrons from there this is historically this is the first reactor in Asia apsala reactor this was this is a pool type reactor in simple words pool means the core is sitting inside a water pool and the water pool acts as a coolant as a moderator for this code same thing is true for apsala this is an external view of the pool it's the reactor is inside this so you can look at it from top this is how the pool looks like so this is the reactor is hanging from top and there was a facility where we could move it from one part of the pool to other to feed to various different experiments so this was a pool type reactor which is now not operational and new apsala reactors come up that is also pool type in the Bhava Atomic Research Center Mumbai campus that is called new apsala so apsala has given rise to new apsala this was the first research reactor in Asia and India happened to be the first country to take up neutron scattering neutron scattering scattering technique to characterize the condensed matter using apsala reactor so I have taken it from the online from the online source and this is how it looked like apsala now this is the present reactor what we use is a Dhruva reactor this you this just to give you a view you can see the people doing experiments are here so this gives the scale of the things one thing you notice that there is something which I call reactor block so this reactor actually it has got a pool which is typically around typically three meter diameter and three meter height so this is a cylindrical object the reactor and this is the outside the cylindrical block that you are seeing but because every time we have a spallation reaction or you have a fission reaction lots of unwanted radiation unwanted means they can be harmful to human beings they also come with fission as fast neutron so in a reactor you have thermal neutrons but every time new neutrons are being born and there are fast neutrons so there are fast neutrons there are thermal neutrons there are huge intensity of gamma reflux because in fission you also have excited nuclear with the de-excite and give out gamma rays so when we propose plan to do experiments at such places we need to have a huge huge shielding so that the personnel can act safely work safely inside a reactor hall or in the experimental hall of an spallation neutron source so here this reactor code first it is surrounded by a D2 reflector and the moderator is also D2 in this reactor so there is no physical boundary but there is a region where you have D2O this D2O reflector when the neutrons leak out to them sends them back inside the code so they are acting as reflector and the D2O in the code is acting as moderator moderator so those same D2O one is acting as a moderator to bring their energy of the neutrons down a part of the thing is acting as reflector to send the or the reflect the neutrons back inside the reactor code so that we don't have any unwanted leakage from the code this is followed by a light water vault H2O and then it is followed by a huge biological shielding or it's a concrete shielding surrounding it what you are looking at is the outer face of the concrete shielding and this whole thing this is approximately 5 meters thick so if the reactor code is 3 meters so the radius so 3 plus 3 6 plus 10 so what you are looking at is a cylindrical block which is nearly 16 meters in diameter 16 meters in diameter and outside that we have a large number of instruments right now I can just tell you the names I will come to each and every instruments later in my talks and explain what kind of experiment what kind of characterization are done at that so there is a powder diffractometer using position sensitive detectors there's a single crystal diffractometer there's a magnetic diffractometer there's a triple axis spectrometer and there is something called neutron guides for neutron transport and you also have a through tube in this reactor and also we have instruments run by an organization which is an university grants commission UGC and DAE collaboration and they also run some instruments here so this will make you understand how the reactor code looks like so this is a schematic this is the core of the reactor where neutrons are getting produced and then you have these beam lines these beam lines these beam lines these beam lines are nothing but they are actually what should I say holes intentionally provided in this cylindrical reactor block and some of them are radial that means if this is the core they are coming out from the core radially and there are some which are tangential we they don't come out radially but they are added as a tangent to the reactor core and all this and these things which are shown this huge drum like objects these are known as monochromatic drums and I am shown here as a circle in this plan view and also there are two guide tubes to guide the neutrons out we shall briefly explain to you so that means this is the core this is the heavy water reflector then the water vault and then you have this biological shielding and outside all these are done so that the reactor remains critical and the people remain safe in this spot which is the reactor hall because experimenters have to work in the reactor hall in a radiation environment where the radiation level is continuously monitored and are within the limits safe limits set up by international council for radiation protection ICRP and this is strictly followed continuously they are being monitored and people who work here they also use radiation badges which gives cumulative dose one maybe might be receiving enough in any experiment at the time they spend inside the reactor core and we have two parts of this experimental facility one is the reactor hall and another one is an adjoining hall known as guide tube laboratory so many experiments are done in the reactor hall but there are also experiments which are done in the guide hall so guide halls at the present day they are an integral part of any neutron source whether spallation neutron source or reactor where the neutrons are taken away from the core by using neutron guides this I will explain to