 Welcome to the lecture on detectors in this series of lectures on neutron scattering techniques. I have discussed with you various components of devices that are required to write from tailoring the beam in the reactor core to the beam falling on the sample, whether in the last leg of this description, I will narrate to you how neutron detectors work because this is important for understanding various neutron scattering techniques, how you design them and how we use various detectors and in different kinds of sources. So with this brief introduction, let me get to the topics that I covered. Regarding neutron beam design, I started with reactor configuration, I discussed with you how beams are made in the reactor and then how they are tailored to hot or cold site using cold or hot neutron sources using moderators at cryogenic temperatures like hydrogen, deuterium, methane and also using hot moderators like a graphite at 2000 degrees centigrade. Once the neutron beam has been tailored in this manner from thermal to cold to hot as well as thermal, of course, I discussed with you how the beam lines are designed to take these neutrons out and also the neutron guides that are the optical fibers for neutrons I am arping because they can take neutrons far away from the source to neutron guide holes. I also showed you the guide of photographs and then I discussed the beam tailoring in the beam site, the beam line using in-pile collimators as well as using filters. In-pile collimators restrict the geometry of the beam and the filters when the neutron beam passes through these filters, certain undesirable neutron energies are removed with good transmission for the desirable neutrons. So that is the basic definition of a filter which takes out the undesirable neutrons and allows the desirable neutrons to pass through it. Then I discussed solar collimators. Solar collimators are collimators which can bring two contradicting requirements together. That is we need a large beam from neutrons, something like 5 centimeters by 5 centimeters in drova. But at the same time we need good resolution, angular resolution for the beam to define our wave vector key. So that is done as I showed you earlier that the beam is broken into vertical slits and this vertical slits are separated by as narrow as 1 millimeter distances and that this 1 millimeter distance between the slits and the length of the collimator typically around say half a meter dictates what is the resolution of the k and at the same time the beam remains large because the entire solid collimator, collier collimator covers wider beam. Then I discussed one of the most important aspects in neutron beam, the monochromators. The monochromators they actually select the neutron energy from the polychromatic beam often after filtration because by filtration you can remove the high energy unwanted background creating fast neutrons. Then we select the desirable wavelength by Bragg's scattering. But there are other kinds of monochromators I will discuss it later but in general the monochromators are silicon, germanium, pyrolytic graphites and they allow you to choose the desirable wavelength or energy depending on the Bragg angle that you choose in the monochromatic drum. Now comes the last piece of device which is a neutron detectors and monitors. So after all in a counting experiment in a scattering experiment you need to count the number of neutrons that are being scattered and for that we need neutron detectors and in this lecture I will be describing to you the neutron detectors. Neutron detectors of course an important part of neutron scattering because that is the final object which gives me the scattered intensity. But the thing is that these neutron detectors they vary with respect to the way we do the experiments because as I mentioned earlier in the present day we have not just nuclear reactors we also have spallation neutron sources and spallation neutron sources use a different kind of spectroscopy called time of flight spectroscopy which we will discuss later. Whereas in case of reactors they use what is known as monochromatic neutron beam and then the spectroscopy is with respect to monochromatic neutron beam. In case of spallation neutron sources we use time of flight with respect to a polychromatic beam that means we don't use a monochromatic beam but you use a beam with many wavelengths and the detectors will be different for all these kinds of experiments. So I will be briefly in this lecture mention to you how the experiments are done and how the detectors are working. So we are all familiar with gas detectors and gas detectors are omnipresent for almost all scattering experiments. We are aware of nuclear experiments, radiochemistry and neutron scattering which we are discussing now. Now in most of us most many of us during the master's days we have done nuclear experiments and we have done counting and we have used Geiger-Muller counters we know. I know that many of you have done experiments using Geiger-Muller counters. Now regarding radioactive materials charge particles are there like alpha rays and beta rays coming from the nucleus and also there are gamma rays and also external nuclear x-rays. So all of these the charge particles and the electromagnetic radiation they can cause ionization in a medium and ultimately the detectors role is to convert it to a current or voltage process which can be measured. So every time a charge particle like alpha or beta ray come inside the detector it is detected how I will tell you just now and finally registered as a current. Now interestingly alpha rays and beta rays are charged rays. Alpha rays are helium nuclei nucleus 2 he4 and beta ray is an electron but it is coming from the nucleus and x-ray is an electromagnetic radiation of wavelength typically around 1 to 10 angstrom. So all these can cause ionization but neutron is a neutral particle so it needs to be converted and for that our detectors have to be spatial unlike other detectors. So we need a nuclear reaction to produce a charge cloud and energy in an exothermic reaction and that can be detected in a detector. So alpha and beta is called once a charge particle enters a gas medium. I am primarily talking about gas detectors but it is also true for solid medium. Alpha is a very narrow range, beta has wheel of longer range because alpha particles are heavier, beta particles are lighter, lighter particles travel longer distance, heavy particles travel lesser and they can cause ionization and finally stop in the medium and through ionization charge is produced which is counted. X-ray and gamma rays also ionize but have longer range compared to particles and we are aware that they interact through photoelectric absorption, Compton scattering and pure production. So photoelectric absorption is basically absorption of an X-ray in an atom and then a