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A radioactive
nucleus that undergoes positron emission has a proton in its nucleus
convert into a neutron and an positron, then it ejects the positron. A positron
is an anti-electron: it has the same mass, but has a +1 change instead of a -1
charge. The remaining nucleus has one less proton and one more neutron: the atomic number
decreases by one and the mass number stays the same. (See beta emission
for what happens if the nucleus ejects an electron.)
For example: 12456Ba undergoes positron decay. Its
product is an isotope of cesium
- 12456Ba -> 01e +
12455Cs
Positrons are often denoted with the symbol
01e- or e+. In the above reaction,
adding an atomic number of 55 (Cs) and 1 (e+) gives 56, the original
atomic number.
Example: 12557La is a radioactive isotope of
lanthanum. It is a positron emitter: what is the product?
Solution: We keep the mass number the same (125) but decrease the
atomic number by one (57-1 = 56) Atomic number 56 is barium, so
- 12557La -> 12556Ba +
01e
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