arXiv subjects

T. Materna

Publications and source records attributed to T. Materna.

33 records · Page 2Linked to original sources

arXiv2018

The STEREO Experiment

The STEREO experiment is a very short baseline reactor antineutrino experiment aiming at testing the hypothesis of light sterile neutrinos as an explanation of the deficit of the observed neutrino interaction rate with respect to the predicted rate, known as the Reactor Antineutrino Anomaly. The detector center is located 10 m away from the compact, highly $^{235}$U enriched core of the research nuclear reactor of the Institut Laue Langevin in Grenoble, France. This paper describes the STEREO site, the detector components and associated shielding designed to suppress the external sources of background which were characterized on site. It reports the performances in terms of detector response and energy reconstruction.

physics.ins-det↗

arXiv2017

Reactor antineutrino shoulder explained by energy scale nonlinearities?

The Daya Bay, Double Chooz and RENO experiments recently observed a significant distortion in their detected reactor antineutrino spectra, being at odds with the current predictions. Although such a result suggests to revisit the current reactor antineutrino spectra modeling, an alternative scenario, which could potentially explain this anomaly, is explored in this letter. Using an appropriate statistical method, a study of the Daya Bay experiment energy scale is performed. While still being in agreement with the γ calibration data and Boron 12 measured spectrum, it is shown that a O(1%) deviation of the energy scale reproduces the distortion observed in the Daya Bay spectrum, remaining within the quoted calibration uncertainties. Potential origins of such a deviation, which challenge the energy calibration of these detectors, are finally discussed.

hep-ex↗

arXiv2014

Experimental reconstruction of primary hot isotopes and characteristic properties of the fragmenting source in the heavy ion reactions near the Fermi energy

The characteristic properties of the hot nuclear matter existing at the time of fragment formation in the multifragmentation events produced in the reaction $^{64}$Zn + $^{112}$Sn at 40 MeV/nucleon are studied. A kinematical focusing method is employed to determine the multiplicities of evaporated light particles, associated with isotopically identified detected fragments. From these data the primary isotopic yield distributions are reconstructed using a Monte Carlo method. The reconstructed yield distributions are in good agreement with the primary isotope distributions obtained from AMD transport model simulations. Utilizing the reconstructed yields, power distribution, Landau free energy, characteristic properties of the emitting source are examined. The primary mass distributions exhibit a power law distribution with the critical exponent, $A^{-2.3}$, for $A \geq 15$ isotopes, but significantly deviates from that for the lighter isotopes. Landau free energy plots show no strong signature of the first order phase transition. Based on the Modified Fisher Model, the ratios of the Coulomb and symmetry energy coefficients relative to the temperature, $a_{c}/T$ and $a_{sym}/T$, are extracted as a function of A. The extracted $a_{sym}/T$ values are compared with results of the AMD simulations using Gogny interactions with different density dependencies of the symmetry energy term. The calculated $a_{sym}/T$ values show a close relation to the symmetry energy at the density at the time of the fragment formation. From this relation the density of the fragmenting source is determined to be $ρ/ρ_{0} = (0.63 \pm 0.03 )$. Using this density, the symmetry energy coefficient and the temperature of fragmenting source are determined in a self-consistent manner as $a_{sym} = (24.7 \pm 3.4) MeV$ and $T=(4.9 \pm 0.2)$ MeV.

nucl-ex↗

arXiv2014

A novel determination of density, temperature and symmetry energy for nuclear multi-fragmentation through primary fragment yield reconstruction

For the first time primary hot isotope distributions are experimentally reconstructed in intermediate heavy ion collisions and used with antisymmetrized molecular dynamics (AMD) calculations to determine density, temperature and symmetry energy coefficient in a self-consistent manner. A kinematical focusing method is employed to reconstruct the primary hot fragment yield distributions for multifragmentation events observed in the reaction system $^{64}$Zn + $^{112}$Sn at 40 MeV/nucleon. The reconstructed yield distributions are in good agreement with the primary isotope distributions of AMD simulations. The experimentally extracted values of the symmetry energy coefficient relative to the temperature, $a_{sym}/T$, are compared with those of the AMD simulations with different density dependence of the symmetry energy term. The calculated $a_{sym}/T$ values changes according to the different interactions. By comparison of the experimental values of $a_{sym}/T$ with those of calculations, the density of the source at fragment formation was determined to be $ρ/ρ_{0} = (0.63 \pm 0.03 )$. Using this density, the symmetry energy coefficient and the temperature are determined in a self-consistent manner as $a_{sym} = (24.7 \pm 1.9) MeV$ and $T=(4.9 \pm 0.2)$ MeV

nucl-ex↗

arXiv2010

Critical behavior of the isotope yield distributions in the Multifragmentation Regime of Heavy Ion Reactions

