 So, now we will switch to another subject, the NXN reactions. So here's the outline of my talk. I will give a short introduction on NXN reactions. Then very, very short, not so short, but I will give the application where, why this kind of reaction have to be studied. Okay, the facility, I will be short because now you know everything about that. But I will give three kinds of experiments to a manner to measure these NXN reactions. Very shortly give a very few words about double differential measurements and I will finish by giving some examples of measurements where what can be used to this kind of reaction. So what is an NXN reaction? So you send a neutron on a nuclei, you produce X neutrons and you produce this daughter nuclei. So X can be one, two or more. So it's an evidence but you produce an isotope. It means that you can't separate chemically except if this daughter nuclei decay by beta for example in another in other spaces. That's a special reaction. Okay, I won't speak about elastic reaction. What we call NN prime, it's when X equals one and in fact the neutrons give one part of its energy to the nuclei and excited the nuclei. So the Q value will depend on the first excited level of the nuclei. So now you can calculate the Q value on the energy special just by knowing the mass excess of the different nuclei. So just to show here I just put the special reaction on silicon iron on lead as a function of the X. Okay. And you can see that the heavier the nuclei is, the lower the special reaction is. And the special of course increases with the energy with the X N-train reaction of a special higher than N2N and so. So just an illustration on the lead, you see in a special on N2N reaction it's around 7MV, N2N around 14MV and N4N around 22MV. So what happens when you make a reaction? So you had another view two days ago by Jan when the neutrons interacts with neutrons it produces composed nuclei which can decay in different channels. But before the equilibrium is rich, before the creation of the composed nuclei, you have a premium phases during which particles, neutrons but also protons can be emitted, okay. And for reaction you have the direct component, you have the repulsion process and the premium process. And you can connect this kind of component to this kind of spectrum, it's an energy spectrum, the direct reaction are the high energy part, evaporation part are the low energy part, statistically evaporation and between you have the premium process. So you can if you reproduce, you want to reproduce, okay I didn't mention, it's also related to the time of reaction. So this reaction are the fastest and the evaporation are the shortest. But when we say short, it's still very short time of reaction compared to the time of your reaction during the experiment. So and here you have a spectrum of a neutron emitted in a reaction with 14MV neutrons. So you can see you have the elastic contribution, the contribution due to evaporation and here the contribution to the premium. So you have to take into account all these processes if you want to reproduce your kind of things. Here you have the premium process is taking into account here and the evaporation here. So and this reaction are of course important in the energy range between let's say 1MV and few tens of MV. Here you have different cross-section and reaction on bismuth. The black one is the total reaction, cross-section reaction and you have here the inelastic cross-section in red, the N2N and the N3N and you can see that in the energy range, okay this cross-section is quite important. It's an important contribution to the total reaction. So if now you switch to a facile nuclei and you're in 8, in addition to the previous spectrum you had to add the fission contribution but again this reaction are quite important you have to take into account of course. So here it's to illustrate the importance of again this reaction compared to the reaction cross-section and here it's to illustrate I spoke about the prequibrium and you need to take into account neutrons emitted during the prequium phases if not you are not able to reproduce your cross-section. So you have cross-section measure on nubium 83 and 2N and 3N and N prime as a function of the energy and different calculation perform okay perform a code close to talis not exactly talis it's Gnash but you see that if you don't take into account the prequium emission you can't reproduce the right cross-section. So now what where this kind of reaction take a role? So we can mention the reactor and also the accelerator driven system you have a large amount of neutrons and when you produce an NXN reaction it contributes to the energy loss of the neutrons and the neutron will lose energy. You win in some cases multiply your number of neutrons if you have a N2N you start with N2N and you have two neutrons so and you produce a daughter nuclei which can be radioactive so all these features have to be taken into account of course for the balance of neutrons in your core or in your accelerator driven system and also for the production of values of the term. It's the same in fusion technology where a large part of neutrons is a 14 mm neutrons so you will produce this kind of reaction NXN and you will produce for example waste undesirable nuclei in your facility. Now when you make a nuclear data evaluation you need to know all the channels because you need to make something consistent so and if you don't know the NXN cross-section you can't perform reliable evaluations and another point I will show an example just after you can use you can need this kind of reaction in order to characterize a neutron field. You have a neutron field you want to measure its flux on its energy distribution you can do it by using this kind of reaction. So it means also that for a reliable evaluated databases you need accurate measurements. So how can we characterize a neutron field in fact you can use this kind of reaction to determine the differential flux from a different source it could be an accelerator it could be a reactor. You ready yet a sample from a tachy with a flux phi that's a flux you want to determine. You measure the activity of each one of your samples the activity is related to the flux you want to measure the cross-section of the reaction you are using and it's proportional to these parts. Now you will simulate a flux ski the one you supposed to be representative of the flux you want to measure and you can calculate the same activity but this one is simulated. And now by comparing this simulated and this measurement okay you try to minimize the difference by adjusting this see flux and by this way you can determine by iterative method you can determine the flux see representative to the one you want to measure. But to do that you will need to have a several set of cross-section okay with several set of samples and you have to choose this cross-section this reaction sorry in order to cover the energy balance of interest of your flux okay. So it will depend of the energy special of your cross-section and you have also to find some reaction where you can produce measurement product you can measure by you can measure it means it should be radioactive you should be have a period