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VERSION:2.0
CALSCALE:GREGORIAN
PRODID:UW-Madison-Physics-Events
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SEQUENCE:0
UID:UW-Physics-Event-1779
DTSTART:20100219T220000Z
DURATION:PT1H0M0S
DTSTAMP:20260506T064039Z
LAST-MODIFIED:20100215T172008Z
LOCATION:2241 Chamberlin Hall (coffee at 3:30 pm)
SUMMARY:Turning Photons into Polarized Nuclei\, Physics Department Col
 loquium\, Thad Walker\, University of Wisconsin\, Department of Physic
 s
DESCRIPTION:Spin-exchange optical pumping (SEOP) transfers the angular
  momentum of circularly polarized light to noble gas nuclei through a 
 series of steps.  First\, an opaque vapor of alkali atoms (transmissio
 n~exp[-100]) is rendered nearly transparent by optical pumping into an
  atomic dark state with an efficiency of 1 photon per atom. Collisions
  between the alkali atoms are nearly spin conserving and produce a spi
 n-temperature distribution with a temperature of about -0.06 K\, or\, 
 in angular momentum units\, -0.2 hbar.  The highly spin-polarized atom
 s then transfer their angular momentum to noble gas nuclei through a w
 eak hyperfine interaction occurring in binary collisions or formation 
 of weakly bound van der Waals molecules.  The cross sections for this 
 process are tiny by atomic standards\, 10^-24 cm^2\, but this is compe
 nsated for by having an extremely large collision rate and long nuclea
 r spin-relaxation times.  According to these arguments\, it should be 
 possible to transfer angular momentum from laser light to nuclei with 
 an efficiency of about 25%\, producing >95% polarized nuclei.  The res
 ulting high density\, hyperpolarized noble gas vapors are of considera
 ble interest for medical imaging\, spin-polarized targets\, neutron sp
 in-filters\, and precision measurements. In practice\, the performance
  of SEOP falls substantially below expectations.  When Wisconsin enter
 ed the field in the mid '90s\, the efficiencies were routinely much le
 ss than 1% and the polarizations in the mid 50%s.  Through a series of
  experiments and technological developments\, we have increased the ef
 ficiencies by an order of magnitude and produced polarizations as high
  as 80%.  This colloquium will describe these experiments and their co
 nsequences. 
URL:https://www.physics.wisc.edu/events/?id=1779
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