 Hello everyone. So today I discuss the sort of applications of nuclear chemistry in determination of analytes in different types of matrices using technique called nuclear electrical techniques. Nuclear electrical techniques are those techniques which are based on a nuclear phenomenon like for example nuclear reaction. So, when we have a we have a tool in the form of a neutron you have a charged particle beam depending upon whether you have a neutron as a projectile and you want to do some analytical chemistry using neutrons then we will call it neutron activation analysis. That means you activate a sample with neutrons and the radioactive radiations emitted by these activated products you are monitoring and that tells you about the concentration of the elements in the sample. So, that technique is called a neutron activation analysis and in today's lecture I will deal with neutron activation analysis. In the first part I will tell you about the the principles, the fundamentals of this technique and in the second part I will be talking about the some of the applications of this technique just to bring home the point how important this techniques can be solving many problems. And the second part I will take next time that is the ion beam analysis where we we use an accelerator providing the charged particle beams or protons, alpha particles or even other ions, heavy ions and so these charged particle beams can induce reactions or they can be just simply scattered at the upward angle or backward angle and based on these interactions again we can characterize a sample for its constituent elements or you can even understand processes involving these elements. So, is both these techniques are called as nuclear analytical techniques and in fact you will see later on how kind of information that will obtain both these techniques some of them are not possible by other techniques. So, I will discuss both of them one by one. So, the first technique as I was mentioning is the neutron activation analysis as the name itself implies that if you want to characterize a sample for its constituent elements then we can activate the stable isotopes of the sample by neutrons and as you know already by that neutron is captured by the different isotopes and you can get radioactive isotopes. In some cases even if you do not get radioactive isotopes you can determine the proper gamma ray by the constituent elements. This technique was developed by George Hevesy the scientist who in fact also gave the tracer concept which I discussed in the last lecture. George Hevesy along with Hilde Levy developed this technique in 1936 and since then it has become so popular among analytical chemists not only in the industry but there are many many fields in which this technique is being utilized. The best part of this technique is that of course at the same time you know if you just radiate one sample you can determine simultaneously several elements in the sample. Whichever elements are amenable to neutron activation analysis you bombard them and you get radioactive isotopes. Sometimes even if you can get prompt gamma rays so then you can determine the concentration of these elements and yet diverse matrices can be studied by neutron activation analysis. It is highly sensitive because the signal that we get in terms of the gamma rays you will you will get high yield particularly nowadays if you have a availability of high flux nuclear reactors. So, the reactors these days give neutrons of the order of 10 power 13, 10 power 14 neutrons per second centimeter square per second and because of that you can you can I say even the nanogram in some cases even picogram quantities of the isotopes. Secondly this availability of high flux nuclear reactors coupled with the high efficiency high filter germinated reactors. So, then you require high efficiency I will discuss the in details what are the factors that govern the sensitivity of this technique. In the periodic table if you see as many as 70 percent of the elements the periodic table are amenable to neutron activation analysis like for example sodium potassium magnesium calcium radium aluminium and so on. So, there is a endless list almost you know there are odd errors many of the actinides they are amenable to neutron activation analysis. You see it is not necessary that every study has to be done using neutron activation analysis, but it has got an edge over other techniques in many cases and in those cases you find this is very very useful. So, neutron activation first of all it is not destructive because you can preserve the sample for subsequent analysis and that is why it finds particularly not but useful the like archaeological samples geological samples which you do not want to destroy do not want to dissolve and analyze. So, you want to preserve them for any further artifacts. So, such a cases neutron activation analysis finds a lot of practice. So, you can have biological samples, geological samples, environmental samples, forensic analysis samples, archaeology, even medicine and of course, neutron nuclear technology. So, there are a lot of fields in which the determination of elements are required for it can be forensic application it can be just chemical, politic control of the materials and even other areas like archaeology, geology. So, neutron