Positron Help

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1. Jan 27, 2014

Antigone

Hi

What is a positron? Is it a electron that moves faster? Why has it positive charge (has it anything to do with its speed?)

When scientists at Cern do these "measurements", what can they use as evidence that a "positron" isnt just a electron? How do they know what positron is?

How long can we observe a positron, Before it "vanishes"?

Antigone

2. Jan 27, 2014

ChrisVer

The speed is completely irrelevant.

The positron is electron's antiparticle. The positron was first introduced as an attempt to explain the Dirac equation's solutions. In the Dirac equation, when you introduce a spinor Ψ as a solution, you end up that you have 4 components. From QM you already know that spin1/2 particles have 2 component spinors. So you have to explain what would the 4 components mean.
Well the 2 of them correspond to positive energy solutions with spin up or down and the other two correspond to negative energy ones with spin up or down.
That is kind of problematic though- if you allow negative energies, then the positive energy particles would not be stable (they'd transit in negative energies). So as an interpretation to avoid that, Dirac introduced the idea of Dirac's sea. The particles you deal with (spin 1/2) are fermions, and thus they must obey the exclusive principle of Pauli. The only way to avoid transitions from positive energies (where you have electrons) to negative ones, would be by assuming that the negative energy states are all occupied by electrons with negative energy.
This would though allow the negative energy electrons to absorb a photon and jump in the positive energy region. The missing "negative energy electron" seen as a hole in the negative energy sea, can be interpreted as a positive energy particle with opposite charge to that of electron.
That was interpreted as a positron. Afterwards a transition from positive energy would be allowed to fill in that hole by emission of photons. So an electron would fall in that hole, and emit photon of the energy difference... this is the interaction $e^{-}e^{+} -> γγ$

In order to understand how experimentally we identify positrons to electrons, you have to see what are their differences. They are the same particle (meaning that they have the same mass, the same spin etc etc) but with different charges. So how would you distinguish between them two? By how they interact with an electromagnetic field. For example electrons inside an homogeneous magnetic field would make circles to the right (let us say) but the positrons would make circles to the left. The circles will look identical but they'd have different orientations. Thus the particles would have the same mass etc, but different charges.

It depends with what process the positron will be annihilated with the electron. For example in the system of positronium (http://en.wikipedia.org/wiki/Positronium) you can read that it lives for 125 picoseconds (2 gamma) or 142 nanoseconds (3 gamma) under the circumstances it states.

Last edited: Jan 27, 2014
3. Jan 27, 2014

Astronuc

Staff Emeritus
A positive particle will move opposite of a negative particle while traveling through a perpendicular magnetic field. A positron has the same e/m as an electron.

Beside motion in a magnetic field, a positron will anihilate with an electron producing two gamma rays.

One could store positrons in a storage ring, but generally positrons and electrons find each other and anihilate.

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/lepton.html#c2

4. Jan 27, 2014

Staff: Mentor

5. Jan 27, 2014

Antigone

So, a positive particle like the positron, is a electron that move in opposite direction? Instead of moving "forward", it moves "across" it? (Of course, they are both moving "forward", but in a perpendicular way the move "across" eachother, right?). Could you please explain that a Little bit, because it would be really interesting to know!

6. Jan 27, 2014

ChrisVer

what do you mean again by "moving forward" or "across". You have particles moving in SUCH a magnetic field as already stated. The force on them is a centrifugal force, that also depends on the charge of the particle. The rest dependencies are the same for both electrons and positrons.
If the charge of the one is minus the charge of the other, the centrifugal force of the one will be minus the other (but the same). So the one will tend to circulate to the right and the other to the left (depending on how you give the velocities and magnetic field) on same radius circles

7. Jan 27, 2014

Staff: Mentor

Suppose you have a magnetic field that is vertically upwards, and you shoot an electron horizontally away from you. The magnetic force makes the electron's path curve to the left.

Now do the same thing with a positron. Its path curves to the right.

That's what Astronuc means by "opposite."

