The highest-energy cosmic-rays – the past, the present and the future

Abstract

The greater part of this paper is concerned with a historical discussion of the development of the search for the origins of the highest-energy cosmic-rays together with a few remarks about future prospects.Additionally, in section 6, the situation with regard to the mass composition and energy spectrum at the highest energies is discussed. It is shown that the change of the depth of shower maximum with energy above 1 EeV, measured using the Telescope Array, is in striking agreement with similar results from the Auger Observatory. This implies that either the mean mass of cosmic rays is becoming heavier above ~4 EeV or that there is a change in details of the hadronic interactions in a manner such that protons masquerade as heavier nuclei. A long-standing controversy is thus resolved: the belief that pure protons dominate the mass distribution at the highest energies is no longer tenable.  Corresponding author: a.a.watson@leeds.ac.uk After World War I, the work of Hess was greatly extended by the German physicist, Erich Regener. At a Royal Society Discussion Meeting in 1931 [3] Ernest Rutherford noted that 'there was now reliable data on cosmic ray absorption in water thanks to the far-reaching experiments of Regener'. Shortly after, Regener pioneered balloon flights in which Geiger counters were carried to great altitudes. He encased his equipment in cellophane to protect against low temperatures: his equipment cooled only to 17⁰ C at the highest altitudes. This was an important technical advance and solved the problems of temperature stability that had blighted similar work by Millikan. In 1935 Regener and his student Pfotzer, succeeded in flying three Geiger counters, arranged to limit the solid angle for detection to within ±20⁰, to an altitude of 22 km (~40 g cm -2 ) [4], the altitude limit probably being set by the destructive interaction of ozone with the rubber balloons. At an over-burden of ~130 g cm -2 they discovered that the coincidence rate maximised, a feature commonly referred to as the 'Pfotzer maximum'. While Pfotzer did follow up and extend this work alone, after Regener had lost his position at the University of Stuttgart because of his political views and because his wife was Jewish, it seems appropriate to refer to this, at the very least, as the Regener-Pfotzer maximum [5].This discovery, combined with the transition curves reported by Rossi [6], were explained by Bhabha and Heitler [7] who used the newly invented quantum electrodynamics to describe shower phenomena. Schmeiser and Bothe [8] were the first to point out that Rossi's curves implied that there should also be showers in airnaming them 'Luftschauer' -and showed that particles in showers were separated by up to 40 cm. Clearly Regener's ionisation curve forced the same conclusion. Kolhörster et al [9] extended the separation over which coincidences were found to nearly 80 m. Quite independently, and somewhat serendipitously, Pierre Auger and his colleagues also discovered the air-shower phenomenon. Auger's assistant, Roland Maze, a...

How, where and when do cosmic rays reach ultrahigh energies?

“…Together with Scarsi, Linsley measured the energy spectrum above 1 EeV and even performed an analysis of arrival directions with a sample of 97 events. Since these pioneering results, decades of experimental and theoretical work have been dedicated to understanding the phenomenology and physics of UHECRs (see historical reviews by Watson [2,3])the highest energy particles in nature. Despite Herculean efforts, the sources of UHECRs are not yet known, nor is the physics of their acceleration understood.…”

“…The current state of the art UHECR observatories are the Pierre Auger Observatory (PAO), in Malargüe, Argentina (detection area ≈ 3000 km 2 ), and the Telescope Array (TA) in Millard County, Utah, USA (detection area ≈ 700 km 2 ). Both observatories have been critical for measuring the spectrum, composition and anisotropy of UHECRs over the past decade.…”