keV Cascade Research Articles

Improvement of double photon emission Compton imaging using angular and polarization correlations in cascade photons

A Compton camera is a gamma ray imaging system that produces images by superposing Compton cones calculated from Compton kinematics. However, the reconstructed images exhibit a high background. In this study, the angular correlation and polarization correlation were used to decrease the background. 111In, which emits 171 keV and 245 keV cascade gamma rays, was imaged using Compton camera modules in the ring structure. The 3D list-mode ordered subset expectation maximization method was used to reconstruct the images. Probability modeling of events reflecting angular and polarization correlations was applied to the image-reconstruction algorithm. The results were compared with those of a typical Compton camera method using only 245 keV Compton cones. The sensitivity of the proposed method was 1.03% of that of the typical method. However, compared to the typical method, the application of angular and polarization correlations improved the signal-to-background ratio. The proposed method is expected to be useful in capturing low-background images wherein a ring-type Compton camera and cascade photon-emitting nuclides can be used.

Displacement cascades database from molecular dynamics simulations in tungsten

In this work, we have carried out a series of displacement cascade simulations in tungsten (W), including seven different primary-knock-on atom (PKA) directions, and PKA energy covers 1 to 300 keV at 363 K; 10, 50, 100, and 200 keV cascades are considered at 600 K, and 10 and 50 keV cascades are simulated at 300 K. Then, a systematic database of W cascade simulations has been established, containing more than 2000 cascades. The number of Frenkel pairs, the type of defect, and its spatial distribution were all determined. The effects of PKA direction, PKA energy and temperature have been discussed in terms of defect generation, defect clustering, and dislocation loops. It is found that, the PKA direction may have an effect on the configuration of defects, such as the formation probability of 〈100〉 interstitial loops, C15 clusters and the sub-cascades, and also has an effect on the number of Frankel pairs when PKA energy larger than 50 keV. Furthermore, from 363 K to 600 K, the number of Frankel pairs is reduced by ∼10%, and high temperature can promote the clustering of interstitial. We further found that temperature may affect the distribution of dislocation loops, and the number of 〈100〉 interstitial dislocation loops is more at 363 K than at 600 K. These results can provide a database of key input parameters for the subsequent larger-scale simulations, and give an important reference for the prediction of defect behaviour, mechanical and thermal properties of W-based materials under fusion neutron irradiation.

Effect of radiation and substitution of Ce4+ at Zr site in Y4Zr3O12 using collision cascades: a molecular dynamics simulation study

ABSTRACT This study employed classical molecular dynamics (MD) simulations to investigate radiation damage and Cerium (Ce4+) incorporation as surrogate of plutonium (Pu) at Zirconium (Zr4+) site in delta (δ) phase Y4Zr3O12 material. The results indicate that Oxygen (O) disorder is dominant due to its higher number of survived defects with 2 keV, 5 keV and 10 keV cascade collision. The number of survived defects produced varies with primary knock-on atom (PKA), incident irradiation energy direction and increases with incident energy. A maximum survived defects of 28 along [103] and a minimum of 17 along [222] directions were generated among the eight different crystallographic directions studied using 5 keV cascade. Additionally, cascade overlap with 5 keV leads to substantial decrease in survival defects. Doping of certain number of Ce atoms at Zr site of δ-Y4Zr3O12 may improve its radiation resistance in terms of survived defects formation using a 5 keV cascade. Furthermore, the results showed that the survived defects vary with the number of Ce atoms added, and the highest occurred at x = 0.5 (at equal number of Ce and Zr). Therefore, this work expands understanding of radiation damage effects mechanism in δ-Y4Zr3O12 material for potential nuclear technology application.

