Development of Two Components for the Neutron Electric Dipole Moment Experiment
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2009-07-09
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Abstract
The discovery of a nonzero electric dipole moment for the neutron (nEDM) would have a fundamental impact upon the current understanding of the weak and strong nuclear interactions. The Standard Model predicts a value for the electric dipole moment that is several orders of magnitude lower than that which can currently be probed. Thus, a non-zero value at the order expected in the proposed experiment
(10-28 e cm) would indicate new sources of T and CP violation, thereby either extending or disproving aspects of the Standard Model.
The current neutron electric dipole moment experiment takes advantage of the known neutron magnetic moment. The neutrons are placed in liquid helium, in a plane perpendicular to parallel magnetic and electric fields B0 and E0, causing them to precess with frequency vn, giving energy
hn = -2dnB0 – 2dnE0
with dn and un the neutron magnetic and electric dipole moments respectively. The impact of the electric field upon the precession of the neutron can thus be used to characterize the neutron charge distribution.
The liquid helium is doped with trace amounts of 3He, which functions as both a detector via the reaction
and as a comagnetometer. The neutron absorption cross section of 3He is strongly spin dependent. Thus, for each test run, it is necessary to polarize the 3He before using it to dope the isotopically pure helium. Scintillation pulses originate when ionizing radiation creates excited electronic states in the helium. Specifically, when the proton and 3H recoil in the helium, both the molecular singlet and triplet states become populated. The track length of these particles in the liquid is approximately 1 mm, whereas background events from electrons that arise from neutron decay or Compton scattered gamma ray events have track lengths of order 1 cm. The differing track lengths, and thus energy deposition per unit length, changes the dynamics of the relative amounts of singlet and triplet states produced. The use of the ratio of the singlet to triplet events shows great promise in distinguishing the nEDM signal from events associated with background ionizing radiation.
As part of the measurement process, the helium is doped with 3He that serves as the comagnetometer. As this helium depolarizes, it is necessary to remove this 3He before inserting more polarized 3He. We are developing an evaporative purification technique that can facilitate this removal, therby lowering the concentration of 3He in 4He from approximately 10-10 to 10-12. The operating temperature for the experiment, 400 mK to 600mK, places a constraint upon the effectiveness of this technique. It is therefore necessary to design and optimize an appratus capable of performing the above mentioned purfication in a time much less than the measurement time of approximately 22 minutes. Additional experimental concerns involve minimizing the introduction of heat to the system and effectively removing the evaporated gas from the testing area.
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neutron electric dipole moment cryogenics 3He 4He
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PhD
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Physics