measurable and with with a decay you can measure okay and which kind of reaction you can use you can reaction NP and alpha or nn prime of nxn so here is an example and of course if you want to be able to determine your flux phi you need to have a good knowledge of your cross-section. Here is an example of reaction you can use up to 20 MeV so here you have the cross-section of different reaction so there's not only nxn reaction and there are also nalpha and np but you have nn prime and n2n reactions so and you see that if you have a flux up to 20 MeV with this indium you will cover the first part the slow energy part of the flux and with this reaction for example the n2n you will be sensitive only to the high energy part of your flux that one of the application of that. So now if you go at higher energy than 20 MeV here I put an example of neutron spectrum produced in a in a heavy ion reaction at high energy and 100 MeV per nucleon. So the energy spectra was measured by using some samples and by using a different type of sample and different neutron induced reaction here and you see in order to go higher in energy if you use different reaction on bismuth it's n2n and n4n and n5n5n6n so you see that here you have the threshold energy of this each of these reactions so and with this reaction you can determine the flux at the high energy part okay and okay here you have all the parameters you need for example the half-life which must be measurable on also the energy of the gamma ray. So here you have an example of the result which were obtained with this kind of measurement the solid line are the flux they have deduced from them for them irradiation so it was measured for this different reaction. So for example in the case there are probably no other solution to do that because if you want to measure this kind of spectrum by time of light techniques for example as you have neutron up to 110mV you need a quite long prior path and you need adapted the frequency and it's probably it's sometimes not not available at the facilities okay and you can also deduce the angular distribution if you place your samples at different angles. So now I will go to the facility which you use to take this kind of measurements so I will go very fast because on that part you you know almost everything we can produce neutron in accelerator in photo fission or with a nuclear fission in a reactor what are the important parameters the energy range of course I say you okay I put 5mV but for nn prime it's less than 5mV so the genre of interest will be this energy range the energy distribution will be very important because depending on your energy distribution you will adapt your measurement techniques can be monokinetic or it can be continuous of spectrum the fluence of course because it's important in order to determine your continuates and the time structure do you have pulse beam or not and we will distinguish two type of facilities the open field or the conlimated beam. So a reactor you know we produce neutron by fission and just okay in this that case we will use not water reactor not PWR reactor where neutrons are slowly down but the reactor where you have only pure fission neutrons you have picture such a kind of reactor and what is interesting that if you see you look at the neutron spectrum of the neutron produce in the reactor it's a pure fission neutron spectrum so you can see that you cover for example the nn prime energy range the energy range of the nn prime reaction of your 5 and also you cover the opening of the end-to-end channel so it means that this kind of flux can be used for also for end-to-end reactions so you can use brainstorm facilities okay I don't go much in the detail you know what is it now we'll have a look later on different facilities here it's NLB and NGELINA you heard about about that for higher energy we can use quasimonautic spectrum in that case use mostly proton beam eating a lithium converter and you can produce neutron spectrum with this quasimonautic component which is equal to the proton energy minus 2 MeV on a long day at low at low energy okay so and you can also have a continuous spectrum in that case you use a thick converter or not a thin converter and you produce continuous spectrum as this kind of shape okay in that case the proton you stop the beam proton of the electron you stop it in the thick target okay and you have some facilities which propose a both kind of spectrum so here you have a scheme of different quasimonautic facilities you see we saw this one yesterday I think so you have you produce your neutron here and here you have some kilometer which allows to define you a neutron beam line so you can have some details on a report concerning this and you can also have a list of facilities in in this report from HIE concerning the time of fly facility so okay this okay really I think you you know that quite well now prediction of quasimonautic neutron with accelerator that's what I want to mention here you have a neutron spectrum performed on DD reaction here you use a deterrent of more than 3 MeV at a zero degree you have an energy of 6.8 MeV and when you increase the deterrent energy you you have break up reaction on your spectrum is no more purely monokinetic but for any xn reaction for and this for n2 interaction sometimes it's not a problem because your threshold of reaction is somewhere here and this part of neutrons doesn't impact your reaction it was mentioned I think yesterday already okay and if you increase the energy also increase the part of the of the break up reaction but for the same reason it's sometimes still usable so you can also use palatine reactions okay I think you had also explanation I think Monday by a by Nicola here you have a neutron spectrum of n2 you have a continuous spectrum and you cover low energy part which is not very interesting for any xn reaction but you cover energy part above 100 of k up to several tens of MeV in order to study the reaction we are interested in so just a few words about the type of beam so you can have what I call open field that's the case for example in in PTB but you can find that also in other facilities around the world and you have the neutron source somewhere it's a target okay and neutron are emitted in 4 pi that's why you call that an open source okay and the consequences is that your detector is not protected and you have back on in your detector that exactly the case I mentioned in my previous talk with the terium neutron induced fission studies okay and but the advantage is that you can put your samples usually very close to the neutron source in order to have a large flux you have a collimated beam in that case I repeat it's neutron are produced here you have collimators and you have a neutron beam which will a neutron beam the advantage is you can put your sample here and your detector close to the sample but quite outside of the neutron beam not in the bag but the problem in that case in this case in this case it as you are quite far from the target you decrease your available flux and there is another another type of facility which produce what I call a conical beam I'm not sure it's a good name but you use a reaction where for kinematic reason neutrons are produced