Table of contents

The present volume contains the lectures and short talks given at the XXIII International school on nuclear physics, neutron physics and applications.

The meeting was held on September 22-28, 2019 in Riviera Beach Hotel at the resort Golden Sands near Varna, Bulgaria.

The Varna school was organized by the Institute for nuclear research and nuclear energy of Bulgarian academy of sciences.

In keeping with a long-standing tradition, the school has been held every second year since 1973. The school's program has been restructured to encompass our new international links and today it is more like an international conference than to a classical nuclear physics school. More than sixty participants from fifteen countries enjoyed the scientific presentations and fruitful discussions during the pleasant autumn days at the Beach sea shore.

The program of the school spanned the latest results in fundamental areas such as nuclear structure and reactions as well as examples of the latest applications of nuclear methods. The main topics of the presentations were:

• Exotic nuclei and their properties

• Nuclear structure

• Super heavy elements, synthesis and properties

• Rare processes and decays

• Radioactive beams, production and research programs

• Evolution of shell structure

• Collective modes of the nucleus

• Nuclear astrophysics

• Applications of exotic beams in materials research

• Current and future nuclear facilities

We would like to thank all participants, members of the advisory and local organizing committees for their contribution to the success of the XXIII International school on nuclear physics, neutron physics and applications. We are also grateful to the reviewers for their positive contribution to the high standard of the articles presented in this volume.

Finally, we would like to thank the Bulgarian national science fund, the Bulgarian national regulatory agency and the Bogoliubov Laboratory for theoretical physics of the Joint institute for nuclear research, Dubna, Russia, for their financial support.

Dimitar Tonev, Sevdalina Dimitrova, Chavdar Stoyanov

Co-Chairs of the Organizing committee

of the XXIII International school on nuclear physics, neutron physics and applications

All papers published in this volume of IOP Conference Series: Materials Science and Engineering have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Magnetic form factors from odd-A spherical and deformed nuclei corresponding to elastic electron scattering are calculated in the plane-wave Born approximation. The nuclear structure of the target is described within a deformed self-consistent mean-field calculation with effective interactions of Skyrme type and pairing correlations in the BCS approximation. We focus our attention to stable nuclei where experimental information is available. It is shown that the deformed formalism improves the agreement with experiment in deformed nuclei, while reproducing equally well spherical nuclei by taking properly the spherical limit of the deformed model. Effects of the collective rotation and nucleon-nucleon correlations are also studied. These results demonstrate the ability of the method to address electron scattering from unstable nuclei to be measured in future experiments on electron-ion beam colliders.

Starting from the quasi particle random phase approximation based on an effective Skyrme interaction, we study the effects of phonon-phonon coupling (PPC) on the low-lying and high-lying collective nuclear states. Many examples of such effects are presented. It is shown that to reproduce experimental data one needs to take into account PPC in many cases.

We report on the extension to odd nuclei of a microscopic multiphonon approach known as equation of motion phonon method and its application to the odd neighbors of the neutron rich ²²O. A calculation using the chiral potential NNLO_opt and encompassing up to two phonon basis states provides a description of the spectroscopic properties which is good quantitatively for ²³O and qualitatively for ²¹O and ²¹N. Serious discrepancies between theory and experiments occur in ²³F. A possible recipe for curing them is under investigation.

We investigate the temperature dependence of the volume and surface components of the nuclear symmetry energy (NSE) and their ratio in the framework of the local density approximation. The results of these quantities for finite nuclei are obtained within the coherent density fluctuation model (CDFM). The CDFM weight function is obtained using the temperature-dependent proton and neutron densities calculated through the HFBTHO code that solves the nuclear Skyrme-Hartree-Fock-Bogoliubov problem by using the cylindrical transformed deformed harmonic-oscillator basis. We present and discuss the values of the volume and surface contributions to the NSE and their ratio obtained for the Ni, Sn, and Pb isotopic chains around double-magic ⁷⁸Ni, ¹³²Sn, and ²⁰⁸Pb nuclei. The results for the T-dependence of the considered quantities are compared with estimations made previously for zero temperature showing the behavior of the NSE components and their ratio, as well as with the available experimental data. The sensitivity of the results on various forms of the density dependence of the symmetry energy is studied. We confirm the existence of "kinks" of these quantities as functions of the mass number at T = 0 MeV for the double closed-shell nuclei ⁷⁸Ni and ¹³²Sn and the lack of "kinks" for the Pb isotopes, as well as the disappearance of these kinks as the temperature increases.

