UC Berkeley

The Cryogenic Dark Matter Search (CDMS) is searching for weakly-interacting massive particles (WIMPS), which could explain the dark matter problem in cosmology and particle physics.

By simultaneously measuring signals from deposited charge and the energy in non-equilibrium phonons created by particle interactions in intrinsic germanium crystals at a temperature of 40 mK, a signature response for each event is produced. This response, combined with phonon pulse-shape information, allows CDMS to actively discriminate candidate WIMP interactions with nuclei from electromagnetic radioactive background which interacts with electrons.

The challenges associated with these techniques are unique. Carrier scattering is dominated by the spontaneous emission of Luke-Neganov phonons due to zero-point fluctuations of the lattice ions. Drift fields are maintained at only a few V/cm, else these emitted phonons would dominate the phonons of the original interaction. The dominant systematic issues with CDMS detectors are due to the effects of space charge accumulation. It has been an open question how space charge accrues, and by which of several potential recombination and ionization processes.

In this work, we have simulated the transport of electrons and holes in germanium under CDMS conditions. We have implemented both a traditional Monte Carlo technique based on carrier energy, followed later by a novel Monte Carlo algorithm with scattering rates defined and sampled by vector momentum. This vector-based method provides for a full anisotropic simulation of carrier transport including free-flight acceleration with an anisotropic mass, and anisotropic scattering rates.

With knowledge of steady state carrier dynamics as a function of applied field, the results of our Monte Carlo simulations allow us to make a wide variety of predictions for energy dependent processes for both electrons and holes. Such processes include carrier capture by charged impurities, neutral impurities, static dipoles, and capture forming ``anion'' (D-/A+) states. We also generate predictions for impact ionization of shallow impurities and of impact ``neutralization'' of D-/A+ states. We use measurements of carrier capture performed on CDMS detectors to validate a plausible model for electron and hole capture due to neutral shallow impurities and their charged D-/A+ states. This model, along with carrier drift and diffusion parameters from Monte Carlo simulation, can be used as the foundation for simulations of space charge evolution in CDMS detectors, simultaneously solving continuity equations with Poisson's equation.