New isotopes, secondary reactions and spectroscopy for medium-mass nuclei at the proton drip line

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A588 (1995) 171c-178c

N e w isotopes, secondary reactions and spectroscopy for m e d i u m - m a s s nuclei at the proton drip line B. Blank ~, S. Andriamonje ~, T. Brohm c, S. CzajkowskP, F. DavP, R. Del MoraP, C. Donzaud b, J.P. Dufour% A. Fleury a, A. Grewe c, R. Grzywaczd, E. HanelV, A. Heinz c, Z. Janas e, T. Josso ~, A. Junghans c, M. Lewitowicz~, A. MusquSre ~, A. Piechaczek b, M.S. Pravikoff% M. Pffitzner d, E. Roecklb, C. RShl c, J.-E. SauvestreJ, K.-H. Schmidt b, S. SteinhS.user ¢, K. S/immerer b, W. Trinder b, B. Voss ~, M. Weber b CEN de Bordeaux-Gradignan and IN2P3, F-33175 Gradignan Cedex, France b Gesellschaft f/Jr Schwerionenforschung, Planckstr. 1, D-64291 Darmstadt, Germany c Inst. f/Jr Kernphysik, TH Darmstadt, Schlot3gartenstr. 9, D-64289 Darmstadt, Germany d Inst. of Exp. Physics, University of Warsaw, PL-00-681 Warsaw, Ho~a 69, Poland GANIL, B.P. 5027, F-14021 Caen Cedex, France f Centre d'Etudes de Bruy~res-le-Chgtel, B.P. 12, F-91680 Bruy~res-le-Chgtel, France This paper reviews two experiments performed at GSI and GANIL. Using a primary beam of SSNi at 650 MeV/nucleon impinging on a beryllium target, production cross sections of proton-rich fragments from projectile fragmentation have been measured at the projectile-fragment separator FRS at GSI. The production rates measured demonstrate that counting rates much higher than expected can be obtained at the proton drip line. The results from spectroscopy measurements show that no Thomas-Ehrmann effect is present in our data which means that the decay energies and masses can be well predicted by using e.g. the IMME. The secondary reactions evidence a slight increase of the interaction cross sections when approaching the proton drip line. Finally, in an experiment performed at the SISSI/LISE facility at GANIL using a ~SKr primary beam, we have observed the 5 new isotopes S°Ga, 64As, sg'T°Kr, and 74Sr. However, we have not found any evidence for 69Br which was reported to be observed with a few counts at MSU. These new findings change our understanding of the path and of the ending point of the rp process.

1. I n t r o d u c t i o n As in the light-nuclei region, project!le-fragmentation reactions allowed to discover many new isotopes also in the medium-mass region [1,2]. Beyond the discovery of new isotopes, the relativistic-energy interactions also allow for the determination of matter distributions of radioactive nuclei by comparing the interaction cross sections to results from Glauber-type calculations, a method until now only used for light nuclei. For medium-mass nuclei, Webber et al. [3] found that the charge-changing cross sections exhaust about 90% of the total interaction cross sections for isotopes close to stability. Approaching the proton drip line, one expects that this fraction still increases reaching 100% at the proton drip line. Since the protons feel the Coulomb repulsion, one does not 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0375-9474(95)00135-2

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expect proton halos comparable in size to the neutron halos found for 11Li and llBe [4]. However, the finite thickness of the Coulomb barrier for medium-mass isotopes may allow for an asymptotic behaviour of the wave functions of loosely-bound protons different from those of neutrons. In an experiment performed at the fragment separator FRS [5] of GSI, we measured production cross sections for isotopes produced in SSNi fragmentation at 650 MeV/nucleon. These results together with a comparison to model calculations will be presented in section 2. These secondary beams are then used for secondary-interaction studies. The results of these studies are described in section 3. The spectroscopy of exotic nuclei is one way to deduce the mass excess of these nuclei by using e.g. the isobaric-multiplet mass equation (IMME) or the Coulomb-displacement method [6]. Beyond the proton drip line, Comay et al. [7] reported a decrease of the mass excess due to the Thomas-Ehrmann effect for nuclei with Z<20, i.e. a lowering of the Coulomb repulsion due to the larger average distance of the last unbound protons from the core. The observation or non-observation of this effect also for elements with 24 < Z < 30 is important for the prediction of candidates for new decay mode like the two-proton ground-state emission. We discuss some of our results on ~3-delayed proton spectroscopy in section 4. Section 5 will be devoted to an experiment performed in September 1994 at the SISSI/LISE facility of GANIL. Using a primary beam of 7SKr, we searched for new isotopes around the ending point of the astrophysical rp process. Preliminary results will be presented from this experiment.

