458 P. Agnes et al. / Physics Letters B 743 (2015) 456–466
Fig. 1. The nested detector system of DarkSide-50. The outermost gray cylinder is
the WCD, the sphere is the LSV, and the gray cylinder at the center of the sphere is
the LAr TPC cryostat.
at a depth of 3800 m.w.e. [14], in close proximity to and sharing
many facilities with, the Borexino solar neutrino detector [15,16].
The
LAr TPC can exploit pulse shape discrimination and the ra-
tio
of scintillation to ionization to reject β/γ background in favor
of the nuclear recoil events expected from WIMP scattering [17,5].
It can also exploit the TPC’s spatial resolution to reject surface
backgrounds and to reject multi-sited events. Events due to neu-
trons
from cosmogenic sources and from radioactive contamination
in the detector components, which also produce nuclear recoils,
are suppressed by the combined action of the neutron and cosmic
ray vetoes. The liquid scintillator also provides additional rejection
of γ -ray background from the detector materials. The water-plus-
liquid
scintillator design was motivated in part by the success of
this shielding concept in achieving very low backgrounds in Borex-
ino
[15,18,19].
The
WCD is an 11 m-diameter, 10 m-high cylindrical tank filled
with high purity water. The tank was originally part of the Borex-
ino
Counting Test Facility. The inside surface of the tank is covered
with a laminated Tyvek-polyethylene-Tyvek reflector [20]. An ar-
ray
of 80 ETL 9351 8
PMTs, with 27% average quantum efficiency
(QE) at 420 nm, is mounted on the side and bottom of the wa-
ter
tank to detect Cherenkov photons produced by muons or other
relativistic particles traversing the water.
The
LSV is a 4.0 m-diameter stainless steel sphere filled with
30 t of borated liquid scintillator. The scintillator consists of equal
amounts of pseudocumene (PC) and trimethyl borate (TMB), with
the wavelength shifter Diphenyloxazole (PPO) at a concentration of
2.5 g/L. The sphere is lined with Lumirror [21] reflecting foils. An
array of 110 Hamamatsu R5912 8
PMTs, with low-radioactivity
glass bulbs and high-quantum-efficiency photocathodes (37% aver-
age
QE at 408 nm), is mounted on the inside surface of the sphere
to detect scintillation photons.
The
neutron-capture reaction
10
B(n, α)
7
Li makes the borated
scintillator a very effective veto of neutron background [22]. The
TMB, B(OCH
3
)
3
, contains
nat
B which has a 20% natural abundance
of
10
Bwith its large (3840 b) thermal neutron capture cross sec-
tion.
The thermal neutron capture time in the borated scintillator
is calculated to be just 2.2 μs, compared to 250 μs for pure PC [15].
The
10
Bneutron capture proceeds to the
7
Li ground state with
branching ratio 6.4%, producing a 1775 keV α particle, and to a
7
Li excited state with branching ratio 93.6%, producing a 1471 keV
α particle and a gamma-ray of 478 keV. Because of quenching,
the scintillation light output of the capture to
7
Li(g.s.) is expected
to be in the β/γ -equivalent range 50 to 60 keV [23,24]. Prelim-
Fig. 2. The DarkSide-50 liquid argon time projection chamber.
inary measurements with our scintillator appear consistent with
this expectation. The measured LSV photoelectron (PE) yield is
(0.54 ± 0.04) PE/keV, making this quenched energy readily de-
tectable.
The high
14
Cdecay rate in the LSV and the fact that its
spectrum covers the signal expected from the α’s from neutron
capture on
10
B severely reduced the effectiveness of the neutron
veto in the present data set. The rejection power is estimated from
simulations to be 40 to 60 instead of the design value of 200 [22].
The
DarkSide-50 TPC, as shown in Fig. 1, is contained in a stain-
less
steel cryostat that is supported at the center of the LSV on
a system of leveling rods. Its design was based on that of the
DarkSide-10 prototype, which operated for 502 days at LNGS [6].
A cut-away view of the TPC is given in Fig. 2.
Ionizing
events in the active volume of the LAr TPC result in
a prompt scintillation signal called “S1”. Ionization electrons es-
caping
recombination drift in the TPC electric field to the surface
of the LAr, where a stronger electric field extracts them into an
argon gas layer between the LAr surface and the TPC anode. The
electric field in the gas is large enough to accelerate the electrons
so that they excite the argon, resulting in a secondary scintilla-
tion
signal, “S2”, proportional to the collected ionization. Both the
scintillation signal S1 and the ionization signal S2 are measured
by the same PMT array. The temporal pulse shape of the S1 signal
provides discrimination between nuclear-recoil and electron-recoil
events. The S2 signal allows the three-dimensional position of the
energy deposition to be determined and, in combination with S1,
provides further discrimination of signal from background. A sig-
nificant
fraction of events also exhibit an “S3” signal. The S3 pulse
resembles S2 in pulse shape but is typically ∼1000 times smaller
and always follows S2 by a fixed delay equal to the maximum drift
time in the LAr TPC. S3 is believed to result from electrons released
from the cathode (at the bottom of the TPC) when struck by the
bright S2 UV light.
The
active LAr is contained in a cylindrical region viewed by 38
Hamamatsu R11065 3
low-background, high-quantum-efficiency
PMTs, nineteen each on the top and the bottom. The average quan-
tum
efficiency of the PMTs at room temperature is 34% at 420 nm.
The PMTs are submerged in liquid argon and view the active LAr
through
fused-silica windows, which are coated on both faces with
transparent conductive indium tin oxide (ITO) films 15 nm thick.
This allows the inner window faces to serve as the grounded
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