Nucleosynthesis of Heavy Elements by the Photonuclear Reaction

削川川・川川川川川川町川畑山倒閣司川川川川川削川間耐叫u山山間山目 DEnuFOnu4. J ζ 9qu JAERI-Conf 2002-007 5. 5. Nucleosynthesis of heavy elements by the photonuclear reaction Takehito Takehito HAYAKAWA Adv α nced Photon Re 何 αrch θeη teT Jαpα n Atornic E'nergy Research Institute ，Toka ，i.. 1b o.1' 仏k1: 319 時 1195 ，Jαpαη Nucleosynthesis Nucleosynthesis of heavy elements is import 乱nt for understanding of the site mechanism in the sむe11ar and cosmochronology ， The nuclei h州判例 than iron have been synthesized thesized mainly the s-process and the s-decay after the r-proce 胤 The light isotope p-nuclei p-nuclei produ じed by the pho むonuclear reaction in Type II supemovae explosions ‘ In order der to understand ちhe role of each process ， the ratios of the processes are calculated. 1 propose propose the experimental plan using the photon sources. KEYWORD: nucleosynthesis ，isomeric state ，heavy element ，photonuclear reaction ，s-processes ，r-process ，p-process ，γ-process ，nuclear structure ，β-decay ， γ-ray spectroscopy. 1. 1. Introduction Nu c1 eosynthesis of heavy elements is important fo 1' understanding the site mechanism in in the stellar and cosmochronology [1]. The nu c1 ei heavier than iron have been mainly synthesized synthesized by neutron capture (1 附) reaction ， which are called to be r- and s-processes ， in in the stellar environment. The nU c1 ei produced by 出 e s-process are located at 七he center of of stability line. The s-process in thermally pulsing stars on the asymptotic giant branch has has recent1y been discussed. On 七he other hand ， the isotopes have also been produced by the s- decay after the r-process ， which are considered to have happened in Type II supernovae supernovae exp1osion. In the pro 七on rich side ，many isolated even-even stab1e isotopes exist exist in the ou 七side of the s-process path and are shielded from 七he s- decay chain of pos も r-process. r-process. These p-nuclei have been considered to be produced by 七he EC/β+ decay after the the pro 七on capture process. However ， the production mechanism for p-nuclei has recently been proposed to be the photodisintegration by (T， n) reaction in Type II supernovae [2 ，3 ， 4]. In order to inves 七igate 七he contribution of these processes in the solar system ， analysis analysis has been carried out. 2. 2. Ratio of the r-process and s-process The iso 七opes consis 七of the s-nuclei ，r-nuclei and p-nuclei produced from the different processes. processes. Some isotopes have pure s-nuclei andJor r-nuclei. The pure s-nu c1 ei are shielded by the stable isobar from the post r-process. On the other hand ， the pure r-nuclei are located located on the neutron rich side and the unstable isotope ，whose life-time of the s-decay is is sho 1'も er than the mean time of the neutron cap むure in the typical s- process site ， located between 七he s .，. nuclei and the pure r-nuclei. Theore 七ical studies show tha 七the r-process is considered considered to be dominant for nucleosynthesis of heavy elemen 白う and 七hat the estimation of of the lighter mass region is smaller than natura1 abundance. We think the ro1e of the s-process s-process is more important もhan the r-process in 七he light mass region. In order 色o check 出is problem ，も he calculation of the ratio of s- and r-processes have been carried out. There -17 - JAERI-Conf 2002-007 are are three types of the combinations. These are the pure s- and r-nuclei ， pure s-nuclei and mixed nuclei ， pure r-nuclei and mixed nuclei. These ratios are derived from the followed equations. equations. 158Dy 158Dy 159Dy 160Dy 165Dyl 161:+Dr 161:+Dr 》 162:+Dr 》 163;+Dr y 16 埜4D 「V p S s-r r s -r r s1"" r 155Tb 155Tb 156Tb 157 丁b 158Tb 159τb 160Tb 161Tb 162Tb 1 164Tb s+r s+r 吋 154Gc 154Gc 155Gd 156Gc 157Gd 158Gc 159Gc 160Gd 161Gd ¥ S s+r S十r s+r s+r 1J，，、，一園、E CM ~ I(s+r 、、個J ω ，，圃園園 E SJ 一一同、 Fig Fig . 