text: * Carson Jeffries, "Dynamic Nuclear Orientation", New York, Interscience Publishers, 1963
  * Anatole Abragam and Maurice Goldman, "Nuclear Magnetism: Order and Disorder", New York : Oxford University Press, 1982
  * Tom Wenckebach, [http://www.wenckebach.net/html/dnp-book.html "Essentials of Dynamic Nuclear Polarization"],  Spindrift Publications, The Netherlands, 2016