|Robert Oppenheimer, 1958|
“At the Institute for Advanced Study in Princeton, Dr J Robert Oppenheimer jumped for me, the arm outstretched and the hand extended toward the ceiling. ‘What do you read in my jump?’ he asked,” wrote Halsman. “’Your hand pointed upward,’ I hazarded, ‘maybe you were trying to show a new direction, a new objective.’” But the theoretical physicist denied any symbolism. “’No,’ said Dr Oppenheimer, laughing, ‘I was simply reaching.’” (Credit: Philippe Halsman/Magnum Photos)
The lunar Askaryan technique is a method to study the highest-energy cosmic rays, and their predicted counterparts, the ultra-high-energy neutrinos. By observing the Moon with a radio telescope, and searching for the characteristic nanosecond-scale Askaryan pulses emitted when a high-energy particle interacts in the outer layers of the Moon, the visible lunar surface can be used as a detection area. Several previous experiments, at Parkes, Goldstone, Kalyazin, Westerbork, the ATCA, Lovell, LOFAR, and the VLA, have developed the necessary techniques to search for these pulses, but existing instruments have lacked the necessary sensitivity to detect the known flux of cosmic rays from such a distance. This will change with the advent of the Square Kilometre Array. The SKA will be the world’s most powerful radio telescope. To be built in southern Africa, Australia and New Zealand during the next decade, it will have an unsurpassed sensitivity over the key 100 MHz to few-GHZ band. We introduce a planned experiment to use the SKA to observe the highest-energy cosmic rays and, potentially, neutrinos. The estimated event rate will be presented, along with the predicted energy and directional resolution. Prospects for directional studies with phase 1 of the SKA will be discussed, as will the major technical challenges to be overcome to make full use of this powerful instrument. Finally, we show how phase 2 of the SKA could provide a vast increase in the number of detected cosmic rays at the highest energies, and thus to provide new insight into their spectrum and origin.
The problem of searching for highest-energy cosmic rays and neutrinos in the Universe is reviewed. Possibilities for using the radio method for detecting particles of energies above the Greisen–Zatsepin–Kuzmin (GZK) cut-off are analyzed. The method is based on the registration of coherent Cherenkov radio emission produced by cascades of most energetic particles in radio-transparent lunar regolith. The Luna-26 space mission to be launched in the nearest future involves the Lunar Orbital Radio Detector (LORD). The potentialities of the LORD space instrument to detect radio signals from showers initiated by ultrahigh-energy particles interacting with lunar regolith are examined. The comprehensive Monte Carlo calculations were carried out within the energy range of 1020 to 1025 eV with the account for physical properties of the Moon such as its density, lunar-regolith radiation length, radio-wave absorption length, refraction index, reflection from the lower regolith boundary, and orbit altitude of a lunar satellite. The design of the LORD space instrument and its scientific potentialities for registration of low-intense cosmic-ray particle fluxes of energies above the GZK cut-off up to 1025 eV are discussed, as well. The designed LORD module (including the antenna, amplification, and data-acquisition systems) now is under construction. The LORD space experiment will make it possible to obtain important information on the highest-energy particles in the Universe, to verify modern models for the origin and the propagation of ultrahigh-energy particles. It is expected that the LORD space experiment will surpass in its apertures and detection capability the majority of well-known current and proposed experiments that deal with the detection of both ultrahigh-energy cosmic rays and neutrinos. The future prospects in the study of ultrahigh-energy particles by orbital radio detectors are also considered, namely, a multi-satellite lunar systems and space missions to largest ice planets of the solar system.
The Moon provides a huge effective detector volume for ultrahigh energy cosmic neutrinos, which generate coherent radio pulses in the lunar surface layer due to the Askaryan effect. In light of presently considered lunar missions, we propose radio measurements from a Moon-orbiting satellite. First systematic Monte Carlo simulations demonstrate the detectability of Askaryan pulses from neutrinos with energies above 1020 eV, i.e. near and above the interesting GZK limit, at the very low fluxes predicted in different scenarios.
Prospects for Lunar Satellite Detection of Radio Pulses from Ultrahigh Energy Neutrinos Interacting with the Moon(Submitted on 10 Apr 2006 (v1), last revised 15 Feb 2007 (this version, v2))