First Direct Observation of Two Protons in the Decay of ${}^{45}\mathrm{Fe}$ with a Time-Projection Chamber

The decay of the ground-state two-proton emitter ${}^{45}\mathrm{Fe}$ 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 ${}^{43}\mathrm{Cr}$, a known $\beta$-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 $\beta$-delayed two-proton radioactivity.

Figure 1

Schematic representation of the time-projection chamber developed at the CEN Bordeaux-Gradignan. The nuclei of interest traverse first two silicon detectors (one standard detector with a thickness of $150\mu \mathrm{m}$ and a second position-sensitive detector ($150\mu \mathrm{m}$) with resistive readout) and are then implanted in the active volume of a gas-filled chamber, where they decay by emission of charged particles. The charge cloud created by the energy loss of either the heavy ions or the decay products drifts in the electric field of the chamber towards a set of four GEMs (not shown), where the charges are multiplied. The electric potential finally directs the charges onto a two-dimensional detection plane, where two orthogonal sets of 768 strips (pitch of $200\mu \mathrm{m}$) allow a measurement of the charges deposited on each strip. Every second strip is equipped with an ASIC readout yielding signal height and time stamp. The other half of the strips is connected in groups of 64 strips to standard preamplifiers and shapers.

Figure 2

Upper part: The figure shows a typical implantation event for ${}^{45}\mathrm{Fe}$. The ion enters parallel to the $X$ strips (left figure) and stops at a certain depth in the $Y$ direction (right figure). This plot allows one to determine the stopping position of the ion, where then the decay has to occur. Lower part: The decay of ${}^{45}\mathrm{Fe}$ takes place by two-proton emission. The decay tracks start, within the position resolution of about 6–7 mm (FWHM), at the stopping point of the implantation event described above. The arrows are indicative only of the tracks of the two protons. A horizontal arrow means that the proton propagates perpendicular to the strips, whereas a vertical arrow indicates that the proton track is parallel to the strips. The shaded zone corresponds to the error bars due to the gain matching procedure.

Figure 3

The figure shows three of the ten decay events observed after implantation of ${}^{45}\mathrm{Fe}$. We have chosen here events where the two protons can be clearly identified. For two of the ten events, the two protons cannot be separated, as they fire the same strips.

Figure 4

(a) Total decay energy of the ${}^{45}\mathrm{Fe}$ events as determined with the last GEM. The low-energy events (in black) show the ${}^{45}\mathrm{Fe}$ decay, whereas the high-energy events are from a triple-$\alpha$ source. The energy of the ${}^{45}\mathrm{Fe}$ events corresponds roughly to the expected decay energy of 1.15 MeV [13]. (b) Half-life of ${}^{45}\mathrm{Fe}$ as determined from the time difference of the implantation events and the decay events (black events). The half-life is ${3.5}_{-0.8}^{+1.6}\mathrm{ms}$. The events in gray are the daughter-decay events which were not included in the fit.

Figure 5

Upper part: Implantation event of a ${}^{43}\mathrm{Cr}$ nucleus in the TPC. The nucleus is identified by means of energy loss and time-of-flight measurements. Lower part: Decay of ${}^{43}\mathrm{Cr}$ as observed with the TPC. The traces of the two protons can be seen in the $X$ and $Y$ directions.