Radiation damage buildup and basal vacancy cluster formation in hcp zirconium: A molecular dynamics study

Irradiation-induced damage buildup in alpha zirconium is investigated using Molecular Dynamics (MD) simulations of cascade overlap. It is found that the damage accumulates very quickly at low dose and then reaches a steady level after around 0.1 dpa. We also observe that high temperature is able to significantly decrease the steady damage while recoil energy exhibits a small effect on the defect accumulation. Most importantly, we find that cascade overlap in hcp Zr naturally creates basal vacancy clusters, which have been seldom observed in simulations of single cascades. Based on our calculations of binding energies of defect clusters and simulations of single 50 keV cascades in strained crystals, we show that the local strain induced by the defect debris in the heavily-irradiated crystal is a possible mechanism for the formation of basal vacancy clusters in hcp Zr. Finally the reliability of the used potential for investigating strain effects is discussed based on calculations of stacking fault energy (SFE) under strains.

Radiation damage effects in amorphous zirconolite

We report the results of a large-scale modelling study of radiation damage effects in the nuclear waste form zirconolite. We particularly focus on the effects of radiation damage in amorphous zirconolite and have developed a new way to analyse the damaged structure in terms of local coordination statistics. On the basis of this analysis, we find that the amorphous structure responds to radiation damage differently from the crystal. Amorphous zirconolite is found to be “softer” than crystalline zirconolite with a much larger number of atoms becoming displaced and changing coordination during a 70 keV cascade. The local coordination and connectivity analysis shows that the amorphous structure continues to evolve as a result of repeated radiation damage, changes which cannot be identified from globally averaged properties such as pair distribution functions. We also find large density inhomogeneities at the local level which we suggest may play an important role for future developments in nuclear waste storage. Finally, we find a correlation between the changes in enthalpy and local coordination, suggesting that measurements of enthalpy change can be linked quantitatively to structural radiation damage. Our results raise an interesting possibility of whether an evolution of the amorphous structure due to radiation damage can converge to a new equilibrium amorphous regime, posing the fundamental question of what that structure may be.

A novel displacement cascade driven irradiation creep mechanism in α-zirconium: A molecular dynamics study

Zirconium alloys used in nuclear reactors undergo irradiation creep, which consists of a visco-plastic deformation activated by irradiation occurring under constant load. However, the fundamental underlying mechanisms have not been unraveled yet. A new high-stress irradiation creep mechanism for recrystallized Zircaloy-4 has recently been proposed based on in situ ion irradiation deformation experiments. A displacement cascade is assumed to induce a direct unpinning of a dislocation from an irradiation defect if the cascade occurs within an effective volume around the pinning point. In the present work, a systematic molecular dynamics study was performed to investigate the effect of a 20 keV cascade occurring near <a>-screw dislocation pinned on an interstitial <a>-loop. The direct release of dislocations by displacement cascades is predicted by the simulations. This release is more likely around the pinning points and with increasing stress, in agreement with experimental observations. The effective volume is roughly estimated for three stress levels equal to 0.89τc, 0.93τc and 0.97τc, with τc being the critical stress for unpinning (without irradiation). A mechanism is proposed suggesting that unpinning occurs when the displacement cascade encompasses, at its peak of damage, the two pinning points of the dislocation. A methodology requiring acceptable computation time has been implemented to estimate the effective volume for various cascade energies and loop characteristics, faithfully reproducing the MD results. The mean pinning lifetime calculated using the model provides values of the same order of magnitudes as in the in situ deformation experiments under ion irradiation at high stress levels.

Molecular dynamics study of primary radiation damage in PuO2 and (U0.5Pu0.5)O2

In this study, primary radiation damage in PuO2 and (U0.5Pu0.5)O2 is investigated as a function of temperature (300–1500 K) using MD simulations with 5 keV cascade. UO2 data from our previous work are used as reference. The interatomic potential develop by Cooper et al. has been used in this study. Time evolution of cascade generated defects is similar for different directions of primary knock on atom (PKA). Our results shows that the number of defects remaining after the cascade events in PuO2 is higher than that in UO2 at all temperatures. Defect concentration in (U,Pu)O2 is observed to be in between that of the pure oxides. The number of residual defects decreases as the temperature increases and finally tends to approach a plateau at elevated temperatures. The concentration of residual anion defects is considerably higher than that of cations in all oxides, while a decrease in the ratio of anion to cation defects with increasing temperature is observed. At lower temperature, the annihilation efficiency (ξ) of defect follows the trend ξPuO2 < ξUPuO2 < ξUO2. It is reported that both pure and mixed oxide fuels form defect clusters in the residual damage. The fraction of interstitial clusters is higher than the vacancy clusters; however, the size of these clusters is smaller than that of voids.

