A large enhancement in the production of neutron-rich projectile residues is
observed in the reactions of a 25 MeV/nucleon 86Kr beam with the neutron rich
124Sn and 64Ni targets relative to the predictions of the EPAX parametrization
of high-energy fragmentation, as well as relative to the reaction with the less
neutron-rich 112Sn target. The data demonstrate the significant effect of the
target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi
energies. A hybrid model based on a deep-inelastic transfer code (DIT) followed
by a statistical de-excitation code appears to account for part of the observed
large cross sections. The DIT simulation indicates that the production of the
neutron-rich nuclides in these reactions is associated with peripheral nucleon
exchange. In such peripheral encounters, the neutron skins of the neutron-rich
124Sn and 64Ni target nuclei may play an important role. From a practical
viewpoint, such reactions between massive neutron-rich nuclei offer a novel and
attractive synthetic avenue to access extremely neutron-rich rare isotopes
towards the neutron-drip line.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

A. Keksis

C. J. Pethick

D. V. Shetty

D. J. Morrissey

E. Martin

G. A. Souliotis

G. Chubarian

H. Geissel

L. Trache

M. Veselsky

R. F. Casten

S. J. Yennello

V. Borrel

Physical Review Letters

English

arXiv

Crossref

Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral Collisions at Fermi Energies

Souliotis, G. A.

Veselsky, M.

Chubarian, G.

Trache, L.

Keksis, A.

Martin, E.

Shetty, D. V.

Yennello, S. J.

OAKTrust Digital Repository (Texas A&M Univ)

