arXiv subjects

A. M. Lallena

Publications and source records attributed to A. M. Lallena.

At least 19 recordsLinked to original sources

arXiv2025

Status of the HENSA collaboration at the Canfranc Underground Laboratory: results from two years of measurement of the neutron flux in hall B

Thiswork deals with the characterization of the neutron flux in hall B of the CanfrancUnderground Laboratory (LSC) employing the High Efficiency Neutron Spectrometry Array (HENSA). The ultimate goal of this measurement is to set a limit on the corresponding effects of the neutron flux in the background of the ANAIS-112 experiment. The preliminary neutron counting rates of two years of measurement are reported. Various data analysis techniques, including pulse shape discrimination, are discussed. The first results on the spectral reconstruction of the neutron flux are also presented.

physics.ins-det↗

arXiv2022

Effect of short- and long-range correlations on neutron skins of various neutron-rich doubly magic nuclei

We study the effects of correlations beyond the independent particle model in the evaluation of neutron skins of various neutron-rich doubly magic nuclei. We consider short- and long-range correlations to take into account the presence of the strongly repulsive core of the bare nucleon-nucleon interaction and collective nuclear phenomena, respectively. Despite the strong sensitivity on the structure of the nucleus considered, our results indicate that, in general, correlations increase the values of the neutron skins.

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arXiv2022

Charge radii of Ca isotopes and correlations

We study the effects of short- and long-range correlations on the charge radii of Ca isotopes. We start our investigation with an independent particle model consisting in Hartree-Fock plus Bardeen-Cooper-Schrieffer calculations with finite-range effective nucleon-nucleon interactions of Gogny type. The short-range correlations effects are evaluated by considering all the terms of a cluster expansion containing a single correlation line. The long-range correlations are taken into account by including the coupling with the quasi-particle random phase approximation phonons. While the effects of the short-range correlations are negligible, those of the long-range correlations largely modify the independent particle model results and improve the agreement with the experimental data.

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arXiv2021

Tensor force and deformation in even-even nuclei

The variational principle is used to build a model which describes open shell nuclei with ground state deformations. Hartree-Fock equations are solved by using single particle wave functions whose radial parts depend on the projection of the angular momentum on the quantization axis. Pairing effects are taken into account by solving Bardeen-Cooper-Schrieffer equations in each step of the minimisation procedure. The Gogny D1S finite-range interaction and an extension of it that includes tensor terms are consistently used in both parts of our calculations. The model is applied to study a set of isotopes with 34 protons and of isotones with 34 neutrons. Total energies, density distributions, their radii and single particle energies are analysed and the results of our calculations are compared with the available experimental data. We focused our attention on the effects of the deformation and of the tensor force on these observables. Our model describes open shell nuclei from a peculiar perspective and opens the possibility of future theoretical developments.

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arXiv2020

Nuclear structure uncertainties in coherent elastic neutrino-nucleus scattering

The effects of the nuclear structure uncertainties on the description of processes induced by coherent scattering of neutrinos on nuclei are investigated. A reference calculation based on a specific nuclear model is defined and the cross sections and also the expected number of events produced by neutrinos generated by the explosion of a supernova in our galaxy, and by a spallation neutron source are evaluated. By changing the input parameters of the reference calculation their relevance on cross sections and on the number of the detected events is estimated. Seven spherical nuclei with different proton to neutron ratios are considered as possible targets of the neutrinos in the detector, the lightest being $^{12}$C and the heaviest $^{208}$Pb. The effects generated by the uncertainties of the nuclear model are much smaller than those due to the supernova neutrino flux models. This makes the coherent elastic neutrino-nucleus scattering a reliable tool to investigate the details of the neutrino sources, the neutrino-nucleus interaction, and, eventually, also to extract information about neutron distributions in nuclei.

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arXiv2019

Shell closure at $N=34$ and the $^{48}$Si nucleus

By using a non-relativistic independent particle model we investigate the mechanism promoting 34 as new magic number. We carried out Hartree-Fock plus Bardeen-Cooper-Schrieffer and Quasi-particle Random Phase Approximation calculations by consistently using the same finite-range interaction in all the three steps of our approach. We used four Gogny-like interactions, with and without tensor terms. We find that the shell closure for $N=34$ neutrons appears in isotones with $Z<26\,$ protons. The smaller is the proton number, the more evident is the shell closure at $N=34$. An ideal nucleus to investigate this effect should be $^{48}$Si, as it has been recently suggested. However, some discrepancies occur between the results obtained with the four effective interactions we used concerning the position of the two-neutron drip line and, therefore, the existence of $^{48}$Si. The experimental identification of this nucleus could shed light about the shell evolution in nuclei far from the stability valley and put stringent tests on nuclear structure theories.

