Astrophysics

Basic research activities include experiments for nuclear astrophysics using (mainly) the indirect methods as well as the experiments concerning exotic nuclei, nuclei far from stability line. An important aspect of the basic experimental research is cooperation. It not only allows to concentrate efforts and competences of different group, but it also opens an access to various facilities and tools that otherwise would not be possible to operate by one sole group.

ANC for ¹⁴C isotope

The ¹⁴C(n, γ)¹⁵C reaction plays an important role in inhomogeneous big bang models. In Timofeyuk et al. [Phys. Rev. Lett. 96 162501 (2006)] it was shown that the ¹⁴C(n, γ)¹⁵C radiative capture at astrophysically relevant energies is a peripheral reaction, i.e., the overall normalization of its cross section is determined by the asymptotic normalization coefficient (ANC) for ¹⁵C→¹⁴C+n. Here we present new measurements of the ¹⁴C(d, p)¹⁵C differential cross sections at deuteron incident energy of 17.06 MeV and the analysis to determine the ANCs for neutron removal from the ground and first excited states of ¹⁵C. The results are compared with previous estimations.

ANC for ¹⁵N isotope

The ¹⁵N(p,γ)¹⁶O reaction provides a path from the CN cycle to the CNO bi-cycle and CNO tri-cycle. The measured astrophysical factor for this reaction is dominated by resonant capture through two strong J^π=1- resonances at E_R=312 and 962 keV and direct capture to the ground state. Asymptotic normalization coefficients (ANCs) for the ground and seven excited states in ¹⁶O were extracted from the comparison of experimental differential cross sections for the ¹⁵N(³He,d)¹⁶O reaction with distorted-wave Born approximation calculations. Using these ANCs and proton and α resonance widths determined from an R-matrix fit to the data from the ¹⁵N(p,α)¹²C reaction, we carried out an R-matrix calculation to obtain the astrophysical factor for the ¹⁵N(p,γ)¹⁶O reaction. The results indicate that the direct capture contribution was previously overestimated. We find the astrophysical factor to be S(0)=36.0±6.0 keV b, which is about a factor of 2 lower than the presently accepted value. We conclude that for every 2200±300 cycles of the main CN cycle one CN catalyst is lost due to this reaction.

• A. M. Mukhamedzhanov et al., Phys.Rev. C 83, 044604 (2011), Phys.Rev. C 78, 015804 (2008)
• Asymptotic normalization coefficients from the ¹⁴C(d, p)¹⁵C reaction

Astrophysical nuclear reactions - indirect methods

A cooperation with foreign partners is an important factor for the experimental basic research. It not only allows to concentrate efforts and competences of different groups from different countries but it also simplifies the access to various facilities and tools that would not be possible to operate by one sole group. Basic research activities include experiments for nuclear astrophysics using (mainly) the indirect methods as well as the experiments concerning exotic nuclei and nuclei far from stability line.

After a decade of experimental efforts, the eited state 0₂+ in ³⁴Si isotope was found. This neutron rich nucleus is close to Island of Inversion, where classical magic numbers are eroded and a coexistence of spherical and deformed states appears at low excitation energies. The observed stated has an excitation energy 2719(3) keV and a half-life of 19.4(7) ns, the deformation beta was determined to 0.29(4). Moreover,the experimental method allowed to identify that this state is fed from newly observed 1+ state from 34Al with half-life of 26(1) ms.

• collaboration - IFIN-HH Magurele,Bordeaux,GANIL
• F. Rotaru et al., Phys.Rev.Lett.109, 092503 (2012)

The low lying states of 18Na were studied by means of resonant elastic scattering of H(¹⁷Ne,p)¹⁷Ne at energy of 4 A.MeV on radioactive ion accelerator SPIRAL. Supporting arguments for existence of two very narrow low lying states in ¹⁸Na were found. This would result in understanding of two-proton ¹⁹Mg decay proceeds simultaneously - as the so-called true 2p decay.

• collaboration - GANIL/SPIRAL2
• M. Assie et al., Phys.Lett. B 712, 198 (2012)

Astrophysical S(E) factor for ¹¹B(p,alpha)⁸Be reaction was studied in an energy range from 600 keV down to zero by indirect THM method, which repsents the first measurement of astrophysical S factor at the Gamow peak. This energy region covers region of stellar burning of light elements LiBeB and future aneutronic fusion power plants using ¹¹B+p cycle. The measured S factor 2.07 (41) MeVb support the pviously extrapolated one and the experimentally determined screening potential is compatible with the pvious value, confirming that theoretical adiabatic limit gives lower value.

