(updated: march 2013)
Date: July 2008
The two protons emitted in the decay of 54Zn have been individually observed for the first time in a time projection chamber . The total decay energy and the half-life measured in this work agree with the results obtained in a previous experiment. Angular and energy correlations between the two protons are determined and compared to theoretical distributions of a three-body model. Within the shell model framework, the relative decay probabilities show a strong contribution of the p2 configuration for the two proton emission. After 45Fe , the present result on 54Zn constitutes only the second case of a direct observation of the ground state two-proton decay of a long-lived isotope.
The study of two proton radioactivity is a recent tool to probe the nuclear structure at the proton drip-line. In particular, studies on correlations between the two protons emitted in the decay can give information about the wave function of the emitter. Angular and energy correlations have been calculated in a three-body model developed recently . In order to study experimentally these correlations, a time projection chamber has been developed at CENBG to detect individually each proton emitted in the decay and reconstruct their tracks in 3D. After a successful experiment on the decay of 45Fe , the 2P radioactivity of 54Zn has been studied with this TPC.
The 54Zn nuclei were produced by quasi-fragmentation at GANIL. A primary 58Ni26+ beam with an energy of 74.5 MeV/nucleon was fragmented in a natNi target. The fragments were selected by a magnetic-rigidity, energy-loss, and velocity analysis by means of the LISE3 separator. Two silicon detectors located at the end of the spectrometer allowed to identify individually the fragments by means of an energy-loss and time-of-flight analysis. The fragments were finally implanted in the TPC. The TPC is a gas detector where the heavy ions are stopped and their decay observed. The electrons, produced by the slowing down of either the incoming ions or the decay products, drift in the electric field of the TPC towards a set of four gas electron multipliers (GEM) where they are multiplied and finally detected in a two dimensional strip detector. The analysis of energy signals allows to reconstruct the tracks of the particles in two dimensions; the drift time analysis gives the third one. Details can be found in .
Figure 1 shows an example of a 2P event from the decay of 54Zn. Energy signals (top) corresponding to the energy loss of the protons along their tracks are shown as a function of the strip number in X direction (left) and Y direction (right). These spectra were analyzed by fitting a function which was showed to be a good approximation of the Bragg curve (more details in ). This first analysis allowed to determine the projection of the tracks on the detection plane. The second step consists in the analysis of the time signals (bottom of figure 1) which gives the third component Z of each track. In total, seven 2P decay events could be reconstructed in three dimensions.
Figure 1: Top: Energy signals as a function of the strip number in X direction (left) and Y direction (right). The black line corresponds to the fit of the spectrum, the solid and dashed vertical lines correspond to the starting and stopping points of the tracks, respectively. Bottom: Corresponding time spectra for each dimension. Only the Y dimension is analysed because the tracks of the two protons are not distinguishable on the X dimension.
The energy fraction distribution of the individual protons as determined from the energy signals is plotted in the left part of figure 2. As expected in a simultaneous emission, the two protons share the decay energy equally in order to favour the barrier penetration. Concerning the width of the distribution which depends strongly on the charge of the emitting nucleus, the experimental one is in a very good agreement with predictions of the model.
The complete analysis of the decay events allowed to measure angular correlations between the protons. The top right part of the figure 2 shows the experimental angular distribution. The bottom right part of figure 2 shows the predictions of the three-body model for different configurations of the two protons in the initial nucleus. W(p2) corresponds to the weight of a p2 configuration compared to the f2 configuration. A quantitative comparison has been done between the experimental and theoretical distributions for 54Zn, yielding W(p2)= 30+46-22%. The standard shell model predicts a W(p2) of 80% but considering the very high error bars due to poor statistics, the interpretation of the result is extremely limited.
Figure 2: Left: Experimental energy distribution compared to the predictions of the three body model (red line). Top right: Experimental angular distribution. Bottom right: Predicted angular distributions for different mixing of configurations of the two protons in the emitter.
Another observable accessible from this experiment was the half-life of the nucleus 54Zn, determined from the time difference between an implantation event and its subsequent decay as 1.59+0.60-0.35 ms. This value is in agreement with the one determined in a previous experiment . By combining the values of the half-lives and the branching ratios of both experiments, the experimental partial half-life for the 2P branch is 1.98+0.73-0.41 ms. The theoretical half-life has been calculated by combining the three-body model which is adapted to treat the dynamics of the 2P emission and the shell model which is more appropriate to describe the nuclear structure. Therefore, the spectroscopic factors of the shell model and the partial half-lives from the three-body model were used to determine the relative decay probabilities of the two l2 configurations, leading to a predicted half-life of 1.6 ms. This value is in excellent agreement with the experimental one obtained in this work.
In summary, the two protons emitted in the decay of 54Zn were observed for the first time in a TPC. Energy and angular distributions could be obtained and allowed a first rough comparison with theoretical models yielding information about nuclear structure. This experiment showed that the study of 2P radioactivity can be a tool to probe the structure of nuclei at the limit of existence. However, to establish a detailed picture of the decay process and extract precise information about nuclear structure, higher statistics of implantation-decay events are needed, which can be obtained in future experiments. In parallel, improvements of theoretical model predictions are essential to elucidate the decay mechanism which governs two-proton radioactivity
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