High-spin rotational excitation in173Hf

Abstract

As is well known, in-beam y-ray spectroscopy offers a means of observing transitions from nuclear states with large spin values, which cannot be excited in most radioactive decays. Since the high spin of the compound nucleus produced in (i, xn) reactions results from the absorption of the angular momentum of the incident particle, heavyion beams (mainly ~-particle beams) are generally used. Here we want to report an instance in which a beam of protons was employed with an acceptable measure of success. The nuclide we chose to study was 173Hf. This odd-A odd-N nucleus belongs to a region of permanent nuclear eccentricity and, therefore, can provide information on the excitation of rotational bands. As yet, no data are available on high-spin states in 173Hf, nor in other odd-A neutron-deficient nuclides with the same number of neutrons. A sample of natural Lu (97.41~ 175Lu and 2.590/0 17SLu) was exposed to the external proton beam of the Milan AVF cyclotron. XTaHf was produced through the 17SLu(p, 3n) reaction and the y-rays associated with it were observed by means of a 35 cm 3 Ge(Li) spectrometer. The 25 mg/cm 2 thick target and the detector were carefully shielded with lead to reduce radiation background. Energy and efficiency calibrations of the spectrometer were made with standard sources; additional reference for the energies was provided by some well-known transitions in 174Hf, which was produced via the 175Lu(p, 2n) reaction in the same experimental set-up. The energy of the proton beam was varied from 20 to 40 MeV in steps of 2 MeV, in order to determine with satisfactory accuracy the excitation functions of the y transitions. The knowledge of these functions was relied upon to discriminate the y-rays associated with the (p, 3n) reaction against those arising from competing activities. The study of the same functions contributed also helpful, even if not definite, information on the angular momenta of the de-exciting levels, since for increasing momenta the functions are shifted toward higher energies. On account of the anisotropy of the emitted y-rays, the detector was placed at a laboratory angle of 126 ~ with respect to the beam. With this choice, second-order terms in the Lcgendre-polynomial expansion of the angular distribution are compensated. A summary of results is presented in Table I, where the energies and intensities of the observed y-rays are given. In Fig. 1, a typical y spectrum is shown, for the incident-proton energy of 26.1 MeV. Examples of excitation functions of some y peaks are reported in Fig. 2.