Cobalt 55

I promised to tell the story of Cobalt 55. Just to get you into the picture, this is me then, Oxford 1971. The leather jacket is made of plastic.

My doctoral research was in the field of nuclear orientation, at the Clarendon laboratory. My supervisor was Nick Stone and our grandfatherly mentor, Nicholas Kurti. We used adiabatic demagnetisation of paramagnetic salts to get down to 10 milliKelvin, at which temperature the hyperfine interaction would overwhelm the Boltzmann energy causing radioactive nuclei in a ferromagnetic host metal, attached to the cold finger of the apparatus, to align, so directing their gamma rays preferentially along the quantisation axis. The degree of orientation could be used as a measure of the interaction strength, so that nuclear magnetic moments or local fields could be measured. But by far the best way of precisely measuring that interaction was by sweeping an RF field in the vicinity of the Larmor frequency. At resonance, the gamma rays would suddenly change their angular distribution and the axial count rate would change dramatically.

We mostly made our own isotopes. My story is about Cobalt-55, a positron emitter with an 18 hr half life. Here's how our paper in "Nuclear Physics A" described the procedure. A 99.8 % enriched Fe-56 1 mg/cm (1.3 micron) foil was used as the target in a 35 MeV proton irradiation at the cyclotron of the Faculté des Sciences, University of Paris, Orsay. After waiting 12 h for short-lived product decay the sample was transferred to Oxford and the experiment performed during the next 24 h.

Let me tell you how it was transferred! In a small lead pot big enough to fit in my pocket so that when I boarded the BA flight from Paris to London, no-one would know I was carrying many many milliCuries of radioactive source. Once I got to Heathrow I got into my car (MG1100), raced up to Oxford, and worked non-stop for the next 24 hours as the isotope decayed.

I managed to measure the magnetic moment of the Cobalt-55 nucleus, as the nuclear magnetic resonance diagram above, lower panel, shows. Is the world a better place for it? Hardly. Are the scientific frontiers advanced? Well, 10 people have cited it in 37 years, so I would have to say not. But there it is, a piece of nuclear magnetism I measured (and for the record, along with the magnetic moments of Antimony 118m, 120m, 122 and 124).

Finally I must tell you about my Paris host, Daniel Spanjaard. I have managed to find only one very poor picture of him. Daniel took me to his parents' place for dinner, in the Sevres district. They had original impressionists on the wall and the meal was made and served by their cook. To a kiwi lad in the 1970s, this was amazing.



I know what some of you are thinking. Did that radioactive source knock his DNA about so that later in life, he developed cancer? I haven't a clue, but I doubt it.