• collaboration - INFN LNS Catania
• L. Lamia et al., J. Phys.(London) G39, 015106 (2012)

The cross section of the radiative proton capture reaction on the drip line nucleus ¹²N was investigated using the asymptotic normalization coefficient (ANC) method. The ¹⁴N(¹²N, ¹³O)¹³C proton transfer reaction at 12 MeV/nucleon was used to extract the ANC for ¹³O → ¹²N + p and calculate from it the direct component of the astrophysical S factor of the ¹²N(p, γ)¹³O reaction. The optical potentials used and the distorted-wave Born approximation analysis of the proton transfer reaction were discussed. For the entrance channel, the optical potential was inferred from an elastic scattering measurement carried out at the same time as the transfer measurement. From the transfer, the square of the ANC, C₂ (p1/2,¹³Og.s.) = 2.53 ± 0.30 fm-1 was determined, and hence a value of 0.33(4) keVb was obtained for the direct astrophysical S factor at zero energy. Constructive interference at low energies between the direct and resonant captures leads to an enhancement of Stotal(0) = 0.42(6) keVb. The ¹²N(p, γ)¹³O reaction was investigated in relation to the evolution of hydrogen-rich massive Population III stars, for the role that it may play in the hot pp-chain nuclear burning processes, possibly occurring in such objects.

• Phys. Rev. C79, (2009), 025805

Shape coexistence in heavy Si isotopes

After a decade of experimental efforts, the excited state 0₂⁺ in ³⁴Si isotope was found. This neutron rich nucleus is close to Island of Inversion, where classical magic numbers are eroded and a coexistence of spherical and deformed states appears at low excitation energies. The observed stated has an excitation energy 2719(3) keV and a half-life of 19.4(7) ns, the deformation beta was determined to 0.29(4). Moreover, the experimental method allowed to identify that this state is fed from newly observed 1⁺ state from ³⁴Al with half-life of 26(1) ms.

• collaboration - IFIN-HH Magurele,Bordeaux,GANIL
• F. Rotaru et al., Phys.Rev.Lett.109, 092503 (2012)

Search for ¹⁸Ne excited states for two-proton decay of ¹⁹Mg

The low lying states of ¹⁸Na were studied by means of resonant elastic scattering of H(¹⁷Ne,p)¹⁷Ne at energy of 4 A.MeV on radioactive ion accelerator SPIRAL. Supporting arguments for existence of two very narrow low lying states in 18Na were found. This would result in understanding of two-proton ¹⁹Mg decay proceeds simultaneously - as the so-called true 2p decay.

• collaboration - GANIL/SPIRAL2
• M. Assie et al., Phys.Lett. B 712, 198 (2012)

S factor in ¹¹B p,alpha reaction by THM method

Astrophysical S(E) factor for ¹¹B(p,alpha)⁸Be reaction was studied in an energy range from 600 keV down to zero by indirect THM method, which repsents the first measurement of astrophysical S factor at the Gamow peak. This energy region covers region of stellar burning of light elements LiBeB and future aneutronic fusion power plants using ¹¹B+p cycle. The measured S factor 2.07 (41) MeVb support the previously extrapolated one and the experimentally determined screening potential is compatible with the previous value, confirming that theoretical adiabatic limit gives lower value.

• collaboration - INFN LNS Catania
• L. Lamia et al., J. Phys. (London) G39, 015106 (2012)

Direct radiative capture ¹²N(p,gamma)

The cross section of the radiative proton capture reaction on the drip line nucleus ¹²N was investigated using the asymptotic normalization coefficient (ANC) method. The ¹⁴N(¹²N, ¹³O)¹³C proton transfer reaction at 12 MeV/nucleon was used to extract the ANC for ¹³O → ¹²N + p and calculate from it the direct component of the astrophysical S factor of the ¹²N(p, γ)¹³O reaction. The optical potentials used and the distorted-wave Born approximation analysis of the proton transfer reaction were discussed.

For the entrance channel, the optical potential was inferred from an elastic scattering measurement carried out at the same time as the transfer measurement. From the transfer, the square of the ANC, C² (p1/2,¹³Og.s.) = 2.53 ± 0.30 fm^-1 was determined, and hence a value of 0.33(4) keVb was obtained for the direct astrophysical S factor at zero energy. Constructive interference at low energies between the direct and resonant captures leads to an enhancement of S_total(0) = 0.42(6) keVb. The ¹²N(p, γ)¹³O reaction was investigated in relation to the evolution of hydrogen-rich massive Population III stars, for the role that it may play in the hot pp-chain nuclear burning processes, possibly occurring in such objects.

• collaboration - Texas A&M University
• A. Banu, et al. Phys. Rev. C79, (2009), 025805

Neutron, Charged particles

Fast neutron generators (FNG) based on the reactions of the accelerated protons (cyclotron U120-M) with the Li and Be targets present the experimental base for the research activities of our group. The irradiation of the Li target with protons of energies ranging from 20 MeV to 35 MeV produces quasi-monoenergetic neutron spectra (half of the produced neutrons in the peak – 18-33 MeV depending on the proton energy, typical peak neutron flux 10⁹ n/cm²/s). The Be target irradiated with 35 MeV neutrons produces continuous neutron spectrum of high intensity (10¹¹ n/cm²/s in the closest irradiation position).

