2 ms·

>in, … 12.3 years…, half of the tritium available today will have decayed into helium-3. When I saw this, I thought that sounded odd, shouldn’t the atomic numb

by dhosek 4y ago

>in, … 12.3 years…, half of the tritium available today will have decayed into helium-3. When I saw this, I thought that sounded odd, shouldn’t the atomic number go down with radioactive decay? but it turns out that in this case what happens is one of the neutrons splits into a proton, an electron and an electron neutrino so the atomic number goes up by one (unlike the more familiar fission reactions of the heavier elements where the atomic number decreases). So much physics I’ve either forgotten or never learned.

db48x 4y ago
Yea, beta decay is weird. On the other hand, transmutation of the elements!
adrian_b 4y ago
This is the standard beta radioactive decay, which increases Z by 1, while the alpha radioactive decay decreases Z by 2 (and gamma decay does not change Z). For every A (the number of nucleons in a nucleus) there is a ratio between (A-Z) and Z (i.e. between the number of neutrons and the number of protons) for which the mass of the nucleus is minimum. Any other isobaric nuclei have an excess of mass over the nucleus with the optimal neutron/proton ratio, so they will decay towards it. The nuclei with too many protons will capture electrons or emit positrons, while the nuclei with too many neutrons will emit electrons, increasing the number of protons with each emitted electron, until the optimal neutron/proton ratio. For the decay products of uranium and thorium, there are always too many neutrons, so the normal beta decay is what always happens. It is possible to artificially produce nuclei with too many protons, and there are a few such unstable isotopes that are produced naturally, which decay by the reverse beta decay (electron capture or positron emission), where Z decreases by 1 for every captured electron / emitted positron. For A = 3, the nucleus with minimal mass is helium-3. Tritium has too many neutrons in comparison with helium-3, and it must get rid of them by emitting an electron.

by dhosek 4y ago

db48x 4y ago

Yea, beta decay is weird. On the other hand, transmutation of the elements!

adrian_b 4y ago

This is the standard beta radioactive decay, which increases Z by 1, while the alpha radioactive decay decreases Z by 2 (and gamma decay does not change Z). For every A (the number of nucleons in a nucleus) there is a ratio between (A-Z) and Z (i.e. between the number of neutrons and the number of protons) for which the mass of the nucleus is minimum. Any other isobaric nuclei have an excess of mass over the nucleus with the optimal neutron/proton ratio, so they will decay towards it. The nuclei with too many protons will capture electrons or emit positrons, while the nuclei with too many neutrons will emit electrons, increasing the number of protons with each emitted electron, until the optimal neutron/proton ratio. For the decay products of uranium and thorium, there are always too many neutrons, so the normal beta decay is what always happens. It is possible to artificially produce nuclei with too many protons, and there are a few such unstable isotopes that are produced naturally, which decay by the reverse beta decay (electron capture or positron emission), where Z decreases by 1 for every captured electron / emitted positron. For A = 3, the nucleus with minimal mass is helium-3. Tritium has too many neutrons in comparison with helium-3, and it must get rid of them by emitting an electron.