Antoine Lemasson

Performance of the large-acceptance gas-filled spectrometer VAMOS

Antoine Lemasson — 2010-04-03T06:18:51Z

A new gas-filled operation mode of the large-acceptance spectrometer VAMOS at GANIL is reported. A beam rejection factor greater than 10¹⁰ is obtained for the ⁴⁰Ca+¹⁵⁰Sm system at 196 MeV. The unprecedented transmission efficiency for the evaporation residues produced in this reaction is estimated to be around 80% for alpha xn channels and above 95% for xn-yp channels. A detailed study of the performance of the gas-filled VAMOS and future developments are discussed. This new operation mode opens avenues to explore the potential of fusion reactions in various kinematics.

C. Schmitt, M. Rejmund, A. Navin, B. Lecornu, B. Jacquot, G. de France, A. Lemasson, A. Shrivastava, P. Greenlees, J. Uusitalo, K. Subotic, L. Gaudefroy, Ch. Theisen, B. Sulignano, O. Dorvaux, L. Stuttgé, Accepted for Publication in Nucl. Instr. Meth. A 621 (2010) 558-565

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Keywords: Gas-filled spectrometer, Fusion reactions, Beam rejection, Transmission

A 21st century Rutherford experiment

Antoine Lemasson — 2009-11-30T22:23:38Z

Our recent Phys. Rev. Lett. "Modern Rutherford experiment : Tunnelling of the most neutron-rich nucleus" was the subject of a Physics "Viewpoint" : A 21st century Rutherford experiment from Dieter Ackermann and Haik Simon.

Physics highlights exceptional papers from the Physical Review journals.

Read the Viewpoint on physics.aps.org

Press Release : L'expérience fondatrice de Rutherford revisitée

Read the CEA (Commissariat à l'Energie Atomique) Press Release
Read the CNRS (Centre National de la Recherche Scientifique) Press release

See online : A 21st century Rutherford experiment

Modern Rutherford experiment : Tunnelling of the most neutron-rich nucleus

Antoine Lemasson — 2009-11-30T17:52:00Z

A modern variation of the Rutherford experiment to probe the tunneling of exotic nuclear matter from the measurement of the residues formed in the bombardment of 197Au by extremely neutron-rich 8He nuclei is presented. Using a novel off-beam technique the most precise and accurate measurements of fusion and neutron transfer involving re-accelerated unstable beams are reported. The results show unusual behavior of the tunneling of 8He compared to that for lighter helium isotopes, highlighting the role of the intrinsic structure of composite many-body quantum systems and pairing correlations.

A. Lemasson, A. Shrivastava, A. Navin, M. Rejmund, N. Keeley, V. Zelevinsky, S. Bhattacharyya, A. Chatterjee, G. de France, B. Jacquot, V. Nanal, R. G. Pillay, R. Raabe, and C. Schmitt

Received Tuesday Sept 15, 2009
Accepted Friday Oct 16, 2009
Published Monday Nov 30, 2009

Phys. Rev. Lett. 103, 232701 (2009)

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Viewpoint : A 21st century Rutherford experiment

Read On the Physics webpage.

Press Release : L'expérience fondatrice de Rutherford revisitée

Read The press Release on GANIL, CEA, CNRS, IN2P3 websites.

Prompt γ-ray spectroscopy of isotopically identified fission fragments

Antoine Lemasson — 2009-11-27T18:19:00Z

Measurements of prompt Doppler-corrected deexcitation γ-rays from uniquely identified fragments formed in fusion-fission reactions of the type 12C(238U,134Xe)Ru are reported. The fragments were identified in both A and Z using the magnetic spectrometer VAMOS. States built on the characteristic neutron configurations forming high spin isomers (7^− and 10+) in 134Xe are presented and compared with the predictions of shell-model calculations using a new effective interaction in the region of Z > 50 and N < 82.

A. Shrivastava, M. Caamano, M. Rejmund, A. Navin, F. Rejmund, K.-H. Schmidt, A. Lemasson, C. Schmitt, L. Gaudefroy, K. Sieja, L. Audouin, C.O. Bacri, G. Barreau, J. Benlliure, E. Casarejos, X. Derkx, B. Fernandez-Domınguez, C. Golabek, B. Jurado, T. Roger, and J. Taieb

Phys. Rev. C 80, 051305(R) (2009) .

