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Posted by on May 26, 2011 in Some reminiscences | 0 comments

Connecting with Felix Bloch and the Golden age of Physics

Kamerlingh Onnes Lab, Leiden Holland

The Lorentz Institute in Leiden, Holland was on the top floor of the Kamerlingh Onnes Laboratory. At that time I was a post doc and in 1972, Professor Felix Bloch was Lorentz Professor for six months.  It was a great experience for me to meet and talk to the person credited, along with Purcell and Pound, with the discovery of NMR.  His thesis supervisor was Werner Heisenberg. Bloch left for Stanford in 1933 at age 28 after the Nazi rise to power.  For his discovery of Nuclear Magnetic Resonance (NMR) he received the Nobel Prize in Physics in 1952.

My post doc work was in the large experimental Molecular Physics group of Jan Beenakker. I was the sole theoretician trying to interpret their experiments.  Essentially the group studied the effects of electric and magnetic fields on the transport properties of gases (the Sentfleben-Beenakker effect).  I had studied the gas phase NMR experiments of Myer Bloom and his student, Ronald Dong for my PhD under the direction of Robert Snider (University of British Columbia).  I was therefore somewhat awed at the prospect of learning how Bloch discovered NMR.

He gave a Colloquium Erenfestii in which he recalled the post war research at Stanford and where the first NMR experiment was done.  He said something like:

We were not under a lot of pressure.  We did not work weekends nor late at night and took our coffee breaks.

None-the-less, they were successful in seeing the first absorption of energy between the two levels of the spin of ½ magnitude which are split in the presence of a magnetic field—the Zeeman Effect.

The experiment consisted of an electromagnet, a resonance coil and an oscilloscope that could detect the absorption of energy between the two Zeeman levels.  They calculated the frequency needed to cause the transition and tuned the magnetic field to that strength: actually the frequency was fixed, and they chose the field strength to give the right splitting.

There was a big toggle switch which was used to turn the magnetic field on and off. When everything was set up, they turned on the field and looked at the oscilloscope.  The line was flat, so no absorption.

Let’s go for coffee.

Bloch apparently said.

So someone closed the big toggle switch to turn off the field.  The person sitting at the oscilloscope said,

There’s a blip!

They had calculated the magnetic field strength too high, and when the field dropped, it scanned past the correct strength and the absorption was observed.

Turn it on again.

and as the field grew and passed the resonance, the blip was again observed.

Apparently they did this several times, so the first NMR experiment consisted of Felix Bloch and his students watching a blip come and go as someone toggled the electromagnet on and off.

The Bloch Equations express the relaxation in terms of T1 and T2, well known to all NMR users.  Bloch did a lot more for physics than I can touch on here. As he spoke, however, I knew he had known and studied with many of those from that Golden Age of Pre-War physics.

It was this link to the past that he gave me, something I think everyone treasures.

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