Elvers, M

Hasper, J

Kern, L

Lindenberg, K

Müller, S

Sonnabend, K

Zilges, A

CERN Document Server

PoS(NIC-IX)057Photodissociation as a Tool for NuclearAstrophysicsK. Sonnabend, S. Müller∗, M. Elvers, J. Hasper, L. Kern, K. Lindenberg, A. ZilgesInstitut für Kernphysik, TU Darmstadt, Schlossgartenstraße 9, 64289 Darmstadt, GermanyE-mail: kerstin@ikp.tu-darmstadt.dePhotodissociation reactions are of crucial interest for the network calculations of stellar nucle-osynthesis. On the one hand, they are directly used in p-process scenarios changing s- and r-process seed distributions to the observed abundance patterns. On the other hand, the study of(γ ,n) reactions helps to constrain theoretical predictions on the (n,γ) cross sections of the branch-ing points on the s-process path. The current status of different experimental efforts is presentedand recent developments are discussed.International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX25-30 June 2006CERN∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)057Photodissociation as a Tool for Nuclear Astrophysics S. Müller1. IntroductionThree different processes are sufficient to explain the nucleosynthesis of most of the heavyelements: the s, r, and p process. The s and r process are based on neutron capture events followedby β decays back to the valley of stability. While the s process takes place during stellar burningphases and is characterized by low neutron densities (nn ≈ 2− 4 · 108 cm−3) and temperatures(T ≈ 1−3 ·108 K) [1], the r-process is thought to occur in explosive scenarios like, e.g. supernovaeand, thus, deals with high neutron densities (nn > 1020 cm−3) and temperatures (T ≈ 1−2 ·109 K)[2, 3]. The (n,γ) cross sections of the so-called branching points on the s-process path must beknown with good accuracy to predict the isotopic abundance patterns in modern nucleosynthesismodels with sufficient correctness. The possibility to constrain the theoretical predictions usingphotodissociation data is explained in section 2.The proton-rich isotopes between Se and Hg that cannot be produced by either of these pro-cesses are called p nuclei [4, 5] and are thought to originate from explosive events lasting a fewseconds at temperatures of about 2−3 ·109 K. These p nuclei are produced from s- and r-processseed nuclei by photodissociation reactions, like (γ ,n), (γ ,p), and (γ ,α), in a process sometimesreferred to as γ process [5]. However, the light p nuclei, like e.g. 92,94Mo and 96,98Ru, are under-produced in this model framework. The efforts to solve this problem include new nucleosynthesisprocesses, like e.g. the rp process [6], as well as suggestions of revised cross section data, like e.g.an enhanced 22Ne(α ,n) cross section leading to an increase of the s-process seed [7]. On that score,the knowledge about photodissociation cross sections plays a crucial role in the prediction of theabundances of the p nuclei. Besides the current status on the experimental efforts, some futureprojects are presented in section 3.2. Cross Sections of s-Process Branching PointsThe neutron capture cross sections of most of the stable isotopes on the s-process path havebeen measured with high accuracy [8]. But the experimental knowledge about the cross sections ofthe branching points, i.e. unstable isotopes with a decay rate λβ comparable to the neutron capturerate λn, is quite scarce. Recently, the cross sections of some long-living branching points withhalf-lives in the order of many years were successfully determined using either activation or directmethods (e.g. 155Eu [9] and 151Sm [10, 11]). Severe discrepancies to the theoretically predictedvalues were found and, therefore, it seems to be favourable to constrain these predictions for theshort-living branching points, too.Most of the short-living branching points play an important role in the final determination ofthe isotopic abundance patterns due to the fact that the branching is only