1. Ratio of r-process and s-process nucle i. 1(r) 1(r) A(r) 1(s) 1(s) A(s) 1(r) 1(r) A(r 十 s) - A(s) 1(s) 1(s) A(s) 1(r) 1(r) A(r) 1(s) 1(s) A(r + s) - A(r) This This ratio is plotted in the figure 2. The U and Th is not showed in this figure ， because the the Bi isotopes are heavy nuclei produced by the s-process. There are four valleys in connection connection with the magic numbers of Z=50 ， 82 and N=50 ， 82. The depth of valley of the the neutron magic number is larger than one of proton magic numbers. We obtained the following following equation from the χ2 fitting. I(r) I(r) 一一ー 0.026eO.073x 1(s) 1(s) = The r-process is dominant in the heavy mass region ， but s-process is also important in in mass region lighter than A=150. - 18- JAERI-Conf JAERI-Conf 2002-007 100 100 10 10 (ω) ¥(L)】】 0.1 0.1 0.01 0.01 20 30 40 50 60 70 80 90 Z Fig Fig . 2. Ratio of r-process and s-process nuclei. 3. 3. Ratio of the p-process and s-process The contribution of the p-process is smaller than the s- and r-processes. However ， p-nuclei p-nuclei are not synthesized by neutron capture reactions. In a typical case ， the lighter isotope isotope (N = N s - 1) than the lightest s-nuclei (N = N s) is unstable and the p-nuclei (N = N s - 2) have small abundance. The ratio between s- and p-nuclei is plotted in the figure figure 3. This is our main founding. 1000 1000 Ce 100 100 (a) 】 ¥(ω) 10 】 Ni Ni 0.1 0.1 20 30 40 50 60 70 80 Z Fig Fig . 3. Ratio of s-process and p-process nucle i. - 19- JAERI-Co ぱ 2002- ∞7 It It should be noted that ， the ratio of I(s)jI(p) is constant at about 22 from A=50 to 180 180 mass region except for some large valleys and peaks. The first valley appears in the Ni isotopes. It is known the binding energy of 58Ni is lowest. There are some theoretical approaches ，and it is indicated that the Kr ，Mo and Ru isotopes could not be explained by the γ-process. On 七he other hand ，it is easy 七o explain two big peaks of the Ce and W isotopes. The s-nuclei are not shielded for the post r-process and combined of the s- decay decay after the r-process. The Hf isotopes show a small difference for 七he systematics. The reason is the s- decay of the Lu nucleus whose half-life is about 3x 10 10 y. 4. 4. Experimental plan The only data for the ， -process is for the Pt isotopes. The cross sections of the Pt isotopes have been measured by the activation method using bremss 七rahlung photon sources sources [5 ，6]. It is pointed out that the measurement of the excitation function near the lower lower limit of neutron separation energy is very important. However ， the spectrum of bremsstrahlung bremsstrahlung has a wide range and the end point is not clea r. The measurement of the many elements using the sharp energy photon f1. ux is important. The experimental 七echnique for 七he production of p-nuclei by photonuclear reactions using using photon sources has recently been developed. In order to investigate the systematics of of the ratio of p- and s-processes in a wide mass region ，an experiment using the activation technique technique afi 七er photon irradiation is proposed. Stacked targets of some metals will be irradiated irradiated by high energy γ-rays. The lightest s-process isotopes will be changed to s unstable unstable nuclei by the (γ ，n) photonu c1 ear reaction. After the photon irradiation ， the X-rays X-rays and γ-rays af 七er the s decay of the targe 七will be measured by one Ge detector. We can measure these 同 rgets sequentially ， because these unstable isotopes have different half-lifes half-lifes in a wide range between hours and days. There is small u Il certainty in the measurement of the relative (r， n) reaction cross sections among targets. References [1] [1] M. Arnould and T. Takahashi ， Rep. Prog. Phys. 61 ，395(1999). [2] [2] S. E. Woosley ，S. E. and W. M. Howard ， Astrophys. J. Supp l. 36 ， 285(1978). [3] [3] W. Michael et αl. ， Astrophys. J. 373 ， L5(1991). [4] [4] M. Rayet ， et αl.， Astron. Astrophys. 298 ， 517(1995) [5] [5] P. Mohr et αl. ， Phys. Lett. B 488 ，127(2000). [6] [6] K. Vogt et αl. ， Phys. Rev. C63 ，055802-1(2001). - 20 一.

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