137,138,139^{137,138,139}La(nn, γ\gamma) cross sections constrained with statistical decay properties of 138,139,140^{138,139,140}La nuclei

The nuclear level densities and $\gamma$-ray strength functions of $^{138,139,140}$La were measured using the $^{139}$La($^{3}$He, $\alpha$), $^{139}$La($^{3}$He, $^{3}$He$^\prime$) and $^{139}$La(d, p) reactions. The particle-$\gamma$ coincidences were recorded with the silicon particle telescope (SiRi) and NaI(Tl) (CACTUS) arrays. In the context of these experimental results, the low-energy enhancement in the A$\sim$140 region is discussed. The $^{137,138,139}$La($n, \gamma)$ cross sections were calculated at $s$- and $p$-process temperatures using the experimentally measured nuclear level densities and $\gamma$-ray strength functions. Good agreement is found between $^{139}$La($n, \gamma)$ calculated cross sections and previous measurements

Impact of the 138,139La radiative strength functions and nuclear level densities on the galactic production of 138La

Thesis (PhD)--Stellenbosch University, 2014.ENGLISH ABSTRACT: 138La is a very long-lived and low abundant p-isotope. Most p-nuclei with Z > 54 are thought to be produced through photodisintegration of s- and r-process seed nuclei. However, this p-process cannot satisfactorily explain the observed abundance of 138La, and more exotic processes, such as ve + 138Ba → 138La + e− have to be considered. This v-process can reproduce the observed solar abundance of 138La, but the significance of the p-process cannot be ruled out due to very high uncertainties in its predicted reaction rates. These errors have been discussed to be mainly due to the unavailability of the experimental nuclear level densities and radiative strength functions of 138,139La, which are critical ingredients for astrophysical reaction rate calculations based on the Hauser-Feshbach approach. Thus, nuclear physics measurements are necessary to place the nuclear properties on a strong footing, in order to make statements regarding the importance of p- and v-processes. In this research project the experimental nuclear level densities and radiative strength functions of 138,139La were measured below the neutron thresholds. From this new experimental data, the Maxwellian averaged cross sections for the 137La(n, y) and 138La(n, y) reactions, at the p-process temperature of 2.5⇥109 K, were computed with the TALYS code. Using these reaction rates the nucleosynthesis calculations in the O/Ne-rich layers of Type II supernovae were performed. The results imply that the standard p-process still under-produces 138La, which puts the v-process on a very strong footing as the main production process for 138La.AFRIKAANSE OPSOMMING: 138La is ’n p-isotoop met ’n baie lang halfleeftyd. Daar word tans vermoed dat p-nukiede met Z > 54 geproduseer word deur fotodisintegrasie van sen r-proses saadnukliede. Nogtans verklaar hierdie p-proses die waargenome natuurlike voorkoms van 138La nie behoorlik nie, en meer eksotiese prosesse soos byvoorbeeld ve+ 138Ba → 138La + e− moet in aanmerking geneem word. Hierdie v-proses kan die waargenome natuurlike voorkoms van 138La verklaar, maar die belangrikheid van die p-proses kan nie afgewys word nie weens die onsekerheid in die voorspelde reaksie snelheid. Sodanige onsekerhede word bespreek en word hoofsaaklik toegeskryf aan die gebrek aan eksperimentele vlakdigthede en stralings sterkefunksies van die kerne 138,139La, wat van kritiese belang is vir berekeninge van astrofisiese reaksie snelhede gebaseer op die Hauser-Feshbach benadering. Kernfisiese metings is derhalwe noodsaaklik om die eienskappe van kerne op ’n stewige grondslag te plaas sodat uitlatings gemaak kan word omtrent die belangrikheid van p- en v-prosesse. In hierdie esperimentele navorsingsprojek is die kern vlakdigthede en stralings sterkefunksies van 138,139La onder die neutron reaksiedrumpels gemeet. Die nuwe gemete data maak dit moontlik om die Maxwell-gemiddelde kansvlakke vir die 137La(n, y) en 138La(n, y) reaksies by ’n p-proses temperatuur van 2.5 x 109 K met die TALYS program te bereken. Hierdie reaksie snelhede is daarna gebruik om berekeninge van elementvorming in die O/Ne-ryke lae van Tipe-II supernovas te maak. Die resultate wys uit dat die stadaard p-proses nie genoegsame 138La produseer nie, wat derhalwe die v-proses op ’n baie stewige grondslag plaas as die hoof produksie proses vir 138La

