# Photon+deuteron-&gt;p+n , E(photon)=?

1. Mar 3, 2008

### Ene Dene

[SOLVED] photon+deuteron--&gt;p+n , E(photon)=?

Problem:
What is the minimal energy of photon for disassembling deuteron on proton and neutron (photon+d --> p+n)? By how much is that energy larger than energy of binding of deuteron? Binding energy of deuteron is E=2.225MeV, mass of deuteron is 1875.628MeV.

This means that m(d)-m(p)-m(n)=2.225MeV.
I don't understand why would do you need a photon of greater energy than 2.225MeV if that is the energy that binds proton and neutron together.

2. Mar 3, 2008

### Staff: Mentor

Binding energy is not energy that binds nucleons, but rather it is the energy given off when a nucleon combines with another nucleon or nucleus, i.e. binding energy is actually the energy require to unbind a nucleon from the nucleus.

In the case of the deuteron, the energy required to unbind the neutron and proton is 2.225 MeV. That would be approximately the energy of a gamma-ray given off when a proton and neutron combine (neutron capture) to form a deuteron.

3. Mar 3, 2008

### kdv

But if Iunderstand correctly the OP, th equestion was ho wmuch above the binding energy must the photon have. And the OP was wondering about why some energy above the binding energy was required.

The answer is of course conservation of 4-momentum. Some of the photon energy goes into breaking apart the deuteron but some of it is needed to give some kinetic energy to the neutron and proton.

4. Mar 3, 2008

### pam

For a photon of energy p (with c=1),
$$p+M_d=\sqrt{(m_p+m_n)^2+p^2}$$.
Square and solve for p.

5. Mar 4, 2008

### Ene Dene

Yes, I did think that these energies were the same, that's why I asked a question, why aren't they the same.
Yes, you did understand correctly but I also wanted to know why is that so.
And here is why:
Thank you all, now I understand.