Abstract
To convincingly demonstrate the viability of MAGIS for enriching stable isotopes in a scalable manner, we needed to measure substantial Li-7 enrichment at throughputs that could scale to macroscopic quantities simply by scaling the apparatus while collecting a sizable fraction of Li-7 feedstock. With MAGIS requiring little energy consumption (particularly requiring no energy consumption for magnetic deflection), our goal was to achieve enrichment and throughput on par with the calutron. In this chapter, we outline a thorough set of measurements using a variety of tools that benchmarks MAGIS performance against the calutron. A quadrupole mass spectrometer together with laser-induced fluorescence imaged onto a CCD yield Li-7 enrichment beyond the magnet array, while a surface-ionization detector and quartz crystal thickness monitor yield absolute throughput and deflection efficiency.
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Notes
- 1.
The ionization probability appears to be particularly sensitive to the partial pressure of oxygen. In our case, given the high operating temperature for the ribbon and low vapor pressure in the magnet chamber, we suspect our ionization probability to be less than 1 % based on previous work by Delhuille et al.
- 2.
In this case, the isotope-specific throughput must be weighted by the relative abundances given by the enriched Li-6 in the source.
- 3.
Figure 4.10 shows the source-to-aperture and source-to-cross distances for G3 while Fig. 4.11 gives measurements using G2. The distance between the heated aperture and the cross is the same in both cases, and the source-to-aperture distances are short enough for both sources that the aperture on the gasket at the cross entrance should be the feature defining the beam width.
- 4.
Equivalently, the nozzle might not be co-axial with the remaining beam line. Maintaining square joints during welding can be challenging.
- 5.
The outermost measurements in Fig. 4.11 might also be an artifact of the sensor itself. We sometimes observed a larger-than-expected deposition rate (particularly when measuring the sensor response function) under certain conditions upon immediately following a high-rate measurement. For this reason, we typically tried to make measurements from positions giving lower deposition rates to positions yielding higher rates.
- 6.
The discrepancy between these points could be a consequence of several factors. For instance, we took the first set of data just after re-loading and degassing the oven. Contaminants in the source might have contributed more heavily when taking the first set of measurements.
- 7.
This relationship is valid immediately around the center of the beam line where the gasket apertures do not interfere with the cosine-dependence of the angular distribution for trajectories originating at the heated aperture [3].
- 8.
The lower efficiency in this case likely stems from the oven position being further away from line-of-sight.
- 9.
Uncertainty in the isotopic fraction of Li-7 of just 1 % around the nominal value can yield large uncertainties when extracting a value for Li-6 suppression (with this ultimate uncertainty worsening for better Li-6 suppression).
- 10.
Part of this offset might be due to slight hysteresis in the stepper motor upon reverting the wire detector back to its initial position upon completing a scan. We usually manually check the starting position (using a position readout on the linear actuator) prior to running, but occasionally will miss a slight offset in starting position.
- 11.
For analog scans, we use software provided by SRS for operating the RGA. For single-mass measurements, we use an NI LabVIEW program that we developed for controlling the RGA.
- 12.
In fact, during its procedure the RGA selects the largest ion current within the 0.6 amu window (not computing an average).
- 13.
For these measurements we use custom software (built using NI LabVIEW) that samples single masses.
- 14.
We used σ − polarization in this case (defining a quantization axis accordingly) in an effort to truly optically pump atoms into the \( F = 3/2 \), \( \mbox{ m}_{\mbox{ F}} = -3/2 \) state.
- 15.
We temporarily introduced a polarizing beamsplitter cube in front of the waveplate. Using a power meter, we adjusted the waveplate to balance the powers at the outputs of the cube. We then removed the cube.
- 16.
The pressure should be given as the pressure at the source aperture which can be related to the pressure in the source using geometrical factors [4].
References
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Mazur, T.R. (2016). Measurements. In: Magnetically Activated and Guided Isotope Separation. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-319-23956-9_4
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