Development on dynamic nuclear polarized targets. Page: 4 of 14
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get material is hydrogenous and deuterated ammonia. Also, chemically doped
hydrocarbons like butanol are used in present targets [14,15].
The DNP process requires the temperature range of 0.3-1 K. These temper-
atures can be achieved with a 4He or 3He evaporator or with a dilution re-
frigerator. Commercially suitable 4He and 3He evaporators are available, but
dilution refrigerators still have to be designed and constructed by DNP target
groups. The main problem is to have a refrigerator with large enough cooling
power to remove heat created by the microwave irradiation during the DNP
process and in some cases, by an intense beam. Depending on the target ma-
terial, magnetic field, and operating temperature, the heat load varies from
1-20 mW per gram of material. A more complicated mode of operation of a
DNP target is a frozen-spin mode where the DNP process is turned off and
the target is cooled to 50 mK or less. At these temperatures and at a 0.3 T
holding field the polarization decay time constants of days have been achieved
[16].
An uncertainty in the target polarization affects directly the overall uncer-
tainty of an experiment. The measurement of the absolute value of target
polarization using CW NMR is a technique that has been developed over the
whole period that DNP targets have been in use. The NMR system for these
experiments has been largely standardized and is well understood [17]. The
systematic uncertainties in a polarization measurement in a proton (deuteron)
target are typically in the range of 2 to 7%. This uncertainty consists of errors
in the polarization calibration measurement about 1% (0.8%), in the tem-
perature measurement during the polarization calibration about 1% (0.4%),
and then the 1.2% (1.8%) systematic error in the NMR measurement of the
enhanced signal [14]. In the latest DNP experiments, the target polarization
uncertainty was a significant part in the overall experiment error, and, there-
fore, more development work is needed to reduce the target uncertainty [17].
3 Polarized Deep Inelastic Scattering Experiments with DNP Tar-
gets
The PDIS experiments of the spin structure function measurements used
longitudinally (transversely) polarized DNP targets and longitudinally po-
larized muons at CERN and electrons at SLAC. The experiments measured
the lepton-nucleon asymmetries
o_ - OT _ 1 CN (N. - N+)
A y = -- + ARC (2)
Al Uf + 'fir .RC ,fb t (N- + N+)3
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Penttila, S. I. (Seppo I.). Development on dynamic nuclear polarized targets., article, January 1, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc927160/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.