of alpha-particle and fast-proton tracks. In 1937, Marietta Blau and her former student Hertha Wambacher discovered nuclear disintegration stars (Zertrümmerungsterne) due to spallation in nuclear emulsions that had been exposed to cosmic radiation at a height of 2300m on the Hafelekarspitze above Innsbruck. This discovery caused a sensation in the world of nuclear and cosmic ray physics, which brought the nuclear emulsion method to the attention of a wider audience. But the onset of political unrest in Austria and Germany, leading to World War II, brought a sudden halt to progress in that field of research for Marietta Blau. In 1938 the German physicist Walter Heitler, who had escaped Germany as a scientific refugee to live and work in England, was at Bristol University researching a number of theoretical topics, including the formation of cosmic ray showers. He mentioned to Cecil Powell, at that time considering the use of cloud chambers for cosmic ray detection, that in 1937 the two Viennese physicists, Blau and Wambacher, had exposed photographic emulsions in the Austrian Alps and had seen the tracks of low energy protons as well as 'stars' or nuclear disintegrations caused by cosmic rays. This intrigued Powell, who convinced Heitler to travel to Switzerland with a batch of llford half-tone emulsions and expose them on the Jungfraujoch at 3,500 m. In a letter to 'Nature' in August 1939, they were able to confirm the observations of Blau and Wambacher. Following on from those developments, after World War II, Powell and his research group at Bristol University collaborated with Ilford (now Ilford Photo), to further optimise emulsions for the detection of cosmic ray particles. Ilford produced a concentrated ‘nuclear-research’ emulsion containing eight times the normal amount of silver bromide per unit volume (see External Link to 'Nuclear emulsions by Ilford'). Powell's group first calibrated the new ‘nuclear-research’ emulsions using the University of Cambridge Cockcroft-Walton generator/accelerator, which provided artificial disintegration particles as probes to measure the required range-energy relations for charged particles in the new emulsion. They subsequently used these emulsions to make two of the most significant discoveries in physics of the 20th century. First, in 1947 Cecil Powell, César Lattes, Giuseppe Occhialini and Hugh Muirhead (University of Bristol), using plates exposed to cosmic rays at the Pic du Midi Observatory in the Pyrenees and scanned by Irene Roberts and Marietta Kurz, discovered the charged Pi-meson. Second, two years later In 1949,