Is this an annihilation process , I am so confused now Watch

NicolasLee
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A positron with a kinetic energy of 2.0 keV collides with an electron at rest, creating two photons that have equal energy.

Show that the energy of each photon is 8.2 × 10−14 J.
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Luqman.ldn
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pair production.
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NicolasLee
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Why do you said so? Particles and Anti-particles meet each other and converted their mass into a pair of gamma ray photons (Original post by Luqman.ldn)pair production
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nzy
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Now I'M confused.
When a particle and antiparticle meet they annihilate, so the energy of EACH photon should be equal to half of the total energy of the particle and antiparticle.
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NicolasLee
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Yea thats right, but I am not sure why the rest mass of electron has to double?

Total energy = 2keV + 2 × 511 keV = 1024 keV✔

= 1024 × 1.6 × 10−19 = 1.64 × 10−13J✔

Energy of each photon = 1.64 × 10−13/2 = 8.19 × 10−14 (J) ✔
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nzy
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(Original post by NicolasLee)
Yea thats right, but I am not sure why the rest mass of electron has to double?

Total energy = 2keV + 2 × 511 keV = 1024 keV✔

= 1024 × 1.6 × 10−19 = 1.64 × 10−13J✔

Energy of each photon = 1.64 × 10−13/2 = 8.19 × 10−14 (J) ✔
Yeah I have no idea. The only reason I could think of for doing that is if the total energy of the positron equals its kinetic energy PLUS its rest energy, but I don't remember ever seeing that anywhere.
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NicolasLee
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(Original post by nzy)
Yeah I have no idea. The only reason I could think of for doing that is if the total energy of the positron equals its kinetic energy PLUS its rest energy, but I don't remember ever seeing that anywhere.
I probably figured it out why this is a pair production because this process is required kinetic energy but not for annihilation.
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Sinnoh
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(Original post by Luqman.ldn)
pair production.
Wrong. Pair production is when a photon is converted into a particle and an antiparticle. The process described in the OP is indeed annihilation, as a particle (the electron) is colliding with an antiparticle (the positron), converting their rest energies and kinetic energies into photons.
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nzy
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(Original post by NicolasLee)
I probably figured it out why this is a pair production because this process is required kinetic energy but not for annihilation.
what
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Sinnoh
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The total energy before the interaction is equal to the total energy after the interaction, as per the law of conservation of energy.

So the energy before impact is given by the mass of the electron at rest (multiplied by c2 ), the mass of the positron (again multiplied by c2) plus the kinetic energy of the positron.
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NicolasLee
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(Original post by Sinnoh)
The total energy before the interaction is equal to the total energy after the interaction, as per the law of conservation of energy.

So the energy before impact is given by the mass of the electron at rest, the mass of the positron plus the kinetic energy of the positron.
Wow many thanks!
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Luqman.ldn
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(Original post by Sinnoh)
Wrong. Pair production is when a photon is converted into a particle and an antiparticle. The process described in the OP is indeed annihilation, as a particle (the electron) is colliding with an antiparticle (the positron), converting their rest energies and kinetic energies into photons.
Yeah , my bad. Covered it during the summer so I forgot which way round it is.
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