Tuesday, July 3, 2012

Sheets of gold and DNA strands used in unconventional dark matter detector

A combined team of physicists and biologists aim to build a directional dark matter detector using strands of DNA and gold.

Dark matter is a hypothesised type of matter which accounts for much of the mass of the universe. It cannot be seen, but its existence is inferred from its gravitational influence on visible matter and the structure of the universe. Some of the most popular models of dark matter suggest that it exerts itself on galaxy clusters and surrounds the Earth like a sea as it travels around the Sun, which in turn is slowly travelling towards the constellation Cygnus as it rotates around the galactic centre.

If this is the case, Earth should experience a "headwind" of dark matter in front of it (coming form the direction of Cygnus) for half of the year and a tailwind for the other half of the year, depending on where it is on its orbit around the Sun.

Many different groups are working to try and detect dark matter using expensive detectors in deep underground caverns, which protect them from radiation that could otherwise pollute the signal. They are focusing on finding the unique signature that the "sea" of dark matter supposedly produces as the Earth orbits the Sun. This should change depending on what point in the year it is and also throughout the day as the Earth rotates on its axis. A dark matter detector should be able to sense the direction change as the Earth rotates each day.

A combined team including Katherine Freese, an astrophysicist from the University of Michigan and geneticist George Church from Harvard say they can overcome challenges with detecting dark matter by using DNA to find the dark matter particles, called weakly interacting massive particles, or WIMPs.

They have created a detector using a thin gold sheet with many single strands of DNA hanging from it. The theory is that a particle of dark matter will smash into the heavy gold nucleus, pushing it out of the gold sheet and through into the DNA "forest", knocking the strands out as it travels.

These strands fall onto a collection tray. Each of them has a unique identifier showing where they were located on the gold sheet, so researchers can reconstruct the path of the gold particle with incredible precision. The detector is made up of hundreds of thousands of these sheets placed between mylar sheets, using around a kilogram of gold and 100g of single-strand DNA on a metre-square array.

DNA is useful in this context because its structure will separate vertically with nanometre resolution -- it wills separate to the nearest nucleotide -- the smallest structural units of DNA. This is many orders of magnitude better than is currently possible. Secondly, the detector can work at room temperature, rather than needing cooling. Finally, the mylar sheets make the detector directional -- each sheet should be able to absorb the gold nucleus of its energy after it has passed through the "DNA forest". Higher energy nuclei from background radiation would pass through several of the leaves of mylar, allowing them to be identified and excluded.

If a dark matter particle hits a gold nucleus in on direction, it will propel it into the DNA forest. If it strikes in the other direction, it will head straight into the mylar sheet and be absorbed.

This highly unconventional approach has a number of major challenges. Firstly, it is not clear how rapidly-moving gold nuclei will interact with the DNA. The team will need to study this before building any such detector. Secondly, it will be challenging to make DNA strands long enough. At the moment, off-the-shelf DNA strands have around 250 bases. The detector would need strands consisting of at least 10,000 bases in order to absorb the energy of the gold nucleus. They would also have to hang straight down and not curl up, which would require some sort of DNA comb or "hair straightener". One suggestion is to place a tiny magnet at the end of each strand that would allow it to be pulled downwards.v

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