Characterizing the Sensitivity of 40 GHz TES Bolometers for BICEP Array
  The BICEP/Keck (BK) experiment aims to detect the imprint of primordial
gravitational waves in the Cosmic Microwave Background polarization, which
would be direct evidence of the inflation theory. While the tensor-to-scalar
ratio has been constrained to be r_0.05 < 0.06 at 95% c.l., further
improvements on this upper limit are hindered by polarized Galactic foreground
emissions and removal of gravitational lensing polarization. The 30/40 GHz
receiver of the BICEP Array (BA) will deploy at the end of 2019 and will
constrain the synchrotron foreground with unprecedented accuracy within the BK
sky patch. We will show the design of the 30/40 GHz detectors and test results
summarizing its performance. The low optical and atmospheric loading at these
frequencies requires our TES detectors to have low saturation power in order to
be photon-noise dominated. To realize the low thermal conductivity required
from a 250 mK base temperature, we developed new bolometer leg designs. We will
present the relevant measured detector parameters: G, Tc, Rn, Psat , and
spectral bands, and noise spectra. We achieved a per bolometer NEP including
all noise components of 2.07E-17 W/sqrt(Hz), including an anticipated photon
noise level 1.54E-17 W/sqrt(Hz).
