Pulse radiolysis photocathode electron guns

Table 1. Comparison of specifications of photocathode electron gun accelerators for picosecond pulse radiolysis.

Photocathode-based picosecond electron accelerators are conceptually simpler than pre-bunched thermionic systems, although they require reasonably powerful, multicomponent femtosecond or picosecond laser systems to drive the photocathode. In addition, the availability of synchronized laser pulses allows the development of advanced detection capabilities with unprecedented time resolution. The combination of ease of use and powerful detection methods has stimulated strong interest in photocathode gun systems. Since the installation ofthe first photocathode electron gun pulse radiolysis system at BNL [5,13], four additional photocathode-based facilities have become operational and two more are in progress. The operational centers include the ELYSE facility at the Universite de Paris-Sud XI in Orsay, France [7,8], NERL in Tokai-Mura, Japan [9,10], Osaka University [11,12], and Waseda University in Tokyo [13]. Facilities under development are located at the Technical University of Delft, the Netherlands, and the BARC in Mumbai, India. 

The RF photocathode electron gun is the newest type of accelerator used for pulse radiolysis. Such devices have been under development since the mid-1980s as electron beam sources for experimental physics facilities and free-electron laser development. They are typically used to produce electron beams in the 4-10-MeV range. The unique quality (low emittance and clean position-momentum relationships) of the electron beams they produce makes extremely sophisticated beam manipulation possible. 

James F. Wishart is a Chemist in the Chemistry Department of Brookhaven National Laboratory (BNL), Upton, New York. He received his Ph.D. degree in inorganic chemistry from Stanford University under Professor Henry Taube, and his S.B. in chemistry from the Massachusetts Institute of Technology. He served as project manager for the construction of the new pulse radiolysis facihty at BNL, which is based on a 10 MeV radio-frequency photocathode electron gun, the first of its Idnd in the world to be dedicated to pulse radiolysis. 

The new accelerator at Brookhaven is based on an RF photocathode gun with one or more resonant cavities in which microwaves create transient electric fields up to 1 MeV cm [104], A pulse of laser light is used for generating photoelectrons which are accelerated to 9 MeV in a distance of 30 cm. The laser pulse can also be used as the analyzing light source this means it is closely synchronized with the electron pulse. The time resolution of the electron pulse is therefore that of the laser pulse, so that subpicosecond pulse radiolysis is possible. A similar system is planned at Argonne National Laboratory [146],... 

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