Time Evolution of Electric Fields in CDMS Detectors

The Cryogenic Dark Matter Search (CDMS) utilizes large mass, 3″ diameter×1″ thick target masses as particle detectors. The target is instrumented with both phonon and ionization sensors, the later providing a ∼1 V cm−1 electric field in the detector bulk. Cumulative radiation exposure which creates ∼200×106 electron-hole pairs could be sufficient to produce a comparable reverse field in the detector thereby degrading the ionization channel performance, if it was not shielded by image charges on the electrodes. To study this, the existing CDMS detector Monte Carlo has been modified to allow for an event by event evolution of the bulk electric field, in three spatial dimensions. Surprisingly, this simple model is not sufficient to explain the degradation of detector performance. Our most recent results and interpretation are discussed.