activation analysis if you see in analytical chemistry and other fields will find is very popular technique only thing is required to have a source of neutrons. Okay. So, let me just discuss what is the fundamental principle of neutron activation analysis. When a isotope captures a neutron I have written it as mass number A, capital A, capital X is the element. So, when it captures a neutron the isotope of the same element is formed A plus 1x and what I have shown is the sort of potential well of the target the compound neutral that is formed. So, the reaction involves capture of neutron by AX to give you A1 A plus 1x and in this capture reaction, energy equivalent to the binding energy of neutron in this nucleus is released. Like when a neutron is captured by a nucleus AX certain energy is released that Q value is positive for all neutron capture reactions and so that energy that is released the Q value is equivalent to the binding energy of neutron in that nucleus. So, this nucleus is excited for example, we have a nucleus 59 cobalt and gamma 60 cobalt this cobalt 60 nucleus is excited let us say 60 cobalt excited state and this is the ground state. So, this excited cobalt 60 nucleus having excessive energy of about 7 MeV first we will emit the gamma range. So, this gamma rays that are shown here they are called the prompt gamma rays because the excited state will decide its gamma rays within picoseconds. So, very quickly within picoseconds either one or several gamma rays may be emitted this prompt gamma rays themselves can be counted online by a HPE detect. So, you can determine the concentration of this isotope using the measurement of prompt gamma rays emitted by the sample and this technique is called prompt gamma neutron activation analysis. Now, since these gamma rays are emitted instantaneously upon capture of a neutron this is essentially is an online version means while the neutrons are reducing the sample in the reactor you have to do the measurement. So, how can you do that? So, for that neutron beams are taken out of the reactor and you have an arrangement where the neutrons are available to you in the outside the beam reactor hall and where you can put your sample and the detector system and then you can do the measurement of gamma rays. In such a situation in PG NAA so, you suppose you are going on are you count the sample for say let us say some time say half an hour. So, that counts that you will get in that half an hour is number of target atoms in the sample because all atoms are being exposed the cross section for the neutron capture the flux of the neutrons in that position the time of radiation efficiency of detection of the gamma ray and the intensity of that gamma. So, you can see here if you know if you irradiate if you do experiment for a certain time t and whatever counts you get you can find out the number of atoms of the target element and hence the concentration of that. You could cross section in the capture cross section which you know you have to do some experiment you need some monitors. There are some flux monitors some standard samples where concentration is known you can find out the flux a detection efficiency you should know using standards you can find out and the intensity of the gamma ray per say per how many you know times this gamma is limited if that atom is formed. So, all these are nuclear data which are available in the literature. So, this is the pronged gamma-neutronicism analysis you would need to do that experiment in the you know online either in a reactor or if you have a neutron source like a californium source or you can generate neutrons in an accelerator site by expelation neutron sources. So, you can do that way. But if you measure the gamma rays that are after beta minus decay then these gamma rays are actually called offline gamma rays or beta delayed gamma rays. In that case for example, koval 60 having half life of 5.27 years you irradiate for some time take out the sample of the reactor take it to the laboratory and it is a half life of koval 60 is 5.27 years. So, you can comfortably do the measurement in the laboratory. So, these are measurement of delayed gamma rays and this is the offline gamma rays. The bulk of the neutron activation analysis is usually done using delayed gamma measurements this is called NAA neutron activation analysis. Now, you are actually measuring the concentration of a radioactive sample. So, you are counting the radioactivity and what you get in the counts per second. You take a gamma spectrum for a particular length of time and divide the peak area by the time you got counts per second. That count per second is a familiar equation and sigma pi the rate of the reaction neutron capture the saturation time for particular radiation time the decay after the end of irradiation radioactive sample decaying with its own half life efficiency of detection and the intensity of gamma ray. So, this you can get the number of target atoms in the sample. So, how do you get the concentration? This is the mass of the element of interest number of atoms of target element is mass of the target element of interest the now it can be a multi isotopic sample like you know for example, cobalt is mono isotopic. So, this is only one isotope, but you have till you have many