LIBRA: The Nuclear Astrophysics Program

The INP's Nuclear Astrophysics research activities up to 2005

    The research program of the Institute of Nuclear Physics (INP) of NCSR "Demokritos", related to stellar nucleosynthesis was launched in the early-nineties. Since then the group has carried out measurements at the in-house Tandem accelerator as well as abroad in collaboration with various well-established nuclear astrophysics groups. In the first years, the group studied key nuclear reactions relevant to inhomogeneous big-bang nucleosynthesis as well as to the CNO and MgAl cycles. It is worth mentioning that, among the most important achievements of the INP Nuclear Astrophysics group is the realisation early-on of the importance of scientific exchange among physicists from the different domains of nuclear physics and astrophysics. This realisation led to the co-organization of the first European conference on Nuclear Astrophysics (12-18 June 1988, Crete), with the title "Quests in Nuclear Astrophysics and Experimental Approaches". The success of this conference stimulated a subsequent series of international conferences, which have been coined "Nuclei in the Cosmos" and is, now, the most esteemed Nuclear Astrophysics conference series worldwide. Until 2000, the INP group has participated in two large Nuclear Astrophysics Networks, i.e. a) the NATO SCIENCE Network Program SC1-0065 entitled "Nuclear Astrophysics: Experimental and Theoretical Studies" (1990-1993) and b) the NACRE Network, supported by the Human Capital and Mobility Programme of the EC from 1995 to 1999. The latter network produced the famous NACRE Compilation of charged-particle-induced thermonuclear reaction rates published in Nuclear Physics A (Vol. 656, 1999, pp. 3-183) which is now a reference work for every nuclear astrophysicist. The INP group obtained additionally a NATO grant within the Collaborative Research Grants Programme entitled "Reaction rates of the nucleosynthetic p-process in the A»t;90 mass region" which was realized between 1997 and 2000 in collaboration with the Institut für Strahlenphysik of the University of Stuttgart, Germany. Within the aforementioned collaborations, the INP group gained widely acclaimed expertise in this field.

    Between 2000 and 2005, the group has participated in experimental studies of the 12C(α,γ)16O key reaction in stellar helium burning. This project was realized at the Dynamitron accelerator of the University of Stuttgart, Germany in the framework of a wide international collaboration comprising 8 European research institutions. The 12C(α,γ)16O reaction is considered as the most important thermonuclear reaction in non-explosive astrophysical sites. It determines helium burning in massive stars after the production of 12C in the triple-alpha process and, therefore, has a strong impact on the carbon/oxygen ratio in the cosmos, the nucleosynthesis of most heavy elements, the composition of white dwarfs, and the mechanism of supernova explosions. The aim of this project was to determine the rate of the 12C(α,γ)16O reaction with the best possible accuracy. This was made possible by measuring γ-angular distributions in the center-of-mass energy range from 0.89 to 2.8 MeV using for the first time in nuclear astrophysics studies, a multi-detector array of 9 BGO-shielded HPGe detectors borrowed from the EUROGAM collaboration. The sensitivity of these measurements was raised by a factor of 10 to 100 compared to previous investigations and the astrophysical reaction rate was finally determined with a considerably improved accuracy. The results of these measurements have motivated new abundance calculations worldwide that include the new rates as input parameters. In the same period, the INP group competed for and got two bilateral research grants: a) Greek-Hungarian bilateral scientific agreement entitled "Experimental investigations of radiative capture reactions of key importance for the understanding of the p-nuclei abundances in the solar system", in collaboration with the Institute for Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), Debrecen, Hungary, and b) Greek-German academic exchange program IKYDA entitled "Capture reactions in the explosive nucleosynthesis", in collaborations with the Institut für Experimentallphysik III (group of Prof. Claus Rolfs) of the Ruhr-University, Bochum, Germany. [top]

Current research program in Nuclear Astrophysics

    The current research program of the INP group on stellar nucleosynthesis is focused on the study of nuclear reactions related to the understanding of the so-called p-process nucleosynthesis, i.e. the synthesis of certain neutron-deficient nuclei heavier than iron (p nuclei) in explosive stellar environments. These activities include mainly systematic measurements of proton and alpha-particle capture reactions relevant to the p-process. These measurements were carried out initially at the in-house Tandem accelerator and, as mentioned above, at the Dynamitron accelerator of the University of Stuttgart, Germany. The latter laboratory has in the meantime been closed and the relevant activities were interrupted. Since 2003, additional measurements were performed at the Dynamitron Tandem Laboratory (DTL) of the Ruhr-University of Bochum, Germany. These systematic measurements aim at establishing an extended cross-section database for capture reactions that will enable us to derive global input parameters for statistical model calculations at energies relevant to p-process nucleosynthesis as well as to investigate the impact of nuclear physics uncertainties on the abundance calculations of the p nuclei. So far, the major challenge in the study of the p nuclei is to define their origin, which is still one of the most puzzling tasks to be solved by any model of heavy-element nucleosynthesis.

