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Name: Andrew
Status: educator
Grade: 6-8
Location: NH
Country: USA
Date: Summer 2012

Why does beta radiation change the mass of an isotope?

Hi Andrew,

A beta particle(â-) is a highly energetic electron emitted from the nucleus of a radioactive atom, coincident with an antineutrino(í). Beta electrons originate from a proton of a neutron rich isotope. When a proton(2U,1D quarks, +1 charge) ejects a beta particle, it is transformed into a neutron(1U,2D quarks, neutral).

That means the original isotope has lost a proton, therefore the element has changed and gained a neutron, so the mass will remain the same. We have a change of an element and an isotope.

K40 yields â- + í and Ca40.

Hope this helps! PEHughes, Ph,D. Milford, NH

Beta particles have mass... so when they are ejected, the remaining mass is lower. Simple conservation of mass... also, see this answer from the Newton archives:

However, it does not change the 'mass number' (the sum of protons and neutrons), as beta emission simply 'converts' a neutron to a proton. The 'mass number' does not distinguish the mass of neutrons from protons. (there is a little more complexity to the situation, but I think this simplification is sufficient for here).

Hope this helps, Burr Zimmerman

Because the mass of the electron (the "beta" particle) is not in the nucleus anymore. Neither is the mass of the anti-neutrino that also was emitted. Also, the kinetic energy that those particles have is no longer in the nucleus.

Tim Mooney

A “beta ray” is an electron. The nucleus of an element can gain ( or lose ) an electron, depending upon what nuclear process is taking place. In addition, this gain (or loss ) of the electron can “trigger” further nuclear processes, leading to further gain or loss of nuclear mass.

Vince Calder


Please see this article for details:

The two types of Beta radiation (particles) are electrons and positrons. Electron rest mass from:

is about 9.11 x (10^(-31)) Kilograms.

Positron rest mass is approximately the same.

So beta radiation reduces the mass of the radiating substance.

Sincere regards, Mike Stewart

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