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論文

Review of even element super-heavy nuclei and search for element 120

Hofmann, S.*; Heinz, S.*; Mann, R.*; Maurer, J.*; M$"u$nzenberg, G.*; Antalic, S.*; Barth, W.*; Burkhard, K. G.*; Dahl, L.*; Eberhardt, K.*; et al.

European Physical Journal A, 52(6), p.180_1 - 180_34, 2016/06

 被引用回数:59 パーセンタイル:7.58(Physics, Nuclear)

The reaction $$^{54}$$Cr+$$^{248}$$Cm was investigated at the velocity filter SHIP at GSI with the intention to study production and decay properties of isotopes of element 120. Three correlated signals were measured, which occurred within a period of 279 ms. The heights of the signals correspond with the expectations for a decay sequence starting with an isotope of element 120. However, a complete decay chain cannot be established, since a signal from the implantation of the evaporation residue cannot be identified unambiguously. Measured properties of the event chain are discussed in detail. The result is compared with theoretical predictions. Previously measured decay properties of even element super-heavy nuclei were compiled in order to find arguments for an assignment from the systematics of experimental data. In the course of this review, a few tentatively assigned data could be corrected. New interpretations are given for results which could not be assigned definitely in previous studies. The discussion revealed that the cross-section for production of element 120 could be high enough so that a successful experiment seems possible with presently available techniques. However, a continuation of the experiment at SHIP for a necessary confirmation of the results obtained in a relatively short irradiation of five weeks is not possible at GSI presently. In the summary and outlook section we also present concepts for the continuation of research in the field of super-heavy nuclei.

論文

In-beam fissio study at JAEA for heavy element synthesis

西尾 勝久; 池添 博; Hofmann, S.*; Ackermann, D.*; 有友 嘉浩*; Comas, V. F.*; D$"u$llmann, Ch. E.*; Heinz, S.*; Heredia, J. A.*; He${ss}$berger, F. P.*; et al.

AIP Conference Proceedings 1524, p.68 - 72, 2013/04

 パーセンタイル:100

Fission fragment mass and kinetic energies distributions were measured in the heavy-ion induced reactions using $$^{238}$$U target nucleus at the energies around the Coulomb barrier. We observed strong variation of the fragment mass distribution with incident energy, which was explained by an energy dependence of the probabilities for fusion and quasifission. A calculation based on a fluctuation-dissipation model was carried out to determine the fusion probability in the reactions of $$^{30}$$Si+$$^{238}$$U and $$^{34}$$S+$$^{238}$$U. The fusion probabilities were consistent with those determied from the evaporation residue cross sections for both reactions, showing that in-beam fission measurement and analysis can give a reasonable estimation for the cross sections to produce super-heavy nuclei. We also discss the results for in-beam fission measurement of $$^{48}$$Ca+$$^{238}$$U.

論文

$$beta$$-delayed fission of $$^{192,194}$$At

Andreyev, A. N.*; Antalic, S.*; Ackermann, D.*; Bianco, L.*; Franchoo, S.*; Heinz, S.*; He${ss}$berger, F. P.*; Hofmann, S.*; Huyse, M.*; Kalaninov$'a$, Z.*; et al.

Physical Review C, 87(1), p.014317_1 - 014317_8, 2013/01

 被引用回数:17 パーセンタイル:17.67(Physics, Nuclear)

By using the recoil-fission correlation technique, the exotic process of beta-delayed fission ($$beta$$DF) was unambiguously identified in the very neutron-deficient nuclei $$^{192,194}$$At in experiments at the velocity filter SHIP at Gesellschaft f$"u$r Schwerionenforschung (GSI). The upper limits for the total kinetic energy release in fission of $$^{194,196}$$Po, being the daughter of $$^{192,194}$$At after $$beta^{+}/EC$$ decay, were estimated. The possibility of an unusually high $$beta^{+}$$DF probability $$^{192}$$At is discussed.

