A low-threshold analysis of CDMS shallow-site data Page: 3 of 18
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3
of some categories of background may also increase at low
energies. Finally, special care is required to account for
the effects of non-zero energy resolution at energies near
the electronic-noise level.
In this work we describe a new analysis of data from
the Cryogenic Dark Matter Search (CDMS) experiment,
with special attention to events of low recoil energy. In
order to access this low-energy parameter space, we forgo
the pulse-shape discrimination techniques used to reject
near-surface background events in previous CDMS anal-
yses. In contrast to previous CDMS results, the signal
region of this analysis will be populated by a number
of background events. We adopt an inclusive philoso-
phy that maximizes the detection efficiency at low en-
ergy while limiting the rate of nonphysical sources of
background, such as electronic noise. This low-threshold
analysis sacrifices some of the strengths of the CDMS ex-
periment's traditional background discrimination meth-
ods for a chance to probe previously untested low-mass
WIMP parameter space.
II. EXPERIMENTAL SETUP AND TECHNIQUE
A. Apparatus
The data reported on here were recorded during the
final exposure of the first six CDMS II detectors at the
Stanford Underground Facility (SUF) [17, 18]. The SUF
setup provided a high level of shielding against external
sources of radiation. The SUF was a shallow site with
~17 meters water equivalent overburden, effectively stop-
ping hadronic cosmic rays and reducing the muon flux by
a factor of 5. The remaining incident muons were tagged
with a high-efficiency, hermetic plastic scintillator muon
veto, allowing offline rejection of muon-coincident detec-
tor interactions. The muon veto enclosed several layers
of tightly packed passive shielding. A 15cm-thick outer
lead shield and 25 cm-thick outer polyethylene shield sur-
rounded the detector cold volume to attenuate external
photons and degrade external neutrons, respectively. In-
side the radiopure copper walls that delineated the in-
nermost 20 mK cold volume, 1 cm of ancient lead and an
additional 11 kg of polyethylene surrounded the detector
assembly, providing further shielding.
In the center of the apparatus six Z-sensitive
Ionization- and Phonon-mediated (ZIP) detectors [19]
were arranged in a vertical stack ("tower"), with adja-
cent detectors separated by 2.2mm with no intervening
material. Table I indicates the detector names, materials,
masses and relative positions within the tower.
Each detector has two ionization electrodes deposited
on its bottom surface; a circular inner electrode ("q-
inner") covering 85% of the physical area, and an an-
nular outer electrode ("q-outer") which permits identifi-
cation and rejection of events with energy depositions
within the outer detector volume. Photolithographed
onto the top side of each Ge (Si) detector were 4144TABLE I. The first six CDMS II detectors are listed in order
of their relative positions (from top to bottom) within the
detector tower, indicating each detector's name, material, and
mass.
Name Material Mass (grams)
Z1 Ge 230.5
Z2 Ge 227.6
Z3 Ge 219.3
Z4 Si 104.6
Z5 Ge 219.3
Z6 Si 104.6
(3552) Al and W superconducting Quasiparticle-trap-
assisted Electrothermal-feedback Transition-edge sensors
(QETs). The 1036 (888) QETs in a given Ge (Si) detec-
tor quadrant are electrically connected, resulting in four
individually read out phonon sensors whose shared bor-
ders orthogonally bisect the surface. By measuring tim-
ing and pulse height differences between the sensors, we
can reconstruct an event's position in the plane parallel to
the detector's top and bottom surfaces ("xy-position").
Following amplification by a SQUID array [20] and
room temperature electronics, two copies of each detec-
tor's four phonon signals were generated at the hard-
ware level. A band-pass filtered analog sum of one set
of phonon signals (the "triggering phonon energy") was
compared to a low-level discriminator threshold. The
resulting logical pulses were OR'ed across all six detec-
tors to form the experimental trigger. The second set of
phonon signals were individually digitized, subjected to
a software optimal filter, and summed to constitute the
"reconstructed phonon energy." The hardware band-pass
filter's poles were chosen to resemble the software filter-
ing as closely as possible. The energy deposition used to
trigger the data acquisition system is therefore very sim-
ilar to the energies evaluated in software during offline
analysis, but not exactly the same. The discriminator
thresholds were carefully tuned to the lowest levels possi-
ble to ensure that the overall trigger rate of <1 Hz during
WIMP search was dominated by true particle interac-
tions, while simultaneously allowing occasional triggers
due to electronic-noise fluctuations.
B. Measurement Technique
Energy deposited by recoils causes two types of sig-
nals in our detectors. Most of the energy is deposited
as a spectrum of high frequency athermal phonons. In
addition, electron-hole pairs are created. The average
deposited energy per pair created is cr3.0 (3.8) eV for
Ge (Si) [21] (see also Appendix C in [22] for a de-
tailed discussion of E). To measure the ionization, the
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Akerib, D. S.; Attisha, M. J.; Baudis, L.; Bauer, D. A.; Bolozdynya, A. I.; Brink, P. L. et al. A low-threshold analysis of CDMS shallow-site data, article, October 1, 2010; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1012252/m1/3/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.