This is a module of the latest generation of PICASSO detectors as installed at SNOLAB. It is a 4.5l module with 80g of active mass of C4F10. Droplets are suspended in an elastic polymer. Signals are recorded by 9 piezo electric sensors . Events are localized by GPS-like triangulation. Presently PICASSO is installing a new experiment with 32 detector modules as shown to the left and with an active mass of 2.6 kg. (Credit: Alex Pepin, Martin Auger)

The detection of dark matter particles, named WIMPs (weakly interacting massive particles), is an endeavor occupying many researchers. Though there is ample indirect evidence suggesting that nearly a quarter of the universe is made up of this mysterious substance, it has yet to be observed directly. Since the Milky Way, like all other galaxies, is nestled in a dark matter halo, researchers hope to capture collisions between WIMPs and atoms in detectors.

 

The PICASSO (Project in Canada to Search for Super-Symmetric Objects) dark matter experiment at SNOLAB, a facility for the study of astroparticle physics in Ontario, Canada, uses the superheated droplet technique to detect dark matter. This technique exploits a liquid’s reaction to energy deposits, which varies with pressure and temperature. Operating conditions can be tuned so that interactions with energies below a certain threshold go undetected, thus preventing them from interfering with the interactions that are of interest. This feature is critical for dark matter detection, because dark matter induced events are very rare and any other radiation present in the detector environment must be suppressed.  

 

But despite PICASSO’s immunity to gamma and beta radiation, it is sensitive to energy losses of alpha particles because they are in the WIMP energy region. Alpha particle emissions are in fact the most important background source because their temperature dependence is similar to that of the WIMPs’ interactions. Therefore, an event by event discrimination between the two types of interactions would constitute an important background suppression feature to the dark matter search.

Illustration of alpha particle emission during alpha decay (Credit: Inductiveload)

Recent calibration runs using new PICASSO detectors suggest such suppression is possible. This stems form the fact that the interactions’ acoustic signals contain information about the primary events, the alpha signals being more intense than those of the WIMPs. At present the cause of this difference is unknown, though one possible explanation has to do with the number of nucleation sites contributing to the signal.

 

Experiment spokesperson Viktor Zacek (Universite de Montreal) said, “When we looked at our calibration data taken with neutrons and compared them with our alpha background data we saw a peculiar difference which we attributed first to some detector instabilities or gain drifts in our electronics.  However, when we checked the data and refined the analysis the discrimination effect became even more pronounced.”

 

This discrimination effect came as a surprise, and further understanding is needed to fully utilize it. Additional system adjustments can also assist in enhancing the effect and refining the dark matter particles’ isolation. This effect can also be deployed for other causes, such as the detection of traces of alpha emitting actinides in biological samples and low neutron level counting.

 

TFOT reported on the development of various instruments intended for the detection of dark matter and other elusive substances. NASA’s High-Resolution Soft X-Ray Spectrometer will study the extreme environments of the universe and help researchers explore dark matter on a large scale as well as the evolution of large galactic structures. Teams at the French Atomic Energy Agency and the French National Center for Scientific Research have completed construction of Antares, the first underwater neutrino telescope. UK astronomers, as a part of the dark energy survey collaboration, are constructing one of the largest ever cameras to detect dark energy.

 

Further information on the recent advancement can be found in the October 16^th issue of the New Journal of Physics and in the Institute of Physics press release.