Study of Defects Produced by Displacement Cascades in Tantalum Monocarbide

We used the binary collision approximation code Marlowe to simulate displacement cascades in tantalum monocarbide. We investigated the damage production, the spatial configurations of the resulting defects, and the vacancy clustering. Statistics over 5000 cascades were accumulated, and primaries with kinetic energies up to 20 keV were launched from lattice sites. Elastic collisions between atoms were modeled by the universal Ziegler–Biersack–Littmark, Moliere, and Born–Mayer potentials. The Lindhard–Scharff–Schiott theory was used to account for the inelastic energy losses. Principal components analysis was utilized to evaluate the volume of the damaged zone. Analysis of the simulation results shows that no more than 40% of the displaced Ta and C atoms constitute permanent damage. The number of surviving tantalum Frenkel pairs is about twice the carbon one. The cascade volume distributions deviate from a Gaussian distribution showing high degree of dispersion. Only small vacancy clusters are observed within the investigated range of the primary energies, and 33% of the produced vacancies are considered as isolated point defects in 20 keV cascade.

VECC array for Nuclear fast Timing and angUlar corRElation studies (VENTURE)

The VECC array for Nuclear fast Timing and angUlar corRElation studies (VENTURE) has been developed using several fast Cerium-Bromide (CeBr3) scintillators coupled to Hamamatsu R9779 Photomultiplier tubes. The CeBr3 detector has been characterised for the spectroscopic properties like energy response, energy resolution, timing resolution and detection efficiency. The response and efficiency of the detector have been compared with the results obtained from a Monte Carlo simulation with GEANT3 package. A time resolution of 144(1) ps and 109(1) ps was obtained for a single detector using 622–512 keV and 1173–1332 keV cascades respectively. The Generalised Centroid Difference (GCD) method has been employed with CeBr3 detectors by measuring the level lifetimes for the 511.9 keV level of 106Pd and the 160.6 and 383.8 keV levels of 133Cs. The angular correlation measurement was performed for the 1173–1332 keV cascade in 60Ni and the 228–49 keV cascade of 132I nucleus, populated from the decay of 132Te produced via 238U(α, f) reaction.

The effect of alloying nickel with iron on the supersonic ballistic stage of high energy displacement cascades

Previous experimental and theoretical studies suggest that the production of extended defect structures by collision cascades is inhibited in equiatomic NiFe, in comparison to pure Ni. It is also known that the production of such extend defect structures results from the formation of subcascades by high-energy recoils and their subsequent interaction. A detailed analysis of the ballistics of 40 keV cascades in Ni and NiFe is performed to identify the formation of such subcascades and to assess their spatial distribution. It is found that subcascades in Ni and NiFe are created with nearly identical energies and distributed similarly in space. This suggests that the differences in production of extended defect structures is not related to processes taking place in the ballistic phase of the collision cascade. These results can be generalized to other, more chemically complex, concentrated alloys where the elements have similar atomic numbers, such as many high-entropy alloys.

