1주: 물질과 힘 그리고 측정(Matter and forces, measuring and counting)
W1.0 환영(Welcome)
W1.1 물질(Matter)
W1.2 힘(Forces)
W1.2a 자연단위(Natural units)
W1.2b 특수 상대론과 4-벡터(Special relativity and four-vector)
W1.2c 가상입자(Virtual Particles)
W1.3 확률과 단면(Probability and cross section)
W1.3a 광자 빔의 감쇄(Attenuation of a photon beam)
W1.4 러더포드 실험(Rutherford experiment)
W1.4a 러더포드 단면(Rutherford cross section)
W1.4b 산란율 계산(Counting rate Rutherford)
W1.5 양자 산란(Quantum scattering)
W1.6 러더포드 실험 실습(Rutherford experiment in practice)/동영상/영문자막
>> 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 of 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?
첫주 과정의 마지막 편은 제네바 대학교의 핵물리 실험실을 방문하여 실제로 학생들이 러더포드 실험 실습을 어떻게 하는지 살펴보겠습니다. 이 실험은 우리 학과의 전임교수이신 알레산드로 브라바 박사님이 담당하고 계십니다. 중성미자 물리 그룹에 참여 하면서 뮤온의 특성을 정밀하게 측정하는 일을 합니다.
>> 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.
사실 이 실험장치를 생각해낸 사람은 러더포드가 아니라 가이거와 마스덴이죠.
>> 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.
하지만 가이거와 마스덴의 실험을 해석한 사람이 러더포드 입니다.
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.
산란 실험장치는 진공용기 안에 있는데 알파 입자가 공기의 입자들과 반응하지 않도록 진공 조건하에서 실험을 해야 합니다.
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.
알파 입자 방출기(AM-2)는 아메리시움을 사용합니다. 알파 입자가 두께 1 마이크론의 금 박박으로 향해 조사되죠. 금 핵과 상호작용(쿨롱 력 반발)하여 산란되어 여러각도로 날아가죠. 산란된 알파 입자는 실리콘 다이오드에서 감지됩니다.
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 femto-coulombs 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?
산란율은 각도에 따라 크게 차이가 나겠지요? 잘 알려진 대로 1/(sin(θ/2))^4 가 잘 맞습니까?
>> 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.
만일 알파 입자가 입사하는 각도에 정면에서 측정 하면 수 킬로 헬쯔(초당 수천개)의 알파입자가 감지 됩니다. 이 경우 알파 입자는 산란되지 않고 통과한 입자들이 포함되죠.
>> This also gives us the intensity of the incident beam.
>> What is the 45 degree count rate, roughly?
그럼 45도 각도로 감지기를 놓으면요?
>> It's very small, one event per minute so we need to measure over several days to accumulate sufficient statistics.
감지되는 입자수는 아주 작아지죠. 분당 한개 꼴이 되기도 합니다. 통계적으로 의미있는 측정을 하려면 몇일씩 걸리기도 합니다.
>> 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?
그럼 학생들이 날밤 세워 실험 하나요?
>> 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.
각도를 바꿔가며 측정을 하는데 보통 수시간씩 걸립니다. 각도를 높히면 일주일씩 측정하기도 합니다.
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.
그래서 실험을 보여주기 위해 검출기 각도를 0에 놓죠(알파 입자 방출기와 정면으로 향함).
>> 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 femto-coulombs. 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.
앞서도 말했지만 검출되는 신호는 아주아주 약합니다. 수 펨토-쿨롱쯤 되요. 그래서 증폭기를 써서 약 1만배가량 증폭 시킵니다. 증폭된 신호의 모습은 이렇습니다. 전하를 띈 알파 입자가 검출기를 급격히 충전 시켰다가 서서히 방전되는 모습이죠.
>> 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.
그럼 알파 입자를 세는 방법은 검출기에 지정된 전압이상으로 올라올때 마다 그 횟수를 세는군요.
>> 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.
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