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論文

Neutron capture cross sections of light neutron-rich nuclei relevant for $$r$$-process nucleosynthesis

Bhattacharyya, A.*; Datta, U.*; Rahaman, A.*; Chakraborty, S.*; Aumann, T.*; Beceiro-Novo, S.*; Boretzky, K.*; Caesar, C.*; Carlson, B. V.*; Catford, W. N.*; et al.

Physical Review C, 104(4), p.045801_1 - 045801_14, 2021/10

中性子過剰核に対する中性子捕獲断面積は、$$r$$過程元素合成を理解する上で直接必要なデータであるものの、中性子標的が存在しないことと中性子過剰核の半減期が短いため、その実験を行うことは困難である。その代替手段として、$$(n,gamma)$$反応の逆過程である$$(gamma,n)$$反応のデータから導出する方法が行われている。本論文では、中性子数20領域の中性子過剰核に対し、クーロン分解反応を用いて$$(gamma,n)$$反応の断面積を測定し、そこから$$(n,gamma)$$反応の断面積を得た。その結果、$$^{28,29}$$Naと$$^{34}$$Alに対する中性子捕獲断面積については、統計模型で予測された値よりも小さく、$$^{32}$$Mgに対しては大きな値となることがわかった。

論文

Competition between allowed and first-forbidden $$beta$$ decays of $$^{208}$$At and expansion of the $$^{208}$$Po level scheme

Brunet, M.*; Podoly$'a$k, Zs.*; Berry, T. A.*; Brown, B. A.*; Carroll, R. J.*; Lica, R.*; Sotty, Ch.*; Andreyev, A. N.; Borge, M. J. G.*; Cubiss, J. G.*; et al.

Physical Review C, 103(5), p.054327_1 - 054327_13, 2021/05

 被引用回数:1 パーセンタイル:74.98(Physics, Nuclear)

The structure of $$^{208}$$Po populated through the EC/$$beta^{+}$$ decay of $$^{208}$$At is investigated using $$gamma$$-ray spectroscopy at the ISOLDE Decay Station. The presented level scheme contains 27 new excited states and 43 new transitions, as well as a further 50 previously observed $$gamma$$ rays which have been (re)assigned a position. Through the analysis using the shell model calculations approximately half of the $$beta$$-decay strength of $$^{208}$$At is found to proceed via allowed decay and half via first-forbidden decay. The first-forbidden transitions predominantly populate core excited states at high excitation energies, which is qualitatively understood using shell model considerations.

論文

Investigation of the $$Delta$$$$n$$ = 0 selection rule in Gamow-Teller transitions; The $$beta$$-decay of $$^{207}$$Hg

Berry, T. A.*; Podoly$'a$k, Zs.*; Carroll, R. J.*; Lic$u{a}$, R.*; Grawe, H.*; Timofeyuk, N. K.*; Alexander, T.*; Andreyev, A. N.; Ansari, S.*; Borge, M. J. G.*; et al.

Physics Letters B, 793, p.271 - 275, 2019/06

 被引用回数:3 パーセンタイル:52.47(Astronomy & Astrophysics)

Gamow-Teller $$beta$$ decay is forbidden if the number of nodes in the radial wave functions of the initial and final states is different. This $$Delta$$$$n$$ = 0 requirement plays a major role in the $$beta$$ decay of heavy neutron-rich nuclei, affecting the nucleosynthesis through the increased half-lives of nuclei on the astrophysical $$r$$-process pathway below both $$Z$$ = 50 (for $$N >$$ 82) and $$Z$$ = 82 (for $$N >$$ 126). The level of forbiddenness of the $$Delta$$$$n$$ = 1 $$nu$$1$$g_{9/2} rightarrow pi$$0$$g_{7/2}$$ transition has been investigated from the $$beta^{-}$$ decay of the ground state of $$^{207}$$Hg into the single-proton-hole nucleus $$^{207}$$Tl in an experiment at the ISOLDE Decay Station. From statistical observational limits on possible $$gamma$$-ray transitions depopulating the $$pi$$0$$g^{-1}_{7/2}$$ state in $$^{207}$$Tl, an upper limit of 3.9$$times$$10$$^{-3}$$ % was obtained for the probability of this decay, corresponding to log$$ft >$$ 8.8 within a 95% confidence limit. This is the most stringent test of the $$Delta$$$$n$$ = 0 selection rule to date.

論文

Beta-delayed proton emission from $$^{20}$$Mg

Lund, M. V.*; Andreyev, A.; Borge, M. J. G.*; Cederk$"a$ll, J.*; De Witte, H.*; Fraile, L. M.*; Fynbo, H. O. U.*; Greenlees, P. T.*; Harkness-Brennan, L. J.*; Howard, A. M.*; et al.

European Physical Journal A, 52(10), p.304_1 - 304_14, 2016/10

 被引用回数:13 パーセンタイル:75.95(Physics, Nuclear)

Beta-delayed proton emission from $$^{20}$$Mg has been measured at ISOLDE, CERN, with the ISOLDE Decay Station setup including both charged-particle and $$gamma$$-ray detection capabilities. A total of 27 delayed proton branches were measured including seven so far unobserved. An updated decay scheme, including three new resonances above the proton separation energy in $$^{20}$$Na and more precise resonance energies, is presented. Beta-decay feeding to two resonances above the Isobaric Analogue State in $$^{20}$$Na is observed. This may allow studies of the 4032.9(2.4) keV resonance in $$^{19}$$Ne through the beta decay of $$^{20}$$Mg, which is important for the astrophysically relevant reaction $$^{15}$$O($$alpha$$,$$gamma$$)$$^{19}$$Ne. Beta-delayed protons were used to obtain a more precise value for the half-life of $$^{20}$$Mg, 91.4(1.0) ms.

論文

Fast-timing study of the $$l$$-forbidden 1/2$$^+$$ $$rightarrow$$ 3/2$$^+$$ $$M1$$ transition in $$^{129}$$Sn

Lic$u{a}$, R.*; Mach, H.*; Fraile, L. M.*; Gargano, A.*; Borge, M. J. G.*; M$u{a}$rginean, N.*; Sotty, C. O.*; Vedia, V.*; Andreyev, A.; Benzoni, G.*; et al.

Physical Review C, 93(4), p.044303_1 - 044303_7, 2016/04

 被引用回数:5 パーセンタイル:46.38(Physics, Nuclear)

The levels in $$^{129}$$Sn populated from the $$beta^-$$ decay of $$^{129}$$In isomers were investigated at the ISOLDE facility of CERN using the newly commissioned ISOLDE Decay Station. The lowest 1/2$$^+$$ state and the 3/2$$^+$$ ground state in $$^{129}$$Sn are expected to have configurations dominated by the neutron $$s_{1/2}$$ ($$l$$=0) and $$d_{3/2}$$ ($$l$$=2) single-particle states, respectively. Consequently, these states should be connected by a somewhat slow $$l$$-forbidden $$M1$$ transition. Using fast-timing spectroscopy we have measured the half-life of the 1/2$$^+$$ 315.3-keV state, $$T_{1/2}$$ = 19(10) ps, which corresponds to a moderately fast $$M1$$ transition. Shell-model calculations using the CD-Bonn effective interaction, with standard effective charges and $$g$$ factors, predict a 4-ns half-life for this level. We can reconcile the shell-model calculations to the measured $$T_{1/2}$$ value by the renormalization of the $$M1$$ effective operator for neutron holes.

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