On the Observations of the Penetrating Radiation during Seven Balloon Flights

Abstract: At the beginning of the twentieth century, the Austrian (later naturalized American) Victor Hess among others developed a brilliant line of research, leading to the final determination of the extraterrestrial origin of part of the atmospheric radiation. Before his work, the origin of the radiation today called "cosmic rays" was strongly debated, as many scientists thought that these particles came integrally from the crust of the Earth. There was however an active and rich research on the topic. Victor (at that time Viktor) Hess measured in 1912 the rate of discharge of an electroscope that flew aboard an atmospheric balloon. Since the discharge rate increased as the balloon flew at higher altitude, he concluded that the origin of part of the natural radiation could not be terrestrial. For this discovery, Hess was awarded the Nobel Prize in 1936, and his experiment became legendary. The fundamental article published by Victor Hess in 1912, reporting a significant increase of the radiation as altitude increases, is translated and commented here. The decisive measurement was performed in the last of the seven flights on aerostatic balloon described.

• The paper demonstrated that ionization rates increased with altitude, providing early evidence of cosmic radiation.
• The paper employed three distinct detectors during balloon flights up to 5350 m to differentiate between gamma and beta rays.
• The paper's findings challenged terrestrial radiation explanations and paved the way for future cosmic ray research.

Insights into V.F. Hess's Balloon Experiments on Penetrating Radiation

Victor F. Hess's 1912 paper detailing his observations during seven balloon flights represents a pivotal moment in the study of atmospheric radiation. Conducted in response to conflicting hypotheses regarding the source of ionizing radiation, these experiments offered empirical data suggesting that at least part of this radiation is extraterrestrial in origin. The following discussion outlines the key findings and implications of Hess's work.

Experimental Setup and Methodology

Hess utilized three distinct radiation detectors with varying wall thicknesses to segregate the impacts of different types of radiation. This meticulous approach aimed to differentiate $\gamma$ rays, which were expected to penetrate the thick-walled chambers, from softer $\beta$ rays addressed by a thinner-walled device. By conducting measurements at varied altitudes up to 5350 meters, Hess collected comprehensive data on ionization rates at different atmospheric layers.

Prior to ascents, baseline ionization rates were established on the ground. An intriguing assumption was applied to some measurements, adjusting for atmospheric pressure changes associated with altitude. Although this adjustment was deemed somewhat uncertain, unreduced data were analyzed for additional insights.

Key Observations

The results from these flights yielded several observations:

1. Ionization Trends with Altitude: A discernible increase in radiation was observed at altitudes above 1000 meters, contrasting with the expected decrease if radiation solely originated from the Earth's crust. At the highest altitude reached during the seventh flight, ionization rates exceeded ground-level values by a factor of over two in some cases.
2. Cosmic Origin Hypothesis: The patterns of increasing radiation contradicted the premise that radioactive substances in the Earth's crust or even atmospheric layers were solely responsible. The data strongly indicated an additional source of highly penetrating radiation entering the atmosphere from above, consistent with what would later be known as cosmic rays.
3. Influence of Meteorological Variations: Despite potential fluctuations due to atmospheric conditions, consistent results were achieved across different trials, reinforcing the robustness of the observed trends.
4. Temporal Fluctuations: Some temporal fluctuation in radiation was noted, hinting at dynamic changes in these cosmic radiations. However, the frequency and magnitude of such fluctuations were not sufficiently characterized.

Theoretical Implications and Legacy

Hess's conclusions challenged the then-dominant view that all penetrating radiation was terrestrial, opening the door to a pioneering understanding of cosmic radiation. The realization that cosmic rays significantly contribute to atmospheric ionization revolutionized theories around atmospheric electricity and radiation physics.

A curious finding was the apparent lack of solar influence, as neither solar eclipse experiments nor diurnal variations showed differences in radiation levels, tentatively ruling out the sun as the direct source of cosmic radiation.

Future Directions

The implications of Hess's work spurred further investigations into the nature of cosmic rays, including their composition, propagation, and impact on the atmosphere. The advent of more advanced detection technologies in subsequent years allowed for more precise studies of cosmic phenomena, solidifying the foundation laid by Hess. His experiments paved the way for groundbreaking discoveries in particle physics and astrophysics, influencing our understanding of high-energy processes beyond our planet.

Conclusion

Victor Hess's insights, derived through his systematic and rigorous balloon experiments, provided compelling evidence for the existence of cosmic radiation, ultimately culminating in his receipt of the Nobel Prize in Physics in 1936. His work illustrates the power of in-situ experimentation in unraveling the complexities of the natural world, with long-lasting impacts that continue to echo through scientific inquiry into cosmic phenomena.