Weakly Interacting Massive Particles (WIMPs) may constitute most of the matter in the Universe. While there are intriguing results from DAMA/LIBRA, CoGeNT and CRESST-II, there is not yet a compelling detection of dark matter. The ability to detect the directionality of recoil nuclei will considerably facilitate detection of WIMPs by means of "annual modulation e ect" and "diurnal modulation e ect". Directional sensitivity requires either extremely large gas (TPC) detectors or detectors with a few nanometer spatial resolution.
In this paper we propose a novel type of dark matter detector: detectors made of DNA could provide nanometer resolution for tracking, an energy threshold of 0.5 keV, and can operate at room temperature. When a WIMP from the Galactic Halo elastically scatters off of a nucleus in the detector, the recoiling nucleus then traverses thousands of strings of single stranded DNA (ssDNA) (all with known base sequences) and severs those ssDNA strands it hits. The location of the break can be identified by amplifying and identifying the segments of cut ssDNA using techniques well known to biologists. Thus the path of the recoiling nucleus can be tracked to nanometer accuracy. In one such detector concept, the transducers are a few nanometer-thick Au-foils of 1 meter by 1 meter, and the direction of recoiling nuclei is measured by "DNA Tracking Chamber" consisting of ordered array of ssDNA strands. Polymerase Chain Reaction (PCR) and ssDNA sequencing are used to read-out the detector. The detector consists of 1 kg of gold and 0.1 kg of DNA packed into 1 cubic meter. By leveraging advances in molecular biology, we aim to achieve about 1,000-fold better spatial resolution than in conventional WIMP detectors at reasonable cost.
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The entire detector consists of hundreds or thousands of these sheets sandwiched between mylar sheets, like pages in a book. In total, a detector the size of a tea chest would require about a kilogram of gold and about 100 grams of single-strand DNA.
The advantage of this design is manifold. First, the DNA sequence determines the vertical position of the cut to within the size of a nucleotide. That kind of nanometre resolution is many orders of magnitude better than is possible today.
Second, this detector works at room temperature, unlike other designs which have to be cooled to measure the energy that dark matter collisions produce.
And finally, the mylar sheets make the detector directional. Each sheet should absorb the gold nucleus of this energy after it has passed through the DNA forest. Any higher energy nuclei, from background radiation or cosmic rays for example, should pass through several 'pages', which allows them to be spotted and excluded.
With the device facing in one direction, a dark matter particle strikes a gold nucleus, propelling it into the DNA forest. But in the other, the gold nucleus is propelled into mylar sheet where it is absorbed. That's what makes it directional--the detector should only record events coming from one direction.
This should allow the device to spot the change in dark matter signal each day, which in turn should make the detection much less statistically demanding.
That's a fascinating idea that's likely to generate much interest. However, it's not without some challenges of its own.
First up, nobody really knowns how rapidly-moving, highly-ionised gold nuclei will interact with single strands of DNA or indeed with forests of them. This is something the team plans to study in some detail before a detector can be built.
Then there is the challenge of making DNA strands that are long enough to present a reasonable 'forest' for gold nuclei to pass through. Church, Freese and co say they'd like strands consisting of 10,000 bases to create a forest that entirely absorbs the energy of a gold nucleus passing through it.
By contrast, off-the shelf arrays offer DNA strands with only 250 bases or so. These guys say they'll probably have to settle for strands of about 1000 bases.
The DNA strands also have to hang straight down, rather than curled up. That's a tall order over the area of a square metre or so that the detector will cover. At this scale, electric and magnetic fields trump gravity and these are likely to be a nuisance, particularly when it comes to collecting the severed DNA.
So the team will have to devise some kind DNA 'comb' that straightens the hair. One idea is attaching a tiny magnet to the free end of each strand, allowing it to be pulled downward.
The DNA strands will also have to be made from carbon-12 and 13, since carbon-14 is naturally radioactive and would otherwise produce an unwanted hiss of background noise. Using only very old carbon, in which all the carbon-14 has decayed, should do the trick.