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arXiv2019

Neutron gas and pairing

We study the emergence of neutron gas effects in the description of nuclei with large neutron excess within the Bardeen-Cooper-Schrieffer approach. We consider Ni and Sn isotopes where, in the literature, these effects have been found. We investigate the role of the single particle states with positive energy generating the neutron gas, and we find that the contribution of these states is numerically irrelevant for the various observables that we evaluate.

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arXiv2018

Computation of the electron beam quality $k_{{\rm Q,Q}_0}$ factors for the NE2571, NE2571A and NE2581A thimble ionization chambers using PENELOPE

The quality correction factor $k_{Q,Q_{0}}$ for electron beams was calculated for three thimble ionization chambers, namely, NE2571, NE2571A and NE2581A. The Monte Carlo code PENELOPE was used to estimate the overall correction factor $f_{\rm c,Q}$ of these chambers for electron beams with nominal energies ranging between 6 and 22 MeV, corresponding to a Varian Clinac 2100 C/D. A $^{60}$Co beam was used as reference quality ${\rm Q}_0$. Also eight monoenergetic electron beams reproducing the quality index $R_{50}$ of the Clinac beams were considered. The $k_{Q,Q_{0}}$ factors were calculated as the ratio between $f_{\rm c,Q}$ and $f_{{\rm c,Q}_0}$. Those obtained for the NE2571 ionization chamber show a nice agreement with those calculated by Muir and Rogers with EGSnrc. As it occurred to other ionization chambers analyzed in previous works, the $k_{{\rm Q,Q}_0}$ factors found for the monoenergetic beams are larger (smaller) than those corresponding to the Clinac beams at low (high) $R_{50}$ values, the differences being slightly above $0.5\%$. Finally, the $k_{{\rm Q,Q}_0}$ factors obtained in the case of the NE2571A chamber are systematically ~0.5% below those of its predecessor chamber, the NE2571.

physics.med-ph↗

arXiv2018

Air density dependence of the response of the PTW SourceCheck 4pi ionization chamber for $^{125}$I brachytherapy seeds

Purpose: To analyze the air density dependence of the response of the new SourceCheck 4pi ionization chamber, manufactured by PTW. Methods: The air density dependence of three different SourceCheck 4pi chambers was studied by measuring 125I sources. Measurements were taken by varying the pressure from 746.6 to 986.6hPa in a pressure chamber. Three different HDR 1000 Plus ionization chambers were also analyzed under similar conditions. A linear and a potential-like function of the air density were fitted to experimental data and their achievement in describing them was analyzed. Results: SourceCheck 4pi chamber response showed a residual dependence on the air density once the standard pressure and temperature factor was applied. The chamber response was overestimated when the air density was below that under normal atmospheric conditions. A similar dependence was found for the HDR 1000 Plus chambers analyzed. A linear function of the air density permitted a very good description of this residual dependence, better than with a potential function. No significant variability between the different specimens of the same chamber model studied was found. Conclusion: The effect of overestimation observed in the chamber responses once they are corrected for the standard pressure and temperature may represent a non-negligible ~4% overestimation in high altitude cities as ours (700 m AMSL). This overestimation behaves linearly with the air density in all cases analyzed.

physics.med-ph↗

arXiv2018

A study of the irradiation technique used for the external beam radiotherapy of retinoblastoma

Purpose: Retinoblastoma (RB) is the most common eye tumor in childhood and can be treated external radiotherapy. The purpose of this work is to evaluate the adequacy of Monte Carlo simulations and the accuracy of a commercial treatment planning system by means of experimental measurements. Dose measurements in water were performed using a dedicated collimator. Methods: A 6MV Varian Clinac 2100 C/D and a dedicated collimator are used for RB treatment. The collimator conforms a D-shaped off-axis field whose irradiated area can be either 5.2 or 3.1cm$^2$. Depth dose distributions and lateral profiles were measured and compared with Monte Carlo simulations run with PENELOPE and with calculations performed with the analytical anisotropic algorithm (AAA) using the gamma test. Results: PENELOPE simulations agree well with the experimental data with discrepancies in the dose profiles less than 3mm of distance-to-agreement and 3% of dose. Discrepancies between the results of AAA and the experimental data reach 3mm and 6%. The agreement in the penumbra region between AAA and the experiment is noticeably worse than that between the latter and PENELOPE. The percentage of voxels passing the gamma test when comparing PENELOPE (AAA) and the experiment is on average 99% (93%) assuming a 3mm distance-to-agreement and a discrepancy of 3% of dose. Conclusions: Although the discrepancies between AAA and experimental results are noticeable, it is possible to consider this algorithm for routine treatment planning of RB patients, provided the limitations of the algorithm are known and taken into account by the medical physicist. Monte Carlo simulation is essential for knowing these limitations. Monte Carlo simulation is required for optimizing the treatment technique and the dedicated collimator.

