S. YOKOYAMA 1;2. Abstract. Light particle-unstable nuclei were studied along the neutron. B is a possible candidate for neutron

Transcription

1 Submitted to the Proceedings of Hirschegg Workshop XXIV on \Extremes of Nuclear Structure", January -20, NUCLEAR STRUCTURE OF PARTICLE UNSTALE NUCLEI M. THOENNESSEN, 1;2 A. AZHARI, 1;2 T. AUMANN, 1;2 J. A. ROWN, 1 J. CAGGIANO, 1;2 M. HELLSTR OM, 1 J. H. KELLEY, 1;2y R. A. KRYGER, 1 H. MADANI, 3 E. RAMAKRISHNAN, 1;2z D. RUSS, 3. M. SHERRILL, 1;2 M. STEINER, 1;2 T. SUOMIJ ARVI, 1y P. THIROLF, 1 AND S. YOKOYAMA 1;2 1 National Superconducting Cyclotron Laboratory East Lansing, Michigan 48824, USA 2 Department of Physics & Astronomy, Michigan State University East Lansing, Michigan 48824, USA 3 Department of Chemistry, University of Maryland College Park, Maryland 20742, USA Abstract Light particle-unstable nuclei were studied along the neutron and the proton driplines. is a possible candidate for neutron radioactivity and the lifetime was measured to be smaller than 191 ps, setting a new limit. On the proton-rich side, rst experimental evidence for an s-wave ground state of 11 Nwas found. 1. Introduction The availability of radioactive nuclear beams has opened up the possibility to study nuclei at and even beyond the proton and neutron driplines. The cross sections for single particle transfer reactions coupled with the detector eciencies are suciently large enough to utilize rather small beam intensities. The lifetimes of nuclei beyond the driplines (unstable with respect to neutron or proton decay) are extremely short ( 10,20 s) and thus can not be measured directly with standard methods. Wehave studied single-nucleon stripping and pick-up reactions with radioactive beams to populate nuclei near and beyond the driplines. Proton stripping from C and 17 C along the neutron dripline was used to populate and Present address: Gesellschaft fur Schwerionenforschung, D Darmstadt, Germany. y Present address: Institut de Physique Nucleaire, IN2P3-CNRS, Orsay, France. z Present address: Cyclotron Institute, Texas A&M University, College Station, TX

2 Counts Mass (arb. units) Energy (MeV) 800 Figure 1. Isotope identication spectrum (left) and energy spectrum for (right) following proton stripping reactions from a C beam., respectively. The decay energy spectra of proton-unbound states of N, O and 11 Nwere measured in a coincidence measurement of the proton with the corresponding fragment. These nuclei were populated by single particle transfer reactions of a (secondary) 12 N beam. 2. Neutron Dripline The search for neutron radioactivity is extremely dicult. Due to the missing Coulomb barrier neutron radioactivity relies solely on the angular momentum barrier. One possible case is where the last neutron probably populates a d 5=2 state. is known to be particle unstable because it was not observed in the spallation of uranium with 4.8 GeV protons [1] nor in the fragmentation of 40 Ar at 44 MeV/nucleon [2]. From the length of the ight path and the velocity of the fragments an upper limit on the lifetime of 9 ns can be extracted. On the other hand, a recent measurement of the multiple transfer reaction 14 C( 14 C, 12 N) found a state very close to the threshold at 4060 kev [3]. Assuming a width of <100 kev results in a lower limit on the lifetime of > ,21 s. Simple shell model calculations predict tohave a lifetime of 3:7 10, s and 1:110, s for decay energies of 10 kev and 1 kev, respectively, assuming a d 5=2 conguration. Thus, the range of possible lifetimes is quite large and extremely dicult to measure. In a rst attempt, we tried to lower the limit of the present experimental range from the long-lived side by designing an experiment similar to the fragmentation measurements. The signicantly reduced time-of-ight, compared to the previous measurements, allowed a reduction on the upper limit of the life time. We used a single proton stripping reaction from a radioactive beam