論文

The Reaction $$^{48}$$Ca + $$^{248}$$Cm $$rightarrow$$ $$^{296}$$116$$^{*}$$ studied at the GSI-SHIP

Hofmann, S.*; Heinz, S.*; Mann, R.*; Maurer, J.*; Khuyagbaatar, J.*; Ackermann, D.*; Antalic, S.*; Barth, B.*; Block, M.*; Burkhard, H. G.*; et al.

European Physical Journal A, 48(5), p.62_1 - 62_23, 2012/05

 被引用回数:104 パーセンタイル:1.27(Physics, Nuclear)

The synthesis of element 116 in the reactions of $$^{48}$$Ca+$$^{248}$$Cm was studied at the GSI-SHIP. Four decay chains from the isotope $$^{292}$$116 and one decay $$^{293}$$116 were detected at the reaction leading to the excitation energy of 40.9 MeV. The cross sections were 3.4 and 0.9 pb, respectively. The decay character reproduced the reported data at FLNR, Russia. In the $$alpha$$ decay chain, we observed the higher $$alpha$$-particle energy in the decay of $$^{289}$$114, which suggested the population and the decay of a quasiparitcle state.

論文

Investigation of fission properties and evaporation residue measurement in the reactions using $$^{238}$$U target nucleus

西尾 勝久; 池添 博; Hofmann, S.*; Ackermann, D.*; Antalic, S.*; 有友 嘉浩; Comas, V. F.*; D$"u$llmann, Ch. E.*; Gorshkov, A.*; Graeger, R.*; et al.

EPJ Web of Conferences (Internet), 17, p.09005_1 - 09005_4, 2011/10

 被引用回数:1 パーセンタイル:46.63

Fission fragment mass distributions after full momentum transfer of the projectile were measured in the reactions using a $$^{238}$$U target nucleus bombarded by $$^{30}$$Si, $$^{31}$$P, $$^{34,36}$$S, $$^{40}$$Ar, $$^{40,48}$$Ca projectiles. We observed a strong variation of the distribution as function of bombarding energy. The results were interpreted by the effects of nuclear deformation of the target nucleus on the competition between fusion and quasifission. The symmetric fission includes fusion-fission, wheres as the asymmetric fission represents quasifission. The fusion probabilities were determined with the help of fluctuation-dissipation model. The estimated evaporation residue cross sections ($$^{263,264}$$Sg and $$^{267,268}$$Hs) produced in $$^{30}$$Si + $$^{238}$$U and $$^{34}$$S+$$^{238}$$U reproduced the experimental data.

論文

Nuclear orientation in the reaction $$^{34}$$S+$$^{238}$$U and synthesis of the new isotope $$^{268}$$Hs

西尾 勝久; Hofmann, S.*; He${ss}$berger, F. P.*; Ackermann, D.*; Antalic, S.*; 有友 嘉浩; Comas, V. F.*; D$"u$llmann, Ch. E.*; Gorshkov, A.*; Graeger, R.*; et al.

Physical Review C, 82(2), p.024611_1 - 024611_9, 2010/08

 被引用回数:54 パーセンタイル:4.71(Physics, Nuclear)

Synthesis of isotopes of the element hassium was studied using the reaction $$^{34}$$S+$$^{238}$$U$$rightarrow$$$$^{272}$$Hs*. At a kinetic energy of 163.0 MeV in the center of mass system we observed one $$alpha$$-decay chain starting at the isotope $$^{267}$$Hs. The cross-section was 1.8$$^{+4.2}_{-1.5}$$pb. At 152.0 MeV one decay of the new isotope $$^{268}$$Hs was observed. It decays with a half-life of 0.38$$^{+1.8}_{-0.17}$$s by 9479$$pm$$16 keV $$alpha$$-particle emission. Spontaneous fission of the daughter nucleus $$^{264}$$Sg was confirmed. The measured cross-section was 0.54$$^{+1.3}_{-0.45}$$pb. In-beam measurements of fission fragment mass distributions were performed to obtain information on the fusion probability at various orientation of the deformed target nucleus. The distributions changed from symmetry to asymmetry when the beam energy was changed from above-barrier to sub-barrier values, indicating orientation effects on fusion and/or quasifission. It was found that the distribution of symmetric mass fragments originates not only from fusion-fission, but has a strong component from quasifission. This result significantly influences the interpretation of data obtained from measurements of symmetric mass distributions with respect to extracting evaporation residue cross-sections.

