An adventure in physics

by Marian DANYSZ (Delta 1/1975)

Over forty years ago I was employed by the Radiological Laboratory in Warsaw, headed by professor Ludwik Wertenstein. The main equipment in the laboratory was in fact a gift from Maria Curie-Skłodowska: 60 mg of radium, the source of all kinds of activity we used to investigate. The 1930s, when I worked there, was a very interesting period indeed. That's when the neutron and the positon were discovered, and Frederic Joliot-Curie discovered radioactivity excited by photoactivating various materials with alpha particles. I recall that one day I turned to prof. Wertenstein, saying with some regret: "Professor, actually we know all that. We know the world around us consists of atoms, the atoms have nuclei with electrons orbiting around, and the nuclei are built of protons and neutrons. We are only left with details." Wertenstein smiled, slapped me on the shoulder and said: "Don't worry, there will be enough to do for you, youngsters".


Some months after Joliot's discovery of radioactivity photoactivated by alpha particles, I was working with Michał Żywy, using a small source of alpha particles. Actually, we prepared them ourselves in a glass apparatus, containing a water solution of the 60 mg of radium in the form of radium chloride. We drew out the emanation of radium from over the solution, cleaned it and condensed it in a small glass tube with a platinium plate set into its end. After the plate was left for a day or two in a radon atmosphere, its surface was activated by radon decay products. When radon had been eliminated, the tube cut off, the plate taken out and duly protected placed against a thin slot, which stopped rebound nuclei while letting pass alpha particles, we had our little source of alpha particles.


Now, photoactivating various targets with alpha particles, we stated a very strange effect, which we could not understand. Independently of what was photoactivated, the material became radioactive. Absorption implied that electrons were emitted as a product of decay. What was most astonishing - independently of the material, the half-period was always the same: about one minute. Wertenstein even suggested that maybe something is going on with the alpha particle. And then one evening, when we were discussing the problem on our walk home, we realised that there was one more factor common to all our experiments: air atmosphere. Photoactivation was performed in the presence of air, so maybe the effect observed was due to the reaction of alpha particles with the nuclei of air components?


So we started asking questions. We photoactivated a target in vacuum - the effect disappeared. Very well, so it depends on air. Photoactivation in the presence of oxygen - no effect again. In a nitrogen atmosphere the effect reappeared. Thus we knew that the reason was the interaction alpha-nitrogen. By then we also knew that substances containing nitrogen become a source of neutrons when bombarded with alpha particles. Therefore we could reasonably assume that the process we had to do with was . It should have been fluorine, but stable fluorine had 19 nucleons in its nucleus -- so it would be fluorine with neutron deficiency, and if so, most probably its nucleus was unstable. But then if the nucleus was unstable, probably one of the protons was transformed into a neutron and fluorine turned into stable oxygen . Therefore we ought to have a closer look at activity, i.e., the emission of positively loaded particles.


So we took our photoactivated plates, the Geiger counter we made by ourselves and we placed them in a magnetic field in such a way that the counter favoured detection of negatively loaded particles at one determined direction of the field, favouring in turn detection of positively loaded ones when the field changed. Measurements proved that we witness emission of positively loaded particles. We were right: the strange effect was caused by the interaction of alpha particles with the nitrogen in the air.


We finished our experiment by 9 a.m., which means that we spent the whole night in the laboratory. I remember that leaving the place in the morning I was deeply impressed by this chance to establish dialogue with nature, to ask questions, to get answers and that one night may suffice to solve a problem, corroborate or falsify a hypothesis. Looking back at the important moments of my life, I must admit that this was probably the most fantastic adventure I have ever experienced.