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First observation of 2p radioactivity in the decay of 45Fe - 2001

Date: July 2001



CENBG, University of Warsaw, GSI Darmstadt, Oak Ridge National Laboratory, GANIL Caen, University of Edinburgh


Two-proton emission modes

Ground-state two-proton (2p) emission can proceed in three different ways:
o sequentially: one proton is emitted after the other
o simultaneously with out any particular correlation. This is called three-body decay
o simultaneously with a strong angular and energy correlation between the two protons. It is called 2He emission


Figure 1: Different decay mode for 2p emission: sequential, three-body decay, 2He emission


History of 2p searches

* ground-state two-proton (2p) emission was predicted by Goldanskii in 1960 [1]
* modern theories predicted 45Fe, 48Ni, and 54Zn to be the best candidates [2,3,4]
* 45Fe was first identified at GSI in 1996 by our group [5]


Figure 2: First identification of 45Fe in an experiment at GSI in 1996. Together with 45Fe, 42Cr and 49Ni were also observed for the first time.


* a first decay study was attempted at GANIL in 1999. Due to problems with the data acquisition the decay could not reliably studied. Nevertheless counts at about 1.1 MeV where the 2p signal is expected were observed, however, could not be interpreted as such [6].


Production and selection of exotic nuclei with the FRS at GSI

* projectile fragmentation of 58Ni primary beam at 600 MeV/nucleon in FRS target
* selection of exotic species with fragment separator FRS
* implantation in detection setup consisting of silicon detectors surrounded by a NaI barrel


Figure 3: Principle of fragment production at the FRS by projectile fragmentation and separation


Detection set-up


Figure 4: The detection set-up consists of seven silicon detectors for energy-loss and residual energy measurements, for decay measurements and a NaI barrel for γ detection


Identification of fragments of interest

* measurement of up to energy loss, residual energy, time-of-flight, and position in focal plane of the FRS
* representation as two-dimensional matrix
* identification of 6 implantations of 45Fe [6]


Figure 5: Isotope identification for the setting on 45Fe. Six events of 45Fe were implanted in a 6 days experiment.


Correlation of implantation of exotic nuclei and their decays * implantation in silicon detector telescope
* correlation in time and in location of decays after a preceding implantation of a well identified 45Fe isotope
* almost background-free decay spectra for this exotic isotope


Figure 6: Decay energy spectrum for 45Fe. The counts at about 1.1 MeV are due to 2p decay of 45Fe with a half-life of 3.2 ms. The event at 6.1 MeV is due a β-delayed decay of one 45Fe isotope.


Decay scheme of 45Fe


Figure 7: 45Fe decay with an 80-90% branch by 2p emission, whereas about 10-20% of the time it decays by β-delayed decay modes. Half-life and decay energy are also indicated. The data from this experiment are in agreement with the GANIL experiment on the same subject [7]


Future studies

* higher statistics data for 45Fe
* search for new 2p emitters like 48Ni, 54Zn
* measurement of more detailed information for 2p emitters like individual proton energies and relative emission angle between the two protons


Physics subjects to be studied with 2p radioactivity

* test mass predictions beyond the proton drip line
* determine single particle level sequence beyond proton drip line
* study the j-content of the wave function
* study pairing in atomic nuclei
* test models for tunnelling with changing deformation


[1] V.I. Goldanskii, Nucl. Phys. 19, 482 (1960)
[2] B.A. Brown, Phys. Rev. C43, R1513 (1991)
[3] W.E. Ormand, Phys. Rev. C53, 214 (1996)
[4] B.J. Cole, Phys. Rev. C54, 1240 (1996)
[5] B. Blank et al., Phys. Rev. Lett. 77, 2893 (1996)
[6] M. Pfützner et al., Eur. Phys. J. A14, 279 (2002)
[7] J. Giovinazzo et al., Phys. Rev. Lett.89, 102501 (2002)