    The class of p nuclei consists of 35 stable nuclei that are heavier than iron and cannot be synthesized by the two neutron-capture processes referred to as s and r processes. To date, these nuclei have been observed only in the solar system. The reproduction of p-nuclei abundances on the basis of astrophysical processes occurring outside the solar system, in exploding supernovae (SNII) or on He-accreting white dwarves with sub-Chandrasekhar mass, will enable us not only to understand the nuclidic composition of the solar system but also to further elucidate our fundamental picture of its creation. So far, all the models of p-process nucleosynthesis are able to reproduce most of the p-nuclei abundances within a factor of 3, but they fail completely in the case of the light p nuclei. Due to the huge number of reactions involved in abundance calculations, the latter have to rely almost completely on the reaction cross-section predictions of the Hauser-Feshbach (HF) theory. It is therefore of key importance, on top of any astrophysical model improvements, to investigate the uncertainties in the nuclear data, and in particular in the nuclear level densities (NLD), nucleon-nucleus optical model potentials (OMP), and γ-ray strength functions entering the HF calculations. In view of these problems, the INP group has performed several in-beam cross sections measurements of proton- and since 2003 of alpha-capture reactions in the Ni-Te region at energies well below the Coulomb barrier. So far, more than 30 nuclear reactions have been investigated. It is worth mentioning that before this systematic work of the INP group, cross section data were very scarce. Based on the results of our systematic measurements of proton-capture reactions carried out over the last 7 years, we can conclude that the uncertainties affecting nuclear input (NLD, OMP) in the calculations give rise to at most 40% uncertainties in the involved reaction rates. It appears that HF predictions are more sensitive to OMPs rather than to NLDs. Although the significant discrepancies between observed solar system p-nuclei abundances and predicted ones are mainly attributed to uncertainties in the pure astrophysics modelling of the p-process, the nuclear physics uncertainties associated with alpha-capture reaction rates are still quite large and could decisively affect the abundance patterns. These uncertainties are largely due to our poor knowledge of the alpha-nucleus OMP at low energies. More reliable global alpha-OMPs are therefore needed, and consequently more data on reaction cross sections to constrain the parameters of the optical model.

   In view of these needs, the INP group has recently developed, in collaboration with the DTL Group in Bochum, an advanced method coined 4π-γ-summing, which is based on the measurement of angle-integrated γ-ray fluxes and allows, for the first time, to determine very low cross sections of α-particle capture reactions. In addition, the 4π-γ-summing method enables us to study any reaction within relatively short beam times, in contrast to other experimental approaches, i.e., the activation technique and the γ-angular distribution measurements, respectively. The 4π-γ-summing method has proved to be very efficient in measuring cross sections of nuclear reactions of stable nuclei. The primary goal, however, to develop global microscopic α-particle OMP requires to extend the measurements to heavy mass regions with much smaller cross sections and also to regions away from the stability valley where the nuclei of interest are unstable. To achieve this goal, there exist different approaches, amongst which the (α,γ) measurements at sub-Coulomb energies in inverse kinematics using state-of-the art detectors combined, if possible, with modern recoil separators is very transparent and, from the experimental point of view a challenging one. Such measurements have in the meantime been proposed by the INP group to be carried out in two large-scale European radioactive beam facilities, i.e. REX-ISOLDE at CERN and, when operational, in SPIRAL2. In the first case, the aim is to determine cross sections of (α,γ) reactions in inverse kinematics through γ-ray spectrometry, whereas in the latter one by employing particle-spectrometry. In both cases, state-of-the art instrumentation, i.e. the MINIBALL multi-array of Ge detectors and the LISE velocity filter, respectively, is proposed to be employed. It is worth notying that the INP group has already tested MINIBALL's response to capture events. These tests have shown that MINIBALL is indeed a very appropriate tool to perform this type of measurements. Meanwhile, the INP group has submitted a proposal to the Scientific Advisory Committee (SAC) of SPIRAL2 whose suggestion was to "begin this type of studies as soon as possible by using the stable beams currently available at GANIL". Indeed, the INP Group has done so and the Program Advisory Committee (PAC) of GANIL has approved the first test measurements. In parallel, the INP group submitted a research proposal to the PAC of the Cyclotron laboratory of the University of Jyväskylä, Finland, to perform cross section measurements of (α,γ) reactions in inverse kinematics by impinging a heavy 124Xe beam on an 4He implanted Al-foil. This proposal was also approved and the realization of the project is in progress. One has to stress that the programs underway serve mainly to familiarize with the experimental difficulties before launching measurements with radioactive beams. [top]

