Constraints on Fermionic Dark Matter Absorption from Radiochemical Solar-Neutrino Measurements

We reinterpret classic radiochemical solar-neutrino measurements as ``rate meters'' for additional, non-negative capture-like contributions induced by fermionic dark matter absorption. Using the chlorine and gallium production-rate data, we build a Bayesian likelihood that accounts for the dominant uncertainties in the solar-neutrino capture-rate prediction (solar fluxes, oscillation parameters, and capture cross sections). Solar-model metallicity systematics are made explicit by presenting results for both the B16--GS98 and B16--AGSS09met solar-model realizations. From the 1D marginalized posteriors of the joint $(R_{\chi,\mathrm{Cl}},R_{\chi,\mathrm{Ga}})$ analysis, we obtain 90\% upper limits on additional capture-like rate contributions, dominated by chlorine: $R_{\chi,\mathrm{Cl},90}\simeq 0.388~\mathrm{SNU}$ (B16--GS98) and $0.588~\mathrm{SNU}$ (B16--AGSS09met). In the charged-current V--A benchmark, we map these constraints onto upper bounds on $y\equiv m_\chi^2/(4\pi\Lambda^4)$ for $m_\chi$ above the ${}^{71}$Ga and ${}^{37}$Cl capture thresholds, using a pep-normalized operator mapping anchored to solar-neutrino capture inputs, where $m_\chi$ is the dark matter mass and $\Lambda$ is the effective scale suppressing the charged-current operator. At $m_\chi\simeq 1~\mathrm{MeV}$, we find $y_{90}\simeq 4.88\times 10^{-49}~\mathrm{cm}^2$ (B16--GS98) and $7.08\times 10^{-49}~\mathrm{cm}^2$ (B16--AGSS09met). These radiochemical bounds are complementary to xenon-based absorption searches and collider interpretations by probing distinct nuclear targets with minimal reliance on spectral reconstruction.