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Mass Measurement of Co-51 by Using Charge Resolved Isochronous Mass Spectrometry
Update time: 2014-12-25
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Masses of short-lived nuclei paly an important role in researches of nuclear physics and astrophysics. The isochronous mass spectrometry (IMS) based on a heavy-ion storage ring has proven to be a powerful tool for direct mass measurements of short-lived nuclides. The mass of 51Co was measured for the first time at CSRe using IMS, and the importance of isospin nonconserving (INC) interactions in the fp-shell nuclei was investigated.  

Conventionally the ions stored in the ring are identified by their mass-over-charge ratios through revolution times. Since the 51Co27+ and 34Ar18+ have a very close mass-over-charge ratio (Δ(m/q)/(m/q) 5×106), they cannot be resolved by their revolution times. A new method based on the analysis of intensities of secondary electrons from the time-of-flight detector was developed which enables determination of the charge of each ion. Due to different atomic numbers of Co and Ar ions, they are nicely resolved by different signal amplitudes, as seen in Fig. 1. The mass excess of the short-lived 51Co is determined for the first time to be ME(51Co) =−27,342(48)keV by applying the new method. The accurate mass value enables us to determine the experimental one-proton separation energy Sp(51Co) =+142(77) keV. Taking into account systematic behaviour of Sp values vs mass number A in this region, 51Co is most probably the lightest proton-bound cobalt isotope with even neutron number. 

The study of Coulomb displacement energy (CDE) - the energy necessary to replace a neutron with a proton in isobaric analogue states - has been a long-standing topic in nuclear physics. It is essentially the binding energy difference of mirror nuclei, a pair of nuclei with the proton number Z and neutron number N interchanged. The behaviour of the staggering of differential ΔCDE values was used to investigate the importance of isospin nonconserving (INC) interactions in the fp-shell nuclei. The new mass of 51Co is essential for this study since it belongs to the region (A =45-51) where shell model calculations turned out to be sensitive to the inclusion of INC forces. The obtained results point to the necessity of including INC interactions in the calculations of fp-shell nuclei. 


Fig. 1 Three dimensional plot (average signal amplitude vs revolution time and vs the number of stored ions) of 51Co27+, 34Ar18+ and neighboring ions. 


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