Enriched Xenon Observatory
Encyclopedia
The Enriched Xenon Observatory (EXO) is a particle physics experiment searching for double beta decay
of xenon
-136. The experiment uses a large amount of xenon, isotopically enriched in xenon-136. This isotope is theorized to undergo ordinary double beta decay (with the emission of two neutrino
s) to barium
-136, though past experiments have only placed limits on the half-life.
If neutrinoless double beta decay is detected for the first time, it will be definitive proof of the Majorana
nature of neutrinos. EXO intends to measure the effective Majorana neutrino mass (if it exists) with a sensitivity close to 0.01 eV
. The actual measurement will be the rate of events, which is equivalent to a measurement of the half-life. Currently only lower limits exist for both the 2-neutrino and neutrinoless double beta decay modes of xenon-136. Observation of the 2-neutrino mode does not provide information about neutrinos, though it is interesting for nuclear theory. Measurement of the half-life of the neutrinoless mode can be converted to an effective neutrino mass using calculated nuclear matrix elements. If the neutrinoless mode is not seen, a lower limit can be placed on the half-life, which corresponds to an upper limit on the neutrino mass.
If a limit on the effective neutrino mass is placed at the 0.01 eV mass range, it answers the question of the ordering of neutrino masses. While the differences between neutrino masses is known, it is not known which neutrino is the heaviest. The effective neutrino mass is dependent on the lightest neutrino mass in such a way that a limit at the 0.01 eV level indicates the neutrino masses lie in the normal hierarchy.
EXO currently consists of two facets: a 200-kilogram liquid time projection chamber
dubbed "EXO-200" and R&D efforts into a ton-scale xenon experiment. While EXO-200 serves as a testing ground for liquid xenon techniques, the ton-scale experiment may take a different form.
design in order to gather information about the decay. Xenon is a scintillator
, so the prompt light provides time information of the event. A large electric field is set up to drive ionization electrons to wires for their collection. The difference in time between the light and the first ionization collection determines the z coordinate of the event, while a grid of wires determines the radial and angular coordinates. Scintillation light is collected by avalanche photodiode
s.
EXO-200 has been designed with a goal of less than 40 events per year within two standard deviations from the expected energy. In order to accomplish this, all materials were selected and screened based on radiopurity. Originally the vessel was to be made of Teflon, but the final design of the vessel uses thin, ultra-pure copper.
The relocation of EXO-200 from Stanford to WIPP
began in the summer of 2007. Further assembly and commissioning is expected to continue to the end of 2009 with data taking beginning in 2010. Photos of the EXO-200 laboratory and cryostat installed underground at the WIPP site are shown here.
Double beta decay
Double beta decay is a radioactive decay process where a nucleus releases two beta rays as a single process.In double-beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron antineutrinos are emitted...
of xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
-136. The experiment uses a large amount of xenon, isotopically enriched in xenon-136. This isotope is theorized to undergo ordinary double beta decay (with the emission of two neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s) to barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
-136, though past experiments have only placed limits on the half-life.
If neutrinoless double beta decay is detected for the first time, it will be definitive proof of the Majorana
Majorana fermion
In physics, a Majorana fermion is a fermion which is its own anti-particle. The term is used in opposition to Dirac fermion, which describes particles that differ from their antiparticles...
nature of neutrinos. EXO intends to measure the effective Majorana neutrino mass (if it exists) with a sensitivity close to 0.01 eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
. The actual measurement will be the rate of events, which is equivalent to a measurement of the half-life. Currently only lower limits exist for both the 2-neutrino and neutrinoless double beta decay modes of xenon-136. Observation of the 2-neutrino mode does not provide information about neutrinos, though it is interesting for nuclear theory. Measurement of the half-life of the neutrinoless mode can be converted to an effective neutrino mass using calculated nuclear matrix elements. If the neutrinoless mode is not seen, a lower limit can be placed on the half-life, which corresponds to an upper limit on the neutrino mass.
If a limit on the effective neutrino mass is placed at the 0.01 eV mass range, it answers the question of the ordering of neutrino masses. While the differences between neutrino masses is known, it is not known which neutrino is the heaviest. The effective neutrino mass is dependent on the lightest neutrino mass in such a way that a limit at the 0.01 eV level indicates the neutrino masses lie in the normal hierarchy.
EXO currently consists of two facets: a 200-kilogram liquid time projection chamber
Time projection chamber
In physics, a time projection chamber is a particle detector invented by David R. Nygren, an American physicist, at Lawrence Berkeley Laboratory in the late 1970s...
dubbed "EXO-200" and R&D efforts into a ton-scale xenon experiment. While EXO-200 serves as a testing ground for liquid xenon techniques, the ton-scale experiment may take a different form.
EXO-200
EXO-200 uses a cylindrical time projection chamberTime projection chamber
In physics, a time projection chamber is a particle detector invented by David R. Nygren, an American physicist, at Lawrence Berkeley Laboratory in the late 1970s...
design in order to gather information about the decay. Xenon is a scintillator
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...
, so the prompt light provides time information of the event. A large electric field is set up to drive ionization electrons to wires for their collection. The difference in time between the light and the first ionization collection determines the z coordinate of the event, while a grid of wires determines the radial and angular coordinates. Scintillation light is collected by avalanche photodiode
Avalanche photodiode
An avalanche photodiode is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint,...
s.
EXO-200 has been designed with a goal of less than 40 events per year within two standard deviations from the expected energy. In order to accomplish this, all materials were selected and screened based on radiopurity. Originally the vessel was to be made of Teflon, but the final design of the vessel uses thin, ultra-pure copper.
The relocation of EXO-200 from Stanford to WIPP
Waste Isolation Pilot Plant
The Waste Isolation Pilot Plant, or WIPP, is the world's third deep geological repository licensed to permanently dispose of transuranic radioactive waste for 10,000 years that is left from the research and production of nuclear weapons...
began in the summer of 2007. Further assembly and commissioning is expected to continue to the end of 2009 with data taking beginning in 2010. Photos of the EXO-200 laboratory and cryostat installed underground at the WIPP site are shown here.