CNRS Université Bordeaux

Accueil du site > ANGLAIS > Research > Exotic Nuclei > Research topics > Experiments > GANIL (Caen) > First observation of 2p radioactivity in the decay of 45Fe (E312b) - 2000

First observation of 2p radioactivity in the decay of 45Fe (E312b) - 2000

Date: June - July 2000

Collaboration :

CENBG, GANIL Caen, IAP Bucharest, University of Warsaw


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]
* 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].


Figure 2: First decay energy spectrum correlated with the implantation of 45Fe. The counts at 1.1 MeV are most likely due to 2p radioactivity; however, they could not be interpreted as such.


Production and selection of exotic nuclei with SISSI/ALPHA/LISE at GANIL:

* projectile fragmentation of 58Ni primary beam at 75MeV/nucleon in SISSI target
* selection of exotic species with ALPHA spectrometer and LISE3 separator
* implantation in detection setup consisting of silicon detectors surrounded by germanium detectors


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


Detection set-up:


Figure 4: The detection set-up consists of micro-channel plate detectors for time-of-flight measurements, silicon detectors to measure energy loss and time-of-flight, a double-sided silicon strip detector as the implantation device, a Si(Li) detector for β particles and Germanium detectors for γ detection


Identification of fragments of interest :
* measurement of up to 10 parameters like energy loss, residual energy, time-of-flight, and position in focal plane of LISE
* representation as two-dimensional matrix
* purification with other parameters
* almost background-free identification


Figure 5: Isotope identification for the setting on 45Fe. Up to ten parameters were measured to identify unambiguously the isotopes of interest. 22 45Fe were implanted in a 36 h experiment.


Correlation of implantation of exotic nuclei and their decays:
* implantation in double-sided silicon strip detector (16*16 strips)
* correlation in time of decays in the same x-y pixel after a preceding implantation of a well identified isotope
* almost background-free decay spectra for all identified exotic isotopes [7]


Figure 6: Decay energy spectrum for 45Fe. The peak at about 1.14 MeV is due to 2p decay of 45Fe with a half-life of 4.7 ms. The red events are events without any β particle detected. The blue events at high decay energies are decays of the 2p daughter nucleus 43Cr.

* β particles were searched for in coincidence with the decay of events in the 2p peak, however, none of them has a coincident β particle


Figure 7: Spectrum from the Si(Li) detector in coincidence with the 2p peak from the implantation detector (a). No signal above the detector noise is observed. For a similar analysis of the neighbouring 46Fe nucleus (b), β particles above an energy of 500 keV are due to a β-delayed proton emission mode for the decay of this nucleus. Insets show the decay energy spectrum and the full-statistics b spectrum for the two nuclei.


Additional pieces of evidence are :
o no broadening of the 2p peak due to pile-up with the energy loss of the β particle in the implantation detector
o the decay characteristics of the daughter decay, i.e. half-life and decay energies, are in agreement with the known decay characteristics of 43Cr
o measured decay energy agrees nicely with theoretical predictions [2,3,4]

A few events with decay energies above 6 MeV are observed, an indication that 45Fe decays partially by β-delayed modes.


Decay scheme of 45Fe:


Figure 8: 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. These results are in agreement with the GSI experiment [8]

Comparison with theory

* di-proton model calculates emission of structure-less 2He particle through a Coulomb barrier in a relative s state
* 3-body model of Grigorenko [9] uses realistic proton-proton and proton-core interactions and assumes emission from a p or f orbital


Figure 9: Comparison with two models: The di-proton model which predicts half-lives (right axis) much shorter than the experimental value for a given decay energy ET. Due to the fact that this model does not contain any nuclear structure, it is expected to give only a lower limit. The three-body model predicts half-lives in reasonable agreement with experiment for a p-wave emission of the two protons. However, the protons are expected to be emitted mainly from f orbitals. But a small contribution from a p wave may be sufficient to dominate the decay.


Future studies

* higher statistics data for 45Fe, 48Ni, and 54Zn
* 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 need for a time projection chamber TPC


Figure 10: Schematic view of the TPC under construction at the CENBG for a detailed study of 2p emitters. By means of a time projection of the proton tracks on a two-dimensional detector a 3D view of an event can be obtained.


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] J. Giovinazzo et al., Eur. Phys. J. A10, 73 (2001)
[7] M. Pfützner et al., Eur. Phys. J. A14, 279 (2002)
[8] J. Giovinazzo et al., Phys. Rev. Lett.89, 102501 (2002)
[9] L. Grigorenko et al., Phys. Rev. C64, 054001 (2001)