IFIN-HHNuclear Physics Department

Nuclear Astrophysics (NAG group)

Contact person: Livius Trache, Dan Filipescu, Adriana Raduta

Nuclear Astrophysics is high in the scientific programs of all large nuclear physics institutes in the world and so it is in the DFN program. We are doing experimental work and theory. A recent review of all nuclear astrophysics activities in IFIN-HH was published recently and is available at https://www.epj-conferences.org/articles/epjconf/abs/2020/03/epjconf_enas2020_01016/epjconf_enas2020_01016.html.
Most of the activities described in the paper are concentrated in DFN. We work with home facilities, and collaborate with fellow experimentalists and theoreticians from different laboratories outside Romania, on 3 continents: Texas A&M University (TAMU), Washington University at St. Louis (WU), LNS Catania, ATOMKI Debrecen, RIKEN, Wako, Konan University and NewSUBARU, Japan, LUTH, Observatoire de Paris, France, FIAS Frankfurt, Germany; University of Coimbra, Portugal.

Nuclear Astrophysics Group (NAG): NAG is a group in DFN working in experimental nuclear physics for astrophysics. To accomplish our program, we work locally with our 9 MV tandem accelerator, with the accelerators (3 MV and 1 MV tandetrons) of the Department of Applied Nuclear Physics (DFNA), and with one theoretician from DFT.

Experimental activities
NAG was established and built after 2012.
Activities are concentrated in two types of experiments and their interpretation:

A. Direct measurements using IFIN-HH’s own installations.

B. Indirect measurements for Nuclear Astrophysics using RIBs in international nuclear physics centers.

We also have a consistent outreach program, with national and international audiences (section C below).

A. Direct measurements for Nuclear Astrophysics

A.1 Measurements using the tandetron & the underground laboratory
In 2013 and 2014 it became clear that doing direct measurements for nuclear astrophysics may be a possibility afforded by the newly installed 3 MV tandetron accelerator of IFIN-HH, which was just installed at the time [2]. The idea of coupling it with the use of the ultra-low background laboratory that IFIN-HH has in the salt mine at Slănic-Prahova [3] turned out to be a valuable one that makes the group competitive in this field. The accelerator tests of 2013 and the experiments of the following years proved the idea correct: this combination makes the group competitive for direct measurements with alpha particle and light ion beams. These “home experiments” turned out to be also an excellent training opportunity for the young students in the group. At first, the main emphasis was on the experiment ¹³C+¹²C, which was done with groups from China (IMP Lanzhou, and CAS Beijing) and from other departments of IFIN-HH: DAT, DFNA and DFVM.

During the experiment, the ¹³C beam in the laboratory energy range of E_lab= 11.0 – 4.6 MeV (E_cm=5.28-2.21 MeV), with steps of 0.2 MeV, impinged on 1.5 mm thick natural carbon targets. Intensities in the range of 0.02-15 pµA were used in different runs. We did activation measurements (both in Magurele and in the salt mine in Slănic) and in-beam prompt gamma-ray measurements. Thick target yield for the ¹²C(¹³C, p)24Na fusion reaction was determined through the measurement of the gamma-ray yield following the beta-decay of 24Na (T1/2=15 h) at the low background laboratory GammaSpec (at ground level in IFIN-HH), in NAG’s own setup and in the ultra-low background laboratory μBq in Unirea salt mine at Slănic (Figures 2 & 3). The characteristics of the combined facility and the main results of this study were published [4,5].

Figure 2. Comparison between collected background from laboratories(NAG, GammaSpec and µBq)

Later, similar measurements were made with alpha-beams for the reactions Ni(α,x) and Zr(α,y). More recently we developed a special device to allow a more efficient measurement of activities with shorter lifetime. It is a beta-gamma coincidence setup we dubbed BEGA (Figure 4). It allows easier, reproducible de-activation measurements for isotopes with lifetimes as low as 1 min.

With it we started a wider program to study reaction mechanisms for ion-ion fusion at energies below the Coulomb barrier. This may take several years, and prompt gamma-ray and de-activation measurements at home will be combined with indirect methods at outside laboratories.

