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- 1. Productions of heavy neutron-rich nuclei around neutron shell closure N=126 in the reaction 136 Xe+208 Pb Oleg Rudakov Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna JYFL User Meeting in Jyväskylä, Finland 7-8 March 2012
- 2. Chart of the nuclides Top part of the nuclear map. The three problems are indicated: How to reach the island of stability, how to fill the gap and how to explore the blank spot of the nuclear map?
- 3. Nuclide map demonstrating the possibility of proton transfer in the reaction 136 Xe+208 Pb at energy closed to the Coulomb barrier. North east area of the nuclides chart
- 4. Target 208 Pb F1 F2 solid angle – 0.3 sr angular resolution – 0.30 TOF-start detector Beam 136 Xe position sensitive stop detector x, y, TOF TOF-start detector the two-armed time-of-flight reaction product spectrometer CORSET composed of microchannel plates •Time resolution δt 150 ps •Mass resolution δΜ/Μ 7 amu •Angular resolution δΘ, δϕ ±0.3° •Solid angle of each arm 150 msr •Range of measured angles: 25° -70° Basic characteristics of the CORSET spectrometer
- 5. Mass-energy distributions of binary fragments for the reaction 136Xe+208Pb Mass-energy distributions of the primary binary fragments obtained in the reaction 136Xe+208Pb at c.m. energies of 423, 526 and 617 MeV.
- 6. • The Q-value (Q = MTLF+MPLF-MT- MP) is not very large for this reaction (−10÷5 MeV). The difference between the center-of- mass kinetic energies in the entrance and exit channels is manly transformed into internal energy and in a first approximation equals to the excitation energy for both fragments. • The mass distributions for fragments with energy-losses greater than 40 MeV are shown in the right panel. The red points correspond to the mass distribution after de-excitation process, the black ones are mass distribution before de-excitation. Mass-energy distributions of binary fragments for the reaction 136Xe+208Pb
- 7. Mass-energy distributions of binary fragments for the reaction 136Xe+208Pb 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 40MeV÷100MeV; 100MeV÷150MeV 150MeV÷200MeV; 200MeV÷250MeV Eloss >250MeV Ec.m. = 617 MeV Yield(%) 100 120 140 160 180 200 220 240 10 -4 10 -3 10 -2 10 -1 10 0 10 1 Ec.m. = 526 MeV Mass (u) Yield(%)
- 8. Production cross section for the heavy nuclei (A>200 u) in the reaction 136 Xe+208 Pb at Elab=870MeV 200 205 210 215 220 225 230 235 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 calculated by Zagrebaev (all events) E losses > 40MeV experimental Yield(mb) Mass (u)
- 9. Angular distributions of binary fragments for the reaction 136Xe+208Pb a) Laboratory angular distributions of the Xe-like reaction products; b) laboratory angular distributions of the Pb-like reaction products for the mass region 200-216 u. Elab , MeV σexp , b 700±14 0.2±0.1 870±17 1.1±0.4 1020±20 1.3±0.4
- 10. Activation analysis Laboratory angular distributionsLaboratory angular distributions of the Pb-like reaction products.of the Pb-like reaction products. Irradiation time 3.07 d Beam intensity ∼9 nA Solid angles 4 ×23 mstr Detection efficiency 9% 30 35 40 45 50 55 60 65 70 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 136 Xe+ 208 Pb Elab =1020 MeV Elab =870 MeV Elab =700 MeV dσ/dΩ(b/sr) Θlab (deg)
- 11. • 224 Ra(3.66d α)→220 Rn(55.6s α) →216 Po(145ms α)→ 212 Pb(10.64h β-)→212 Bi(1.01h β-)→212 Po(299ns α)→ 208 Pb(st) • 222 Rn(3.82d α)→218 Po(3.10m α) → 214 Pb(26.8m β-)→214 Bi(19.9m β-) →214 Po(164.3µs α)→ 210 Pb(22.3 a β-) → 210 Bi(5.01d β-)→210 Po(138.38 α)→ 206 Pb(st) 0 1 2 3 4 5 214 Po 212 Po 0.2 d 0.0 0.3 0.6 0.9 Counts(Hz) 214 Po 212 Po 1.3 d 216 Po 220 Rn 218 Po212 Bi 222 Rn224 Ra 0.0 0.1 0.2 0.3 0.4 214 Po 212 Po 5.6 d 216 Po 220 Rn 218 Po212 Bi 222 Rn224 Ra 210 Po 4000 5000 6000 7000 8000 9000 10000 0.00 0.01 0.02 0.03 0.04 bkg Eα (keV) 214 Po 212 Po 216 Po 220 Rn 218 Po 222 Rn224 Ra 210 Po Alpha spectra obtained in activation analysis
- 12. The calculated production cross section for primary (dash lines) and survived (solid lines) isotopes of Po and Rn. The points correspond to the estimated cross sections for 210Po, 222Rn and 224Ra.