in a certain cone on outside of the cone at larger angle you don't have any more neutrons so it means that you can put your detector outside of the neutron beam so now we'll look how we can measure this cross section of this kind of experiment so I will show you three techniques activation techniques the nn prime the nx and gamma reaction measurement and the direct measurement of a secondary ray neutrons so activation technique it was explained I think it was two days by Vitaly okay you read yet samples of nuclei high by neutrons and you produce these daughter nuclei and you measure the activity of this nuclei and if you know the flux the number of atom you have in your samples from the activity you can deduce the cross section but but to to be able to use this technique your daughter should be radioactive or there are other solution if you use a mass spectrometer but it's not exactly the same technique but in the same in most of case you use radioactive technique adductive samples sorry so and again I'll repeat it was mentioned before to be available you need measurable periods you need a decay mode which is measurable gamma alpha electrons and you need a good feeling ratio because if you take a nuclei which decay 10 to the minus 4 to 2 photons for example to be difficult to measure the activity by detecting photons and also the value of the ratio is important but the knowledge of the feeding ratio is also important okay what is okay I didn't mention sometimes it's also called what your chemical technique what is a what are those advantage so as you measure the activity mainly by gamma pectrosopy or about alphas petrosopy you can clearly identify your daughter nuclei so it means that your target do not need to be isotropic if you have several isotope in your target okay it doesn't matter you can identify the one you are interested you don't need a pulse beam because you just put on your radiation and then you make a contingent and you're ready you just need to know how long you have irradiated and how many particular many neutrons you have you have some so if your if your beam is not constant sometimes you could have to make some correction in respect to to the life you are measuring but what is the drawback the drawback is as you don't have any information on the time of your reaction you can't measure the incident neutron by time of flight so it means that you need to have a monokinetic neutrons if you don't have you will have to make some corrections you can measure only one energy at one time of course and you need one target for each energy okay it seems okay not a problem but if you want to work with actinide target with a radioactive target with a rare target if you did sometimes it's difficult to have several targets so i have put an example of the n2n reaction on americium 248 so i didn't knew but you have a poster on this on this subject just here it was a measurement performed at ERMM you can find some detail in this phg thesis and it's quite special because your your your target is radioactive americium 241 is quite highly radioactive so it means that you can't have a really a large amount of materials so and it is radioactive so you will have a lot of background in your in your detection system so you see you have your americium 249 you capture a neutron you emit two neutrons and you detect the gamma ray emitted by the americium 240 okay so you need to detect this kind of gamma okay and this one have a period of only 50 50 hours here you have a gamma spectrum measured before the radiation and after the radiation and you see that there are succeed to measure gamma ray at 987 kV which correspond to this to this transition okay you measure the activity and you can deduce the cross section and with this you can determine cross section at different energy and cross perform with monokinetic neutrons sources okay so you can find more detail i think on the on the poster you have here so now we switch to another technique which in this technique we measure the nxn gamma cross section not the nxn so our how it looks like you you have your samples you have your neutron neutron beam and you detect with a set of gamma detector you detect the gamma rays not not the neutron but the gamma rays emitted during this reaction so what is the advantage of this such a reaction so as you detect gamma rays you have a time a time signals so it means that if your beam is pulsed by time of flight you can measure the incident energy the energy of the incident neutrons it means also that you can use it this technique with a continuous neutron beam and as a consequence you can measure several incident neutron energy in the same in the same measurement okay again you identify the gamma rays from the the nuclei so again you can you don't need a monokinetic monoisotopic target so you can even measure several isotopes in the same in the same experiment now the main drawback of this of this technique is that you don't measure the nxn cross section but you measure the nxn gamma cross section in fact you measure just one part of the cross section you are looking at so here you have an example of such experiment also perform at gale but it time at galeela i'm not at the vondograph so you already know this this facility and here you have the setup this set of of a germanium detector surrounding the target and in addition there are also other detector at dedicated angle okay the target is here the beam come from that position i think so and here's the other detector here you can have a lot of detail in the so many publication was done we're done we have a lot of different cross section of nn prime on n2n which have been measured on this on these isotopes here also you have the list of the measure or the isotope which are scheduled to be to be measured so here is an example of the measurement of nn prime on n2n gamma cross section on your m5 here you take here the detail of of the target okay and as you have the time of flight between your reaction and your beam you have the the time of flight of the incident neutrons and you can select which kind of the energy you are interested on for example here it's the time of arriving of the of the gamma rays okay you have gamma produce during the photo photo reaction and the accelerator and you detect gamma also in your detector that's the time of of your gamma and then when the time increases you have the neutrons and the farther you are here the lower energy are the neutrons so if you select this energy range you have in you are in the nn prime energy gate and you select this part it corresponds to the energy of the n2n reaction now if you look at the spectrum of the gamma spectrum this gamma spectrum on nn prime gamma reaction you can identify some transition in fact in this case it one transition the one 129 kV which corresponding to an excited level of your number 235 if you identify this three energy transition three gamma ray transition it corresponds to three transition of level of your number 234 because in that case you produce the n2n gamma reaction okay so if you count the number of gammas of this energy range of this energy you can deduce the nxn gamma cross section so how do you do okay you have your target your beam you have your