In order to obtain information on a possible two-neutron component in the 6He wavefunction, we investigated the 4He(6He, 6He)4He elastic scattering at beam energies of 29.1, 29.6 and 40 MeV and at center-of-mass angles between 20 and 163 degrees. The experiments were performed at the ARENAS3 Radioactive Ion Beam Facility in Louvain-La-Neuve. We used a4He-implanted aluminium foil as a target. A 4He thickness of about 2-3 × 1017 particles/cm2 was obtained implanting 4He nuclei at energies between 20 and 80 keV in 0.7-μm-thick foils. Data collected in a larger angular range than previous measurements give additional evidence for the 2n-transfer process to take place in the 4He(6He, 6He)4He scattering. Our results demonstrate the possibility of using a 4He-implanted target in experiments with low-intensity beams, like radioactive beams, where deterioration of the target is irrelevant.</p

Andreyev, A.

Angulo, C.

Baye, D.

Bradfield Smith, W.

Cherubini, S.

Davinson, T.

Descouvemont, P.

Di Pietro, A.

Huyse, M.

Laird, A. M.

Mueller, W. F.

Musumarra, A.

Ostrowski, A.

Piechaczek, A.

Raabe, R.

Shotter, A.

Terwagne, Guy

Van Duppen, P.

Weissman, L.

Wöhr, A.

Bradfield-Smith, W.

Terwagne, G.