isotopes iron you have many isotopes. So, abundance of that particular isotope is theta avocado number and the atomic mass. So, if it is a multi element multi isotopic sample then atomic average atomic mass will different from the mass number the mass number and so, what you can determine is the concentration m, m is the parameter of interest to you. Now, quickly I will just give you that the theta is the isotopic abundance. So, you can see these are the mono isotopic elements where only one isotope is present in the sample. So, 100 percent abundance of arsenic 75, manganese 55, scandium 46 today we go through and the remaining ones are you will find the even ones even z elements. So, they have more than one isotope stable isotope aluminum is mono isotopic. So, the abundance of the isotopes will vary these are the cross sections for the particular isotope neutron capture reaction and so, they are in bond and these are the products that are formed after neutron capture, they are formed, they are their half-lives the gamma ray and then the intensity of gamma. So, you can see here gamma ray intensity some of them have 100 percent means every decay it could be beta minus solid tone capture will give you one gamma ray. So, abundance is under gamma ray abundance or gamma ray intensity is under percent. So, you can see here the sensitivity of the detection of the elements depends upon the isotopic abundance, the capture cross section and the intensity of gamma. So, essentially under the gamma ray peak you get a peak area that peak area will be more if the isotopic abundance is more, if the cross section for neutron capture is more and if the intensity of the gamma ray is more other parameter is flux. So, that does not depend upon nuclear data of the isotope, but another quantity the neutron flux in the reactor. So, these parameters you need to maximize to get high sensitivity even these are the other isotopes. So, there are several isotopes as I mentioned are amenable to neutron activation analysis. You can see some of them you see the mono isotopic ones. So, they will have more no sensitivity because there is only one isotope present in the stable isotope. So, let us discuss the sensitivity and detection limit of neutron activation analysis. I just mentioned the count rate in the neutron when you when you irradiate a sample with neutrons. Now, we are discussing about now neutron activation analysis that means you do an offline nuclear irradiation. Take the sample out of the reactor or neutron source and count the activity of the radioact stop in the level. So, that is the offline measurement. So, again this is the same equation which I mentioned in the previous slide. The this essentially gives you the number of target elements. This is the cross section and the flux, the saturation time, the pooling time, de decay time, efficiency and abundance of the gamma ray, intensity of the gamma ray. So, what is sensitivity? Suppose you irradiate one microgram of the sample, how many counts per second you get? That is called the sensitivity. So, counts per second per unit mass, let us say per microgram. So, sensitivity can be written as CPS per microgram or CPS per milligram or CPS per nanogram. If you have a sample containing one microgram or one nanogram or one milligram sample, what is the counts in your detectors? Suppose this is the gamma spectrum. So, this peak, this peak area you can put the limits and so this is the peak area of the gamma ray peak and this is the background. Below this is the background. So, you have to subtract the background from the gross count. So, when you put the two limits area under this graph will be gross counts and then you do a linear subtraction of this background. So, in this trapezium you subtract you get the net peak area. So, this peak area will be high if flux is high, projection is high, efficiency is high, gamma ray intensity is high, isotopic abundance is high. So, that determines the sensitivity of the technique. Second quantity is the detection limit. How much is the minimum quantity that you can determine by this technique is called the detection limit. Now, how do you define the detection limit? So, normally now when you will suppose there is no sample, if you take out a gamma spectrum, you will get a flat background counts or there will be some bump here. So, that bump, small bump it may be confused at the sample, the gamma ray spectrum due to the sample. So, you do not know or how much of the amount that will give you a reasonable peak area. So, what is essentially done is called the Curie's law. That means, if you take a background of a sample, suppose you have a sample, you don't have a sample and you just record the gamma ray spectrum. Then in the position of this gamma ray peak, you will get and you define this limits, upper limit and lower limit of the peak, you will get some background counts. And you suppose you take multiple times the background, there is if you and then over now, next time you count the sample. So, suppose you get some higher counts, how you make sure that there is a detectable amount of that particular element. So, the thumb rule for that is the level of detection is the 3 sigma background. Now, 3 sigma, what is the background of the standard deviation, sigma b means standard deviation of