Ultracold Neutrons


A new high-density ultracold neutron (UCN) source is currently being constructed at TRIUMF with a targeted operational date of 2016 for the highest density UCN worldwide. The flagship experiment for the UCN source is a neutron electric dipole moment (nEDM) experiment with the following motivation: a non-zero value of the nEDM would violate time-reversal symmetry. Through the Charge-Parity-Time (CPT) reversal theorem, the current level of CP-violation in the standard model predicts a value of 10^(-31) e-cm for the nEDM. New sources of CP-violation are predicted by modifications to the standard model that seek to explain the baryon asymmetry (matter-antimatter asymmetry) of the universe. These standard model updates also increase the nEDM value by five orders of magnitude, tantalizingly close to the current experimental upper limit of < 2.9*10^(-26) e-cm. With the approach taken by the UCN at TRIUMF, we project a measurement of the nEDM under this level of uncertainty, thus offering the potential of experimentally confirming a number of theories beyond the standard model.


We are part of the Canadian Team in the UCN project and responsible for development of a comagnetometer for the nEDM experiment. We are developing the dual-species Xe-129/Hg-199 comagnetometer. This solution has several advantages over the traditional single-species Hg-199 comagnetometer, the most important being that it offers a cross-check on the anticipated largest systematic error in the experiment. A Xe-129 comagnetometer has yet to be demonstrated and thus entails our initial efforts.


The dual-species Xe-129/Hg-199 comagnetometer represents two separate comagnetometer systems operating simultaneously. In each, the relevant atomic species is polarized by means of optical pumping. The atoms are then introduced into the EDM measurement volume, along with the UCN. RF pulses near p/2 initiate precession of the spins of the atoms and UCN. The UCN spins are probed by the application of a second p/2-pulse and then passing them through an analyzer magnet to measure their transmission (using the Ramsey technique). The Xe and Hg, on the other hand, are probed continuously by observing the modulation of transmitted probe light, at 253.5 nm, for Hg, and emission in NIR (823 nm and 895 nm) for Xe by exciting at 252.4 nm.

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Ultracold Neutron



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