Repository of the University of Namur

RESEARCH OUTPUTS / RÉSULTATS DE RECHERCHEAuthor(s) - Auteur(s) :Publication date - Date de publication :Permanent link - Permalien :Rights / License - Licence de droit d’auteur :Bibliothèque Universitaire Moretus PlantinInstitutional Repository - Research PortalDépôt Institutionnel - Portail de la Rechercheresearchportal.unamur.beMeasurement of the 4He(6He,6He)4He cross section with a 4He-implanted Al targetRaabe, R.; Weissman, L.; Andreyev, A.; Angulo, C.; Baye, D.; Bradfield Smith, W.; Cherubini,S.; Davinson, T.; Descouvemont, P.; Di Pietro, A.; Huyse, M.; Laird, A. M.; Mueller, W. F.;Musumarra, A.; Ostrowski, A.; Piechaczek, A.; Shotter, A.; Terwagne, Guy; Van Duppen, P.;Wöhr, A.Published in:Nuclear physics APublication date:2002Document VersionPeer reviewed versionLink to publicationCitation for pulished version (HARVARD):Raabe, R, Weissman, L, Andreyev, A, Angulo, C, Baye, D, Bradfield Smith, W, Cherubini, S, Davinson, T,Descouvemont, P, Di Pietro, A, Huyse, M, Laird, AM, Mueller, WF, Musumarra, A, Ostrowski, A, Piechaczek, A,Shotter, A, Terwagne, G, Van Duppen, P & Wöhr, A 2002, 'Measurement of the 4He(6He,6He)4He cross sectionwith a 4He-implanted Al target', Nuclear physics A, vol. 701, pp. 387c-393c.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 13. Nov. 2019Nuclear Physics A 701 (2002) 387c–393cwww.elsevier.com/locate/npeMeasurement of the 4He(6He, 6He)4He cross sectionwith a 4He-implanted Al targetR. Raabe b,∗, L. Weissman b,1, A. Andreyev b,2, C. Angulo a, D. Baye c,W. Bradfield-Smith d, S. Cherubini a, T. Davinson d,P. Descouvemont c, A. Di Pietro d,3, M. Huyse b, A.M. Laird d,W.F. Mueller b, A. Musumarra a,3, A. Ostrowski d, A. Piechaczek b,4,A. Shotter d, G. Terwagne e, P. Van Duppen b, A. Wöhr b,5a Institut de Physique Nucléaire, Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgiumb Instituut voor Kern- en Stralingsfysica, University of Leuven, B-3001 Leuven, Belgiumc Physique Nucléaire Théorique et Physique Mathématique, CP 229, Université Libre de Bruxelles,B-1050 Brussels, Belgiumd Department of Physics & Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdome Laboratoire Interfacultaire d’Analyses par Réactions Nucléaires,Facultes Universitaires Notre-Dame de la Paix, B-5000 Namur, BelgiumAbstractIn order to obtain information on a possible two-neutron component in the 6He wavefunction, weinvestigated the 4He(6He, 6He)4He elastic scattering at beam energies of 29.1, 29.6 and 40 MeVand at center-of-mass angles between 20 and 163 degrees. The experiments were performed atthe ARENAS3 Radioactive Ion Beam Facility in Louvain-La-Neuve. We used a 4He-implantedaluminium foil as a target. A 4He thickness of about 2–3 × 1017 particles/cm2 was obtainedimplanting 4He nuclei at energies between 20 and 80 keV in 0.7-µm-thick foils. Data collectedin a larger angular range than previous measurements give additional evidence for the 2n-transferprocess to take place in the 4He(6He, 6He)4He scattering. Our results demonstrate the possibility of* Corresponding author.E-mail address: riccardo.raabe@fys.kuleuven.ac.be (R. Raabe).1 Present address: ISOLDE, CERN, EP Division, CH-1211 Geneva 23, Switzerland.2 Permanent address: FLNP, Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia.3 Present address: Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud, I-95123 Catania, Italy.4 Present address: Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA70803, USA.5 Present address: Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom.0375-9474/02/$ – see front matter  2002 Elsevier Science B.V. All rights reserved.PII: S0375-9474(01) 01 61 6- 5388c R. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393cusing a 4He-implanted target in experiments with low-intensity beams, like radioactive beams, wheredeterioration of the target is irrelevant.  2002 Elsevier Science B.V. All rights reserved.Keywords: Halo nuclei; Implanted targetHalo nuclei have so far initiated a large number of experimental and theoreticalinvestigations [1,2]. While 11Li is the showcase of a halo nucleus, 6He is at this momenta much better candidate for understanding the experimental results by detailed three-bodycalculations (4He, n, n). This is due to the fact that the 4He–n and n–n interactions arewell-known, while the 9Li–n interaction is not. This three-body approach allows also theprediction of properties of the 2n-component in 6He; however, experimental data that canreveal information on this aspect is rare.We performed an experiment at the ARENAS3 [3] Radioactive Ion Beam Facility inLouvain-La-Neuve, Belgium, to search for the elastic transfer of the 2n-cluster in theelastic scattering of 6He on a 4He target in inverse kinematics. The energy range of the6He radioactive beam in Louvain-la-Neuve enlarges the domain available for investigationsof elastic-scattering and fusion processes (studied at higher energies in Dubna [4,5]),allowing measurements around the Coulomb barrier. Very accurate results can be achievedexploiting the high intensity and purity of the beam.Measurements were carried out at beam energies of 29.1, 29.6 and 40 MeV, corre-sponding to center-of-mass energies on the 4He target of 11.6 and 15.9 MeV. Assumingthe cluster structure of 6He, elastic two-neutron transfer is expected to occur between thetwo α cores [6]. Its observation and interpretation should provide information on the spatialand momentum correlation of the two valence neutrons in 6He.In our first measurements we used a gas cell of 1 cm thickness, containing 4He at500 mbar pressure between an entrance Al window and an exit Kapton or Al window.Reaction products were registered using a segmented silicon detector system [7]. Detailsabout the experimental method, results