AsianScientist (Jul 31, 2014) – Scientists in China have designed and built PandaX, a large liquid-xenon detector designed to search for direct evidence of dark matter interactions. The technical design of the facility located deep underground in the southwestern Chinese province of Sichuan has been published in the journal Science China Physics, Mechanics & Astronomy.
The detector’s central vessel was designed to accommodate a staged target volume increase from an initial 120 kg (stage I) to 0.5 t (stage II) and ultimately to a multi-ton scale.
While noting that cosmologists generally agree that 80 percent of the matter in the universe is made up of some form of “dark matter,” these researchers also acknowledge that so far, no physicist has ever produced experimental data that provides convincing evidence for the existence and structure of dark matter.
“The standard model of particle physics, which has been very successful in explaining the properties of ordinary matter, can neither explain dark matter’s existence nor its properties,” said Professor Ji Xiangdong from the Institute of Nuclear and Particle Physics, Astronomy and Cosmology at Shanghai Jiao Tong University. “Yet the discovery and identification of dark matter would have a profound impact on cosmology, astronomy, and particle physics.”
“A leading dark matter candidate consistent with all astrophysical data is a weakly interacting massive particle (WIMP),” the authors write in the report. “WIMPs could be studied in standard particle physics through either observations of ordinary matter particles produced through dark matter annihilations in the halo of the Milky Way, production of dark matter particles through high-energy collisions in accelerators such as the Large Hadron Collider, or WIMPs could be detected through their interactions with atomic nuclei in specially designed detectors.”
Direct detection experiments are deployed in underground laboratories around the world. When WIMPs interact with nucleons in a detection medium, it is predicted they will recoil and generate kinetic motion of atoms (heat), ionization (free electrons) and scintillation (de-excitation of excited electrons). Direct detection experiments measure one or two or even possibly three of these signatures, depending on the choice of material.
In the case of noble liquid detectors, a light signal is measured by photo multiplier tubes; ionization electrons drifting in an external electric field are either detected through their charge or through electroluminescence. For heat measurements, the detector has to be kept at very low temperature, typically at tens of milli Kelvin, which is a cryogenic challenge, particularly for large masses.
Among all the direct detection experiments, the xenon dual-phase technology appears to be particularly promising. Over the last three years, the XENON100 experiment, using liquid xenon (LXe), has produced the best limits over a wide range of WIMP masses.
The new PandaX liquid-xenon facility is likewise aimed at the detection of both prompt scintillation and ionization electrons in a dual-phase mode, which allows for discrimination between nuclear recoils and electron recoils.
“Xenon does not have long-lived radioactive isotopes and can be highly purified,” explains the team of researchers. “Xenon has a large atomic mass, which entails a large WIMP scattering cross section. Furthermore, xenon liquefaction temperature is around 100°C, thus making cryogenics relatively easy to manage.”
A crucial property of xenon as a WIMP detector is its outstanding background discrimination. A particle interacting with liquid xenon produces both xenon excitation states and electron-ion pairs. The decay of excited states to the ground state results in scintillation light (S1) at a vacuum UV wavelength of about 175 nm.
Collaboration on the Particle and Astrophysical Xenon (PandaX) experiment started in 2009 involving physicists at Shanghai Jiao Tong University, Shandong University and the Shanghai Institute of Applied Physics, Chinese Academy of Sciences. Researchers at the University of Maryland, Peking University, and the University of Michigan joined two years later.
Most of the PandaX Dark Matter Experiment’s sub-systems were developed in the particle physics laboratory of Shanghai Jiao Tong University, and were transported to the China Jinping Deep Underground Laboratory in August 2012. After successful installation, two engineering runs were carried out in 2013. The system has been collecting science data since late March 2014. A small prototype for PandaX was developed and is running in the particle physics laboratory at Shanghai Jiao Tong University.
Initial results from the PandaX Dark Matter Experiment could be released late this year.
The article can be found at: Cao et al. (2014) PandaX: a Liquid Xenon Dark Matter Experiment at CJPL.
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Source: Science China Press; Photo: PandaX Dark Matter Experiment.
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