detectors and you deduce your cross section at an angle that's that by the number of new of gammas you have in your peak number of atom your flux your detection efficiency the relation time so you have to take into account the the dead time of your acquisition okay so but here you have measured the cross section at one angle and what you want is the total cross section over all the angle so for that you can use you have to the cross section you want to measure is this one you have to integrate this cross section over 4 pi so you make this kind of integration over a solid angle then you can integrate over phi by 2p you have 2 pi it's phi and then you can change of interference coming from theta to cosine i cosine i theta and you have an integration between minus one and one of this of this formula and then you can use the Gaussian correlation approximation which gives you the value of this integral it's equal to this one as a condition that your theta here or your cos theta here correspond to have a value for which the Legendre polynomial is equal to zero so an example if you take the Legendre polynomial of order n equal 4 you have four roots which are this one that's the roots the roots are the value for which the polynomial is equal to zero and if you put your your detector at angles for which the cosine i of the angle is equal to this value you can apply this formula in fact you can apply this formula you you have the value you have measured to your two angles okay the angles for which the polynomial is equal to zero and you have some coefficient which are which are known and so by placing your detector at these two angles 3 3 30 degrees and 70 degrees you can deduce from the measurement of only with two only two angles you can deduce the the cross section of nx and gamma reaction okay so it's very well explained in this in this reference so now the problem and it's the drawback of this method in fact that you don't measure nxn reaction in that case n2n but you measure n2n gamma reaction so you measure the only one part of the cross section and you have to to go from here or here you have the results here for different energy energy bin you have the cross section measure of maybe you can't see but it's for a transition uh is a 6 plus to 4 plus this one is a 6 plus to 8 plus to 6 plus on this one 10 plus to 8 plus so and to go from this transition to the nxn transition you need to make a uh a theoretical calculation and when you make this calculation with a talis code for example you see that uh the the ratio between this transition it's the first transition at 150 kv to the n2n cross section you see you have a large factor of correction so it means that if you want to to to be able to perform this kind of measurement you must trust your simulation from going from the nxn gamma to the nxn reaction so another kind of fraction which looks like a little bit to the previous one that's measurement per format nlb and nlb it's in rossendorf in germany and neutron are produced by photoefficient and you have a channel for for neutrons and here you have the energy range covered is up to 10 in v but a little bit a little bit less again i didn't knew but again you have a poster on this kind of measurement here on the study of an information of an iron and in that case there is a a coincidence measurement between the gamma rays and a neutron emitting okay so so here you have a picture of the facility the the sample is is here here you have plastic scintillator for neutron detection okay that scintillator with two photo tube the same type as i mentioned in the previous lecture and here you have bf3 detector to detect the gamma ray so the flux is determined sorry the incident flux is determined by a fission chamber sorry i hope i will finish the lecture alive i find some all the detail in the in the thesis oh also you can find detail on the on the poster on the on the reaction so rather the distance okay and here is a typical spectrum which is measured here you have the time of flight between the the accelerator on the on the detection this peak correspond to to the gamma flash photo flash and here correspond with this time of flight you can deduce the neutron energy and here you have different that's the gamma energy detected and you can distinguish different gamma line which correspond to different transition of the iron 56 okay you can see also for example this is this line due to potassium 40 you have any idea where this potassium 40 come from okay nothing to do with iron in fact you have potassium 40 in every concrete in all the concrete so and you have concrete all around the cave so you always have this gamma line at at 1047 466 so so but again what you measure it's not the nxn reaction but the nxn gamma reaction and here you have some results obtained all the transition on different states an energy here can you give you the cross section of the transition of two of two states and then again to switch from these results to the nxn results you have to make a talis calculations or talis or other calculations but usually we use the talis code so now third part in that case we will okay we have measured the activation of the daughter nuclei here we have measured gamma rays or consonants for neutron gammas in order to detect to measure the nxn gamma cross section now if we can also measure the directly the neutron emitting and measure directly the nxn right so what are the advantage in that case is a direct measurement we don't need a calculation of feeding ratio or alimentation it's applicable to all nuclei you don't need to have a daughter radioactive okay what is a drawback the back is you need in that in that case you need a mono is a topic target because as you don't detect gamma rays you can't identify the nuclei on which the nucleus on which you made your reaction and also you need a neutron detector of high efficiency and two kind of detector can be used the neutron balls and the neutron spectrometer and the goal is to measure each neutron you are emitting if you want to measure n to n reaction you have to measure the two neutrons and to n the three neutrons emitted in the reaction so if you were about neutron balls what is a neutron ball a neutron ball is a neutron detector a large neutron detector spherical of different size but different shape but it doesn't matter your your sample is in the center and it's a tank which is filled by a scintillator liquid and it is surrounded by okay here picture on the old one which was used in the 70s at that time you you could buy it directly to to the producer now you have to to build it yourself this one orion was used in the past at ganil and this was it four cubic meters of volume and karman he was he was built 15 years ago in ca it's a little bit less one 1000 meter and 24 photo tube but it's a four pi neutron detector it's a very high efficient and also high uh sensitive you can use it but for any send cross section measurements i will show you how you can also measure to measure new bar new bar it's average number of neutrons emitted in fission okay and you can also use for the study of old nuclei because old nuclei decayed mainly by neutron emission okay how it works i show i said you it's filled by a liquid scintillator