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what does it mean that a beta particle has a 1.6 MeV of energy , does it mean that it can ionize 120000 hydrogen particle with ionization energy of 13.6 MeV ?

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It is a measure of either the electron's kinetic energy or it's total energy (including mass). Because the electron's mass is $511\text{ keV} = 0.511\text{ MeV}$, it matters which in this case. By default particle physicists mostly talk about total energy, but nuclear physicist often talk about kinetic energy. In either case you'd have to ask to be sure.

Assuming that is all kinetic energy, then in principle it could be used to ionize more than one hundred-thousand hydrogen atoms with each electron being asymptotically free but having no additional energy; in practice the energy could not be brought to bear in so ordered a manner. If the particle was directed into a region of high purity hydrogen gas it would actually ionize many fewer atoms, but the ejected electrons would have positive asymptotic kinetic energy.

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An electron volt (eV) is just a unit of energy like a Joule. In fact 1eV = 1.602 $\times$ 10$^{-19}$J. Historically the eV originated as the work you have to put in to move 1 electron through a potential difference of 1 volt, hence the name. It's used in Physics because it's a very convenient unit. If we used joules we'd be having to write powers of ten all over the place.

You might also see the eV used as a mass. This is simply done by converting energy to mass using Einstein's famous formula $E = mc^2$.

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i know what an eV is , i wanted to know if this was its ionizing energy , can we predict how many ions will be ionized by the particle? – Abdelrahman Esmat Feb 4 '13 at 10:45

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