Why does all the excess BE go to the radiated particle?

1. Sep 4, 2011

kenshi64

Well all textbooks probably state something like this 'A Nucleus loses energy by radiation'

I'm cool with that statement except the fact that all the excess BE or energy is lost only to the eg.alpha particle! and no mention of some being given to the new nucleus!

Look at this for example:
Mass Helium(Alpha)=4.002602u
Change in mass=0.005198u
Energy equivalent=4.84MeV

Note: Energy of alpha=5MeV
Resemblance of 4.84MeV to 5MeV-Uncanny
So this is going to say that the lost BE is all given to the alpha, WHY?
(or am I supposed to consider 5-4.84 for the energy given to the new nucleus)

Last edited: Sep 4, 2011
2. Sep 4, 2011

Simon_Tyler

Solving the conservation of momentum and energy in this case tells you that the kinetic energy of the alpha particle will be approx 55.5 times more than that of the radon nucleus.

So I guess that to a good approximation, you can ignore the recoil of the daughter nucleus.

3. Sep 4, 2011

sambristol

Agreed Simon_Tyler, Kenshi64 the daughter nucleus MUST recoil in the opposite direction to the alpha to conserve momentum. It's not just energy which must be conserved.

PS Even if the radiation is a gamma (so no rest mass) it still has momentum given by E/c
where E is its energy and c as usual is the speed of light

4. Sep 4, 2011

sambristol

Kenshi64 see attached for all the details

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5. Sep 4, 2011

kenshi64

Did you type that out yourself? Cool, Thanks! I got most of it but the end was above me, and anyway its sufficient if you answer the doubt in words, since I'm not doing Physics in much detail.

6. Sep 4, 2011

kenshi64

I appreciate both the reply, but sadly I don't understand either, could you address the question directly without jumping into other complicated things! Thanks, I'm truly sorry for the inconvenience. :)

7. Sep 4, 2011

sambristol

Tell me exactly what you don't understand and I will try my best to bridge the gap.

That is ask me specific questions

Physics is written in the language of maths. Separate the maths from the physics and much more importantly do not be afraid of either, the fear of the maths has hampered many a young physicist – not helped by professors who 'drop in' the maths so students dont know what is physics and what is maths

Regards

Sam