The study of fusion reactions at extreme sub-barrier energies has seen an increased interest in recent years, although difficult to measure due to their very small cross sections. Such reactions are extremely important for our understanding of the production of heavy elements in various environments. In this article, the status of the field is reviewed covering the experimental techniques, the available data, and the theoretical approaches used to describe such reactions. The fusion hindrance effect, first discovered in medium-mass systems, has been found to be relevant also for lighter systems. In some light systems, resonance structures are found to be important, while for heavy systems, the fission process plays an important role. In the near barrier region, couplings to collective excitations in the fusion participants and transfer reactions have been found to give a good description of the measured fusion cross sections and it results in a distribution of fusion barrier heights. New physics ingredients, related to the overlap process of the two projectiles, have to be introduced to describe the hindrance behavior. In addition, it has recently been found that the fusion cross section in both near-barrier and sub-barrier regions can be described very well in many cases using simple, analytical forms of the barrier-height distributions or a modified version of the classic Wong formula.

/pdf/heavy-ion-fusion-reactions-at-extreme-sub-barrier-energies-4vqm9xqtqv.pdf

Heavy-ion fusion reactions at extreme sub-barrier energies

This paper reports on the effects of Liþ, Naþ, Mg2þ, Ca2þ, Sr2þ, and Ba2þ co-doping on the scintillation properties of LaBr3:5%Ce3þ. Pulse-height spectra of various gamma and X-ray sources with energies from 8 keV to 1.33 MeV were measured from which the values of light yield and energy resolution were derived. Sr2þ and Ca2þ co-doped crystals showed excellent energy resolution as compared to standard LaBr3:Ce. The proportionality of the scintillation response to gamma and X-rays of Ca2þ, Sr2þ, and Ba2þ co-doped samples also considerably improves. The effects of the co-dopants on emission spectra, decay time, and temperature stability of the light yield were studied. Multiple thermoluminescence glow peaks, decrease of the light yield at temperatures below 295 K, and additional long scintillation decay components were observed and related to charge carrier traps appearing in LaBr3:Ce3þ with Ca2þ, Sr2þ, and Ba2þ co-doping.

/pdf/improvement-of-labr3-5-ce-scintillation-properties-by-li-na-4q6yhisfaq.pdf

Improvement of LaBr3:5%Ce scintillation properties by Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+ co-doping

Advances in gamma radiation detection systems for emergency radiation monitoring

In-beam spectroscopy of heavy elements

Recent advances obtained in the field of near and sub-barrier heavy-ion fusion reactions are reviewed. Emphasis is given to the results obtained in the last decade, and focus will be mainly on the experimental work performed concerning the influence of transfer channels on fusion cross sections and the hindrance phenomenon far below the barrier. Indeed, early data of sub-barrier fusion taught us that cross sections may strongly depend on the low-energy collective modes of the colliding nuclei, and, possibly, on couplings to transfer channels. The coupled-channels (CC) model has been quite successful in the interpretation of the experimental evidences. Fusion barrier distributions often yield the fingerprint of the relevant coupled channels. Recent results obtained by using radioactive beams are reported. At deep sub-barrier energies, the slope of the excitation function in a semi-logarithmic plot keeps increasing in many cases and standard CC calculations over-predict the cross sections. This was named a hindrance phenomenon, and its physical origin is still a matter of debate. Recent theoretical developments suggest that this effect, at least partially, may be a consequence of the Pauli exclusion principle. The hindrance may have far-reaching consequences in astrophysics where fusion of light systems determines stellar evolution during the carbon and oxygen burning stages, and yields important information for exotic reactions that take place in the inner crust of accreting neutron stars.

