Introduction

Abstract

Just over a century ago cosmic rays were discovered. Around the turn of the C20th it was recognised that the Earth’s atmosphere contained ions but the consensus was that these originated from radiation coming from radioactive elements in the Earth’s crust. Which was largely true. A dramatic confirmation of this comes from gamma-ray detectors launched under balloons, which experience a rapid reduction in counting rate as they move up a few hundred metres through the atmosphere. In 1912 Victor Hess took three electroscopes underneath a balloon up to an altitude of ∼ 5 km and discovered that the radiation level at altitude was around three times that on the ground. By the end of the 1920s it was generally agreed that the extra radiation was coming from beyond the Earth. An exciting new field was born.