Status and prospects of nuclear clustering studies by dissociation of relativistic nuclei in nuclear track emulsion are presented. The unstable ⁸Be and ⁹B nuclei are identified in dissociation of the isotopes ⁹Be, ¹⁰B, ¹⁰C and ⁿC, and the Hoyle state in the cases ¹²C and ¹⁶O. On this ground searching for the Hoyle state and more complex α-particle states in the dissociation of the heavier nuclei is suggested. A detailed study of a low-density baryonic matter arising in dissociation of the heaviest nuclei is forthcoming long-term problem. An analysis of nuclear fragmentation induced by relativistic muons is proposed to examine the mechanism dissociation.

We present the cyclotron project of the Institute for Nuclear Research and Nuclear Energy which aims to centralize national the production of radioisotopes and radiopharmaceuticals and to provide opportunities for interdisciplinary research and education in the fields of Physics, Chemistry, Biology and Nuclear Energy. The human resources needed for the successful operation of the production program of the centre are also described in this article. An account of the ongoing research related to the radiation protection and radiation shielding of the cyclotron is made.

The Institute for Nuclear Research and Nuclear Energy at the Bulgarian Academy of Sciences is building an accelerator laboratory to operate a TR-24 cyclotron. Thus, a preliminary radiation shielding analysis of the accelerator bunker is required. For this purpose Monte-Carlo simulations have been performed and dosimetric (Dose equivalent) quantities have been estimated in two model geometries - simplified spherical geometry and full-scale bunker, respectively during operation and after the end of the cyclotron life. Our current efforts are directed to the production of ¹⁸F thus in all the conducted simulations a water target enriched with ¹⁸O is considered. NiGa₃ as a target for production of ⁶⁸Ge has also been simulated and a comparison of the radiation shielding characteristics of the bunker for the two targets has been made.

We have studied the behavior of charge and neutron radii across the shell-closure at N = 82 near the neutron dripline in a comparative approach which includes Relativistic Mean Field (RMF) Theory and the nonrelativistic Skyrme-Hartree-Fock (SHF) Theory. We have investigated the extent and the effects of the spin-orbit interaction on the ground state properties of neutron-rich nuclei in the extreme region. RMF and SHF calculations have been performed for nuclei in the isotopic chains from Kr (Z = 36) to Sn (Z = 50) near the neutron dripline across the magic number N = 82. We present the results of shell-effects on charge radii, neutron radii, and the neutron skin thickness. We have examined the single particle energies (SPE) in the context of the kinks in the radii. It is shown that RMF theory produces a kink in charge radii, similar to what was seen experimentally for Pb (Z = 82) isotopic chains. This is in contrast to the Skyrme forces, which do not produce such a kink.

The experimental nuclear reaction data compiled in the EXFOR library are loaded to databases and accessed by users. The EXFOR library covers not only the cross section but also various other nuclear reaction quantities such as fission yield and thick target yield. It is not always trivial to find an appropriate set of keywords extracting the data sets of interest neither too many or too few. Sometimes users are too specific in their queries, and find nothing in the database. This article presents some examples of EXFOR retrievals for better access to the databases.

Activation method was employed to measure the cross section for the ⁵⁸Ni(n,pα)⁵⁴Mn and ⁶³Cu(n,pα)⁵⁹Fe reactions in the energy range from the threshold up to 21 MeV. The quasi-monoenergetic neutrons in the energy range from 14 to 21 MeV were produced via the ³H(d,n)⁴He reaction at 1, 2, 3 and 4 MeV incident deuteron energies. High purity Ni and Cu samples with natural isotopic composition and dimensions of ϕ12x5 mm and ϕ20x5 mm respectively were utilized. The gamma-ray spectrometry using HPGe detector was utilized to measure the activity induced in the sample. The cross sections were determined relative to the standard cross section the ²⁷Al(n,α)²⁴Na reaction. The results for the ⁶³Cu(n,pα)⁵⁹Fe reaction cross section were obtained for the first time.