論文

Orientation effects of deformed $$^{238}$$U target nuclei on the fusion probability for the heavy element synthesis

西尾 勝久; Hofmann, S.*; 池添 博; He${ss}$berger, F. P.*; Ackermann, D.*; Antalic, S.*; Comas, V. F.*; Gan, Z.*; Heinz, S.*; Heredia, J. A.*; et al.

Nuclear Physics A, 805(1-4), p.516 - 518, 2008/06

The effects of the orientation of the deformed $$^{238}$$U target nuclei were investigated in the reaction of $$^{30}$$Si+$$^{238}$$U. The cross-sections of the system to overcome the Coulomb barrier were determined by measuring the fission cross-sections at JAEA-tandem accelerator, from which the Coulomb barrier height for the collision of $$^{30}$$Si to the polar side and the equatorial side of $$^{238}$$U were determined. The evaporation residue (ER) cross-sections were determined at the velocity filter SHIP of GSI. Significant enhancement of the cross-sections of $$^{264}$$Sg(4n) in the sub-barrier energy indicates the lowering of the Coulomb barrier at the Polar collisions. However, competition between fusion and quasifission was implied. The cross-sections for $$^{263}$$Sg(5n) measured at the above barrier energy indicated that there is no significant fusion hindrance at the equatorial collisions. In this contribution, the systematic behavior of the orientation effects of $$^{238}$$U is discussed by using additionally the data of $$^{16}$$O+$$^{238}$$U and $$^{48}$$Ca+$$^{238}$$U.

論文

Measurement of evaporation residue and fission cross sections of the reaction $$^{30}$$Si + $$^{238}$$U at subbarrier energies

西尾 勝久; Hofmann, S.*; 池添 博; He${ss}$berger, F. P.*; Ackermann, D.*; Antalic, S.*; Comas, V. F.*; Gan, Z.*; Heinz, S.*; Heredia, J. A.*; et al.

Journal of Nuclear and Radiochemical Sciences, 8(2), p.73 - 78, 2007/10

Effects of the prolate deformation of $$^{238}$$U on fusion were studied in the reaction $$^{30}$$Si + $$^{238}$$U at bombarding energies close to the Coulomb barrier. The fission cross section was measured at JAEA tandem accelerator to see the enhancement of the cross section in the subbarrier energy due to the lowering of the Coulomb barrier at the collision of the projectile to polar side of $$^{238}$$U. Evaporation residue cross sections were measured at UNILAC of GSI by using the velocity filter SHIP to determine the fusion probability. In the subbarrier region, we could produce isotope $$^{264}$$Sg by 4n channel, but fusion hindrance was implied in the analysis. At above barrier energies, we produced $$^{263}$$Sg by 5n channel. The analysis suggests that there is no fusion hindrance at the equatorial collisions.

論文

Identification of a K isomer in $$^{252}$$No

Sulignano, B.*; Heinz, S.*; He${ss}$berger, F. P.*; Hofmann, S.*; Ackermann, D.*; Antalic, S.*; Kindler, B.*; Kojouharov, I.*; Kuusiniemi, P.*; Lommel, B.*; et al.

European Physical Journal A, 33(4), p.327 - 331, 2007/09

 被引用回数:56 パーセンタイル:4.37(Physics, Nuclear)

The $$K$$-isomer with a half-life of 100 ms has been discovered in the nucleus $$^{252}$$No. The nucleus was produced in the fusion-evaporation reacion $$^{206}$$Pb($$^{48}$$Ca,2n)$$^{252}$$No. The experiment was carried out at the velocity filter SHIP of GSI. The isomeric state is located at an excitation energy of 1254 keV and is interpreted as a two quasi-neutron state with $$K$$=8$$^{-}$$. The isomeric state has direct connection to the rotational states of 8$$^{+}$$ in $$^{252}$$No. Also the rotational states of 6$$^{+}$$ and 4$$^{+}$$ was feeded via the octupole band with $$K$$=2$$^{-}$$. The isomeric state and the ground state was populated with the equal intensity in the fusion-evaporation reaction.

