2.6 Radioactivité en pratique

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En provenance du cours de University of Geneva

Physique des particules - une introduction

51 notes

University of Geneva

Physique des particules - une introduction

51 notes

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.

À partir de la leçon

Physique nucléaire

Pendant ce deuxième module on parle de la physique du noyau atomique et de ses applications. On visitera les travaux pratiques nucléaire de l’UniGe pour voir les expériences sur la radioactivité. A la fin du module, on visitera un réacteur expérimental à fusion et une centrale nucléaire.

2.6 Radioactivité en pratique8:53

Rencontrer les enseignants

Martin Pohl
Professeur ordinaire
Département de physique nucléaire et corpusculaire
Mercedes Paniccia
Collaboratrice scientifique
Département de Physique Nucléaire et Corpusculaire

[MUSIQUE]

>> In this video,

we'll introduce you to the handling of radioactive sources of type α,

β and γ, and radiation protection measures

to protect both

the people who handle sources and the environment.

Let me present Dr. Alessandro Bravar, who is senior lecturer

in our particle physics department,

and who directs the nuclear physics

lab course for our students.

Dr. Bravar is an active member of the neutrinos research group

in our department, and he prepares

an experiment on muon physics

at the Paul Scherrer Institute in Villigen, near Zurich.

Alessandro, can you explain to us

what the rules of the game are, when handling radioactive sources?

>> First, in this laboratory, we are using several radioactive sources.

In general, they are stored in a safe, to block

all the radiation, and also to restrict

access to the sources because we cannot allow anyone to use

radioactive sources, unless they have had adequate preparation.

So, I will take out some radioactive sources,

a β source, and a γ source, and we will see how they are protected,

how they are stored, and also how we measure

the radiation coming out, with shielding and without shielding.

>> Very good.

So let's go!

[BIP] [BIP] So, first thing, I open the safe.

This is a first source, a β source,

here we have a γ source,

and as I'll show you an old watch,

where one used radium to make the needles visible.

[NO AUDIO]

>> So, that's three sources, you see there

β- source, 90Sr,

you see a γ source, 137Cs, and

an old alarm clock that incorporates a very, very weak source of radium.

>> So, Alessandro, can you explain why the sources are

packaged like this and how students

are protected from excessive radiation?

>> Okay. So I begin with the first source,

it's a β- source, which emits electrons.

So all the sources we use here are sealed sources.

That is to say that there is a small layer of protection around the source to avoid

physical contact and thus contamination.

But the protective layer is not sufficient to block

all radiations.

Depending on the type of radiation, different materials and also

materials of different thickness are used to block the radiation.

In this case, there is a β- source so it emits electrons.

To stop the electrons, one needs about 2cm of plastic,

and therefore, the source is contained in this plastic object.

Here, if you look, you can see the silvery part,

it is the radioactive part that emits electrons.

To show that the source is well shielded,

you can use a Geiger counter to measure the radiation level.

So I turn on the counter, and as we see,

there are not many counts coming from the source.

On the contrary, if I remove the protection

(as I said, 2cm of plastic), and placing the counter on the source, we see compteur sur la source et on voit que on

that the count rate goes up a lot, say several thousand hits per second.

So I put the cap back on and we see that there is no more

radiation from the source.

The second source I will show you is a γ source,

gammas are photons of a certain energy,

rather high energy compared to visible light photons.

To protect photon sources,

one needs a lead shielding.

To stop the photons,

it takes a rather heavy material with a rather high Z, such as lead.

Here is the lead container, inside

we have a second lead container, the source is hidden inside.

To remove the source, I use a pair of tweezers,

I do not touch the source with my fingers.Inside there is

this little piece of lead, which contains the source in its tip.

So if I place a Geiger counter just in front of the source,

we see that the count rate is quite high.

On the contrary, if I put the source in its containers

almost no radiation comes out.

So to protect from a source of γ-type, one does not just choose the good material,

but also a thickness of the shielding according to the intensity of the source.

The more the source intense, the more lead

I need to stop all photons emerging from the source.

>> In fact, we did a little calculation of the absorption of photons

in the minimum of their cross section, in a previous video.

In the old times, in the 1950s,

everyone was much more relaxed

with respect to radioactive sources, right?

One even put them into objects of every day use.

Can you explain what was the purpose of the radioactive source in this clock?

>> This is an old alarm clock,

and to make the hands visible during the night,

they used radium to make them phosphorescent.

Today, we use different materials, such as phosphorus,

but in the past, people used radium, and this is a very, very dangerous substance.

The reason why the radium

is so dangerous is that it decyas into radon.

Radon is a radioactive element, it is a gas,

which is everywhere in nature, and we can breathe radon.

Breathing air one also takes in a portion

of radon in our lungs, where radon decays by emitting

alpha particles, which come in direct contact with the tissues

of our body and can do much damage.

In fact, radon is one of the concerns we have around the world,

in homes, also in Switzerland.There are regions in Switzerland, especially the Alps

and the Jura, where there are rocks outgassing a lot of radon, while

in big cities, like in Geneva or Zurich etc., there is not much radon.

>> That is to say that one should not spend the night in a basement in the Alps.

in a basement in the Alps.

>> Exactly. Houses must be properly ventilated.

>> And, that's why we have banned the use of radium since a long time.

To show the effect, although there is a minimum amount of radium,

which has been deposited on the clock, with the same Geiger counter,

which I used before, one can reach a certain number of counts, which tells us

that there is a lot of radium decays from this clock

And so it's not a good idea to sleep with such a clock by the bedside.

>> And that's why, we have long since banned

using radium in clocks.

Thank you very much.

[MUSIQUE]

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