Cross sections for the strongest γ-ray emission lines produced in α-particle reactions with C, Mg, Si, Fe have been measured in the range E_α = 50-90 MeV at the center for protontherapy at the Helmholtz-Zentrum Berlin. Data for more than 60 different γ-ray lines were determined, with particular efforts for lines that are in cross section compilations/evaluations with astrophysical purpose, and where data exist at lower projectile energies. The data are compared with predictions of a modern nuclear reaction code and cross-section curves of the latest evaluation for gamma-ray line emission in accelerated-particle interactions in solar flares.

A discussion on the quasi-exact solution of the Bohr Hamiltonian with sextic oscillator potential is made by attracting the attention on some recent results of its application to the phase transition from spherical vibrator to a γ-unstable system. More precisely, it is underlined the importance of the solvability order on the structure of the states, especially in the critical point, respectively, in the deformed region of the phase transition.

A time-dependent variational principle with an angular momentum coherent state as a variational state, is used to describe the dynamics associated to a triaxial rotor Hamiltonian with rigidly aligned high-j quasiparticles. Solving the variational principle within a stereographic parametrization of the coherent state, one obtains a classical energy function and a set of canonical equations of motion expressed in terms of the azimuth angle and a canonical conjugate coordinate represented by the third projection of the total angular momentum. The system's rotational dynamics is investigated through the evolution on total angular momentum of the canonical variables as well as spherical angles corresponding to minima of the constant energy surface. The unique minimum energy conditions are spin-dependent and define phases with specific dynamic behaviour. The transition between phases is investigated for a single and two aligned quasiparticle spins. The discrete energy levels and corresponding wave-functions are obtained through a quantization procedure applied to the classical energy function. The formalism is used for numerical applications to ¹³⁵Pr and ¹³⁴Pr nuclei.

The Time-of-Flight (TOF) subsystem, one of the core detectors of the CBM experiment, is dedicated to the identification of all charged hadrons produced in beam-target interaction. The targeted system time resolution of 80 ps and an efficiency above 90% should be maintained at a particle flux up to 30 kHz/cm² in the region of low polar angles. In order to cope with these challenging requirements, CBM-TOF wall will be equipped with Multi-Gap Resistive Plate Counters with Multi-Strip readout (MSMGRPC).

Our R&D activity has been focused on the development of a MSMGRPC prototype for the most demanding region of the CBM-TOF wall, the region of low polar angles (from 2.5° to ∼12° around the beam pipe). The results obtained in heavy ion in-beam tests have demonstrated the performances of the developed prototypes in conditions of exposure of the whole active area to high flux and high multiplicity reaction products. The latest developed prototypes have an optimized design which fulfills simultaneously two important requirements for MSMGRPCs, the granularity for the inner zone of the CBM-TOF subdetector and the impedance matching to the front-end electronics. Based on the obtained in-beam test results and the architecture of the developed prototypes, a modular structure of 12 units called modules is proposed. A total number of 470 MSMGRPCs (~30000 readout channels) assures an uniform coverage of the active area.

Magnetars form a subclass of neutron stars characterized by magnetic fields of order 10¹⁴ − 10¹⁵ G at their surface. According to numerical simulations, the magnetic fields in their interior could potentially be even stronger. Such magnetic fields are so extreme that the internal constitution of neutron stars may be altered. The effects of Landau-Rabi quantisation of electron motion on the equation of state and on the equilibrium composition of the crust of a neutron star are investigated for a wide range of magnetic field strengths. Both the outer and inner parts of the crust are treated in a unified and consistent way within the nuclear-energy density functional theory.

An extended discussion of the deuteron-induced reactions concerning full parametrization procedure as well as consideration of the theoretical models associated to the deuteron interaction process is carried out. The key role of direct interactions, i.e., breakup, stripping and pick-up processes is stressed out by comparison of the available experimental data with theoretical and evaluation predictions.