論文

The Reaction $$^{48}$$Ca+$$^{238}$$U $$rightarrow$$ $$^{286}$$112$$^{*}$$ studied at the GSI-SHIP

Hofmann, S.*; Ackermann, D.*; Antalic, S.*; Burkhard, H. G.*; Comas, V. F.*; Dressler, R.*; Gan, Z.*; Heinz, S.*; Heredia, J. A.*; He${ss}$berger, F. P.*; et al.

European Physical Journal A, 32(3), p.251 - 260, 2007/06

 被引用回数:202 パーセンタイル:0.36(Physics, Nuclear)

The fusion reaction of $$^{48}$$Ca projectiles with$$^{238}$$U target nuclei was studied at the velocity filter SHIP of GSI. Two decay chains were measured, which fully confirm data that were previously assigned to the isotope $$^{283}$$112 in experiments at the Flerov Laboratory in Dubna. Two other events are consistent with a 50 % spontaneous fission (SF) branch of this isotope. The mean value obtained for the half-life of $$^{283}$$112 is (6.9$$^{+6.9}_{-2.3}$$) s, the $$alpha$$ energy is (9.520$$pm$$0.015) MeV, and the total kinetic energy (TKE) of SF is (238$$pm$$14) MeV. The half-life of the $$alpha$$ decay daughter nucleus $$^{279}$$Ds is (0.18$$^{+0.32}_{-0.07}$$) s, and the TKE of SF is (210$$^{+32}_{-11}$$) MeV. The cross-section deduced from all four events is (0.72$$^{+0.58}_{-0.35}$$) pb, measured at an excitation energy of 34.6 MeV of the compound nucleus $$^{286}$$112.

論文

Measurement of evaporation residue cross-sections of the reaction $$^{30}$$Si + $$^{238}$$U at subbarrier energies

西尾 勝久; Hofmann, S.*; He${ss}$berger, F. P.*; Ackermann, D.*; Antalic, S.*; Comas, V. F.*; Gan, Z.*; Heinz, S.*; Heredia, J. A.*; 池添 博; et al.

AIP Conference Proceedings 891, p.71 - 79, 2007/03

重イオン融合反応$$^{30}$$Si+$$^{238}$$UによりSg同位体を合成し、この生成断面積を調べた。実験は、ドイツ重イオン研究所(GSI)で行った。反応エネルギーE$$_{c.m.}$$=144MeVにおいて、$$^{263}$$Sgから始まる3つの$$alpha$$崩壊連鎖を観測し、断面積67pbを得た。サブバリヤエネルギーE$$_{c.m.}$$=133MeVで、新同位体$$^{264}$$Sgを3つ合成し、断面積10pbを得た。これは自発核分裂で崩壊し、半減期は120msであった。これら断面積を統計モデル計算と比較した。融合過程においては、$$^{238}$$Uの変形を取り入れたチャンネル結合法で捕獲断面積を決定した。この値は、原子力機構のタンデム加速器を用いて行った$$^{30}$$Si+$$^{238}$$Uの核分裂断面積とよく一致した。E$$_{c.m.}$$=133MeVでの断面積の値は、一次元モデルに比べて10$$^{4}$$倍大きな値であり、ウランの変形によって断面積が増加することがわかった。しかし、計算との不一致から、準核分裂が何割か存在することが示唆された。クーロン障壁より高いエネルギーE$$_{c.m.}$$=144MeVでは$$^{30}$$Siは$$^{238}$$Uの赤道面と接触できる。$$^{263}$$Sgの生成断面積67pbは、統計モデル計算値によく一致し、融合に阻害がなかった。これは、赤道面衝突が融合に有利であることを示唆している。

論文

Alpha-$$gamma$$ decay studies of $$^{255}$$Rf, $$^{251}$$No and $$^{247}$$Fm

He${ss}$berger, F. P.*; Hofmann, S.*; Ackermann, D.*; Antalic, S.*; Kindler, B.*; Kojouharov, I.*; Kuusiniemi, P.*; Leino, M.*; Lommel, B.*; Mann, R.*; et al.

