 I have discussed various components of beam design with you, starting right from beam tailoring inside the reactor or at the source in espalation neutral source to compile collimeters to solar collimeters. And finally, the detectors. We have discussed this, but I now will introduce you to a few more devices which are essential, especially when we go for time of flight spectroscopy, mostly in pulse neutral sources, but also in case of time of flight spectroscopy in reactors. So I'll introduce you some more devices. And then I will discuss with you the difference in the experiment using time of flight technique and a continuous neutron source in a reactor. So far, I have gone from reactor configurations, how a reactor core is placed, and cold and hot neutron sources to all the way to neutron detectors and monitor counters in our discussion so far. Now I want to introduce you to a few more devices. One is very common in most of the sources known as neutron choppers. So I'll talk to you about neutron choppers, neutron velocity selectors, and an important component of neutron beam, neutron polarizers. Neutron polarizers, actually, they help us to get one spin component out from a given beam of unpolarized neutrons. Neutron is a spin-half particle. So with respect to any direction, you can have two possible spins, plus half or minus half. And there are devices like neutron super mirrors or Heusler and other monochromators which can remove one of these polarization components and allows us to have a polarized neutron beam. And the polarized neutron beam is important for studying magnetic structure and dynamics in materials. So neutron polarizers are an important component of magnetic neutron diffractometer, neutron reflectometer. Because there, apart from the physical structure, we are also looking for magnetic structure in our samples. So I'll start with something called neutron choppers. And I've written in the bracket for time of flight. What is a neutron chopper? It's a very simple device. A neutron chopper is a device that creates a periodic burst of neutrons when rotated or we chop the beam. So let me just start with the simple thing. So a neutron beam, in this beam path, I can put a disk material with a slot in it. The slot in it. And I rotate it. So in the beam path, you can see whenever this slot comes aligned and this thickness is typically, I just show, it's a cylinder of small thickness. So now whenever the beam sees this opening, it goes out. It goes out from the other side. Rest of the time, the material of the chopper, it is usually neutron absorbing material, neutron absorbing material. So it can be cadmium, encased in stainless steel, many other things, and it is rotated. So when it is rotated, that means in one rotation, every time it comes, I get a neutron beam. So this was a continuous beam. If it's a reactor, if it is a continuous beam, then here I have got a beam which might look in time somewhat like this. So if I may say, in every rotation, it goes one rotation. And again, I have a beam. Again, one rotation. And I have got a beam. So from a continuous beam, depending on the rotation speed, I have converted into a periodic neutron beam. Why we need to chop the beam? The reason is if we want to do time of flight spectroscopy, that means if I want to time of flight spectroscopy is when I measure the time the neutron takes to travel a certain length in a more general term. For that, I just like a race in Olympics. You need a start time and end time to calculate the velocity v of the neutrons. So that is start time and stop time. So the periodicity is brought in with the chopper in place. So here you see there is called a disk chopper. The thing is that there is a slot here as you can see through which the neutron can pass. This is a disk chopper. And there is something called a Fermi chopper. I will come to it right now. So usually, a cylindrical neutron absorbing material with a slit and rotated, it is not 1,000. I mean, not only 1,000, we can go even up to 20,000 RPM, 20,000 hertz in our rotation. But you must remember that there is a mechanical limitation on the rotation speed. So it has a limitation on how many neutron pulses you can get in a second. Another fact is that in case of reactor, the allowance for one pulse in a rotation, rest of the time, the neutron beam is present there. But you are not allowing it to pass through. So we lose neutrons. So it's poor for the neutron economy. But for some experiments, we can sacrifice neutron economy and go for time of flight in case of reactors. In case of pulse neutron sources, for example, when you have a palatial neutron source, then there are a number of pulses which come every second fixed for the source. For example, the ISIS neutron source at Rutherford-Appleton laboratory in UK, they have a 50 hertz beam. That means the proton hits a target 50 times in a second and generates a neutron beam. And that neutron beam comes with a frequency of 50 hertz, which is after generation, it is moderated. And then it can still... Then why do we need a chopper? Because already we have the periodicity of the beam. The fact is that then, in this case, the chopper can allow a band of neutrons which we want to get through it to the respective beam line. So to measure the time of flight, we can't have a continuous source, whether reactor or any other place. And we need to make it periodic using choppers. So and the Fermi chopper has certain some other... So this is a disk chopper. I've taken it from the site of Miratron, which is a commercial company. And you can see that this disk chopper is... Basically, it's a disk. It uses a band of neutron energy to go through. I have given the reference here. You can see the photo. These are the photograph. Now, coming to Fermi chopper. This is a little more interesting. You can see that the Fermi chopper... Of course, it's a cylindrical body which is being rotated. But most interestingly, it has got a curved structure, curved