LIBRA activities in Nuclear Astrophysics

    The further development of the research program with measurements relevant to the understanding of p-process nucleosynthesis, is of increased scientific interest. The study of capture reactions, notably at energies well below the Coulomb barrier, is quite challenging both in terms of scientific motivation and the experimental approach. It reflects a long-standing astrophysical problem, i.e. how certain heavy-isotopes (p nuclei) observed, so far, only in the solar system are formed in the Universe. Moreover, p-nuclei abundances are the signatures of the creation mechanism(s) of our solar system. It is worldwide accepted, that this issue still remains open and that there is much research work left to be done in this respect. Existing p-process models are still unable to reproduce the observed p-nuclei solar abundances and certain nuclear physics models describing global parameters entering the calculations are inaccurate and unreliable. It is therefore of paramount importance on top of any astrophysical model improvements to perform a reliability check of nuclear physics predictions related to the understanding of the p-nuclei abundance pattern. The importance of this scientific program has independently been documented in the last Long Range Plan of NuPECC published in 2004 (pages 128,129) with an explicit reference to the contributions of the INP Group. The latter have also been recognized by the external committee that has evaluated INP in 2005. In the corresponding evaluation report one reads that "The committee noted the NuPECC recommendation for research lab accelerators to extend dedicated nuclear astrophysics programmes, to which Demokritos has a strong possibility to contribute significantly...".

    To date, the INP group has achieved to clarify several aspects of the planned measurements with stable as well as radioactive beams and has developed clear ideas concerning the type of targets and detectors required to implement successfully the aforementioned research programme. In conclusion:

  1. A state-of-the art 4π-γ-summing detector is necessary to perform at the in-house Tandem accelerator, or elsewhere, cross-section measurements of alpha-particle capture reactions.
  2. He-implanted solid targets with certain properties (stability, high 4He-fluences, etc) as well as a compact He-filled gas-cell target-system have to be developed for complementary measurements at different European centres of excellence in nuclear physics (GANIL, REX-ISOLDE/CERN, LN-Legnaro, Jyväskylä and others) using the advanced 4π-γ-detector arrays available in these centres.
  3. An advanced, low-cost, and "mobile" instrument to count the recoils, instead of the gammas, emitted by a capture reaction can provide a breakthrough for cross section measurements of astrophysical relevance. In this direction, a Time Projection Chamber (TPC) based on the MICROMEGAS technology may offer a novel solution. The MICROMEGAS-TPC will be employed not only in experiments abroad but also at INP to measure cross sections of neutron-induced reactions of relevance, such as (n,α) reactions

    Complementary to these activities, the INP group is planning to extend its nuclear astrophysics to low-energies using a low-energy (<250 keV) high-current accelerator known within the European community as PAPAP (Petit Accelerateur Pour AstroPhysique). This 250 kV electrostatic single-stage accelerator is currently installed at the Centre de Spectrometrie Nucleaire et de Spectrometrie des Masse (CSNSM) at Orsay, France. It is equipped with one beam line and a multicusp-type source system that is capable of providing proton and deuteron beams with intensities up to 0.5 mA. The construction and installation of PAPAP was funded by the French funding agencies IN2P3 and INSU. The Institute of Nuclear Physics of NCSR "Demokritos" also contributed with construction funds as well as with man power during the calibration and testing phases of PAPAP. This Orsay-Demokritos collaboration is documented in the first publication about PAPAP from 1994. Unfortunately, the collaboration was interrupted due to lack of mobility funds on the Greek side.  Recently, however, CSNSM Orsay has officially offered PAPAP to the INP of "Demokritos" due to lack of man-power required to run the facility. The installation and operation of PAPAP at INP, with some improvements in its source system that will allow accelerating α-particles as well, will enhance significantly the research potential of INP and will decisively strengthen the existing collaboration with the French group of CSNSM. What's more, it will attract many users from abroad. PAPAP will mainly be employed to determine nuclear reaction rates of key importance in explaining the observed abundances of the light elements in the universe that provide direct signatures of the Big-Bang Nucleosynthesis (BBN). Though the standard BBN theory has been very successful in explaining the observed abundances of the light elements in the universe, at least approximately, the associated reaction network calculations are often confronted with uncertainties affecting strongly the final result. Apart from BBN relevant reactions, such as 2H(4He,γ)6Li, there are others related to the modelling of the various Nova models, such as 23Na(p,γ)24Mg or 17O(p,γ)18F and its "competitor" 17O(p,α)14N, which though intensively studied in the last 3 to 5 years are still affected with signifficant uncertainties in their reaction rates. The relevant scientific community is currently carrying out an ambitious program to eliminate these uncertainties in order to better understand stellar evolution. In such a program, low-energy high-intensity beam accelerators like PAPAP are most desirable. The INP group is planning to install and upgrade PAPAP in collaboration with scientists and technicians from CSNSM.

