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

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22 CHAPTER 1. INTRODUCTION mass. ZEPLIN only measures the scintillaton produced in the Xe. An electron recoil produces more ionization which releases light when the free electron recombines with the Xe. The slight delay due to recombination makes it possible for a signal due to nuclear recoils to be distinguished from a signal due to electron recoils through statistical pulse shape analysis. The ZEPLIN experiment does not see any nuclear recoil signal. However, the robustness of this result is sometimes questioned due to the lack of an in-situ neutron calibration. EDELWEISS and CDMS I The detector technology for EDELWEISS and CDMS I is virtually identical. The target mass is Ge. Electrodes on the detector surface measure the ionization produced by a recoil while thermistors simultaneously measure the heat that is produced. These detectors can discriminate between nuclear recoils and electron recoils on an event by event basis since the heat measurement is an absolute measurement of the recoil energy while the ionization is reduced or quenched for nuclear recoils. The detectors have one electromagnetic background. Electron recoils near the electrodes can have deficient charge collection resulting in reduced ionization measurements. Neither experiment observes a signal from dark matter [40, 41]. The sensitivity of CDMS I was limited by an irreducible cosmogenic neutron background at the Stanford Underground Facility. CRESST The target mass of CRESST is a crystal of CaWO 4 . CRESST simultaneously measures both the scintillation and the athermal phonons produced by a recoil. Discrimination between nuclear and electron recoils is possible since the scintillation signal is reduced for nuclear recoils. Unlike the detectors for CDMS and EDELWEISS, the CRESST detector does not suffer from a background due to electron recoils near the detector surface. However, the presence of three different nuclei within the crystal makes it difficult to fully characterize the crystal’s response through the use of in-situ calibrations. Preliminary measurements by CRESST find no dark matter signal.
1.3. DIRECT DETECTION OF DARK MATTER 23 CDMS II - SUF The CDMS collaboration has developed another detector technology for dark matter detection. The detectors are crystals of Ge and Si with sensors that simultaneously measure the ionization and athermal phonons produced by a recoil. These detectors are described in more detail in Ch. 2. They are capable of discrimination due to the reduced ionization for a nuclear recoil. The detectors are designed to also measure timing information from the athermal phonon signal making it possible to reconstruct the location of the interaction. The ability to reconstruct the position of an event enables the rejection of the background from electron recoils near the surface. The use of both Si and Ge detectors allows for an estimate of neutron backgrounds since the cross section for neutrons in Si is 5-7 times larger in Si than in Ge. The CDMS II experiment [42] observes no dark matter detection at the Stanford Underground Facility. The sensitivity was again limited by the irreducible neutron background.
Page 1 and 2: THE CRYOGENIC DARK MATTER SEARCH (C
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cdms-ii - CDMS Experiment - University of California, Berkeley

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