Aksel Hallin, PhD

Area of Study / Keywords

Astroparticle physics dark matter neutrino physics

About

Research

I enjoy designing and building detectors to measure interesting astroparticle physics phenomena.  I also enjoy the challenges of analyzing and understanding the data from such apparatus. The phenomena I focus on are identitying the dark matter in the universe and understanding the properties of neutrinos, which are among the highest priority physics issues (see, for instance, the Canadian long-range plan, or the European Astroparticle Physics Strategy).

I am an active member of the Global Argon Dark Matter Collaboration, which is seeking to understand the nature of galactic dark matter.  In particular, I have worked on the design and construction of two instruments called DEAP 3600 and Darkside 20k. 

DEAP 3600 is a running experiment and consists of an 1.7 meter diameter spherical acrylic vessel that can contain 3600 kg of ultrapure liquid argon that is operating inside the SNOLAB facility, 2 km underground in Vale's Creighton Mine near Sudbury, Ontario. We are sensitive to dark matter that consists of particles with masses similar to atoms and interaction strengths similar to neutrinos.  However, other experiments as well as our original measurements have shown no evidence for dark matter, and we need to look with increasing sensitivities and larger masses. Dark matter with velocities characteristic of particles in our galaxy and appropriate masses would scatter on Argon nuclei, causing recoils that generate several hundred vacuum ultraviolet photons as they interact with the liquid.  Those photons travel with little interaction in the argon until they strike a thick wave length shifter coated on the inner surface of the acrylic vessel, which converts them to visible photons.  The visible photons are converted into electrical pulses by 255 20 cm diameter photomultiplier tubes densely packed around the sphere.  At the University of Alberta, we designed and built the inner detector, and in particular the acrylic components.  As far as we know, this is the first large scale acrylic cryostat. We have also made several major contributions to analysis and simulations, including reconstruction algorithms and background rejection techniques. The capital construction of DEAP 3600 was funded by a CFI grant, and started operations in 2017.

In 2023/2024 we are designing and building the next generation detector, Darkside-20k, which will consist of 50 megagrams of underground liquid argon, configured as a Time Projection Chamber, and operating in Italy's Gran Sasso laboratory.  Special argon, extracted from a well in Colorado, doesn't contain the radioactive argon-39 nucleus, which occurs in atmospheric argon and is the dominant radioactivity in DEAP 3600.  In the time projection chamber, we impose an electric field across the detector.  If a dark matter interaction occurs, there will be both a flash of light and a cluster of electrons generated.  The flash of light is detected with arrays of silicon photomultipliers. The electrons are accelerated by the electric field, and drift upwards until they reach a small argon vapour gap.  An enhanced field at the top of the chamber moves the electrons from liquid to gas, and also causes them to electroflouresce within the gas, generating about 20 visible photons for every electron.  This second flash is also measured with the same silicon photomultipliers, but is very local and enables precise position reconstruction.  The time between original and second flash enable us to know the vertical position of the event.  At Alberta, we are responsible for the design and construction of the inner acylic vessel that comprise the anode and cathode for the time projection chamber.  

I work on the SNO+ experiment, which studies the nature of neutrinos. In particular, we are searching for a rare process called neutrinoless double beta decay, which has been postulated as a way in which the matter-antimatter asymmetry in the universe could be explained. If observed, it would also enable us to measure the tiny neutrino masses. In addition, SNO+ observes neutrinos from the sun, within the earth's interior, and from nuclear reactors.  These processes tell us about the detailed properties of neutrinos and how they propagate, and also about fusion reactions in the sun and the content and radioactivity deap within Earth.  The detector consists of a 12 meter acrylic vessel filled with linear alkyl benzene scintillator, a special material that generates optical photons when ionized by radiation.  The detector is viewed by about 9000 20 cm diameter photomultiplier tubes that enable us to count the photons from the scintillator.  At Alberta, we took responsibility for building an ultrapure rope net and anchor system, that is used to hold the detector down and offset the 1.50 giganewton bouyant force as the 12 meter diameter acrylic sphere is kept submerged under water.  We are also responsible for measuring acrylic/scintillator compatibility, for measuring the radon in the cover gas above the detector, and for building some of the calibration apparatus.  In analysis we have developed various reconstruction algorithms and algorithms within the simulation framework.  

I am also a member of the SNO experiment which measured neutrinos from the sun and demonstrated that neutrinos oscillate and have mass.  

To enable our measurements we require ultrapure materials, to eliminate as much natural radioactivity as possible.  To that end our laboratories include several instruments to measure radioactivity, and in particular detectors for radon and gamma rays.  In addition, we have made a radon-free clean room at the UofA, in which we can build apparatus without exposing it to the radon in the air, which generates a layer of radioactive lead.