A.2 Experiments at the 9 MV tandem-pelletron
The astrophysical p-process involves the transformation of pre-existing stable nuclei located at the bottom of the valley of nuclear stability - s nuclei – and those located on the neutron-rich side of the valley – r nuclei - into proton rich species by series of (γ, n), (γ, p), (γ, α) photodisintegrations and beta decays [6]. Cross section measurements on proton and α captures at sub-Coulomb energies provide constrains on the γ-ray strength function, especially the E1-strength distribution, which directly determines the total photodisintegration cross section and therefore plays a decisive role in model calculations for nuclear astrophysics.

Our group measures (α, γ) [7-9] and (p, γ) [10] absolute cross sections on medium mass targets in the energy range close to the Gamow window. The experiments typically measure activation γ-rays, using a counting setup of two large volume high-pure germanium detectors (HPGe) with relative efficiencies 55% and 100%, placed head-on in a passive lead shielding. Recently we also started to investigate the possibility of using the RoSPHERE detection array for in-beam cross section measurements and spectroscopic studies dedicated to astrophysics. Experimental measurements are complemented by theory efforts to provide systematics of α-particle optical potentials at low and very low energies by Vlad and Marilena Avrigeanu of our department.

Together with collaborators from the Department of Nuclear Physics at Oslo University, we launched an extensive experimental study on nuclear spin distributions. Using proton beams delivered by the 3 MV IFIN-HH Tandem accelerator, we measure proton capture cross sections for the residual nucleus formation in the ground and metastable states. The measured isomeric ratio is particularly useful for probing the spin dependence of nuclear reactions and can shed light on the spin distribution of the compound nucleus [11]. The spin distribution is a key ingredient for the Oslo method [12] of simultaneous extraction of nuclear level densities (NLD) and Gamma-ray Strength Functions (GSF) in the astrophysical relevant low energy range, below the particle separation threshold. Sample spectra below, in Figure 5.

Figure 5. Background and activation spectra of proton + ^natZr reactions for the lowest proton beam energy E_p=1800 keV.

B. Indirect methods

There are many indirect methods that can be used in nuclear physics for astrophysics. We use a few of them, depending on the topic in our attention, of the collaboration possibilities and the available beams. Most of these are with RIBs and here we include the spectroscopy of resonances (B1) and the breakup of loosely bound proton-rich projectiles (B2), but there are cases in which stable beams are used, like in the reactions studied with the Trojan Horse Method (B3) in collaboration with our Catania group collaborators. In many cases one needs good theoretical descriptions and parameters (B4) of the reactions used to use information obtained in indirect measurements at higher energies to estimate astrophysical S-factors or reaction rates at NA relevant energies or temperatures.

B.1. Beta-delayed proton decay
The resonant capture of protons is a two-step process where the proton incident on a nucleus populates first a metastable state in the compound nucleus (1^st step) that then de-excites (2^nd step) by gamma-ray emission. The corresponding astrophysical reaction rates are given by the properties of the narrow, isolated resonances only: spin and parity, energy, and resonant strength ωγ. To study these resonances at astrophysical energies by direct measurements is not always easy or even possible. An alternative is to populate the same metastable states and determine their spectroscopic properties by other means. One way is the decay spectroscopy: we chose an exotic nucleus that will beta-decay to these same states. These are schematically illustrated in Figure 3 below. The conditions were this is possible are listed on the left and right sides of the figure: Q-values and the appropriate selection rules.
We have studied β-delayed proton decay (βp) at Texas A&M University using ASTROBOX2, an improved version of and early gas detector ASTROBOX-1, both developed with TAMU and CEA/IRFU Saclay [13] A clone of this new detector is at IFIN-HH, for use in European laboratories, ASTROBOX2E.
Resonance spectroscopy studies can be made using the beams from the 9 MV tandem-pelletron and the possibilities of the RoSPHERE array, combining gamma-ray detectors and neutron detectors, e.g.