physics.med-ph↗

arXiv2018

Testing Monte Carlo absolute dosimetry formalisms for a small field `D'-shaped collimator used in retinoblastoma external beam radiotherapy

Purpose: To investigate the validity of two Monte Carlo simulation absolute dosimetry approaches in the case of a small field dedicated `D'-shaped collimator used for the retinoblastoma treatment with external photon beam radiotherapy. Methods: The Monte Carlo code {\sc penelope} is used to simulate the linac, the dedicated collimator and a water phantom. The absolute doses (in Gy per monitor unit) for the field sizes considered are obtained within the approach of Popescu {\it et al.} in which the tallied backscattered dose in the monitor chamber is accounted for. The results are compared to experimental data, to those found with a simpler Monte Carlo approximation for the calculation of absolute doses and to those provided by the analytical anisotropic algorithm. Our analysis allows for the study of the simulation tracking parameters. Two sets of parameters have been considered for the simulation of the particle transport in the linac target. Results: The change in the tracking parameters produced non-negligible differences, of about 10\% or larger, in the doses estimated in reference conditions. The Monte Carlo results for the absolute doses differ from the experimental ones by 2.6\% and 1.7\% for the two parameter sets for the collimator geometries analyzed. For the studied fields, the simpler approach produces absolute doses that are statistically compatible with those obtained with the approach of Popescu {\it et al.} The analytical anisotropic algorithm underestimates the experimental absolute doses with discrepancies larger than those found for Monte Carlo results. Conclusions: The approach studied can be considered for absolute dosimetry in the case of small, `D'-shaped and off-axis radiation fields. However, a detailed description of the radiation transport in the linac target is mandatory for an accurate absolute dosimetry.

physics.med-ph↗

arXiv2018

Monte Carlo study for designing a dedicated `D'-shaped collimator used in the external beam radiotherapy of retinoblastoma patients

Purpose: The purpose of the present article is to propose a modified version of the D-shaped collimator used at the Univ. Hosp. of Essen for retinoblastoma treatment that reduces even further the irradiation field with the scope to reduce the risk of radio-induced secondary malignancies. The new dedicated D-shaped collimator is easier to build and produces dose distributions that only differ on the field size with respect to the dose distributions obtained by the current collimator in use. Methods: The Monte Carlo code PENELOPE was used to study the effect that the structural elements of the collimator have on the absorbed dose distribution. The radiation transport through a Varian Clinac 2100 C/D operating at 6MV was simulated in order to tally phase-space files which were used as radiation sources to simulate the considered collimators and the subsequent dose distributions. Results: The proposed collimator delivers a dose distribution which is 2.4cm wide along the inferior-superior direction of the eyeball. This width is 0.3cm narrower than that of the dose distribution obtained with the collimator currently in use. The other relevant characteristics of the dose distribution obtained with the new collimator, namely, depth doses, penumbrae width and shape of the lateral profiles, are statistically compatible with the results obtained for the collimator in use. Conclusions: The smaller field size delivered by the proposed collimator still fully covers the planning target volume with at least 95% of the maximum dose at a depth of 2cm and provides a safety margin of 0.2cm, so ensuring an adequate treatment while reducing the dose absorbed by surrounding structures of the eye.

physics.med-ph↗

arXiv2018

Matter distribution and spin-orbit force in spherical nuclei

We investigate the possibility that some nuclei show density distributions with a depletion in the center, a semi-bubble structure, by using a Hartree-Fock plus Bardeen-Cooper-Schrieffer approach. We separately study the proton, neutron and matter distributions in 37 spherical nuclei mainly in the $s - d$ shell region. We found a relation between the semi-bubble structure and the energy splitting of spin-orbit partner single particle levels. The presence of semi-bubble structure reduces this splitting, and we study its consequences on the excitation spectrum of the nuclei under investigation by using a quasi-particle random-phase-approximation approach. The excitation energies of the low-lying $4^+$ states can be related to the presence of semi-bubble structure in nuclei.