Recent experimental results in sub- and near-barrier heavy ion fusion reactions

This paper presents the results of our measurements and detailed simulations using GEANT4 to investigate the performance of LaBr 3 : Ce detectors. In the first part of the paper we present the results of our experimental studies using cylindrical 1 in . × 1 in . LaBr 3 : Ce scintillator. We have measured the energy and timing resolutions and also the absolute detection efficiencies, both, total detection and photopeak, using known activity sources. The energy dependence of the resolution has been studied using a variety of low energy gamma rays sources. The measured absolute efficiencies have been compared with the simulated values using GEANT4. We have extended our simulations to reproduce the recent measurements (Favalli et al., 2008) [15] of absolute photopeak efficiencies of a 1.5 in . × 1.5 in . cylindrical LaBr 3 : Ce detector up to 5 MeV. The second part of the paper is devoted to a detailed comparative study of the energy dependent absolute efficiencies, both total detection and photopeak, of cylindrical LaBr 3 , NaI(Tl), and BaF 2 scintillators of different sizes for a wide range of gamma rays from 662 keV to 50 MeV. We have also simulated the energy dependent efficiencies for compact assemblies of: (a) LaBr 3 : Ce and NaI(Tl) and (b) LaBr 3 : Ce and BaF 2 in two different configurations. The efficiencies, so simulated, for these configurations, have been compared with the efficiencies of single scintillators of equivalent volumes. The usefulness of such combined detector assemblies using LaBr 3 crystals of available volumes with NaI ( Tl ) / BaF 2 in measurements of high-energy gamma rays (5–50 MeV) is discussed.

Experimental measurements and GEANT4 simulations for a comparative study of efficiencies of LaBr3:Ce, NaI(Tl), and BaF2

This paper presents the results of our measurements and detailed simulations using GEANT4 to investigate the performance of a large volume (946 cm3) cylindrical ( 3.5 in . diameter × 6 in . length) LaBr3:Ce detector. The properties of the detector have been studied using γ - rays from radioactive sources and in-beam reaction, from few hundred keV to 22.5 MeV. The salient features, which have been studied in-depth, are the uniformity and internal activity of the crystal, the energy and timing resolutions, linearity of the response up to 22.5 MeV, and efficiencies. A highly linear response has been observed by extracting the energy signal from a lower dynode and operating the PMT at a low voltage. The detector is to be primarily used for measuring high energy γ - rays spectra from Giant Dipole Resonance (GDR) decay studies.

Studying the properties and response of a large volume (946 cm3) LaBr3:Ce detector with γ-rays up to 22.5 MeV

Close‐geometry efficiency calibration has been performed for a 25.4 mm×25.4 mm cylindrical LaBr 3 (Ce) detector using calibrated point sources of 137 Cs and 60 Co placed on the top of the detector surface. The absolute total detection efficiency and the absolute photo-peak efficiency, obtained with 60 Co, are corrected for coincidence-summing effect. The measured efficiency for 137 Cs and the corrected efficiencies for 60 Co are reproduced by GEANT4 simulations, considering the detector in a realistic geometry. In addition, the simulations also faithfully reproduce the entire measured spectra for both the sources.

Efficiency calibration and simulation of a LaBr3(Ce) detector in close‐geometry

Effect of angular momentum on giant dipole resonance observables in the 28 Si+ 116 Cd reaction

Efficiency calibration and coincidence summing correction have been performed for two large arrays of NaI(Tl) detectors in two different configurations. They are, a compact array of 32 conical detectors of pentagonal and hexagonal shapes in soccer-ball geometry and an array of 14 straight hexagonal NaI(Tl) detectors in castle geometry. Both of these arrays provide a large solid angle of detection, leading to considerable coincidence summing of gamma rays. The present work aims to understand the effect of coincidence summing of gamma rays while determining the energy dependence of efficiencies of these two arrays. We have carried out extensive GEANT4 simulations with radio-nuclides that decay with a two-step cascade, considering both arrays in their realistic geometries. The absolute efficiencies have been simulated for gamma energies from 700 to 2800 keV using four different double-photon emitters, namely, 60Co, 46Sc, 94Nb and 24Na. The efficiencies so obtained have been corrected for coincidence summing using the method proposed by Vidmar et al. [11] . The simulations have also been carried out for the same energies assuming mono-energetic point sources, for comparison. Experimental measurements have also been carried out using calibrated point sources of 137Cs and 60Co. The simulated and the experimental results are found to be in good agreement. This demonstrates the reliability of the correction method [11] for efficiency calibration of two large arrays in very different configurations.

Efficiency calibration and coincidence summing correction for large arrays of NaI(Tl) detectors in soccer-ball and castle geometries

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