Analysis of the differential cross sections of scattering of neutron-rich nuclei ^12,14Be on ¹²C at 56 MeV/nucleon and on protons at energy near 700 MeV/nucleon is carried out within the microscopic model of optical potential (OP). The real part of the ^12,14Be+¹²C OP is calculated by using the double-folding procedure accounting for the anti-symmetrization effects, while the imaginary part of OP is obtained in the framework of the high-energy approximation (HEA). As to the ^12,14Be+p scattering at relativistic energies, calculations of the both real and imaginary OPs were made within the HEA approach. In this framework, the only free parameters of OP are the depths of its real and imaginary parts obtained by fittingto experimental data. The role of the ^12,14Be density models is considered when reproducing the experimental data. A contribution of the inelastic channels with excitations of 2⁺ and 3⁻ states in ¹²C when calculating the quasielastic cross sections is analysed. Also, the ¹⁴Be cluster model, in which this nucleus consists of a 2n-halo and the ¹²Be core, is applied to calculate the cross sections of diffraction breakup and stripping reactions in the ¹⁴Be+¹²C collisions at the energy of 56 MeV/nucleon. A good agreement of the theoretical results with the available experimental data of both scattering and breakup processes is obtained.

The pion-nucleus elastic scattering cross sections are calculated and compared to the data of pion scattering on ²⁸Si, ⁵⁸Ni, ²⁰⁸Pb at energies 162 and 291 MeV by using two kinds of the microscopic optical potentials (OPs). The first one is the folding OP, where the known nuclear density distributions are used while parameters of the elementary pion-nucleon amplitude of scattering are fitted to the data with the aim to estimate the in-medium effect, when pions are scattered not on the free but on the nuclear bounded nucleons. The analysis given when calculations are made using the local Kisslinger type OP, whose parameters are fitted to the data. The cross sections for both OPs are obtained by solving the Klein-Gordon wave equation. The role of a surface region of potentials is discusses.

The lifetime of the ${4}_{1}^{+}$ state in ²⁰⁸Po has been measured using γ − γ fast-timing technique with LaBr₃(Ce) Detectors. The experiment was performed at the FN Tandem facility at the University of Cologne. The preliminary results are discussed in the scope of the systematic behavior of the transition strengths between yrast states in polonium isotopes.

In this study we present the preliminary results about the lifetimes of the ${2}_{2}^{+}$, ${4}_{1}^{+}$ states of ²⁰⁸Po and the upper limit of the lifetime of the ${2}_{1}^{+}$ state. For measuring the lifetimes of the ${2}_{2}^{+}$ and ${4}_{1}^{+}$ states the Recoil Distance Doppler Shift (RDDS) method and for the lifetime of the ${2}_{2}^{+}$ state the Doppler Shift Attenuation method (DSAM) were used. The resulting absolute transition strength $B(M1; {2}_{2}^{+}\to {2}_{1}^{+})\ge 0.122(20) {\mu }_{{N}}^{2}$ reveals the predominant isovector nature of the ${2}_{2}^{+}$ state of ²⁰⁸Po.

A new setup for production and investigation of exotic heavy neutron rich nuclei located in the "north-east" region of nuclear map is under construction at Flerov Laboratory for Nuclear Reactions (FLNR) of JINR, Dubna. The setup will use the available U-400M cyclotron beams for low energy multi-nucleon transfer reactions. Products of 4.5 to 9 MeV per nucleon heavy-ion collisions, such as ¹³⁶Xe beam on ²⁰⁸Pb target, will be captured in a gas cell, selectively laser-ionized, transported towards mass separator by electrostatic ion guide extraction system and then to detecting system.

In recent years new facilities have been developed for quasifree knockout studies of unstable atomic nuclei. This has stimulated renewed interest in several unresolved issues from earlier work on stable targets. With selected examples the present paper provides a brief explanation to what extent results from the traditional investigations are reliable. As is known, current insight suggests a problem with the mean-field approximation to nuclear matter. To some extent this result is vulnerable to sensitivity to input ingredients of the theoretical model used to arrive at this conclusion. This review also implicitly provides some guidance in this respect.