valid during the highneutron densities provided by the 22Ne(α ,n) neutron source in the thermal pulses [12]. However,the preparation of targets enriched in material with half-lives in the order of some dozens of daysis nearly impossible. Therefore, data of the inverse (γ ,n) reaction can be used to observe the samesample of reactants.Several branching points were investigated at the S–DALINAC using the photoactivationmethod. After the naturally composed targets were typically irradiated for 24 hours at the nu-clear resonance fluorescence setup, the number of the produced unstable isotopes is determined by2PoS(NIC-IX)057Photodissociation as a Tool for Nuclear Astrophysics S. Müllerthe γ rays emitted after their β decay. The yield Y of events in the photo-peak is connected to thecross section σ(E) and the photon intensity Nγ(E,Emax) viaY ∝ Ntarget ·∫ EmaxSnσ(E) ·Nγ(E,Emax)dE (2.1)with the number of target atoms Ntarget and the neutron separation energy Sn. The factor of propor-tionality consists of the detector efficiency, the γ intensity, and some time-dependent factors thatare explained in [13] in detail.Two setups consisting of a HPGe detector and two Low Energy Photon Spectrometers (LEPS),respectively, can be used according to the energy of the emitted γ radiation. Results were alreadypublished for 185W [14] and 186Re [15, 16] and are in good agreement to direct measurementsperformed at AIST, Japan [17]. During a recent beam-time, the branching points 147Nd, 153Gd, and191Os were investigated.Besides the observation of the short-living branching points, the usage of the inverse (γ ,n)reaction is also helpful to observe the role and importance of low-lying exited states of the long-living branching points [18]. In the laboratory, only the cross section of the ground-state is normallymeasured and both, the population numbers and the cross section of excited states remain to bepredicted by theory. The excited states are populated in the decay channel of the inverse (γ ,n)reaction and, thus, the corresponding matrix elements can be studied in this way.However, the long half-lives and small γ intensities that encourage direct cross section mea-surements are a hindrance for a classical activation experiment due to the fact that the decay of theunstable nuclei cannot be observed. A possible solution is the usage of accelerator mass spectrom-etry (AMS) for the determination of the yield of unstable nuclei. This method has already beenused successfully, e.g. in the case of 62Ni(n,γ) and 40Ca(n,γ) [19]. Therefore, it is planned to ex-amine some naturally composed targets that were activated during former beam-times with regardto the isotopes 93Zr, 193Pt, and 205Pb. In a recent beam-time, the reaction 152Sm(γ ,n) was studiedin order to complement the direct measurements of the (n,γ) cross section of the branching point151Sm [10, 11].The combination of activation and AMS enables merely the comparison of all matrix elementsinvolved in the (γ ,n) reaction to the corresponding theoretical prediction. Therefore, it is plannedto study the population of excited states in the daughter nucleus of the (γ ,n) reaction and the cor-responding matrix elements by direct measurements at the NEPTUN tagger system which is underconstruction at the S-DALINAC.3. Reaction Rates for p-Process Network CalculationsUsing the photoactivation technique at the S–DALINAC, it is possible to calculate the ground-state reaction rates for p-process network calculations from activations at different energies withoutany assumption about the photodissociation cross section. This method was explained in detail in[13] and results on several isotopes (e.g. 190,192,198Pt, 197Au, 196,198,204Hg, and 204Pb) were pre-sented in [20].Although the sensitivity of the activation technique enables to measure the reaction rates of thevery low-abundant proton-rich nuclei in some isotopic chains (e.g. the natural abundance of 196Hg3PoS(NIC-IX)057Photodissociation