Search

The idea that clusters of nucleons might exist in the nucleus has a long history and dates back to the earliest days of nuclear physics. Recently there have been significant advances in our theoretical treatment of clustering and in the experimental methods available to identify cluster structure in nuclei. These developments are reviewed with particular reference to the24Mg nucleus where a rich variety of cluster configurations is observed.

A Residue Implantation Detection System has been used in conjunction with the Daresbury Recoil Mass Separator in a search for radioactivity from the unknown nuclides108I and105Te. In the reaction of 260 MeV58Ni ions with a54Fe target two new decay lines at energies of 3.730±0.025 MeV and 3.885±0.025 MeV corresponding to cross sections of approximately 0.3 µb and 0.4 µb respectively were identified in theA=108 region of the separator's focal plane. These decay lines are tentatively assigned to the alpha decays of108I. No evidence for a proton decay branch of this nuclide could be found. In a second reaction in which a50Cr target was bombarded with 230 MeV58Ni ions, no alpha decay peak from105Te could be identified. Cross section dependent half life limits were determined from which correspondingQ-value limits were deduced.

The ground-state masses of35Si and34Si have been measured using the reactions64Ni(36S,35,34Si)65,66Zn at a36S beam energy of 198 MeV.34,35Si14+ ions were analysed and identified in a QMG/2 magnetic spectrometer and gas-filled focal-plane detector. The experimental mass excess of35Si was determined to be -14.58±0.120.07 MeV while that of34Si was measured as -19.961±0.034 MeV. A comparison is made with the results of mass model predictions.

The reaction64Ni(36S,34Si)66Zn atE=198 MeV has been used to measure the mass excess of34Si; a value of -19.971-0.044+0.037 MeV was obtained. This result which is in agreement with the previous value (-19.85±0.30 MeV) is in excellent agreement with recent shell-model mass predictions. The result is discussed within the context of the proposed region of deformation aroundZ=11,N=20.

The MP tandem Van de Graaff accelerator at the University of Rochester's Nuclear Structure Research Laboratory¿ has been employed in part since May 1977 as an ultrasensitive mass spectrometer to directly detect 14C and 36Cl from natural samples and to generally investigate many of the technical details involved in this new technique. The sensitivity that has been achieved for measuring the 14C/C ratio is 3×10-16 and for the 36Cl/Cl ratio is 3×10-15. Both the radioisotopes were detected for the first time by direct extraction methods at Rochester. Carbon sample sizes are 3 mg or less and for chlorine the compound AgCl was employed in amounts of 70 mg or less. Both the sensitivity and the sample size represent many orders of magnitude improvement over even the most advanced conventional mass spectrometric methods. In the case of 14C the carbon sample sizes required are at least a thousand times less than required by the conventional radiocarbon measuring technique. A brief summary of published results and the most recent results on both radioisotopes will be described and the use of the method for other isotopes and for other applications will be discussed.

A new atom identification and detection technique for ultrasensitive mass spectrometry has been applied to the radionuclide 36Cl. Measurements of the 36Cl/Cl ratio in AgCl precipitated from six different natural water samples fell in the range 1.2 × 10−13−2 × 10−12with a background level below 3 × 10−15. For each sample 70 mg of AgCl was used, requiring 1–51 of water.

The breakup of24Mg into16O and8Be fragments has been studied using the reactions12C(24Mg,16O8Be)12C and12C(20Ne,16O8Be)8Be. In the latter case, discrete states are observed near 24–28 MeV of excitation in24Mg and the yield from this reaction is an order of magnitude greater than that of the former. This implies the excited configuration populated in24Mg is favoured by the transfer of an alpha-particle and would therefore suggest an association with a 4-particle 4-hole configuration. This suggests a link with the octupole stabilised deformed minimum which appears in Nilsson-Strutinsky calculations of the potential energy surface in24Mg, and also with thea —16O —a structure predicted in cranked cluster model calculations. In the excitation spectrum no states appear above 31 MeV indicating a possible band termination in disagreement with recent results using the16O+12C reaction. These results are discussed in terms of the Harvey model.

The fusion reaction74Se+106Cd?180Pb* at a compound nucleus excitation energy Ex ? 40 MeV, has been used in a search for the decay of the unknown nucleus177Tl. Evaporation residues were velocity and mass analysed prior to implantation into a position sensitive silicon surface barrier detector. No evidence was discovered for the proton or alpha-decay of177Tl for a cross-section > 10-5+10 nb. This indicates either that177Tl decays too rapidly (t1/2 < 1 µs) by proton emission for the decay to be detected, or that177Tl is produced with a cross-section less than the limit established in the present experiment. The following nuclear decay half-lives were measured with improved accuracy;177Hg (t1/2 = 130±5 ms),178Hg (t1/2=250±25 ms) and177Au (t1/2=1180±70 ms). The experiment also provided the first direct confirmation of the correct mass assignment of the alpha-decay, Ea=6.26 MeV, to the decay of176Au. Two additional halflife measurements of proton-rich Osmium isotopes are also presented from a previous similar experiment;165Os (t1/2=73±8 ms) and166Os (t1/2 = 194±17 ms).