Spectroscopy of Very Neutron Rich Nuclei Produced in the QuasiFragmentation of a 36S Beam

• Introduction
• Experimental Details
• Results
• Collaborators

Nuclei in the vicinity of the neutron drip line are weakly bound and exhibit unique physical properties, which are of considerable interest for testing theories describing exotic nuclei. Steady progress has been achieved in mapping the location of the neutron drip line recent years but the basic decay properties such as half-lives and beta-delayed neutron emission probabilities are undetermined for almost all of the exotic species and even less is known about their spectroscopy. These important measurements can reveal details of the underlying microscopic structures which can provide a test of modern shell model calculations.

Experimental Details

   An experiment was therefore carried out at GANIL using the LISE3 Spectrometer which can separate out nuclei according to their:

• magnetic rigidity (effectively A/Z),
• time of flight (TOF) through the spectrometer which corresponds to N/Z,
• total kinetic energy and energy loss in a silicon detector (proportional to Z).

(Nuclei contain Z protons and N neutrons and have mass number, A, equal to N+Z.)

   A telescope containing six silicon detectors was used to measure the energy loss and the time of flight of the nuclei which were implanted in the fifth detector. The exotic nuclei were produced by fragmenting a high energy (77.7 MeV/A) beam of the rare isotope ³⁶S and then selecting these fragments using the LISE3 spectrometer. Owing to the high energies the nuclei are fully stripped of their electrons. The diagram above demonstrates the different forms of selection possible and using these it is possible to pick out the different nuclei. A typical nuclide identification spectrum with LISE3 optimised for ²⁶F is shown here, where the x-axis is TOF and the y-axis corresponds to Z. Each cluster of events corresponds to a distinct nuclide. Other spectrometer settings were also used to optimise for other nuclei of interest.

   The region of nuclei studied with this experiment is the heavy nitrogen, oxygen, fluorine, neon, sodium and magnesium isotopes. A chart of the nuclei produced is shown above, from which it can be seen many exotic nuclei were produced in the A/Z=3 region. Since hardly anything is known of the spectroscopy of the nuclei in this region or their beta-decay properties, the silicon telescope was surrounded by forty two individual ³He proportional counters for detecting beta-delayed neutrons and four 70% germanium detectors for measuring the energies of beta-delayed gamma-rays.

Results

   The data are currently being analysed at Liverpool, Orsay and Dubna, with the analysis at Liverpool concentrating on the beta-delayed gamma-decays of ^25,26F and ^27,28Ne which are being studied for the first time in the present work.

   Preliminary gamma-ray spectra are shown for ²⁶F, ²⁸Ne and ³¹Na. The beta-delayed gamma-rays for ³¹Na are already known and these are marked on, as is the 511keV line.

The analysis of these data is ongoing at the moment.

Collaborators

    This work was a collaboration between: University of Liverpool, IPN Orsay, FLNR-JINR Dubna, LPC Caen, GANIL, IAP Bucharest, NPI Rez and University of Surrey.

The Liverpool group members are: Alan Reed, Robert Page, Roger Allatt, Alex Keenan and Paul Nolan.

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Last modified: 20 September 1999