- 13. Daughter nuclei produced by successive β decay of Os parent isotopes. In red unknown isotopes, in green β unstable nuclei, in white stable nuclei. Decays of osmium isotopes
- 14. Detection system with cooled surface of deposition Beta detector Gamma detector Detectors Gas mixture Beam 136 Xe 820 MeV Rotating target 208 Pb Gas catcher Reaction chamber Electric furnace (8500 ) Aerosol filter Basement MAP cave General layout Production of osmium isotopes
- 15. Summary The mass-energy and angular distributions of binary fragments produced in the reaction 136Xe+208Pb have been measured in the energy range close to the Coulomb barrier. The obtained results demonstrate the following. ● Low-energy collisions of 136Xe with 208Pb can be really used for the production of new neutron-rich heavy nuclei located along the closed neutron shell N=126 (the last astrophysical waiting point). ● The yield of nuclei with masses heavier than target mass was found to be larger than predicted by the theoretical model. This makes even more promising the production of new neutron-rich SH nuclei in the multi-nucleon transfer process at low-energy collisions of heavy actinide nuclei.
- 16. Summary Multi-nucleon transfer reactions are to be used for synthesis of neutron enriched long-living SH nuclei close to beta-stability line. 48Ca and 136Xe beams are insufficient. Uranium-like beam is needed !
- 17. O.Rudakov1 , S Dmitriev1 , P. Greenlees2 , F. Hannape3 , I.M. Itkis1 , M.G.Itkis1 , S. Khlebnikov4 , A. Knyazev1 , G. Knyazheva1 , E.Kozulin1 , T.Loktev1 , A. Di Nitto5 , K. Novikov1 , O.Petrushkin1 , E. Rasinkov1 , S.Smirnov1 , W.H.Trzaska2 , E. Vardaci5 , V.Zagrebaev1 1 Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, (FLNR JINR) Dubna, Russia 2 Accelerator Laboratory of University of Jyväskylä (JYFL), Jyväskylä 3 Universite Libre de Bruxelles (ULB), Bruxelles, Belgium 4 Khlopin Radium Institute(KRI), St. Petersburg, Russia 5 Dipartamento di Scienze Fisiche and INFN (INFN-Na), Napoli, Italy
- 18. 18
- 19. Production of Os
- 20. 136Xe + 208Pb Figure 3. (Left panel) – total cross section of the formation of heavy fragments d2у/dZdN (mb), the number of the contour lines) in the reaction 136Xe+208Pb at
- 22. 197 Os 198 Os 199 Os 200 Os 197 Os 201 Os 202 Os 203 Pt 204 Pt 200 Ir 201 Ir 202 Ir 203 Ir 194 Re 195 Re 196 Re 197 Re 198 Re 199 Re 200 Re 201 Re
- 24. Astrophysical r-process and heavy neutron rich nucleiAstrophysical r-process and heavy neutron rich nuclei
- 26. 26 Use of low-energy Radioactive Ion BeamsUse of low-energy Radioactive Ion Beams for production of neutron rich superheavy nuclei ?for production of neutron rich superheavy nuclei ? No chances today. But in future ?
- 27. 27 Formation of SH elements in astrophysical r-processFormation of SH elements in astrophysical r-process Strong neutron fluxes are expected to be generated by neutrino-driven proto-neutron star winds which follow core-collapse supernova explosions or by the mergers of neutron stars. The question: How large is the neutron flux?
