Probe measurements of the electron distribution function in an electron-beam-produced ytterbium plasma
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Probe Measurements of the Electron Distribution Function in an ElectronBeamProduced Ytterbium Plasma A. A. Bobrova, A. E. Dubinov, M. I. Esin, S. V. Zolotov, A. N. Maksimov, V. D. Selemir, I. I. Sidorov, and A. Yu. Shubin AllRussia Research Institute of Experimental Physics, Russian Federal Nuclear Center, pr. Mira 37, Sarov, Nizhni Novgorod oblast, 607188 Russia Received April 29, 2010
Abstract—A nonequilibrium anisotropic plasma produced by an electron beam in the residual air with a low content of ytterbium vapor was investigated by the probe method. It is found that a minor (at a level of a few ppm) admixture of ytterbium to lowpressure air substantially modifies the electron energy distribution func tion (EEDF): the main peak corresponding to thermal electrons broadens, and new peaks appear. It is shown that the observed change in the EEDF is caused by the low ionization energy of ytterbium, due to which one beam electron can ionize several ytterbium atoms. The new peaks in the EEDF correspond to the final ener gies of a beam electron after each subsequent ionizing collision with ytterbium atoms. DOI: 10.1134/S1063780X10121013
1. INTRODUCTION
At present, a production line (the Afina facility) for laser enrichment of natural ytterbium with the Yb168 isotope is being created at the Russian Federal Nuclear Center, AllRussia Institute of Experimental Physics. In this facility, atomic vapor enters a special vacuum chamber, where it is ionized, thereby forming a steadystate plasma.
The Yb169 radioactive isotope is very promising for the use in medicine. Thus, it finds application in car diology, entering into the composition of stents implanted permanently in blood vessels because of its capability of emitting Xrays into the surrounding tis sues over a fairly long period of time (hν = 50– 308 keV, 〈hν〉 = 93 keV, and T1/2 ~ 32 days), due to which risk of various complications is reduced sub stantially. The Yb169 isotope is also used in oncology as a component of the needles inserted into cancerous growths in order to diminish and destroy them. Unfortunately, Russia is far behind developed countries in providing people with services of radionu clide therapy, including those utilizing Yb169. There fore, it is very challenging to create new domestic cen ters accumulating the Yb169 isotope in order to increase its overall yield.
The process of ytterbium evaporation, ionization, and separation requires permanent monitoring of the parameters of vacuum, vapor, plasma, and products deposited on the collectors. For this purpose, the facil ity is equipped with various diagnostics, which are tested using auxiliary setups, the physical conditions in which are very close to those in the Afina facility. These setups are also used to train the staff and con duct physical experiments. The goals of the present study, which was carried out in the framework of this program, were to elabo rate and adjust probe diagnostics and experimentally study specific features of the electron energy distribu tion f
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