3 Counts Mass (arb. units) Energy (MeV) 800 Figure 2. Isotope identication spectrum (left) and energy spectrum for (right) following proton stripping reactions from a 17 C beam. of 17 C to produce. Fragment separation following this reaction was not necessary due to the small background. The radioactive 17 C beam was produced via fragmentation of 80 MeV/nucleon 18 O and then selected with the A1200 fragment separator at the NSCL. A E silicon detector in front of the secondary 114 mg/cm 2 thick carbon target coupled with the ight time through the A1200 allowed the identication of the 17 C isotopes. Directly after the target a E-E-E-E telescope with thicknesses of 303 m, 498 m, 5mm and 5 mm stopped the beam and the reaction products. The distance between the secondary target and the E detector was only 5 cm. Further experimental details can be found in Ref. [4]. The setup was tested with the reaction nat C( C, ). Figure 1 shows the particle identication (left) and the energy spectrum of the fragments (right) which resulted from the one proton stripping reaction. The arrow corresponds to pure fragmentation reactions where the fragments continue with the initial C beam velocity corrected for energy loss in the target. The peak due to the transfer reaction 12 C( C, ) Nwould be 20 MeV higher. The observed broad peak between these two energies suggests that both reaction types contribute. The corresponding spectra for the reaction nat C( 17 C, ) is shown in gure 2. The absence of a peak in the left part of the gure conrms that is unbound. The right part of gure 2 shows the energy spectrum of the 67 (background) events which appear in the gate of the particle spectrum. Most of the events are at much lower energies compared to the arrow at which fragmentation events should have occurred. Only 4 events still qualify when an additional gate on the energy is applied. The (Figure 2, left) spectrum

4 from the 17 C fragmentation actually shows a peak at the same position as in the C reaction (Figure 1, left) indicating that these events are predominantly due to one-proton stripping followed by neutron decay [4]. Using the measured cross section for these reactions and assuming that the four events in the energy gated window are indeed, a new upper limit of 191 ps for the life time of was extracted. Although this is a factor of 50 smaller than the previous value, it narrows the possible window of lifetimes only by a small fraction. The exact determination of the lifetime therefore remains a major experimental challenge. 3. Proton Dripline One of the most interesting exotic decay modes on the proton rich side is the ground-state di-proton emission. In heavy nuclei, proton emission can occur with fairly long lifetimes due to the large Coulomb and angular momentum barriers. However, in the light mass region, both the Coulomb barrier as well as the angular momentum barriers are smaller and thus the lifetimes are extremely short. The search for correlated two-proton emission (di-proton) has been unsuccessful so far. The most recent investigation of the groundstate decay of 12 O established a limit of < 7% for the contribution of di-proton emission [5]. The decay was consistent with a sequential emission of the two protons. However, this sequential decay depends critically on the intermediate system of 11 N. The lowest measured state in 11 Nisap 1=2 state with a decay energy of 2.24 MeV and the s 1=2 ground state was assumed to be at 1.9 MeV which was deduced from the isobaric mass multiplet equation (IMME) [6,7]. With these values the sequential decay of 12 O should be strongly suppressed and the calculated decay width is inconsistent with the measured width of kev for 12 O [5]. Thus, we have studied the decay of 11 N in order to search for the ground state and determine its decay energy. We used the single neutron stripping reaction 9 e( 12 N, 11 N) with a radioactive beam of 12 N. This secondary beam was produced from a 80 MeV/nucleon O beam and separated in the A1200 and the RPMS velocity lter. The secondary target of 37 mg/cm 2 9 e was located in front of a telescope consisting of a PPAC, a segmented silicon E detector and a segmented 5mm thick silicon E detector. This telescope was surrounded by a silicon E detector segmented into annular and radial segments which was backed by plastic phoswich detectors. The eciency of this arrangement was optimized for decay energies between 1 and 3 MeV [8]. In addition to the single neutron stripping reaction populating 11 N, the proton decays of unbound states following other transfer reactions were observed.