arXiv:nucl-ex/0302011v2  13 May 2003Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral Collisionsat Fermi EnergiesG.A. Souliotis, M. Veselsky, G. Chubarian, L. Trache, A. Keksis, E. Martin, D.V. Shetty, and S.J. YennelloCyclotron Institute, Texas A&M University, College Station, TX 77843(Dated: June 13, 2018)A large enhancement in the production of neutron-rich projectile residues is observed in thereactions of a 25 MeV/nucleon 86Kr beam with the neutron rich 124Sn and 64Ni targets relative tothe predictions of the EPAX parametrization of high-energy fragmentation, as well as relative tothe reaction with the less neutron-rich 112Sn target. The data demonstrate the significant effect ofthe target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi energies. A hybrid modelbased on a deep-inelastic transfer code (DIT) followed by a statistical de-excitation code accountsfor part of the observed large cross sections. The DIT simulation indicates that the production ofneutron-rich nuclides in these reactions is associated with peripheral nucleon exchange in which theneutron skins of the neutron-rich 124Sn and 64Ni target nuclei may play an important role. Froma practical viewpoint, such reactions between massive neutron-rich nuclei offer a novel syntheticavenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.PACS numbers: 25.70.-z, 25.70.Hi,25.70.LmExploration of the nuclear landscape towards theneutron-drip line [1] is currently of great interest in or-der to elucidate the evolution of nuclear structure withincreasing neutron-to-proton ratio (N/Z) [2, 3] and un-derstand important nucleosynthesis pathways [4], mostnotably the r-process [5]. Reactions induced by neutron-rich nuclei provide invaluable information on the isospindependence of the nuclear equation of state [6, 7]. Ex-tremely neutron-rich nuclei offer the unprecedented op-portunity to extrapolate our knowledge to the propertiesof bulk isospin-rich matter, such as neutron stars [8, 9].The efficient production of very neutron-rich nuclides isa key issue in current and future rare isotope beam fa-cilities around the world [10, 11, 12] and, in parallel, thesearch for new synthetic approaches is of exceptional im-portance.Neutron-rich nuclides have traditionally been pro-duced in spallation reactions, fission, and projectile frag-mentation [13]. In high-energy fragmentation reactions,the production of the most neutron-rich isotopes is basedon a “clean-cut” removal of protons from the projectile.The world’s data on fragmentation cross sections are wellrepresented by the empirical parametrization EPAX [14].EPAX is currently the common basis for predictions toplan rare beam experiments and facilities. In additionto the widely used projectile fragmentation approach,neutron-rich nuclides can be produced in multinucleontransfer reactions [15] and deep-inelastic reactions nearthe Coulomb barrier (e.g. [16, 17, 18]). In such reactions,the target N/Z significantly affects the production crosssections, but the low velocities of the fragments and theensuing wide angular and ionic charge state distributionsrender practical applications rather limited.The Fermi energy regime (20–40 MeV/nucleon) [19] of-fers the unique opportunity to combine the advantages ofboth low- and high-energy reactions. At this energy, thesynergy of projectile and target enhances the N/Z of thefragments, while the velocities are high enough to allowefficient in-flight collection and separation. Early workon neutron-rich fragment production from heavy projec-tiles at Fermi energies [20, 21, 22] provided no produc-tion cross sections, hampering quantitative comparisonsto parametrizations or reaction simulations. In order toexplore the possibilities offered at Fermi energies, we re-cently undertook an experimental study of the produc-tion cross sections of projectile residues from the reaction25 MeV/nucleon 86Kr+64Ni, in which enhanced produc-tion of neutron-rich fragments was observed [23].In this Letter, we focus on a subsequent detailed studyof near-projectile residues from the reactions of 86Kr(25 MeV/nucleon) with 124Sn and 112Sn targets andpresent a comparison with the results from 86Kr+64Ni[23], as well as with the expectations of EPAX [14]. Wedemonstrate that the production of neutron-rich projec-tile residues is substantially enhanced in the reactions in-volving the most neutron-rich systems and we provide aninterpretation based on a deep inelastic transfer model.The measurements were performed at the CyclotronInstitute of Texas A&M University, following the exper-imental scheme of our previous work [23]. A concise de-scription is given in the following. A 25 MeV/nucleon86Kr22+ beam (∼1 pnA) from the K500 superconduct-ing cyclotron interacted with 124Sn