B.2. The breakup of 9C
In 2001 two of us proposed to use nuclear breakup data to determine the ANC (Asymptotic Normalization Coefficients) for the breakup of nuclei Y->X+p and from there to evaluate the astrophysical S-factors for radiative proton capture reactions X(p,g)Y. Important NA reactions data like S17 [14] and S18 [15] were evaluated using data from literature. Later a dedicated experiment at GANIL was used to obtain NA data for the reactions 22Mg(p,γ)23Al and 23Al(p,γ)24Si [16].
In proposal NP1412-SAMURAI29R1 approved by the PAC of RIBF at RIKEN, Wako, Japan, we use the same scheme: we study the nuclear and Coulomb breakup of 9C to evaluate the astrophysical S-factor for radiative proton capture on 8B. The experiment was carried out in 2018 and the data are being analyzed.

B.3. Trojan Horse Method measurements
Trojan Horse Method was introduced and demonstrated as a valuable method for NA by the group of prof. Claudio Spitaleri from the University of Catania and INFN LNS. They have proposed and test measurements were done in collaboration in Bucharest, at the 9 MV FN tandem accelerator together with us to check if the reaction ¹²C + ¹²C can be studied using the Trojan Horse Method. The reaction proposed was 12C(¹⁶O,α²⁰Ne)α. The test was done using an ¹⁶O beam on ¹²C targets, but the answer was NO, this reaction cannot be used because it turns out that the ground state of the projectile does not have a good clusterisation in the α+¹²C channel. An alternative experiment using ¹⁴N beam was carried out at LNS Catania and resulted in important results [17]. We continue this path for other reactions with light ion-ion fusion reactions in stars.

B.4. Optical Model Potentials for nucleus-nucleus collisions
We have a long-term program to understand and describe nucleus-nucleus collisions in terms of one interaction potential, the optical model potential (OMP). LT has worked on the problem for almost two decades with dr. F. Carstoiu. The motivation is that a good understanding of all phenomena occurring in the elastic nucleus-nucleus scattering, which are used typically to extract OMP, and the interpretation of the origin of different aspects, including the well know potential ambiguities, are of crucial importance for finding and justifying the procedures used for predicting nucleus-nucleus OMP in the era of radioactive nuclear beams (RNB) (see ours based on double folding in Ref. 18). The reliability of these potentials is crucial in the correct description of reactions, from elastic to transfer, to breakup, at energies ranging from a few to a few hundred MeV/nucleon. Of interest for us is to support the absolute values of the calculated cross sections for reactions used in indirect methods for nuclear astrophysics, see references [19] for some examples.

B.5 Others
An exotic and recent subject can be considered the approach to cosmochemistry. Our group member Iuliana Stanciu is a PhD student on the topic "Search for Supernova R-process actinides in fossilized reservoirs", PhD Supervisor Prof. Shawn Bishop, Physik-Department, E68, Technische Universität München. An important part of her work is done in the institute, including a few other research departments.

C. Carpathian Summer Schools of Physics

NAG was and remains also instrumental in the organization of the most recent editions of the Carpathian Summer Schools of Physics, a tradition that begun in 1964. Seven latest editions were a series dedicated to nuclear and particle astrophysics, in relation to exotic nuclei and to physics with small accelerators. The Carpathian school is part of the European Network of Nuclear Astrophysics Schools (ENNAS), together with the European Summer School on Experimental Nuclear Astrophysics, ESSENA (Catania, Italy) and the Russbach Winter School on Nuclear Astrophysics, RWSNA (Russbach am Pass Gschütt, Austria). In agreement with those schools’ organizers, we created an established network of periodic events that responds to the need of preparing and educating the younger generations of physicists in the cross disciplinary fields of nuclear physics and astrophysics.

Below are links to the latest editions of CSSP:
CSSP12: http://cyclotron.tamu.edu/cssp12
CSSP14: http://cssp14.nipne.ro/
CSSP16: http://cssp16.nipne.ro/
CSSP18: http://cssp18.nipne.ro/

The 2020 edition was scheduled for July 2020, but had to be canceled due to the coronavirus crisis and will probably be postponed with exactly one year CSSP20: http://cssp20.nipne.ro/
The Proceedings of the schools were published, 6 of them with AIP Publishing. The latest was published in 2019 [20]