as a Tool for Nuclear Astrophysics S. MüllerFigure 1: Scheme of the NEPTUN setup.equals 0.15%) unstable isotopes cannot be examined. Therefore, it is planned to use CoulombDissociation (CD) in inverse kinematics as it is performed at the SIS/FRS/LAND setup at GSIDarmstadt [21, 22]. The (γ ,n) cross sections of the isotopes 92,93,94,100Mo have been studied incollaboration with the research centers Rossendorf and Karlsruhe. To establish the accuracy of theCD method, the cross sections of 92Mo(γ ,n)+(γ ,p) [23] and 100Mo(γ ,n) were observed at the ELBEaccelerator at Rossendorf and at the S–DALINAC at Darmstadt, respectively.The data of the activation experiments is compared to the results of the CD measurementsto estimate the influence of several potential complications on the interpretation of data taken inCD experiments. The contribution of nuclear break-up and the fraction of different multiplicitiesto the cross section have to be taken into account as well as the final-state interactions known aspost-Coulomb acceleration to extract a reasonable result of the CD data.Furthermore, the experimental knowledge about the (γ ,p) and (γ ,α) reactions in the Gamowwindow corresponding to typical p-process temperatures is very scarce. Thus, it is planned to usetagged photons at the new setup NEPTUN at the S–DALINAC to fill this gap. The scheme ofthe tagger is shown in Fig. 1 and the characteristics of the NEPTUN setup are listed in Tab. 1.The NEPTUN setup will be used for the study of (γ ,x) reactions with x being scattered photons,neutrons, protons or α particles. First test experiments are planned in 2006.4. Conclusion and OutlookPhotodissociation reactions play an important role in nuclear astrophysics. Besides the directinfluence of (γ ,n), (γ ,p), and (γ ,α) reactions on the nucleosynthesis in p-process scenarios, thestudy of (γ ,n) reactions helps to constrain theoretical predictions for the inverse (n,γ) cross sectionsof branching points on the s-process path. In the past, the study of photodissociation was realized4PoS(NIC-IX)057Photodissociation as a Tool for Nuclear Astrophysics S. MüllerRadiator target Ni, Al, . . .Thickness ≈ 10 mg/cm2Magnet 180◦ Clam-Shell magnetFocal plane detectors scintillating fibers (width: 1 mm)Number of detectors 32 (upgrade: 128)Energy range 8 MeV ≤ Eγ ≤ 20 MeVEnergy resolution ∆E = 25 keV @ 10 MeVPhoton current ≈ 104 keV−1s−1Table 1: Designed characteristics of the NEPTUN setup.using different photon sources, like e.g. bremsstrahlung or Compton backscattered photons, anddifferent methods, like e.g. activation or direct measurements.New techniques to study the cross sections of unstable isotopes are the usage of CoulombDissociation in inverse kinematics and the study of the inverse reactions [24]. Besides the extensionof the experimental efforts to unstable isotopes, new methods and setups will be used to increasethe accuracy of the “classical” measurements, e.g. the usage of accelerator mass spectroscopy inactivation experiments and the availability of tagged photons in a suitable energy range for directmeasurements.AcknowledgmentsWe thank T. Rauscher, E. Khan, and S. Goriely for valuable discussions concerning theoreticalaspects. F. Käppeler and A. Mengoni are cordially acknowledged for their support in experimentalquestions. This work was supported by the DFG under contract SFB 634.References[1] F. Käppeler, The Origin of the Heavy Elements: The s Process, Prog. Part. Nucl. Phys. 43 (1999) 419.[2] K.-L. Kratz, J.-P. Bitouzet, F.-K. Thielemann, P. Möller, B. Pfeiffer, Isotopic r-process abundancesand nuclear structure for from stability: implications for the r-process mechanism, Astrophys. J. 403(1993) 216.[3] G. Wallerstein, I. Iben, P. Parker, A.M. Boesgaard, G.M. Hale, A.E. Champagne, C.A. Barnes,F. Käppeler, V.V. Smith, R.D. Hoffman, F.X. Timmes, C. Sneden, R.N. Boyd, B.S. Meyer,D.L. Lambert, Synthesis of the elements in stars: forty years of progress, Rev. Mod. Phys. 69 (1997)995.