and 112Sn targets(2 mg/cm2). Projectile residues were analyzed with theMARS recoil separator [24] offering an angular accep-tance of 9 msr and momentum acceptance of 4%. Thebeam struck the target at 4.0o relative to the optical axisof MARS. Thus, fragments were accepted in the polarangular range 2.7o–5.4o (lying inside the grazing angle of6.5o for the Kr+Sn reaction at 25 MeV/nucleon). At theMARS focal plane, the fragments were collected in a two-element (∆E, E) Si detector telescope. Time of flight was2measured between two PPACs (parallel plate avalanchecounters) placed at the MARS dispersive image and atthe focal plane, respectively. The horizontal positionfrom the first PPAC and the field of the MARS firstdipole magnet determined the magnetic rigidity, Bρ. Thefragments were characterized event-by-event by energy-loss, residual energy, time of flight, and Bρ. With theprocedures described in Ref. [23], the atomic number Z,the ionic charge q, the mass number A and the velocitywere obtained with high resolution (0.5, 0.4, 0.6 units and0.3%, respectively). After summation over ionic chargestates (with corrections for missing charge states), frag-ment cross sections with respect to Z, A and velocity wereobtained in the angular range 2.7o–5.4o and Bρ range1.3–2.0 Tm. To extract total cross sections, the mea-sured yield data were corrected for the limited angularacceptance and magnetic rigidity range covered. The cor-rections were obtained from the ratio of the total to thefiltered cross sections (with angular and momentum ac-ceptance cuts) calculated using the simulation approachdescribed below. The corrections involved an azimuthalangular factor of 10 and mass-dependent polar factorsranging from 2 to 4 for A=85 to 50, respectively (result-ing in, e.g., a total factor of 20 for fragments very closeto the projectile). The systematic uncertainty in the ex-tracted cross sections is estimated to be a factor of 2 [23].Fig. 1 shows the results of the mass distributions(cross sections) of elements Z=30–35 from the reactionsof 86Kr (25 MeV/nucleon) with 124Sn (full circles) and112Sn (full squares). The dotted lines are the predictionsof the EPAX parametrization [14]. We observe that theneutron-rich projectile fragments produced in the reac-tion with the more neutron-rich 124Sn (N/Z=1.48) targethave considerably larger yields compared to those ob-tained in the reaction with the less neutron-rich 112Sntarget (N/Z=1.24). Typical ratios are 3–10 for the mostneutron-rich isotopes observed in both reactions. In mostof the cases of the present data, the most neutron-richnuclides have been observed only in the 86Kr+124Sn re-action. Compared to EPAX, the yields from 86Kr+124Snare substantially larger, whereas those from 86Kr+112Snare in reasonable agreement with EPAX. It should bepointed out that EPAX represents an abrasion-ablationtype of fragmentation scenario in which the target isa mere spectator having (almost) no effect on the pro-duction cross section (apart from a geometrical factor)[14]. To quantify the comparison between the presentdata and EPAX, we show in Fig. 2a the ratio of themeasured cross sections of fragments with Z=28–35 from86Kr+124Sn with respect to EPAX expectations. Thesame ratio is presented in Fig. 2b for our previous dataon 86Kr+64Ni [23]. In the EPAX calculation, we ex-tended the prediction up to one neutron pick-up productsto allow comparison with the present data. However, ourdata on 86Kr+124Sn and 86Kr+64Ni show the produc-tion of nuclides with up to 3–4 neutrons picked-up fromZ=349085807570(25MeV/nuleon)86Kr +124;112SnZ=35908580757010001001010.10.010.0010.0001Z=328580757065Z=33858075701001010.10.010.0010.00011e-05Z=308075706560Z=31Mass Number ACrossSetion(mb)807570651001010.10.010.0010.00011e-05FIG. 1: Mass distributions of elements Z=30–35 from thereaction of 86Kr (25MeV/nucleon) with 124Sn and 112Sn. Thedata are shown by full circles for 86Kr+124Sn and full squaresfor 86Kr+112Sn. The dotted lines are EPAX expectations [14].the target, along with usual proton removal products. Inboth figures, we observe that for nuclei far from the pro-jectile (Z=28–30), as well as for N/Z less than that ofthe projectile (N/Z=1.39, arrow in Fig. 2), the experi-mental cross sections are in remarkable agreement withEPAX (within a factor of ∼2). However, for heavier el-ements (Z=31–35) and for progressively higher N/Z, astriking increase in the ratio is observed, especially fornear-projectile products involving neutron pick-up. Theobserved yield amplification over EPAX for neutron-richnuclei demonstates the dramatic effect of the neutron-rich target on the production mechanism at Fermi ener-gies, which involves substantial nucleon exchange [25].To gain insight into the mechanism of