and so we did by photo tube you have your target which emits neutrons neutrons enter in this liquid scintillator okay the liquid scintillator is mainly composed of hydrogen nuclei and carbon nuclei the neutron eats on the hydrogen nuclei and protons and lose its energy by several uh it's in that proton heated recoil and when they recoil they stop in the liquid and it produces light because it's a scintillator okay and this light is detected by the photo tube this process is quite short several nanoseconds several tenths of nanosecond so it means we have a so-called prompt picked then neutrons continue to be slowed down in the scintillator until it's thermalized and then when it's thermalized it continues to live until it's captured by gadolinium nuclei i didn't mention but in this scintillator they are loaded by less than one percent of gadolinium why gadolinium because gadolinium has a very very large the huge uh neutron conception uh neutron capture conception more than uh two uh two two hundred fifty thousand of bounds for one of the results okay so then when the neutron is captured by the gadolinium the gadolinium is exhaled and they excite by emitting gamma rays in average three gamma rays and for total energy of around 80 mev and these gamma rays they also interact with the electron of the liquid and produce light in the scintillator and this light is detected by the photo tube so what is uh the interest of this detector the interest is you you put very small quantity of gadolinium it means that the time between the thermalization on the capture is quite long here you have the time the solution of the capture the capture time the solution of the neutron by gadolinium nuclei and as the concentration is slow the time of capture is quite long you need 50 macro seconds in order to capture 99 percent of the neutron you will capture and why do we use a so low time distribution it means that if you have several neutrons emitted at the same times all your capture won't uh happen at the same times and you can count your capture independently so it means that when you have a reaction you wait several tens of nanoseconds 108 nanoseconds that this point peak has been detected and then you open a gate to count the delete the delete signals and here you count the delixinal or in that case you count four delixinals in 50 macro seconds so you have four capture of neutrons if the concentration have been larger this this distribution would be shorter and you have a probability that two neutrons will be captured at the same times and you can't distinguish so and which is so low uh concentration you can measure very high neutron multiplicity it has no interest for n2 and n3 interaction but when you use spallation reaction and you use uh uh evion reaction you have a multiplicity of 10 20 50 you can measure it so but this kind of detector is quite large it is also very sensitive to background because it's sensitive to neutrons but also to photons so what we do after the measurement of this neutron is the first gate we open a second gate later which is uh which measure evaluates the background which because this gate is not correlated to to this one in time okay so uh the advantage also of this detector because it's very efficient here you have a the efficiency detection efficiency as a function of the neutron energy in the center and you see that you have close to 80 percent of detection efficiency at low energy this efficiency decreases with with energy for for two reasons the first one is that when neutrons energy increases the probability of the neutron to escape for detector increases but the main reason is that uh when you increase the energy you have new uh channel reaction which can open and this reaction uh you your neutron disappears so that's why uh we could say okay i will increase my volume in order to increase efficiency but it's not a good solution because you want you will increase a little bit your efficiency but you will mainly increase the background sensitivity of your detector and you will increase the volume of liquid which is expensive which is a poison which is explosive which is uh cancerous okay it's uh the people of safety that don't like at all this kind of detector okay how we can measure the efficiency of this kind of detector again we use a californium source you put your californium source in the center of the detector uh you put the source on a silicon detector when you detect efficient fragments you know that in average you have 3.78 neutrons emitted by the californium and also the advantage of the kind of detector that okay you can measure your neutrons but you measure also you don't measure the average number of neutrons but you measure the neutron multiplicity even by event it means that here you have a spectrum of the multiplicity measurement you measure the number of events with zero neutrons emitted one neutron emitted two neutrons three neutrons you measure the neutron multiplicity okay you have to correct for multiplicity of background of course but you have to do that and you do that even by event so how we use it for this this kind of detector for nxn measurement so you put your sample your detector here the samples in in the center you need uh yeah you need absolutely a collimated neutron beam because the detector is very sensitive to all all those neutrons and you see you you need a very clean error in terms of background it means also that if you put a wall here it won't work because your neutron won't go to the wall and go back to to the detector and create a large background okay you need to measure your flux by your monitor okay and what you will measure you measure the number the multiplicity of uh of uh the number of events with multiplicity x for example if you are looking of a n2n reaction you will look on your distribution the number of counts with multiplicity two and you have to correct uh for the efficiencies the number of atoms on the flux okay so one of the problem is you have a passive target okay so you you you have to use a pulse beam you pulse a beam and you open a gate in order to count the number of neutrons so in in the this kind of experiments which are presented here they use three three successive uh uh pulse in order to increase the statistic uh in fact you have no chance on the probability of having a reaction in two or three uh successive beam is very very very low but you have to open a gate quite long uh here uh around 40 microsecond so uh by using this kind of technique you can measure uh n2n on any section that you have some results uh which are obtained by this kind of detector again it's done by monokinetic neutrons and you have a lot of uh point to do and when you arrive to some kind of energy above uh 11 or 12 mv i while i said you before your spectrum is not purely monokinetic so you have to make some correction of reaction which have occurred at energy a little bit lower okay that's other measurement performed with another uh uh other team but in this uh in these plots you have data obtained by this technique but also data obtained by activation so other uh possibility to to measure the this reaction is to use a neutron spectrometer it's uh it was mainly used for nn prime reaction so what is uh