photoelectric a photoelectron comes out in the process and the X-ray is absorbed. Compton scattering is when you take the photon picture and the X-ray or neutron or gamma ray it undergoes a momentum conserving scattering process and finally pure production happens when the gamma ray has energy more than 2 into 511 kiloelectron volts because that is the rest mass of an electron. So electrons are produced, positrons are produced in a pure production and these are charged particles which can be again counted and finally it causes ionization or scintillation. I will talk about ionization first scintillation later. Now for neutrons we need a strong neutron absorber and a strong neutron absorber like boron 10, 5 boron 10 or helium 3. Now 5 boron 10 in form of gas in BF3 the boron 10 enriched BF3 or helium 3 gas in a detector. Helium 3 you can see it is an isotope of the helium 2H4. This isotope is much more expensive than normal helium 4 but an extremely important component of neutron beam experiment because helium 3 detectors are used heavily and lithium 6 as an isotope along with z and s scintillators causes a nuclear reaction interacting with 0 and 1. It produces one tritium and one proton. Uranium fission where neutron causes breaking up of a nucleus also produces charged particles with high energy and that can also be used but usually when we use fission the efficiency of the detectors are not so good and they are used as monitor detector. What are monitor detectors? I will complete later. They are right now they are using low efficiency detection but when you want to have more than 80 percent detection efficiency that means in a scattering experiment the neutron which has gone out gone in and produced a charge cloud there we use gas detectors mostly gas detectors and in spallation neutron sources of course scintillation detectors are also used but first I will discuss the gas detector and how they work. So I have just given a table here for the reactions so let us take the reaction helium 3 with neutron. So I just write the reaction like this. This is a neutron plus 2H3 we need to conserve these numbers so it becomes so and forth. This is a proton this is tritium. These two both of them come out in this reaction and they are charged. So these charged particles when they move through the gas they produce ionization which is collected. Similarly let me just take the boron 10 reaction so it is this is the sensor we may say or the isotope which actually converting the neutron to charged particle. So this becomes plus a lithium isotope. So these are again they come out with some kinetic energy as charged particles and they produce a charge cloud which is detected but we must remember this neutron energy in most of these experiments I will come to the energies for these experiments later they are of the order of millilectron volts to electron volts they are much much lesser than the kinetic energy of the product. So detection of neutron does not give us the energy of the neutron because ultimately the charge clouds are produced by the products and not by the neutron itself not by the neutron itself. So the neutron we know we can find out the neutron energy but you can detect them and for finding out the neutron energy there are techniques which I will come to later. So similarly with lithium-6 isotope it can the neutron can interact and produce tritium and similarly I have shown here the fission of uranium-235. Gatellinium gives neutron to gamma conversion and we can count the gap we can we can excite or we can detect them on a photographic plane. So they are also used for imaging techniques in the foil and I have given the absorption cross section you can see that helium has a very high scattering absorption cross section of 5,330 bonds where one bond is 10 to the power minus 24 centimeter square always we talk about scattering cross section in terms of area because classically you can see if I have a disc of radius r it gives a resistance of pi r square in the path of a pure classical picture but absorption cross section absorption cross section cross section is in units of length square always. So I have given it in bonds and this is the exothermic energy listed here they come out from the products they have this energy so 764 kilo electron volts in case of helium-3 in case of boron it is 2.31 mega electron volts much more energy and similarly the energies are given in case of fission we have hundreds of we know and thing is that the helium-3 and bf-3 they are used in the gas mode mostly gas detector boron 10 can also be used as a solid coating so after detection the alpha particle and the lithium they cause tracks or charge in the coating and they can be used as a scintillation detector then lithium-6 can also be used as a crystal the fission chambers can be solid and gadolinium can be used as a foil which will produce gamma rays so I think that a target basic principle is that target nucleus absorbs a neutron and then two more charged nuclei are produced with a with kinetic energy sharing between them which is the q value for the reaction so this is not proportional to the neutron energy as I told you earlier this q value is fixed and it doesn't depend on the energy of the neutron the absorption cross section as I mentioned just now that it is in bounce which is to the minus 24 centimeter square for the thermal neutron the absorption cross section sigma is 1 by v where v is the neutron velocity and neutron wavelength is given by h by mv longer wavelength have higher slightly higher absorption cross section but this is the thermal range at the higher energy ranges that kv's we also have resonance absorption because these energies can match with the nuclear energies in the shell model and then they can be absorbed through resonance absorption that means higher absorption probability at certain resonance energies so now I'll quickly give a recap of the gas detectors so a gas detector is usually of cylindrical geometry I am saying usually because I will tell you later that in case of position sensitive some of the position sensitive detectors they can be also square geometry but for the time being I will just discuss the cylindrical geometry so in case of cylindrical geometry you can see that there is a central wire known as a node on which the charge is collected there is a body which is usually grounded and called as a cathode so the charge that we collect finally is mostly electron an electron shower comes and hits the anode the anode of the detector and is connected to preamplifier amplifier to the counters it goes and that makes interesting that in a cylindrical geometry it is easier to engineer and it's also fact and in a cylindrical coaxial cylinder which is the anode wire and the cathode the field at a distance is given by v upon r log b by a so the field is larger near the anode that means