SISAK Liquid-Liquid Extraction Experiments with Preseparated 257Rf

Abstract

The SISAK liquid-liquid extraction system was used to extract 4.0-s 257 Rf. The 257 Rf was produced in the reaction 208 Pb( 50 Ti, 1n) 257 Rf with 237-MeV beam energy on target, separated in the Berkeley Gas-filled Separator (BGS) and transferred to a gas jet using the Recoil Transfer Chamber (RTC). The activity delivered by the gas jet was dissolved in 6-M HNO3 and Rf was extracted into 0.25-M dibutyl-phosphoric acid in toluene. This was the first time a transactinide, i.e., an element with Z ≥ 104, was extracted and unequivocally identified by the SISAK system. Thus, this pilot experiment demonstrates that the fast liquid-liquid extraction system SISAK, in combination with liquid- scintillation detectors, can be used for investigating the chemical properties of the transactinides. The extraction result is in accordance with the behaviour shown by the Rf group IV homologues Zr and Hf. and output. 2-4 During recent years, work has been undertaken to deploy SISAK in studies of the chemical properties of the transactinide elements (Z ≥ 104). 2, 5, 6 The only suitable detection method found was liquid-scintillation (LS) detection. 7 This was mainly because of the rather high flow rates (0.5-2.0 mL/s) encountered in the SISAK system. Other detection methods usually require either thin, dry samples or very thin liquid films. No suitable method was found to prepare such samples without an unac- ceptable time delay between separation and detection. LS counting has high efficiency, but suffers from relatively poor energy resolution (about 300-keV FWHM at 7-MeV α en- ergy) and is sensitive to β particles and γ rays. This puts very stringent demands on the separation system, because most of the β- and γ-emitting contaminants are produced with orders of magnitude higher yield than the transactinide under study. A huge effort was made to develop methods for LS detection of very low levels of α activity under these conditions. The most important techniques implemented are: • Pulse-shape discrimination is used to reduce the β back- ground in the α spectra by a factor >1000. Without this tech- nique the β- and γ-induced background from the surroundings and from nuclides produced in the target will interfere with the α spectrum to such an extend that it becomes useless. 8 • Real-time scintillation-yield monitoring by continuous mea- surement of the Compton edge of 662-keV 137 Cs γ rays. The position of the Compton edge is used to automatically adjust the energy calibration. Based on this adjustment the event gates used for switching the detection cells between mother and daughter mode (open or closed valve, respectively) will always be set on the correct energy range. 9