Production of neutron-rich isotopes by cold fragmentation in the reaction 197Au + Be at 950 A MeV

doi:10.1016/S0375-9474(99)00386-3

Nuclear Physics A

Volume 660, Issue 1, 15 November 1999, Pages 87-100

https://doi.org/10.1016/S0375-9474(99)00386-3 Get rights and content

Abstract

The production cross sections and longitudinal-momentum distributions of very neutron-rich isotopes have been investigated in the fragmentation of a 950 $A$ MeV $^{179}$ Au beam in a beryllium target. Seven new isotopes ( $^{193}$ Re, $^{194}$ Re, $^{191}$ W, $^{192}$ W, $^{189}$ Tl, $^{187}$ Hf and $^{188}$ Hf) and the five-proton-removal channel were observed for the first time. The reaction mechanism leading to the formation of these very neutron-rich isotopes is explained in terms of the cold-fragmentation process. An analytical model describing this reaction mechanism is presented.

Introduction

The possibility to extend the limits of the chart of the nuclides towards the neutron-rich side opens new opportunities for nuclear-structure investigations. Halo or skin nuclei, double and single shell closures far from stability or new regions of deformation are some examples of studies accessible by the production of neutron-rich nuclei. Many of these nuclei have special interest for the stellar nucleosynthesis problem.

The first difficulty of any experimental study in this field is the production of neutron-rich isotopes. Two reaction mechanisms are mainly used for the production of these isotopes: fragmentation and fission. While fission has successfully been used to produce medium-mass neutron-rich nuclei [1], [2], [3], fragmentation reactions are better suited to approach the neutron drip line for light [4], [5] and heavy fragments [6], [7].

From a technical point of view, two different approaches have been used for the production of non-stable nuclei: the isotopic separation on-line (ISOL) and the in-flight separation. While the ISOL technique benefits from higher primary-beam intensities, the efficiency of this method is strongly influenced by the chemical properties and by the half-life of the isotope to be extracted from the ion source. Projectile fragmentation using relativistic heavy-ion beams and an in-flight fragment separator take profit of the inverse kinematics and the short time needed for the isotope identification. For the present study, we applied the latter method.

Peripheral nuclear collisions at relativistic energies are subject to large fluctuations in the $N$ over $Z$ ratio of the abraded nucleons and in the excitation energy acquired. Very neutron-rich nuclei result from reactions where mostly protons are abraded and only few neutrons are evaporated. As pointed out in Ref. [8], the proton-removal channel is a key process for the understanding of the production of very neutron-rich heavy nuclei in relativistic fragmentation reactions. These reaction products which keep all neutrons of the projectile are supposed to result from extremely cold fragmentation reactions where only protons are abraded. Since the excitation energy acquired stays below the particle threshold, any evaporation which would predominantly lead to a loss of neutrons is prohibited.

In the present work, a careful study of the cold-fragmentation mechanism at relativistic energies is presented, based on the measurement of production cross sections of heavy neutron-rich isotopes and their longitudinal-momentum distributions up to the five-proton-removal channel. The data are compared to calculations with a modern version of the abrasion-ablation model [9]. For describing the production of extremely neutron-rich nuclei by cold fragmentation, an analytical formulation is presented.

Section snippets

Experiment and data analysis

The experiment was performed at the SIS synchrotron of GSI which delivered a 950 $A$ MeV $^{197}$ Au pulsed beam with a pulse length of 2 s and a repetition rate of 1/15 Hz. The intensity of the beam was of the order of $4×10^{7}$ ions per pulse. It was measured with a secondary-electron monitor. The beam impinged on a 1023 mg/cm $^{2}$ thick beryllium target placed at the entrance of the projectile-fragment separator FRS.

The FRS was operated as a momentum-loss achromat spectrometer [10], [11]. In addition to the

Production cross sections

In order to obtain production cross sections, the primary-beam intensity was calibrated using an ionisation chamber as reported in Ref. [17] with an accuracy better than 10%. The final cross sections were corrected for transmission losses of the FRS, dead-time, ionic charge changing and secondary reactions in the layers of matter inside the FRS.

