January 12, 2016

"Detecting dark matter is exceedingly hard. So much so that we’ve never actually done it."

hackaday: A sufficiently large detector like the LUX can hope to catch just a few interactions per year. by Mike Szczys

'To make sure that these aren’t missed, it’s important to filter out as many non-events as possible. This is the reason behind the underground location; 4,850 feet of earth stand between the detector and open air to reduce cosmic rays by a factor of 1 million. That doesn’t get rid of everything but it becomes possible to discern false readings.

'The ultra pure Xenon further filters this noise in the system. What is left is a very dark environment waiting for Weakly Interactive Massive Particles (WIMPs) to pass through it. Theory tells us that somewhere between millions and billions of the WIMP dark matter particles move through one square centimeter of space every second. But one square centimeter on a subatomic scale is a vast and empty space. With the noise filtered out, researchers are just waiting for a WIMP to collide with a Xenon nucleus.

'Xenon is a scintillator; when the nucleus is struck by a fast-moving subatomic particle, it gives off a photon. Ionizing electrons are also a result of the interaction and they in turn create secondary scintillation. The pattern of primary and secondary light is specific to the type of interaction and, if measured properly, can be used to distinguish a dark matter interaction from other events. The good news is that we’re really good at measuring light. In fact, you can probably already guess what mechanism is used in the measurement: a Photomultiplier Tube (PMT).

'PMTs are used in all kinds of scientific measurement equipment, and also appear often in medical devices and photography equipment.LUX has 122 PMTs in its sensor array. A fair amount of signal conditioning sets the levels before being fed into the custom triggering system. Then things really start to get interesting. Readings are summed into 16 groups of PMTs which are then processed by the triggering system to establish if the pattern is that of a possible dark matter interaction. If you need to do a lot of very fast, parallel processing, what kind of hardware do you choose? You’re right, you reach for an FPGA and build up a system around it.'

FPGA-based Trigger System for the LUX Dark Matter Experiment here

Frank L.H. Wolfs (University of Rochester) pages dedicated to the triggering hardware here

No comments: