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論文

Compound-nucleus and doorway-state decays of $$beta$$-delayed neutron emitters $$^{51,52.53}$$K

Xu, Z. Y.*; Grzywacz, R.*; Andreyev, A. N.; 他49名*

Physical Review Letters, 133(4), p.042501_1 - 042501_7, 2024/07

https://doi.org/10.1103/PhysRevLett.133.042501
 被引用回数：1 パーセンタイル：0.00(Physics, Multidisciplinary)

We investigated decays of $$^{51,52,53}$$K at the ISOLDE Decay Station at CERN in order to understand the mechanism of the $$beta$$-delayed neutron-emission ($$beta$$n) process. The experiment quantified neutron and $$gamma$$-ray emission paths for each precursor. We used this information to test the hypothesis, first formulated by Bohr in 1939, that neutrons in the $$beta$$n process originate from the structureless "compound nucleus". The data are consistent with this postulate for most of the observed decay paths. The agreement, however, is surprising because the compound-nucleus stage should not be achieved in the studied $$beta$$ decay due to insufficient excitation energy and level densities in the neutron emitter. In the $$^{53}$$K $$beta$$n decay, we found a preferential population of the first excited state in $$^{52}$$Ca that contradicted Bohr's hypothesis. The latter was interpreted as evidence for direct neutron emission sensitive to the structure of the neutron-unbound state. We propose that the observed nonstatistical neutron emission proceeds through the coupling with nearby doorway states that have large neutron-emission probabilities. The appearance of "compound-nucleus" decay is caused by the aggregated small contributions of multiple doorway states at higher excitation energy.

論文

New isomeric transition in $$^{36}$$Mg; Bridging the $$N = 20$$ and $$N = 28$$ islands of inversion

Madurga, M.*; Christie, J. M.*; Xu, Z.*; Grzywacz, R.*; Poves, A.*; King, T.*; Allmond, J. M.*; Chester, A.*; Cox, I.*; Farr, J.*; et al.

Physical Review C, 109(6), p.L061301_1 - L061301_6, 2024/06

https://doi.org/10.1103/PhysRevC.109.L061301
 被引用回数：2 パーセンタイル：78.45(Physics, Nuclear)

ミシガン州立大学の国立超伝導サイクロトロン研究所にて$$^{48}$$Caからのフラグメンテーション反応によって中性子過剰核$$^{36}$$Mgを生成、分離し、そのアイソマーを探した。その結果、168keVのガンマ線を出して$$2^+$$に半減期約90nsで脱励起するアイソマーを発見した。その性質を議論するため、大規模殻模型計算の結果と比較した。Mg同位体では中性子数20から28領域にかけて広く変形核となることが知られているが、理論的には、前者の領域は$$sd$$殻から2個の中性子が励起することによって変形し、後者の領域は中性子数28の殻ギャップからの中性子励起によって変形する。$$^{36}$$Mgはその中間に位置し、両者の状態が近くに存在することで、励起エネルギーの低い$$0^+$$が出現すると理解される。

論文

Evidence of nonstatistical neutron emission following $$beta$$ decay near doubly magic $$^{132}$$Sn

Heideman, J.*; Grzywacz, R.*; Xu, Z. Y.*; Madurga, M.*; Escher, J. E.*; 河野 俊彦*; Algora, A.*; Andreyev, A. N.; 他41名*

Physical Review C, 108(2), p.024311_1 - 024311_9, 2023/08

https://doi.org/10.1103/PhysRevC.108.024311
 被引用回数：3 パーセンタイル：45.98(Physics, Nuclear)

$$beta$$-delayed neutron emission was observed in the study of the $$beta$$ decay of $$^{134}$$In at ISOLDE CERN. Neutron emission probabilities from the unbound states in $$^{134}$$Sn to known low-lying, single-particle states in $$^{133}$$Sn were measured. The neutron energies were determined using the time-of-flight technique and the decay from excited states in $$^{133}$$Sn was studied by detecting its $$gamma$$ rays. Individual $$beta$$-delayed neutron emission probabilities were determined by correlating the relative intensities and energies of neutrons and $$gamma$$ rays. The new results suggest the violation of the assumption that neutrons are emitted statistically via the intermediate compound nucleus. This impacts the neutron-emission probabilities and other properties of nuclei participating in the $$r$$-process. A model of neutron emission, which links the observed neutron emission probabilities to nuclear shell effects, is proposed.

論文

$$beta$$-delayed neutron spectroscopy of $$^{133}$$In

Xu, Z. Y.*; Madurga, M.*; Grzywacz, R.*; King, T. T.*; Algora, A.*; Andreyev, A. N.; 他43名*

Physical Review C, 108(1), p.014314_1 - 014314_9, 2023/07

https://doi.org/10.1103/PhysRevC.108.014314
 被引用回数：8 パーセンタイル：87.99(Physics, Nuclear)

Nuclei in the vicinity of doubly magic nuclei away from the line of $$beta$$ stability provide a tremendous testing ground to study the evolution of nuclear shell structure as a function of the unbalance between proton and neutron numbers. The decay $$^{133}$$In (Z=49, N=83), just one proton below and one neutron above doubly magic $$^{132}$$Sn, were studied at the ISOLDE Decay Station. Using the RILIS laser ion source, separate measurements of the 9/2$$^+$$ ground state and 1/2$$^-$$ isomer were carried out. With the use of $$beta$$-delayed neutron and $$gamma$$-ray spectroscopy, the decay strengths above the neutron separation energy were quantified for the first time. The spins and parities to the neutron-unbound states based were assigned based on the $$beta$$-decay selection rules, the log ft values, and systematics. The experimental findings greatly extend the current knowledge of the $$^{133}$$In decay from previous works, providing the $$beta$$-strength distribution southeast of $$^{132}$$Sn.