The low-lying collective states in osmium isotopes are investigated microscopically by means of the boson expansion theory with the self-consistent effective interactions. The potential energy surfaces and the structures of boson wave functions are illustrated. Theoretical level structures and electromagnetic properties are compared with the available experimental data. The prolate-oblate shape transition predicted at around N = 116 is also discussed in terms of the evolution of the theoretical potential as the neutron number changes.

The proton-neutron symplectic shell-model approach is applied to the description of the microscopic structure of the low-lying negative-parity states of the ${K}^{\pi }={0}_{1}^{-}$ and ${K}^{\pi }={1}_{1}^{-}$ bands in ¹⁵⁴Sm and ²³⁸U without the introduction of additional degrees of freedom that are inherent to other approaches to odd-parity nuclear states. A good description of the energy levels of the two bands under consideration, as well as the reproduction of some energy splitting quantities which are usually introduced in the literature as a measure of the octupole correlations, is obtained for these two nuclei. Additionally, the low-energy B(E1) transition strengths between the states of the ground band and ${K}^{\pi }={0}_{1}^{-}$ band for the two nuclei are calculated in the extended proton-neutron symplectic model and compared with experiment. The results obtained reproduce well the experimental data for the two nuclei under consideration without the use of an effective charge, which could be considered as a significant achievement of the present approach.

Solid experimental evidence for triaxial nuclear shape is difficult to obtain and is quite rare. Such evidence can be provided by the phenomena of chiral rotation and wobbling motion, as these can occur only in triaxially deformed nuclei. Although nuclear chirality is well documented in different mass regions, experimental evidence for multiple chiral bands as well as for wobbling motion has been reported so far only in a few nuclei. Pd and Rh nuclei in the A ≈ 100 mass region have been recently studied, and transverse wobbling motion as well as multiple chiral band structures were identified for the first time in this region. These observations provide experimental evidence for the predicted triaxial shape in this mass region, and new data which enable a better understanding of the studied phenomena.

Within the SEASTAR III campaign at the Radioactive Isotope Beam Factory, at the RIKEN Nishina Center, neutron-rich isotopes in the vicinity of ⁵³K were produced from the fragmentation of the primary ⁷⁰Zn beam on a ⁹Be target. After nucleon knockout reactions on the secondary liquid hydrogen MINOS target the known γ rays of the neutron-rich ⁵⁵Sc isotope were observed (shown in this proceedings) and γ rays from ^57,59Sc isotopes have been identified for the first time. The evolution of the occupied nucleon orbitals of these nuclei in the ground and excited state is investigated under the prism of the tensor force.

A projectile Coulomb-excitation experiment was performed at the radioactive ion beam facility HIE-ISOLDE at CERN. The radioactive ¹⁴⁰Nd and ¹⁴²Sm ions were post accelerated to the energy of 4.62 MeV/A and impinged on a 1.45 mg/cm²-thin ²⁰⁸Pb target. The γ rays depopulating the Coulomb-excited states were recorded by the HPGe-array MINIBALL. The scattered charged particles were detected by a double-sided silicon strip detector in forward direction. Experimental γ-ray intensities were used for the determination of electromagnetic transition matrix elements. Preliminary results for the reduced transition strength of the $\rm {B}(M1; {2}_{3}^{+}\to {2}_{1}^{+})=0.35(19) {\mu }_{\rm {N}}^{2}$ of ¹⁴⁰Nd and a first estimation for ¹⁴²Sm have been deduced using the Coulomb-excitation calculation software GOSIA. The ${2}_{3}^{+}$ states of ¹⁴⁰Nd and ¹⁴²Sm show indications of being the main fragment of the proton-neutron mixed-symmetry ${2}_{1,\rm {ms}}^{+}$ state.

