Hydrogen or deuterium? Pulsed or continuous wave? Integrated amplifier or not? Metrolab, the uncontested specialist for NMR (Nuclear Magnetic Resonance) teslameters, discloses some key information for understanding this unique technology – the only one capable of measuring strong magnetic fields with a relative error of less than 0.1 ppm.
What is small, contains a coil and a drop of rubber, and is capable of measuring very strong magnetic fields with incredible accuracy? An NMR probe, of course! Certain technicians and research scientists are very familiar with these 20 cm long copper tubes, indispensable when a magnetic field value has to be known with great precision. One of the uses is the production and installation of MRI scanners (see our article on Siemens Magnet Technology). With a relative error of 0.1 ppm, NMR technology is by far more accurate than any other. It can be used for total field measurements for field strengths between 0.05 and 20 T, as long as the field is strictly homogeneous and varies slowly or not at all.
The measurement principle is well known to physicists: nuclei of atoms whose spins are aligned by a magnetic field are stimulated by an electromagnetic wave at their resonant frequency, which is directly proportional to the strength of the magnetic field.
The ingenuity lies in building an industrial-quality instrument that does all this simply, reliably and reproducibly. A technological tour de forcethat Metrolab has made one of its specialities:"For 20 years, we have been able to propose a reliable and industrialised solution – that’s made our success", says Metrolab's Pascal Sommer.
After a first series of probes known as 1040 and 1050 (no longer available), Metrolab developed a miniature probe called 1060. 8 probes cover the range of 0.043 to 13.7 T. The first five probes, from 0.043 to 2.1 T, are fitted with rubber samples, a rich source of hydrogen nuclei. The remaining three probes, designed to measure intense magnetic fields of 2.1 to 13.7 T, are based on heavy water; this contains deuterium, an isotope of hydrogen that has a much lower resonant frequency. In all the probes, the sample is surrounded by an RF coil that ensures the electromagnetic excitation of the nucleus, as well as a modulation coil to induce a slight variation of the magnetic field. A clever design allows the same probe head to be used for axial as well as transverse fields without any change or commutation. This series of probes is connected to the instrument itself via an amplifier (Type 1030), needed to amplify the RF wave going to the probe and the very weak NMR signal on its way back to the main unit.
In a next generation, the model 1062 probes, the external amplifier was integrated into the probes, providing an improved signal-to-noise ratio and less sensitivity to external perturbations – with no change in size! "The 1062 probes are highly recommended for measurements requiring a very long cable, up to 100 m, between the probe and the teslameter", adds Pascal Sommer.
Versions of the 1060 and 1062 series with miniaturised heads (external diameter of 10 mm) resulted in the 1080 and 1082 subminiature probes. These probes also cover field strengths between 0.043 and 13.7 T; however, they are only sensitive to transverse magnetic fields.
The excitation mode is "continuous wave", as opposed to the more conventional pulsed mode. "In pulsed mode, a few microsecond pulse of the appropriate frequency excites the protons of the sample material, then one switches the RF coil to a receiver and measures the frequency of the response", explains Pascal Sommer. "In contrast, with continuous wave, the material is irradiated permanently. When the frequency reaches the resonant value, the response of the protons leads to a very small absoption of energy which is detected, filtered and amplified more than 3300x." Advantages of this system: the probe is greatly simplified as it no longer has a switch, and searching for the field value is easier. Needless to say, there is a price to pay … In order to be able to detect the nuclear magnetic resonance, the field must be modulated by means of a second coil! But rest assured, this is of no concern to the user, who can confidently read off the value displayed on the screen of his PT2025 or PT2026 Teslameter.
With the recent development of the PT2026 NMR teslameter, which has a digital frequency generator, Metrolab has taken things a step further by producing a new generation of probes (Model 1126). "This time, the probe excitation frequency is modulated directly, making the field modulation obsolete",explains Pascal Sommer. The field modulation may perturb certain types of experiments being performed in the magnet. That disadvantage vanishes with the frequency modulation technique. Moreover, the modulation coil is removed, allowing a smaller probe head. "The disadvantage of this approach is that the tuning of the excitation coil is much more complex. This is why the new probe is equipped with a built-in tuning system, needed to improve the sensitivity of the probe." Once again, the probe size stays the same.
At the same time, Metrolab is researching flowing water technology. This allows a single probe to measure a wide range of fields, including very low fields, with the high precision that characterize NMR measurements. "We are also currently exploring NMR pulsed probes", indicates Pascal Sommer. Finally, to measure weak fields (down to around 500 µT), a technique similar to NMR can be used, with a similar degree of accuracy: ESR (Electron Spin Resonance). Here it is the electrons that resonate, at a very high frequency, on the order of 28 GHz/T. Metrolab used to build this type of probe, but had to suspend production: there were no longer any suppliers proposing ESR material! But that's another story…
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