there is if I plot the field as the distance r from the center considering the anode is of zero diameter it falls drastically like this so the field is high near the anode very high and it falls as we go out in the detector so the field is high near the anode very high and is less and when we go out to the cathode and in this case the charge cloud which is produced close to the anode is collected so let me just now bring it to your notice that you might have done it before for other gas detectors that there are various regions when he increase the voltage in a gas detector the applied voltage so it looks like this if this is a number and this is the voltage first you have this region recombination this recombination region is that the charge produced initially that means whatever charge particles were produced in case case after absorption of neutron they have produced charge and they have caused ionization in the medium and in this range actually you are collecting those ionized particles the cathode and the anode but in this process as we increase the voltage the number increase because at lower voltages there is a probability that these electrons and ions they recombine and then I will lose them as we increase the voltage the field increases acceleration towards the anode increases and so ultimately start collecting all the charges and you reach a playthrough so in this region we don't collect all the charges in the playthrough region we collect all the charges so this is the anode this is the cathode so the positive ions go here and the electrons negative electrons come here now when I increase the voltage further increase the voltage further in this increase the voltage further now the the number of particles that are collected are proportional to the voltage because now there is a multiplication so let us consider the electron and for the timing forget the ion and electron has entered the high charge field over here high high field region over here sorry and then here it starts producing showers starts producing showers and they are there so there is a multiplication so one electron entering here might give you a million electrons after the shower and they are collected here so that means the number collected the voltage as we increase the voltage improves so check this so the detected charge for the detector it increases here because there is a proportionality with respect to the voltage as I was showing so the showers are produced here and increases but if I keep on increasing the voltage then this proportionality is lost and we go to something called Geiger region this is the Geiger region so this is recombination plate 2 this is a proportional region here basically what I mean is the number of charges collected at the detector not the number of neutrons please remember this and then we go to Geiger region in the Geiger region since it is not proportional to the incident energy of the charge particles so in case of I mean in case of alpha rays gamma rays beta rays them so where they produce the ionization directly but in the Geiger region this non-linearity tells us that it is not proportional to the incident energy in case of neutron anyway the incident energy that of the charges they are produced so it doesn't come from the Geiger region we can just count the particles but we cannot find out the energy whereas in the proportional region we can get a voltage pulse which is commensurate with the energy of the particle and we can detect the energy of the particle once we calibrate our detector so in the Geiger region we just count it's just a counter so we call it a Geiger counter Geiger counter so in this case we are generally in the ionization and the proportional region for the neutron detection but I must mention that here when I say the detected charge is proportional to the energy of the particles we can find out energy of gamma rays and energy of beta rays in this detector in the proportional region because they directly create the ionization whereas in case of neutrons it is faster nuclear reaction which is independent of the energy of the neutron and then the Q value of the of the reaction is decided before we known before and it is same so we don't try to find out neutral we cannot find out neutral energy from here for their other processors there but this plot shows you for proton the charge produces more that's why again for electron the charge produced is less but this plot shows you the various zones in a gas detector where you are detecting the particle this is the recombination region the play to where you are collecting all the initial charges then they are proportional they are the number of charge collected at the detector increases proportional to the energy of the incident neutron or in sorry not neutron incident charge particle which is proton electron as shown here and then we go to a Geiger region where it is not proportional there is a quick recap of the gas detectors and we here also use gas detectors using bf3 gas or helium 3 gas so I just to remind you that neutron absorption reaction with boron 10 please know that when boron 10 absorbs a neutron it creates lithium and alpha particle which is 2he4 and 2.31 mega electron volt and this is the excited state that's why star is there 94% and 6% is the ground state with this energy so basically this energy is shared by the lithium and alpha so here I have pictorially I'm showing this so 94% goes to an excited state lithium then it gives out a gamma ray because it comes from excited state to the ground state and the 6% is directly produced in the in the ground state and this energy 2.70 or 2.31 mega you can see that it is shared between the two products of the nuclear products products of this reaction and the heavier one has lesser energy and the lighter one has got more energy same true here and that is we know very well from classical mechanics that here the neutron energy or direction is immaterial because these energies are much higher compared to the neutron energy which is typically in a milli electron volt or electron volt region but once these are produced so basically entire energy is shared by them and they travel in the opposite direction so this is a reaction for the boron tank similarly the reaction for helium here it is gives out the exothermic reaction the energy kinetic energy of these two products the proton and the titan is 764 kilo electron volt and it shares in the same manner as I told you earlier the lighter one has higher kinetic energy and the heavier one has lesser kinetic energy and these are the particles which are detected in the gas detector that I showed you and neutron energy is immaterial here it is detected these their energies are fixed more or less whatever be the neutron energy and we can just detect neutrons in these gas detectors but this covers the first portion of the talk