The differential cross-sections of deuteron elastic and inelastic scattering on ¹³C target were measured at the cyclotron U-150M INP, Almaty, Kazakhstan. Accelerated deuteron ion energy was 14.5 MeV. The results of the study are new experimental data for the d + ¹³C process of elastic and inelastic scattering to the 3.68 (3/2⁻), 6.86 (5/2+) and 7.55 (5/2⁻)MeV excited states of the ¹³C nucleus. The optical model with either a Woods-Saxon potential or a double folding model were used for analyzing the experimental results on elastic scattering. We performed theoretical calculations within the framework the coupled channel (CC) method for the indicated excited states. The optimal deformation parameters for the excited states of ¹³C nucleus were extracted.

Formation of the ⁷He heavy isotope was studied in the reactions of stopped pion absorption by light nuclei ⁹Be, ^10,11B and ^12,14C. Measurements were performed using the two-arm multilayer semiconductor spectrometer. Contrary to the results of other studies, we observed several narrow highly excited states (E_x > 16 MeV) of ⁷He. From the analysis of the continuous excitation spectrum in reactions ⁹Be(π⁻, d)X and ¹¹B(π⁻, dd)X it was obtained first indication that the ⁴He+3n structure is not present in the ground state of ⁷He.

Calculations of the probability densities and energies for Nα-cluster nuclei ¹²C (3α), ¹⁶O (4α), ²⁰Ne (5α), ²⁴Mg (6α), ²⁸Si (7α) and for nuclear molecules ⁹Be (α + n + α), ¹⁰Be (α + n + n + α), were performed by the Feynman's continual integrals method using parallel computing based on NVIDIA CUDA technology. New effective method for the solution of the hyperradial equations with cubic splines approximation is proposed and tested for exactly solvable 4-body system.

Differential cross sections for the formation of oxygen isotopes in the reaction ¹⁸O + ¹⁸¹Ta have been measured at projectile nucleus energy 10 A⋅MeV on the high-resolution magnetic spectrometer MAVR. Theoretical analysis of product yields has been performed in the finite-range distorted-wave Born approximation formalism using the FRESCO code under the assumption of sequential neutron transfer mechanism. The results of calculations are in good agreement with experimental data.

The analysis of the available data on the α + ⁹Be elastic scattering in range from 18.4 to 104 MeV, including recent measurements at energies of 40 and carried out. The experimental data on elastic scattering were analyzed within the of the optical model using Woods-Saxon potential and the double folding one.

The presented paper discusses capability of recently developed version 2 revision 1, patch 1 of ASTEC computer code to predict a fuel behaviour at late phase of severe accident progression. The conducted assessment is part of the ASTEC validation based on available information from performed tests in QUENCH program for these purposes. It has been selected QUENCH-12 experiment for simulation of investigated phenomena as fuel cladding, oxidation and hydrogen generation, including temperature escalation during heat up followed by quenching overheated surface fuel cladding.

The QUENCH-12 experiment was proposed by FZK together with RIAR Dimitrovgrad and IBRAE Moskow (Russia). The test was conducted at Forschungszentrum Karlsruhe on 27 December, 2006 in the frame of EC-supported ISTC program.

The selected QUENCH-12 experiment was performed to extend knowledge gathered in previous QUENCH test to fuel behaviour in other LWRs slightly different from well investigated western type PWR with squared lattice. The fuel assemblies with a hexagonal lattice and containing fuel rod claddings made of Zr1%Nb (E 110) using in the VVER reactors are investigated in the experiment.

The developed QUENCH-12 model for ASTEC code successfully predicted the entire sequence from initial heat-up to quench phase of experiment, except hydrogen generation during quenching phase at the end of transient.

During the last 15 years at JINR, Dubna a large number of experiments have been done to investigate the neutron field generated in big spallation targets. The Energy plus Transmutation project has two set-ups: big U/Pb [1] and Quinta [2,3]. These set-ups are irradiated by high energy deuteron and proton beams. The project continues with the next generation "quasi infinite target", a cylinder of 21 tons of depleted uranium packed in a steel cage (the experiments have not been done yet) [4]. Such a target has simple requirements: all generated neutrons deliver more than 90% of their energy in the target volume and the leakage ranges between 5 and 10% of the generated neutrons. There is a significant number of experimental results and simulations with different transport codes MCNPX, FLUKA, GEANT 4 for big U/Pb target and Quinta set ups. The comparison of simulations with reaction rate measurements shows very good agreement with an error range of 10-20%. These results provided the basis for additional simulations for similar set-ups. In the big U/Pb set-up the lead target is replaced by ^natU and the polyethylene (PE) cage is replaced by ^natPb. Similar calculations are done with the Quinta set-up. The ^natPb shield is replaced with polyethylene (PE). The integral number of neutron induced reactions, neutron production and leakage are calculated and compared.

