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Accueil du site > ANGLAIS > Research > Exotic Nuclei > Research topics > Experiments > GANIL (Caen) > Two-proton radioactivity of 54Zn, 48Ni, and 45Fe (E312c, E312d, E312e) - 2004


Two-proton radioactivity of 54Zn, 48Ni, and 45Fe (E312c, E312d, E312e) - 2004

Date: April - May 2004

Collaboration

CENBG, GANIL Caen, IAP Bucharest, University of Santiago de Compostella, IPN Orsay, KU Leuven, USTHB Alger

 

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]
* its was observed recently by our group and collaborators [5,6] in the decay of 45Fe
* new searches turned their attention therefore to 48Ni and 54Zn

 

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

 

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

 

Identification of fragments of interest

* measurement of up to 8 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 2: Isotope identification for two settings: left for 54Zn with the identification of seven 54Zn; right for 48Ni with three 48Ni identified. The plot for 54Zn contains only runs where a 54Zn isotope was detected. The production rate was 2 per day for 54Zn, 1 per day for 48Ni, and 10 per days for 45Fe

 

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
* background subtraction by means of time characteristics of decaying isotope
* almost background-free decay spectra for all identified exotic isotopes

 

Figure 3: Decay energy (left) and decay time (right) spectra for 54Zn. The peak at about 1.36 MeV is due to 2p decay of 54Zn with a half-life of 3.6 ms. The red events are events without any b particle detected. The green events are second decay events, i.e. decays of the 2p daughter nucleus 52Ni. The black events are decays where the beta particle was explicitly observed.

 

Figure 4: Decay energy (left) and decay time (right) spectra for 45Fe. The peak at about 1.14 MeV is due to 2p decay of 45Fe with a half-life of 1.6 ms. The red events are events without any beta particle detected. The green events are second decay events, i.e. decays of the 2p daughter nucleus 43Cr. The black events are decays where the beta particle was explicitly observed.

 

Decay of 48Ni

* two decays resemble b-delayed decays. A β particle is observed in one of the adjacent silicon detectors
* third event has all characteristics of a 2p emission
* 48Ni could decay by a weak 2p branch
* more statistics needed to confirm or infirm

 

Comparison with theory for 54Zn

* di-proton model calculates emission of structure-less 2He particle through a Coulomb barrier in a relative s state
* 3-body model of Grigorenko [7]uses realistic proton-proton and proton-core interactions and assumes emission from a p or f orbital

 

Figure 5: 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.

 

Future studies

* higher statistics data for 45Fe, 48Ni, and 54Zn
* search for new 2p emitters like 59Ge
* 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 6: 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.

 

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