 So, again the core as a core or in this case the spalation target once moderated, it has got a maxillian at near room temperature or at lower temperatures using cold neutrons. So I will quickly tell you the role of cold neutrons are the family to remoderate the neutrons from the thermal to low energies because for some experiments we need neutrons with lower energies for some experiments we need neutrons in the thermal energy. So we can in some parts of the reactor or the spalation target we can insert a cryogenic moderator, cryogenic moderator, moderator that means a moderator at low temperature. The typical moderators are hydrogen, liquid hydrogen at 20 Kelvin, liquid deuterium around the same temperature around 20 Kelvin in rather for dephotron laboratory also have liquid methane at around 100 Kelvin. So these moderators they capture neutrons, re-thermalize them that means this maxillian is re-thermalized into another maxillian at raw temperature and in the process you don't multiply the total number of neutrons but in a certain region of energy in a certain region of energy you have a gain in number of neutrons at the cost of high energy neutrons and these sources can be used for experiments where low energy neutrons are preferred. So these are known as cold neutron sources which gives us cold neutrons. I just show you the photograph available with me this is the photograph of the through our neutron guides. Now what are neutron guides I will briefly tell you. Neutron guides basically the reactor is on that side of the there's a wall here this wall here I showed you the reactor hall let me just show the photograph. This is the reactor hall this is how the neutron guides are taking neutrons out from the reactor hall and now in here the neutrons are coming from there and they're traveling through the guide like this. So these are neutron transporting devices you can call but they are static devices there are no moving parts in them. The way they work is using the principle of total external reflection of neutrons. This is necessary because one it is difficult to input to place all the experimental facilities in the reactor hall. So you need more space. So that is possible with neutron guides because in general neutrons are neutron they're coming out isotropically the isotropically from the reactor core they're coming out isotropically. So there is no directionality so they go in all possible directions so every point of the reactor core behaves like a 4 pi source. So if it's a 4 pi source you can see that the fall of intensities will be 1 by 4 pi r square we know that. So that means if I go 5 meters or 500 centimeters 500 centimeters then the flux falls by 1 by 4 pi is around 12 into 25 into 10 to the power 4 so 12 into 25 is 600 300 so one third into 10 to the power minus 4 3 7. So it falls by a factor of 10 to the power minus 7. This is too smaller value and so if you start with 10 to the power of 14 neutrons as I told you if I just allow it to go out by 5 meters which is typically the distance it has to travel to reach the outside of the reactor block I get neutrons only which is number is about typically 10 to the power 7 neutrons. Neutrons per centimeter square per second. So neutron intensity falls very fast because they are coming out isotropical. Now how to take them say tens of meters away 10 20 30 or even hundreds of meters away how do I carry them all the neutrons I cannot carry but some neutrons I can how that I will just quickly table. So if we can assemble neutron reflecting elements like this so consider how light travels in an optical fiber you know that optical fiber is a cylindrical thing and once the light enters it can follow the optical fiber whether you bend it or keep it straight through total internal reflection total internal reflection. In case of neutron the similar thing is done but for neutron the refractive index n is slightly less than 1. So for light the refractive index is typically 1.5 1.3 so it is more than 1. So you have total internal reflection in case of neutron because n is less than 1 you have total external reflection. So these are nothing but I'll show you how typically a neutron guide looks like it's an assembly which looks like this. So a rectangular assembly where these walls these walls they are made of some good neutron reflecting material. Neutron reflecting material. So I mean reflection means optical reflection so it follows Sren's law. Neutron impinges on a surface and then reflects but because n is less than 1 up to a certain angle neutron gets total externally reflected and the typical neutron guide is a nickel. So you quote typical very flat glass pieces known as float glass and quote it with a good material for neutron reflection. Nickel is good because nickel has got a reasonably large critical angle. So and also a good metal to handle. So the guides that I showed you here in Dhruva these are the plan view of the guides these are the nickel coated pieces nickel coated pieces in case of Dhruva the dimension of the dimension of this guide is dimension is guide is 25 millimeter by 100 