PACSThe production cross sections and the kinematical properties of fission fragment residues have been studied in the reaction $^{238}$U(1 A.GeV) + p. Isotopic distributions were measured for all elements from O(Z=8) to Gd(Z=64). The distribution of fission velocities and of production cross sections as function of Z of the fragments, provide relevant informations on the intermediate fissioning nucle

Armbruster, P.

Benlliure, J.

Bernas, M.

Boudard, A.

Casajeros, E.

Czajkowski, S.

Enqvist, T.

Legrain, R.

Leray, S.

Napolitani, P.

Pereira, J.

Rejmund, F.

Ricciardi, M.-V.

Schmidt, K.-H.

Stéphan, C.

Taieb, J.

Tassan-Got, L.

Volant, C.

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Fission of 1 A GeV 238U-ions on a hydrogen-targetM. Bernas, P. Napolitani, F. Rejmund, C. Ste´phan, J. Taieb, L. Tassan-Got,P. Armbruster, T. Enqvist, M.-V. Ricciardi, K.-H. Schmidt, et al.To cite this version:M. Bernas, P. Napolitani, F. Rejmund, C. Ste´phan, J. Taieb, et al.. Fission of 1 A GeV 238U-ionson a hydrogen-target. C.Wagemans, J. Wagemans, P. D’hondt. Seminar on Fission- Pont d’OyeV, Sep 2003, Habay-la-Neuve, Belgium. World Scientific, pp.223-231, 2004. <in2p3-00024868>HAL Id: in2p3-00024868http://hal.in2p3.fr/in2p3-00024868Submitted on 10 Oct 2005HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.October 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoieTHE DISTRIBUIION OF COLLISION PRODUCTS IN THETHE REACTION OF 1 A GEV 238U-IONS ON AHYDROGEN-TARGETM. BERNAS, P. NAPOLITANI, F. REJMUND, C. STEPHAN, J. TAIEB,L. TASSAN-GOTI.P.N. d’Orsay, F-91406 Orsay Cedex, FranceE-mail: bernas@ipno.in2p3.frP. ARMBRUSTER, T. ENQVIST, M.-V. RICCIARDI, K.-H. SCHMIDTG.S.I. Darmstadt, D-64291 Darmstadt, GermanyJ. BENLLIURE, E. CASAJEROS, J. PEREIRAUniv. of Santiago de Composteela, E-15706 Santiago, SpainA. BOUDARD, R. LEGRAIN, S. LERAY, C. VOLANTDAPNIA/SPhN CEA/Saclay F-91191 Gif-sur-Yvette Cedex, FranceS. CZAJKOWSKIDAPNIA/SPhN CENBG, IN2P3 F-33175 Gradignan, FranceThe production cross sections and the kinematical properties of fission fragmentresidues have been studied in the reaction 238U (1 A.GeV) + p. Isotopic dis-tributions were measured for all elements from O (Z = 8) to Gd (Z= 64). Thedistribution of fission velocities and of production cross sections as function of Z ofthe fragments, provide relevant informations on the intermediate fissioning nuclei.1. IntroductionThe spallation and fission of the excited nuclei produced in collisions of 1A.GeV U on a hydrogen target has been investigated in order to extendthe domain of data already obtained by our collaboration, namely Au +p 1,2 and Pb + p 3 to a fissile projectile. These studies are part of anexperimental program to collect nuclear data relevant for ADS4,5, RNBand spallation neutron sources. For technical applications, reliable and1October 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie2accurate cross sections are needed and a special care was devoted to theexperiment in order to fulfill these requirements. We report here on themeasurement of fission residues left after the cascade of nucleon-nucleoncollisions induced by the proton in the U-nucleus. The excited fragmentsproduced cool down by evaporation of nucleons or light particles and mayundergo fission. Evaporation residues have been analysed in a parallel work6. The experimental set up used at GSI is briefly described and the mainresults are presented. We show how from the kinematical properties and theisotopic distribution of fission fragments (f.f), the intermediate fissioningparent nucleus can be inferred. The balance between evaporation residuesand f.f. provides a test of the energy dissipation of the excited intermediatesystem.2. ExperimentUsing inverse kinematics at relativistic energy, the f.f. are totally strippedof their electrons and forward focused. They can be separated by the FRag-ment Separator (FRS) 7 and identified by the associated detection systemi.e. an ionisation chamber and a time of flight for Z and A determination,respectively. The performances of the equipement were demonstrated bythe identification of 117 new n-rich fission fragments in U (0.75 GeV) + Becollisions. 