# School Me on Mass Difference

1. Apr 18, 2006

### jaster

(first post on these forums). I am a nuclear physics sophomore and I come to you with what is, perhaps, a question I should already know the answer to. Several nuclear reactions release energy in accordance with some mass difference between reactants and products. My question, however, is about the physical meaning of this. Where does this mass come from? How might I visualize this process? If I wanted to visualize, with some model, the difference between (for example) a free neutron and a bound one (bound in a system with some binding energy relating to the mass difference between the bound system and the sums of the masses of the individual particles)? If I was to be given some visualization/information about a random proton is there any way (speaking fromt theory not instrumentation) I would be able to tell if it was bound or free? Sorry for the rambling nature of the question, it's been a long day :yuck: Thanks and Peace

2. Apr 18, 2006

### Meir Achuz

Try to make the question more specific.
Would it help for me to say the the mass of a nucleus is given by
the sum of its constituent's masses, and the total potential and kinetic energy.

3. Apr 18, 2006

### Astronuc

Staff Emeritus
Particles, such as nucleons like the proton and neutron, have rest mass. A proton and a neutron can combine in a bound structure, which is a deuteron, or nucleus of a deuterium atom (H2). If one measures the rest mass of a deuteron, one will find that the mass is less than the sum of the rest masses of the neutron and proton. This energy is given up as a gamma-ray.

Similarly neutrons can be aborbed by any nuclei, with varying levels of success (probability) and a gamma-ray is usually released promptly. The new nucleus has a slightly lower rest mass than the rest masses of the initial nucleus and free neutron.

Fusion occurs when light nuclei combine. In most cases the rest masses of the reactant nuclei are greater than the product nuclei, and this difference in energy is manifest in the kinetic energy of the product nuclei, rather than gamma photons.

At extremely high energy, e.g. in particle colliders (accelerators), colliding protons can be used to create a variety of heavy partices, including anti-protons. In this case, the kinetic energy is transformed into new particles, which also have rest mass.