obtained and their interpretation can be foundelsewhere [8]. We focus here on the development of a 4He-implanted aluminium targetand the results obtained using it in a new measurement of the 4He(6He, 6He)4He crosssection.The use of a gas target ensured a large 4He thickness (1.2×1019 atoms/cm2), providinggood statistics for the events of interest. However, it had some disadvantages; the main onesare: the large size of the cell, which introduced uncertainties in the kinematic reconstructionof the scattering events; the relatively thick (1.8 mg/cm2 Kapton or 2.3 mg/cm2 Al) exitwindow, which stopped particles scattered with low energies, preventing the detection ofscattering events at small and large center-of-mass angles. Due to the latter, data pointsobtained with the gas target were limited to the range from 50 to 140 degrees in the center-of-mass. Therefore, the development of a 4He-implanted Al target was undertaken in orderto measure cross-section data in a larger angular range.The use of ion-implanted targets is not a novel idea. An important limiting factor forthe application of this method is probably the deterioration of the targets due to beambombardment [9] (such limitations are not important for experiments not dedicated tocross-section measurements [10,11]). However, intensities of radioactive beams are ordersR. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393c 389cof magnitude lower compared to stable beams, even in the best cases. This makes theproblem of target deterioration irrelevant.Details about the development of the implanted targets can be found in [12]. They wereprepared at the Leuven off-line isotope separator by using 4He beams of various energies.We chose thin Al foils as implantation host, since we needed to minimize the energy lossesof the scattered 4He and 6He particles in the host material itself. The Al-foil thicknesswas measured by weighting the samples, and was found to be 0.68 ± 0.02 µm. Severalprobe targets were prepared, and the details on the fluences and beam energies are listed inTable 1. During implantation the foils were mounted on an Al plate to provide heat transfer.Unfortunately due to the small thickness and fragility of the host foils it was very difficultto provide a reliable thermal contact with the backing plate. Therefore, in order to keep thetarget heating low, the He+ current density was kept below 4 µA/cm2.To check the retained He content we have performed proton RBS measurements usingthe new Tandetron accelerator at the LARN laboratory of the University of Namur [13].The RBS measurements were carried out using a 2.2 MeV proton beam. The energy waschosen to enhance the 4He(p, p)4He cross-section σHe exploiting the strong resonance at2.5 MeV proton energy, corresponding to the ground state of 5Li. The 27Al(p, p)27Al crosssection is equal to the corresponding Rutherford value at this particular proton energy.The ratio of σHe to the Rutherford cross section at 160 degrees of proton scattering angle,where two Si detectors were placed, amounts to a factor of 250 [14]. In Fig. 1 the RBSspectrum from a nonimplanted sample (a) is compared with the one from target #3 (b). Inboth spectra, beside the strong RBS Al peak, two resolved peaks of 16O and two resolvedpeaks of 12C corresponding to the aluminium oxide and carbon layers at the front and therear foil surfaces are clearly visible. In the low-energy region the background is very small,which allowed an accurate measurement of the He content. A weak, well-separated peakTable 14He fluences in the Al foils. For samples #3, #8, #9 and #10 (#9 was used in the 6He–4He experiment)implantation was performed on both sidesFoil At 20 keV At 30 keV At 50 keV At 80 keV Total# 1 1.0× 1017 0 0 1.0× 1017# 2 0 0 1.0× 1017 1.0× 1017# 3 2× 1.2× 1017 2× 1.0× 1017 2× 0.3× 1017 5.0× 1017# 4 1.0× 1018 1.0× 1018# 5 1.0× 1018 1.0× 1018# 6 2.0× 1018 2.0× 1018# 7 1.0× 1018 1.0× 1018# 8 2× 2.0× 1017 4.0× 1017# 9 2× 4.0× 1017 8.0× 1017# 10 2× 1.1× 1017 7.1× 1017# 11 3.0× 1018 3.0× 1018390c R. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393cFig. 1. The RBS spectra of (a) the nonimplanted Al sample and (b) target #3. See text for details.Table 2Results of the RBS measurements: measured 4He thickness in the different foilsFoil 4He content (at/cm2) 4He at. % Retention (%)# 1 (4.98± 0.18)× 1016 1.2 50# 2 (3.02± 0.14)× 1016 0.7 30# 3 (2.20± 0.07)× 1017 5.4 44# 4 (5.51± 0.03)× 1016 1.3 6# 5 (1.12± 0.06)× 1017 2.7 11# 6 (1.11± 0.06)× 1017 2.7 6# 7 (1.43± 0.07)× 1017 3.5 15# 8 (1.32± 0.07)× 1017 3.2 33# 9 (2.72± 0.14)× 1017 6.6 34# 10 (2.16± 0.11)× 1017 5.3 31# 11 (1.61± 0.14)× 1017 2.7 5around 900 keV corresponding to a nonelastic 27Al(p, p′)27Al∗ scattering is also observed.The strong 4He peak is observed in the implanted foil, see Fig. 1b.The He content was determined from the ratios of the integrals of the He and AlRBS peaks, knowing the values of the 4He(p, p)4He and 27Al(p, p)27Al scattering crosssections and the thickness of the Al foils. The main error comes from the uncertainty onthe foil thickness (3%). The results are shown in Table 2 and Fig. 2: here the retention(ratio between the number of atoms staying in the foil and the total number of implantedatoms) is shown as function of the implanted dose. The points in Fig. 2 are ratherscattered, because implantations were carried out in different conditions (implantationR. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393c 391cFig. 2. 