Photonuclear reaction & Conservation of momentum

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Discussion Overview

The discussion centers around the photonuclear reaction involving deuterium and the conservation of momentum. Participants explore the implications of including photon momentum in the momentum balance equation for the reaction.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a momentum balance equation for the reaction γ+2D → 1H + 1n, questioning its validity due to the omission of photon momentum.
  • Another participant challenges the clarity of the original momentum balance, suggesting that the momenta of the outgoing particles should sum to the momentum of the photon if deuterium is stationary.
  • A later reply acknowledges the need to consider the momentum of the photon, indicating that both incoming and outgoing particles are in motion.
  • One participant notes that for typical photon energies, the momentum of the photon is negligible compared to that of the proton and neutron.
  • Another participant points out that if photon momentum is ignored, the outgoing neutron and proton will have equal but opposite momenta.
  • A final contribution mentions a rough calculation showing a significant difference in momentum between a photon and a proton with energy of 1 MeV, suggesting that photon momentum is indeed much smaller.

Areas of Agreement / Disagreement

Participants express differing views on the significance of photon momentum in the momentum balance equation, indicating that the discussion remains unresolved regarding its impact on the reaction analysis.

Contextual Notes

There is a lack of consensus on how to properly account for photon momentum in the momentum balance, and assumptions about the motion of the particles involved may affect the analysis.

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γ+2D ----> 1H + 1n

In this photonuclear reaction, is it possible to write the momentum balance equation as it follows,

MHvH = Mnvn

It somehow seems to wrong to me, since the incoming photon has a certain momentum, which is not taken account in the balance equation.

Thank you in advance.
 
Last edited:
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Your original "equation" is not. The momentum balance is not too clear - what particles are you referring to? The outgoing particles (n and p) have momenta which sum to the momentum of te photon, assuming D is stationary.
 
I am sorry about the "+" mark between D and H. just edited it. Both incoming and outgoing particles are in motion. We should consider the momentum of the photon, should not we?
 
For typical photon energies, the photon momentum is tiny compared to the momenta of the proton and neutron.
 
If you ignore the photon momentum, the outgoing (n and p) have the same momentum,but in opposite directions.
 
Thank you all for answering. I should have thought this before. I have made a rough calculation. There is around eight orders of magnitude difference between the momentum of a photon and a proton with energy of 1 MeV.
 

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