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The nuclear level lifetimes in the range of picoseconds to femtoseconds cannot be measured directly with any conventional technique available to us. For such small lifetimes, the methods based on the Doppler’s effect of light have been used historically. The excited states for which the level lifetimes are of the order of a few 10s to a few 100s picoseconds, the recoil distance Doppler shift method (RDM) [1, 2] is used. In order to perform the RDM measurements, a special device, called the plunger setup is used. The success of an RDM lifetime measurement heavily depends on the quality of the target and the stopper foils used in the experiment and therefore the fabrication of a quality target in such an experiment becomes challenging. In plunger measurements always a smooth, wrinkle-free, surface target and stopper foils are required. Also, in such measurements, as the target and the stop-per foils need to be mounted and then carefully stretched on the target holder cones (to ensure smoothness of the surface), so the target and stopper foils needs to satisfy some more properties, like they should have good mechanical strength (to sustain the stretching), uniform thickness (to keep distance between the target and stopper same at all points), and good thermal conductivity (to avoid burnout of the target due excessive deposition of energy imparted by the heavy ion beam). Preparing such a high-quality target is a real challenge, and therefore, some special care is required during every step in the fabrication process of the target. In one such activity, an isotopically pure thin 128Te sandwiched target has been made at the Inter-University Accelerator Center (IUAC), New Delhi. For this purpose, a thin layer (thickness ∼ 1.1 mg/cm2) of isotopically enriched 128Te material was sandwiched be-tween an ultra-thin (thickness ∼ 0.073 mg/cm2) and a relatively thick (thickness ∼ 3.4 mg/cm2) layers of highly pure gold via the evaporation technique. The isotopic purity of the target material has been verified with material characterization techniques such as Energy Dispersive X-ray Fluorescence (EDXRF) and the Rutherford Back Scattering (RBS) techniques. The prepared target has been used successfully in a recent lifetime measurement experiment with the plunger technique to extract the level lifetimes of the excited nuclear states in the 151Dy nuclei. The results of the experiment are encouraging and indicate the overall success of the RDM lifetime measurements with the fabricated sandwiched target.
[1] T.K. Alexander and A. Bell, Nuclear Instruments and Methods 81, 22 (1970).
[2] A. Dewald et al., Progress in Particle and Nuclear Physics 67 (3) (2012) 786.