Ce cours vous introduit à la physique subatomique, c'est à dire à la physique du noyau et à celle des particules élémentaires. Plus spécifiquement les questions adressées sont les suivantes : - Quels sont les concepts de la physique des particules et comment sont-ils implémentés? - Quelles sont les propriétés du noyau atomique et comment peut on les utiliser? - Comment accélérer et détecter des particules et mesurer leurs propriétés? - Qu’est-ce qu’on apprend à partir des réactions de particules à haute énergie et leurs désintégrations? - Comment fonctionnent les interactions électromagnétiques et comment peut-on les mettre à contribution? - Comment fonctionnent les interactions fortes et pourquoi sont-elles difficiles à comprendre? - Comment fonctionnent les interactions faibles et pourquoi sont-elles spéciales? - Quelle est la masse des objets au niveau subatomique, et comment y intervient le Higgs? - Que peut-on apprendre de la physique des particules concernant l’astrophysique et l’Univers tout entier? Le cours est structuré en sept modules. Suivant le premier module qui introduit notre sujet, les modules 2 (Physique nucléaire) et 3 (Accélérateurs et détecteurs) dépendent peu du reste du cours et peuvent être étudiés séparément. Les modules 4 à 7 approfondissent les notions de la matière et des forces élémentaires.
1.6 Expérience Rutherford en pratique
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En provenance du cours de University of Geneva
Physique des particules - une introduction
51 notes
À partir de la leçon
Matière et forces, mesurer et compter
Pendant ce premier module on introduira notre sujet en faisant le tour des objets de la physique des particules, c’est à dire la matière, les forces et l’espace-temps. On discutera aussi comment on définit l’intensité d’une interaction entre particules, par le biais de la section efficace, qui est une notion centrale de la physique des particules. A la fin de ce module, on visitera les travaux pratiques nucléaires de l’UniGe pour voir l'expérience Rutherford.
Rencontrer les enseignants
Martin PohlProfesseur ordinaire
Département de physique nucléaire et corpusculaire
Mercedes PanicciaCollaboratrice scientifique
Département de Physique Nucléaire et Corpusculaire
[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]
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