Search for the admixture of heavy neutrinos in the recoil spectra of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow/><mml:mrow><mml:mn>37</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="normal">Ar</mml:mi></mml:math>decay

Neutrino-induced recoil spectra of ${}^{37}\mathrm{Cl}$ ions produced in the electron capture (EC) decay of ${}^{37}\mathrm{Ar}$ were measured and searched for the presence of massive neutrinos admixed to the dominant electron neutrino. Fractions of a monolayer of ${}^{37}\mathrm{Ar}$ were physisorbed on Au and on several underlayers of ${}^{40}\mathrm{Ar}$ adsorbed on both Au and graphite substrates cooled to $&lt;~20 \mathrm{K}$ under ultrahigh vacuum conditions. Time-of-flight spectra of the recoiling ions were recorded in coincidence with x rays and Auger electrons emitted following the EC decay. By searching these spectra for peaks with energies between 7.6 eV and 3.6 eV upper limits were placed on the mixing probability of the electron neutrino with heavy neutrinos in the 370--640 keV mass range. These limits vary from 1 to 4 %, at the 90% confidence level.