論文

Development of a reference database for beta-delayed neutron emission

Dimitriou, P.*; Dillmann, I.*; Singh, B.*; Piksaikin, V.*; Rykaczewski, K. P.*; Tain, J. L.*; Algora, A.*; Banerjee, K.*; Borzov, I. N.*; Cano-Ott, D.*; et al.

Nuclear Data Sheets, 173, p.144 - 238, 2021/03

https://doi.org/10.1016/j.nds.2021.04.006
 被引用回数：33 パーセンタイル：95.41(Physics, Nuclear)

核分裂の発見以降、核分裂片から放出される遅発中性子の存在には、数多くの関心が集められてきた。原子力発電では、遅発中性子は原子炉動特性の計算や炉の安全運転において非常に重要な役割を演じている。それに加え、遅発中性子は、核構造や天体核、特に新世代のRIビーム施設の中心課題である安定線から離れた原子核でも、大きなインパクトをもっている。現在、ベータ崩壊半減期と遅発中性子分岐比に関するいくつかの文献が利用可能であるが、測定と評価によって実施されるべき十分な考証は、それらの文献ではしばしば抜けている。この欠落部に注目し、遅発中性子の核データ評価とそれに関する新しい文献作成の作業が、国際原子力機関(IAEA)の援助の基に着手され、「ベータ遅発中性子用データベースの開発」に関するCoordinated Research Project (CRP)が結成された。本論文では、CRPで行われたこれまでの結果について報告をする。

論文

Search for $$alpha$$ decay of $$^{104}$$Te with a novel recoil-decay scintillation detector

Xiao, Y.*; Go, S.*; Grzywacz, R.*; Orlandi, R.; Andreyev, A. N.; 浅井 雅人; Bentley, M. A.*; de Angelis, G.*; Gross, C. J.*; Hausladen, P.*; et al.

Physical Review C, 100(3), p.034315_1 - 034315_8, 2019/09

https://doi.org/10.1103/PhysRevC.100.034315
 被引用回数：19 パーセンタイル：83.66(Physics, Nuclear)

A search for super-allowed $$alpha$$ decay of N=Z nuclei $$^{104}$$Te and $$^{108}$$Xe was carried out using a novel recoil-decay scintillator detector at the tandem accelerator facility at Japan Atomic Energy Agency. Inorganic crystal scintillation material of YAP:Ce (Yttrium Aluminium Perovskite) coupled to position-sensitive photo-multiplier tube (PSPMT) was implemented for the first time in a radioactive decay experiment. Residues from the fusion-evaporation reaction $$^{58}$$Ni+$$^{54}$$Fe$$rightarrow$$ $$^{112}$$Xe$$^*$$ were separated by the JAEA Recoil Mass Separator (RMS) and implanted into the YAP:Ce crystal. $$alpha$$ decays of neutron-deficient tellurium isotopes were identified and proton-emission of $$^{109}$$I was observed. No conclusive evidence was found for the decay chain $$^{108}$$Xe$$rightarrow$$$$^{104}$$Te$$rightarrow$$ $$^{100}$$Sn within 3 days experiment. The cross section limit of 130 pb was obtained for production of two events of $$^{108}$$Xe, about an order of magnitude below the expectation based on earlier cross section measurements and HIVAP calculations.

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

https://doi.org/10.1140/epja/i2016-16180-4
 被引用回数：200 パーセンタイル：93.94(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

Remarks on the fission barriers of super-heavy nuclei

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

European Physical Journal A, 52(4), p.116_1 - 116_12, 2016/04

https://doi.org/10.1140/epja/i2016-16116-0
 被引用回数：31 パーセンタイル：87.07(Physics, Nuclear)

Shell-correction energies of super-heavy nuclei are approximated by using Q$$_{alpha}$$ values of measured decay chains. Five decay chains were analyzed, which start at the isotopes $$^{285}$$Fl, $$^{294}$$118, $$^{291}$$Lv, $$^{292}$$Lv and $$^{293}$$Lv. The data are compared with predictions of macroscopic-microscopic models. Fission barriers are estimated that can be used to eliminate uncertainties in partial fission half-lives and in calculations of evaporation-residue cross-sections. In that calculations, fission probability of the compound nucleus is a major factor contributing to the total cross-section. The data also provide constraints on the cross-sections of capture and quasi-fission in the entrance channel of the fusion reaction. Arguments are presented that fusion reactions for synthesis of isotopes of elements 118 and 120 may have higher cross-sections than assumed so far.

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