- 28. O.Rudakov1 , S Dmitriev1 , P. Greenlees2 , F. Hannape3 , I.M. Itkis1 , M.G.Itkis1 , S. Khlebnikov4 , A. Knyazev1 , G. Knyazheva1 , E.Kozulin1 , T.Loktev1 , A. Di Nitto5 , K. Novikov1 , O.Petrushkin1 , E. Rasinkov1 , S.Smirnov1 , W.H.Trzaska2 , E. Vardaci5 , V.Zagrebaev1 1 Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, (FLNR JINR) Dubna, Russia 2 Accelerator Laboratory of University of Jyväskylä (JYFL), Jyväskylä 3 Universite Libre de Bruxelles (ULB), Bruxelles, Belgium 4 Khlopin Radium Institute(KRI), St. Petersburg, Russia 5 Dipartamento di Scienze Fisiche and INFN (INFN-Na), Napoli, Italy
- 29. Activation analysis Laboratory angular distributionsLaboratory angular distributions of the Pb-like reaction products.of the Pb-like reaction products. Irradiation time 3.07 d Beam intensity ∼9 nA Solid angles 4 ×23 mstr Detection efficiency 9% 30 35 40 45 50 55 60 65 70 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 136 Xe+ 208 Pb Elab =1020 MeV Elab =870 MeV Elab =700 MeV dσ/dΩ(b/sr) Θlab (deg)
Hinweis der Redaktion
- The present limits of the upper part of the nuclear map are very close to stability while the unexplored area of heavy neutron-rich nuclides is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleogenesis. The neutron shell N=126 (and Z~70) is the last “waiting point” on the r-process path. The half-lives and other characteristics of these nuclei are extremely important for the r-process scenario of the nucleosynthesis. As a rule, new isotopes located far from the stability line are obtained in the fragmentation processes at intermediate colliding energies, in fission of heavy nuclei and in low-energy fusion reactions. The first two methods are extensively used today for the production of new isotopes in the light and medium mass region. Due to the “curvature” of the stability line, in fusion reactions of stable nuclei it is possible to produce only proton-rich isotopes of heavy elements. This is the main reason for the impossibility of reaching the center of the “island of stability” (Z~114, 120 and N~184). Because of that we also have almost no information about neutron-rich isotopes of heavy elements located in the “north-east” part of the nuclear map. We propose to fill this “blank spot” of the nuclear map by the production of heavy neutron rich nuclei located along the neutron closed shell N=126 obtained in multi-nucleon transfer reactions in low-energy collisions of 136Xe with 208Pb.
- The idea is to use the stabilizing effect of the closed neutron shells in both nuclei, N=82 and N=126, respectively. The proton transfer from lead to xenon might be rather favorable here because the light fragments formed in such a process are well bound (stable nuclei) and the reaction Q-values are almost zero. In these reactions the formed reaction fragments have excitation energy about 20-40 MeV. Therefore, obtained in these reactions nuclei stay still neutron-rich after emission of 2-3 neutrons.
- The experiment was carried out at the Flerov Laboratory of Nuclear Reactions using the beam of 136Xe ions extracted from the U-400M cyclotron at energies of 700, 870 and 1020 MeV. Beam intensity on target was 5 nA. Each arm of the spectrometer consists of a compact start detector and a position-sensitive stop detector, both based on microchannel plates. The arms of the spectrometer were positioned in an optimal way according to the kinematics of the reaction. The positions of the arms of the spectrometer have been changed several times during the experiment. This arrangement allows to detect the coincident binary fragments over the angular range 25-70 in the laboratory system. As a result of the experiment mass-energy, angular distributions and differential cross sections of binary products were measured. On the next picture I will show You mass-energy distributions obtained in this experiment.
- Mass-energy matrices of binary fragments for the reaction 136Xe + 208Pb at c.m. energies of 423, 526 and 617 MeV that were measured on CORSET setup are presented on the picture. Figure shows the measured TKE-mass distributions integrated over the center-of mass angle 40-140 of the primary fragments in the reaction 136Xe+208Pb. PLF and TLF can be identify as elastic and quasi-fission events at mass-energy matrices. Besides the elastic and quasi-elastic components, a significant part of events has a large dissipation of the initial kinetic energy which indicates the presence of strongly damped collisions. Let consider projections of mass-energy matrices with different TKE.
- The distributions of the total kinetic energy lost for all detected events are shown in Figure. Lower TKEL values correspond to quasi-elastic processes; higher TKEL values correspond to more damped events. If the quasi-elastic component is represented by a Gaussian curve we observe that most of the damped events are localized at TKEL values above 40 MeV. The primary mass distributions of fragments with energy lost greater than 40 MeV are shown in the right panel. Due to such selection of TKEL, most of the quasi-elastic events have been removed. All fragments in Figure are considerably excited and are likely to de-excite by neutron evaporation. Since we are interested in the production cross section of the secondary fragments, we can provide a first estimate of such cross section by correcting the primary mass distribution for neutron emission assuming that the available excitation energy Ef* is divided between the two fragments according to their mass ratio and each neutron takes away on the average 10MeV. On the next picture let observe development of mass-energy distributions by cutting out TKEL gates.