European Physical Journal A, 30(3), p.561 - 569, 2006/12

 被引用回数:39 パーセンタイル:9.44(Physics, Nuclear)

$$^{255}$$Rf, $$^{251}$$No及び$$^{247}$$Fmについて、$$alpha$$線と$$gamma$$線の検出による崩壊核分光実験を行った。実験は、ドイツ重イオン研究所の線形加速器施設UNILACにおける反跳生成核分離装置SHIPを用いて行った。SHIPで分離された蒸発残留核を焦点に設置したシリコン検出器に打ち込んだ。この検出器で$$alpha$$崩壊を検出し、また近くに設けたGe検出器で$$gamma$$線を検出した。これら同位体は、$$^{207}$$Pb($$^{50}$$Ti,2n)$$^{255}$$Rf及びこの$$^{255}$$Rfの$$alpha$$崩壊核種として生成し、さらに$$^{206}$$Pb($$^{48}$$Ca,3n)$$^{251}$$No反応及びこの$$^{251}$$Noの$$alpha$$崩壊核種として生成した。解析から次のことがわかった。(1)$$^{251}$$Noには半減期1.0秒のアイソマがあることがわかっていたが、このエネルギー準位が106keVであることがわかった。(2)また、$$^{251}$$Noには2$$mu$$sの半減期を持つ第2のアイソマがあり、$$^{206}$$Pb($$^{48}$$Ca,3n)$$^{251}$$No反応で直接生成されることがわかった。このアイソマは、2本の$$gamma$$線とコインシデンスし、解析から励起エネルギーが1700keV以上の準位にあることが示唆された。この2本の$$gamma$$線は、$$^{255}$$Rfの$$alpha$$崩壊につづく$$gamma$$遷移と一致した。

論文

Measurement of evaporation residue cross-sections of the reaction $$^{30}$$Si+$$^{238}$$U at subbarrier energies

西尾 勝久; Hofmann, S.*; He${ss}$berger, F. P.*; Ackermann, D.*; Antalic, S.*; Comas, V. F.*; Gan, Z.*; Heinz, S.*; Heredia, J. A.*; 池添 博; et al.

European Physical Journal A, 29(3), p.281 - 287, 2006/09

 被引用回数:51 パーセンタイル:6.26(Physics, Nuclear)

重イオン融合反応$$^{30}$$Si+$$^{238}$$UによりSg同位体を合成し、この生成断面積を調べた。実験は、ドイツ重イオン研究所(GSI)で行った。反応エネルギーE$$_{c.m.}$$=144MeVにおいて、$$^{263}$$Sgから始まる3つの$$alpha$$崩壊連鎖を観測した。断面積は、67pbであった。サブバリヤエネルギーE$$_{c.m.}$$=133MeVで、新同位体$$^{264}$$Sgを3つ合成し断面積10pbを得た。これは自発核分裂で崩壊し、半減期は120msであった。これら断面積を統計モデル計算と比較した。融合過程においては、$$^{238}$$Uの変形を取り入れたチャンネル結合法で捕獲断面積を決定した。この値は、原子力機構のタンデム加速器を用いて行った$$^{30}$$Si+$$^{238}$$Uの核分裂断面積とよく一致した。E$$_{c.m.}$$=133MeVでの断面積の値は、一次元モデルに比べて10$$^{4}$$倍大きな値であり、ウランの変形によるサブバリヤでの断面積の増加で説明できた。しかし、計算との不一致から、準核分裂が何割か存在することが示唆された。クーロン障壁より高いエネルギーE$$_{c.m.}$$=144MeVでは$$^{30}$$Siは$$^{238}$$Uの赤道面と接触できる。$$^{263}$$Sgの生成断面積65pbは、統計モデル計算値によく一致し、融合に阻害がなかった。これは、赤道面衝突が融合に有利であることを示唆している。

