Precision Measurements to Test the Standard Model and for Explosive Nuclear Astrophysics by Adrian A. Valverde

Author:Adrian A. Valverde
Language: eng
Format: epub, pdf
ISBN: 9783030307783
Publisher: Springer International Publishing

Fig. 3.2Photograph of the gas cell installed at the NSCL

3.3 Offline Ion Sources

In addition to rare isotope beam from the gas stopping area, LEBIT can also take beam from two offline production sources. The first offline source installed in LEBIT is a modified commercial ion source from the Colutron Research Corporation, which can produce stable isotopes in two ways, either through surface ionization or from a plasma. When operated as a surface ionization source, the filament in the Colutron source is heated, vaporizing either the alkali impurities present in the filament itself or an alkali earth metal loaded into the ceramic holder and inserted into the center of the heated filament coil. Once vaporized, the alkali or alkali earth metal vapor is ionized by contact with the hot filament, which is biased to ∼100 V, which causes the ions to accelerate away from the filament. These are then extracted and accelerated into the LEBIT cooler-buncher. This source has been used in this way to produce alkali earth isotopes like 48Ca [5, 6] for the study of neutrinoless double β decay, and the alkali isotopes produced from the filament (commonly 39K, 41K, 85Rb, and 133Cs) have been used as either reference ions for rare isotope measurements (e.g., as in Ref. [7]) or for the calibration of mass-dependent shifts in Penning trap mass measurements [8]. For use as a plasma ion source, a gas is injected into the chamber through a needle valve and the filament is negatively biased to ∼− 100 V, causing it to emit electrons. At a pressure of ∼10−6 mbar, a continuous discharge occurs, ionizing the gas and generating a plasma. This mode of operation can also be used to produce ions of elements with low melting points, where a plasma is generated using helium gas, and the element of interest is vaporized in a ceramic holder and then ionized by the plasma. The source has been used in this way to produce ions such as 82Se [9] for neutrinoless double β decay experiments and for the production of 14N in the direct measurement of the 14O Q EC value [10].

The second offline source at LEBIT is a laser ablation source, or LAS [11, 12]. Here, a Quantel Brilliant pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) laser with a second harmonic module produces 532 nm light with a pulse duration of 4 ns. The power is usually limited to well below its maximum, commonly achieving a density of ∼5 × 108 W/cm2. This laser is focused on a solid, rotating target, where the laser pulse vaporizes target material, and the high temperature of the target caused by the laser irradiation results in the emission of positive ions and electron, resulting in a multi-stage plasma expansion [11]. This has been used to produce ionized clusters of 12C ions, which are extremely useful as a calibrant due to the definition of the atomic mass unit, and for the production of ions of 50Ti, 50V, 50Cr [13] 96Zr [8], 113Cd [14], and 190Pt [15], all

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