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ULTRAFAST CHEMICAL SEPARATIONS
The SISAK system can be manipulated to study specific longer-lived nuclides. Björnstad and coworkers [Bjö77a] have reported the study of 14.2-min 143La using the SISAK system. The precursor of 143La has a half-life of 14 s, while 144La, the longest-lived heavier lanthanum isotope, has a half-life of 42 s. The chemical separation system they used is shown in Fig. 37. Three mixer-centrifugal separator units were used. As in the study of 144-146La, the fission products carried by the gas-jet stream were dissolved and degassed; in the first unit, the lanthanides and other fission products were extracted by 0.3M HDEHP. The aqueous phase from unit 1 containing the precursor of 143La flowed through a delay unit of 15 s before entering the second mixer-centrifugal separator unit. The delay allowed 50 to 60% of the 143Ba to decay to 143La. In the second unit, lanthanum was extracted into 0.3M HDEHP; the organic phase flowed through a 200-s delay line before entering the third separator unit. This delay allowed 144–146La to decay. In the third unit, the organic phase was contacted with an oxidizing aqueous phase; lanthanum (143La) was back-extracted, leaving cerium (144Ce) in the organic phase.
Figure 37.Chemical separation system for the isolation of143La. C1, C2, C3 = mixer-centrifugal separation units; D11, D12 = delaylines; GJ = gas jet (N2+ C2H4); M = static mixer for gas and liquid; DG = degassing unit; NG =noble gases; FP = fission products; C = counting cell (Dowex-50WX4, 50–100 mesh); D = detectors. [Bjö77a; reprinted with permission fromJ. Inorg. Nucl. Chem.]
The technetium procedure reported by Broden and coworkers [Bro81] shows the wide applicability of the SISAK technique in nuclear studies. Stachel and coworkers [Sta84] utilized the technetium procedure to perform γ-γ angular correlation measurements on 5-s 108Tc. Figure 38 shows a schematic of the technetium separation procedure they used. The fission products carried by the KCl aerosol in the gas jet were dissolved in 0.1 M HNO3 – 0.1 M KBrO3. The solution was heated to 80°C to accelerate the oxidation of technetium to pertechnetate. Technetium was extracted by 0.05M Alamine-336 in CHCl3 in the first mixer-centrifugal separator unit. The organic phase contained zirconium, niobium and molybdenum. In the second stage, technetium was stripped using 2M HNO3. Traces of zirconium and niobium were also stripped (<1%). The angular correlation measurements were performed with three detectors. The total transport time was 5 s. Additional decontamination can be achieved by including a mixer-centrifugal separator stage to extract zirconium and niobium with 0.5M HDEHP in CHCl3; this increased the transport time to 7 s.