slits inside the chopper. So this is because a neutron is travelling in a straight line. But when you put it in a rotating frame of a chopper, then the straight line in the frame of reference of this chopper, chopper becomes a helix, becomes a helix. So it's helical. And Fermi chopper has a slit assembly. This is because if there is a slit like this, the beam is coming and the chopper is rotating at such a speed that by the time the neutron reaches this point, the chopper is rotated enough to allow it, or neutrons of certain in lambda, certain lambda with certain spread, that they can travel at a speed matching with the rotation speed of the disk that it can clear the chopper based on the rotation speed of the chopper and the velocity of these neutrons. So this is like a band pass filter. So it allows certain frequencies passed through the chopper. And this Fermi chopper has this slit. So it's some kind of a chopped, chopped wide monochromator. So I can say wide monochromator. So we allow a band of neutrons to pass and still we have the issue of wide wavelength band. So this is a Fermi chopper and there are many, for example, there's a quasi elastic spectrometer in NISD at NCNR, NISD Center for Neutron Research USA. The Fermi chopper allows an energy range of 2.2 millilectron volt to 15 millilectron volt energies to pass through. And that depends on the curvature of the helix and the band you want, you have a polychromatic beam from which you want to select a particular band which matches with the rotational speed of the chopper and gets to the other side. So this is how a Fermi chopper works which is like we can say not just a chopper which chops a neutron beam but also monochromatizes it to some extent. And now I come to the next kind of devices which is a neutron velocity selector. It really does not chop the beam. But it rather selects a wide monochromatic band from the given polychromatic beam. So it can be used in a continuous source also. Instead of a monochromator, now this velocity selector acts as a monochromator. But since it's again we have to rotate the device. So there's a limit on the how many rotations per minute we can actually do mechanically. And it is preferable when we want to use the incident neutrons to be slow neutrons or cold neutrons. So this is good for those spectrometers which use slow neutrons or cold neutrons for the experiments. So how it looks like. So as I told you earlier you can see that this is a cylindrical you can say body on surface of which there are slots of angle alpha and it follows a helix. So as I told you that in a rotating frame the neutron will follow a helix, helical structure, a helical path. And this helical path, the rotation speed dictates the helical path and the wavelength that you allow to pass. Of course with again because there's a width of the slot with a certain wavelength width. So now I can take a material and can make parallel helical slots like this and put it in the beam path. This is the beam path. I can show you the beam path which passes through it. And ultimately this velocity selector or I can V and lambda you know that so they are related to each other. So velocity selector is also a wavelength selector. It's basically a wavelength selector though you call it a velocity selector but it's a wavelength selector also. And so there are helical slots on this on the body of this cylinder in which the neutron impinges and there are many of such helixes machine on the surface of this and then they provide a continuous neutron beam of certain lambda plus minus delta lambda for the experiments. At the moment the small angle neutron scattering instrument at Dhruva it uses a velocity selector as a monochromator for its experiment. So this is how again another image that is a helical slot on a cylindrical body and as I told you earlier that this velocity of the surface is given by r into omega, omega r. Omega is a rotational velocity and delta theta is the width of a slot and this width decided by r delta theta and if you allow a neutron beam to come through it it will choose the wavelength depending on the rotational speed of this velocity selector because the velocity times t should match with the omega times t in the same time it should angular coverage should be such that that neutron can follow the helix as it passes through this thing. If it is not rotated then electrons will get into the absorbing material and get absorbed. When I rotate it then as say as a neutron beam let me just move one second if there is a helix like this helix like this and the neutron beam is travelling if I rotate it as the neutron beam goes forward the helix also comes in the beam path and gives the opening as it goes further this helix comes in the beam path the opening in the form of helix comes to the beam path and allows the neutron to pass through only of certain wavelength and band. So this is a velocity selector which I have shown. So the thing is that neutron scattering is done in a continuous source like reactor and usually we do in the reactors experiments using monochromatic neutron beams and nowadays we are also using PhD based setups for the diffraction experiment specifically where we are trying to find out structure sometimes we also use time of flight techniques in reactor sources but then if to do this we have to use a chop beam at the cost of neutron economy as I told you earlier a chopper throws away a lot of a lot of neutrons and allows only a certain burst of neutrons to go through it in general the major pulse sources they are spallation neutron sources of course there are still some pulse sources based on electron accelerators so pulse neutron sources they have pulse nature and time of flight is the natural choice of spectrometers for the pulse spallation sources I must mention that earlier there were also electron accelerator based sources like one is there even now at Hokkaido University so in case of electron accelerator based sources