5 N 12 C + p O 12 N + p Figure 3. Decay energy spectra for N (left) and O (right) following the single particle pick-up reaction 9 e( 12 N, N) and 9 e( 12 N, O), respectively. The decay energies and widths of previously measured states were used to calibrate and determine the resolution of the set-up. Figure 3 shows the decay spectra of N and O following the reactions 9 e( 12 N, N) and 9 e( 12 N, O) and the corresponding decay schemes. The decay spectrum of Nto 12 C shows a peak at a decay energy of 1.5 MeV which corresponds to decays of the second and third excited state of N which are located at excitation energies of 3.51 MeV and 3.55 MeV with widths of 62 kev and 47 kev, respectively [9]. The decay energy of the rst excited state at 2.37 MeV is too small (421 kev) to be detected by the present arrangement. O also has no bound excited states and the proton decay of the rst three excited states at 2.75 MeV, 4.21 MeV and 6.02 MeV [9] can be seen in the decay-energy spectrum in gure 3. Only the width of the third excited state has been previously been determined (1.2 MeV) [9]. The widths of the rst two excited states were previously unknown and from a preliminary analysis of the present data, values of 400 kev and 500 kev, respectively, were extracted. The decay-energy spectrum of 11 N is shown in gure 4. It exhibits one broad peak around 2 MeV with a strong asymmetry at lower energies. Assuming that this peak is dominated by the previously measured p 1=2 state at 2.24 MeV, it is clear that an additional peak at lower energies is necessary in order to t the data. In the t shown in gure 4, the 2.24 MeV peak was kept xed with a width of 740 kev [6]. The low-energy part could be tted by adding a peak at 1.5 MeV with a width of 2.5 MeV. The data could not be described with

6 one single peak with an intermediate energy. Even when the apparent p 1=2 state was lowered to 2 MeV, an additional component was necessary to t the data. The detailed values of this peak at low decay energy have to be determined bya 2 -search including a variation of the upper peak. The present observation of a state 1.5 MeV with a rather broad width is consistent with recent theoretical predictions. A reanalysis of the isobaric analog states in 11 and 11 C Figure 4: Decay energy spectrum of 11 N by Sherr indicated that the IMME predict the s 1=2 state to be located between MeV [10]. H.T. Fortune et al. calculated the level structure of 11 N using a Woods-Saxon plus Coulomb plus spin-orbit potential [11]. They predict the p 1=2 state to be at 2.48 MeV, about 200 kev higher than the measured value, and the s 1=2 state at MeV excitation energy. The existence of this state at signicantly lower decay energy in 11 N could also explain the sequential decay nature of the two-proton decay of 12 O. The suppression of the sequential decay is reduced and the measured large decay width of 12 O could be consistent with the present results. The detailed simulations for this decay including the new values for the structure of 11 Nhave still to be performed. 4. Conclusions Decay studies of nuclei along the dripline can be a powerful tool to search for exotic decay modes. A rst experiment to search for the possible neutron radioactivityof established a new upper limit on the lifetime of 191 ps. The exact determination of the lifetime of will be extremely dicult because the possible lifetimes still cover a range of approximately 10 orders of magnitude. In addition the necessary time/energy resolutions are not sucient to measure lifetimes of 10, s. The observation of the ground state of 11 N at a decay energy of 1.5 MeV will most likely be able to explain the large decay width of 12 O and thus establish the sequential nature of this decay. Thus the search for di-proton emitters has to be extended to heavier masses. The next possible candidate would be Ne.

7 REFERENCES 1. J. D. owman, A. M. Poskanzer, R. G. Korteling, and G. W. utler, Phys. Rev. C9, 836 (1974). 2. M. Langevin et al., Phys. Lett. 0, 71 (1985). 3. H. G. ohlen et al., Nucl. Phys. A583, 775c (1995). 4. R. A. Kryger, A. Azhari, J. rown, J. Caggiano, M. Hellstrom, J. H. Kelley,. M. Sherrill, M. Steiner and M. Thoennessen, Phys. Rev. C53, in press (1996). 5. R. A. Kryger et al., Phys. Rev. Lett. 74, 860 (1995). 6. W. enenson et al., Phys. Rev. C9, 20 (1974). 7. F. Ajzenberg-Selove, Nucl. Phys. A506, 1 (1990). 8. M. Thoennessen et al., Proc. of the Int. Conf. on Exotic Nuclei and Atomic Masses, Arles, France, June (1995). 9. F. Ajzenberg-Selove, Nucl. Phys. A523, 1 (1991). 10. R. Sherr, private communication. 11. H. T. Fortune, D. Koltenuk and C. K. Lau, Phys. Rev. C51, 3023 (1995).