8,9.The systematic scanning of the FRS magnetic rigidity allows to recon-struct the center of mass velocity spectra, truncated by the FRS angularacceptance α, as shown in Fig. 1b. Note that 4 to 5 isotopes of all of the 36elements populated by fission are scanned simultaneously. It reduces therelative uncertainties due to beam intensity calibration or dead time cor-rections. The laboratory velocity of f.f. results from the Lorentz additionof the projectile velocity with the fission velocity.From the gap between the first momenta of the two peaks, that is twicethe apparent velocity Vapp, knowing α and its variance σα, we calculate theangular transmission and the fission velocity Vf for each isotope, see10.3. KinematicsFission velocities varies as the inverse of the atomic number Z due to mo-mentum conservation in fission. For each element, velocities of 15 to 25isotopes are investigated. They decrease slightly with the mass of the iso-tope (1 to 2 %) except for the 4 lightest isotopes where they drop by 10to 20%. This fall confirms that the 4 lightest isotopes are produced byOctober 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie300.20.40.6x 10-5-2 -1 0 1 2V (cm/ns) dn(V) / dVFigure 1. a) Schematic view of the experimental parameters shaping the measuredvelocity spectrum in the frame of the fissioning system. Vf is the fission-fragmentvelocity, α is the angular acceptance of the FRS, and σα its variance. b) Velocityspectrum of 128Te in the frame of the fissioning system. The velocity V = 0 refers tothe projectile frame. Vapp is the apparent fission velocity defined in the text.secondary break-up of heavier fission fragments 11.A mean value of the velocity per element is presented on Fig. 2, to-October 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie4Figure 2. Fission fragment velocities as a function of Z. The lines are obtained usingthe calculation of velocity for fissioning parent nuclei of Z0 = 80 (full line), Z0 = 86(dashed line) and Z0 = 92 (dotted line) with the parametrisation of ref. 12 and 13 anda deformation of β = 0.65 for both fragments.gether with the 3 straight lines expected in case of fission of U, Rn and Hg.The paramerization of 12,13 and a deformation of β = 0.65 is assumed forcalculating the three distributions of velocity as a function of Z of the f.f..A small decrease of the measured values relative to the expected slope fordecreasing Z is found. The data are compatible with 84 < Z < 90 for thefissioning nuclei.4. Isotopic Yields: Symmetric and asymmetric fissionIsotopic cross sections have been obtained for all isotopes of the 36 elementsproduced by U + p fission, down to a threshold value of 0.1 mb. Theycover a range of 0.1 to 14 mb. The measurements can be extended furtherfor neutron-rich isotopes since they are not contaminated by secondarybreak-up, contrary to cross sections on the neutron-deficient side. Threedistributions are shown here on Fig. 3 to illustrate our results. A systematicuncertainty of 10% is calculated, mostly due to the beam monitoring. TheOctober 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie5error bars reported on Fig. 3 correspond to the relative uncertainties.Figure 3. Isotopic cross sections for fission-fragments in the reaction 238U 1 AGeV + pfor elements 30Zn, 40Zr and 52Te. The arrows indicate a pair of isotopes, the productionof which is enhanced in asymmetric fission.The isotopic cross section distributions show a bell shape with a shoul-der on the neutron-rich side, more prominent in case of isotopes stronglypopulated in asymmetric fission. The ”asymmetric” regime of fission, hasbeen analysed in various experiments. In our previous