Knocking on Heaven's Door - Lisa Randall [185]
Fortunately for dark matter detection, plenty of mines and tunnels exist. The DAMA experiment, along with experiments called XENONIO and the bigger version XENONIOO—as well as CRESST, a detector that uses tungsten—take place in the Gran Sasso laboratory, situated in a tunnel in Italy about 3,000 meters underground. A 1,500-meter-deep cavern in the Homestake mine in South Dakota, originally built for gold excavation, will be home to another xenon-based experiment known as LUX. This experiment will take place in the very same cavity where Ray Davis discovered neutrinos from nuclear reactions taking place in the Sun. The CDMS experiment is in the Soudan mine, about 750 meters underground.
Still, all that rock above the mines and tunnels is not enough to guarantee that the detectors are radiation-free. The experiments further shield the actual detectors in a variety of ways. CDMS has a layer of surrounding polyethylene that will light up if something too strongly interacting to be dark matter comes through from the outside. Even more memorable is the surrounding lead from an eighteenth-century sunken French galleon. Older lead that has been underwater for centuries has had time to shed its radioactivity. It is a dense absorbing material that is perfect for shielding the detector from incoming radiation.
Even with all these precautions, a lot of electromagnetic radiation still survives. Distinguishing radiation from potential dark matter candidates requires further discrimination. Dark matter interactions resemble nuclear reactions that occur when a neutron hits the target. So opposite the phonon readout system is a more conventional particle physics detector that measures the ionization created when the alleged dark matter particle passed through the germanium or silicon. Together, the two measurements, ionization and phonon energy, distinguish nuclear events—the good processes that might be the result of dark matter—from events due to electrons, which are just radioactivity induced.
Other beautiful features of the CDMS experiment include the excellent position and timing measurements that it can make. This is nice because although the position is only directly measured in two directions, the timing of the phonons gives the position in the third coordinate. So experimenters can locate exactly where the event happened and discard background surface events. Another nice feature is that the experiment is segmented into the stacked hockey-puck-size detectors. A true event will occur in only one of these detectors. Locally induced radiation, on the other hand, won’t necessarily be confined to a single detector. With all these features and an even better design to come, CDMS has a good chance of finding dark matter.
Nonetheless, impressive as it is, CDMS is not the only dark matter detector and cryogenic devices are not the only type. Later on in the week, Elena Aprile, one of the xenon experiment pioneers, gave comparable details about her experiments (XEN0N10 and XEN0N100), as well as other experiments performed with noble liquids. Since these would soon be the most sensitive detectors for dark matter, the audience paid rapt attention to her talk too.
Xenon experiments record dark matter events through their scintillation. Liquid xenon is dense and homogenous, has a large mass per atom (enhancing the dark matter interaction rate), scintillates