204 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



The method is applicable only to steady fields or to time-varying 

 fields of a frequency which is low compared to the frequency of the 

 a-c current supplied to the coil C 1 . The more uniform the field 

 strength, the sharper are the pulses induced in C 2 ; the method fur- 

 nishes poor results in inhomogeneous fields. 



The dimensions of the ferromagnetic probe M are not critical; the 

 ratio of length to diameter should be large to obtain narrow and well- 

 defined pulses. The magnetic properties of most suitable ferromag- 

 netic materials are quite dependent upon temperature and mechan- 

 ical deformation; they vary with the magnitude of the applied field 

 and the frequency of the a-c generator. Kelly 1 recommends the use 

 of a molybdenum-permalloy wire of 5 cm length and 0.005 cm 

 diameter, annealed in a hydrogen atmosphere inside a quartz tube 

 and sealed after annealing. 



3-14. Nuclear Magnetic Resonance Systems 



a. Nuclear-absorption Method. Protons and neutrons possess an 

 intrinsic magnetic moment, and hence atomic nuclei have, in 

 general, a magnetic moment of a magnitude which depends upon the 

 number and the arrangement of the protons and neutrons in the 

 nucleus. If a nucleus with a magnetic moment is placed in a magnetic 

 field, the field will exert a force on the nucleus. Because of the quanti- 

 zation of the nuclear angular momentum which always accompanies 

 the magnetic moment, the nucleus can assume only certain orien- 

 tations in a magnetic field, each orientation corresponding to a 

 certain level of energy. In the simplest case of a proton (H 1 ) which 

 has only two permitted orientations in the field, there will be two 

 states of slightly different energy corresponding to the two orienta- 

 tions. The energy difference AE between the two energy levels is 

 proportional to the magnetic field strength. Transition from the 

 lower to the higher energy level is accomplished by an absorption of 

 energy. The energy can be supplied by an electromagnetic field of 

 frequency ("resonance frequency") 



AE 



'• = T 



where h is Planck's constant. The energy AE is proportional to the 

 external magnetic flux density B, and hence 



h = yB (i) 



Values of y are: proton in H 2 0, 4257.76 ± 0.1 cps/gauss; lithium 7 , 

 1 J. M. Kelly, Rev. Sci. Instr., 22, 256 (1951). 



