The central detector in the MuSun experiment is a pad-plane time projection ionization chamber that operates without gas amplification in deuterium at 31 K; it is used to measure the rate of the muon capture process $mu^- + d rightarrow n + n + u_mu
$. A new charge-sensitive preamplifier, operated at 140 K, has been developed for this detector. It achieved a resolution of 4.5 keV(D$_2$) or 120 $e^-$ RMS with zero detector capacitance at 1.1 $mu$s integration time in laboratory tests. In the experimental environment, the electronic resolution is 10 keV(D$_2$) or 250 $e^-$ RMS at a 0.5 $mu$s integration time. The excellent energy resolution of this amplifier has enabled discrimination between signals from muon-catalyzed fusion and muon capture on chemical impurities, which will precisely determine systematic corrections due to these processes. It is also expected to improve the muon tracking and determination of the stopping location.
Measuring the muon flux is important to the Sanford Underground Laboratory at Homestake, for which several low background experiments are being planned. The nearly-vertical cosmic ray muon flux was measured in three locations at this laboratory: on t
he surface (1.149 pm 0.017 x 10^-2 cm^-2 s^-1 sr^-1), at the 800-ft (0.712 km w.e.) level (2.67 pm 0.06 x 10^-6 cm^-2 s^-1 sr^-1), and at the 2000-ft (1.78 km w.e.) level (2.56 pm 0.25 x 10^-7 cm^-2 s^-1 sr^-1). These fluxes agree well with model predictions.