nucl-th↗

arXiv2017

Pairing in spherical nuclei: quasi-particle random phase approximation calculations with the Gogny interaction

We investigate the effects of the pairing in spherical nuclei. We use the same finite-range interaction of Gogny type in the three steps of our approach, Hartree-Fock, Bardeen, Cooper and Schrieffer, and quasi-particle random phase approximation calculations. We study electric and magnetic dipole, quadrupole and octuple excitations in oxygen and calcium isotopes and also in isotopes with 20 neutrons. We investigate the pairing effects on single particle energies and occupation probabilities, on the excitation energies, $B$-values and collectivity of low-lying states including the isoscalar electric dipole and the magnetic dipole excitations, and also the giant resonances. The inclusion of the pairing increases the values of the excitation energies in all the cases we have studied. In general, the effects of the pairing are too small to remarkably improve the agreement with the available experimental data.

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arXiv2016

Self-consistent continuum random-phase approximation with finite-range interactions for charge-exchange excitations

The formalism of the continuum random-phase approximation theory which treats, without ap- proximations, the continuum part of the single-particle spectrum, is extended to describe charge- exchange excitations. Our approach is self-consistent, meaning that we use a unique, finite-range, interaction in the Hartree-Fock calculations which generate the single-particle basis and in the con- tinuum random-phase approximation which describes the excitation. We study excitations induced by the Fermi, Gamow-Teller and spin-dipole operators in doubly magic nuclei by using four Gogny- like finite-range interactions, two of them containing tensor forces. We focus our attention on the importance of the correct treatment of the continuum configuration space and on the effects of the tensor terms of the force.1

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arXiv2015

Electric quadrupole and magnetic dipole moments of odd nuclei near the magic ones in a self-consistent approach

We present a model which describes the properties of odd-even nuclei with one nucleon more, or less, with respect to the magic number. In addition to the effects related to the unpaired nucleon, we consider those produced by the excitation of the closed shell core. By using a single particle basis generated with Hartree-Fock calculations, we describe the polarization of the doubly magic-core with Random Phase Approximation collective wave functions. In every step of the calculation, and for all the nuclei considered, we use the same finite-range nucleon-nucleon interaction. We apply our model to the evaluation of electric quadrupole and magnetic dipole moments of odd-even nuclei around oxygen, calcium, zirconium, tin and lead isotopes. Our Random Phase Approximation description of the polarization of the core improves the agreement with experimental data with respect to the predictions of the independent particle model. We compare our results with those obtained in first-order perturbation theory, with those produced by Hartree-Fock-Bogolioubov calculations and with those generated within the Landau-Migdal theory of finite Fermi systems. The results of our universal, self-consistent, and parameter free approach have the same quality of those obtained with phenomenological approaches where the various terms of the nucleon-nucleon interaction are adapted to reproduce some specific experimental data. A critical discussion on the validity of the model is presented.

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arXiv2014

Charge-exchange excitations with finite range interactions including tensor terms

We study charge-exchange excitations in doubly magic-nuclei by using a self-consistent Hartree-Fock plus Random Phase Approximation model. We use four Gogny-like finite-range interactions, two of them containing tensor forces. We investigate the effects of the various parts of the tensor forces in the two computational steps of our model, and we find that their presence is not negligible and improves the agreement with the experimental data.

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arXiv2013

Coulomb and spin-orbit interactions in random phase approximation calculations

We present a fully self-consistent computational framework composed by Hartree-Fock plus ran- dom phase approximation where the spin-orbit and Coulomb terms of the interaction are included in both steps of the calculations. We study the effects of these terms of the interaction on the random phase approximation calculations, where they are usually neglected. We carry out our investigation of excited states in spherical nuclei of oxygen, calcium, nickel, zirconium, tin and lead isotope chains. We use finite-range effective nucleon-nucleon interactions of Gogny type. The size of the effects we find is, usually, of few hundreds of keV. There are not simple approximations which can be used to simulate these effects since they strongly depend on all the variables related to the excited states, angular momentum, parity, excitation energy, isoscalar and isovector characters. Even the Slater approximation developed to account for the Coulomb exchange terms in Hartree-Fock is not valid in random phase approximation calculations.

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