Proton radioactivity from deformed nuclei

One of the most exciting subjects in contemporary nuclear physics is the search for the nuclear stability limit. Recent experiments have measured the proton decay of well-deformed nuclei such as 140,141 Ho and 131 Eu. All the proton emitters with Z < 63 and with Z > 67 are expected to have strongly deformed ground states. A detailed analysis of deformed proton emitter 57<Z<69 in the framework of the relativistic Hartree-Bogoliubov theory (RHB) has been presented by Vretmer et.al. (P.R.L. 82, 4595 (1999)). The RHB results from proton separation energies are found to be in very good agreement with recent experimental data on direct proton decay of 131 Eu, 141 Ho and 147 Tm. However, in the region 50 < Z < 64 the predicted proton emitters are expected to be quite deformed in their ground state, among them 117 La is predicted to have β2=0.29. We measured the 117 La decay by proton emission at the Legnaro National Laboratory. In this case the reaction was 64 Zn(58 Ni,p4n)117 La at 310 MeV; the target was a 1 mg/cm2 self supporting foil. The Tandem-LINAC accelerator delivered the beam with an average intensity of 1.5 pnA. The experimental setup used the Recoil Mass Spectrometers with its maximum solid angle acceptance (10 msr) together with a DSSD placed at 1 m downstream of RMS focal plane. Combining the RMS A/q selectivity the spectral energy information given by the DSSD and an absolute clock (4 MHz) for half-life measurements it was possible to measure the decay properties of nuclei with half lives between ∼ 20 μs and few seconds. DSSD energy calibration was obtained from an α source with three peaks, a pulser signal and a known proton decay. The latter one was the 147 Tm p decay which presents two protons (1051 MeV and 1131 MeV and was produced after (p,2n) evaporation from the fusion reaction 58 Ni+92 Mo (ENi 261 MeV). An example is given of data from the calibration run on 147 Tm. The low energy peak is the ground state proton decay of 147 Tm. Calculations performed for a deformed proton emitter reproduce quite well the experimental results confirming that 117 La is strongly deformed (β2 = 0.3). Spin and parity of the two proton-decaying levels have been determined as well: 3/2+ for the ground state and 9/2+ for the Ex 151(12)keV excited state. Another main objective of our project was to search for new strongly deformed proton emitter in order to get further information on this nuclear region. As we emphasized before in the region 50 < Z < 67 the predicted proton-emitters are expected to be quite deformed in their ground state, this making their interpretation more difficult due to the complexity of the emitted proton wave function. Anyhow, a complete explanation of the proton-radioactivity process should be able to describe this decay as a function of deformation parameter. In this case one of the most significant goal of the project was to obtain information referring to the decay by proton emission of the proton very rich 126 Pm exotic nucleus. The experiment has been carried out at the Legnaro National Laboratory. The reaction was 58 Ni(74 Se,p5n)126 Pm at 340 MeV. The beam was delivered by the Tandem + LINAC accelerator with an average intensity of 1.5 pnA. The experimental setup used the RMS with a DSSD placed at 1 m downstream of the focal plane. The DSSD has (40x40) strips, 1 mm wide and a thickness of 60 μm. Among all the recoils focused on the RMS focal plane, implanted in the DSSD were only those falling in the range (A-1)q - (A+1)q. The signals were treated differentially in the two sides: in vertical direction (x side) strips are acquired to get both position and energy information, while horizontal direction (y side) all strips are read together. From the experimental data Qp = 819 ± 9 keV was obtained. Though it is not a high statistics peak, its analysis gives positive results; we also made calculations using mass models predictions by Moeller and Nix and obtained Qp = 0.869. In summary, the proton radioactivity of the very rich proton nuclei 117 La and 126 Pm has been studied characterizing the energy and the half-lives of the measured decays. (authors)