- With increasing TKEL, more neutrons are evaporated and the TLF and PLF masses drift toward lower masses, respectively. TLF drift at a faster rate than PLF because the mechanism arbitrarily chosen to split the excitation energy awards more energy to the heavier fragment. It is important to remark that the mass drift in Figure is an artificial effect which comes from the attempt to reconstruct the secondary fragment mass distributions from the primary ones on the hypothesis of Qgg = 0. In fact, a large (negative) Qgg would reduce the available excitation energy and fewer neutrons would be evaporated. Consequently, the rate of mass drift would not be as fast as in Figure. This means that mass transfers with large negative Qgg values end up in a larger survival probability against neutron evaporation. This case is particularly important for mass transfer from Xe to Pb where larger negative Qgg contribute to further reduce the excitation energy.
- Therefore, according to next Figure, the contribution of the heavy fragments with masses greater than 220 u is relatively large as compared with calculated events even after the worst-case correction for neutron emission.
- The angular distributions of light reaction products of deep-inelastic collision in the laboratory system are shown in Figure a). At energies of 1020 and 870 MeV the cross sections are focused around angles slightly forward of the grazing angles. The cross section drops quickly at deviation from this angle. At energy of 700 MeV (below the Coulomb barrier) the cross section increases at increasing the scattering angle. Such behavior of the angular distributions of the reaction products is typical for fast peripheral process. The angular distributions of the Xe-like fragments obtained in the reaction 136Xe+209Bi at energies of Xe-ions of 940 and 1140 MeV are show for comparison. Similar to the present experiment the cross sections peaked around angles slightly forward of the grazing angles. Total reaction cross sections exp were deduced with angle integration of these angular distributions. The experimental cross section less than total reaction cross section R. The reason for this reduction of the experimental cross section is possible sequential fission of excited Pb-like reaction fragments. The excitation of formed fragments is relatively high, especially at energy of Xe-ions of 1020 MeV. The de-excitation of the fragments may come by neutron emission or fission. Unfortunately, in this experiment the fragments detected only in coincidence mode and it is impossible to derive the number of events, lost due to sequential fission. In a similar to our case reaction 136Xe+209Bi it was found that the integrated cross section of the Bi-like nuclei is only (1.5±0.4)b at energy of Xe-ions 940 MeV, which is 30% smaller than the reaction cross section deduced from the light fragments. In this work the reaction products were detected in two E-E telescopes in single and coincidence modes.
- From the present measurements of the mass distributions it was found that the yield of the fragments with masses greater than 210 u is relatively high. The main part of elements with A>210u suffer α-decay process. To obtain the production cross section for these isotopes, the experiment using the activation analysis of the products of the reaction 136Xe+208Pb at 850MeV incident energy has been done. The experiments took place in the CORSET chamber at the U-400M cyclotron. The aluminum foils catching the reaction products were at angles 45°-55° to the beam to optimize the measurements for the highest production cross section for heavy target-like fragments. Under this condition the reaction fragments knocked into the foil to the deepness of about 1µm. The α-particles emitted from these fragment lose about 150 keV passing through the foil. The foils were irradiated for 3.07days. The solid angle covered by the foils was 92msr. After irradiation, aluminium foils with caught fragments were removed from the chamber. The energies of α-particles emitted from the foils were measured by silicon detectors. The total observation period was 6 days. The detection efficiency was about 9%. The measured energy spectra of α-particles normalized to measurement period are shown in the next Figure.
- The background spectrum was measured at the same condition and for the foils identical to the irradiated ones. The major part of the events is located between 5 and 9 MeV. The α-particle energies for the Xe-like nuclei are less than 4 MeV and only nuclei with Z > 68 may give a contribution to energies larger than 4 MeV in the α-particle energy spectra. Due to the high density of lines between 4 and 10 MeV and the relatively poor statistics a unique identification of all peaks is not possible. Since the most part of heavy nuclei following α–decay have the half-life from µs up to several minutes, they are not accumulated in the foils in a reasonable quantity during irradiation. Only long-lived nuclei and their daughters may be observed. The measurement period and background conditions allow to observe only the chains of 224Ra and 222Rn. All α-lines energies of these chains have been identified. The production cross sections for all nuclei in these chains are used as variable parameters while only nuclei with half-life more that several minute play a role in the fitting procedure. Therefore, the production cross sections may be estimated with reasonable accuracy only for these nuclei. One can see that the evaluation gives a good agreement with measured spectra. The values of about 0.2mb, 20 µb and 5µb were obtained for 210Po, 222Rn and 224Ra nuclei, respectively. In the next Figure calculated cross sections for primary and survived isotopes of Po and Rn are shown. The estimated production cross sections for 210Po, 222Rn and 224Ra nuclei obtained from measured α-energy spectra are also presented in the same figure.