T1. Dense Matter Equation of State and Compact Objects

Cold mature nuclear stars are described by a one-parameter equation of state (EoS) that relates pressure to energy density. In contrast, the studies of the dynamics of core-collapse supernovae (CCSN), proto-neutron star (PNS) evolution, stellar black-hole (BH) formation and binary neutron star mergers (BNS) require as an input an EoS at non-zero temperature and out of (weak) β-equilibrium, i.e., the pressure becomes a function of three thermodynamic parameters. For describing all of the above mentioned astrophysical objects one needs to consider baryon number densities ranging from sub-saturation densities up to several times the nuclear saturation density (10^-15 < n < 10 fm^-3), temperatures 0 < T <100 MeV and charge fractions 0 < Yq < 0.6.
To the uncertainties related to the isoscalar and isovectorial channels of nuclear matter, add up uncertainties related to the possible population of exotic degrees of freedom (strange baryons, baryonic resonances, condensates and quarks) for which little experimental constraints exist despite significant experimental efforts. The rapid progress of multi-messenger astrophysics allows one to constrain the neutron star (NS) EoS in domains unattainable in our terrestrial laboratories.
In this context the study of dense matter’s EoS is of major interest for both nuclear physics and astrophysics communities.
Our research concerns:

EoS of cold catalyzed matter with hyperonic degrees of freedom,

thermal evolution of isolated NS and accreting NS in quiescence,

development of finite temperature EoS with exotic degrees of freedom,

stability of NS with respect to gravitational collapse into BH,

universal scaling in hot star matter.

Most recent publications are [21-25].