Finally, there is the significant engineering challenge in making metre square DNA arrays, collecting trays that catch the severed DNA strands and fitting them altogether into a working detector.
There are more than a few unknowns in this approach which makes it high risk. But there is also high potential pay off because other designs for directional dark matter detectors are huge, complex and potentially vastly more expensive to build and run. That makes this approach exciting.
DNA Tracking Chamber
The detector is modular and consists of a series of identical units stacked on top of each other. It is like a book and the WIMP travels sequentially through the pages. Each module consists of the following layers. On the top is a 1 m layer of mylar (which is inactive from the point of view of incoming WIMPs). Next is a 5-10 nanometer thick layer of gold, corresponding to roughly 10 atoms of Au in thickness. It is with these Au nuclei that the WIMPs will interact, giving the Au nuclei a kick of 10 keV out of the plane. Hanging from the gold plane is an ordered periodic array of ssDNA strands, which can be thought of as a curtain of DNA through which the recoiling Au nuclei will travel. Whenever an Au nucleus strikes one of the ssDNA strands, it breaks the ssDNA. More accurate studies of this breaking will be required, e.g. by calibrating the response of the ssDNA to heavy ions of a given energy (such as 5, 10, 30 keV Ga ions that may be obtained from an ion implementation machine). The required amount of energy to break the strand 4 is estimated to be 10eV, but more accurate values must be obtained experimentally. Thus we estimate that it will take hundreds to thousands of direct hits of Au on ssDNA, corresponding to a comparable number of breaks of ssDNA strands, to stop a gold nucleus. Currently o ff the shelf technology consists of arrays containing ssDNA strands that are 250 bases in length (manufactured by Illumina Inc.). The average length of single-stranded DNA is up to about 0.7 nm per base when fully stretched. Thus this corresponds to about 100 to 200 nm length DNA strands.
In 2nd generation detectors, ideally one would like to have ssDNA strands that are 10,000 bases in length, as this setup would then be optically thick to Au nuclei; i.e. all the Au nuclei would be stopped in the DNA Tracking Chamber and one could obtain the maximum information in reconstructing the particle's track. More realistically we envision ssDNA strands consisting of 1000 bases, or equivalently 0.3-0.7 microns in length. The goal is to have the ssDNA strands completely periodically ordered, with 10 nanometer distances between them.
A major step forward in the fi eld of direct detection would be the development of detectors with directional capability. By contrasting the count rates in a detector in the direction toward and away from the Galactic WIMP "wind" that the Sun is moving into, the statistical requirements on the number of detected WIMPs drops to 100 rather than thousands without the directional sensitivity. In the paper we proposed using DNA as a detector material that can provide nanometer resolution tracking. We presented a particular design consisting of modules of thin gold planes with single stranded DNA hanging down from each plane. The required amount of material is 1kg of gold and 0.1kg of ssDNA. The DNA strands all consist of (almost) identical sequences of bases (combinations of A,C,G,T), with an order that is well known. An incoming WIMP from the Halo of our Galaxy strikes one of the gold nuclei and knocks it out of the plane with 10 keV of energy. The Au nucleus moves forward into the strands of DNA, traverses thousands of these strands, and whenever it hits one, breaks the ssDNA. The locations of the breaks are easy to identify, using PCR to amplify the broken segments a billion fold followed by DNA sequencing to locate the break.
In this way the path of the recoiling nucleus can be tracked to nanometer accuracy.
We note that this design is not restricted to the use of Au nuclei, which can be interchanged with many di erent nuclei with high atomic number (so as to maximize the SI interaction rate). By using a variety of di erent materials, it should be possible to identify the mass and cross-section of the interacting WIMP. In addition, although the speci c detector design may be modi ed, the important new development is the idea of using DNA in lieu of other detector materials to provide better tracking resolution so that directionality of the WIMPs can be determined. More generally, it is easy to imagine multiple applications for nanometer tracking beyond that of WIMP detection.
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