Michigan State University, East Lansing MI48824, USA INTRODUCTION

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1 Two-Proton Decay of the First Excited State of 17 Ne M.J. Chromik 1;2,P.G. Thirolf 1;2, M. Thoennessen 1, M. Fauerbach 1, T. Glasmacher 1, R. Ibbotson 1, R.A. Kryger 1, H. Scheit 1, and P.J. Woods 3 1 National Superconducting Cyclotron Laboratory and Department of Physics & Astronomy, Michigan State University, East Lansing MI48824, USA 2 Sektion Physik, Ludwig-Maximilians Universitat Munchen, D-8748 Garching, Germany 3 Department of Physics, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom Abstract. The rst excited state of 17 Ne has been populated via intermediate energy Coulomb excitation with a radioactive beam of 17 Ne on a 197 Au target to search for two-proton decay, which is in competition with the -decay back to the ground state of 17 Ne. The reconstructed invariant mass spectrum of the outgoing O in coincidence with two protons shows evidence for a two-proton transition from the rst excited state in 17 Ne as well as for transitions from higher excited states in 17 Ne, which will decay via the emission of two sequential protons. INTRODUCTION So far all experimental attempts to identify two-proton radioactivity at or near the proton dripline have been unsuccessful (e.g. [1]). A promising candidate is 17 Ne, where the rst excited state (J =3=2,,E =1:288 MeV) is bound by 168 kev with respect to one-proton emission but unbound with respect to two-proton emission by 344 kev (for details see [2,3]). Therefore this state can decay via a simultaneous emission of protons to O, because the widths of the low lying states in 16 F are too small (' 40keV) for a sequential decay through the tails. The two-proton decay is in competition with the -decay to the ground state of 17 Ne. In a recent intermediate energy Coulomb excitation experiment the -decay from the rst excited state to the ground state (J =1=2, ) has been measured and the experimental yield has been compared to the theoretically expected cross section. The measured -ray yield accounts for only 43% of the predicted yield from an excitation cross section of 18.7 mbarn, thus encouraging the investigation of a potential two-proton decay branch [2]. EXPERIMENTAL PROCEDURE The experiment to search for the two-proton decay of 17 Ne was performed at the National Superconducting Cyclotron Laboratory at Michigan State University. A 60MeV/u radioactive 17 Ne beam was produced from a primary Ne beam using the A10 fragment separator. A Wien lter was used to further purify the secondary beam and % pure beam with an intensity of' Ne particles/s was achieved. In order to identify the two-proton decay from the rst excited state in 17 Ne a complete reconstruction of the decay kinematics in the center-of-mass system (CM) was necessary. Thus the interaction point on the target as well as the energies and directions of all outgoing decay particles had to be measured. Fig. 1 shows the experimental setup. Two position sensitive parallel plate avalanche counters (PPAC) in front of the target served to determine the interaction point of the 17 Ne in the target and a thin plastic scintillator 40m upstream was used for time-of-ight (ToF) measurements to identify the incoming particle. The target was surrounded by the MSU-NaI-array [4] to simultaneously measure the -ray decay. The particle fragments were analyzed in a multiple stage particle telescope, which was positioned at 0 relative tothe

2 Pin 16x16 Si 17 Ne 60 MeV/A O, 2p 40x40 Si PPAC Au-Target Si(Li) CsI-array FIGURE 1. Experimental Setup beam axis. The position of the heavy fragment was measured in a 00m thick 4040 double-sided silicon strip detector and the E in a Si-Pin Diode (00m), which also delivered a time signal. The heavy fragments were then stopped in a mm thick Si(Li)-Detector. The light fragments penetrated these detectors and were then detected in a 0 m thick 1616 double-sided silicon strip detector, which served as a E-detector as well as for position measurements. Finally the protons were stopped in a 44 CsI array, (consisting of 16 crystals 1.7cm1.7cm x cm, read out by photo diodes). These detectors were packed in a close geometry and placed 16.3cm behind the target, thus covering an opening angle from 0 to 7. A 6.cm thick lead collimator (with a central opening adapted to the size of the 4040 strip detector) was placed in front of the detectors in order to shield the surrounding NaI-array from -rays originating from interactions with the detector material. DATA ANALYSIS AND RESULTS The incoming 17 Ne particles had to be identied and separated from the % of contamination which primarily consisted of O. Fig. 2(a) shows the clear separation in the ToF spectrum between the plastic scintillator (40m upstream) and the Si-PIN. In addition the 17 Ne was spatially separated from the O after a bending magnet at the end of the Wien-lter so it also could be separated in the 4040 strip detector (Fig. 2(b). FIGURE 2. Particle Identication of the incoming beam. The incoming 17 Ne was identied with TOF-measurements (a) and the position in the 4040 strip detector (b). In the two-dimensional E(PIN)-E(Si) spectrum (not shown) it is possible to identify O fragments from the ( 17 Ne, Opp) reaction and separate them clearly from the 17 Ne beam. After identifying the O from the reaction it is necessary to determine two-proton events. In Fig. 2 the sum of the E in all the strips of the 1616 strip detector is plotted against the sum of the deposited energy in all CsI detectors. In a spectrum gated on incoming 17 Ne only (a), one can clearly see the proton- and the

