Experimental studies of the two-step scheme with an intense radioactive $^{132}\mathrm{Sn}$ beam for next-generation production of very neutron-rich nuclei

Experimental studies of the two-step scheme with an intense radioactive ${}^{132}{Sn}$ beam for next-generation production of very neutron-rich nuclei

H. Suzuki, K. Yoshida, N. Fukuda, H. Takeda, Y. Shimizu, D. S. Ahn, T. Sumikama, N. Inabe, T. Komatsubara, H. Sato, Z. Korkulu, K. Kusaka, Y. Yanagisawa, M. Ohtake, H. Ueno, T. Kubo, S. Michimasa, N. Kitamura, K. Kawata, N. Imai, O. B. Tarasov, D. Bazin, J. Nolen, and W. F. Henning

Phys. Rev. C 102, 064615 – Published 18 December 2020

Abstract

The study of nuclei in the yet-unexplored, very neutron-rich, mid- to heavy-mass region of the nuclear chart, comprising perhaps half of all nuclei predicted to exist, is leading to new and/or upgraded facilities, as well as to research into the underlying production processes. In the present study, the usefulness of the two-step scheme, an alternate method to produce very neutron-rich nuclei, by a combination of an isotope-separation online (ISOL) system as a first step, and in-beam fragmentation of re-accelerated radioactive isotopes (RIs) as a second step, was investigated with a ${}^{132}{Sn}$ beam. Very neutron-rich RIs around the neutron-rich neutron number $N = 82$ region were produced from the 278-MeV/nucleon ${}^{132}{Sn}$ beam impinging on a 5.97-mm Be target, and their production cross sections were measured. Yields were then estimated for the two-step scheme with the ${}^{132}{Sn}$ beam relative to the ones by a one-step scheme, in-flight fission of a ${}^{238}U$ beam, for 1-MW proton and ${}^{238}U$ beams at respective RI-beam facilities. This comparison suggests that the two-step scheme with the ${}^{132}{Sn}$ beam provides yields $> 40$ -times higher than those with the one-step scheme for the very neutron-rich $N = 82$ region. Moreover, by using various RI beams over the nuclear chart from ISOL, certain kinds of very neutron-rich RIs around the supernova $r$ -process path can be produced with greater yields than by the one-step approach.

Received 28 June 2020
Revised 4 November 2020
Accepted 17 November 2020

DOI:https://doi.org/10.1103/PhysRevC.102.064615

Physics Subject Headings (PhySH)

Nuclear fragmentation

Radioactive beams

Nuclear Physics

Authors & Affiliations

H. Suzuki^1,*, K. Yoshida ¹, N. Fukuda ¹, H. Takeda ¹, Y. Shimizu¹, D. S. Ahn¹, T. Sumikama¹, N. Inabe¹, T. Komatsubara¹, H. Sato¹, Z. Korkulu¹, K. Kusaka¹, Y. Yanagisawa¹, M. Ohtake¹, H. Ueno¹, T. Kubo¹, S. Michimasa ², N. Kitamura², K. Kawata², N. Imai², O. B. Tarasov ³, D. Bazin ³, J. Nolen⁴, and W. F. Henning^4,5

¹RIKEN Nishina Center, 2-1 Hirosawa Wako, Saitama 351-0198, Japan
²Center for Nuclear Study, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
³National Superconducting Cyclotron Laboratory, Michigan State University, 640 South Shaw Lane, East Lansing, Michigan 48824-1321, USA
⁴Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
⁵Technische Universität München, D-85748 Garching, Germany

^*hsuzuki@ribf.riken.jp

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Vol. 102, Iss. 6 — December 2020

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Figure 1
Flow chart of the two-step and one-step schemes of very neutron-rich radioactive isotope (RI) beams. Cases of ${}^{128}{Pd}$ -beam productions are shown. In the two-step scheme, a long-lived neutron-rich RI ( ${}^{132}{Sn}$ ) is produced by proton-induced spallation, in the first step. Then, it is extracted by ISOL and re-accelerated. In the second step, a more neutron-rich RI beam ( ${}^{128}{Pd}$ ) is produced by projectile fragmentation of ${}^{132}{Sn}$ . In the one-step scheme, ${}^{128}{Pd}$ is produced by in-flight fission of ${}^{238}U$ directly.
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Figure 2
Schematic of the BigRIPS separator and ZeroDegree spectrometer with targets, degrader, and particle-identification (PID) detectors. The labels Fn indicate foci. The ${}^{132}{Sn}$ beam was separated at the first stage (F0–F2) of BigRIPS and identified at the second stage (F3–F7) and the subsequent F7–F8 matching section by deducing two mass-to-charge ratios ( $A / Q_{35}$ and $A / Q_{58}$ ). The secondary target of Be was located at F8, the entrance of ZeroDegree. Projectile fragments produced from the ${}^{132}{Sn}$ beam were identified at ZeroDegree by deducing $A / Q, Z$ , and $A$ . See text.
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Figure 3
(a) Correlation plot of $A / Q_{35}$ versus $A / Q_{58}$ for the ${}^{132}{Sn}$ beam in BigRIPS. The subscripts 35 and 58 refer to the $A / Q$ values obtained at F3–F5 and F5–F8, respectively. A gate for the ${}^{132}{Sn}^{50 +}$ beam is shown by a solid pink contour. (b) PID plot of $Z$ versus $A / Q$ in ZeroDegree for very neutron-rich RIs produced in the reaction of ${}^{132}{Sn} +$ Be (5.97 mm) at 278 MeV/nucleon obtained in the ${}^{128}{Pd}$ -main setting. Fully stripped and hydrogen-like Pd isotopes are indicated by solid pink contours and dashed lime contours, respectively. The radii of the ellipses are $2.4 σ$ in distribution along each axis. $2.8 σ$ separation was achieved between ${}^{128}{Pd}^{46 +}$ and ${}^{125}{Pd}^{45 +}$ . (c) $A$ versus $A / Q$ plot of Pd isotopes in the ${}^{128}{Pd}$ -main setting. $\approx 4 σ$ separation was achieved between fully stripped and hydrogen-like ions.
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Figure 4
Measured production cross sections of very neutron-rich RIs produced in the reaction of ${}^{132}{Sn} +$ Be at 278 MeV/nucleon. The red circles and blue squares indicate the RIs produced in the ${}^{126}{Pd}$ -main and ${}^{128}{Pd}$ -main settings, respectively. The filled and open symbols indicate that the distribution peaks were located inside and outside the slit opening at F9, respectively. The black triangles show the data from GSI [23]. The solid pink lines and dashed lime lines show the predictions by the COFRA1.0 [44] and EPAX3.1a [45] codes, respectively. Errors shown are statistical only.
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Figure 5
(a) Yield comparisons between the two-step scheme with a ${}^{132}{Sn}$ beam (red circles) and the one-step in-flight fission of a ${}^{238}U$ beam (blue squares) for Pd ( $Z = 46$ ) isotopes. See text for the assumptions of the primary-beam intensities, production targets, and transmissions in a separator. (b) Expected yields by the two-step scheme with a ${}^{132}{Sn}$ beam. The cross-section data are from this work and from GSI [23]. Black squares show stable nuclei. Red lines indicate the magic numbers; ${}^{132}{Sn}$ is located at their cross point. The neutron drip-line predicted by ktuy05 [46] is shown by black lines. The supernova $r$ -process path [1] predicted by etfsi-q [47] is shown by cyan square dots. (c) Expected yields by the one-step scheme. The cross-section data are from Refs. [48, 49, 50] measured at RIKEN. (d) Ratios of the two-step yields in panel (b) to the one-step yields in panel (c). Moving away from the stability line, the ratio systematically increases. It reaches values of $\approx 40$ at ${}^{128}{Pd}$ .
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Figure 6
(a) Expected yields by the two-step scheme with various kinds of RI beams supplied from ISOL. The RI-beam intensities from ISOL are estimated by using the ISOLDE database [27]. The cross-section data are estimated by using the cofra code [44] with the experimental results from this work and from GSI [23] for the ${}^{132}{Sn}$ beam. See text for other assumptions. The yield of each nuclide ( $Z_{2step}, N_{2step}$ ) plotted was chosen to be the highest one among all fragment yields produced from heavier RI beams ( $Z \geq Z_{2step}, N \geq N_{2step}$ ) supplied from ISOL. (b) Expected yields by the one-step scheme. The cross-section data are from Ref. [28, 48, 49, 50, 53] measured at RIKEN. For the very exotic regions, where cross sections are not measured yet, we assumed that the cross sections decrease by 1/50 with each added neutron. (c) Ratios of the two-step yields for various kinds of RI beams supplied from ISOL in panel (a) to the one-step yields in panel (b). The two-step scheme is expected to be more useful than the one-step scheme in the very neutron-rich regions around $N = 50$ , 60, 82, and 90.
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Physical Review C

covering nuclear physics

Experimental studies of the two-step scheme with an intense radioactive ${}^{132}{Sn}$ beam for next-generation production of very neutron-rich nuclei

H. Suzuki, K. Yoshida, N. Fukuda, H. Takeda, Y. Shimizu, D. S. Ahn, T. Sumikama, N. Inabe, T. Komatsubara, H. Sato, Z. Korkulu, K. Kusaka, Y. Yanagisawa, M. Ohtake, H. Ueno, T. Kubo, S. Michimasa, N. Kitamura, K. Kawata, N. Imai, O. B. Tarasov, D. Bazin, J. Nolen, and W. F. Henning

Phys. Rev. C 102, 064615 – Published 18 December 2020

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Physical Review C

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Experimental studies of the two-step scheme with an intense radioactive Sn132 beam for next-generation production of very neutron-rich nuclei

H. Suzuki, K. Yoshida, N. Fukuda, H. Takeda, Y. Shimizu, D. S. Ahn, T. Sumikama, N. Inabe, T. Komatsubara, H. Sato, Z. Korkulu, K. Kusaka, Y. Yanagisawa, M. Ohtake, H. Ueno, T. Kubo, S. Michimasa, N. Kitamura, K. Kawata, N. Imai, O. B. Tarasov, D. Bazin, J. Nolen, and W. F. Henning

Phys. Rev. C 102, 064615 – Published 18 December 2020

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Experimental studies of the two-step scheme with an intense radioactive ${}^{132}{Sn}$ beam for next-generation production of very neutron-rich nuclei