The transmission was evaluated by using the method described in Ref. [18]. This correction ranges from 0% to 50% depending on the positions of the

The cold-fragmentation process.

Heavy-ion reactions at relativistic energies can be described as a two-step process consisting of a fast interaction between the projectile and the target and the subsequent deexcitation of the reaction products. The initial stage can be described by a Glauber-type model where only the nucleons in the overlapping region between the projectile and the target, the “participants” interact strongly, while nucleons in the non-overlapping zone, the ”spectators”, continue to move almost undisturbed

Conclusion and outlook

The production cross sections and longitudinal-momentum distributions of very neutron-rich isotopes produced by the fragmentation of a $^{197}$ Au (950 $A$ MeV) beam in a beryllium target have been investigated. Seven new isotopes as well as the five-proton-removal channel have been observed for the first time. The standard deviations of the longitudinal-momentum distributions are well described by the Goldhaber model indicating that these neutron-rich nuclei are produced in a cold-fragmentation

Acknowledgements

The authors are indebted to K.H. Behr, A. Brünle and K. Burkhard for their technical support to this experiment. This work was partially supported by the European Union under contract ERBFMGECT950083 and by the regional Department of Scientific Research of Galicia under contract number XUGA20603A98.

References (29)

M. Bernas et al.
Phys. Lett. B
(1994)
M. Pfützner et al.
Phys. Lett. B
(1998)
P. Van Duppen et al.
Nucl. Instr. and Methods B
(1998)
K.-H. Schmidt et al.
Nucl. Phys. A
(1992)
J.-J. Gaimard et al.
Nucl. Phys. A
(1991)
H. Geissel et al.
Instr. and Methods B
(1992)
K.-H. Schmidt et al.
Nucl. Instr. and Methods A
(1987)
M. de Jong et al.
Nucl. Phys. A
(1998)
B. Voss et al.
Nucl. Instr. and Methods A
(1995)
M. Pfützner et al.
Nucl. Instr. and Methods B
(1994)

A.R. Junghans et al.

Nucl. Instr. and Methods A

(1996)

A.S. Goldhaber

Phys. Lett. B

(1974)

K.-H. Schmidt et al.

Phys. Lett. B

(1993)

K.-L. Kratz, Proc. of Fission and Properties of Neutron-Rich Nuclei, Sanibel Island, Nov. 1997 (World Scientific,...

Cited by (108)