Isotope yields have been analyzed within the framework of a Modified Fisher Model to study the power law yield distribution of isotopes in the multifragmentation regime. Using the ratio of the mass dependent symmetry energy coefficient relative to the temperature, $a_{sym}/T$, extracted in previous work and that of the pairing term, $a_{p}/T$, extracted from this work, and assuming that both reflect secondary decay processes, the experimentally observed isotope yields have been corrected for these effects. For a given I = N - Z value, the corrected yields of isotopes relative to the yield of $^{12}C$ show a power law distribution, $Y(N,Z)/Y(^{12}C) \sim A^{-τ}$, in the mass range of $1 \le A \le 30$ and the distributions are almost identical for the different reactions studied. The observed power law distributions change systematically when I of the isotopes changes and the extracted $τ$ value decreases from 3.9 to 1.0 as I increases from -1 to 3. These observations are well reproduced by a simple de-excitation model, which the power law distribution of the primary isotopes is determined to $τ^{prim} = 2.4 \pm 0.2$, suggesting that the disassembling system at the time of the fragment formation is indeed at or very near the critical point.

nucl-ex↗

arXiv2010

The Isospin Dependence Of The Nuclear Equation Of State Near The Critical Point

We discuss experimental evidence for a nuclear phase transition driven by the different concentration of neutrons to protons. Different ratios of the neutron to proton concentrations lead to different critical points for the phase transition. This is analogous to the phase transitions occurring in 4He-3He liquid mixtures. We present experimental results which reveal the N/A (or Z/A) dependence of the phase transition and discuss possible implications of these observations in terms of the Landau Free Energy description of critical phenomena.

nucl-ex↗

arXiv2010

A novel approach to Isoscaling: the role of the order parameter m = (N-Z)/A

Isoscaling is derived within a recently proposed modified Fisher model where the free energy near the critical point is described by the Landau O(m^6) theory. In this model m = (N-Z)/A is the order parameter, a consequence of (one of) the symmetries of the nuclear Hamiltonian. Within this framework we show that isoscaling depends mainly on this order parameter through the 'external (conjugate) field' H. The external field is just given by the difference in chemical potentials of the neutrons and protons of the two sources. To distinguish from previously employed isoscaling relationships, this approach is dubbed: m - scaling. We discuss the relationship between this framework and the standard isoscaling formalism and point out some substantial differences in interpretation of experimental results which might result. These should be investigated further both theoretically and experimentally.

nucl-ex↗

arXiv2010

Isocaling and the Symmetry Energy in the Multifragmentation Regime of Heavy Ion Collisions

The ratio of the symmetry energy coefficient to temperature, $a_sym/T$, in Fermi energy heavy ion collisions, has been experimentally extracted as a function of the fragment atomic number using isoscaling parameters and the variance of the isotope distributions. The extracted values have been compared to the results of calculations made with an Antisymmetrized Molecular Dynamics (AMD) model employing a statistical decay code to account for deexcitation of excited primary fragments. The experimental values are in good agreement with the values calculated but are significantly different from those characterizing the yields of the primary AMD fragments.

nucl-ex↗

arXiv2010

Isobaric Yield Ratios and The Symmetry Energy In Fermi Energy Heavy Ion Reactions

The relative isobaric yields of fragments produced in a series of heavy ion induced multifragmentation reactions have been analyzed in the framework of a Modified Fisher Model, primarily to determine the ratio of the symmetry energy coefficient to the temperature, $a_a/T$, as a function of fragment mass A. The extracted values increase from 5 to ~16 as A increases from 9 to 37. These values have been compared to the results of calculations using the Antisymmetrized Molecular Dynamics (AMD) model together with the statistical decay code Gemini. The calculated ratios are in good agreement with those extracted from the experiment. In contrast, the ratios determined from fitting the primary fragment distributions from the AMD model calculation are ~ 4 and show little variation with A. This observation indicates that the value of the symmetry energy coefficient derived from final fragment observables may be significantly different than the actual value at the time of fragment formation. The experimentally observed pairing effect is also studied within the same simulations. The Coulomb coefficient is also discussed.