the background counts. So, suppose you have b counts in the background, sigma b is the square root of the counts is the standard deviation because variance is equal to me or the detection when you are doing gamma ray any counting you do radiation counting, they follow the Poisson's distribution. And so Poisson's distribution variance is equal to me that means sigma square equal to me and so the standard deviation is equal to square root of the mean value. Now, how do you want to quantify that if it is this much time of standard deviation, we say there is a confidence limit this much probability that there is sample contains some element of and so 3 sigma, 3 sigma essentially is a suppose you have a Gaussian distribution of the background counts. So, this is the mean background counts and this is the 1 sigma sigma b you have 2 sigma and we have 3 sigma. So, these are corresponding to different confidence level this is called the 68 percent plus minus 1 sigma, 95 percent and 99 percent. So, 99 percent of the time that means your sample contains that particular element. So, now if it is limit is 3 sigma means it has to be more than background plus 3 sigma. So, 3 sigma is essentially is an indication of a confidence limit of 99 percent. You are 99 percent sure that your sample contains that element that is the limit of detection 3 times the square root of the background counts under the peak. So, you can see here or the detection limits for different elements are given here in terms of picograms real values like dysprosium and europium have a detection limit of one picogram. That means if you have a picogram of a sample of these elements in a sample by Newton activation you will get measurable. So, over the background you will see a bump that you can quantify. So, this is called that you can detect that just it contains that element. There is something called limit of quantification that is more than 3 sigma is equal to 6 times the sequence then you want to quantify all the concentration. Here you want to just it is you can detect it 1 to 10 picogram, indium, lutecium, magnesium, so this is it tells you the decay characteristics, nuclear data, cross section for Newton capture, the gamma intensity, isotopic abundance and so on. 10 to 100, gold, olbium, 100 to 1000, 1000 to 10,000, 10,000 to 1 lakh, 1 lakh to 1 million and 10 million. So, different elements have different sensitivity of detection in the Newton activation analysis. So, you can choose. It is not necessary that we have to follow the Newton activation technique to determine concentration. These days you will find there are very sensitive techniques like inductively coupled plasma atomic mass spectrometer, deeply equal plasma atomic emission spectroscope. You can have a source, you can excite, you can generate a plasma and the excited atoms decay and then you can measure the gamma rate that the visible photons emitted by these atoms or you can generate plasma and then see emission lines of this one or you can do the mass spectrometer. So, depending upon the what you measure you can have ICP AES or ICP MS and this ICP MS techniques can go up to picogram per gram parts per billion. So, there are techniques now, but then what you need to do you need to dissolve the sample and you need to do the analysis, but in this particular case you just take the solid sample and you radiate in the reactor and you can get the gamma rate. So, the sample is intact, you can repeat the experiment or you can preserve it for future analysis. That is the beauty of this Newton activation analysis. Okay, let us discuss the how do we determine the concentration. So, there are mainly three methodologies I will be discussing. First is the absolute method of NAA Newton activation analysis. Absolute method means you use this equation counts per second. This is that to concentrate the amount of the isotope of element interest isotopic abundance. So, this is the NT sigma phi saturation factor and the decay factor efficiency and detection and the intensity of the gamma i gamma. So, you want to determine m in the absolute method. So, you need to know the section you anyway know in the reactor what is the flux at that position this factor can be determined easily efficiency detection efficiency gamma rate intensity is known. These are also not are known. So, what we can find out the m from this expression provided you know sigma and phi. So, sigma and phi sometimes you know you may not know exactly like the flux the reactor is a big place where the flux to the particular position may not be accurately known because the flux is not constant all around the reactor. So, this absolute method requires that you should have the data on cross section and phi. So, generally you know this is not a preferred method, but sometimes you may not have any choice that you have to go for absolute method. The most common method is comparative method or relative method. That means if you have a sample where the concentration of that particular element is known that is called the standard and with respect to that you can determine the concentration in the sample. So, what you do in this you irradiate the sample and the standard together. So, the irradiation time is same you are determining the gamma rate of the same isotope. So, the saturation factor is