Some time ago we discovered a paper which said the NMR frequency for iron is different where the NMR frequency is stable at 45.5Mhz in varying magnetic field strengths, which gives us the chance to align the iron atoms spin and resonate it at it's NMR frequency, with the hope of destabilizing the atom and releasing some energy in the form of beta radiation and dropping to a lighter isotope.
With iron we have a penetration problem due to skin depth, I hope to alleviate this by passing a current through the pure iron powder to heat it above it's Currie point.
Many references state that
"The absolute frequency of iron is 3.237778 MHz with respect to 100.00 MHz for TMS".
That refers to the NMR frequency of
individual iron nuclei subjected to an external magnetic flux of such density that it causes the single protons of the Tetramethylsilane to precess at 100MHz. This happens at the magnetic flux density of 2.34 Tesla.
At a lower flux density, e.g. at 0.5T, the
individual Iron nuclei resonate at 690kHz.
The dependence of the NMR frequency on the magnetic field is better captured by the Gyromagnetic Ratio which precisely relates how the NMR frequency of
individual nuclei varies in response to the density of the externally applied magnetic flux.
For
individual iron nuclei, this Gyromagnetic Ratio is 1381.56Hz/mT. You can see the values for other metals
here.
However, for the ferromagnetic bulk metallic iron, the effective Gyromagnetic Ratio and resulting NMR frequencies are
wildly different because of its huge negative internal hyperfine field which affects the iron nuclei in addition to the externally applied magnetic field.
The authors of
this paper experimentally measured metallic Iron's NMR frequency as 45.525MHz. ( half of it is 22.763MHz ) in the absence of an external magnetic field (and in presence of its -33.02T internal hyperfine field).
Unlike non-ferromagnetic compounds, the metallic Iron's nuclear resonance frequency is relatively independent of external magnetic fields because the internal -33.02 Tesla hyperfine field swamps any externally applied fields.
An external magnetic field >0.75T saturates the bulk metallic Iron (i.e. coalesces and orients all its magnetic domains in one direction) and the flux density of this field does not need to be precisely controlled/correlated with the nuclear resonance frequency (unlike with non-ferromagnetic materials) because the internal -33T hyperfine field of Iron affects its resonance frequency much more than any external field, which we mere mortals could apply.
For example, the application of a 1T external magnetic field decreases the total magnetic field to which the Iron nuclei are subjected to, to -32.7T
* which decreases the Iron's nuclear resonance frequency by only ~350kHz, and for external fields well below the Iron's saturation level (< 0.6T) that frequency changes negligibly (by only -0.033%). See the hollow squares graph line of
f vs.
BEXT below:
It is important to remember that even when an external magnetic field is not applied, increasing the temperature of the Iron metal significantly decreases its nuclear resonance frequency, so if the oscillator does not track the temperature then periodic cool-downs are required.
The Curie temperature (TC) of pure iron is 1040°K* The ferromagnetic magnetization (domain rotation and coalescence) takes 0.75T to happen. After that, the remaining external flux density directly subtracts from the internal hyperfine field.
Author
Topic: Towards a 45.525MHz 16 Watt Amp (Read 38728 times)
