Deep underground laboratory for particle astrophysics
We have built a new, deep underground laboratory called SNOLAB, 6800 feet
underground at the SNO site near Sudbury. This facility will host a battery of
new experiments to address several unanswered questions about the properties of
neutrinos, and the nature dark matter and dark energy, and how these relate to
the origins of our universe. The underground laboratory is necessary to look
for the rare events that are signals for the neutrino and dark matter
interactions that are used to probe the cosmos.
Below are summaries of the different Queen's astroparticle experiments, many
of which take advantage of the SNOLAB facilities. See the individual project
home pages for more details and references to published papers.
The Sudbury Neutrino Observatory
The SNO experiment was designed to look at neutrinos from the
sun and led to the discovery that neutrinos change flavour in passing from
the core of the sun to the earth.
Although SNO data acquisition stopped in November, 2006, data analysis will
continue until 2010. It will be focused on the neutral current detector phase
of the experiment and the combination of all three phases. It is possible for
students who arrive in 2007 to work on SNO analysis; typically in conjunction
with work on some other aspect of the detector.
Neutrinos may be the most common particle in the universe, yet they are also
among the most difficult to detect. The majority of neutrinos detected here on
earth are produced in the Sun by stellar burning processes (about 2% of the
total energy from the sun is emitted in the form of neutrinos), however they can
also be the result of radioactive decay, supernova explosions (where 99% of the
released energy is in the form of neutrinos), or as relics from the big
bang.
The SNO detector observes tiny flashes of light resulting from neutrino
interactions using an array of 9600 20cm-diameter photomultiplier tubes (PMTs)
each of which are sensitive to single photons. At the core of the detector is
1000 tonnes of heavy water which is used to detect neutrinos via three different
nuclear interactions. Through careful analysis, these measurements provide
insight into previously unknown properties of this elusive particle.
Dark matter search with superheated droplets
PICASSO is a direct dark matter search experiment. It uses the
superheated droplet detector technique to find evidence for dark matter in our
solar system. The experiment is located at SNOLAB in Sudbury, Ontario. The
Queen's group is involved in the design, installation, and operation of the
system. Queen's students are also at the forefront of the data analysis of
PICASSO WIMP search data.
Supersymmetry theories favored by particle physicists today predict the
existence of a stable heavy particle that only interacts weakly. These
particles are called WIMPs (weakly interacting massive particles).
PICASSO uses tiny (200μm) liquid droplets of freon suspended in a gel as
medium for detecting these WIMPs. The droplets are kept in a superheated state,
and when a WIMP hits a droplet the freon changes phase to a gaseous bubble.
This transition creates a shock wave that is detected by a piezo-electric
sensor.
A single detector has 9 piezo-electric sensors and contains 4.5 litres of
gel. The detectors are housed in a temperature-controlled enclosure and
surrounded with water shielding to reduce background radiation. Currently, 29
detectors are operational at SNOLAB, with plans to add 3 more.
Liquid scintillator detector for low energy neutrinos
SNO+ is a proposed project that would be a follow-up to SNO. Using
most of the existing SNO detector but replacing the heavy water with a
"new" liquid scintillator made from linear alkylbenzene, SNO+ would be
sensitive to solar neutrinos with lower energies than SNO, and it
would also be able to detect antineutrinos produced by nuclear
reactors and by the decays of the natural radioisotopes present in the
Earth. This would give SNO+ the ability to make measurements that are
important not only to neutrino physics, but also to solar physics,
geophysics and geochemistry.
By measuring the survival probability of the pep solar neutrinos with
precision, SNO+ would probe the coupling between neutrinos and matter
in the region most sensitive to new phenomena. This could reveal the
presence of new physics such as non-standard couplings to new
particles, or the presence of sub-dominant effects in oscillations
from a sterile neutrino.
We can load the liquid scintillator with neodymium, a double beta
decay isotope. With 1 tonne of neodymium dispersed in the detector,
SNO+ could detect neutrino-less double beta decay. This would shed
light on the charge conjugation nature of the neutrino and on the
absolute neutrino mass scale, both impacting on our understanding of
the evolution of the Universe.
As with SNO, Queen's is leading the development of this project.
Mechanical construction during the transition, scintillator
purification and engineering, liquid scintillator optics, detector and
physics simulations - there are opportunities to get involved in many
aspects of this project.
Dark matter search with liquid argon
With the prototype we plan to demonstrate a discrimination of events that are
backgrounds to the dark matter search (beta and gamma events) in liquid argon at
the level of one in a billion. With this very low background level, the large
detector is projected to be sensitive to cross-sections down to
10-46cm2, and will increase the current experimental
sensitivity to dark matter particles by a factor of 1000.
Commissioning of the large detector underground at SNOLAB is planned for
2009. The DEAP group at Queen's is currently active in cryogenics design and
construction, liquid argon purification and scintillation studies, Monte-Carlo
simulation, detector calibration and analysis (for DEAP-1) and on the conceptual
and engineering design for the 1000 kg detector. We are planning several
R&D activities for the large detector, including bonding of a large acrylic
sphere in an ultra-clean environment, cold and cryogenic tests of
photomultiplier tubes, and techniques for radon mitigation for critical detector
components.
Cryogenic Dark Matter Search
CDMS is designed to detect the very rare interaction of Weakly
Interacting Massive Particles (WIMPs), that are proposed to solve
the almost 80 year old dark matter problem, with atomic nuclei.
The detectors in use are germanium (and silicon) single crystals
kept at very low temperatures, so that a low energy WIMP interaction
leads to a measurable increase in temperature. In addition an
ionization signal is recorded; this additional signal together with
a sophisticated analysis of the signal shapes an timing allows a
very efficient discrimination of disturbing interactions from
environmental radioactivity.
CDMS is presently running a total of about 5 kg of target mass in
the Soudan underground laboratory in Minnesota and so far is the
most sensitive experiment in the field. Plans for the next phase
(SuperCDMS 25 kg) are to increase the target mass to 25 kg, to
improve the detector performance and to move to SNOLAB, which
provides a better shielding against cosmogenic radiation.
Queen's recently joined the CDMS collaboration. We plan to install
a cryostat at Queen's to test and characterize the new detectors,
and contribute to the analysis of the running experiment and to the
installation of the new setup at SNOLAB.
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