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Workshop LEA Colliga

Antoine Lemasson — 2009-11-20T07:26:00Z

23rd - 24th November 2009

"The Collaboration INFN-Ganil on nuclear structure, nuclear reactions and future ISOL based accelerators – (COLLIGA)"

Oral Contribution

Title :

Interplay between structure and reaction mechanisms around the Coulomb barrier : Tunneling of the most neutron-rich nucleus ⁸He

Fete de la Science 2009 - Atelier du Chercheur

Antoine Lemasson — 2009-11-19T08:04:00Z

Lors de cette édition 2009 de l'atelier du chercheur, j'ai présenté introduction à la physique nucléaire ainsi que mon travail de doctorant à des élèves de première et de seconde du Lycée Marie Curie de Vire.

Qu'est que l'Atelier du Chercheur

Organisés par Relais d'sciences, les {{{Ateliers du chercheur}}} sont l'occasion pour des doctorants, des chercheurs et des ingénieurs de sillonner la Basse-Normandie pour présenter leurs travaux de recherche à des collégiens ou des lycéens, directement dans les établissements scolaires.

Source : Relais d'Sciences

Exploring fusion at extreme sub-barrier energies with weakly bound nuclei

Antoine Lemasson — 2009-11-03T19:05:06Z

Results of measurement of residues formed in fusion of 6Li with 198Pt in the energy range of 0.68< E/Vb <1.3 using a new sensitive off-beam technique are reported. The fusion excitation function and the derived average angular momenta do not indicate a change of slope at deep sub- barrier energies, contrary to recent observations. The present results for a system with weakly bound projectile confront the current understanding of the fusion hindrance at these low energies, underlying the role of internal re-organization on the dynamical path towards fusion.

A. Shrivastava, A. Navin, A. Lemasson, K. Ramachandran, V. Nanal, M. Rejmund, K. Hagino, T. Ichikawa, S. Bhattacharyya, A. Chatterjee, S. Kailas, K. Mahata, V. V. Parkar, R. G. Pillay, and P. C. Rout

Phys. Rev. Lett. 103, 232702 (2009)

Received 3 July 2009; published 3 December 2009

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Ecole Joliot Curie 2009

Antoine Lemasson — 2009-10-12T20:26:08Z

Oral Contribution

(Webpage)

Title :

Modern Ruterford experiment : Tunneling of the most neutron rich nucleus ⁸He

Abstract :

Nearly a hundred years ago the scattering of alpha particles (⁴He) from gold atoms by Rutherford led to the discovery of the atomic nucleus. Today exotic nuclear species with weak binding, unusual neutron/proton asymmetry and extended spatial distributions (halos), can be created and studied in the laboratory using short-lived radioactive ion beams [1]. Fusion reactions around the Coulomb barrier are not only a unique laboratory for probing the various aspects of multidimensional quantum tunnelling [2], but are also relevant to the synthesis of Super Heavy Elements and reactions of astrophysical interest.

In this talk, a modern variation of the Rutherford experiment where exotic ⁸He nuclei interact with a gold target was presented. The induced radioactivity, either from gold isotopes produced after the transfer of neutron(s) or from thallium isotopes resulting from fusion of the projectile and target, probes facets of the tunnelling of exotic nuclear matter. Employing a novel technique [3], precise measurements of the excitation functions involving 8He show that for this neutron-rich nucleus, neutron transfer modifies the tunnelling process in a different way to “normal” stable nuclei. A systematic isotopic comparison within the Helium chain show that the tunnelling of ⁸He is unusually similar to ⁶He, but as expected different from the stable tightly bound⁴He [4]. These results shed light on the complex quantum phenomena of tunnelling of composite objects [5], that is of fundamental interest in fields ranging from molecular processes to transport in superconductors.

[1] D. F. Geesaman et al. Ann. Rev. Nucl. Part. Sci. 56, 53–92 (2006).

[2] A. B Balantekin and N. Takigawa, Rev. Mod. Phys. 70, 77–100 (1998); M. Dasgupta et al. Ann. Rev. Nucl. Part. Sci. 48, 401–461 (1998); N. Keeley et al. Prog. Part. Nucl. Phys. 59, 579–630 (2007).

[3] A. Lemasson et al. Nucl. Instr. Meth., A 598, 445–449 (2009).

