I. Stefan2, F. de Oliveira Santos, M.G. Pellegriti, M. Fadil, S. Grévy, M. Lenhardt, M. Lewitowicz, L. Perrot, M.G. Saint Laurent, I. Ray, O. Sorlin, C. Stodel, J.C. Thomas1
G. Dumitru, A. Buta, R. Borcea, F. Negoita, D. Pantelica2 J.C. Angélique, M. Angélique3
E. Berthoumieux4
A. Coc, J. Kiener, A. Lefebvre-Schuhl, V. Tatischeff5 J.M. Daugas, O. Roig6
T. Davinson7 M. Stanoiu8
1) GANIL, CEA/DSM - CNRS/IN2P3, BP 55027, F-14076 Caen Cedex 5, France 2) Institute of Atomic Physics, P.O. Box MG6, Bucharest-Margurele, Romania
3) Laboratoire de Physique Corpusculaire, IN2P3-CNRS, ISMRA et Université de Caen, F-14050 Caen, France 4) CEA Saclay, DSM/DAPNIA/SPhN, F-91191 Gif-sur-Yvette, France
5) CSNSM, CNRS-IN2P3, Université Paris-Sud, F-91405 Orsay, France 6) CEA/DIF/DPTA/PN, BP 12, F-91680 Bruyères le Châtel, France
7) Department of Physics and Astronomy University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom 8) IPN Orsay, IN2P3-CNRS, Université Paris-Sud, F-91406 Orsay, France
Contact person: oliveira@ganil.fr 1. Key question and initial goals
Astrophysical motivations. Gamma-ray emission from classical novae is dominated, during the first hours, by positron annihilation resulting from the beta decay of radioactive nuclei. The main contribution comes from the decay of 18F (half- life of 110 minutes) and hence is directly related to 18F formation during the outburst.
A good knowledge of the nuclear reaction rates of production and destruction of 18F is required to calculate the amount of 18F synthesized in novae and the resulting gamma-ray emission. The rate relevant for the main mode of 18F destruction (i.e., through 18F(p,α)15O) has been the object of many recent experiments. Despite certain progress, this reaction rate still remains badly known; the rate uncertainty is about a factor 100 in a large range of temperature. This clearly supports the need of new experimental studies to improve the reliability of the predicted gamma-ray fluxes from novae.
Very high-energy resolution. Achieving very high-energy resolution (better than 4 keV) with charged particles is an experimental challenge, and this challenge is stronger when dealing with radioactive beams.
Gas target. In our experiment, we had to develop a thin helium gas cell that can sustain intense radioactive beams and the experimental constrains (energy resolution).
The method we used to investigate the spectroscopy of the 19Ne nucleus is the resonant elastic scattering. At low energies, it is known that the elastic scattering shows up anomalies (resonances), which are related to the structure of the compound nucleus. In this way, the spectroscopy of 19Ne can be achieved by the measurement of the 15O(α,α)15O reaction. We can use a silicon detector to detect the
emitted alpha. Since 15O is radioactive, we have to measure this elastic reaction in inverse kinematics α(15O,α)15O using a thin helium gas target. In fact, the excitation function is measured in one time, from the entrance energy down to the outgoing energy of the heavy ions after they have crossed the gas target. The inverse geometry and small specific energy loss of the alpha strikingly reduce the influence of the beam spread and straggling on the final resolution. This method is very well suited for secondary beams since the limited intensity is compensated by the large cross sections (several 100 mbarn/sr). From the shape of the resonances, one can obtain the angular momentum of the reaction, and finally deduce the spin assignment of the states. Another advantage in that case is that the first excited state in 15O is at
very high energy, at 5183 keV, which means inelastic scattering contributions are forbidden. In fact, all inelastic processes producing alpha particles are totally forbidden. The energy broadening of the alpha is minimum at zero degree. At zero degree, with a very narrow angle aperture of 1° (~ 1 msr), using a high-energy resolution silicon surface barrier detector, with a very thin helium gas target (100 µg/cm²), we can achieve a resolution better than 4 keV in centre of mass frame, a real improvement in the spectroscopy of this nucleus.
3. Results and achievements
α(15O,α)15O
The analysis of this reaction is still going on (the experiment was scheduled in April 2005). Unfortunately, the gas target suffered several problems, mainly a continuous change of the windows thickness due to the degradation by the beam interaction. This main problem prevented us to achieve the expected energy resolution.
p(14N,p)14N
We performed several measurements to calibrate our silicon detector and to validate the analysis. The p(14N,p)14N reaction was one of the reactions we used for
the calibration. The results obtained for this reaction are shown in Fig. 1. We achieved a very nice resolution (better than 4 keV in CM) since we used a solid target. The R-matrix analysis (continuous line) is in perfect agreement with the data.
Resonance Coulomb
Figure 1: Excitation function for the p(14N,p)14N reaction. A very nice resolution was obtained since a narrow resonance in 15O is observed at ~ 1.1 MeV with a width of
~3.6 keV.
p(15O,p)15O
This reaction is one of the reactions we wanted to use for the calibration, but we found new and interesting results for the unbound compound nucleus 16F. See Fig. 2.
Figure 2: Excitation function for the p(15O,p)15O reaction. Peaks correspond to states in 16F.
New reaction pathway to bypass the 15O waiting point
We have proposed a new and very exotic process involving unbound nuclei. Using the new results obtained for 16F, we applied the new ideas to this case. We have proposed a new reaction pathway to bypass the 15O waiting point in astrophysics. It is the sequential reaction process
15O(p,gamma)(beta+)16O. This exotic reaction is found to have a
surprisingly high cross section, approximately 1010 times higher than the
15O(p,beta+)16O. The large cross section can be understood to arise from
the more efficient feeding of the low energy wing of the ground state resonance by the gamma decay, see Fig. 3.
Figure 3: The new proposed reaction pathway. Red and blue lines correspond to two different gamma transitions from the first excited state to the ground state resonance of 16F.
4. Conclusion and prospective
In conclusion, it was not possible to achieve a high-energy resolution measurement of the resonant elastic scattering reaction 4He(15O,alpha)15O. This was mainly due to the fact that the thin helium gas target was not homogeneous. We measured the properties of the low-lying states in 16F with H(15O,p)15O, using a solid target. We applied the new results into a new reaction pathway
15O(p,gamma)(beta+)16O. This reaction is very promising.
There are a lot of subjects in prospective:
- To achieve the best energy resolution (better than 100 eV?) with our method
- To develop a homogeneous helium gas target that can sustain intense beams
- To develop the (p,gamma)(beta) reaction ideas and related ideas: non exponential decay, quasi-bound unbound nucleus, test of the Heisenberg relationship etc…
5. References
Tours Symposium on Nuclear Physics VI, Tours 2006, AIP Conference Proceedings 891, to be published