2. Production of SSNi fragments A primary beam of 5SNi with an intensity of about 5x107 particles per second accelerated by the SIS synchrotron at GSI, Darmstadt, to an energy of 650 MeV/nucleon impinged on a 9Be target (4 g/cm 2) at the entrance of the projectile-fragment separator FRS [5]. Due to a degrader thickness of only 1.5 g/cm 2 of aluminum, the different settings of the FRS allowed for the simultaneous transmission of up to 20 different isotopes. These isotopes have been identified by a standard AE - time-of-flight - Bp analysis. Fig. 1 shows a schematic drawing of the detection set-up. After correction of losses due to i) the efficiency of the detection set-up, ii) secondary reactions in the target and the degrader, iii) the dead time of the acquisition, and iv) the limited transmission of the FRS, we determined production cross sections for isotopes between scandium (Z=21) and nickel (Z=28) for isotopes close to stability as well as at the drip line [8]. The results for nickel and iron are shown in Fig. 2. The lowest data point for nickel correspond to three events of the yet unobserved isotope 5°Ni. A comparison of the experimental results to calculations with the statistical abrasionablation model [9] demonstrates that this model nicely reproduces the data. However, due to CPU-time limitations, the model cannot predict cross sections below the #b level. The EPAX code [10] shows a rough overall agreement with the experimental data. At the limits of stability, the EPAX formula underestimates the production cross sections by several order of magnitude (a factor of 750 for 46Fe).

B. Blank et a L I Nuclear Physics A588 (1995) 171c-178c

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3. Total c h a r g e - c h a n g i n g cross sections After the identification of the different isotopes at the exit of the FRS, these isotopes impinge on a secondary target. Behind this target, a second AE measurement allowed to determine the charge-changing cross sections (see Fig. 1). We performed measurements of the total charge-changing cross sections for stable isotopes as well as for isotopes at the proton drip line with low-Z targets (CH2, C, A1) and with a high-Z target (Pb) at an energy of about 440 MeV/nucleon. The measurements and the necessary corrections are described in details in Ref. [11]. The results for cobalt and iron secondary beams impinging on a carbon target are shown in Fig. 3. For isotopes close to stability, the total charge-changing cross sections for the carbon target are constant within the error bars. Although these error bars are large for the most exotic isotopes, a significant increase is visible. This tendency is also present in the measurements with the other targets as well as with the other secondary beams.

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A comparison to calculations with a Glauber model and to the semi-empirical para-

B. Blank et al. / Nuclear Physics A588 (1995) 171c-178c

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metrization of Shen et al. [13] yielding both the total interaction cross section shows that the total charge-changing cross sections exhaust more and more the total interaction cross sections when approaching the proton drip line. At or close to the drip line, the experimental cross sections exceed even the model predicitions for the total interaction cross sections. Such a signature cannot be explained within these two model calculations. The Shen formula [13] assumes the same shape for exotic nuclei as for stable nuclei. The Glauber calculations have been performed with density distributions from mean-field calculations [12]. Although these calculations predict a slight increase of the proton rootmean-square (rms) radius as compared to the neutron rms radius, this increase seems not to be sufficient. A lower limit of the total charge-changing cross sections has been calculated by using only the proton density distribution of the projectile in the Glauber calculations. This procedure does not take into account that protons might be evaporated in the deexcitation step of the interaction. Therefore, this procedure uses only the first step of the interaction, i.e. the abrasion. Indeed, most of the data points lie in between the curves for the total interaction cross sections and for the total charge-changing cross sections, approaching the lower curve for the neutron-rich isotopes and approaching or even exceeding the upper curves for the most neutron-deficient isotopes. This is in agreement with the expectation that proton evaporation after the abrasion of neutrons becomes less important for stable or neutron-rich isotopes while it is very probable for the most neutron-deficient isotopes. The total charge-changing cross sections indicate that the changes of the density distribution at the proton drip line are small. However, our data show a slight tendency to increased total charge-changing cross sections near the proton drip line. Due to the limited accuracy of our experimental data, we cannot give a final answer concerning the quantitative increase of the total charge-changing cross sections at the proton drip line. An additional experiment which should result in a considerably increased statistical precision for the most proton-rich nuclei seems to be desirable in order to confirm the indications for a slightly extended proton-density distribution of nuclei at the proton drip line. 4. /3-delayed p r o t o n s f r o m 4SFe One of the most exciting topics in nuclear-struture physics is the search for new decay channels, especially for the two-proton ground-state emission. The most promising candidate seems to be 4SFe [14]. However, due to the question whether or not the ThomasEhrmann effect described above exists in this mass region, the value for the mass excess of this isotope has a large uncertainty which influences directly the half-life for 2p emission. In order to test mass predictions in the iron region, we performed fl-delayed proton measurements for 4s,49Fe, S°Co, and 52Ni [15]. The experimental spectrum for 4SFe is shown in Fig. 4. A proton peak is observed at (0.959+0.033) MeV which belongs to a proton emission from the isobaric analogue state (IAS) in 4SMn populated via a super-allowed/3 decay to the 47Cr ground state. The predicted energy from the IMME is (0.961+0.055) MeV which means that no deviation from the systematic behaviour has been observed. This finding is supported by the other candidates studied in our experiments. However, the branching ratio for proton emission