it is an n gamma reaction so neutron not neutron I am sorry n gamma gamma n reaction so an electron it is a gamma n reaction that means it is a photo reaction gamma n reaction and what is done actually an accelerated electron beam is impinging on a target as it impinges a metal target the electron beam gets decelerated and what is known as Bremstrahlung gamma rays come and this gamma rays cause photo fission and giving rise to neutron but this efficiency is quite low and you have to give almost 1000 mega electron volt per electron per neutron per neutron is required at electron based sources so electron based sources suffer from this problem this gamma ray which is causing photo fission can also cause hitting of the target hitting of the target and that causes the problem of raising the power or raising the flux output of these sources so these are usually low flux pulse neutron sources but the major sources today are spellation neutron sources here we have proton beam of high energy hitting a target and generating neutrons by a spellation reaction now usually these sources are pulse so ICs at Rutherford Appleton Laboratory UK who are in the Oak Ridge SNS at Oak Ridge in UK, USA and also in Japan we have these spellation neutron sources one is coming up at Europe joint effort called ESS European spellation neutron source which is coming up in one of the Nordic countries mostly they use time of flight techniques but there are some exceptions like sometimes we can do time of flight in reactors similarly there are some spellation neutron sources which act like a continuous source and the example is the PSI source in Switzerland this uses a large current of the order of milli ampere impeach on a target and it acts like a reactor so all the reactor based spectroscopy or scattering techniques are equally applicable to PSI with some time of flight based techniques now I would like to go back to the most general instrument design in case of a reactor so as I told you it's a monochromatic beam based technique so as I showed you earlier also there is a collimator which allows the direction of the beam to be fixed then there is a monochromator which through Bragg angle so there is a direction which is fixed then there is a monochromator which uses a Bragg angle 2D sin theta equal to lambda so these can be pyrolytic graphite silicon jamming and makes a monochromatic beam simple and then the outgoing beam the direction and the energy can be detected by putting an analyzer crystal which is going round the sample so this analyzer crystal again using the same principle of Bragg diffraction so this is monochromator this is the analyzer principles are same so from this scattered beam it selects a beam and as you keep rotating this analyzer around its own axis you can change the sin theta you can detect different lambda and by going around the sample you can go to various values also in various directions so then you scan it at various angles so one is the analyzer can go round the sample and change this wave vector wave vector transfer because these angles with respect to incident angles are different these angles are different this is theta 1 this is theta 2 this is theta 3 so we can go for various angles by taking the analyzer around the sample and by rotating the analyzer around its own axis I can change the sin theta for the same dispacing and for every angle I can detect the final energy lambda final and this is the lambda incident so I know e incident minus e final from this data so that's how earlier also I said this is axis 1 sample is axis 2 and analyzer is axis 3 and if I don't want the information about the final energy that means if I am doing a diffraction experiment then I can forget analyzer crystal so I have the monochromator of course monochromator it falls on the sample but now I don't have the analyzer and what I have today in most of the instruments in Dhruva and other places we have position sensitive detectors position sensitive detectors covering a large angle and collecting the scattered data over a large angular ring in my detector so this is the diffraction experiment in a reactor when I want to do inla 6 scattering that means I want to know about the dynamics when I want to measure the energy differences then this is not the way I need to have analyzer and one more step to find out the energy of the outgoing neutron and then that is the most generalized spectrometer as I told you monochromator sample analyzer not used in case of diffraction and then analyzer gives out the energy of the outgoing neutron and usually analyzer is on an end on position because I have to move it when I am rotating the analyzer the detector also has to rotate around the rotating the analyzer around its own axis the detector has to follow to catch the neutron beam and then it is put in an end on position that means end on position and PSD these are the two differences I will explain it briefly in a scattered beam usually PSD is put like this this is the one meter long position sensitive detector so the beam enters and gets detected this is the beam path but if we do a serial scanning then the detector rotates around here what I was showing with the analyzer position analyzer is rotating and the detector is rotating in a theta to theta coupled mode and the detector is here which is end on position where the neutron enters like this and traverses the length of the detector whereas in this case the neutron traverses the radial distance for the detector and this is the main reason for PSDs are using helium 3 gas because if you remember helium 3 the sigma absorption for helium is much larger than sigma absorption for boron so because in this geometry the neutron traverses much shorter length in the detector than the end on position where the neutron traverses a much longer path in the detector the only issue is that in this case it is a serial scanning here if I may say the scanning at the same time you can see parallel scanning