Displacement cascades in Fe[sbnd]Ni[sbnd]Mn[sbnd]Cu alloys: RVP model alloys

Primary damage due to displacement cascades (10–100 keV) has been assessed in Fe1%Mn1%Ni-0.5%Cu and its binary alloys by molecular dynamics (MD), using a recent interatomic potential, specially developed to address features of the FeMnNiCu system in the dilute limit. The latter system represents the model matrix for reactor pressure vessel steels. The applied potential reproduces major interaction features of the solutes with point defects in the binary, ternary and quaternary dilute alloys. As compared to pure Fe, the addition of one type of a solute or all solutes together does not change the major characteristics of primary damage. However, the chemical structure of the self-interstitial defects is strongly sensitive to the presence and distribution of Mn and Cu in the matrix. 20 keV cascades were also studied in the FeNiMnCu matrix containing 〈100〉 dislocation loops (with density of 1024 m−3 and size 2 nm). Two solute distributions were investigated, namely: a random one and one obtained by Metropolis Monte Carlo simulations from our previous work. The presence of the loops did not affect the defect production efficiency but slightly reduced the fraction of isolated self-interstitials and vacancies. The cascade event led to the transformation of the loops into ½〈111〉 glissile configurations with a success rate of 10% in the matrix with random solute distribution, while all the pre-created loops remain stable if the alloy's distribution was applied using the Monte-Carlo method. This suggests that solute segregation to loops “stabilizes” the pre-existing loops against transformation or migration induced by collision cascades.

Molecular dynamics modeling of atomic displacement cascades in 3C–SiC: Comparison of interatomic potentials

We used molecular dynamics modeling of atomic displacement cascades to characterize the nature of primary radiation damage in 3C–SiC. We demonstrated that the most commonly used interatomic potentials are inconsistent with ab initio calculations of defect energetics. Both the Tersoff potential used in this work and a modified embedded-atom method potential reveal a barrier to recombination of the carbon interstitial and carbon vacancy which is much higher than the density functional theory (DFT) results. The barrier obtained with a newer potential by Gao and Weber is closer to the DFT result. This difference results in significant differences in the cascade production of point defects. We have completed both 10 keV and 50 keV cascade simulations in 3C–SiC at a range of temperatures. In contrast to the Tersoff potential, the Gao-Weber potential produces almost twice as many C vacancies and interstitials at the time of maximum disorder (∼0.2 ps) but only about 25% more stable defects at the end of the simulation. Only about 20% of the carbon defects produced with the Tersoff potential recombine during the in–cascade annealing phase, while about 60% recombine with the Gao-Weber potential. The Gao-Weber potential appears to give a more realistic description of cascade dynamics in SiC, but still has some shortcomings when the defect migration barriers are compared to the ab initio results.

Slow relaxation of cascade-induced defects in Fe

On-the-fly kinetic Monte Carlo simulations are performed to investigate slow relaxation of nonequilibrium systems. Point defects induced by 25 keV cascades in $\ensuremath{\alpha}$-Fe are shown to lead to a characteristic time evolution, described by the replenish-and-relax mechanism. Then, we produce an atomistically based assessment of models proposed to explain the slow structural relaxation by focusing on the aggregation of 50 vacancies and 25 self-interstitial atoms in 10-lattice-parameter $\ensuremath{\alpha}$-Fe boxes, two processes that are closely related to cascade annealing and exhibit similar time signatures. Four atomistic effects explain the time scales involved in the evolution: defect concentration heterogeneities, concentration-enhanced mobility, cluster-size-dependent bonding energies, and defect-induced pressure. These findings suggest that the two main classes of models to explain slow structural relaxation, the Eyring model and the Gibbs model, both play a role in limiting the rate of relaxation of these simple point-defect systems.

Perturbed Angular Correlation Study of the Static and Dynamic Aspects of Cadmium and Mercury Atoms Inside and Attached to a C60 Fullerene Cage

Abstract 30 keV 111mCd and 50 keV 199mHg beams from ISOLDE were used to implant on preformed targets of C60 with a thickness of 1 mg cm-2. Endofullerene compounds, viz. 111mCd@C60 and 199mHg@C60 formed during implantation were separated by filtration through micropore filter paper followed by solvent extraction. Dried samples of the endofullerene compounds were counted for the time differential perturbed angular correlation (TDPAC) measurement using the coincidence of the 151 - 245 keV cascade of 111mCd and the 374 - 158 keV cascade of 199mHg on a six LaBr3(Ce) detector system coupled with digital electronics. The results for 111mCd@C60 indicate a single static component (27%) and a fast relaxing component (73%), the latter implying that the cadmium atom moves rapidly inside the cage at room temperature. The quadrupole interaction frequency and asymmetry parameter of the cadmium atom occupying the static site in C60 are wQ=8.21(36) Mrad s-1 and η = 0.41(9), respectively. The fast relaxation constant is 0.0031(4) ns-1. Similarly, mercury atoms also exhibit a single static and a fast component. The static site has a quadrupole frequency wQ=283.0(12.4) Mrad s-1 and η =0 with a fraction of 30%. The fast relaxation constant is 0.045(8) ns-1 with a fraction of 70%, very similar to that of cadmium.