論文

Alpha-$$gamma$$ decay studies of $$^{255}$$No

He${ss}$berger, F. P.*; Hofmann, S.*; Ackermann, D.*; Antalic, S.*; Kindler, B.*; Kojouharov, I.*; Kuusiniemi, P.*; Leino, M.*; Lommel, B.*; Mann, R.*; et al.

European Physical Journal A, 29(2), p.165 - 173, 2006/08

 被引用回数:33 パーセンタイル:11.58(Physics, Nuclear)

The decay of $$^{255}$$No was investigated by means of $$alpha$$-$$gamma$$ spectroscopy. The experiment was carried out by using the velocity filter SHIP at GSI. The isotope was produced in the reactions $$^{208}$$Pb($$^{48}$$Ca,n)$$^{255}$$No, $$^{209}$$Bi($$^{48}$$Ca,2n)$$^{255}$$Lr (EC)-$$>$$ $$^{255}$$No, and $$^{238}$$U($$^{22}$$Ne,5n)$$^{255}$$No. Decay data for $$alpha$$-$$gamma$$ coincidence measurements were obtained, and the level structures of the daughter nucleus $$^{251}$$Fm were obtained, including the tentative assignment of (1) 9/2$$^{-}$$ for ground state and (2) 5/2$$^{+}$$ for the 200 keV isomeric state with $$T_{1/2}$$=21 $$mu$$s.

論文

$$alpha$$-decay of the new isotope $$^{187}$$Po; Probing prolate structures beyond the neutron mid-shell at N = 104

Andreyev, A. N.*; Antalic, S.*; Ackermann, D.*; Franchoo, S.*; He${ss}$berger, F. P.*; Hofmann, S.*; Huyse, M.*; Kojouharov, I.*; Kindler, B.*; Kuusiniemi, P.*; et al.

Physical Review C, 73(4), p.044324_1 - 044324_8, 2006/04

 被引用回数:29 パーセンタイル:13.82(Physics, Nuclear)

The new neutron-deficient isotope $$^{187}$$Po has been identified in the complete fusion reaction $$^{46}$$Ti+$$^{144}$$Sm $$rightarrow$$ $$^{187}$$Po+3n at the velocity filter SHIP of GSI. Striking features of the $$^{187}$$Po $$alpha$$ decay are the strongly-hindered decay to the spherical ground state and unhindered decay to a surprisingly low-lying deformed excited state at 286 keV in the daughter nucleus $$^{183}$$Pb. Based on the potential energy surface calculations, the $$^{187}$$Po ground state and the 286 keV excited state in $$^{183}$$Pb were interpreted as being of prolate origin.

論文

$$alpha$$-decay spectroscopy of the new isotope $$^{192}$$At

Andreyev, A. N.*; Antalic, S.*; Ackermann, D.*; Franchoo, S.*; He${ss}$berger, F. P.*; Hofmann, S.*; Huyse, M.*; Kojouharov, I.*; Kindler, B.*; Kuusiniemi, P.*; et al.

Physical Review C, 73(2), p.024317_1 - 024317_11, 2006/02

 被引用回数:20 パーセンタイル:20.91(Physics, Nuclear)

Decay properties of the new neutron-deficient nuclide $$^{192}$$At have been studied in the complete fusion reaction $$^{144}$$Sm($$^{51}$$V,3n)$$^{192}$$At at the velocity filter SHIP of GSI. Two isomeric states with half-lives of 88(6) ms and 11.5(6) ms, respectively, and with complex $$alpha$$-decay schemes were identified in $$^{192}$$At. The decay pattern of one of the isomers suggests that it is based on the oblate-deformed $$pi$$2f7/2 $$times$$ $$nu$$1i13/2 configuration, which confirms the expected onset of deformation in the At isotopes by approaching the neutron midshell at N=104.