what is called a neutron spectrometer so you have your production of neutrons here for example a proton beam on the target or the electron beam on the target okay here are the samples you want to study and here you put some neutron detector at different angles okay and you will use uh pulse beam in order to measure the energy or at least the fly path of the electron from here to here okay and one of the difficulties here you need to fix so you need to be quite closed if you want to be closed you can't use a collimated beam okay so you have to be in open source and the problem that you what you want to measure is the neutron coming from there not the one from the source on the one has much more numerous than this one so that's why you need to make uh very a lot of shielding in order that your detectors see neutron coming from the samples are not coming from the source okay here you have to put a monitor in order to be able to normalize your your experiments okay here you see you have the fly path here which is also important i will come it later what it looks like okay here you have a picture of a neutron spectrometer which was used at uh at ca in the eighties here you have the the beam line the accelerator is somewhere here okay and you have a special uh treatment of your uh your signal in order to reduce uh the the time pulse and and they they succeed to have a the time resolution uh below one nano signals okay the sample is here and here your your detector are inside this big shielding which are composed of uh power fine on the listium six uh on uh on boron okay and here you have some collimator also now if i will look more here you have the shallow cone in order to define here the channel for detection of the neutrons and here you have some some shielding because here the source is here and you don't want to see the neutrons on here and the samples can be from out or in here okay okay uh in addition at that time uh this target was a tritium gas target but there is a tritium or also deuterium target in order to to to cover different energy so neutron energy okay you already know uh what is a neutron bi-term of light but what i want to to just point out something in that case we never measure an absolute time okay because we don't know uh the length of the cable we don't know the time spent in the electronics what you measure it's a difference okay so the time of fly is the time you measure minus a certain constant due to the electronics and minus the times of the neutrons between the emission and the samples so and if you want to to be able to measure really the the time of life of neutrons this time of neutrons must be constant it means that you must have constant monokinetic neutrons in order that this time must be constant and then what you measure the time of flight you you you measure you have a constant what is this constant in fact we determine this concern by using the gamma rays because the gamma rays they always take the same time to to the to to fly on this path and this time is equal to the distance divided by the by the by the velocity of light so uh it was explained uh yesterday or not two days ago by al but what i want to stress that what we measure it's a difference of time between neutrons on gammas okay here you have a direct time offline spectrum that corresponds to the rise of gamma rays i measure this time because i can measure in the same cables in the same electronics you just have to know the ratio between your channel or your your time measurement in your in your tac or in your tdc okay and uh with this measurement this difference you can measure your neutron in time of flight so let's go back to this spectrometer so here you have some results on osmium but it doesn't matter what we have measured in fact in this neutron it's a time of flight okay and here you have this kind of time of flight uh that it's in channel but okay you don't have the the number and here again it's rising inverse time of flight method it means that the the fastest neutrons arrive the fastest electron arrive at long time of flight because we start the measurement by the detection of neutrons and we stop by the accelerator which is deling okay so it doesn't matter so here you have the time distribution corresponding to a elastic scattering of neutron on osmium in that case is osm 190 and here these neutrons have a little bit less energy because they have been uh emitted uh from the first uh for for this uh the decay of this excited states okay and if you know the time you see the you know the time between different peaks you can deduce the energy corresponding corresponding energy of the different time okay and in that sense you can deduce this one is elastic this one is the first excited state this one is the one that's excited and so and so here you have the results on osmium uh 190 and this one is on 192 okay okay another application on the uranium uh nn prime on uranium 238 and this one is a little bit special because you can see that the first excited level is at 45 kiwi it's quite small it means that between your elastic on your first excited state you are only 45 kiwi in difference it means that your time of flight will be very very close one for each other i made just a small calculation with a fly path of eight meters correspond to the to the spectrometer i show you uh and here that results at 1.5 mev okay at 1.5 mev the time of flight is 295 nanoseconds and the first excited state in only five nanosecond later that means that if you want to be able to measure this kind of measurement you need a spectrometer of a very very good energy resolution and good energy resolution is good time resolution on long flight path and long flight path it means low counting rate just put you here a contrary estimation okay if you put uh a flux which can be okay you can reach a 10 3 10 to the 8 neutron percent per square centimeter no per star anion per second if you put a sample of 30 gram i i put the example of osmium okay if you put a distance of 7 centimeter here i put a cross section of 10 milliband per star anion but in some for some angles it can be less than that okay and we put a data eight meters a detector of a radius of five inches you know of a diameter of five inches so you can calculate the number of counts you will will have on it's more or less one point per per second so now you can also perform this kind of any set measurement by using a multicell detector uh in that case the detector are not in the shielding they are around here it's a the figure of facility which is used at at Los Alamos at the WNR facilities and in that case they use a continuous neutron beam and they need a start signal that's why they use this kind of detector a bf3 uh to detect the gamma ray so in that case uh they use it for nn prime measurements so they use they measure the neutron but they measure also the gammas in order to be able to to measure that so it's okay the detector are also a liquid ventilator detector so uh very short i i just spoke to you about cross section measurement but you are also interesting not only to measure the cross section but also to measure the energy of the neutron emitting all the technique is quite the same than the the one i show you previously