[MUSIC] >> To conclude the first module of this course, we visit the lab course on nuclear physics at University of Geneva, to see how one does an experiment à la Rutherford. I present Dr. Alessandro Bravar, a senior lecturer in our department, who is responsible for this lab course. He is also an active researcher, like all our collaborators, who is part of our neutrino physics group and also has his own experiment, which he prepares to measure the detailed properties of muons. It will take place in a few years at the Paul Scherrer Institute in Villigen, near Zurich. So in front of us we have a typical set-up for a Rutherford experiment. Alessandro, can you please explain the ingredients of this set-up and the way it works? >> Okay. Here, we try to repeat the Rutherford measurements, but with modern equipment and a technique that looks a bit like the experiments that we make today to study, for example, the electron structure. So we have, in our case, an alpha emitter, which emits alpha particles sent to a target, and we are detecting alpha particles scattered by the target. >> Indeed, it was not Rutherford, who did it first, it was Geiger and Marsden who have implemented his idea. >> But it was Rutherford who interpreted the observations of Geiger and Marsden. So to make this experiment, we have to work in a vacuum. This will require to open this box in a moment, to show all the ingredients. The reason that one works in vacuum, is that we use alpha particles and alpha particles are already stopped by an air layer of about ten centimeters. So that the alpha particles can propagate from the source to the target and our detector, we need to remove all the air that otherwise would stop the particles. We have therefore put in in this box with a pump that pumps the air; it makes the noise you hear now. What we will do now is to stop the pump and open the cover to see all the ingredients of the experiment. >> Let's go. You open the valve? >> Yes, first thing, open the valve and let the air in, otherwise we cannot lift the cover. >> We hear the faint sound of air entering the vacuum chamber. I think this is it. >> Not yet. We have to wait a while. Here we go. So we open the vacuum chamber and the three main components of the experiment can be seen. Here we have a source of americium that emits alpha particles. Alpha particles travel to a target, here we use a gold foil with a thickness of 1 micron. Alpha particles interact with the gold nuclei and are scattered by the gold nuclei in different directions. To detect these alpha particles, we use a silicon detector, a diode placed in the tube here. We will see the silicon detector in a little more detail. Thank you Martin. That's our silicon detector. The central part we see here is the active part of the detector, while the rest serves to define the acceptance of the detector. Behind it there is a small connector to measure the electrical signal generated by alpha particles, which are completely absorbed in this detector. >> The signal is then propagated. >> Yes, it goes through this cable and an amplifier we have here, because the charge left by an alpha particle in the silicon detector is only about a few femtocoulombs and we need to amplify this signal for detection. >> Then the experiment is to measure the count rate, the rate of alpha particles, which have been deflected, depending on the angle that the detector made with incident alpha beam. The target may be varied, it may be gold of different thicknesses, and can also be made of other metals, such as iron here, 3 microns of iron, nickel, a thin foil 3 microns thick, or different thicknesses of gold, as here, a gold foil of half a micron. One varies two things: the target, that is to say the Z and the thickness Δx of the target in the direction of the beam, and one varies the angle. How many different angles does one typically measure in an experiment? >> About a dozen different angles One puts the detector on both sides of the alpha beam to ensure, that we make a symmetric measurement relative to the direction of incident alpha beam. >> Obviously, the count rate varies very quickly with the angle. This is the famous 1/sin^4(θ/2) at work. Then, the count rate varies between what and what, roughly? >> If you make the measurement in the direction of the incident beam, you measure, in principle, all the alpha particles from the source, those that were scattered to very small angle, but also those, which have not interacted in target. And thus the count rate, in this case, is roughly a few kilohertz. So we can measure very quickly and acquire enough events in a very short period. >> This also gives us the intensity of the incident beam. >> Exactly. Then, to measure the count rate at different scattering angles, we move the silicon detector up to about 30 degrees, on déplace le détecteur à silicium sometimes 45 degrees, relative to the axis of alpha particles. It is clear that the count rate drops dramatically as we vary the angle. >> What is the 45 degree count rate, roughly? >> It's very small, one event per minute so we need to measure over several days to accumulate sufficient statistics. >> On the contrary, at low angles, it is a lot higher, right? >> A lot higher. As I said earlier, this is about a few kilohertz at zero angle, and at small angles the counting rate is still high. >> Okay. Then, students spend about a day to do this experiment, or a day and a night perhaps? >> Well, if we measure angles in the direction of the particle, the measurement can be done in a few hours. While when we measure at fairly large angles, we leave the experiment running a whole week. Students spend the day to start the experiment, check that all the ingredients are working properly, and afterwards the measurement can be taken, as I said, in a week. So, I put the sensor at zero angle to have a significant count rate and see things in real time. To run the experiment, you need to put all back into vacuum. So we will close the vacuum chamber and switch on the pump again. >> So, once the chamber is closed, the internal pressure drops relatively quickly, and it is also possible to apply the bias voltage to the silicon diode that is used for counting the passage of particles. Here, you use which bias voltage? >> Here we work with 24V, this is due to the thickness. Also, one does not need to deplete the detector completely, as alpha particles stop within the first few microns of detector thickness. >> If the first microns are depleted, this is perfectly adequate for accumulating the signal. So here, we see the signal of alpha particles passing, the typical signal curve shown by a silicon detector; this is the voltage as a function of time. >> As I said before, we have a very, very low signal, a very, very low charge, some femtocoulombs. So we need a first amplifier to amplify the signal about 10’000 times, the signal seen here, so this is a charge amplifier. That is why we see a signal that rises very quickly, with a much longer decay time. >> So the method of measurement would trigger a counter once the voltage exceeds a certain threshold, and count the rate of these events to measure the interaction rate. >> Here, actually, we do something a little more sophisticated. Instead of only measuring the count rate, we measure the charge deposited by the alpha particle in the detector, to be sure that the detected pulse is generated by an alpha particle. To do this, we need to go through a second amplifier that will shape the signal as seen here. In principle, it converts the signal to almost Gaussian shape, so it has a slower rise, and afterwards we use a charge-to-digital converter. This transforms the signal into something that can be used with a computer. >> That is to say that we measure the dE/dx of the particles, which they leave in the thickness of the detector. >> Once we have converted the charge to a binary number, a computer is used to store the data, i.e. the charge deposited by alpha particles, and we obtain an energy spectrum of the alpha particles. >> Okay. Because they are completely absorbed, it is a kind of miniature calorimeter. But at the same time, one obviously saves the count rate that will give us the Rutherford cross section. >> Exactly. So we must also record the time we used to measure, because it gives us the count rate. We record a number of pulses and divide by the measurement time. [MUSIC]