4He retention in the different targets, plotted as function of the fluence. The numbers next to the datapoints identify the target according to Table 1 and 2. The dotted line is only to guide the eye.energy, 4He current, thermal contact) and some foils were in fact to some extent damaged.Nevertheless, a trend is visible: the retention looks constant up to a critical fluence,dropping dramatically beyond this limit. This appears to be around 7–8 × 1017 at/cm2( 7% atomic concentration for a 0.7 µm Al foil), confirming previous studies [15–17].To pin down the influence of the target heating factor on the He retention value wehave implanted a higher dose, 3 × 1018 at/cm2, into a 0.7 mg/cm2 Al foil evaporatedon a thick copper substrate. The latter has been consequently dissolved in a 7N HNO3solution. Such technique provided a good thermal contact during implantation avoidingtarget heating [18]. The RBS analysis of this target, #11 in Tables 1 and 2, yielded alow He content. Therefore, it seems that even with a good heat transfer the He retentiondrops drastically for a high fluence. The content levels of 4He in the different targets weremeasured again after some weeks time, and found to be the same: the long time elapseddid not cause a serious diffusion out of the host foil, even though no particular procedurewas followed to store the foils.We used target #9 for the 4He(6He, 6He)4He cross-section measurement at Louvain-la-Neuve. The 4He thickness was 2.7 × 1017 cm−2, almost a factor 50 less than thethickness of the gas target previously used. However, the increased 6He radioactive beamintensity and irradiation time allowed to perform the measurement successfully. The 6Hebeam (29.1 MeV) had an average intensity of 7.7× 105 pps during about 3 consecutivedays. The experimental setup and subsequent analysis of the collected data were similarto the one described in [8]. The resulting cross-section data are shown in Fig. 3, wherethe absolute 4He(6He, 6He)4He elastic cross-section is plotted against the center-of-massscattering angle. Filled circles correspond to data collected using the implanted target,compared to the ones collected with the gas cell (open circles). The solid curve wasobtained in the analysis of the previous data sets [8] using the double-folding potentialdescribed in [19], including a parity-dependent term which takes into account the two-neutron transfer process. The curve reproduces well the new data points; if a fit of thepotential parameters is performed, no appreciable changes are obtained (dotted curve). Thissupports the conclusions reported in [8], that the elastic 2n-transfer process is present in the392c R. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393cFig. 3. Measured 4He(6He, 6He)4He differential cross section. The full curve was obtained fitting the previoussets of data [8] using a double folding potential [19] including a parity-dependent term. The dotted curve wasobtained fitting the same potential to the new data points.4He(6He, 6He)4He scattering at 11.6 MeV center-of-mass energy. The large cross sectionmeasured between 155 and 163 degrees in the center-of-mass is an additional evidence forthe transfer process taking place in the reaction. These data points cannot be reproduced bya fit using a potential (either the double-folding or an optical model) that does not includethe parity-dependent term. Further investigation is planned using other reaction models.Among others, special attention is dedicated to the parameter-free α+ 6He scatteringpotential calculated in the framework of the resonating group method [20,21].In conclusion, we have used a 4He-implanted Al target to measure the 4He(6He,6He)4He cross section at a beam energy of 29.6 MeV. The thin Al foil allowed low-energy,large-angle scattered particles to escape from the target. The angular range covered usingthis target includes data points from 20 to 163 degrees in the center of mass, addingnew information with respect to our previous measurements. Our results demonstratethe possibility of using a 4He-implanted target in experiments with low-intensity beams,like radioactive beams, where deterioration of the target is irrelevant. An example is themeasurement of (α,γ ) and (α, p) reaction cross sections on radioactive nuclei, which isrelevant to nuclear astrophysics. The use of an implanted target can be especially importantfor laboratories where the separation of the recoiling reaction products is possible.References[1] P.G. Hansen, A.S. Jensen, B. Jonson, Annu. Rev. Nucl. Part. Sci. 45 (1995) 591.[2] I. Tanihata, J. Phys. G 22 (1996) 157.[3] M. Gaelens, M. Huyse, P. Van Duppen, M. Loiselet, G. Ryckewaert, Nucl. Instrum. Methods B 126 (1997)125.R. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393c 393c[4] G.M. Ter-Akopian et al., Phys. Lett. B 426 (1998) 251.[5] R. Wolski et al., JINR Report No. E-15-98-284, 1998.[6] W. von Oertzen, Nucl. Phys. A 148 (1970) 529.[7] P.J. Sellin et al., Nucl. Instrum. Methods A 311 (1992) 217.[8] R. Raabe et al., Phys. Lett. B 458 (1999) 1.[9] T.K. Alexander, G.C. Ball, W.N. Lennard, H. Geissel, Nucl. Phys. A 427 (1984) 526.[10] K.H. Speidel et al., Nucl. Phys. A 552 (1993) 140.[11] U. Grabowy et al., Nucl. Instrum. Methods B 101 (1995) 422.[12] L. Weissman et al., Nucl. Instr. Meth. A, submitted.[13] G. Demortier, Nucl. Instrum. Methods B 66 (1992) 51.[14] J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Material Analysis, Material ResearchSociety, Pittsburgh, USA, 1995.[15] S.E. Donnelly, F. Bodart, K.M. Barfoot, R. Wernz, R.P. Webb, Thin Solid Films 94 (1982) 289.[16] W. Jäger, R. Manzke, H. Trinkaus, G. Crecelius, R. Zeller, J. Fink, H.L. Bay, J. Nucl. Mater. 111–112 (1982)674.[17] K.L. Wilson, G.J. Thomas, J. Nucl. Mater. 63 (1976) 266.[18] G. Terwagne, S. Lucas, F. Bodart, G. Sorensen, H. Jensen, Nucl. Instrum. Methods B 45 (1990) 95.[19] D. Baye, L. Desorgher, D. Guillain, D. Herschkowitz, Phys. Rev. C 54 (1996) 2563.[20] D. Baye, P. Descouvemont, Y. Suzuki, K. Varga, Phys. Rev. C 59 (1999) 817.[21] P. Descouvemont, private communication.