口頭

融合反応$$^{30}$$Si+$$^{238}$$UによるSgの合成

西尾 勝久; 光岡 真一; 池添 博; Hofmann, S.*; He${ss}$berger, F. P.*; Ackermann, D.*; Antalic, S.*; Comas, V. F.*; Gan, Z.*; Heinz, S.*; et al.

no journal, , 

重イオン融合反応$$^{30}$$Si+$$^{238}$$UによりSg同位体を合成し、この生成断面積を調べた。実験は、ドイツ重イオン研究所(GSI)で行った。反応エネルギーE$$_{c.m.}$$=144MeVにおいて、$$^{263}$$Sgから始まる3つの$$alpha$$崩壊連鎖を観測した。断面積は、67pbであった。サブバリヤエネルギーE$$_{c.m.}$$=133MeVで新同位体$$^{264}$$Sgを3つ合成し、断面積10pbを得た。これは自発核分裂で崩壊し、半減期は120msであった。これら断面積を統計モデル計算と比較した。融合過程においては、$$^{238}$$Uの変形を取り入れたチャンネル結合法で捕獲断面積を決定した。この値は、原子力機構のタンデム加速器を用いて行った$$^{30}$$Si+$$^{238}$$Uの核分裂断面積とよく一致した。E$$_{c.m.}$$=133MeVでの断面積の値は、一次元モデルに比べて10$$^{4}$$倍大きな値であり、ウランの変形によってサブバリヤで断面積が増加していることを明らかにした。しかし、計算との不一致から、準核分裂が何割か存在することが示唆された。クーロン障壁より高いエネルギーE$$_{c.m.}$$=144MeVでは$$^{30}$$Siは$$^{238}$$Uの赤道面と接触できる。$$^{263}$$Sgの生成断面積67pbは、統計モデル計算値によく一致し、融合に阻害がなかった。これは、赤道面衝突が融合に有利であることを示唆している。

口頭

Orientation effects of deformed $$^{238}$$U target nuclei on the fusion probability for heavy element synthesis

西尾 勝久; Hofmann, S.*; 池添 博; Ackermann, D.*; Antalic, S.*; Comas, V. F.*; Gan, Z.*; Heinz, S.*; Heredia, J. A.*; He${ss}$berger, F. P.*; et al.

no journal, , 

Effects of the prolate deformation of $$^{238}$$U on the fusion probability was investigated in the reactions of $$^{16}$$O+$$^{238}$$U and $$^{30}$$Si+$$^{238}$$U. The experiment was carried out at the JAEA tandem accelerator and GSI of Darmstadt. For $$^{16}$$O+$$^{238}$$U, the evaporation residue cross-sections for fermium isotopes were measured down to deep sub-barrier. The results indicate that the projectile $$^{16}$$O fuses from every colliding angle to the symmetric axis of the $$^{238}$$U target nucleus. In the reacion of $$^{30}$$Si+$$^{238}$$U, fusion hindrance was implied in the deep sub-barrier energies, but no fusion hindrance was suggested at the above barrier energy where the collision on the equatorial side of $$^{238}$$U starts to contribute. The results suggest that the equatorial collisions has advantage to produce heavy elements in the reaction using heavier projectiles.

口頭

$$^{34}$$S+$$^{238}$$Uのサブバリヤエネルギー融合反応による新同位体$$^{268}$$Hsの合成

西尾 勝久; Hofmann, S.*; He${ss}$berger, F. P.*; Ackermann, D.*; Antalic, S.*; Comas, V. F.*; D$"u$llmann, Ch. E.*; Gorshkov, A.*; Graeger, R.*; Heinz, S.*; et al.