Deflection of Ag-atoms in an inhomogeneous magnetic field

Thesis (MSc)--Stellenbosch University, 2011.ENGLISH ABSTRACT: In the current design of the high temperature gas cooled reactor, a small fraction of coated fuel particles will be defective. Hence, 110Ag may be released from the fuel spheres into the coolant gas (helium) and plate out on the cooler surfaces of the main power system. This poses a radiation risk to operating personnel as well as general public. The objectives of this thesis were to design and construct an apparatus in which silver-109 atoms may be produced and deflected in an inhomogeneous and homogeneous magnetic field, compare experimental and theoretical results, and make a recommendation based on the findings of this thesis to the idea of removing silver-110 atoms from the helium fluid by deflecting them with an inhomogeneous magnetic field onto target plates situated on the inner perimeter of a helium pipe. The experimental results for the deflection of the collimated Ag- atoms with the round-hole collimators showed a deflection of 1.77° and 2.05° of the Ag- atoms due to an inhomogeneous magnetic field when the target plate was positioned 13 and 30 mm away from the magnet, respectively. These values were considerably greater than 0.01° and 0.02° that were calculated for the average velocity of atoms, v = 500 m/s. The case where Ag- atoms were collimated with a pair of slits and the target plate positioned 13mm away from the magnet showed the following: An inhomogeneous magnetic field changes the rectangular shape of the beam to a roughly elliptical shape. The beam of Ag- atoms was not split into two separate beams. This was caused by the beam of Ag- atoms consisting of atoms travelling at different speeds. The maximum deflection of Ag- atoms was 1.16° in the z direction and 1.12° in the x direction. These values were also significantly greater than 0.01 mm calculated at v = 500 m/s. This huge difference between the theoretical and experimental results raised a conclusion that the size of each Ag deposit depended mostly on the exposure time that was given to it. It was noticed that the beam of Ag- atoms was not split into two separate beams, in both cases. The conclusion was that the technique of removing Ag- atoms from the helium stream by means of an inhomogeneous magnetic field may not be effective. This is due to the inability of the inhomogeneous magnetic field to split the beam of Ag- atoms into two separate beams in a vacuum of ~10-5 mbar. It would be even more difficult for an inhomogeneous magnetic field to split the beam of Ag- atoms in helium, due to the Ag- atoms having a shorter mean free path in helium compared to a vacuum.AFRIKAANSE OPSOMMING: In die huidige ontwerp van die hoë temperatuur gas afgekoelde reaktor, is 'n klein fraksie van omhulde brandstof deeltjies foutief. 110Ag kan dus vrygestel word vanaf die brandstof sfere in die verkoelingsgas (helium) wat dan op die koeler oppervlaktes van die hoofkragstelsel presipiteer. Hierdie 110Ag deeltjies hou 'n bestraling risiko vir die bedryfpersoneel sowel as vir die algemene publiek in. Die doelwitte van hierdie verhandeling is eerstens om 'n apparaat te ontwerp en konstrueer wat silwer-109 atome produseer en nie-homogene en homogene magnetiese velde deflekteer,. Tweedens om die eksperimentele en teoretiese resultate met mekaar te vergelyk. Derdens om 'n aanbeveling te maak gebasseer op die bevindinge van hierdie verhandeling rakende die verwydering van silwer-110 atome uit die helium vloeistof deur hulle met 'n nie-homogene magneetveld te deflekteer op die teikenplate binne-in 'n helium pyp. Die eksperimentele resultate vir die defleksie van die gekollimeerde Ag-atome met die ronde gat kollimators toon ‘n defleksie van 1.77° en 2.05° van die Ag-atome as gevolg van ‘n nie-homogene magneetveld wanneer die teikenplaat 13mm en 30mm, onderskeidelik, vanaf die magneet geposisioneer is. Hierdie waardes is aansienlik groter as die teoretiese defleksies van 0.01° en 0.02o wat bereken is vir ‘n gemiddelde snelheid van 500 m/s vir die atome. Die geval waar Ag-atome met 'n paar splete gekollimeer is en die teikenplaat 13 mm weg van magneet geposisioneer is, is die volgende resultate verkry: 'n nie-homogene magneetveld verander die reghoekige vorm van die bondel na 'n rowwe elliptiese vorm. Die bondel Ag-atome is nie volkome twee afsonderlike bundels verdeel nie. Dit is omdat die bondel van Ag-atome bestaan uit atome wat teen verskillende snelhede beweeg. Die maksimum defleksie van Ag-atome is 1.16° in die z-rigting en 1.12° in die x-rigting. Hierdie waardes is ook aansienlik groter as 0.01° bereken teen 500 m/s. Hierdie groot verskil tussen die teoretiese en eksperimentele resultate dui daarop dat die grootte van elke Ag neerslag grootliks afhanklik is van die blootstellingstyd wat daaraan gegee is. Daar is vasgestel dat die straal van Ag-atome in beide gevalle nie in twee afsonderlike bondels verdeel nie. Die gevolgtrekking is dat die tegniek van die verwydering van Ag-atome uit die helium stroom deur middel van 'n nie-homogene magneetveld nie effektief is nie. Dit is te wyte aan die onvermoë van die nie-homogene magneetveld om die bondel Ag-atome te verdeel in twee afsonderlike bondels in 'n vakuum van ~ 10-5 mbar. Dit sou selfs nog moeiliker vir 'n nie-homogene magnetiese veld wees om die bundel Ag-atome in helium te verdeel, weens die korter gemiddelde beskikbare pad van Ag-atome in helium wanneer dit met 'n vakuum vergelyk word

Gamma radiation shielding properties of (x)Bi2O3–(0.5 – x)ZnO–0.2B2O3–0.3SiO2 glass system