Measurement of the 4He(6He, 6He)4He cross section with a 4He-implanted A1 target

In order to obtain information on a possible two-neutron component in the 6He wavefunction, we investigated the 4He(6He, 6He)4He elastic scattering at beam energies of 29.1, 29.6 and 40 MeV and at center-of-mass angles between 20 and 163 degrees. The experiments were performed at the ARENAS3 Radioactive Ion Beam Facility in Louvain-La-Neuve. We used a 4He-implanted aluminium foil as a target. A 4He thickness of about 2'3'1017 particles/cm2 was obtained implanting 4He nuclei at energies between 20 and 80 keV in 0.7-m-thick foils. Data collected in a larger angular range than previous measurements give additional evidence for the 2n-transfer process to take place in the 4He(6He, 6He)4He scattering. Our results demonstrate the possibility of using a 4He-implanted target in experiments with low-intensity beams, like radioactive beams, where deterioration of the target is irrelevant

RESEARCH OUTPUTS / RÉSULTATS DE RECHERCHEAuthor(s) - Auteur(s) :Publication date - Date de publication :Permanent link - Permalien :Rights / License - Licence de droit d’auteur :Bibliothèque Universitaire Moretus PlantinInstitutional Repository - Research PortalDépôt Institutionnel - Portail de la Rechercheresearchportal.unamur.beMeasurement of the 4He(6He,6He)4He cross section with a 4He-implanted Al targetRaabe, R.; Weissman, L.; Andreyev, A.; Angulo, C.; Baye, D.; Bradfield Smith, W.; Cherubini,S.; Davinson, T.; Descouvemont, P.; Di Pietro, A.; Huyse, M.; Laird, A. M.; Mueller, W. F.;Musumarra, A.; Ostrowski, A.; Piechaczek, A.; Shotter, A.; Terwagne, Guy; Van Duppen, P.;Wöhr, A.Published in:Nuclear physics APublication date:2002Document VersionPeer reviewed versionLink to publicationCitation for pulished version (HARVARD):Raabe, R, Weissman, L, Andreyev, A, Angulo, C, Baye, D, Bradfield Smith, W, Cherubini, S, Davinson, T,Descouvemont, P, Di Pietro, A, Huyse, M, Laird, AM, Mueller, WF, Musumarra, A, Ostrowski, A, Piechaczek, A,Shotter, A, Terwagne, G, Van Duppen, P & Wöhr, A 2002, 'Measurement of the 4He(6He,6He)4He cross sectionwith a 4He-implanted Al target' Nuclear physics A, vol. 701, pp. 387c-393c.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 21. May. 2019Nuclear Physics A 701 (2002) 387c–393cwww.elsevier.com/locate/npeMeasurement of the 4He(6He, 6He)4He cross sectionwith a 4He-implanted Al targetR. Raabe b,∗, L. Weissman b,1, A. Andreyev b,2, C. Angulo a, D. Baye c,W. Bradfield-Smith d, S. Cherubini a, T. Davinson d,P. Descouvemont c, A. Di Pietro d,3, M. Huyse b, A.M. Laird d,W.F. Mueller b, A. Musumarra a,3, A. Ostrowski d, A. Piechaczek b,4,A. Shotter d, G. Terwagne e, P. Van Duppen b, A. Wöhr b,5a Institut de Physique Nucléaire, Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgiumb Instituut voor Kern- en Stralingsfysica, University of Leuven, B-3001 Leuven, Belgiumc Physique Nucléaire Théorique et Physique Mathématique, CP 229, Université Libre de Bruxelles,B-1050 Brussels, Belgiumd Department of Physics & Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdome Laboratoire Interfacultaire d’Analyses par Réactions Nucléaires,Facultes Universitaires Notre-Dame de la Paix, B-5000 Namur, BelgiumAbstractIn order to obtain information on a possible two-neutron component in the 6He wavefunction, weinvestigated the 4He(6He, 6He)4He elastic scattering at beam energies of 29.1, 29.6 and 40 MeVand at center-of-mass angles between 20 and 163 degrees. The experiments were performed atthe ARENAS3 Radioactive Ion Beam Facility in Louvain-La-Neuve. We used a 4He-implantedaluminium foil as a target. A 4He thickness of about 2–3 × 1017 particles/cm2 was obtainedimplanting 4He nuclei at energies between 20 and 80 keV in 0.7-µm-thick foils. Data collectedin a larger angular range than previous measurements give additional evidence for the 2n-transferprocess to take place in the 4He(6He, 6He)4He scattering. Our results demonstrate the possibility of* Corresponding author.E-mail address: riccardo.raabe@fys.kuleuven.ac.be (R. Raabe).1 Present address: ISOLDE, CERN, EP Division, CH-1211 Geneva 23, Switzerland.2 Permanent address: FLNP, Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia.3 Present address: Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud, I-95123 Catania, Italy.4 Present address: Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA70803, USA.5 Present address: Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom.0375-9474/02/$ – see front matter  2002 Elsevier Science B.V. All rights reserved.PII: S0375-9474(01) 01 61 6- 5388c R. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393cusing a 4He-implanted target in experiments with low-intensity beams, like radioactive beams, wheredeterioration of the target is irrelevant.  2002 Elsevier Science B.V. All rights reserved.Keywords: Halo nuclei; Implanted targetHalo nuclei have so far initiated a large number of experimental and theoreticalinvestigations [1,2]. While 11Li is the showcase of a halo nucleus, 6He is at this momenta much better candidate for understanding the experimental results by detailed three-bodycalculations (4He, n, n). This is due to the fact that the 4He–n and n–n interactions arewell-known, while the 9Li–n interaction is not. This three-body approach allows also theprediction of properties of the 2n-component in 6He; however, experimental data that canreveal information on this aspect is rare.We performed an experiment at the ARENAS3 [3] Radioactive Ion Beam Facility inLouvain-La-Neuve, Belgium, to search for the elastic transfer of the 2n-cluster in theelastic scattering of 6He on a 4He target in inverse kinematics. The energy range of the6He radioactive beam in Louvain-la-Neuve enlarges the domain available for investigationsof elastic-scattering and fusion processes (studied at higher energies in Dubna [4,5]),allowing measurements around the Coulomb barrier. Very accurate results can be achievedexploiting the high intensity and purity of the beam.Measurements were carried out at beam energies of 29.1, 29.6 and 40 MeV, corre-sponding to center-of-mass energies on the 4He target of 11.6 and 15.9 MeV. Assumingthe cluster structure of 6He, elastic two-neutron transfer is expected to occur between thetwo α cores [6]. Its observation and interpretation should provide information on the spatialand momentum correlation of the two valence neutrons in 6He.In our first measurements we used a gas cell of 1 cm thickness, containing 4He at500 mbar pressure between an entrance Al window and an exit Kapton or Al window.Reaction products were registered using a segmented silicon detector system [7]. Detailsabout the experimental method, results obtained and their interpretation can be foundelsewhere [8]. We focus here on the development of a 4He-implanted aluminium targetand the results obtained using it in a new measurement of the 4He(6He, 6He)4He crosssection.The use of a gas target ensured a large 4He thickness (1.2×1019 atoms/cm2), providinggood statistics for the events of interest. However, it had some disadvantages; the main onesare: the large size of the cell, which introduced uncertainties in the kinematic reconstructionof the scattering events; the relatively thick (1.8 mg/cm2 Kapton or 2.3 mg/cm2 Al) exitwindow, which stopped particles scattered with low energies, preventing the detection ofscattering events at small and large center-of-mass angles. Due to the latter, data pointsobtained with the gas target were limited to the range from 50 to 140 degrees in the center-of-mass. Therefore, the development of a 4He-implanted Al target was undertaken in orderto measure cross-section data in a larger angular range.The use of ion-implanted targets is not a novel idea. An important limiting factor forthe application of this method is probably the deterioration of the targets due to beambombardment [9] (such limitations are not important for experiments not dedicated tocross-section measurements [10,11]). However, intensities of radioactive beams are ordersR. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393c 389cof magnitude lower compared to stable beams, even in the best cases. This makes theproblem of target deterioration irrelevant.Details about the development of the implanted targets can be found in [12]. They wereprepared at the Leuven off-line isotope separator by using 4He beams of various energies.We chose thin Al foils as implantation host, since we needed to minimize the energy lossesof the scattered 4He and 6He particles in the host material itself. The Al-foil thicknesswas measured by weighting the samples, and was found to be 0.68 ± 0.02 µm. Severalprobe targets were prepared, and the details on the fluences and beam energies are listed inTable 1. During implantation the foils were mounted on an Al plate to provide heat transfer.Unfortunately due to the small thickness and fragility of the host foils it was very difficultto provide a reliable thermal contact with the backing plate. Therefore, in order to keep thetarget heating low, the He+ current density was kept below 4 µA/cm2.To check the retained He content we have performed proton RBS measurements usingthe new Tandetron accelerator at the LARN laboratory of the University of Namur [13].The RBS