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Fragmentation of 58Ni (E312) - 1998

Date: November 1998


* CEN Bordeaux-Gradignan (France)
* IReS Strasbourg (France)
* Univ. Brighton (UK)
* Univ. Liverpool (UK)
* Univ. Surrey (UK)
* Univ. Warsaw (Poland)
* GANIL Caen (France)



This experiment on proton-rich nuclei in the A=50 region had three main purposes :

* the search for doubly-magic 48Ni, the only Tz=-4 isotope predicted by mass models, and a good candidate to 2-proton radioactivity ;
* the study of 45Fe decay that is also a very good candidate for 2-proton radioactivity and for which many delayed particle emission channels are energetically open (beta-p, 2p, 3p, 4p, p-alpha, etc.) ;
* the delayed proton emission of Tz=-2 nuclei between Cr and Ni, with gamma coincidence measurements, to get precise structure information on level positions that may lead to mass determinations via the isobaric multiplet mass equation.


Experimental setup

We used a 58Ni beam (73 MeV/A) on a nickel target for production of ions to be sent in the LISE spectrometer. In addition to the standard LISE equipment, we used the following detectors :

* a silicon telescope for energy loss and time-of-flight measurements
* a double sided (x-y) silicon strip detector in which ions are implanted. This detector had a size of 48x48 mm with 16x16 channels and is used for a pixel correlation between implantation and decay signal in order to decrease the background from accumulated activity
* a germanium detector array (4 clovers and a LEPS counter) with a total efficiency of about 10%.


We performed two time-of-flight (ToF) measurements : between the cyclotron high-frequency signal or a micro-channels plate etector (in D4) and the Si telescope (in D6). Since the expected counting rates for 48Ni and 45Fe were very low, this experiment requested a 1 uAe primary beam and the use of SISSI. But the beam intensity was one order of magnitude lower and there were some troubles with SISSI tests, so that we had to run without it. Under these conditions there was no way to hunt for 45Fe and 48Ni.

Data analysis Nevertheless, we could get new data concerning proton emission from proton-rich isotopes in this mass region. This part is under analysis. In addition, we could run for a few hours with a LISE setting optimized on very proton-rich zinc isotopes. The time of flight (HF to D6) versus energy loss (ToF-dE) identification plot clearly shows the existence of 56Zn and 55Zn. The inset of the figure presents this ToF-dE matrix with a filter on 55,56Zn isotopes. The filters are obtained with a fit of Z=f(dE,ToF) and Tz=g(dE,ToF), and reduced to 90% to reject a possible overlap region. In addition, half of the events do not appear on this plot because we selected events with a proper ToF measured with the micro-channel plate detector that had only a 50% efficiency.


ToF versus dE matrix

A few pictures