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T h o r i u m I o n T r a p p i n g |
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Pictured is a segmented linear quadrupole ion trap with hyperbolic electrodes and an einzel lens system for ion injection. |
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In this collaborative effort, we are interested in applying AMO technologies of ion trapping and cooling
and high resolution spectroscopy to the manipulation of nuclear excited
states. While typical nuclear excitation energies are in the keV to MeV
range, there are several exceptional cases where the excitation energies
are much lower. Of particular interest is the thorium-229 isotope, which,
uniquely, has an excited state in the UV optical spectrum.
The existence of a very low-lying isomeric state of 229Th was first
proposed by Kroger and Reich in 1976. They established that the energy of
this state was less than 100 eV above the ground state. In 1994, the
energy of this state was indirectly determined to be 3.5 +/-1 eV, via
spectroscopy of gamma rays emitted from alpha decay of 233U. This
measurement motivated numerous studies relating to the unique prospect of
controlling nuclear matter with optical radiation. Several experimental
searches for optical emission in this range undertaken in the past ten
years were unsuccessful or inconclusive.
Recently, a refined gamma ray spectroscopy measurement employing new
detector technology has established the energy splitting to be 7.6 +/-0.5
eV, likely explaining the failure of the early direct measurements.
In addition to searching in the wrong energy region, the searches were
challenged by the narrow linewidth of the transition. Depending on the size of the contribution of electron bridge processes, the predicted lifetime of this state may be as long as 105 s.
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A false color image of a cloud of ~104 Th3+ ions confined in a linear Paul trap. |
While this may be an
impediment for the initial observation of the state, once it is found, it
offers the tantalizing prospect of an optical clock that is dramatically
less sensitive to external electromagnetic field perturbations. To first
approximation, the shifts of the nuclear clock transition due to spurious
fields are suppressed relative to an optical-electronic clock by the ratio
of the electron to nucleon mass (103). Additionally, it has been
suggested that the nuclear clock transition may be sensitive to variation
of fundamental constants, due to the interplay of the strong and
electroweak interactions inside this nucleus.
We are investigating the nuclear state using trapped and sympathetically
cooled Th3+, which has a level structure that is amenable to 100%
detection efficiencies via laser-induced fluorescence.
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