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# Physics: Pair Creation watch

1. When analysing particle tracks of electron-positron pair creation, why is one spiral always bigger than the other? I thought they would be the same as they have same mass and equal (but opposite) charge...

Thanks!
2. it's just due to the random nature of the decay of kinetic energy. They're spiralling in as they lose their kinetic energy through random collisions within the bubbles chamber. So one is likely to lose it's energy slower than the other, so have a bigger spiral
3. clearly from the graph, the total momentum is non-zero
is it possible that they have different speeds when produced?
4. (Original post by Willa)
it's just due to the random nature of the decay of kinetic energy. They're spiralling in as they lose their kinetic energy through random collisions within the bubbles chamber. So one is likely to lose it's energy slower than the other, so have a bigger spiral
ok... so the positron OR the electron can have the bigger spiral... right?
(Original post by keisiuho)
clearly from the graph, the total momentum is non-zero
is it possible that they have different speeds when produced?
erm... i think pair creation is a result of a moving particle's decay not a collision so there can't be zero momentum... so different speeds happen... most the energy goes to the lone electron and the pair takes little... thats why it spirals...
5. i think the point she was getting at is that the pair come from a gamma ray, and this has non-zero momentum, but she was wondering whether the momentum can be shared unequally between the electron and positron. As far as i'm aware, they will both obtain the same momentum - so it's not that.

as regards to your first question - yes, i believe it can be either the positron or the electron that spiral longer, it jsut depends on which ones loses energy slower
6. (Original post by Willa)
i think the point she was getting at is that the pair come from a gamma ray, and this has non-zero momentum, but she was wondering whether the momentum can be shared unequally between the electron and positron. As far as i'm aware, they will both obtain the same momentum - so it's not that.

as regards to your first question - yes, i believe it can be either the positron or the electron that spiral longer, it jsut depends on which ones loses energy slower
who is "she"? lol...

Thanks you have been a great help
7. How does a gamma ray have momentum?
And if it decays to a positron and electron, why do they go in circles again? the spirals look like the particles have a resultant force because they are accelerating also.
8. (Original post by mik1a)
How does a gamma ray have momentum?
And if it decays to a positron and electron, why do they go in circles again? the spirals look like the particles have a resultant force because they are accelerating also.
it has momentum, but i dont know why.

the momentum perpendicular to the direction of motion is zero initially.
So when they break up, two equal momenta with oppisite directions can result.

Edit: sorry, ive made a mistake here. It should be a perpendicular magnetic field that provides the charged particles with the centripetal forces for their circular motions
9. Ok, but if one spiral is larger then pressumably it's because the mass of this particle is smaller? But them this does not agree with conservation of mass - can an electron and positron have different masses?

And I don't understand how gamma waves can have mass :s, they have no rest mass, which I thought was what determined momentum (as well as velocity).
10. (Original post by mik1a)
Ok, but if one spiral is larger then pressumably it's because the mass of this particle is smaller? But them this does not agree with conservation of mass - can an electron and positron have different masses?

And I don't understand how gamma waves can have mass :s, they have no rest mass, which I thought was what determined momentum (as well as velocity).
more accurately, when matters approach the speed of light, Newton's Laws no longer hold.
According to Einstein's and Planck theories, E=mv^2 and E = hf
mc^2=hf
mc^2=hc/l
mc=h/l
p=h/l
where p is momentum and l is lambda
so it is not necessary to have rest mass

And now we have to talk about conservation of mass-energy
a positron is the anti-particle of an electron
so they must have the same mass
11. (Original post by keisiuho)
more accurately, when matters approach the speed of light, Newton's Laws no longer hold.
According to Einstein's and Planck theories, E=mv^2 and E = hf
mc^2=hf
mc^2=hc/l
mc=h/l
p=h/l
where p is momentum and l is lambda
so it is not necessary to have rest mass

And now we have to talk about conservation of mass-energy
a positron is the anti-particle of an electron
so they must have the same mass
Ok that makes sense (mostly), except why one spirals larger than the other - surely this means one has more velocity than the other (I assume the spirals happen in the same time period)? And why do they spiral, and not just moce with constant velocity? What accelerates them to the centre of the spiral?

So many questions
12. (Original post by mik1a)
Ok that makes sense (mostly), except why one spirals larger than the other - surely this means one has more velocity than the other (I assume the spirals happen in the same time period)? And why do they spiral, and not just moce with constant velocity? What accelerates them to the centre of the spiral?

So many questions
Maybe it's just a crap drawing and they weren't bothered about the nit picky details!
13. if you look at the answer i gave above - it's because the particles lose kinetic energy through collisions. If they never lost kinetic energy, they would go round and round in circles, but since they are losing energy, they must move in to a smaller radius:

f=bqv = mv^2/r
bq = mv/r
so r = mv/bq
so as the particles lose energy, v decreases, and so does their radius!!

The particles do not lose energy at the same rate since the collisions are random!
14. (Original post by lxl_bono_lxl)
Maybe it's just a crap drawing and they weren't bothered about the nit picky details!
willa seems to be right i think...

its not a crap drawing because it is an actual picture of tracks left in a bubble chamber...
the colours were put in after... the tracks were actually there... there was the same pic but without the colours... but this one showed exactly where my confusion was... thats why i used it...
15. the particles do start off with the same momenta, but due to the random nature of the colissions, will lose it unequally.

photons do have momentum; from the reletavistic energy-momentum relation: E² - p²c² = (mc²)², thus photons (with no rest mass m) have momentum p=E/c
16. (Original post by elpaw)
the particles do start off with the same momenta, but due to the random nature of the colissions, will lose it unequally.
yup.. thanks for clearing that... (but willa did it first .... still useful to have someone else confirming it though)

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