Correction Technique for Cascade Gammas in I-124 Imaging on a Fully-3D, Time-of-Flight PET Scanner

It has been shown that I-124 PET imaging can be used for accurate dose estimation in radio-immunotherapy techniques. However, I-124 is not a pure positron emitter, leading to two types of coincidence events not typically encountered: increased random coincidences due to non-annihilation cascade photons, and true coincidences between an annihilation photon and primarily a coincident 602 keV cascade gamma (true coincidence gamma-ray background). The increased random coincidences are accurately estimated by the delayed window technique. Here we evaluate the radial and time distributions of the true coincidence gamma-ray background in order to correct and accurately estimate lesion uptake for I-124 imaging in a time-of-flight (TOF) PET scanner. We performed measurements using a line source of activity placed in air and a water-filled cylinder, using F-18 and I-124 radio-isotopes. Our results show that the true coincidence gamma-ray backgrounds in I-124 have a uniform radial distribution, while the time distribution is similar to the scattered annihilation coincidences. As a result, we implemented a TOF-extended single scatter simulation algorithm with a uniform radial offset in the tail-fitting procedure for accurate correction of TOF data in I-124 imaging. Imaging results show that the contrast recovery for large spheres in a uniform activity background is similar in F-18 and I-124 imaging. There is some degradation in contrast recovery for small spheres in I-124, which is explained by the increased positron range, and reduced spatial resolution, of I-124 compared to F-18. Our results show that it is possible to perform accurate TOF based corrections for I-124 imaging.

Including electronic effects in damage cascade simulations

A method for including the effects of electronic losses and electron–phonon coupling in radiation damage simulations has been developed and implemented for 10 keV cascades in Fe. The MD simulations are coupled to a continuum model for the electronic energy and energy lost by the atoms, due to electronic friction and electron–phonon coupling, is gained by electronic system. Electronic energy transport is described by the heat diffusion equation and energy is returned to the lattice via a stochastic force. Thus the temperature of the atomic system is controlled by a Langevin thermostat at the local electronic temperature, which varies with time and space. The results of simulations with this inhomogeneous thermostat are compared with those of homogeneous (constant temperature) thermostat simulations for a range of electron–phonon coupling strengths. The residual defect concentration was found to have a non-monotonic variation with coupling strength.

Temperature-Dependent Behavior of Impurity Atoms Implanted in Highly Oriented Pyrolytic Graphite –An Application of a New Online TDPAC Method–