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B. Blank et aL / Nuclear Physics A588 (1995) 171c-178c

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is only 3.6-1-1.1%, compared to a calculated branching ratio for the proceeding superallowed Fermi decay of about 50%. The only possible explanation for such a weak proton branch is a de-excitation of the IAS via. 7 emission. This hypothesis is supported by barrier penetration calculations for proton emission which yield approximately the same half-life as for 3, decays with low angular momentum. 5. N e w i s o t o p e s f r o m rSKr f r a g m e n t a t i o n In an experiment performed at GANIL with a primary beam of 7SKr at 70 MeV/nucleon with an intensity of about 1 #Ae, we identified new isotopes by a T O F - A E - E technique. The T O F was measured twice, i) between a PPAC at the exit of LISE and a silicon detector at the exit of LISE 3 [16], as well as ii) between the cyclotron high-frequency and the silicon detector. The AE-E measurement was performed by a silicon detector telescope. This T O F - A E - E identification was checked by measuring the -), decay of the known isomer 71Se with germanium detectors. Fig. 5 shows the A E - T O F plot purified by conditions on the second T O F as well as on the total energy E (part a), together with the projected isospin-projection rows for Tz = -1/2 (b), -1 (c), and -3/2 (d). The new isotopes are indicated by the arrows. We find clear evidence for 6°Ga (about 10 counts in a different setting), 64As, 69,7°Kr, and 74Sr. On the other hand, we have no count at all for 69Br. This seems to be in contradiction with a recent paper from MSU [17] which reported on the observation of this isotope. However, our flight path was about six times longer than the one of the MSU experiment which still allows for a half-life of less than 100 ns. Nevertheless, 69Br is shown to be

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(a) as well as projections of the different isospin-projection for Tz---1/2 (b), for T~=-I (c), and for T~=-3/2 (d). The isotopes [c),d)] as well as the expected position of 69Br [b)]. show evidence for five new isotopes (see text).

proton unbound. This finding together with the observation of 6°Ga and 64As changes our understanding of the astrophysical rp-process in this region, 6SSe being now the most probable ending point [18]. REFERENCES 1. T.J.M. Symons et al., Phys. l{ev. Lett. 42 (1979) 40 2. F. Pougheon et al., Z. Phys. A327 (1987) 17 3. W.R. Webber et al., Phys. ttev. C41 (1990) 520 4. P.G. Hansen, Nucl. Phys. A553 (1993) 89c 5. H. Geissel et al., Nucl. Instr. Meth. B70'(1992) 286 6. M.S. Antony et al., At. Data Nucl. Data Tables 34 (1986) 279 7. E. Comay et al., Phys. Lett. B210 (1988) 31 8. B. Blank et al., accepted for publication in Phys. Rev. C 9. T. Brohm et al., Nucl. Phys. A569 (1994) 821 10. K. Siimmerer et al., Phys. Rev. C42 (1990) 2546 11. B. Blank et al., submitted to Z. Phys. A 12. I. Tanihata et al., Phys. Lett. B289 (1992) 261 and D. Hirata, private communication 13. Shen Wen-qing et al., Nucl. Phys. A491 (1989) 130 14. B.A. Brown, Phys. t{ev. C43 (1991) 1%1513 15. L. Faux et al., Phys. t{ev. C49 (1994) 2440

16. A.C. Mueller et al., Nucl. Instr. Meth. B56 (1991) 559 17. M.F. Mohar et al., Phys. Rev. Lett. 66 (1991) 1571 18. A.E. Champagne et al., Rev. Nucl. Part. Sci. 42 (1992) 39