To study them, he invented an improved electronic coincidence circuit, and travelled to Eritrea to conduct experiments that showed that cosmic ray intensity from the West was significantly larger than that from the East.

In 1929, Rossi read the paper of Walther Bothe and Werner Kolhörster, which described their discovery of charged cosmic ray particles that penetrated 4.1 centimetres (1.6 in) of gold.

With Occhialini's help in the construction of GM tubes, and with the aid of a practical coincidence circuit, Rossi confirmed and extended the results of Bothe, who invited him to visit Berlin in the summer of 1930.

Because this effect is more prominent near the equator, he organised an expedition to Asmara in Eritrea, which was then an Italian colony on the Red Sea at a latitude of 15° N.[22] With Sergio De Benedetti,[23] he set up a "cosmic ray telescope", which consisted of two separated GM counters in coincidence, whose axis of maximum sensitivity could be pointed in any direction.

One was by Thomas H. Johnson,[24] and the other was by Compton and his student, Luis Alvarez, who reported observations at Mexico City, where the latitude is 19° N.[25] Because others had carried out the first experimental exploitation of his important idea of 1930, Rossi was disappointed, but published his results immediately after returning to Padua.

Similarly, Ettore Pancini, who received his laurea under Rossi in 1938, spent the war years alternating between cosmic ray research and active participation in the Italian resistance movements of Padua and Venice.

[41] Immediately after a symposium session on mesotron instability reached a consensus that more definitive observations were needed, Rossi and Compton began to plan an experiment.

He urged Rossi to begin a series of experiments that summer, before snow blocked the road, and to help, enlisted two of his friends, Norman Hillberry and J. Barton Hoag,[42][43] and a student, Winston Bostick.

Because the distance traversed in air was much larger than that in carbon, they interpreted this result as evidence for decay of the mesotron, and taking into account the effect of relativistic time dilation, estimated its mean life at rest as roughly 2 microseconds.

Hall, not only confirmed the proportionality between particle momentum and the mean free path of mesotrons before decay that is expected on the basis of relativity theory, but also presented an improved estimate of the lifetime at rest: (2.4±0.3) microseconds.

[45] These results and those of the previous year were not only the first to show definitively that mesotrons are unstable, but also the first experimental confirmation of the time dilation of moving clocks predicted by relativity theory.

[48] During the summer of 1941, Greisen and physicists from Denver and Boulder accompanied Rossi to Mount Evans, where they refined the knowledge of proportionality between mesotron momentum and lifetime before decay.

At the heart of their experiment, was a "chronometer", which was an electronic circuit that produced a pulse whose height was accurately proportional to the time interval, and which could be recorded by photographing an oscilloscope trace.

[53] After the war, Rossi discovered that his Italian colleagues, Marcello Conversi and Oreste Piccioni, had performed experiments very similar to his and measured a lifetime consistent with his result.

Looking back on what he called the "Age of Innocence", Rossi wrote:How is it possible that results bearing on fundamental problems of elementary particle physics could be achieved by experiments of an almost childish simplicity, costing only a few thousand dollars and requiring only the help of one or two graduate students?

The laboratory's director, Robert Oppenheimer, asked Rossi to form a group to develop diagnostic instruments needed to create the atomic bomb.

They were assisted by approximately twenty young researchers,[58] including Matthew Sands an "electronic wizard", who later earned a PhD under Rossi, and David B. Nicodemus, whom Staub brought from Stanford University, who was an expert on particle detectors.

To address this problem, Rossi and Staub carried out a careful analysis of the pulses that result when individual charged particles create ions within an ionisation chamber.

In his history of the Los Alamos project, David Hawkins wrote: "RaLa became the most important single experiment affecting the final bomb design".

Observations on this time scale were almost beyond the state of the art in 1945, but Rossi designed and built a large cylindrical ionisation chamber whose speed of response was adequate because its coaxial electrodes were separated by a narrow gap of only 1 centimetre (0.39 in).

[68] To record the signal, he installed a very fast oscilloscope, provided as a prototype by DuMont Laboratories, in an underground bunker several hundred feet from the Gadget, where it was photographed.

In 1948, with the aid of a multi-plate cloud chamber in which lead plates alternated with aluminium ones, Gregory, Rossi and Tinlot showed that the source of the electromagnetic component of cosmic ray interactions was predominantly energetic photons, rather than electrons.

[83] In early 1953, with Bridge, Richard Safford and Charles Peyrou, Rossi published results of a comprehensive cloud chamber study of the elementary particles that became known as kaons.

Because the trees played an essential role in suppressing atmospheric convection that would degrade telescopic observations, Harvard and MIT carried out tense negotiations, until an elaborate system of fire protection was installed, and the experiment was allowed to resume.

[96] To eliminate the threat of fire, Clark, Frank Scherb and William B. Smith created a "factory" that made nonflammable plastic scintillator disks, whose thickness was 10 centimetres (3.9 in) and whose diameter was approximately 1 metre (3 ft 3 in).

[100] The most important results were summarized by Rossi as: As the Agassiz experiment came to an end, the group realized that observations near the equator and in the southern hemisphere were needed to extend their conclusion that air shower arrival directions are nearly isotropic.

[105] Because no large enough tract of open land was available near Boston, the array was constructed on a semi-desert property known asVolcano Ranch, about 16 miles (26 km) west of Albuquerque, New Mexico, at an altitude of 1,770 metres (5,810 ft).

[112] Rossi formed a subcommittee which included Thomas Gold, Philip Morrison and biologist Salvador Luria, who agreed that investigations of plasma in interplanetary space would be desirable.

A key innovation was a modulating voltage applied to one of the grids, which converted the signal into an alternating current, proportional to the proton flux and uncontaminated by any contribution of photoelectrons.

At 22 Earth radii, the spacecraft entered a region where magnetic fields were weaker and more irregular, and where a substantial flux of protons was observed coming from the general direction of the Sun.