[4] A. Lemasson et al. Phys. Rev. Lett., (In Press) (2009).

[5] C. A. Bertulani, et al. J. Phys. G 34, 2289–2295 (2007) ; S. Bacca, and H. Feldmeier. Phys Rev.C 73, 054608 (2006).

Isotopic resolution of fission fragments from 238U+12C transfer and fusion reactions

Antoine Lemasson — 2009-10-10T10:43:00Z

M. Caamaño, F. Rejmund, X. Derkx, K.-H. Schmidt, L. Andouin, C.-O. Bacri, G. Barreau, J. Benlliure, E. Casarejos, B. Fernández-Domínguez, L. Gaudefroy, C. Golabek, B. Jurado, A. Lemasson, A. Navin, M. Rejmund, T. Roger, A. Shrivastava, C. Schmitt, and J. Taieb.

4TH INTERNATIONAL WORKSHOP ON NUCLEAR FISSION AND FISSION-PRODUCT SPECTROSCOPY; AIP Conf. Proc. — October 15, 2009 — Volume 1175, pp. 15-22

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Abstract

Recent results from an experiment at GANIL, performed to investigate the main properties of fission-fragment yields and energy distributions in different fissioning nuclei as a function of the excitation energy, in a neutron-rich region of actinides, are presented. Transfer reactions in inverse kinematics between a 238U beam and a 12C target produced different actinides, within a range of excitation energy below 30 MeV. These fissioning nuclei are identified by detecting the target-like recoil, and their kinetic and excitation energy are determined from the reconstruction of the transfer reaction. The large-acceptance spectrometer VAMOS was used to identify the mass, atomic number and charge state of the fission fragments in flight. As a result, the characteristics of the fission-fragment isotopic distributions of a variety of neutron-rich actinides are observed for the first time over the complete range of fission fragments.

Oral Contribution : Colloque GANIL 2009

Antoine Lemasson — 2009-08-26T06:42:25Z

Oral Contribution :

Colloque GANIL 2009 The XVIth "colloque GANIL" will be held from September 6th to 11th, 2009 in Giens (France). (Webpage)

[HAL]

Title :

Interplay between structure and reaction mechanisms around the Coulomb barrier probed using 8He

Abstract:

The availability of low energy Radioactive Ion Beams, has provided the possibility to study the influence of exotic structures and/or weak binding on reaction dynamics far from stability around the Coulomb barrier. Fusion reactions around the Coulomb barrier are not only a unique laboratory for probing various aspects of multidimensional tunneling but are also relevant to the synthesis of Super Heavy Elements and reactions of astrophysical interest. The SPIRAL facility at GANIL provides the most intense beams of 6,8He presently available, giving a unique opportunity to study neutron correlations [1] and the effect of Borromean structure on the reaction processes. The low energy and relatively low intensities ( 10^5 p/s) of the Radioactive Ion Beams make measurements challenging thus requiring the development of innovative methods to each new detections limits [2].

In this talk we will present recent results obtained with low intensity beams of Borromean isotopes of Helium. The complementarity of in-beam and off-beam methods will be discussed. In-beam inclusive and exclusive measurements of gamma-rays, charged particles and neutrons performed in the 8He+65Cu system to obtain fusion cross-sections and the angular distributions of direct reactions at energies above the barrier will be presented (Expt. E403aS). The first sub-barrier cross-sections measurements for fusion and nucleon transfer reactions involving the 8He beams on a 197Au target will also be presented (Expt. E543S). These results were obtained using a new activation technique [3] involving coincidences between characteristic X-rays and gamma-rays. The precision and accuracy reached in this experiment are at the same level as those obtained using stable beams having a million times larger intensity.

Results of these experiments will be discussed within a coupled channel framework using FRESCO [4].The role of channel coupling between the elastic and direct reaction channels, its influence on fusion at energies around the Coulomb barrier arising as a result of the exotic structure of 8He will be illustrated.

[1] A. Chatterjee et al., Phys. Rev. Lett. 101, 032701 (2008)

[2] R.F. Casten, Nuclear Structure from a Simple Perspective, 2nd Edition, Oxford, New York (2005) p. 426.

[3] A. Lemasson et al., Nucl. Instr. Meth., A 598 (2009) 445.

[4] I.J. Thompson, Comp. Phys. Rep. 7, 167 (1988).