References:
[1] Daniela Chesneanu, L. Trache, R. Margineanu, A. Pantelica, D. Ghita, M. Straticiuc, I. Burducea, A. M. Blebea-Apostu, C. M. Gomoiu, and X. Tang, AIP Conference Proceedings 1645, 311 (2015).
[2] I. Burducea, M. Straticiuc, D.G. Ghita, D.V. Mosu, C.I. Calinescu, N.C. Podaru, D.J.W. Mous, I. Ursu, N.V. Zamfir, Nuclear Instruments and Methods in Physics Research B 359, 1219, 2015.
[3] R. M. Margineanu, C. Simion, S. Bercea, O.G. Duliu, D. Gheorghiu, A. Stochioiuand, M. Matei, Appl. Radiat. Isot. 66, 1501(2008).
[4] D. Tudor, L. Trache, Alexandra I. Chilug, Ionut C. Stefanescu, Alexandra Spiridon, Mihai Straticiuc, Ion Burducea, Ana Pantelica, Romulus Margineanu, Dan G. Ghita, Doru G. Pacesila, Radu F. Andrei, Claudia Gomoiu, Ning T. Zhang, Xiao D. Tang, Nucl. Instr. & Meth. in Phys. Res. A 953 (2020) 163178. A facility for direct measurements for nuclear astrophysics at IFIN-HH -- a 3 MV tandem accelerator and an ultra-low background laboratory
[5] N. T. Zhang, X. Y. Wang, H. Chen, Z. J. Chen, W. P. Lin, W. Y. Xin, S. W. Xu, D. Tudor, A. I. Chilug, I. C. Stefanescu, M. Straticiuc, I. Burducea, D. G. Ghita, R. Margineanu, C. Gomoiu, A. Pantelica, D. Chesneanu, L. Trache, X. D. Tang, B. Bucher, L. R. Gasques, K. Hagino, S. Kubono, Y. J. Li, C. J. Lin , et al., Phys. Lett. B 801 (2020) 135170. Constraining the 12C+12C astrophysical S-factors with the 12C+13C measurements at very low energies
[6] M. Arnould and S. Goriely, Physics Reports 384 (2003)1; https://doi.org/10.1016/S0370-1573(03)00242-4
[7] I. Cata-Danil et al., Phys. Rev. C 78, 035803 (2008); https://doi.org/10.1103/PhysRevC.78.035803
[8] D. Filipescu et al., Phys. Rev. C 83, 064609 (2011); https://doi.org/10.1103/PhysRevC.83.064609
[9] A. Oprea et al., EPJ Web of Conf. 146, 01016 (2017); https://doi.org/10.1051/epjconf/201714601016
[10] I. Gheorghe et al. Nucl. Data Sheets 119 (2014); https://doi.org/10.1016/j.nds.2014.08.067
[11] JR Huizenga and R. Vadenbosch, Phys. Rev. 120 (1960)1305; https://doi.org/10.1103/PhysRev.120.1305
[12] A.C. Larsen et al., Phys. Rev. C 83, 034315 (2011); https://doi.org/10.1103/PhysRevC.83.034315
[13] A. Saastamoinen, E. Pollacco, B.T. Roeder, A. Spiridon, M. Daq, L. Trache, G. Pascovici, R. Oliveira, M.R.D. Rodrigues, R.E. Tribble, Nucl. Instr. & Meth. B 376. (2016) 357. AstroBox2-Detector for low-energy beta-delayed particle detection
[14] L. Trache, F. Carstoiu, CA Gagliardi and RE Tribble, Phys. Rev. Lett. 87, 271102 (2001); ibidem, Phys. Rev. C 69, 032802 (2004).
[15] L. Trache, F. Carstoiu, CA Gagliardi and RE Tribble, Phys. Rev. C 66, 035801 (2002).
[16] A. Banu et al., Phys. Rev. C 84, 015803 (2011); Phys. Rev. C 86, 015806 (2012).
[17] A. Tumino, C. Spitaleri, M. La Cognata, S. Cherubini, G. L. Guardo, M. Gulino, S. Hayakawa, I. Indelicato, L. Lamia, H. Petrascu, R. G. Pizzone, S. M. R. Puglia, G. G. Rapisarda, S. Romano, M. L. Sergi, R. Spartá & L. Trache, Nature 557 Issue: 7707 Pages: 687 (2018). https://doi.org/10.1038/s41586-018-0149-4 An increase in the C-12+C-12 fusion rate from resonances at astrophysical energies
[18] L. Trache, F. Carstoiu et al. Phys. Rev. C 61, 024612 (2000).
[19] T. Al-Abdullah, F. Carstoiu, X. Chen, H. L. Clark, C. A. Gagliardi, Y.-W. Lui, A. Mukhamedzhanov, G. Tabacaru, Y. Tokimoto, L. Trache, R. E. Tribble, and Y. Zhai, Phys. Rev. C 89, 025809 (2014); ibidem Phys. Rev. C 89, 064602 (2014).
[20] Livius Trache and Alexandra Spiridon (eds.), Exotic nuclei and nuclear/particle astrophysics (vii) - Physics with small accelerators. Proceedings of the Carpathian Summer School of Physics 2018 (CSSP18). Book Series: American Institute of Physics Conference Proceedings, Volume: 2076, Melville, New York, 2019. https://aip.scitation.org/toc/apc/2076/1?expanded=2076
[21] A. Pascal, S. Giraud, A. Fantina, F. Gulminelli, J. Novak, M. Oertel, Ad. R. Raduta, Phys. Rev. C 101, 015803 (2020). Impact of electron capture rates on nuclei far from stability on core-collapse supernovae.
[22] M. Fortin, Ad. R. Raduta, S. Avancini, C. Providencia, Phys. Rev. D 101, 034017 (2020). Relativistic hypernuclear compact stars with calibrated equations of state.
[23] Adriana R Raduta, Jia Jie Li, Armen Sedrakian, Fridolin Weber, MNRAS 487, 2639 (2019). Cooling of hypernuclear compact stars: Hartree–Fock models and high-density pairing.
[24] Adriana R. Raduta, Armen Sedrakian and Fridolin Weber, MNRAS 475, 4347 (2018). Cooling of hypernuclear compact stars.
[25] Ad.R.Raduta, F.Gulminelli, Nucl. Phys. A 983, 252 (2019). Nuclear Statistical Equilibrium equation of state for core collapse.