neutron-rich nu-clide production at this energy, we performed simulationsusing a hybrid Monte Carlo approach, as in [23]. The dy-namical stage of the collision was described by the deepinelastic transfer (DIT) code of Tassan-Got [26] and the3Z=28Z=29Z=30Z=31Z=32Z=33Z=34Z=3586Kr+124Sn(a)N/Zexper/EPAX1.71.61.51.41.31.21.11302520151050Z=28Z=29Z=30Z=31Z=32Z=33Z=34Z=3586Kr+64Ni(b)N/Zexper/EPAX1.71.61.51.41.31.21.11302520151050FIG. 2: Ratio of measured cross sections of projectile residuesfrom 86Kr (25MeV/nucleon) on 124Sn target (a) (current mea-surement), and on 64Ni target (b) [23] with respect to EPAXexpectations [14]. Arrows indicate the N/Z of the projectile.de-excitation stage was simulated by the statistical codeGEMINI [27]. Comparison of the DIT/GEMINI predic-tions with the data is shown in Fig. 3 for Se (Z=34),Ge (Z=32) and Zn (Z=30) (thick solid lines). We ob-serve that the calculation describes well, in most cases,the central and the neutron-deficient part of the mea-sured distributions. However, it cannot fully account forthe enhancement observed on the neutron-rich sides fornear-projectile elements. Backtracing of the DIT simu-lations indicates that neutron-rich products originate inperipheral collisions in which the projectile–target den-sity distributions overlap by up to 1–1.5 fm. For largerprojectile–target overlaps, the observed enhancement di-minishes and the cross sections agree with DIT/GEMINI[23] and EPAX (Fig. 2). It is worth noting the large crosssections from 86Kr+64Ni [23] relative to 86Kr + 112Sn, al-though the targets have nearly equal N/Z. In this case itis their position with respect to the valley of β-stability(64Ni is neutron-rich and 112Sn is neutron-poor) that de-termines the nucleon flow. Consequently, the potentialenergy surface (appropriately taken into account in theDIT code) plays a definitive role in the production ofneutron-rich fragments at Fermi energies.We should point out that, in the original DIT code[26], the nuclei are assumed spherical with homogeneousneutron and proton density distributions. However, it iswell established that neutron-rich nuclei possess neutron-86Kr +112SnSe908580757086Kr +124SnSe908580757086Kr +64NiSe90858075701001010.10.010.0010.000186Kr +112SnGe858075706586Kr +124SnGe858075706586Kr +64NiGeCrossSetion(mb)85807570651001010.10.010.0010.000186Kr +112SnZn807570656086Kr +124SnZn807570656086Kr +64NiZnMass Number A80757065601001010.10.010.0010.0001FIG. 3: Comparison of experimental mass distributionsof Se (Z=34), Ge (Z=32) and Zn (Z=30) from 86Kr (25MeV/nucleon) on 64Ni [23], 124Sn and 112Sn (present data)with DIT/GEMINI calculations without (thick solid line),and with (thick dashed line) neutron and proton density dis-tributions. The thin solid line is the EPAX prediction [14].skins; namely, their neutron density distributions ex-tend further out relative to the proton distributions. Aswell known, the neutron skin is predicted by a vari-ety of theoretical models including microscopic Thomas-Fermi, Skyrme Hartree-Fock and relativistic mean-fieldapproaches (e.g. [28],[29],[30]). Also, it has recentlybeen demonstrated experimentally in annihilation stud-ies of antiprotonic atoms [31]. Some time ago, Harvey[32] took into consideration the neutron skin of heavytargets (208Pb, 197Au) in a microscopic nucleon-nucleonscattering model to explain the enhanced production ofneutron-rich projectile fragments from 16O and 20Ne atFermi energies relative to those at relativistic energies.His approach also served as an interpretation of the N/Zproperties of projectile fragment yield data from 40Ar(44 MeV/nucleon) on heavy targets [22]. Although theDIT code employed in our simulations is based on nu-cleon exchange (rather than nucleon-nucleon collisions)in the overlap zone, we may attribute its partial successto describe the observed enhanced cross sections to the4inadequate treatment of peripheral nucleon exchange, inwhich the neutron skins of the reaction partners mayplay important roles. To investigate this assumptionfurther, we included neutron and proton density distri-butions into the DIT code. The density distributionswere calculated with a microscopic Thomas-Fermi code[28] and employed in DIT. In the simulation, at a givenprojectile-target overlap and a given nucleon exchangesite, the transfer probabilities were scaled by the local pe-ripheral (“halo”) factors, (ρn/ρp)/(N/Z), of the respec-tive reaction partners. The change of the density profileswas followed during the (sequential) nucleon exchange.The results of this modified-DIT/GEMINI calculationare shown in Fig. 3 by dashed lines. A qualitative im-provement in the description of the neutron-rich sidesof the distributions for near-projectile elements from thevery neutron-rich systems 