no journal, , 

$$^{34}$$S+$$^{238}$$Uのサブバリヤエネルギー融合反応により、超重元素(Z=108)の新同位体$$^{268}$$Hsを合成した。実験に先立ち、原子力機構タンデム加速器においてこの反応の核分裂断面積の励起関数を測定するとともに、核分裂片の質量数分布を測定した。モデル計算により、サブバリヤ領域でも融合反応確率が存在することを予測した。$$^{268}$$Hsの合成実験は、ドイツGSIの線形加速器UNILACに設置された反跳生成核分離装置SHIPを用いて行った。合成に成功し、断面積として0.54pbを得た。$$^{268}$$Hsは$$alpha$$崩壊核種で、半減期0.38 sであることがわかった。

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Nuclear orientation in fusion and synthesis of heavy element at sub-barrier energy

西尾 勝久; 池添 博; Hofmann, S.*; Ackermann, D.*; Antalic, S.*; 有友 嘉浩; Comas, V. F.*; D$"u$llmann, Ch. E.*; Gorshkov, A.*; Graeger, R.*; et al.

no journal, , 

In-beam measurement of fission-fragment mass distributions in $$^{34}$$S + $$^{238}$$U were performed to obtain information on the fusion probability at various orientations of the deformed target nucleus. The experiment was carried out at the JAEA tandem facility. The distributions changed from symmetry to asymmetry when the beam energy was changed from above-barrier to sub-barrier values, indicating the orientation effects on fusion and/or quasifission. We also produced evaporation residues (ERs) as a direct evidence for fusion. The experiment was carried out at GSI by using the velocity filter SHIP. At above-barrier energy, the isotope $$^{267}$$Hs (5n) was produced and the cross section of 1.8 pb was obtained. It was concluded that the distribution of symmetric mass fragments originate not only from fusion fission, but has a strong component from quasifission. At sub-barrier energy, we produced a new isotope $$^{268}$$Hs (4n) with the cross section 0.54 pb. We obtained the fusion probability enough to produce SHN. These results were supported by a calculation based on a dynamical description using Langevin equation, where fusion and quasifission were separately determined. The effects of projectile mass (charge) on fusion from the measured fission properties of $$^{30}$$Si, $$^{31}$$P, $$^{36}$$S, $$^{40}$$Ar, $$^{40,48}$$Ca + $$^{238}$$U are obtained.

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原子力機構における融合核分裂・準核分裂および多核子移行反応の実験研究

廣瀬 健太郎; 西尾 勝久; 西中 一朗; 牧井 宏之; 池添 博*; Orlandi, R.; L$'e$guillon, R.; 塚田 和明; 浅井 雅人; 永目 諭一郎; et al.

no journal, , 

原子核に重イオンを衝突させたとき、入射核が標的核から多数の核子を受け取り、複合核を形成せずに核分裂(準核分裂)する場合や、複合核を形成して核分裂(融合核分裂)する場合がある。これらを弁別する手法を確立するために、原子力機構タンデム加速器施設を使用し、準核分裂および融合核分裂のメカニズムを調べてきた。$$^{238}$$Uに種々の原子核を衝突させ、合成した複合核の核分裂質量分布を測定した。取得した質量分布を、動的計算と比較した結果、よい一致を得た。この理論では原子核のポテンシャル上を系がどのように時間発展していくかを追跡できるため、融合核分裂が全核分裂にしめる割合(融合確率)を計算できる。得られた融合確率により、重元素合成断面積の実験結果をよく再現できた。すなわち重イオン入射反応において系が複合核を形成するか否かを知るための手法を確立した。さらに我々は、多核子移行反応を用いた核分裂の研究を行っている。これまでに、$$^{232}$$Th, $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm標的に$$^{18}$$Oビームを照射し、広い核種・励起エネルギー領域で複合核を合成し、その核分裂における質量分布などを測定した。一度の測定で約20核種を合成できることが多核子移行反応の利点であり、その半数程度の核種の核分裂質量分布は本研究により初めて得られた。これまでに一部の核種では質量分布の測定結果を先の理論計算により再現できることを確認した。今後さらなる比較を行い、理論計算の改良・適用領域の拡大が期待できる。

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