Nuclear Data Sheets for A=194
2021, Nuclear Data Sheets
Experimental nuclear structure and decay data are evaluated for all of 14 known nuclides of mass 194 (Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, At, Rn). Detailed evaluated spectroscopic information is presented for each reaction and decay experiment, and by combining all the available data, recommended values are provided for energies, spins and parities and half-lives of levels, together with energies, branching ratios and multipolarities of gamma radiations from the levels, and characteristics of beta and alpha radiations in radioactive decays. No excited states have yet been identified in ¹⁹⁴Ta, ¹⁹⁴W, and ¹⁹⁴Rn. Only limited spectroscopic data are available for ¹⁹⁴Re, with a 45−μs isomer identified, but its absolute energy yet to be determined. Only two levels at unknown absolute energies are reported for ¹⁹⁴At, both of which are isomeric but neither can be unambiguously identified as the ground state. Data are scarce for ¹⁹⁴Os, ¹⁹⁴Bi and ¹⁹⁴Po, while ¹⁹⁴Pt, ¹⁹⁴Au, ¹⁹⁴Hg, ¹⁹⁴Tl and ¹⁹⁴Pb are well studied through decays and reactions. For ¹⁹⁴Ir, although a major (n, γ), E=thermal study and two particle-transfer studies provide large amounts of spectral data, still several problematic issues are found in the assignment of multipolarities for the secondary gamma transitions in the (n, γ) study by 2008Ba25, 1998Ba85 and 1998Ba42, all from the same group. Some issues have been resolved by the evaluators, but we suggest that further experimental work is needed on ¹⁹⁴Ir structure to resolve standing issues, and verify the conclusions made by 2008Ba25. Our attempt to contact the prime authors of the 2008Ba25 paper was not successful. The present work supersedes all the earlier evaluations of A=194 nuclides by 2006Si17, 1996Br26, 1989Si01 and 1972Au11.
New ion-optical operating modes of the BigRIPS and ZeroDegree spectrometer for the production and separation of high-quality rare isotopes beams and high-resolution spectrometer experiments
2020, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
BigRIPS is a powerful two-stage in-flight separator for the research with exotic nuclei studied in frontier experiments since more than a decade. The ion-optical system is very versatile due to the multi-stage structure of BigRIPS combined with the ZeroDegree spectrometer or the Superconducting Ring Cyclotron (SRC). Various optical modes can be flexibly realized according to the purpose of experiments. Two categories of developments are presented here. One is a new operating mode of BigRIPS aiming at higher ion-optical resolving power. BigRIPS itself has a two-stage structure. Spatial isotope separation is made at both the first and second stages. In the standard operating mode of BigRIPS, at the second stage the two spatial separations with energy degraders are subtractive in their resolving powers. Here, we present the additive mode. With the resulting increased spatial separation power, the isotopic background can be substantially reduced. Higher ion-optical resolving powers of the first and second BigRIPS degrader stages are also investigated with the goals to reduce further the background and to yield access to new isotopes of heavier elements. The other development is a dispersion-matched system with BigRIPS for high-resolution spectrometer experiments. The BigRIPS and ZeroDegree spectrometer are presently two independent, coupled achromatic systems. A new dispersion-matched mode of BigRIPS and ZeroDegree will enable novel experiments. For high-resolution spectroscopy experiments with high-intensity light projectiles, SRC and BigRIPS can be operated as a dispersion-matched system. The described different ion-optical developments are a base for a new category of experiments exploring exotic nuclei and mesic atoms. Characteristic future experiments with these new ion-optical developments are exemplified in this report.
Possible evolutions of rare isotope production methods and the associated instrumentation
2013, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
In recent decades, an impressive progress was made in high intensity light and heavy ion accelerators, reaching now several hundreds kilowatts to megawatt average beam power. Simultaneously, large solid angle spectrometers were developed, preserving good resolution properties by the introduction of sophisticated high order corrections. For further progress in the study of the properties of nuclei far from stability, it will be important to optimize the global layout of the accelerator-experimental devices. It will be more and more important to take radiation safety concerns into account since the very conception of the devices. The global efficiency to obtain a result concerning the properties of a given nucleus depends not only on the detection efficiencies but also on the resolution of the measurement device. Another new frontier may open now, this is the optimization of the use of primary and secondary beams to avoid dumping of possibly useful beams, and utilize them for multiple purposes. Different aspects of this optimization will be discussed.