    The research program related to stellar nucleosynthesis studies described above is strongly linked to nuclear reaction theory. Therefore, it is within the aims of the proposed project to strengthen the relevant research program of INP too. The nuclear reaction theory activities of INP, over the past 5 years have focused on the study of nuclear reaction mechanisms in a broad energy range, and have included the development of both purely theoretical and more phenomenological models for the prediction of the relevant nuclear properties. As a result a considerable experience in the field of nuclear reactions was accumulated, including skills in software development for model calculations. Moreover, through constant and active interaction with other theoretical and experimental physicists, expertise and know-how in providing theoretical support for the planning and analysis of cross section measurements performed by national and international collaborations was acquired. The theoretical description of nuclear reactions, followed by the development of nuclear reaction codes that provide nuclear data over a broad range of energies and nuclear masses is of great importance for a host of applications, from astrophysics to the more practical reactor technologies, transmutation of nuclear waste, space and aviation technologies, and medical physics. As an example, we mention the increasing interest in nuclear data on silicon materials which are needed for the development of radiation-hardened electronic devices used in space and aviation systems. The establishment of accurate and reliable nuclear data libraries for silicon and its neighboring nuclei has enormous implications for the space and aviation industry, and consequently huge economic implications as well.

    Apart from these, the theoretical support of the INP experimental group with activities related to stellar nucleosynthesis is a long-term goal of the INP's overall research program. One of the nuclear astrophysics issues we have been concerned with in the past is the alpha-nucleus optical model potential which is poorly known at low energies and leads to large uncertainties in the predictions of nuclear reaction rates relevant to the p-process nucleosynthesis. In our previous studies, we derived a global semi-microscopic alpha-nucleus optical potential based on all existing reaction data at low energies. One of the accomplishments of our ongoing research program will be the updating of this semi-microscopic optical potential on an extended database of measured cross sections covering a wider range of nuclei not studied before (A≈100). Our efforts will also lead to the determination of a fully microscopic alpha-nucleus optical potential, based on state-of-the-art effective nucleon-nucleon interactions. These optical potentials will be able to reproduce existing data and also predict more reliable reaction cross sections in unknown mass regions. Given these, the main objective to be achieved is to update the semi-microscopic alpha-nucleus optical potential in collaboration with the experimental group of the INP, and to further develop a microscopic alpha-nucleus optical potential based on nuclear matter considerations.[top]

Upcoming LIBRA scientific events in Nuclear Astrophysics

    An International Workshop on "Nucleosynthesis and Nuclear Physics: Open Questions" will take place at the Congress Center of NCSR "Demokritos" for 3 days. Around 30 to 40 expert scientists are expected to participate. An international program committee will decide on the invited speakers. Support will be granted to invited speakers and a number of young researchers with an excellent scientific record. The exact dates will be announced by end of March 2009.

    Apart from the aforementioned Workshop, the 3rd International Conference on "Frontiers in Nuclear Structure, Astrophysics, and Reactions - FINUSTAR 3"will be organized. FINUSTAR is a conference series holding a leading place among international scientific events. It is organized by the INP group together with the Department of Physics of the University of Jyväskylä, Finland. FINUSTAR 1 and 2 were organized in Sept. 2005 in the Isle of Kos, Greece, and in Sept. 2007 in Crete, Greece, respectively. Both conferences have been attended by more than 150 participants. FINUSTAR-3 will be organized either in 23-27 August 2010, on the Isle of Rhodes, Greece. At least 150 participants are expected to attend. The scientific program will be based on the recommendations of an International Advisory Committee consisting of approx. 30 members that will ensure excellent scientific contributions. As in the case of FINUSTAR 1 and 2, there will be approximately 80 oral contributions and more than 40 poster presentations. FINUSTAR 3 will have an interdisciplinary character (nuclear structure-nuclear astrophysics-nuclear reactions) to give the opportunity to many participants not only to be updated in scientific activities in other nuclear physics topics but also establish new collaborations. Two prizes are foreseen for the best 2 posters, one in theory and one in experiment, to be awarded during the conference by the international advisory committee and the participants. In addition, peer-reviewed proceedings will be published by a worldwide known publisher. [top] [NNP Workshop website][FINUSTAR 3 website]

Nuclear Astrophysics


Dr. Sotirios V. Harissopulos
Tandem Accelerator Laboratory,
Institute of Nuclear Physics,
NCSR "Demokritos",
POB 60228, 153.10 Aghia Paraskevi,
Athens, Greece