8. Jan 27, 2014

Antigone

Chris, I dont understand your explanation. I am not a physicist, so Think of it as excercise in speaking to a Little Child. That would help.

jtbell, yes they are moving "opposite" to eachother. But your explanation means that they are moving in opposite directions when traveling "forward". So, if they where at the same Place, they would have taking of "right" and "left"? I thought of it, when I read the Word "perpendicular", that one would have traveled "down wards", and the other to the right.

Wikipedia:

http://en.wikipedia.org/wiki/Perpendicular

9. Jan 27, 2014

ZapperZ

Staff Emeritus
Do you understand that, in an electric field, a + charge moves in one direction while a - charge moves in the opposite direction? There is no "forward" direction here.

This is E&M and not specific to electron and positron.

Zz.

10. Jan 27, 2014

ChrisVer

the magnetic field is perpendicular... so think of electrons moving on the surface of the table...
and then you put a perpendicular magnetic field (it shows upwards outside the table's surface). Then, the electrons moving on the table will start moving in circles on it...
If the electrons will move on the circles clockwise, the positrons will move counterclockwise, but the radius of the circles will be the same...

11. Jan 27, 2014

Staff: Mentor

By "perpendicular", he means that the direction of the magnetic field (vertical in my example) is perpendicular to the direction of motion of the particle (horizontal in my example).

12. Jan 27, 2014

Antigone

Chris and jtbell, so if a magnetic field is traveling in a vertical line, the particle will move in a horizontal line? And if a positron was on the horizontal line, it would travel in direction "X". If a electron was on the same horizontal line, with the same magnetic field, it would travel in opposite of direction "X"?

Have I got it? I really hope so

Last edited by a moderator: Jan 27, 2014
13. Jan 27, 2014

ChrisVer

there is just no reason to say that the magnetic field is traveling ... it can be static..
the particle will move on the perpendicular plane (the surface that will have the lines of your field vertical on it). So avoid thinking in lines for the motion but in 2D surfaces (on a paper, on a table or so on). That's why you cannot get it (you cannot think of a circle on 1 axis X alone)...
So you have X Y Z axis... on Z is your magnetic field lines, so forget about it... an electron moving on XY, will make circles on XY plane clockwise, while the positron will make circles on XY plane counterclockwise.

14. Jan 27, 2014

ChrisVer

as an illustration of what we are talking about...

and look at how the negative and how the positive charges move... the negative moves counterclockwise, while the positive moves clockwise in this case... (forget about the motion towards the magnetic field lines-it won't change the circles, it will only make circles look spiral)

Last edited by a moderator: Sep 25, 2014
15. Jan 27, 2014

Antigone

I dont understand the axis-explanation.

All I am wondering is this, and I will probably open a new thread for this. A positive particle, and a negative particle, are both effected by a magnetic field. Now, one will travel at one direction, the other will travel at opposite direction.

Does this happen to protons, too? Protons are positively charged, and electrons are negativly charged. So the proton would go one way, and the electron the other way?

16. Jan 27, 2014

ChrisVer

See the video and get a glimpse what I mean by axis- and why you need PLANES and not 1 dimensionsional directions to understand it...
Yes a proton will also circulate in different orientation than the electron...BUT the circle it will "draw" will not have the same radius - it will make smaller circles because it has a bigger mass

17. Jan 27, 2014

Antigone

Thank you. I just look it up. Its quite a complicated motion. I have to study some videos, I guess.

Last edited by a moderator: Sep 25, 2014
18. Jan 27, 2014

SteamKing

Staff Emeritus
It's like this: two particles with the same charge (both positive or both negative) will repel one another.

Two particles with opposite charges (one positive and one negative) will attract one another.

For bar magnets, if you try to push both north poles together, they want to repel each other. If you try to stick the north pole of one magnet to the south pole of the other magnet, they snap together.

Doesn't anyone play with magnets anymore? Is it all just cell phones now?

19. Jan 27, 2014

sophiecentaur

Think of a right hand and a left hand thread and the 'axis' is along the length of the screw. You need to look at some diagrams of this (as recommended) and get familiar with some of the terms used. You will need to do some homework if you want to 'get it'.