secondary emission

secondary emission. It is a phenomenon where a primary incident particle, such as an electron or ion, strikes a material surface and causes the emission of secondary particles. This process is fundamental to the operation of various electronic devices, including photomultiplier tubes and image intensifiers. The efficiency of the process is quantified by the secondary emission yield, which depends on the energy and angle of the primary particles as well as the properties of the target material.

Physical mechanism

The physical mechanism involves the transfer of energy from the primary particle to electrons within the conduction band of the material. These excited electrons, known as secondary electrons, can then travel to the surface and escape if their energy exceeds the work function. Key figures like Léon Brillouin and Nevill Francis Mott contributed to the theoretical understanding of electron interactions in solids. The process is influenced by the band gap in semiconductors and the Fermi level in metals, with significant research conducted at institutions like the Cavendish Laboratory. The emission can be enhanced by using materials with a low work function, such as cesium-coated surfaces, or specific compounds like magnesium oxide.

Applications

This phenomenon is exploited in numerous practical devices across scientific and industrial fields. The photomultiplier tube, a critical component in instruments like the Hubble Space Telescope and experiments at CERN, relies on it for signal amplification. It is also essential for the function of image intensifiers used in night vision equipment by the United States Army and in electron multipliers for mass spectrometry. Furthermore, it plays a role in the operation of certain types of cathode-ray tubes and can cause unwanted effects in high-power klystrons and traveling-wave tubes used in radar systems. Research into applications continues at facilities like the Stanford Linear Accelerator Center.

Measurement and characterization

The key parameter for characterization is the secondary emission yield, typically measured using specialized ultra-high vacuum systems to prevent surface contamination. Experimental setups often involve directing a focused beam from an electron gun onto a sample and measuring the resultant current with a Faraday cup. Pioneering measurements were made by scientists such as Philipp Lenard and Robert Andrews Millikan. Modern analysis employs techniques like Auger electron spectroscopy and is supported by organizations like the American Institute of Physics. The yield curve, which plots yield against primary energy, shows a characteristic peak, often studied for materials like aluminum and copper.

History and development

The effect was first observed in the late 19th century during experiments with cathode rays. Significant early investigations were conducted by J. J. Thomson at the University of Cambridge and by Karl Ferdinand Braun. Theoretical advancements followed in the 20th century with the work of Hans Bethe on electron energy loss and Lev Davidovich Landau on electron interactions. The development of the dynode structure for practical amplification is credited to Vladimir Zworykin and his team at RCA. Wartime research during World War II, particularly for radar, greatly accelerated its technological application, with contributions from the Massachusetts Institute of Technology Radiation Laboratory.

Types of secondary emission

Several distinct types are categorized based on the nature of the primary incident particle and the emitted species. True secondary emission involves low-energy electrons ejected directly from the material. Backscattered emission consists of higher-energy primary particles that elastically reflect from the surface. Ion-induced secondary emission occurs when the primary particle is an ion, a process relevant to sputtering and analysis with SIMS. Photon-induced secondary emission, or photoemission, is triggered by ultraviolet or X-ray photons. Each type has distinct yield characteristics and applications, studied in contexts ranging from scanning electron microscope imaging to the Van Allen radiation belt. Category:Electron phenomena Category:Electronic engineering