DarkSide Document 198-v1

Ben Loer's Thesis

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DarkSide-doc-198-v1
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Publications
Submitted by:
Ben Loer
Updated by:
Ben Loer
Document Created:
20 Oct 2011, 09:50
Contents Revised:
20 Oct 2011, 09:50
Metadata Revised:
22 Aug 2012, 20:46
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Abstract:
Scientists have now accumulated overwhelming evidence indicating that over 80% of the mass
of the universe is in the form of dark matter, neutral particles with ultra-weak couplings to
ordinary matter. One compelling candidate is a Weakly Interacting Massive Particle or
WIMP, with mass on the order of 100 GeV.The signal of a WIMP interaction in a detector
is a low energy (typically below ∼100 keV) recoiling atomic nucleus. The expected rate
is at most a few interactions per year per ton of target. The most critical issue for direct
WIMP searches is reducing the background in the detector below this very low rate. Ar-
gon is a promising target because measurements of the scintillation pulse shape and the
scintillation-to-ionization ratio allow the reduction of gamma-induced signals, the largest
source of background, by a factor of 108 or better. One of the major drawbacks of argon
39
is the presence of radioactive Ar, which results in a decay rate of ∼1 Bq/kg in natural
argon. Because 39Ar is produced primarily in the upper atmosphere, the 39Ar fraction can be
reduced significantly by obtaining the argon from underground wells.
Our collaboration, DarkSide, is developing a series of two-phase argon time projection
chambers (TPCs) utilizing this depleted argon, along with passive shielding and active neu-
tron and muon vetoes, for WIMP searches. I present results from a recent campaign of a
10 kg active mass prototype TPC that demonstrate the successful realization of many of
the technical aspects necessary for a full-scale detector, in particular an electron-equivalent
light yield of 4.5 photoelectrons per keV deposited, and a free electron lifetime in excess of
200 microseconds. Based on this successful prototype and Monte Carlo simulations, I then
conclude that DarkSide-50, a 50 kg active mass TPC to be installed in LNGS, can likely ac-
quire data background-free for three years, accumulating a fiducial exposure of ∼100 kg-years
and reaching a sensitivity to the WIMP-nucleon cross section of ∼3×10−45 cm2 .
I also present some details on the analysis of Borexino data that resulted in the first
real-time, spectroscopic measurement of 7Be solar neutrino interactions.
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