same detection of efficiency gamma rate intensity cross section flux all are same, what is different is the m. So, if you see CPS, CPS sample will be relatively proportional to now this all will get cancelled what you have m s is to this d, this factor you are counting at different times ds upon c and c it will be. So, m standard d standard ok. So, this is upon CPS standard. So, you can count count rate of the sample counted on the standard, mask of the element in the sample mask of element in standard decay factor for the sample decay factor for the standard. So, what you get is the m this value mask of the element in the sample, mask of element in standard which is known count rate in the sample upon count rate in the standard decay factor this factor. So, these are all very simple quantities. There are standard reference materials for which the concentrations are known. There have been inter-laboratory membrane experiments done worldwide to establish the concentration of the analytes in samples and those standards can be not even purchased from the source. Then there are certified reference materials and there are working standards. You can prepare a sample standard in your laboratory. Suppose you take a compound of known stoichiometry which is not hydroscopic, you can use for the particular element that standard. Working standards, CRMs, certified reference material and standard reference materials either of them can be used. So this is the most commonly used method but then there are situations when you do not know, suppose you have a new sample which you don't know what are these elements in that. So how can you have a reference for that? Unless you know the elements you don't know what is how to make the standard. So there this new development has taken place called single comparator K0NAA, K0NAA, Newton-Nectar analysis. That means this is a new in the absolute method for every element you have to have a standard amount in the sample. So you require the concentration of the element to be known in the standards. So suppose you have 10 elements, all the standards would have all those 10 elements. But if you have single comparator method, then you determine the concentration of all the elements with respect to the single comparator, like gold you can use as a single comparator or magnetic or standlium. So you irradiate a gold standard with the sample and you determine the nowadays activity of gold and the sample. And there are no methodologies by which you can find out the concentration of the element with respect to gold. Gold is already known so you can determine the percentage. So the single comparator become very popular for unknown samples where you don't know what are the elements present in that particular sample. I am not going to give details of this K0NAA. This is a bit more elaborate methodology where you require to know the flux. In the reactor, you need to calibrate the reactor position for the flux, ask flux to thermal flux. So because the cross-section for the gold, as I was telling now, the cross-section follows one by V law. There are some redundancies at higher energy. So the flux of the neutron or the function of energy of neutron, the cross-section of the element, the isotope as a function of neutron energy are required to be known. So it is little bit more elaborate. So lastly, what are the advantages of neutron activation analysis? One, it is the simultaneous multi-element determination. In the same time, if you radiate a sample, whatever elements in that sample are amenable to NAA determined by this technique, it is generally non-destructive. Suppose there can be cases that you can introduce lot of activity and you may not be able to handle it, but after some time anyway the activity will die down and so you can reuse the sample. So it is in general, you can say non-destructive sample. You can preserve the sample. After the reductivity dies down, you can use it. Matrix effects are largely negligible. Suppose this is like interference. There are very, very few isolated cases where the two elements will give the same gamma ray. Even if they give the same gamma ray, there may be different half-lives. So you can dissolve them. So that way, matrix effects are negligible. Unless you have a very high, an element which has got very high neutrons in the cross-section, the flux may get reduced. Highly selective because the radioactors that are formed, they have distinct gamma rays. So it is distinct. Even if the gamma ray is same, you have different half-lives. Highly selective, high sensitivity because of the high cross-section, high flux, high detection efficiency, they have the high sensitivity. And so high precision and acquisition require reproducible and data can be obtained because of these advantages. Neutronization analysis is called reference techniques. That means for many like forensic applications or many applications, data provided by different experiments are taken as reference data. So they are, even in the, suppose you have forensic data, in a port also, this data can be used as an evidence for the data. So these are the advantages and reproducible, well studied, very common technique, not only the integral chemistry, but many other areas which I will discuss in the next lecture. So I will stop here. Thank you very much.