NAGroup leader: Dr. Livius Trache Senior Researcher		Prof. Adriana Raduta Senior Researcher
Dr. Dan Filipescu Scientific Researcher II		Dr. Florin Carstoiu Senior Researcher (Department of Theoretical Physics)
Dr. Ioana Gheorghe Scientific Researcher		Dr. Alexandra Elena Spiridon Scientific Researcher
Dr. Andreea Oprea Scientific Researcher		PhD Student Alexandra Chilug Research Assistant now IPA fellow RIKEN Nishina Center, Wako
PhD Student Dana Tudor Research Assistant		PhD Student Iuliana Stanciu Research Assistant Currently at Technical University Munich
Ionut Catalin Stefanescu, MS Research Assistant now IPA fellow RIKEN Nishina Center, Wako		Madalina Ravar, MS Currently at Univ. of Cologne-Bonn, Germany

Astroparticle Physics

Contact person: Dr. Alexandra Saftoiu, Dr. Denis Stanca

The Astroparticle Physics group, within DFN, studies astroparticles (a.k.a cosmic rays) from several aspect:

Muon charge ratio of secondary atmospheric muons studies using the method of delayed decays in different media – the WILLI detector. The WILLI detector, built in IFIN-HH Bucharest measures the muon charge ratio by the effective lifetime of muons stopped in the detector.
The bulk of the detector consists of 16 scintillator modules for recording the energy and time signature of the muon and 4 scintillator modules in vertical position, on each side of the stack, acting as anticounters.
Thus the muon trajectory and the electron produced by its desintegration produce a clear signature used for determination of energy, direction and decay time of the muon. The anticounters are used to determine events which have the full history contained in the detector that is any time an anticounter fire the event is rejected.
All counters are made of a 1 cm thick aluminum plate and a plastic scintillator (NE 114) of 90 x 90 x 3 cm³ closed by an aluminum cover of 2 mm. All counters are installed in a rotatory frame such that the whole set-up can be used for measurements in all zenith and azimuthal directions.

Studies of muon charge ratio within extensive air showers (EAS) by correlating the WILLI detector with an array of scintillator stations around the detector – WILLI-AIR (in progress).
During the development of an extensive air shower (EAS) in the atmosphere positive and negative particles are deflected in the Earth’s magnetic field. This induces an asymmetry in the distribution of particle that are detected at ground level depending on the incoming direction of the primary particle that has generated the EAS.

Development of muon detectors using various experimental set-ups designed and developed in house. Development of detectors for muography applications. Muography is a novel technique used for non-invasive investigations of the inner structure of large volumes. It relies on the natural flux of secondary muons, generated by the astroparticles as they interact in the Earth’s atmosphere. These muons are highly penetrating particles and can thus ‘scan’ dense and large volumes, like architectural structures (even as large as pyramids), natural rock volumes (hills, rock overburden of galleries or tunnels), hazardous volumes like nuclear reactors or even cars and trucks for border traffic control. Muography can be done by transmission (recording of all muons that pass through a volume and comparison with those that are recorder above the same volume) of by deviation (recording of deviation of a trajectory of an individual muon as it passes through a high-density material, such as lead). In our group we develop several prototypes of muography detectors aiming to improve resolution, modularity and scalability.

Muon flux measurements above ground and underground (using the mobile detector in various location and in the underground Slanic MicroBq Laboratory).

High energy cosmic ray theoretical and experimental studies at the Pierre Auger Observatory using hybrid detection capabilities.

Detection of cosmic rays using radio waves in air (within the AERA set-up in the frame of the Pierre Auger Observatory).

Detection of high energy neutrinos in dense media using radio waves emitted by the secondary particle shower (in collaboration with the University Politehnica of Bucharest). High energy neutrinos (both of astrophysical and cosmogenic origin) are elusive particles that could provide answers to important questions in Astrophysics and Fundamental Particle Physics, if their detection would be easily accomplished. Considering that the flux of these neutrinos is extremely low and their probability of interaction also low, new methods of detection are investigated.

Astroparticle Physics Team:

Dr. Alexandra Saftoiu, Scientific Researcher III
Dr. Denis Stanca, Scientific Researcher III
Dr. Dana Dumitriu, Scientific Researcher III
Dr. Alexandru Gherghel-Lascu, Scientific Researcher
Dr. Alexandru Balaceanu, Scientific Researcher
Dr. Catalin Vancea, Scientific Researcher
Dr. Mihai Niculescu-Oglinzanu, Scientific Researcher
Toma Mosu, PhD student Research Assistant
Madalina Dobre, Research Assistant
Raluca Smau, Research Assistant
Marian-Mario Popa, Research Assistant