86Kr+64Ni and 86Kr+124Snis found. However, for the 86Kr+112Sn reaction, thestandard DIT/GEMINI calculation agrees better withthe data. These calculations indicate, although quali-tatively, the possible effect of the neutron skin of theneutron-rich targets 64Ni and 124Sn in the observed en-hancement. Guided by these observations, we infer thatan improved deep-inelastic transfer model, or an equiva-lent approach, which self-consistently takes into accountthe neutron and proton density distributions of the reac-tion partners may account for the large cross sections ofthe neutron-rich products observed in such reactions.From a practical viewpoint, the large production crosssection of neutron-rich fragments indicate that such re-actions offer an attractive approach to produce veryneutron-rich nuclides. Apart from direct in-flight orIGISOL-type options, we propose the application of suchreactions as a second stage in two-stage rare beam pro-duction schemes. For example, a beam of 92Kr froman ISOL-type facility (or, from the proposed Rare Iso-tope Accelerator Facility (RIA) [10, 11]) can be acceler-ated around the Fermi energy and interact with a veryneutron-rich target (e.g. 64Ni, 124Sn, 208Pb, 238U) toproduce extremely neutron-rich nuclides that cannot beaccessed by fission or projectile fragmentation. A quan-titative prediction of rates of such nuclides will be possi-ble after further experimental studies and improvementof the description of peripheral collisions between veryneutron-rich heavy nuclei in the Fermi energy regime.In summary, a substantial enhancement in the produc-tion cross sections of neutron-rich projectile fragments isobserved in the reactions 86Kr (25 MeV/nucleon)+124Snand 86Kr (25 MeV/nucleon)+64Ni relative to the EPAXparametrization, as well as relative to the less neutron-rich system 86Kr+112Sn. The hibrid DIT/GEMINImodel provides a partial interpretation of the observedcross sections. The DIT simulations indicate that theenhanced production of neutron-rich isotopes in the re-actions 86Kr+124Sn and 86Kr+64Ni is associated with pe-ripheral nucleon exchange. In such collisions, the neutronskins of the 124Sn and 64Ni target nuclei may play a sig-nificant role. For practical purposes, such reactions offera novel and competitive pathway to produce extremelyneutron-rich isotopes towards the neutron-drip line.We are thankful to L. Tassan-Got for his DIT code anduseful suggestions. We also wish to thank A. Sanzhurfor insightful discussions and R. Charity for using theGEMINI code. We gratefully acknowledge the supportof the operations staff of the Cyclotron Institute duringthe measurements. Financial support for this work wasprovided, in part, by the U.S. Department of Energy un-der Grant No. DE-FG03-93ER40773 and by the RobertA. Welch Foundation under Grant No. A-1266.[1] P. Moller, J. R. Nix, and K. L. Kratz, At. Data Nucl.Data Tables 66, 131 (1997).[2] R. F. Casten and B. M. Sherrill, Prog. Part. Nucl. Physics45, 171 (2000).[3] J. Meng and P. Ring, Phys. Rev. Lett. 80, 460 (1998).[4] C. Sneden and J. J. Cowan, Science 299, 70 (2003).[5] K. Langanke and M. Wiescher, Rep. Prog. Phys. 64, 1657(2001).[6] P. Danielewicz, R. Lacey, and W. G. Lynch, Science 298,1592 (2002).[7] B.-A. Li, Phys. Rev. C 66, 034609 (2002).[8] C. J. Pethick and D. G. Ravenhall, Ann. Rev. Nucl. Part.Sci. 45, 429 (1995).[9] T. Sil et al., Phys. Rev. C 66, 045803 (2002).[10] D. J. Morrissey, Eur. Phys. J. A 15, 105 (2002).[11] J. A. Nolen, Eur. Phys. J. A 13, 255 (2002).[12] D. Guerreau, Eur. Phys. J. A 13, 263 (2002).[13] H. Geissel and G. Munzenberg, Ann. Rev. Nucl. Part.Sci. 45, 163 (1995).[14] K. Su¨mmerer and B. Blank, Phys. Rev. C 61, 034607(2000).[15] L. Corradi et al., Nucl. Phys. A 701, 109 (2002).[16] R. Broda, Eur. Phys. J. A 13, 1 (2002).[17] V. V. Volkov, Phys. Rep. 44, 93 (1978).[18] I. Y. Lee et al., Phys. Rev. C 56, 753 (1997).[19] H. Fuchs and K. Mo¨hring, Rep. Prog. Phys. 57, 231(1994).[20] C. O. Bacri et al., Nucl. Phys. A 555, 477 (1993).[21] D. Bazin et al., Nucl. Phys. A 515, 349 (1990).[22] V. Borrel et al., Z. Phys. A 324, 205 (1986).[23] G. A. Souliotis et al., Phys. Lett. B 543, 163 (2002).[24] R. E. Tribble, R. H. Burch, and C. A. Gagliardi, Nucl.Instr. and Meth. A 285, 441 (1989).[25] M. Veselsky et al., Phys. Rev. C 62, 064613 (2000).[26] L. Tassan-Got and C. Stefan, Nucl. Phys. A 524, 121(1991).[27] R. Charity et al., Nucl. Phys. A 483, 371 (1988).[28] V. M. Kolomietz, A. I. Sanzhur, S. Shlomo, and S. A.Firin, Phys. Rev. C 64, 024315 (2001).[29] B. A. Brown, Phys. Rev. C 58, 220 (1998).[30] F. Hofmann, C. M. Keil, and H. Lenske, Phys. Rev. C64, 034314 (2001).[31] A. Trzcinska et al., Phys. Rev. Lett. 87, 082501 (2001).[32] B. G. Harvey, Nucl. Phys. A 444, 498 (1985).

