To determine the original isotope that undergoes decay to produce strontium-85 and a positron, we need to look at the decay process and identify the isotope that fits the description given in the question.
1. The production of strontium-85 indicates that the original isotope decays by emitting a beta particle (positron). In this process, a neutron in the nucleus is converted into a proton, and an electron antineutrino is emitted along with the positron.
2. Strontium-85 has an atomic number of 38. This means the original isotope must have an atomic number that is one less, as a positron emission increases the atomic number by 1 while keeping the mass number the same.
3. Therefore, the original isotope that could decay to produce strontium-85 and a positron is **rubidium-85** (Rb-85). Rubidium-85 decays by emitting a positron and transforming into strontium-85 (Sr-85).
By understanding the decay process and the properties of the elements involved, we can deduce that rubidium-85 is the most likely original isotope in this scenario.