A complex investigation on Bulgarian mineral waters from certain frequently used sources have been carried out. Specific activities of ²²⁶Ra and concentration levels of natural uranium in mineral waters samples were analyzed using nuclear and analytical methods.

²²⁶Ra is a naturally occurring radioisotope with a period of half live 1 600 years, which specific activity in water was determined through analysis of the daughter ²²²Rn. The measurements were carried out using a low-level liquid scintillation counting in a Packard Tri-Carb 2770 TR/SL liquid scintillation spectrometer. Uranium concentration was measured in water samples using a luminescent method. In addition to ²²⁶Ra and natural uranium, determination of gross alpha and beta activities of waters was made by means of the same liquid scintillation spectrometer. The concentrations values obtained were compared with data reported by other authors in different countries and with reference values accepted for drinking water. The expected annual effective doses for ingestion of natural uranium and ²²⁶Ra in the analysed waters were calculated.

In the paper is demonstrated the use of positron lifetime spectroscopy (PLS) in a comparative study of the results in ferroelectrics. The first part is devoted to the application of PLS as a probe for measurements of ferroelectrics. In Triglycine Sulphate (TGS) (NH₃CH₂COOH)₃H₂SO₄, Rochelle Salt (RS) NaKC₄H₄O₆4H₂O and Potassium Dihydrogen Phosphate (KDP) KH₂PO₄ at different temperatures and gamma irradiation doses is established the existence of three positron lifetime components. This experimental results show that the second long lifetime component as a function of temperature in order-disorder ferroelectrics TGS, RS and KDP is due to the positron trapping in negatively charge defects of ferroelectric. The positron spectroscopy method is effective for measurements of extremely small defects and voids, which is important for the next exploitation period of the ferroelectrics. PLS measurements of ferroelectrics show that about the third lifetime component is due to the local formation of orhto-positronium state on the boundary between the domains. In the second part the PLS analysis has been applied for examination the electronic and defect structure in selected ferroelectrics materials. In gamma irradiated TGS positrons are trapped in the defect states of the oxygen ions of the two radicals CH₂COO^- and NH₃CHCOO^-. In RS positrons are trapped also in the defect states of the oxygen ions and OH groups.

The possible use of the adjoint function as an automated source for weight windows generation in the case of VVER-1000 benchmark transport calculations is presented in this paper. The numerical solution for obtaining the adjoint function is performed using the Discrete Ordinates code TORT. Interface software is used to translate the adjoint function into weight windows' input parameters. A deep penetration problem has been modelled for the VVER-1000 Mock Up, built at the NRI/Rez's LR-0 critical assembly for VVER-1000 benchmark studies. The neutron flux is evaluated in three locations placed on the azimuthal symmetry axis, radially from the reactor core to the shielding. The calculations are performed in three different approaches: by MCNP code using the weight windows obtained through the adjoint function, by MCNP code using weight windows created by the author through the standard procedure and by the TORT code. The results are compared in terms of integral fluxes and the validity of the adjoint function in variance reduction for VVER reactor Monte Carlo calculations is justified.

The history of the discovery of delayed nuclear fission is presented, and the retrospective of investigations into this phenomenon that were performed at various research centers worldwide is outlined. The results obtained by measuring basic delayed-fission features, including the fission probability, the total kinetic energy of fission fragments, and their mass distributions, are analyzed. Recommendations concerning further studies in various regions of nuclear map with the aim of search for atomic nuclei undergoing delayed fission are given. Lines of further research into features of delayed fission with the aim of solving current problems of fission physics are discussed