The behavior of impurity atoms in graphite is one of the most intriguing subjects in current materials science. It is well known, for graphite intercalation compounds, that the type and concentration of intercalated impurities control their physical and chemical properties. It is thus of great importance to investigate the behavior of intercalants in the two dimensional layers for further understanding of their interacting nature with the graphite matrix. In order to investigate such interactions, it is essential to introduce impurities of interest into graphite; and, for that purpose, the ion-implantation method would be the simplest method. Nuclear techniques with radioactive probes are highly suited to such studies on the behavior of dilute impurities because of their high sensitivity. In our recent study, we developed a new detection apparatus for radioactive impurities: an online time-differential perturbed angular correlation (TDPAC) measurement system. The TDPAC method is a spectroscopy using unstable nuclei as the probe; by observing the time-variant angular correlation of rays successively emitted in the disintegration process of the excited nucleus of the probe, we can obtain direct information on the local field in the vicinity of the probe nucleus through electromagnetic interactions between the probe nucleus and extranuclear field. The newly developed system enables the application of shortlived unstable nuclei to the spectroscopy as the parents of probes; they are implanted in samples of interest at high energy immediately after their production by acceleratorbased nuclear reactions. In our previous paper, we demonstrated the applicability of the present online system and reported on the residence site of F( O) implanted in highly oriented pyrolytic graphite (HOPG). In comparison with the previous data obtained at 18K, we here discuss the displacement of the F( O) nuclei at room temperature on the basis of the damping trend of the TDPAC spectra. The online TDPAC experiment was carried out at the RIKEN Nishina Center. Ionized Ne was accelerated by a two-stage acceleration with the AVF cyclotron and the ring cyclotron up to 110MeV/u at a beam intensity of 150 pnA. A variety of radioactive nuclides were produced at a Be production target by projectile-fragmentation reactions; the secondary beam of interest, O (t1=2 1⁄4 26:9 s), was separated out of these heavy ions at a purity of 98% or higher by the RIKEN projectile-fragment separator (RIPS). A wellfocused pulsed beam of O was implanted deep in an HOPG sheet at a beam intensity of 10 s . The online TDPAC measurements were performed at room temperature on the 1357–197 keV cascade rays from F nuclei, whose 197 keV intermediate state with nuclear spin of I 1⁄4 5=2 has a half-life of 89.3 ns. In the present work, the time evolution of the angular correlations of the cascade rays was observed at =2 and directions. In order to obtain better counting statistics, we used sixteen BaF2 detectors. They were arranged in four independent detector planes for the investigation of the sample-to-detector configuration dependence of perturbation patterns for a single-crystalline sample. All the events were taken in a list mode using a CAMAC system so that we could work on off-line data processing searching for the optimum conditions on energy gates for the detected rays of interest. The experimental details are described elsewhere. The TDPAC spectra of F( O) implanted in the HOPG sheet at room temperature are shown in Fig. 1 together with the previous data obtained at 18K. In the present experiment, we derive the time evolution of the angular correlation as a function of the time interval between the cascade -ray emissions, RðtÞ, from the following simple arithmetic operation: -0.2

Monte Carlo simulations of defect recovery within a 10 keV collision cascade in 3C–SiC

A kinetic lattice Monte Carlo (KLMC) model is developed to investigate the recovery and clustering of defects during annealing of a single 10 keV cascade in cubic silicon carbide. The 10 keV Si cascade is produced by molecular dynamics (MD), and a method of transferring the defects created by MD simulations to the KLMC model is developed. The KLMC model parameters are obtained from MD simulations and ab initio calculations of defect migration, recombination, and annihilation. The defects are annealed isothermally from 100 K to 1000 K in the KLMC model. Two distinct recovery stages for close Frenkel pairs are observed at about 200 and 550 K, and the growth of complex clusters is observed above 400 K. These simulation results are in good agreement with available experimental results.

The effect of electron–ion interactions on radiation damage simulations

Classical cascade simulations of radiation damage generally neglect the effect of energyexchange between the lattice and the electrons; however electronic effects increase withincreasing radiation energy. Indeed, even for low energy radiation events the electronscontribute to heat transport and increase the cooling rate, particularly in materials withstrong electron–ion interactions. We use a method described in an earlier publication toinclude these effects in a series of 10 keV cascades in Fe, for a range of electron–ioninteraction strengths. We find a non-monotonic relationship between the number of residualdefects and the strength of the electron–ion interactions and we discuss the mechanismsinvolved.

Temperature dependence of damage accumulation in α-zirconium

Using the input data obtained from molecular dynamics (MD) simulations on defect energetics and cascade damage, we present results obtained on irradiation of hexagonal-close-packed (hcp) α-zirconium under different conditions with a kinetic Monte Carlo (kMC) model. We used three 25 keV cascade databases at temperatures of 100 K, 300 K and 600 K respectively. The evolution of the microstructure during irradiation for a dose rate of 10 −6 dpa/s, at temperatures of 100 K, 300 K and 600 K until a final dose of 0.1 dpa has been studied. We have considered isotropic motion for vacancies and one dimensional movement for interstitials and we have studied how the accumulation of damage is affected considering different temperatures. We present preliminary comparisons with experimental data.