measurements were carried out using a 2.2 MeV proton beam. The energy waschosen to enhance the 4He(p, p)4He cross-section σHe exploiting the strong resonance at2.5 MeV proton energy, corresponding to the ground state of 5Li. The 27Al(p, p)27Al crosssection is equal to the corresponding Rutherford value at this particular proton energy.The ratio of σHe to the Rutherford cross section at 160 degrees of proton scattering angle,where two Si detectors were placed, amounts to a factor of 250 [14]. In Fig. 1 the RBSspectrum from a nonimplanted sample (a) is compared with the one from target #3 (b). Inboth spectra, beside the strong RBS Al peak, two resolved peaks of 16O and two resolvedpeaks of 12C corresponding to the aluminium oxide and carbon layers at the front and therear foil surfaces are clearly visible. In the low-energy region the background is very small,which allowed an accurate measurement of the He content. A weak, well-separated peakTable 14He fluences in the Al foils. For samples #3, #8, #9 and #10 (#9 was used in the 6He–4He experiment)implantation was performed on both sidesFoil At 20 keV At 30 keV At 50 keV At 80 keV Total# 1 1.0× 1017 0 0 1.0× 1017# 2 0 0 1.0× 1017 1.0× 1017# 3 2× 1.2× 1017 2× 1.0× 1017 2× 0.3× 1017 5.0× 1017# 4 1.0× 1018 1.0× 1018# 5 1.0× 1018 1.0× 1018# 6 2.0× 1018 2.0× 1018# 7 1.0× 1018 1.0× 1018# 8 2× 2.0× 1017 4.0× 1017# 9 2× 4.0× 1017 8.0× 1017# 10 2× 1.1× 1017 7.1× 1017# 11 3.0× 1018 3.0× 1018390c R. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393cFig. 1. The RBS spectra of (a) the nonimplanted Al sample and (b) target #3. See text for details.Table 2Results of the RBS measurements: measured 4He thickness in the different foilsFoil 4He content (at/cm2) 4He at. % Retention (%)# 1 (4.98± 0.18)× 1016 1.2 50# 2 (3.02± 0.14)× 1016 0.7 30# 3 (2.20± 0.07)× 1017 5.4 44# 4 (5.51± 0.03)× 1016 1.3 6# 5 (1.12± 0.06)× 1017 2.7 11# 6 (1.11± 0.06)× 1017 2.7 6# 7 (1.43± 0.07)× 1017 3.5 15# 8 (1.32± 0.07)× 1017 3.2 33# 9 (2.72± 0.14)× 1017 6.6 34# 10 (2.16± 0.11)× 1017 5.3 31# 11 (1.61± 0.14)× 1017 2.7 5around 900 keV corresponding to a nonelastic 27Al(p, p′)27Al∗ scattering is also observed.The strong 4He peak is observed in the implanted foil, see Fig. 1b.The He content was determined from the ratios of the integrals of the He and AlRBS peaks, knowing the values of the 4He(p, p)4He and 27Al(p, p)27Al scattering crosssections and the thickness of the Al foils. The main error comes from the uncertainty onthe foil thickness (3%). The results are shown in Table 2 and Fig. 2: here the retention(ratio between the number of atoms staying in the foil and the total number of implantedatoms) is shown as function of the implanted dose. The points in Fig. 2 are ratherscattered, because implantations were carried out in different conditions (implantationR. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393c 391cFig. 2. 4He retention in the different targets, plotted as function of the fluence. The numbers next to the datapoints identify the target according to Table 1 and 2. The dotted line is only to guide the eye.energy, 4He current, thermal contact) and some foils were in fact to some extent damaged.Nevertheless, a trend is visible: the retention looks constant up to a critical fluence,dropping dramatically beyond this limit. This appears to be around 7–8 × 1017 at/cm2( 7% atomic concentration for a 0.7 µm Al foil), confirming previous studies [15–17].To pin down the influence of the target heating factor on the He retention value wehave implanted a higher dose, 3 × 1018 at/cm2, into a 0.7 mg/cm2 Al foil evaporatedon a thick copper substrate. The latter has been consequently dissolved in a 7N HNO3solution. Such technique provided a good thermal contact during implantation avoidingtarget heating [18]. The RBS analysis of this target, #11 in Tables 1 and 2, yielded alow He content. Therefore, it seems that even with a good heat transfer the He retentiondrops drastically for a high fluence. The content levels of 4He in the different targets weremeasured again after some weeks time, and found to be the same: the long time elapseddid not cause a serious diffusion out of the host foil, even though no particular procedurewas followed to store the foils.We used target #9 for the 4He(6He, 6He)4He cross-section measurement at Louvain-la-Neuve. The 4He thickness was 2.7 × 1017 cm−2, almost a factor 50 less than thethickness of the gas target previously used. However, the increased 6He radioactive beamintensity and irradiation time allowed to perform the measurement successfully. The 6Hebeam (29.1 MeV) had an average intensity of 7.7× 105 pps during about 