Furnace ion source with electron ionization for calibrating a spectrometer for precise determination of masses of superheavy elements

Abstract

Precise determination of the isotope masses of superheavy elements obtained by the complete fusion of 48Ca+ ions with Au, Pb, Bi, U, and Pu targets can provide valuable information on the mechanisms of nuclear reactions leading to the formation of these elements. A specialized multi-turn time-of-flight mass spectrometer is to be used to match the goal. The source of the target isotopes is a DC-280 cyclotron, a «factory of superheavy elements» at the Joint Institute for Nuclear Research (JINR, Dubna, Russia). The point of the problem is that ions of superheavy elements are formed quite rarely even in the specialized accelerator of JINR: during the last experiment, the frequency of their appearance was up to 10 events per day. Precise determination of the mass number requires comparing the time of flight of a superheavy ion through a mass analyzer with the same time for a calibrant ion, the mass number and charge of which are precisely known a priori from the process of its formation. Implementation of such a mass spectrometric analysis required the development of an ion source capable of stable operation for a long time. To solve the problem, a number of ion sources were considered. The choice of an ion source with electron ionization is substantiated. A source with a crucible in which the calibrant evaporates during heating was proposed and tested. Various substances (PbBr2, Nd, C60, fullerene soot) were considered as a calibrant. The mass spectra of these compounds are presented, namely, the spectrum of neodymium compounds and the mass spectrum of fullerene soot. It is shown that the use of fullerene soot is optimal, since the mass spectrum of fullerene soot contains ions in the range of mass numbers from 12 to 800 and higher. Moreover, the mass spectrum of fullerene soot contains many peaks in the mass number region of interest 275 – 300 a.m.u. It has been experimentally shown that the ion intensities within the indicated range differ by less than an order of magnitude which makes the use of fullerene soot a standard sample for calibration of a high-resolution time-of-flight mass spectrometer.