Nuclear Data Sheets for A=188
2018, Nuclear Data Sheets
Evaluated nuclear structure and decay data for all nuclei with mass number A=188 (Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po) are presented. The experimental data are compiled and evaluated, and best values for level and gamma-ray energies, quantum numbers, lifetimes, gamma-ray intensities, and other nuclear properties are recommended. Inconsistencies and discrepancies that exist in the literature are noted. This work supersedes the earlier evaluation by Balraj Singh (2002Si10), published in Nuclear Data Sheets 95, 387 (2002). The gamma-ray transition probabilities were calculated using the ENSDF analysis program ruler (www-nds.iaea.org/public/ensdf_pgm/). However, the reader should be warned that the program may produce incorrect results when dealing with non-symmetric uncertainties.
Nuclear Data Sheets for A=193
2017, Nuclear Data Sheets
Evaluated spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented for ¹⁹³Ta, ¹⁹³W, ¹⁹³Re, ¹⁹³Os, ¹⁹³Ir, ¹⁹³Pt, ¹⁹³Au, ¹⁹³Hg, ¹⁹³Tl, ¹⁹³Pb, ¹⁹³Bi, ¹⁹³Po, ¹⁹³At, and ¹⁹³Rn. This evaluation for A=193 supersedes the earlier one by E. Achterberg, et. al. (2006Ac01).
Highlights of this evaluation are the following:
Based on γγ-coincidence measurements by 2005Za15, excited levels 848.93 and 849.083 in ¹⁹³Ir have been merged.
Proposed new spin-parity assignments for $2213.8 + x (J^{π} = 23 / 2^{+})$ , $2426.7 + x (J^{π} = 25 / 2^{+})$ , and $2584.8 + x (J^{π} = 27 / 2^{+})$ by 2011Ba02 (²⁸Si,5nγ) indicate 1ħ lower spin assignments for many of the excited levels in the ¹⁹³Pb Adopted Levels including the Magnetic dipole band 1 based on 2584.8+x level.
2015He27 identified many new structures in ¹⁹³Bi close to the yrast line. Many new states have been added, significantly extending the previously known level scheme of ¹⁹³Bi, including several new rotational bands.
Nuclear Data Sheets for A=189
2017, Nuclear Data Sheets
Evaluated experimental data are presented for 13 known mass 189 nuclides (Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po). Since the 2003Wu02 publication, structure and decay data from 25 new and primary publications have been incorporated in the current work, while adding a new ¹⁸⁹Hf nuclide. New data have been added for all nuclides except ¹⁸⁹Au and ¹⁸⁹Hg. Moreover, several previous datasets were modified for β-decay Q values and conversion coefficients even when no new publications appeared since 2003Wu02.
In spite of large amounts of data available for A=189 nuclides, several deficiencies remain, which are pointed out below in the hope that further experimental work may improve our knowledge of structure of these nuclides. For ¹⁸⁹Hf and ¹⁸⁹Ta, ground-state half-lives, and their decay schemes are unknown. An isomer in ¹⁸⁹Ta has recently been established but its decay characteristics are unknown, even though several gamma rays were connected with its decay. No excited states are known in ¹⁸⁹W and only one in ¹⁸⁹Po. The decay scheme of ¹⁸⁹W is known poorly with most gamma rays left as unassigned. The decay schemes of ¹⁸⁹Au and ¹⁸⁹Pb g.s. suffer from incompleteness, while those for the g.s. and isomer of ¹⁸⁹Tl are almost absent. The decay schemes of g.s. and isomer of ¹⁸⁹Hg are very complex as apparent from the study by 1996Wo04. Evaluators feel that these two decay schemes could be improved with modern gamma-ray detector arrays. While several isomers are known in many of the A=189 nuclides, there is in general lack of information of level half-lives, thus limiting the knowledge of transition probabilities. High-spin structures, including SD bands in ¹⁸⁹Hg and ¹⁸⁹Tl, are known in ¹⁸⁹Re, ¹⁸⁹Ir, ¹⁸⁹Pt, ¹⁸⁹Au, ¹⁸⁹Hg, ¹⁸⁹Tl, ¹⁸⁹Pb and ¹⁸⁹Bi. Single-particle transfer data are available for ¹⁸⁹Re, ¹⁸⁹Os, and ¹⁸⁹Pt; and two-neutron transfer data for ¹⁸⁹Ir.

View all citing articles on Scopus

¹: Present address: Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.

²: present address: Nuclear Physics Laboratory, University of Washinton, Seattle WA 98195, USA.

View full text

Production of neutron-rich isotopes by cold fragmentation in the reaction 197Au + Be at 950 A MeV

Abstract

Introduction

Section snippets

Experiment and data analysis

Production cross sections

The cold-fragmentation process.

Conclusion and outlook

Acknowledgements

Phys. Lett. B

Phys. Lett. B

Nucl. Instr. and Methods B

Nucl. Phys. A

Nucl. Phys. A

Instr. and Methods B

Nucl. Instr. and Methods A

Nucl. Phys. A

Nucl. Instr. and Methods A

Nucl. Instr. and Methods B

Nucl. Instr. and Methods A

Phys. Lett. B

Phys. Lett. B

Production of neutron-rich isotopes by cold fragmentation in the reaction $^{197}$ Au + Be at 950 $A$ MeV