Detection of elusive Radio and Optical emission from Cosmic-ray showers in the 1960s

TL;DR

This paper reviews the pioneering efforts in the 1960s to detect cosmic-ray showers using optical Cherenkov radiation and radio emission, highlighting early challenges and the subsequent revival of these methods with technological advances.

Contribution

It provides a historical overview of the initial attempts and challenges in using optical and radio techniques for cosmic-ray shower detection in the 1960s.

Findings

Radio emission from EAS is highly beamed, limiting detection area.

Optical Cherenkov detection faced significant background rejection issues.

Early efforts laid the groundwork for modern gamma-ray and neutrino astronomy.

Abstract

During the 1960s, a small but vibrant community of cosmic ray physicists, pioneered novel optical methods of detecting extensive air showers (EAS) in the Earth's atmosphere with the prime objective of searching for point sources of energetic cosmic gamma-rays. Throughout that decade, progress was extremely slow. Attempts to use the emission of optical Cherenkov radiation from showers as a basis for TeV gamma-ray astronomy proved difficult and problematical, given the rather primitive light-collecting systems in use at the time, coupled with a practical inability to reject the overwhelming background arising from hadronic showers. Simultaneously, a number of groups experimented with passive detection of radio emission from EAS as a possible cheap, simple, stand-alone method to detect and characterise showers of energy greater than 10^16 eV. By the end of the decade, it was shown that the radio emission was quite highly beamed and hence the effective collection area for detection of high energy showers was quite limited, diminishing the effectiveness of the radio signature as a stand-alone shower detection channel. By the early 1970s much of the early optimism for both the optical and radio techniques was beginning to dissipate, greatly reducing research activity. However, following a long hiatus both avenues were in time revived, the optical in the early 1980s and the radio in the early 2000s. With the advent of digital logic hardware, powerful low-cost computing, the ability to perform Monte Carlo simulations and above all, greatly improved funding, rapid progress became possible. In time this work proved to be fundamental to both High Energy Gamma-ray Astronomy and Neutrino Astrophysics. Here, that first decade of experimental investigation in both fields is reviewed.