Lead (Pb)-based materials are very effective in radiation shielding due to their high density of Pb. However, they pose health risks to humans because of the toxicity of lead. As a result, the investigation of radiation shielding properties of various lead-free glass materials has drawn a lot of attention from researchers. In this work, the γ radiation competence of the Bi2O3–ZnO–B2O3–SiO2 glass network was investigated, for the first time in the 0.015–15 MeV energy range, using Phy-X/PSD and XCOM software systems. The results showed that 45Bi2O3–5ZnO–20B2O3–30SiO2 glass sample has the highest linear attenuation coefficient, mass attenuation coefficient, and effective atomic number, and it has the lowest half-value layer, tenth-value layer, and mean-free path. Therefore, 45Bi2O3–5ZnO–20B2O3–30SiO sample is more effective on γ ray shielding than 10Bi2O3–40ZnO–20B2O3–30SiO, 20Bi2O3–30ZnO–20B2O3–30SiO, 30Bi2O3–20ZnO–20B2O3–30SiO, and 40Bi2O3–10ZnO–20B2O3–30SiO samples. The comparison of the results with the literature also revealed that the 45Bi2O3–5ZnO–20B2O3–30SiO glass sample is even more effective than some of Bi2O3-based glass systems, which were recently developed in the literature, by at least a factor of 2

Deflection of Ag-atoms in a magnetic field

ENGLISH ABSTRACT: An apparatus that generates and deflect silver-109 atoms with an inhomogeneous and homogeneous magnetic field was constructed. The experimental results found using this apparatus were used to make a recommendation, to the idea of removing silver-110 atoms from the helium fluid of Pebble Bed Modular Reactor (PBMR) with an inhomogeneous magnetic field based on the Stern-Gerlach principle. It is shown that experimental results corresponded well with the theoretical predictions. The apparatus was also used for a practical for a course for mechanical engineers in modern physics.Pre-prin

γ-ray strength function for barium isotopes

Photoneutron cross sections were measured for 137 Ba and 138 Ba at energies below two-neutron threshold using quasimonochromatic γ -ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. The photoneutron data are used to constrain the γ -ray strength function on the basis of the Hartree-Fock-Bogolyubov plus quasiparticle random phase approximation using the Gogny D1M interaction. Supplementing the experimentally constrained γ -ray strength function with the zero-limit E1 and M1 contributions, which are unique to the deexcitation mode, we discuss radiative neutron capture cross sections relevant to the s-process nucleosynthesis of barium isotopes in the vicinity of the neutron magic number 82

Nuclear level densities and γ-ray strength functions of 180,181Ta and neutron capture cross sections

The γ -ray strength functions and nuclear level densities in the quasi-continuum of 180,181Ta are extracted from particle-γ coincidence events with the Oslo Method, below the Sn . The data were used as input in the TALYS reaction code for calculations of the astrophysical Maxwellian-averaged (n, γ ) cross-sections to investigate nucleosynthesis of nature’s rarest stable isotope 180Ta

Photoneutron cross sections for Ni isotopes: Toward understanding (n, γ ) cross sections relevant to weak s-process nucleosynthesis

Photoneutron cross sections were measured for 58Ni, 60Ni, 61Ni, and 64Ni at energies between the one-neutron and two-neutron thresholds using quasimonochromatic γ -ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. These photoneutron data are used to extract the γ -ray strength function above the neutron threshold, complementing the information obtained by the Oslo method below the threshold. We discuss radiative neutron-capture cross sections and the Maxwellian-averaged cross sections for Ni isotopes including 63Ni, a branching point nucleus along the weak s-process path. The cross sections are calculated with the experimentally constrained γ -ray strength functions from the Hartree-Fock-Bogolyubov plus quasiparticle–random-phase approximation based on the Gogny D1M interaction for both E1 and M1 components and supplemented with the M1 upbend

Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes

Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes 142,144−151Nd which exhibit such a crossover. These results represent the most complete data set of nuclear level densities to date for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of ≈150 at an excitation energy of 6 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement

Resonances in odd-odd 182Ta

Enhanced γ -decay on the tail of the giant electric dipole resonance, such as the scissors or pygmy resonances, can have significant impact on (n,γ ) reaction rates. These rates are important input for modeling processes that take place in astrophysical environments and nuclear reactors. Recent results from the University of Oslo indicate the existence of a significant enhancement in the photon strength function for nuclei in the actinide region due to the scissors resonance. Further, the M1 strength distribution of the scissors resonances in rare earth nuclei has been studied extensively over the years. To investigate the evolution and persistence of the scissor resonance in other mass regions, an experiment was performed utilizing the NaI(Tl) γ -ray detector array (CACTUS) and silicon particle telescopes (SiRi) at the University of Oslo Cyclotron laboratory. Particle-γ coincidences from the 181Ta(d,p)182Ta and 181Ta(d,d’)181Ta reactions were used to measure the nuclear level density and photon strength function of the well-deformed 181Ta and 182Ta systems, to investigate the existence of resonances below the neutron separation energy