study of U + Pbfission for example 14, it was enhanced due to the strong Coulomb exci-tation of the giant resonance. The mean value of the mass A, A(Z) andthe width σA(Z) were determined coherently with previous works. The iso-topic distributions presently measured are decomposed into contributionsof symmetric and asymmetric fissions. As a result the element distributionshown on Fig. 4a can be shared into both components, as shown on thefigure.The mean value of Z, Z = 45 , is calculated for the total of the distri-bution shown on Fig.4a, down to Z = 28. For Z < 28, the binary break-upis still observed, and assigned by the FRS kinematical selection 15. Theoccurence of the very asymmetric break-up with rising cross sections wasalready observed and discussed in other systems at similar excitation ener-gies 16.The region populated by the two fission regimes are characterized onthe Fig. 4b. Asymmetric fission populates a narrow corridor (σZN = 1.8)in the neutron-rich region, (N/Z = 1.52). The symmetric process drives ina region of less neutron-rich isotopes, towards the valley of stability, whereN/Z (≈1.3) increases slowly with Z, crossing the valley at Z = 57, as shownOctober 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie6Figure 4. a) Distribution of f.f. integrated over N for 238U + p at 1 A.GeV (Full pointsor squares). Light circles: symmetric fission, and dashes: asymmetric component (withlarge error bars). In order to single out the symmetric component, the related valueshave been divided by a factor of 2. b)Mean neutron number divided by Z as a functionof Z for the total distribution and for symmetric and asymmetric fission.on Fig. 4b and σZN increases from 2 for Z = 30 to 4 for Z = 60.From the mean values of the atomic numbers of f.f. Z = 45 and from themean neutron number N = 63 the most probable symmetric fission leadsto two f.f. of 108Rh. A mean fissioning parent nucleus can be recontructedusing the conservation of protons and neutrons in the spallation processand the energy balance between the cascade and the evaporation chainending in fission. The mean cascade in 238U induced by the protons leadsOctober 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie7to a 232Th excited at E∗ = 183 MeV after 3 protons and 4 neutrons havebeen torn away from the incident system. Then a mean number of 11neutrons are evaporated to reach a 221Th at E∗ = 149 MeV. With a meannumber of 5 post fission neutrons emitted, the two symmetric f.f. of 108Rhare obtained. This estimation is based on known energies required to freeneutrons and protons in the cascade and on binding energies of neutrons inthe evaporation process. The simulations to be done still, will substantiatethe result.The fission cross section is calculated by summing elemental cross sec-tions σ(Z) over the whole fission area. For symmetric and asymmetricfission, cross sections of (1.42 ±0.15) b and of (0.105 ± 0.01) b are found,respectively. The total fission cross section amounts to (1.53 ± 0.15) b, avalue which compares well with the results of direct measurements of fissioncross sections (1.46±0.07) b and (1.48±0.06) b 17,18, respectively.Figure 5. Comparison of cross-sections for evaporation and fission residues as a functionof the mass-loss ∆A for the two collision systems Pb + p (triangles) and U + p (fullpoints) at 1 A GeV.5. Mass Yields: Comparison with the Pb (1 A GeV)+ psystemThe mass distribution σ(A) is built by summing isotopic cross sectionsfor A fixed. It shows a roughly symmetric Gaussian shape except for theOctober 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie8shoulders of asymmetric fission. We have plotted on Fig. 5 cross sectionsas function of the mass loss for spallation residues and f.f.. The Pb + presults 3 are reported also to illustrate the importance of the fissility incase of U. Total cross sections are almost the same within 10 %, (see theTable) but the dominant