Texas A&M University

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Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral Collisions
at Fermi Energies
G.A. Souliotis, M. Veselsky, G. Chubarian, L. Trache, A. Keksis, E. Martin, D.V. Shetty, and S.J. Yennello
Cyclotron Institute, Texas A&M University, College Station, TX 77843
(Dated: June 13, 2018)
A large enhancement in the production of neutron-rich projectile residues is observed in the
reactions of a 25 MeV/nucleon 86Kr beam with the neutron rich 124Sn and 64Ni targets relative to
the predictions of the EPAX parametrization of high-energy fragmentation, as well as relative to
the reaction with the less neutron-rich 112Sn target. The data demonstrate the significant effect of
the target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi energies. A hybrid model
based on a deep-inelastic transfer code (DIT) followed by a statistical de-excitation code accounts
for part of the observed large cross sections. The DIT simulation indicates that the production of
neutron-rich nuclides in these reactions is associated with peripheral nucleon exchange in which the
neutron skins of the neutron-rich 124Sn and 64Ni target nuclei may play an important role. From
a practical viewpoint, such reactions between massive neutron-rich nuclei offer a novel synthetic
avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.
PACS numbers: 25.70.-z, 25.70.Hi,25.70.Lm
Exploration of the nuclear landscape towards the
neutron-drip line [1] is currently of great interest in or-
der to elucidate the evolution of nuclear structure with
increasing neutron-to-proton ratio (N/Z) [2, 3] and un-
derstand important nucleosynthesis pathways [4], most
notably the r-process [5]. Reactions induced by neutron-
rich nuclei provide invaluable information on the isospin
dependence of the nuclear equation of state [6, 7]. Ex-
tremely neutron-rich nuclei offer the unprecedented op-
portunity to extrapolate our knowledge to the properties
of bulk isospin-rich matter, such as neutron stars [8, 9].
The efficient production of very neutron-rich nuclides is
a key issue in current and future rare isotope beam fa-
cilities around the world [10, 11, 12] and, in parallel, the
search for new synthetic approaches is of exceptional im-
portance.
Neutron-rich nuclides have traditionally been pro-
duced in spallation reactions, fission, and projectile frag-
mentation [13]. In high-energy fragmentation reactions,
the production of the most neutron-rich isotopes is based
on a “clean-cut” removal of protons from the projectile.
The world’s data on fragmentation cross sections are well
represented by the empirical parametrization EPAX [14].
EPAX is currently the common basis for predictions to
plan rare beam experiments and facilities. In addition
to the widely used projectile fragmentation approach,
neutron-rich nuclides can be produced in multinucleon
transfer reactions [15] and deep-inelastic reactions near
the Coulomb barrier (e.g. [16, 17, 18]). In such reactions,
the target N/Z significantly affects the production cross
sections, but the low velocities of the fragments and the
ensuing wide angular and ionic charge state distributions
render practical applications rather limited.
The Fermi energy regime (20–40 MeV/nucleon) [19] of-
fers the unique opportunity to combine the advantages of
both low- and high-energy reactions. At this energy, the
synergy of projectile and target enhances the N/Z of the
fragments, while the velocities are high enough to allow
efficient in-flight collection and separation. Early work
on neutron-rich fragment production from heavy projec-
tiles at Fermi energies [20, 21, 22] provided no produc-
tion cross sections, hampering quantitative comparisons
to parametrizations or reaction simulations. In order to
explore the possibilities offered at Fermi energies, we re-
cently undertook an experimental study of the produc-
tion cross sections of projectile residues from the reaction
25 MeV/nucleon 86Kr+64Ni, in which enhanced produc-
tion of neutron-rich fragments was observed [23].
In this Letter, we focus on a subsequent detailed study
of near-projectile residues from the reactions of 86Kr
(25 MeV/nucleon) with 124Sn and 112Sn targets and
present a comparison with the results from 86Kr+64Ni
[23], as well as with the expectations of EPAX [14]. We
demonstrate that the production of neutron-rich projec-
tile residues is substantially enhanced in the reactions in-
volving the most neutron-rich systems and we provide an
interpretation based on a deep inelastic transfer model.
The measurements were performed at the Cyclotron
Institute of Texas A&M University, following the exper-
imental scheme of our previous work [23]. A concise de-
scription is given in the following. A 25 MeV/nucleon
86Kr22+ beam (∼1 pnA) from the K500 superconduct-
ing cyclotron interacted with 124Sn and 112Sn targets
(2 mg/cm2). Projectile residues were analyzed with the
MARS recoil separator [24] offering an angular accep-
tance of 9 msr and momentum acceptance of 4%. The
beam struck the target at 4.0o relative to the optical axis
of MARS. Thus, fragments were accepted in the polar
angular range 2.7o–5.4o (lying inside the grazing angle of
6.5o for the Kr+Sn reaction at 25 MeV/nucleon). At the
MARS focal plane, the fragments were collected in a two-
element (∆E, E) Si detector telescope. Time of flight was
2measured between two PPACs (parallel plate avalanche
counters) placed at the MARS dispersive image and at
the focal plane, respectively. The horizontal position
from the first PPAC and the field of the MARS first
dipole magnet determined the magnetic rigidity, Bρ. The
fragments were characterized event-by-event by energy-
loss, residual energy, time of flight, and Bρ. With the
procedures described in Ref. [23], the atomic number Z,
the ionic charge q, the mass number A and the velocity
were obtained with high resolution (0.5, 0.4, 0.6 units and
0.3%, respectively). After summation over ionic charge
states (with corrections for missing charge states), frag-
ment cross sections with respect to Z, A and velocity were
obtained in the angular range 2.7o–5.4o and Bρ range
1.3–2.0 Tm. To extract total cross sections, the mea-
sured yield data were corrected for the limited angular
acceptance and magnetic rigidity range covered. The cor-
rections were obtained from the ratio of the total to the
filtered cross sections (with angular and momentum ac-
ceptance cuts) calculated using the simulation approach
described below. The corrections involved an azimuthal
angular factor of 10 and mass-dependent polar factors
ranging from 2 to 4 for A=85 to 50, respectively (result-
ing in, e.g., a total factor of 20 for fragments very close
to the projectile). The systematic uncertainty in the ex-
tracted cross sections is estimated to be a factor of 2 [23].
Fig. 1 shows the results of the mass distributions
(cross sections) of elements Z=30–35 from the reactions
of 86Kr (25 MeV/nucleon) with 124Sn (full circles) and
112Sn (full squares). The dotted lines are the predictions
of the EPAX parametrization [14]. We observe that the
neutron-rich projectile fragments produced in the reac-
tion with the more neutron-rich 124Sn (N/Z=1.48) target
have considerably larger yields compared to those ob-
tained in the reaction with the less neutron-rich 112Sn
target (N/Z=1.24). Typical ratios are 3–10 for the most
neutron-rich isotopes observed in both reactions. In most
of the cases of the present data, the most neutron-rich
nuclides have been observed only in the 86Kr+124Sn re-
action. Compared to EPAX, the yields from 86Kr+124Sn
are substantially larger, whereas those from 86Kr+112Sn
are in reasonable agreement with EPAX. It should be
pointed out that EPAX represents an abrasion-ablation
type of fragmentation scenario in which the target is
a mere spectator having (almost) no effect on the pro-
duction cross section (apart from a geometrical factor)
[14]. To quantify the comparison between the present
data and EPAX, we show in Fig. 2a the ratio of the
measured cross sections of fragments with Z=28–35 from
86Kr+124Sn with respect to EPAX expectations. The
same ratio is presented in Fig. 2b for our previous data
on 86Kr+64Ni [23]. In the EPAX calculation, we ex-
tended the prediction up to one neutron pick-up products
to allow comparison with the present data. However, our
data on 86Kr+124Sn and 86Kr+64Ni show the produc-
tion of nuclides with up to 3–4 neutrons picked-up from
Z=34
9085807570
(25MeV/nuleon)
86
Kr +
124;112
Sn
Z=35
9085807570
1000
100
10
1
0.1
0.01
0.001
0.0001
Z=32
8580757065
Z=33
85807570
100
10
1
0.1
0.01
0.001
0.0001
1e-05
Z=30
8075706560
Z=31
Mass Number A
C
r
o
s
s
S
e

