cdms-ii - CDMS Experiment - University of California, Berkeley

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26 CHAPTER 2. DETECTORS Figure 2.1: Diagram of a ZIP detector. Bottom right is the “Ionization Side” showing a large inner electrode and an outer guard ring. Bottom left is the “Phonon Side” whose surface is divided into four quadrants labeled A, B, C, and D. The axis shown defines the basis for the x-y coordinate system. Each quadrant has 148 cells (top left) each of which have 24 QETs. A QET (top right) consists of a 1 µ thick strip of W connected to 8 Al collector fins. first class of phonons are ballistic phonons. These phonons are sufficiently low in frequency (0.1-1 THz) that their mean free path is comparable to the size of the crystal. These phonons will not scatter inside the crystal, but will travel in a straight line across the crystal reflecting at the boundaries until they are absorbed by metal on the surface. The second class of phonons have very high frequencies (∼ 3 THz for Ge and ∼ 6 THz in Si) and scatter frequently. The propagation resulting from the combined isotopic scattering and anharmonic decay is referred to as quasi-diffuse
2.1. THE PHONON MEASUREMENT 27 propagation [48]. For quasi-diffuse propagation, the phonon “wave front” moves at ∼ 1/3 the speed of sound. The initial phonon spectrum produced by a recoil consists primarily of phonons of the second class. There are three sources of ballistic phonons. Neganov-Luke [49, 50] phonons are produced when drifting the electric charges across the crystal to the electrodes at the surface. It has been shown that Luke phonons are ballistic and have a total energy equal to the work done in drifting the charges across the crystal [51]. The second source of ballistic phonons is electron-hole recombination at the electrodes. In reality, the phonons released when the electrons relax to the valence band are high frequency phonons. However, since these phonons are at the detector surface, they interact frequently with metal at the surface resulting in a rapid down conversion to ballistic phonons. Similarly, the third source of ballistic phonons is the interaction of the initial quasi-diffuse phonon cloud with the surface resulting in rapid down conversion into ballistic phonons [51]. This third effect is only possible if the interaction is sufficiently close to the surface of the crystal. 2.1.2 Measuring Phonon Energy using QETs Al fins and quasi-particle diffusion When the phonon wave-front encounters the Al fins at the detector surface, ∼30% of the phonon energy will be transferred into the Al. The Al fins are superconducting at our operating temperatures and have a superconducting energy gap of 340 µeV. If the energy absorbed is larger than the energy gap, the absorption results in quasiparticle (broken Cooper pairs) excitations in the Al which diffuse into the W TES. The lower band gap in W traps the quasi-particles in the TES so that they deposit their energy into the W electron system. ETF TES The W strips are operated as ETF TESs (Electro-Thermal Feedback Transition Edge Sensors) [52]. In this configuration, a voltage bias generates a current through the W which, through Joule heating, keeps the temperature of the W electrons somewhere
Page 1 and 2: THE CRYOGENIC DARK MATTER SEARCH (C
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