Shortly after this event professor Pierre Auger from Paris - the discoverer of extensive air showers - visited Kraków. He was interested in the Wieliczka salt mine  as an ideal place to study extensive air showers filtered by a thick layer of ground. He proposed to Kraków physicists a joint research on air showers. This research began, but soon got interrupted by the outbreak of World War II.

In 1946  Prof. Jan Weyssenhoff, professor of the Jagellonian Uniwersity  invited the Cosmic Ray Commission of the Union of Pure and Applied Physics (IUPAP) to convene a conference on cosmic rays and he offered to organize it. The conference took place in 1947 in Kraków and started the series of the International Cosmic Ray Conferences (ICRC) which now are organized around the world on a biennial basis.

Prof. Marian Miesowicz

 Many outstanding experts on cosmic rays participated in the Kraków conference. In particular, Prof. Cecil Frank Powell in his talk "Evidence of existence of mesons with different masses" announced a discovery of a new particle - the pi meson. He got the Nobel Prize in 1950 for this discovery.

During the conference, in discussions with Prof. Pierre Auger, the project to study cosmic rays in the Wieliczka salt mine was revived. The project was carried out in 1948 - 1949, headed by professor M.Miesowicz. The study confirmed existence of a weakly ionizing radiation, interpreted as the muon radiation known at that time. This weak radiation was interpreted as a natural radioactivity of ambient rocks.

The cosmic ray study was continued over next years using the technique of photographic emulsion. Prof. Marian Danysz of the Warsaw University helped o lot in development of nuclear technique research in Kraków. He stayed in Prof. Powell's laboratory in Bristol, England in 1950-1952, where he participated in research with nuclear emulsion technique. In 1952 Professors Danysz and Pniewski observed a cosmic ray interaction in emulsion, in which one of the nuclear fragments contained a hyperon. Such fragments were called hyperfragments. In other words, a hyperfragment (or hypernucleus) is a nucleus in which one or more nucleons are replaced by hyperons. The group headed by Professors Danysz and  Pniewski became a renowned center for hypernuclei research. 

PARTICIPANTS  OF  THE  1st INTERNATIONAL  COSMIC  RAY  CONFERENCE (ICRC), KRAKÓW  1947
	Standing in front row are Professors: P.Blacket, J,Blaton, J.A Wheeler and W.Heitler.
	In front row, sitting are Professors:P.Fleury, M.Forro, Clay and P.Blacket; standing are W.Heitler, J.A.Wheeler. In second row: J.Weyssenhoff, L.Janossy and L.Leprince-Ringuet.

Thanks to Prof. Danysz and to personal contacts of Prof. Miesowicz with Prof. Powell, a  small stack of nuclear emulsions was sent to Kraków. This stack was previously exposed to cosmic rays in one of balloon flights organized by University of Bristol, which were carried out by NASA on Sardinia (Italy). Results obtained by the group led by Prof. M. Miesowicz were so interesting that at the beginning of the 1960-s the Kraków group was invited by the University of Bristol to participate in a new emulsion balloon flight in southern Italy. A gondola with emulsion stacks containing lead plates was designed and built in Kraków. The high-altitude balloon flight was performed on Sardinia. Analysis of cosmic ray interactions recorded in this stack resulted in a series of scientific papers on nuclear interactions and electromagnetic cascades at high energies.

The cosmic ray study was continued over next years. In a series of experiments large quantities of emulsion (up to several tens of liters) were exposed to cosmic rays in high-altitude balloon flights. The emulsion served as both a target and a detector of cosmic ray interactions. These studies were very fruitful and established Kraków's position in cosmic ray and particle physics.

At the International Cosmic Ray Conference, Jaipur, India 1963. Right to left: Professors: O.Czyzewski, C.F.Powell, M.Miesowicz

Particularly important was participation of a group from the Institute of Nuclear Physics in the JACEE experiment, carried out in collaboration with American and Japanese laboratories. In this experiment, the study focussed on two topics: composition and energy spectra of primary cosmic rays as well as general properties of nucleus-nucleus interactions at extremely high energies. The JACEE experiment lasted for about 20 years and provided the best available data on cosmic ray spectrum in the energy range 10¹² - 10¹⁵ eV.