We present the measured baseline ionization resolution of a HEMT-based cryogenic charge amplifier coupled to a CDMS-II detector. The amplifier has been developed to allow massive semiconductor dark matter detectors to retain background discrimination at the low recoil energies produced by low-mass WIMPs. We find a calibrated baseline ionization resolution of $sigma_E = 91,text{eV}_{ee}$. To our knowledge, this is the best direct ionization resolution achieved with such massive ($approx$150 pF capacitance) radiation detectors.
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