Deep and Dark

by Alex Menarry

Surely astronomy looks outwards from the Earth as far as possible? Why should eight astronomers from your very own Society go 1300 metres below the surface of the Earth? Perhaps because it's there. Boulby, near the east coast and close by Skelton, is a salt and potash mine. It was first opened in the 1970's to exploit the deposits laid down 250 million years ago, roughly when the dinosaurs were being zapped by the famous asteroid. These thick beds of evaporites underlie Britain, the North Sea and Germany. Two shafts were dug and now the mine workings go 5 kilometres out under the North Sea. Although very little water finds it's way into the mine, we managed to experience some of that which did.

Who were we? Eight brave souls prepared to risk all in the pursuit of knowledge. Prepared to strip to the buff and don the overalls, helmet and boots provided by Cleveland Potash Ltd, keeping on our own socks remember that important fact. Innocents, not allowed to take our cameras to record the event, taken down the shaft by a mining engineer and a physicist, packed tight like sardines into a mine cage and dropped 1300 metres. It took ages and was almost as bad as flying. The temperature rose and rose as we descended, reaching 36C as we emerged into the tunnels, excavated by the huge salt-eating machines digging like huge moles. The tunnels are about 10 metres wide by 5 metres high, dug in a square, room-and-stoop pattern, to leave a massive pillar to hold up the roof against the unimaginable pressure of the rock above.

Why go there? For a Dark Secret; the pursuit of Dark Matter. Here are the laboratories of the Particle Physics and Astronomy Research Council the famous PPARC. An international collaboration of Universities from Los Angeles, Turin, Moscow, Texas, Wayne State and the Lawrence Livermore National Laboratory as well as UK Universities, running the exotic equipment at the bottom of the mine. The search for Dark Matter is one of the most important tasks in astronomy and particle physics at the present time.

What is Dark Matter and how do you look for it? Those of us who heard Fred Stevenson's lecture on 12 th September will know all about Dark Matter. The stuff that's missing but which must be present to explain the manner in which galaxies and groups of galaxies rotate. There simply isn't enough visible stuff to hold the visible matter together. 90% of the what-ever-it-is, needed to explain how the Universe is expanding and how the galaxies are holding together, is missing. It's dark and invisible. It's been missing since the 1930's, when Keplar's Laws and the Virial Theorem were applied to the problem and revealed the missing material. The known few percent is baryonic matter, the atoms and molecules as we know them, Jim. The non-baryonic stuff is very strange.

MACHOs (massive compact halo objects) is the general name given to a lot of what's missing because they are probably in the haloes of galaxies. The cold dark matter could be WIMPs (weakly interacting massive particles) and the hot dark matter could be neutrinos (do they have a small mass?). The laboratories in Boulby Mine are looking for WIMPs. The Standard Model Theory of the Big Bang allows for particles of many hundreds of thousand of times the mass of a proton. There are billions of them passing through you and me every second but they rarely interact with the nuclei in the atoms of our bodies. Or any other atoms, for that matter (no pun intended!). The occasional impact of a WIMP with a nucleus betrays its presence.

In the Boulby labs are several different types of detectors looking for evidence that the elusive dark matter is there. Zeplin I, to be followed by the more sensitive Zeplins II and III, holds a small quantity of liquid xenon, surrounded by photo-multipliers. If a WIMP hits a nucleus it will recoil and then give out a photon, detected by the photo-multipliers. Another detector is an array of Sodium Iodide crystals,called NAIAID, doing roughly the same thing. The third instrument we saw was the DRIFT detector, producing ionisation tracks caused by the passage of particles. The Earth moves through a "wind" of WIMPS, so if the tracks are in the right direction, they may well be WIMPS.

The trouble is that what is detected may or may not be a WIMP. Hence the reason for the detectors being 1300 metres down, protected from cosmic rays and other confusing things. The mine is also quite low in natural radio-activity, which also helps. Even so, it is considered necessary to add lead shielding around the detectors.

After a most enjoyable time walking through the mine from laboratory to laboratory and looking at these state-of-the-art detectors, it was time to go up again. But first, pick up a few crystals as souvenirs. We obviously went a different way back to the bottom of the shaft because here we were, plodging through water. Water!! I thought this didn't happen. But it does occasionally. So plodge on, up to one's calves in warm salty water. Then remember you have got your own socks on to travel home in. Oh, no! Against all expectations, the lift cage did work and we emerged, blinking in the light at the surface and tasting of salt.

What an excellent visit this was, combining the experience of going down the deepest mine in Europe with discussing the most important problem in current astronomy and particle physics with someone who is actually tackling the problem. It was worth getting your socks wet. Our physicist guide was confident that they would detect their WIMPS in the next few years, despite the competition from Super-Kamiokande in Japan, the Italian detectors under the Gran Sasso and the French Edelweiss cryogenic experiment in the Feizer Tunnel in France. When the Large Hadron Collider starts operating at CERN in Geneva in a few years time, it may well generate these particles, as well as them being detected coming in from space. Then everybody will begin to believe they actually do exist.