so i find it's quite old publication but you you can find another one it's always the same thing you have your your neutron emission your samples you need a shielding in order to that your detector see neutron emitted by your samples and here you have the time of flight measurement and you can show the elastic on the other neutron coming from the other reaction this one is on lead and this one is on carbon you see the different peak it corresponds to the excited level on carbon at minus four mev minus seven and you can translate this kind of a neutron of uh of time of flight spectra uh to a neutron energy spectrum and you have also the elastic the first excited state and so but the problem of this technique is what you measure it's the neutron coming from all the channel where at least one neutron has been emitted it means you measure in the same times an n prime and two n and three n so or and and n b if it exists so you cannot say i have my spectra my new aspect of n two n or my spectra of n three n it's a sum of of the world so another kind of experiment which was performed on on lead in that case and you can see the results here at 60 degree it was 14 mev or you understand why because the first time it's quite easy to have a neutron source and you have the elastic component and you have this component due to n two n okay okay uh in that case in that case they can say it's n two n but uh it could be an n prime also but they they are sure it's not n three n because we are below the threshold of n three n reaction so now i will give you some example of experiment taking into account all you have learned during this this lecture i will go to this n two n reaction on plutonium nine and i will show you that this cross-section have been have been measured with the three techniques i mentioned before okay so again plutonium is a reductive target it's less reductive than americium that is reductive and the daughter nuclei is a plutonium 238 which is quite highly reductive with a period of of 80 87 euro and it emits alpha at this energy and it emits a lot of alphas when you use plutonium 238 usually you are your setup is added hidden by the high alpha activities so uh activation techniques so here you have an example you have the reference here of what was known by activation techniques they did it by uh at 14 mev you have the flux you have an integral experience so what is the difficulties in that case in that case is uh you you have to measure at the end you have to measure plutonium eight okay and the problem in the samples is that you need a very pure samples okay and here that that's a major problem because when you have a sample of plutonium usually you have several isotopes and if you have a pollution of your m 238 you can't succeed your experiment so in this case they had a purity of 6 10 to minus 10 before before the radiation okay and uh what they measure in that case the plutonium they didn't try to measure the gamma rays but they measure the alpha rays because i said to you that is very active in gamma rays so here you have uh the alpha spectrum of the samples before your radiation it was in red okay and after your radiation this this is one of the alpha of the plutonium nine okay so and uh what you saw i made a small calculation of that taking into account this integrated fluency which is given here uh and the cross section and uh we can obtain an activation of two two bachron okay but here we obtain 10 to the nine uh atom of plutonium eight and you have to compare to the number of atom you have at the beginning so that's why you really need a very pure pure target here so second method which has been uh was used it's a nn prime gamma uh method it was uh performed at uh lance it's a lo salamos national center by using the genie detector uh okay again uh neutron army did by uh i produced by spallation your energy proton it in a thick target you have neutron going through these collimators that's a continuous beam so here you are the extended time distribution of neutrons and the target is at the center and you have a lot of germanium detector uh in order to measure the gamma rays and in addition you have a bejo escape suppression to to clean up your your spectrum so and here you have the kind of spectra which have been measured uh as uh you have a pulse beam you can uh select some energy energy domain of interest and you detect the gamma rays uh which are characteristic from transition of the plutonium eight so here you have this big correspond to this transition this one to this one on this one to this one and you detect you you determine the cross section of this level and again you need calculation in order to determine the cross section of the end to end reaction so last method used also for this kind of actinine is to use the famous neutron ball i i show you previously and okay the advantage it's in that case we don't need any theoretical calculation because we directly measure the nixon question but when i say directly okay not so direct why because in your reaction you will have some fission you will have end to end with an end and prime maybe end to end if you have above the special of entry and reaction so what happens if you make this measurement you will have this neutron distribution neutron multiplicity distribution due to fission okay that's typical to a fission so the contribution of the fission of plutonium you will obtain this distribution and in addition you will have okay the reverse okay okay the blue is the distribution you measure the neutron multiplicity you measure and in this distribution you have the contribution of fission and you know that above a multiplicity of two this part is only due to fission okay we know the fission probability of of this of this isotope so you can say that i normalize on that part we can say this is fission and that part is not fission and this this subtraction of the two value allow us to go back to the end to end cost section but as you see so in fact that's that's the formula you use okay the number of fission for multiplicity greater than four is equal to the number you detect divided by the by the fission probability and then you can measure the end to end on end to end if the energy is high enough relative to cost but here's the problem one of the problem of this method that you have to subtract two value okay and the uncertainty can be very huge okay so here it's a summarize of the results obtained on this plutonium two of the end to end reaction on the plutonium line by taking the three methods I just show you before and you see that there are some discrepancies between the data especially in the special energy region where the cross section is quite low okay and it's a reason which is very interesting because it corresponds to to the q of the fission spectrum so that to say that new experiments are forcing for such such so now another special measurement concerning the end to end reaction on deuterium and this one is a little bit special because in the previous plan remind what I said at the beginning one part of the neutron are evaporated okay that's by the compound nuclear and it's evaporation it's a statistical process okay you have a certain energy energy in this case you'll we sent a