First constraints on Helium $^{+}{\rm He}^3$ reionization using the 8.67GHz hyperfine transition

We present the first constraints on the cross-correlation power spectrum of HeII-HeIII ($^{+}{\rm He}^3$) reionization using the redshifted 8.67GHz hyperfine transition between z=2.9 and z=4.1 and with interferometric data obtained from the public archive of the Australia Telescope Compact Array. 210 hours of observations of the primary calibrator source B1934-638 were extracted from data obtained with the telescope from 2014--2021, and coherently combined in a power spectrum pipeline to measure the HeII power across a range of spatial scales, and at three redshifts that span the period of Helium reionization. Our best limit places the brightness temperature fluctuation to be less than 557$\mu$K on spatial scales of 30 arcmin at z=2.91, and less than 755$\mu$K on scales of 30 arcmin at z=4.14 (2-sigma noise-limited). We measure a temperature of 489$\mu$K at z=2.91. ATCA's few antennas and persistent remaining RFI in the data prevent deeper integrations improving the results. This work is a proof of principle to demonstrate how this type of experiment can be undertaken to reach the 1--10$\mu$K level expected for the Helium signal at z~4.