= Argon-37
<Argon-37> is a radioactive <isotope> with $18$ <protons> and $19$ <neutrons>. It undergoes <electron capture> to <chlorine-37>, with an approximately $35$-day <half-life>:
$$
{}^{37}\mathrm{Ar}+e^-\longrightarrow{}^{37}\mathrm{Cl}+\nu_e.
$$
The <mass number> is unchanged while the nuclear charge decreases by one. Atomic vacancies left by the captured <electron> can produce detectable low-energy radiation during atomic relaxation.
Argon-37 made from chlorine was the counted product in the radiochemical solar-neutrino experiment. The method separated rare radioactive <argon> <atoms> from a much larger chlorine target, connecting nuclear counting to a very weak astronomical signal.<Nobel Lecture: A half-century with solar neutrinos by Raymond Davis Jr.>[Chlorine–argon experiment: production, extraction and counting of argon-37]{ref}
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