nucl-ex↗

arXiv2009

beta-decay study of Cu-77

A beta-decay study of Cu-77 has been performed at the ISOLDE mass separator with the aim to deduce its beta-decay properties and to obtain spectroscopic information on Zn-77. Neutron-rich copper isotopes were produced by means of proton- or neutron-induced fission reactions on U-238. After the production, Cu-77 was selectively laser ionized, mass separated and sent to different detection systems where beta-gamma and beta-n coincidence data were collected. We report on the deduced half-live, decay scheme, and possible spin assignment of 77Cu.

nucl-ex↗

arXiv2008

The Quantum Nature of a Nuclear Phase Transition

In their ground states, atomic nuclei are quantum Fermi liquids. At finite temperatures and low densities, these nuclei may undergo a phase change similar to, but substantially different from, a classical liquid gas phase transition. As in the classical case, temperature is the control parameter while density and pressure are the conjugate variables. At variance with the classical case, in the nucleus the difference between the proton and neutron concentrations acts as an additional order parameter, for which the symmetry potential is the conjugate variable. Different ratios of the neutron to proton concentrations lead to different critical points for the phase transition. This is analogous to the phase transitions occurring in $^{4}$He-$^{3}$He liquid mixtures. We present experimental results which reveal the N/Z dependence of the phase transition and discuss possible implications of these observations in terms of the Landau Free Energy description of critical phenomena.

nucl-ex↗

arXiv2006

Experimental determination of the symmetry energy of a low density nuclear gas

Experimental analyses of moderate temperature nuclear gases produced in the violent collisions of 35 MeV/nucleon$^{64}$Zn projectiles with $^{92}$Mo and $^{197}$Au target nuclei reveal a large degree of alpha particle clustering at low densities. For these gases, temperature and density dependent symmetry energy coefficients have been derived from isoscaling analyses of the yields of nuclei with A $\leq 4$. At densities of 0.01 to 0.05 times the ground state density of symmetric nuclear matter, the temperature and density dependent symmetry energies are 10.7 to 13.5 MeV. These values are much larger than those obtained in mean field calculations. They are in quite good agreement with results of a recently proposed Virial Equation of State calculation.

nucl-ex↗

arXiv2006

Properties of the Initial Participant Matter Interaction Zone in Near Fermi-Energy Heavy Ion Collisions

The sizes, temperatures and free neutron to proton ratios of the initial interaction zones produced in the collisions of 40 MeV/nucleon $^{40}$Ar + $^{112}$Sn and 55 MeV/nucleon$^{27}$Al + $^{124}$Sn are derived using total detected neutron plus charged particle multiplicity as a measure of the impact parameter range and number of participant nucleons. The size of the initial interaction zone, determined from a coalescence model analysis, increases significantly with decreasing impact parameter. The temperatures and free neutron to proton ratios in the interaction zones are relatively similar for different impact parameter ranges and evolve in a similar fashion.

nucl-ex↗

arXiv2005

Analysis of fusion-fission dynamics by pre-scission neutron emission in $^{58}$Ni+$^{208}$Pb

We analyzed the experimental data of the pre-scission neutron multiplicity in connection with fission fragments in the reaction $^{58}$Ni+$^{208}$Pb at the incident energy corresponding to the excitation energy of compound nucleus $E^{*}$=185.9 MeV, which was performed by DéMoN group. The relation between the pre-scission neutron multiplicity and each reaction process having different reaction time is investigated. In order to study the fusion-fission process accompanied by neutron emission, the fluctuation-dissipation model combined with a statistical model is employed. It is found that the fusion-fission process and the quasi-fission process are clearly distinguished in correlation with the pre-scission neutron multiplicity.

nucl-th↗

arXiv2005

Tracing the Evolution of Temperature in Near Fermi Energy Heavy Ion Collisions

The kinetic energy variation of emitted light clusters has been employed as a clock to explore the time evolution of the temperature for thermalizing composite systems produced in the reactions of 26A, 35A and 47A MeV $^{64}$Zn with $^{58}$Ni, $^{92}$Mo and $^{197}$Au. For each system investigated, the double isotope ratio temperature curve exhibits a high maximum apparent temperature, in the range of 10-25 MeV, at high ejectile velocity. These maximum values increase with increasing projectile energy and decrease with increasing target mass. The time at which the maximum in the temperature curve is reached ranges from 80 to 130 fm/c after contact. For each different target, the subsequent cooling curves for all three projectile energies are quite similar. Temperatures comparable to those of limiting temperature systematics are reached 30 to 40 fm/c after the times corresponding to the maxima, at a time when AMD-V transport model calculations predict entry into the final evaporative or fragmentation stage of de-excitation of the hot composite systems. Evidence for the establishment of thermal and chemical equilibrium is discussed.

nucl-ex↗