[4] D.L. Lambert, The p-nuclei: abundances and origins, Astron. Astroph. Rev. 3 (1992) 3.[5] M. Arnould, S. Goriely, The p-process of stellar nucleosynthesis: astrophysics and nuclear physicsstatus, Phys. Rep. 384 (2003) 1.[6] H. Schatz, A. Aprahamian, J. Görres, M. Wiescher, T. Rauscher, J.F. Rembges, F.-K. Thielemann,B. Pfeiffer, P. Möller, K.-L. Kratz, H. Herndl, B.L. Brown, H. Rebel, rp-process nucleosynthesis atextreme temperature and density conditions, Phys. Rev. 294 (1998) 167.5PoS(NIC-IX)057Photodissociation as a Tool for Nuclear Astrophysics S. Müller[7] V. Costa, M. Rayet, R.A. Zappalà, M. Arnould, The synthesis of the light Mo and Ru isotopes: hownow, no need for an exotic solution?, Astron. Astroph. 358 (2000) L67.[8] F. Käppeler, s-Process nucleosynthesis and the interior of Red Giants, Nucl. Phys. A752 (2005) 500c.[9] S. Jaag, F. Käppeler, Stellar (n,γ) cross section of the unstable isotope 155Eu, Phys. Rev. C 51 (1995)3465.[10] K. Wisshak, F. Voss, F. Käppeler, M. Krticˇka, S. Raman, A. Mengoni, R. Gallino, Stellar neutroncapture cross section of the unstable s-process branching point 151Sm, Phys. Rev. C 73 (2006) 015802.[11] S. Marrone et al., The n_TOF Collaboration, Measurement of the 151Sm(n,γ) cross section from 0.6 eVto 1 MeV via the neutron time-of-flight technique at the CERN n_TOF facility, Phys. Rev. C 73 (2006)034604.[12] M. Lugaro, A.M. Davis, R. Gallino, M.J. Pellin, O. Straniero, F. Käppeler, Isotopic compositions ofstrontium, zirconium, molybdenum, and barium in single presolar SiC grains and asymptotic giantbranch stars, Astroph. J. 593 (2003) 486.[13] K. Vogt, P. Mohr, M. Babilon, J. Endres, T. Hartmann, C. Hutter, T. Rauscher, S. Volz, A. Zilges,Measurement of the (γ ,n) reaction rates of the nuclides 190Pt, 192Pt, and 198Pt in the astrophysical γprocess, Phys. Rev. C 63 (2001) 055802.[14] K. Sonnabend, P. Mohr, K. Vogt, A. Zilges, A. Mengoni, T. Rauscher, H. Beer, F. Käppeler,R. Gallino, The s-process branching at 185W, Astroph. J. 583 (2003) 506.[15] S. Müller, A. Kretschmer, K. Sonnabend, A. Zilges, D. Galaviz, 187Re(γ ,n) cross section close to andabove the neutron threshold, Phys. Rev. C 73 (2006) 025804.[16] T. Rauscher, Comment on “187Re(γ ,n) cross section close to and above the neutron threshold”,Phys. Rev. C 74 (2006) 019801.[17] T. Shizuma, H. Utsunomiya, P. Mohr, T. Hayakawa, S. Goko, A. Makinaga, H. Akimune,T. Yamagata, M. Ohta, H. Ohgaki, Y.-W. Lui, H. Toyokawa, A. Uritani, S. Goriely,Photodisintegration cross section measurements on 186W, 187Re, and 188Os: Implications for theRe-Os cosmochronology, Phys. Rev. C 72 (2005) 025808.[18] A. Mengoni, Neutron capture cross sections: from theory to experiments and back, Proc. of theInt. Conf. on Nucl. Data for Science and Technology, Santa Fe, NM - September 2004[nucl-th/0501082][19] A. Wallner, R. Golser, W. Kutschera, A. Priller, P. Steier, C. Vockenhuber, AMS – A powerful tool forprobing nucleosynthesis via long-lived radionuclides, Eur. Phys. J. A 27 (2006) 337.[20] K. Sonnabend, K. Vogt, D. Galaviz, S. Müller, A. Zilges, Systematic study of (γ ,n) reaction rates forZ ≥ 78 isotopes, Phys. Rev. C 70 (2004) 035802, Erratum: Phys. Rev. C 72 (2005) 019901(E).[21] T. Aumann, Reactions with fast radioactive beams of neutron-rich nuclei, Eur. Phys. J. A 26 (2005)441.[22] K. Sonnabend, M. Babilon, J. Hasper, S. Müller, M. Zarza, A. Zilges, Photodissociation of neutrondeficient nuclei, Eur. Phys. J. A 27 (2006) 149.[23] M. Erhard, A.R. Junghans, R. Beyer, E. Grosse, J. Klug, K. Kosev, C. Nair, N. Nankov, G. Rusev,K.D. Schilling, R. Schwengner, A. Wagner, Photodissociation of p-process nuclei studied bybremsstrahlung-induced activation, Eur. Phys. J. A 27 (2006) 135.[24] I. Dillmann, M. Heil, F. Käppeler, T. Rauscher, F.-K. Thielemann, Experimental (n,γ) cross sectionsof the p-process nuclei 74Se and 84Sr, Phys. Rev. C 73 (2006) 015803.6

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