3 deuteron-band, and barely the triton-band. An intensive band, consisting of events with twice the E and E-values of the proton band can be identi ed with the two-proton band. At even larger energy losses and energies one can also see the 3He and 4 He bands. An additional condition requiring single (Fig. 2(b)) or double hits (Fig. 2(c)) in the veri es the identi cation of the two-proton band He 4 He Total E in 16x16 /MeV Double-Particle Events in the 16x16 Single-Particle Events in the 16x16 Total E in 16x16 /MeV Total E in 16x16 /MeV Ungated p p 0 d p 2p d FIGURE 3. Sum of the E in all the strips of the strip detector versus the sum of the deposited energy in all CsI detectors with no gates (a), one particle gate (b) and two-particle gate (c). Using the geometric correlation between events in the strip detector and the CsI-array one can then extract the energies and directions for each proton. With the information of the energies of the outgoing fragments and their trajectories one can perform the transformation into the CM-System in order to obtain the invariant mass spectrum which is shown in Fig. 4(a). The Gaussian t curves indicate the preliminary assigned peaks, the lowest one at an energy of 29 40stat 0syst kev corresponding to the simultaneous two-proton decay of the rst excited state in 17Ne. FIGURE 4. Decay energy spectrum (a) and decay scheme (b) of 17 Ne. The measured decay energies agree with the known values []. The arrows between 2 and 3 MeV indicate positions of known transitions, which could not be resolved. The energy resolution is on the order of 00keV, mainly dominated by the error in the determination of the interaction point on the target. Fig. 4(b) shows the decay scheme of 17Ne. The levels of 17Ne up to 6 MeV are from []. In order to transform the decay energies from Fig. 4(a) to the excitation energies one has to add the two-proton separation energy of 944keV. The full circles indicate states that were identi ed in the present experiment. Two new states at 6.3 MeV and 7.6 MeV were observed. In the meantime the 6.3 MeV state has been also identi ed and published

4 in a transfer reaction [6]. Before the lowest energy peak can cleanly be identied as a two-proton decay it has to be shown that the decay of the upper states can be understood as sequential decays via intermediate states in 16 F. As an example we discuss the decay structure of the third peak at a decay energy of 1:7MeV corresponding to the decay of the doublet at an excitation energy of 2.623MeV and 6MeV. Fig. shows dierent proton spectra of events gated on a decay energy of 1.7 MeV. In Fig. (a) the energy of the second (low energy) proton (The protons within one event were ordered by energy). It shows a double humped structure indicating two dierent distinguishable decay path. This decay pattern can be veried by gating on the left (light shaded) and the right (dark shaded) peak and displaying the energy spectra of the rst proton. These spectra are shown in Fig. (b) and Fig. (d) resulting in two peaks with distinctly dierent energies. Another way to display the sequential decay scheme is the dierence spectrum shown in Fig. (c). The low energy second proton together with the high energy rst proton result in a large dierence peak, whereas the high energy second proton and the low energy rst proton have almost the same energy, thus resulting in a very small dierence energy. FIGURE. Decay structure of the peak at 1.7 MeV. The energy spectra of the second proton (a) and the rst proton gated by the low (light area, (b)) and high (dark area, (d)) second proton. The dierence energy spectrum is shown in part (c). A similar analysis has to be performed for the lowest energy peak of Fig. 4(a) in order to determine the decay structure of this two-proton decay. However, in a preliminary analysis of the opening angle of the protons in the CM-system indicates an isotropic distribution, which would be consistent with a simultaneous uncorrelated two-proton emission. A preliminary analysis of the lifetime indicates a two-proton lifetime in the order of picoseconds, compared to a lifetime of ' 0:1 ps for the -decay from the rst excited state in 17 Ne.

5 DISCUSSION AND CONCLUSION In conclusion, we observed several sequential two-proton transitions at their expected energies as well as evidence for two-proton radioactivity of the rst excited state in 17 Ne. The preliminary lifetime of picoseconds is too fast for the emission of two protons from the 1d =2 shell, which is calculated to be a factor of ' 0 slower [2]. Using an improved experimental setup with optimized eciency and energy resolution will help to clarify the remaining uncertainties in the context of the reported rst evidence for two proton radioactivity. One of us (MJC) acknowledges the support and hospitality of the NSCL and the support of the \Studienstiftung des Deutschen Volkes". We acknowledge the help of A. Azhari and S. Yokoyama during the experiment and thank J. Brown, D.J. Morrissey and M. Steiner for producing the radioactive 17 Ne beam. This work was supported by the National Science Foundation under grant PHY REFERENCES 1. R. A. Kryger et al., Phys. Rev. Lett. 74, 860 (199) 2. M. J. Chromik et al., Phys. Rev. C, 1676 (1997) 3. P. Woods et al. Proposal for CYCLONE, Louvain-la-Neuve 199, unpublished. 4. H. Scheit et al., submitted to Nucl. Instr. Meth. A (1998). V. G~uimaraes et al., Z.Phys. A 33, 117 (199) 6. V. G~uimaraes et al., Phys. Rev. C 8, 116 (1998)