For simplicity reasons, the ion production process is disregarded and the detection efficiency εtot of the apparatus is defined as the ratio of detected ions Ndet vs. the
number of ionsNion existing inside the cell at the beginning of each measurement cycle
εtot =
Ndet
Nion
. (5.5)
Assuming no chemical or charge exchange reactions occur during the ion drift, this efficiency can be factorized into 4 contributions according to
εtot =εex·εQP IG·εQM S ·εdet. (5.6) εex is the extraction efficiency discussed before. For the electrode potentials listed in
Tab. 5.1, this amounts to (8±1)% for the 0.5 mm nozzle at 40 mbar argon. As mentioned in Sect. 5.1.4, the interplay between the gas flow and the electrostatic field inside the nozzle cone seems to have a big impact on the extraction efficiency. From this point of view, εex is expected to decrease when working at relatively
εQP IG stands for the ion guiding efficiency. Ions trapped inside the QPIG are effi-
ciently guided through the different pumping sections due to the gas cooling mechanism, which forces the ions to stay at the minimum of the quadrupolar po- tential. However, ions may hit the first QPIG segments while they get extracted in the supersonic gas stream inside the extraction chamber. Therefore, εQP IG is
expected to be in the range between 40 % and 100 % similarly to the guiding efficiency claimed in Ref. [51].
εQM S is the QMS transmission efficiency. It decreases with increasing mass resolution,
due to the fluctuations in the ion oscillations inside the quadrupole potential of the QMS. This efficiency depends also on the ion mass [89] such that it should be determined for each measurement series. At a moderate QMS resolution, εQM S
is estimated to be (30±10)% as has been obtained in Ref. [50].
εdet denotes the detection efficiency of the channeltron detector in conjunction with
the deflection electrodes and the lens system shown in Fig.5.8. In former investi- gations [50], this efficiency was found to have a best value of (56±5)%, which is assumed to hold also for the described apparatus with the optimized potentials listed in Tab. 5.2.
Hence, the overall efficiency is expected to be in the range between (0.5±0.2) % and (1.3±0.5) % at a cell pressure of 40 mbar argon when using the 0.5 mm nozzle, which still allows for mobility measurements using primed filaments in off-line experiments [51,
113]. However, further improvements and tests have to be performed in order to be efficient enough for mobility investigations in the region of actinides and transactinides in online experiments. The ion extraction and detection efficiency achievable can be increased by about a factor of 10 if working at higher cell pressures and using larger extraction nozzles. To achieve this goal, an efficient pumping of the detection part of the apparatus is mandatory.
6 Investigation of the time resolution
by RIS methods
Extensive test experiments using laser resonance ionization techniques at certain lan- thanide elements are carried out to determine the time resolution of the spectrometer in advance of systematic mobility studies. Special emphasis is put on the influence of the cell pressure and of the laser beams on the time resolution as well as on the arrival time distributions. A sizable part of this chapter is dedicated to discuss the consequences of such an influence and to present the strategy followed in this work to guarantee mobility measurements of high precision.
6.1 Creation of sample ions by RIS methods
In order to obtain sample ions in off-line experiments, the elements of interest should be available in a pure form of macroscopic quantities. Using electrochemical deposition techniques [114], lanthanide as well as actinide (up to fermium) filaments can be pro- duced. The elements, usually in the form of hydroxides in solutions, are deposited on a tantalum carrier foil of 25−50µm thickness and subsequently covered by a 1−2µm titanium layer. More details on this method and its benefits for laser spectroscopic investigations at heavy elements can be found in Ref. [114] and [115]. Another alterna- tive may be the vacuum evaporation-condensation techniques described in Ref. [116], which actually allow for the production of radioactive targets for nuclear accelerator experiments. The evaporant is heated in the evaporation source up to a temperature which causes the generation of a vapor cloud. This cloud propagates in vacuum towards the carrier foil, where it is condensed in form of a thin film of desired area densities. More details on this topic can be found in Ref. [117, 118].
Since this work focuses on the mobility of lanthanide elements, neither of both tech- niques has been extensively tested with the developed spectrometer. Instead commer- cially available lanthanide foils of 99.9% purity [71] are used. In the mobility measure- ments carried out, the sample foil is fixed between two clamps of the filament mounting through which a current of <5 A can flow (see Fig. 6.14). The power supply (DELTA Elektronika, ES030-5) used for filament heating is galvanically isolated from the net- work such that a filament potential can be applied according to Fig.3.3 (c). Usually 25µm thin foils (25 mm x 1 mm) are used in order to minimize the heat output inside the drift cell. During mobility experiments, the filament temperatures are determined with an uncertainty of ≤ 50 K using a pyrometer (Keller Pyro, type: PB06 AF3).
Figure 6.1: Left: Laser resonance ionization spectrum for 168Er at 19 mbar buffer gas
pressure (E/n = 1.2 Td). Right: Mass spectrum when ionizing erbium (E/n= 2.4 Td, 20 mbar Ar).
Exploiting the Resonance Ionization Spectroscopy method described in Ref. [30, 53], an almost background-free ionization can be easily achieved, see also Sect. 3.2. With this method, the evaporated atoms of interest are resonantly ionized using laser beams of suitable wavelengths provided by the excimer-pumped dye laser system described in Sect. 4.2. The ionization process takes place only once per single laser shot of ∆t ≈15 ns duration, which allows for a precise determination of the starting time t0. The such created ions drift in a homogeneous electric field inside the cell, get extracted, mass selected by the QMS and subsequently detected by the channeltron detector (see Sect. 4.1 and Sect. 5.2).