contribution of the fission for U is inhibited incase of Pb, whereas for Pb, excited fragments cool down mainly by neutronevaporation.reaction σtot σfis σEV R(barn) (barn) (barn)U + p 1.99± 0.17 1.53 ± 0.13 0.46±0.08Pb + p 1.84± 0.23 0.16 ± 0.07 1.68±0.22Simulations have been performed by our collaboration to reproduce themeasured f.f. cross sections. The best result is obtained by using a cal-culation of the cascade with a new version of INCL 19, the statistical de-excitation code ABLA 20,21, and a fission parametrization PROFI 22The cross sections for (U + p)-evaporation residues 6 cannot be repro-duced by the Bohr and Weeler model and dissipation has to be includedin the calculation. The reproduction of measured data is optimum with adissipation constant β = 2*1021 s−1 23.6. ConclusionThe isotopic cross sections are determined for 1300 isotopes produced inthe reaction U (1 AGeV) + p see Fig. 6. The fission fragments, separatedkinematically from the spallation residues are investigated and productioncross-sections are determined within a range of 2 to 3 orders of magnitudedown to 10 to 100 µb with an accuracy better than 15 %. The kinetic energyof each fragment is determined. Of a total cross section of 2 barns, 77 % isattributed to fission and 23 % to evaporation residues. Multifragmentationcan be neglected. The mean element number and the mean neutron numberof the f.f. allow to reconstruct 221Th as the mean parent fissioning nucleus.October 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie925    mb  5    mb  1    mb  0.2 mb40    µb  8    µb  1.6    µb28202850822 82028 5082126238 U(1 A GeV) + pFigure 6. Two-dimensional plot of the isotopic cross sections for fission fragments (ref10 et 15) and evaporation residues (ref 6) obtained in 1 A GeV 238U + p shown on thechart of isotopes with indication of the stable isotopes. Increasing dark level correspondto decreasing cross sections according to the logarithmic scale indicated.References1. F. Rejmund et al. Nucl. Phys.A683 481 (2001)2. J. Benlliure et al. Seminar on fission, Pont d’Oye IV 223 (1999)3. T. Enqvist et al. Nucl. Phys.A686 481 (2001)4. F. Carminati, R. Klapisch, J. P. Revol, Ch. Roche, J. A. Rubio, C. Rubbia.CERN Report CERN/AT/93-47(ET), (1993).5. G. Fioni, see the contribution therein.6. J. Taieb, PhD Thesis Univ. Paris XI (2000) IPNO-T-00-10 andJ. Taieb et al. Nucl. Phys. A724 413 (2003)7. H. Geissel et al. Nucl. Instrum. MethodsB70 286 (1992).8. M. Bernas et al. Phys. Lett.415B 111 (1997)9. Ch. Engelmann, Thesis Univ. Tu¨bingen (1998), GSI Rep. Diss. 1998-15,and Zeit. Phys.A352 351 (1995)10. M. Bernas et al. Nucl. Phys.725A 213 (2003)October 10, 2005 15:26 WSPC/Trim Size: 9in x 6in for Proceedings pontdoie1011. P. Napolitani et al. Nucl. Phys.727A 120 (2003)12. B.D. Wilkins, E.P. Steinberg, R.R. Chasman, Phys. Rev.C14 1832 (1976)13. C. Bo¨ckstiegel et al. Phys. Lett.398B 259 (1997)14. C. Donzaud Eur. Phys. J.A1 407 (1998)15. M. V. Ricciardi et al. proceedings of the XXXIX Int. Winter Meeting onNucl. Phys., Bormio, Italy (2001).M. V. Ricciardi, PhD Thesis in progress, GSI-Darmstadt.16. D.G. Sarantites et al. Phys. Lett.218B 427 (1989)17. B. A. Bochagov et al. Sov. J. Nucl. Phys.28-2 291 (1978)18. L. A. Vaishnene et al.Z. Phys.302A 143 (1981)19. A. Boudard, J. Cugnon, S. Leray and C. Volant, Phys. Rev.66C 044615(2002)20. J.-J. Gaimard and K.-H. Schmidt Nucl. Phys.531A 709 (1991)21. A. R. Junghans et al. Nucl. Phys.629A 635 (1998)22. J. Benlliure et al. Nucl. Phys.628A 458 (1998)23. B. Jurado, PhD Thesis, Univ. Compostela (2002) and GSI Rep. 2002-10, tobe published

Fission of 1 A GeV $^{238}$U-ions on a hydrogen-target

Fission of 1 A GeV $^{238}$ U-ions on a hydrogen-target

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Fission of 1 A GeV 238^{238}238U-ions on a hydrogen-target

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Fission of 1 A GeV $^{238}$ U-ions on a hydrogen-target