This paper explores the prospect of CMOS devices to assay lead in drinking water, using calorimetry. Lead occurs together with traces of radioisotopes, e.g. Lead-210, producing $gamma$-emissions with energies ranging from 10 keV to several 100 keV when they decay; this range is detectable in silicon sensors. In this paper we test a CMOS camera (Oxford Instruments Neo 5.5) for its general performance as a detector of x-rays and low energy $gamma$-rays and assess its sensitivity relative to the World Health Organization upper limit on lead in drinking water. Energies from 6 keV to 60 keV are examined. The CMOS camera has a linear energy response over this range and its energy resolution is for the most part slightly better than 2 %. The Neo sCMOS is not sensitive to x-rays with energies below $sim!!10 keV$. The smallest detectable rate is 40$pm$3 mHz, corresponding to an incident activity on the chip of 7$pm$4 Bq. The estimation of the incident activity sensitivity from the detected activity relies on geometric acceptance and the measured efficiency vs. energy. We report the efficiency measurement, which is 0.08$pm$0.02 % (0.0011$pm$0.0002 %) at 26.3 keV (59.5 keV). Taking calorimetric information into account we measure a minimal detectable rate of 4$pm$1 mHz (1.5$pm$0.1 mHz) for 26.3 keV (59.5 keV) $gamma$-rays, which corresponds to an incident activity of 1.0$pm$0.6 Bq (57$pm$33 Bq). Toy Monte Carlo and Geant4 simulations agree with these results. These results show this CMOS sensor is well-suited as a $gamma$- and x-ray detector with sensitivity at the few to 100 ppb level for Lead-210 in a sample.
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