t
i
o
n
(
m
b
)
80757065
100
10
1
0.1
0.01
0.001
0.0001
1e-05
FIG. 1: Mass distributions of elements Z=30–35 from the
reaction of 86Kr (25MeV/nucleon) with 124Sn and 112Sn. The
data are shown by full circles for 86Kr+124Sn and full squares
for 86Kr+112Sn. The dotted lines are EPAX expectations [14].
the target, along with usual proton removal products. In
both figures, we observe that for nuclei far from the pro-
jectile (Z=28–30), as well as for N/Z less than that of
the projectile (N/Z=1.39, arrow in Fig. 2), the experi-
mental cross sections are in remarkable agreement with
EPAX (within a factor of ∼2). However, for heavier el-
ements (Z=31–35) and for progressively higher N/Z, a
striking increase in the ratio is observed, especially for
near-projectile products involving neutron pick-up. The
observed yield amplification over EPAX for neutron-rich
nuclei demonstates the dramatic effect of the neutron-
rich target on the production mechanism at Fermi ener-
gies, which involves substantial nucleon exchange [25].
To gain insight into the mechanism of neutron-rich nu-
clide production at this energy, we performed simulations
using a hybrid Monte Carlo approach, as in [23]. The dy-
namical stage of the collision was described by the deep
inelastic transfer (DIT) code of Tassan-Got [26] and the
3Z=28
Z=29
Z=30
Z=31
Z=32
Z=33
Z=34
Z=35
86
Kr+
124
Sn
(a)
N/Z

e
x
p
e
r
/

E
P
A
X
1.71.61.51.41.31.21.11
30
25
20
15
10
5
0
Z=28
Z=29
Z=30
Z=31
Z=32
Z=33
Z=34
Z=35
86
Kr+
64
Ni
(b)
N/Z

e
x
p
e
r
/

E
P
A
X
1.71.61.51.41.31.21.11
30
25
20
15
10
5
0
FIG. 2: Ratio of measured cross sections of projectile residues
from 86Kr (25MeV/nucleon) on 124Sn target (a) (current mea-
surement), and on 64Ni target (b) [23] with respect to EPAX
expectations [14]. Arrows indicate the N/Z of the projectile.
de-excitation stage was simulated by the statistical code
GEMINI [27]. Comparison of the DIT/GEMINI predic-
tions with the data is shown in Fig. 3 for Se (Z=34),
Ge (Z=32) and Zn (Z=30) (thick solid lines). We ob-
serve that the calculation describes well, in most cases,
the central and the neutron-deficient part of the mea-
sured distributions. However, it cannot fully account for
the enhancement observed on the neutron-rich sides for
near-projectile elements. Backtracing of the DIT simu-
lations indicates that neutron-rich products originate in
peripheral collisions in which the projectile–target den-
sity distributions overlap by up to 1–1.5 fm. For larger
projectile–target overlaps, the observed enhancement di-
minishes and the cross sections agree with DIT/GEMINI
[23] and EPAX (Fig. 2). It is worth noting the large cross
sections from 86Kr+64Ni [23] relative to 86Kr + 112Sn, al-
though the targets have nearly equal N/Z. In this case it
is their position with respect to the valley of β-stability
(64Ni is neutron-rich and 112Sn is neutron-poor) that de-
termines the nucleon flow. Consequently, the potential
energy surface (appropriately taken into account in the
DIT code) plays a definitive role in the production of
neutron-rich fragments at Fermi energies.
We should point out that, in the original DIT code
[26], the nuclei are assumed spherical with homogeneous
neutron and proton density distributions. However, it is
well established that neutron-rich nuclei possess neutron-
86
Kr +
112
Sn
Se
9085807570
86
Kr +
124
Sn
Se
9085807570
86
Kr +
64
Ni
Se
9085807570
100
10
1
0.1
0.01
0.001
0.0001
86
Kr +
112
Sn
Ge
8580757065
86
Kr +
124
Sn
Ge
8580757065
86
Kr +
64
Ni
Ge
C
r
o
s
s
S
e

