Spectrometry of the Neutrino Recoils of Argon-37

The ions that recoil from neutrino emission in the electron capture decay of ${\mathrm{A}}^{37}$ have been subjected to magnetic spectrometry with a resolution amounting to 2.8% in energy. They are found to have an energy of 9.63±0.06 ev, which is in agreement with the value 9.65±0.05 ev to be expected on the basis of a two-body breakup, the neutrino having an energy of 815±2 kev as determined by others from the threshold of the ${\mathrm{Cl}}^{37}(p, n){\mathrm{A}}^{37}$ reaction. The momentum balance sets an upper limit of about 5 kev for the rest mass of the neutrino. Auger electron emission following the orbital electron capture leaves the recoils mostly multiply charged, the percentage abundances in charge states 1 to 7 being 6.2±0.1, 15.7±0.4, 39.2±0.5, 26.7±0.4, 10.0±0.2, 1.8±0.1 and 0.4±0.1, respectively. Neutrals were not measured. The natural width at half-intensity of the singly-charged recoil line is 1.7 ev, which is fully accounted for by the thermal motion of the argon atoms. The natural width of the triply-charged recoil line is 2.5 ev, which is mostly but not entirely accounted for by thermal motion plus recoil from the emission of one 2300-ev $K$ Auger electron. The singly-charged recoils are thought to result from $L$-capture and from $K$-capture followed by $K$ x-ray emission, neither of these processes involving recoil from the 2300-ev Auger electron.