- The production cross section for 210Po agrees well with the prediction, however the obtained cross section for 222Rn is at least of one order of magnitude large than the predicted values. According to this calculation the maximum for the production cross section of about 8 µb is expected for the 207-210Rn. The obtained cross section for 222Rn is close to the calculated value for 207-210Rn. This disagreement may be explained that N/Z ratio of multi-nucleon fragments, formed in this reaction, is shifted to more neutron rich isotopes compared with the calculation. In addition, the obtained yield for fragments with mass around 220-224u after correction on neutron emission is about 0.05% or (250±90)µb for 870 MeV.
- The tags for the Os isotopes are the β and γ chains of the daughter nuclei detected in coincidence. By gating on detected known transitions it is possible to obtain spectroscopic information concerning unknown Os isotopes if produced. Life times of Os isotopes can be extracted using known rate equations that connect the production rate of parent and daughter nuclei with their lifetime. From the known lifetime of the daughter nuclei it is possible to estimate the Os isotopes lifetime.
- We plan to perform the measurements with the CORSAR set-up ((correlation setup for the reaction products registration) which was designed for the identification and investigation of the properties of neutron-rich heavy nuclei in the region of nuclei near N = 126. CORSAR setup consists of the following parts: Feeding system and gas transport lines, Reaction chamber with rotating target inside Gas catcher of the recoils, System of gas channel heating, Detection system with cooled surface of deposition. A gas mixture (80% He + 20%O) at the pressure of 1.5 atm is fed to the gas catcher which is located in the reaction chamber. A rotating target of 208Pb is located in front of the gas catcher. The products of the reaction 136Xe (820MeV) + 208Pb are stopped inside the gas mixture of the gas catcher and are transported through an electric furnace which provides oxidation of Os isotopes. Then the gas mixture with OsO4 flows through heated teflon tubes to the detection system where the OsO4 molecules are deposited on copper surfaces kept at the temperature of -130°C. β and γ radiations are measured by Si and Ge detectors respectively. The acquisition system operates in triggerless mode with time stamps. After purification, the gas mixture returns back to the gas catcher Reaction cross section will be estimated by normalisation of elastic scattering cross section measured by semiconductor monitoring detectors located on the outer diameter of the gas catcher.
- The experiments were carried out at the FLNR Dubna using the time-of-flight spectrometer CORSET. The mass-energy and angular distributions of binary fragments produced in the reaction 136Xe+208Pb have been measured in the energy range from the Coulomb barrier to well above it, at 700, 870 and 1020 MeV incident energies of 136Xe-ions. In the mass-energy distributions of binary fragments at all measured bombardment energies, besides the elastic and quasi-elastic components, a significant part of events has a large dissipation of the initial kinetic energy which indicates the presents of strongly damped collisions. An unexpected relatively large yield of fragments heavier than 208Pb has been observed in the mass distributions. Due to large negative values of Qgg for this mass region the fragments are expected to be less excited than more symmetric fragments. It leads to increasing survival probability for these fragments against fission and emission of less neutrons during de-excitation process. The radiochemical analysis has been applied for a firm identification of some alpha-emitted isotopes formed in the reaction 136Xe+208Pb at Xe-ions bombardment energy of 850MeV. The values of about 0.2mb, 20 µb and 5µb were obtained for 210Po, 222Rn and 224Ra nuclei, respectively.
- For further development of the CORSAR setup the scintillator detector was designed and manufactured. It allows to increase efficiency of beta particles detection. This detector and photoelectronic multiplier are presented on the picture.
- From the present measurements of the mass distributions it was found that the yield of the fragments with masses greater than 210 u is relatively high. The main part of elements with A>210u suffer α-decay process. To obtain the production cross section for these isotopes, the experiment using the activation analysis of the products of the reaction 136Xe+208Pb at 850MeV incident energy has been done. The experiments took place in the CORSET chamber at the U-400M cyclotron. The aluminum foils catching the reaction products were at angles 45°-55° to the beam to optimize the measurements for the highest production cross section for heavy target-like fragments. Under this condition the reaction fragments knocked into the foil to the deepness of about 1µm. The α-particles emitted from these fragment lose about 150 keV passing through the foil. The foils were irradiated for 3.07days. The solid angle covered by the foils was 92msr. After irradiation, aluminium foils with caught fragments were removed from the chamber. The energies of α-particles emitted from the foils were measured by silicon detectors. The total observation period was 6 days. The detection efficiency was about 9%. The measured energy spectra of α-particles normalized to measurement period are shown in the next Figure.