millimeter. I will show you the photograph of the guides and the experimental setup that have been put up in the guides in my later talks because specially neutrons of lower energy or experiments where we need low energy neutrons they are taken out by using neutron guides from the reactor hulls and those experimental setups can be set up in a neutron guide hull. There are several advantages several advantages of using neutron guides one is that instead of 1 by 4 power r square loss because it is traveling through total internal reflection you can have typically 70 to 70 percent 60 to 70 percent transmission of the neutrons. So you do have some loss in intensity but the fact is because we have gone outside the reactor hull the number of background neutron counts fall drastically by a factor of 10 to power 4 or 5 so 0.6 transmission with a transmission with a drop in the background of 10 to power 4 gives you an excellent signal to noise ratio for experiments and you can do much cleaner experiments in the neutron guide hull and I will describe such experiments in my later talks when I discuss typical experiments using neutrons of lower energy or cold neutrons. Why cold neutrons are preferably transmitted using neutron guides because the critical angle of reflection the critical angle of reflection depends on the wavelength of the neutrons and longer the wavelength lesser is the energy larger is the critical angle and they are preferably transmitted in the neutron guide. Also another interesting thing instead of making the neutron guides straight we can make the neutron guides slightly bent when you do this then you can see that now this neutron beam which is coming out at this point but you don't look at the reactor core directly you are avoiding looking at the reactor core directly I'm using a little colloquial term like looking at basically along with preferred neutrons desired neutrons which are low energy neutron you also have fast neutrons and gamma rays coming out from the reactor core and they will also travel in this direction. Now we don't want them so the fast neutron means high energy neutrons for our experiments in guide hull we need low energy neutrons and gamma rays because a certain solid angle you are looking at the reactor core directly and those gamma rays are harmful when you make slight curvature the one which is following the neutron guide by total internal reflection can also follow the curvature of the guide and come out here but fast neutrons and gamma rays they cannot follow the curvature of the neutron because fast neutrons will have very very very low critical angle and they don't transmit and gamma rays don't get reflected by the guides so they cannot follow so I can get rid of the gamma ray and fast neutron backgrounds by bending the neutron guides and the guide hull photograph that I showed you that the photograph I showed you here there are two guides there are two guides this one and there's other guide at top you can see there are two guides one has a radius of curvature of 2.9 kilometers so this curvature is very small and the other one has a radius of curvature of 1.9 kilometers so 29 and it's around 2.8 kilometers so 28 hundred meters and 1900 meters are the radius of curvature of the two guides so that we can avoid fast neutrons and gamma rays so we will come to it again when the time comes to discuss experiments using cold neutrons but this is one advantage of using neutron guides and going outside the reactor hull to do experiments at lower background environment and another aspect is that when we want to put more number of instruments you can see that the reactor hull let me just go back to the reactor once again look at this photograph you can see at the reactor hull for setting up experiments we need to shield the outgoing beam and these are known as monochromatic drums which shields the reactor neutron beam and they are called monochromatic drum because each of them at the center of the drum we have a monochromatic which monochromatizes the incoming neutron beam by means of drag reflection and shields the other unwanted radiations it doesn't allow it to come out so but then this monochromatic drums are large they are around 1.5 meter in diameter which doesn't allow too many setups to be made inside the reactor hull and for that the answer is that we can use neutron guides and we can use more number of neutron guides in the beam path where we can put more number of instruments here and also you can have break in the guides where we can put again a monochromator and take the beams out and the rest of the neutron beam can travel so we can put more number of instruments at a low background environment in a neutron guides and today apart from simple nickel guides there are super mirror guides super mirror guides guides we are also planning to install super mirror guides what are super mirrors they are above mirrors that's why they are called super mirrors they are called neutron super mirrors and I will discuss neutron super mirrors in a later lecture today I stop here after familiarizing you with the neutron sources