My Main Research Activities


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After my undergraduate Physics studies at the University of Paris-Sud XI in Orsay, France, I was a postgraduate student at the University of Caen in Normandy, France. I obtained my PhD in 1996 at the Grand Accélérateur National d'Ions Lourds (GANIL) in Caen on high-precision direct mass measurements of very exotic nuclei.
 

An innovative experiment using the CSS2 cyclotron at GANIL as a high-resolution spectrometer [1,2] led to the production and identification of the very exotic nucleus 100Sn with the 50Cr + 58Ni fusion-evaporation reaction at 255 MeV and to its mass-measurement for the first time [3]. The understanding of this doubly-magic N = Z nucleus is one of the cornerstones on which nuclear structure models depend. It was discovered in 1994 at GSI, Germany, and GANIL, France.

By analogy with electrons in the atom, the nucleons (protons and neutrons) in the nucleus are organised into energy levels or shells. These shells can only accept a certain number of nucleons and, each time a shell is filled, the nucleus becomes more tightly bound and more difficult to excite than others. These numbers (2, 8, 20, 28, 50, 82 and 126) discovered in 1949 are called the magic numbers of the nuclear shell model. By analogy with atoms, magic nuclei are similar to the noble-gas atoms of the Mendeleiev periodic table of the elements. 100Sn with its 50 neutrons and 50 protons is called doubly-magic and is one of a dozen nuclei to have both magic numbers for  protons and neutrons, which should greatly strengthen its stability. Of bound nuclei which have the same numbers of neutrons as protons 100Sn is the heaviest and most exotic. It is an extreme case where its magicity competes with its exoticity.
 
 


The direct time-of-flight technique using the SPEG magnetic spectrometer at GANIL was extended to the mass measurement of neutron-deficient nuclei near the N = Z line such as 66As, 68,70,71Se and 71Br [4,5] which are in a region known to provide input for the modelling of the rp-process (rapid proton capture) in nuclear astrophysics and information relevant to nuclear structure in a region of high deformation. Radioactive secondary beams were produced via the fragmentation of a high-energy high-intensity 78Kr beam on a natNi target, using the SISSI device.
 

After my PhD, I took a two-year post-doctoral appointment jointly held between the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) and the Oak Ridge National Laboratory (ORNL), USA, to focus on the study of the structure of light exotic nuclei, in particular nuclei showing a halo structure and unbound nuclei.
 

I studied the break-up of the one-neutron halo nucleus 11Be in 10Li + p by measuring the 9Li fragments in coincidence with 2.7 MeV gamma-rays (corresponding to the 9Li first excited state) which were detected in anarray of 90 BaF2 detectors. These data provided information on the structure of the unbound-nucleus 10Li through the valence-neutron interaction with the 9Li nucleus and showed evidence for a parity inversion in 10Li leading to a dominant s-wave (l = 0) ground state [6]. The results of this experiment are essential to a proper description of the three-body Borromean system 11Li, one of the most interesting of halo nuclei. Another similar experiment showed evidence for an l = 0 ground state in unbound 9He [7]. The secondary radioactive beam of 11Be was produced by fragmentation of a 13C primary beam and purified in the A1200 spectrometer at the NSCL.
 
 


 
 



Following my post-doctoral position in the USA, I became in 1998 a Physics Lecturer at the University of Bordeaux 1 within the Centre d'Etudes Nucléaires de Bordeaux-Gradignan (CENBG), France, where I have extended my research activities to the spectroscopy of nuclei at the proton drip-line.
 

The two-proton radioactivity, that is to say the emission of a 2He nucleus from the ground state of a nucleus for which the emission of a single proton is energetically forbidden has been predicted nearly forty years ago but is yet to be observed. From mass predictions 45Fe and 48Ni are the best candidates for this new radioactivity. With this aim, a projectile-fragmentation experiment was recently performed at GANIL with the LISE3 spectrometer that led to the discovery of doubly-magic 48Ni [8].  Consequent experimentswere performed at GANIL and at GSI on the FRS fragment separator, to search for two-proton radioactivity of 45Fe and 48Ni.
  Odd-odd N = Z nuclei such as 62Ga are good candidates to extend the test of the Conserved Vector Current hypothesis and probe physics beyond the Standard Model of the weak interaction. These nuclei have 0+ ground states and decay by super-allowed Fermi beta-decay. The log ft values of these 0+ to 0+ transitions are used to determine the dominant matrix element of the Cabbibo-Kobayashi-Maskawa matrix which links the quark eigenstates to their mass eigenstates. To obtain the log ft values, high precision beta-decay half-lives and Q-values are needed. As these transitions are ground-state to ground-state transitions, the Q-values can be determined by measuring the masses of parent and daughter nuclei. I collaborate in experiments at the IGISOL facility in Jyväskylä, Finland, and at the on-line mass separator at GSI which aim to perform detailed studies of the decay of 62Ga as well as the precise measurement of its half-life. At the limits of extremely proton-rich nuclei, the direct emission of protons becomes the dominant decay mode and ultimately defines one of the boundaries of observable nuclei in the N-Z plane. In 2002 a new radioactive decay mode was discovered at GANIL and GSI: the ground state two-proton emission from the nuclide 45Fe.  Physicists also plan  to the search for this decay mode in another good candidate: the doubly-magic nucleus 48Ni that was recently discovered at GANIL. These nuclei can emit a di-proton, but are forbidden from expelling a single proton. An interesting challenge for future research is to confirm whether or not the protons are emitted as a 2He, which then breaks up. Theorists predicted this new type of radioactivity forty years ago. Recent progress in the delivery of secondary radioactive ion beams made these discoveries possible.

Since September 2001 I have been a Lecturer in the Department of Physics of the University of Liverpool where I joined the Nuclear Physics group. I have also been awarded a five-year EPSRC Advanced Research Fellowship to develop my research programme within this group.
 

Mass measurements of very short-lived exotic nuclei can be performed with the CSS2 cyclotron of GANIL and the new CIME cyclotron of SPIRAL which are complementary to those possible using the Penning trap of ISOLTRAP or the radio-frequency spectrometer MISTRAL, both at CERN/ISOLDE (Switzerland), or the ESR storage ring at GSI. An experimental proposal, of which I am spokesperson, has recently been approved beam time in order to develop and test the new CIME method, as well as to measure, with a precision better than 10-6, the masses of 31,32Ar radioactive isotopes delivered by SPIRAL.
 
 


 
 

The CIME cyclotron


crédit photo: M. Desaunay/GANIL



 
 
 



 
 
 
 
 

References

1. G. Auger et al, Nucl. Instr. Meth. A 350 (1994) 235.
2. M. Chartier et al, Nucl. Instr. Meth. B 126 (1997) 334.
3. M. Chartier et al, Phys. Rev. Lett. 77 (1996) 2400.
4. M. Chartier et al, Nucl. Phys. A 637 (1998) 3.
5. G.F. Lima et al, to appear in Phys. Rev. C.
6. M. Chartier et al, Phys. Lett. B 510 (2001) 24.
7. L. Chen et al, Phys.  Lett. B 505 (2001) 21.
8. B. Blank et al, Phys. Rev. Lett. 84 (2000) 1116.
 

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maintained by Marielle Chartier

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