you later this is a typical schematic of a spallation neutron source this is the I have taken it from internet this is the icis source at rather for dappleton laboratory in UK UK so this is in a place called harwell located so here you can see first there is a small linear accelerator which accelerates the proton beam up to 70 MeV after that this neutron is fed to a synchrotron synchrotron and this synchrotron boosts the 70 MeV to 800 mega electron volt 10 times and after that you can see it is travels a long path and here there is a target this target need not be uranium because this is not a reactor this is a subcritical assembly it can be any high Z material like tantalum it can also be depleted uranium like uranium 238 but usually a heavy target like tantalum can also he's also used so here the target is typically maybe a few feet by few feet in dimension and in which the proton beam impinges as I told you that this is a very high energy proton beam so it can knock off neutrons with very high energy forward direction so in the forward direction there's a very heavy shielding now neutron for shielding thermal neutrons it is preferable that you use some hydrogenous material they can be polymers they can be wood they can be some often we use something called borated wood borated wood because boron is a strong absorber of neutrons and wood is a very good moderator so we can use the wood to moderate and reflect the neutron beam and we can impregnate wood with boron to absorb and in this case when you have very high energy neutrons going in the forward direction these will not do because the energy of the neutrons is very high so use a very high Z material like stainless steel in the front side which can absorb such a high momentum and erase the neutron going in the forward direction but apart from that if this is the target it's just a schematic I'm trying to just give you an idea maybe 1 or 2 feet 1 or 2 feet so once it impinges here so you have a very large number of neutrons generated in the spallation I told you in part reaction you have nearly 30 neutrons and now these neutrons are at a very high energy so what is done here I'll just let me just try to give you some schematic so basically this is the target this is the target in which the proton beam comes and produces spallation so now these are 10 to 100 mega electron volt neutrons which cannot be used for the purpose of conditional studies so what is done actually there are moderators I'm showing the moderators as blocks moderators below and above this target station so maybe ambient temperature means room temperature of around 300 Kelvin or moderators at very low temperature like 100 Kelvin methane or maybe another moderator is there hydro liquid hydrogen hydrogen at 20 Kelvin these are known as cold sources I'll at least describe to you what are cold sources later and this is they are also several ambient temperature moderators so these moderators when the neutrons they come out in all possible directions from a spallation target they enter these moderators they get thermalized once thermalized then they're ready for use in our experiment and then we go for the experiment now you can see like a reactor I showed you the monoclonal drum surrounding the core here surrounding the small target and the accompanying moderators you have large number of instruments sitting on these circles and you can see some beam parts are very large very long why I will tell you later so the so this is the part which is accelerator and this is the part which is experimental hall there was a single target till a few years back in the other for dappleton laboratory now this beam of protons is diverted to two sources so there is target station one and target station two in rather for dappleton laboratory and experimental facilities are provided at these two places so this is not to teach you everything about rather for dappleton laboratory but this is to tell you typically how an accelerator based source with respect to a reactor source in the reactor source I showed you Dhruva because that is what I'm quite familiar with and this is typically how it looks like for a spallation neutron source in both the places you have moderators but here there is an accelerator there you have the whole assembly a critical assembly so here it is pulse and in case of rather for dappleton lab this isis we have a source which has got frequency of 50 hertz 50 hertz that means every second you have 50 pulses of proton hitting the target so giving us 50 pulses of neutron they are actually first these neutrons are arrested in the moderator or they enter the moderators thermalize and after that they are used for experimental purposes so we have got 50 pulses to work with and a few of the pulses they go to target station two few of the pulses go to target station one and the sharing is dependent then on the kind of experiments and the kind of instrument they are available at the two targets so this is a typical schematic of the spallation neutron source isis at rather for dappleton laboratory similar sources are now there at sns spallation neutron source which is an oak ridge usa and also a very large facility is coming up even some of you if you are interested in doing experiments in future maybe writing proposals for this source known as ess european pn spallation source it is coming up near the university at lund in the north in the scandinavian countries near lund university and these are joint venture of many european nations like sarne or like rather for like ill at granable so these are joint effort and a very large spallation neutron source which is coming up yet to become operational so i showed you the experimental hall in druba reactor this is a experimental hall at rather for dappleton laboratory in uk so again you can see the scale of the things they are big and the facilities are huge and there are basically to work there one needs to familiarize with all the safety rules and how to enter how to come out and they need some training also so these are little elaborate process but the aim is to study condensed matter using spallation neutron sources