Date: september 2006
* CEN Bordeaux-Gradignan (France)
* GANIL Caen (France)
* NIPNE Bucarest (Romania)
* DAPNIA Saclay (France)
The decay of the ground-state two-proton emitter 45Fe was studied with a time-projection chamber and the emission of two protons was unambiguously identified. The total decay energy and the half-life measured in this work agree with the results from previous experiments. The present result constitutes the first direct observation of the individual protons in the two-proton decay of a long-lived ground-state emitter. In parallel, we identified for the first time directly two-proton emission from 43Cr, a known -delayed two-proton emitter. The technique developed in the present work opens the way to a detailed study of the mechanism of ground-state as well as -delayed two-proton radioactivity.
For the most proton-rich nuclei with an even number of protons, Goldanskii  predicted the occurance of a new nuclear decay mode, which he termed two-proton (2p) radioactivity. He suggested that, at the proton drip line and due to the pairing of protons, nuclei exist which are bound with respect to one-proton emission but unbound to two-proton emission. It is interesting that the list of possible 2p emitters proposed by Goldanskii contained already 45Fe, for which ground-state two-proton radioactivity was indeed discovered [2,3] only recently.
However, these experiments did not allow to observe the two protons directly. For this purpose, we constructed a time projection chamber (TPC), which enabled us to visualise the traces of the two protons in three dimensions. This information should allow to determine the decay pattern and distinguish between different schematic picture like the three-body decay or a 2He emission pattern (see Figure 1).
Figure 1: Schematic presentation of uncorrelated three-body decay and 2He emission, both rather handy but also unrealistic pictures of two-proton emission.
Experimental setup: the TPC
The 45Fe nuclei were produced at the SISSI-ALPHA-LISE3 facility of GANIL. A primary 58Ni26+ beam with an energy of 75 MeV/nucleon and an average intensity of 3 A was fragmented in a natNi target (200m) in the SISSI device. The fragments of interest were selected by a magnetic-rigidity, energy-loss, and velocity analysis by means of the ALPHA spectrometer and the LISE3 separator including an energy degrader (500m of beryllium) in the LISE3 dispersive focal plane and transported to the LISE3 final focal plane .
In the focal plane they were implanted in the TPC  (see Figure 2), where their trajectories can be visualised in 3 dimensions. Figure 3 presents an implantation event in X and Y which allows to locate the stopping position of the implanted 45Fe. By means of a fit this position is determined. The 2p emission has to start at the same position.
Figure 4 shows a few of the 45Fe events as registered with the TPC during the 2005 experiment after treatment in the analysis.
From this analysis, we can then determine the 2D projection of the 2p events (Figure 5). The third dimension which is determined from the time projection of the events is presently being analysed.
Figure 5: 2D representation of three two-proton emission events as reconstructed from the TPC signals.
Finally, from the determination of the path length and the integrated signal for each individual proton, the energy of each proton can be determined. This is hown in Figure 6.
The present experiment allowed to visualise for the first time in three dimensions the two protons emitted in two-proton radioactivity of 45Fe. From the data, we have established the equal energy sharing of the two protons and we will determine the emission angle between the two protons. Similar data have been obtained also in a MSU experiment .
 V.I. Goldanskii, Nucl. Phys. 19, 482 (1960).
 J. Giovinazzo et al., Phys. Rev. Lett. 89, 102501 (2002).
 M. Pftzner et al., Eur. Phys. J. A14, 279 (2002).
 J. Giovinazzo et al., Phys. Rev. Lett. 99, 102501 (2007).
 B. Blank et al., Nucl. Instr. Meth. B266, 4606 (2008).
 K. Miernik et al., Phys. Rev. Lett. 99, 192501 (2007).
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