We are mainly focused on the experimental validation of the neutron cross-section libraries for the materials used as neutron monitors (Au, Bi, Co, Nb, Tm, …) and construction material (Fe, Cr, W, Ta, …) in the future thermonuclear technologies (IFMIF-DONES, ITER). The validation is based on the comparison of the evaluated cross-sections (libraries) with the experimental results. These are obtained by the irradiation of the studied material with well-defined quasi-monoenergetic or continuous spectrum neutron beams and subsequent analysis of the induced activities using gamma-spectrometry. The pneumatic tube system is used for the transport of the studied samples from the irradiation position to the gamma-spectrometry laboratory. Few seconds transport time allows the study of short-lived isotopes (l_1/2>10s).

The detailed knowledge of the source neutron spectra and intensity is crucial for our experiments and validations. The calibration measurements of the neutron spectra are performed by several methods (Proton Recoil Telescope, Time-Of-Flight, neutron activation analysis). Furthermore, for the p-Li source the total number of peak neutrons produced during the irradiation is determined by the measurement of the ⁷Be production in the Li foil using gamma spectrometry (ca. 2%). The measurements of the neutron yields are extrapolated to the positions of irradiated samples (few cm from the converter) using Monte Carlo simulations with the MCNPX code and LA150H or JENDL4.0/HE libraries.

The neutron Time-Of-Flight method is based on the pulsation of the cyclotron beam (bunches are extracted at each turn, with frequency ranging from 20 to 25 MHz). Due to the effect of the frame overlap the neutron spectrum at lower energies cannot be determined, but the measurements of its high energy part are accurate. High energy neutron transmission through various materials is performed using Time-Of-Flight – measurements of (n,tot) reaction cross-section (on eg. oxygen).

The p+Be generator produces intense fluxes of continuous-spectrum neutrons (with the maximum energy of 34 MeV) which are used for the neutron hardness tests, for the studies of the primary radiation damage by neutrons, multidisciplinary research, etc. Various electronic components supposed to operate in the neutron flux are tested here (eg. readout electronics for the ATLAS/CERN calorimeter in the cooperation with Max Planck Institute, Munchen). Longer irradiations with intense neutron fluxes produce enough radiation defects to be detected by the Positron Annihilation Lifetime Spectrometry (cumulative flux of 10¹⁶ n/cm² is necessary). For the studied materials the primary radiation damage in the units of the displacement per atom (dpa) can be determined.

In 2018, the collimation system for the neutron beam was designed. The collimated neutron beams make possible the measurements in which the studied sample is exposed to the high neutron flux while the online detectors are placed in few orders of magnitude lower neutron fluxes. Two detection systems are being constructed on the newly acquired collimated neutron beam:

1) the evacuated chamber with the Si telescopes on the rotating table will be used for the studies of the (n,charged particle) reactions and

2) the array of four HPGe detectors placed around the studied sample will serve to study the (n,*g) reactions.

The modulation of the cyclotron beam by the microcontroller allows the measurements of the gamma radiation from isotopes with decay times down to ms as well as the studies of the delayed neutrons from the fission process. The neutrons from the fission products are moderated by the polyethylene setup and detected with the array of the BF₃ detectors. The experimental results from the two types of experiments are used to validate the fission model calculations (eg. GEF) at higher neutron energies.

Our group is taking part in the development of the neutron target station and preparation of first day experiments at the NFS@SPIRAL2 (Ganil, France). In the frame of the research infrastructure SPIRAL2-CZ we prepared the specialized irradiation and measurement station for the studies of the material activated with proton and deuteron beams at the NFS. The irradiation, transport and the measurement of the irradiated samples with the HPGe detector is fully automatized.

The studies of the activation by the charged particles is important in various research fields (eg. international energetic projects). In cooperation with the theoreticians from IFIN-HH (Magurele, Romania), the department performs the activation analysis of the selected materials (Al, Cu, Nb, Ni, Fe, …) with the deuteron beams up to 20 MeV. The results bring important information for the reaction databases for future IFMIF-DONES and similar accelerator based facilities. The results are showing us that the whole complex of the processes has to be taken in account at the description of the experimental results – direct reactions, pre-equilibrium and compound nucleus processes. The results have initiated the intensive improvement of the nuclear models, which is important for the description of the reactions that cannot be measured experimentally at the moment. The improved theoretical models provide safer future energetics and better understanding of the processes of the nucleosynthesis, eg. the explosion of the supernovas.