neutron on the deuterium and you'll turn a proton on two neutrons and in fact you have a three three body reactions and these three reactions these three particles are strongly correlated in energy and in anger so and in order to to try to to simulate this kind of process you have to to solve the FDF equation and it's quite interesting for the theoreticians okay so how do did we proceed to do this kind of experiment we also use neutron ball but what was interesting that we use an active target it means that our target was a liquid ventilator of c6d6 okay that's such a detector such a detector are used for example from from capture measurement at end of for example but in your case it was a target it means that in the target we have deuterium okay and the advantage of that what why is it active target because when you make this reaction you have a proton which is produced and the proton in the ventilator it produced light it means that we have signals we have a trigger and this trigger allows us to measure the neutron energy of the incoming neutrons and it allows also to trigger the detection of the neutrons so when we have a signal here we open or continue it and we come to a number of neutrons your neutron distribution the difference of these neutrons okay one problem in this target there are carbon okay and around the the target there is the the aluminum there is also so to avoid this we have to do the same amount with a c6 h6 target the composition is the same but the hydrogen is replaced by the deuterium is replaced by hydrogen the results here it's a spectrum we have obtained incident spectrum we have obtained the time between our active target and our accelerator okay we are sensitive to gamma rays and the neutrons of interest and are the neutrons coming from the from the from the breaker reactions okay you see we have two gamma rays you see in fact because the beam was was eating something in the beam one or two meter upstream of the of the target that's why we have to today so by time of flight you can say okay I will select this event this event have an energy corresponding to nine mev and I will select this event which have an energy corresponding to three point for seven mev okay so here you have we have the multiplicity district neutron multiplicity distribution we have measured with the c6 target and with the c6 h6 target and here for gammas I select this event and you see I have only multiplicity zero here for this energy neutrons I have multiplicity zero multiplicity one and multiplicity two for c6 d6 and for c6 h6 I have no multiplicity two because at this energy range at nine mev I am below the end to end threshold of carbon the end threshold of carbon is 19 mev okay and at 3.7 mev I have this kind of the solution so you can see that I have no more multiplicity two because I am I think I'm don't remember exactly the threshold threshold is 3.3 mev the threshold of fraction so here at 3.7 I am very close to to the threshold so it means uh why I have multiplicity one one and multiplicity two for several reasons multiplicity one we have the elastic scattering in the target and we have also end to end reaction from one from which only one neutron has been detected okay because the detector is not one percent efficient so again to deduce the cross section you have to know the number of of events with multiplicity two in the c6 d6 minus the one of c6 h6 okay and you need to know your flux your efficiency and the number of atoms and with this method we succeed to measure the cross section at different energy of the end to end reaction of the material so now I will to finish I will present you two experiments a future experiment uh that will be performed at the NFS facility and uh letter of intense have been emitting okay the first one uh is an interesting because we will try to measure an entry and cross section measurement by two different techniques namely the direct the activation technique and the nx and gamma reaction at the same times okay the the experiment will be an entry and reaction on zirconium 90 okay by by one way we measure the gamma rays of different uh transition of zirconium 88 that's the technique I show you we have performed not we uh they have performed at uh at gelina okay but at the end of the experiment in fact this nuclei is already active and decay on the ytrium and so we can measure the activation the activity of this uh decay by the activation technique it means that by the two method we will obtain this end to end cross section that is particularly interesting because uh for this one we just need to know the filling ratio and we will be able to check if the calculation made to go from entry and gamma to entry and reaction are correct or not okay so we'll perform that uh here you have some estimation of the different entry and reaction as a function of energy with the threshold and we'll perform that with the the gain detector on the graphine detector uh again at the NFS facility okay another another experiment we we will try to to do also remember I say you when uh when I spoke about the measurement of neutron spectrum emitted in in this kind of reaction then we cannot distinguish an n prime and two n and free interaction so we will try to do it out here you have neutron spectrum emitted in a reaction emitted in a neutron induced reaction on lead but we could have done another another samples at 14 MeV so and you recognize you have your uh you you have the component of evaporation you have the direct reaction and you have the uh neutron emitted during the pre-equilibrium but as I mentioned uh we want to measure spectrum of end to end on spectrum f reaction so we will use a neutron beam collimatin neutron beam a set of detector here by time of fly we will measure the neutron energy my direct time of fly but in addition we will use this detector to measure the x minus one other neutron emitted in the reaction it means if I detect a neutron here I measure its energy and in coincidence I have a neutron as well I say its end to end reaction if I detect my my detector if I have two other neutrons I say it's an entry interaction because one neutron here and two neutrons here it's entry interactions okay so we just have this the photography is not good but uh we have to just open a little bit to our detector in order that neutrons can be uh emitted from here and can be detected to to here so uh in fact we already did this experiment at low energy at 13 mv and it works quite well what a lot of correction to do but it works it works well and at higher energy namely 30 mv it will be more interesting because the pre-equilibrium more becomes more important and also we can open new channels like entry end on end for end so uh I will finish by summarize what I said to you that any send reaction are important for different topics in reactor in the estimation of waste production in high neutron flux and also in order to have accurate data nuclear data many three techniques exist maybe I forgot others but I think it's the three main techniques used to to measure the cross section and uh just to be clear that all the technique can do can not be used in all the cases because you have some strong limitation by by case thank you for your attention