Proton-Proton Correlations Observed in the Two-Proton Radioactivity of the (21+) Isomer of 94Ag

The (21) isomer occurring in the lightest known isotope of silver, Ag, has properties that are unmatched in the entire nuclear chart. It is characterised by a long halflife of 0.39(4) s [1], a high spin [2] and, although its dominant disintegration modes are β-delayed γ-ray [2] or proton [1] emission, the excitation energy of 6.7(5) MeV [3] makes direct one-proton and two-proton (2p) radioactivity possible. The experimental results on the latter two decay modes have recently been published [3,4], those on the 2p radioactivity will be summarised here. The 2p radioactivity of the (21) isomer was studied at the GSI on-line mass separator by using a Ni(Ca,p3n) fusion-evaporation reaction. After ionisation and acceleration to 55 keV, the A=94 ions were mass separated and implanted into a tape positioned in the centre of an array of segmented silicon and composite germanium detectors. The tape was periodically removed from the implantation position in order to reduce the build-up of long-lived daughter activity. While the silicon detectors were used to record protons the germanium crystals served to 'tag' on known γ-ray transitions in the two-proton daughter, Rh. In this way a 2p branch was discovered, having a decay energy of 1.9(1) MeV and a decay probability of 0.5(3) % which corresponds to a partial half-life of about 80 s. The cross-section for producing the 2p radioactivity in the fusion-evaporation reaction amounts to about 350 pb [5]. The experimental proton-proton correlation data are displayed in Fig. 1 together with predictions obtained from the break-up model [5-8]. The spectra displayed in Fig. 1 were derived out of the Si1-Si2-γ-γ coincidence matrix, with a total of 19 events fulfilling the triple condition set on two Rh γ-transitions and on the 2p sum-energy in the range of 1.8-1.95 MeV. All proton-proton events shown in Fig. 1 are normalized by using their respective solid angles of the detectors. This leads to a normalization of the histogram and therefore the intensity is given in arbitrary units. The experimental uncertainties are calculated as weighted sums of the standard deviations of the respective proton-proton counts. In general, 2p decay can proceed through sequential proton emission involving intermediate Pd states, or through a simultaneous three-particle decay mechanism. The former decay should result in narrow peaks in the spectrum shown in Fig. 1b whose energies depend on the Figure 1: Correlations observed in the 2p decay of the (21) isomer of Ag. Relative-energy spectra for protonproton (Ep-p) and proton-Rh (Ep-Rh) correlations are shown by histograms in a and b, respectively. The solid curves are the predictions of our model of simultaneous proton emission from a deformed nucleus convoluted with an experimental uncertainty of 200 keV. The dashed line represents the fit obtained with a sequential emission mechanism. The dashed-dotted curve shows the calculated distribution when the 2p decay is isotropic in the absence of the mentioned decay mechanisms. NUSTAR-SPECTROSCOPY-08