t
i
o
n
(
m
b
)
8580757065
100
10
1
0.1
0.01
0.001
0.0001
86
Kr +
112
Sn
Zn
8075706560
86
Kr +
124
Sn
Zn
8075706560
86
Kr +
64
Ni
Zn
Mass Number A
8075706560
100
10
1
0.1
0.01
0.001
0.0001
FIG. 3: Comparison of experimental mass distributions
of Se (Z=34), Ge (Z=32) and Zn (Z=30) from 86Kr (25
MeV/nucleon) on 64Ni [23], 124Sn and 112Sn (present data)
with DIT/GEMINI calculations without (thick solid line),
and with (thick dashed line) neutron and proton density dis-
tributions. The thin solid line is the EPAX prediction [14].
skins; namely, their neutron density distributions ex-
tend further out relative to the proton distributions. As
well known, the neutron skin is predicted by a vari-
ety of theoretical models including microscopic Thomas-
Fermi, Skyrme Hartree-Fock and relativistic mean-field
approaches (e.g. [28],[29],[30]). Also, it has recently
been demonstrated experimentally in annihilation stud-
ies of antiprotonic atoms [31]. Some time ago, Harvey
[32] took into consideration the neutron skin of heavy
targets (208Pb, 197Au) in a microscopic nucleon-nucleon
scattering model to explain the enhanced production of
neutron-rich projectile fragments from 16O and 20Ne at
Fermi energies relative to those at relativistic energies.
His approach also served as an interpretation of the N/Z
properties of projectile fragment yield data from 40Ar
(44 MeV/nucleon) on heavy targets [22]. Although the
DIT code employed in our simulations is based on nu-
cleon exchange (rather than nucleon-nucleon collisions)
in the overlap zone, we may attribute its partial success
to describe the observed enhanced cross sections to the
4inadequate treatment of peripheral nucleon exchange, in
which the neutron skins of the reaction partners may
play important roles. To investigate this assumption
further, we included neutron and proton density distri-
butions into the DIT code. The density distributions
were calculated with a microscopic Thomas-Fermi code
[28] and employed in DIT. In the simulation, at a given
projectile-target overlap and a given nucleon exchange
site, the transfer probabilities were scaled by the local pe-
ripheral (“halo”) factors, (ρn/ρp)/(N/Z), of the respec-
tive reaction partners. The change of the density profiles
was followed during the (sequential) nucleon exchange.
The results of this modified-DIT/GEMINI calculation
are shown in Fig. 3 by dashed lines. A qualitative im-
provement in the description of the neutron-rich sides
of the distributions for near-projectile elements from the
very neutron-rich systems 86Kr+64Ni and 86Kr+124Sn
is found. However, for the 86Kr+112Sn reaction, the
standard DIT/GEMINI calculation agrees better with
the data. These calculations indicate, although quali-
tatively, the possible effect of the neutron skin of the
neutron-rich targets 64Ni and 124Sn in the observed en-
hancement. Guided by these observations, we infer that
an improved deep-inelastic transfer model, or an equiva-
lent approach, which self-consistently takes into account
the neutron and proton density distributions of the reac-
tion partners may account for the large cross sections of
the neutron-rich products observed in such reactions.
From a practical viewpoint, the large production cross
section of neutron-rich fragments indicate that such re-
actions offer an attractive approach to produce very
neutron-rich nuclides. Apart from direct in-flight or
IGISOL-type options, we propose the application of such
reactions as a second stage in two-stage rare beam pro-
duction schemes. For example, a beam of 92Kr from
an ISOL-type facility (or, from the proposed Rare Iso-
tope Accelerator Facility (RIA) [10, 11]) can be acceler-
ated around the Fermi energy and interact with a very
neutron-rich target (e.g. 64Ni, 124Sn, 208Pb, 238U) to
produce extremely neutron-rich nuclides that cannot be
accessed by fission or projectile fragmentation. A quan-
titative prediction of rates of such nuclides will be possi-
ble after further experimental studies and improvement
of the description of peripheral collisions between very
neutron-rich heavy nuclei in the Fermi energy regime.
In summary, a substantial enhancement in the produc-
tion cross sections of neutron-rich projectile fragments is
observed in the reactions 86Kr (25 MeV/nucleon)+124Sn
and 86Kr (25 MeV/nucleon)+64Ni relative to the EPAX
parametrization, as well as relative to the less neutron-
rich system 86Kr+112Sn. The hibrid DIT/GEMINI
model provides a partial interpretation of the observed
cross sections. The DIT simulations indicate that the
enhanced production of neutron-rich isotopes in the re-
actions 86Kr+124Sn and 86Kr+64Ni is associated with pe-
ripheral nucleon exchange. In such collisions, the neutron
skins of the 124Sn and 64Ni target nuclei may play a sig-
nificant role. For practical purposes, such reactions offer
a novel and competitive pathway to produce extremely
neutron-rich isotopes towards the neutron-drip line.
We are thankful to L. Tassan-Got for his DIT code and
useful suggestions. We also wish to thank A. Sanzhur
for insightful discussions and R. Charity for using the
GEMINI code. We gratefully acknowledge the support
of the operations staff of the Cyclotron Institute during
the measurements. Financial support for this work was
provided, in part, by the U.S. Department of Energy un-
der Grant No. DE-FG03-93ER40773 and by the Robert
A. Welch Foundation under Grant No. A-1266.
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Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral
  Collisions at Fermi Energies

Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral Collisions at Fermi Energies

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