3 consecutivedays. The experimental setup and subsequent analysis of the collected data were similarto the one described in [8]. The resulting cross-section data are shown in Fig. 3, wherethe absolute 4He(6He, 6He)4He elastic cross-section is plotted against the center-of-massscattering angle. Filled circles correspond to data collected using the implanted target,compared to the ones collected with the gas cell (open circles). The solid curve wasobtained in the analysis of the previous data sets [8] using the double-folding potentialdescribed in [19], including a parity-dependent term which takes into account the two-neutron transfer process. The curve reproduces well the new data points; if a fit of thepotential parameters is performed, no appreciable changes are obtained (dotted curve). Thissupports the conclusions reported in [8], that the elastic 2n-transfer process is present in the392c R. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393cFig. 3. Measured 4He(6He, 6He)4He differential cross section. The full curve was obtained fitting the previoussets of data [8] using a double folding potential [19] including a parity-dependent term. The dotted curve wasobtained fitting the same potential to the new data points.4He(6He, 6He)4He scattering at 11.6 MeV center-of-mass energy. The large cross sectionmeasured between 155 and 163 degrees in the center-of-mass is an additional evidence forthe transfer process taking place in the reaction. These data points cannot be reproduced bya fit using a potential (either the double-folding or an optical model) that does not includethe parity-dependent term. Further investigation is planned using other reaction models.Among others, special attention is dedicated to the parameter-free α+ 6He scatteringpotential calculated in the framework of the resonating group method [20,21].In conclusion, we have used a 4He-implanted Al target to measure the 4He(6He,6He)4He cross section at a beam energy of 29.6 MeV. The thin Al foil allowed low-energy,large-angle scattered particles to escape from the target. The angular range covered usingthis target includes data points from 20 to 163 degrees in the center of mass, addingnew information with respect to our previous measurements. Our results demonstratethe possibility of using a 4He-implanted target in experiments with low-intensity beams,like radioactive beams, where deterioration of the target is irrelevant. An example is themeasurement of (α,γ ) and (α, p) reaction cross sections on radioactive nuclei, which isrelevant to nuclear astrophysics. The use of an implanted target can be especially importantfor laboratories where the separation of the recoiling reaction products is possible.References[1] P.G. Hansen, A.S. Jensen, B. Jonson, Annu. Rev. Nucl. Part. Sci. 45 (1995) 591.[2] I. Tanihata, J. Phys. G 22 (1996) 157.[3] M. Gaelens, M. Huyse, P. Van Duppen, M. Loiselet, G. Ryckewaert, Nucl. Instrum. Methods B 126 (1997)125.R. Raabe et al. / Nuclear Physics A 701 (2002) 387c–393c 393c[4] G.M. Ter-Akopian et al., Phys. Lett. B 426 (1998) 251.[5] R. Wolski et al., JINR Report No. E-15-98-284, 1998.[6] W. von Oertzen, Nucl. Phys. A 148 (1970) 529.[7] P.J. Sellin et al., Nucl. Instrum. Methods A 311 (1992) 217.[8] R. Raabe et al., Phys. Lett. B 458 (1999) 1.[9] T.K. Alexander, G.C. Ball, W.N. Lennard, H. Geissel, Nucl. Phys. A 427 (1984) 526.[10] K.H. Speidel et al., Nucl. Phys. A 552 (1993) 140.[11] U. Grabowy et al., Nucl. Instrum. Methods B 101 (1995) 422.[12] L. Weissman et al., Nucl. Instr. Meth. A, submitted.[13] G. Demortier, Nucl. Instrum. Methods B 66 (1992) 51.[14] J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Material Analysis, Material ResearchSociety, Pittsburgh, USA, 1995.[15] S.E. Donnelly, F. Bodart, K.M. Barfoot, R. Wernz, R.P. Webb, Thin Solid Films 94 (1982) 289.[16] W. Jäger, R. Manzke, H. Trinkaus, G. Crecelius, R. Zeller, J. Fink, H.L. Bay, J. Nucl. Mater. 111–112 (1982)674.[17] K.L. Wilson, G.J. Thomas, J. Nucl. Mater. 63 (1976) 266.[18] G. Terwagne, S. Lucas, F. Bodart, G. Sorensen, H. Jensen, Nucl. Instrum. Methods B 45 (1990) 95.[19] D. Baye, L. Desorgher, D. Guillain, D. Herschkowitz, Phys. Rev. C 54 (1996) 2563.[20] D. Baye, P. Descouvemont, Y. Suzuki, K. Varga, Phys. Rev. C 59 (1999) 817.[21] P. Descouvemont, private communication.

Measurement of the 4He(6He,6He)4He cross section with a 4He-implanted Al target

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Measurement of the <sup>4</sup>He(<sup>6</sup>He,<sup>6</sup>He)<sup>4</sup>He cross section with a <sup>4</sup>He-implanted Al target

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