Recoil Cascades Research Articles

Photocurrent From Single Collision 14-MeV Neutrons in GaN and GaAs

Accurate predictions of device performance in 14-MeV neutron environments rely upon understanding the recoil cascades that may be produced. Recoils from 14-MeV neutrons impinging on both gallium nitride (GaN) and gallium arsenide (GaAs) devices were modeled and compared to the recoil spectra of devices exposed to 14-MeV neutrons. Recoil spectra were generated using nuclear reaction modeling programs and converted into an ionizing energy loss (IEL) spectrum. We measured the recoil IEL spectra by capturing the photocurrent pulses produced by single neutron interactions with the device. Good agreement, with a factor of two, was found between the model and the experiment under strongly depleted conditions. However, this range of agreement between the model and the experiment decreased significantly when the bias was removed, indicating partial energy deposition due to cascades that escape the active volume of the device not captured by the model. Consistent event rates across multiple detectors confirm the reliability of our neutron recoil detection method.

Fabrication of large area nanogap electrodes for sensing applications

ABSTRACTA large area nanogap electrode fabrication method combinig conventional lithography patterning with the of focused ion beam (FIB) is presented. Lithography and a lift-off process were used to pattern 50 nm thick platinum pads having an area of 300μm × 300μm. A range of 30-300 nm wide nanogaps (length from 300μm to 10 mm ) were then etched using an FIB of Ga+at an acceleration voltage of 30 kV at various beam currents. An investigation of Ga+beam current ranging between 1-50 pA was undertaken to optimise the process for the current fabrication method. In this study, we used Monte Carlo simulation to calculate the damage depth in various materials by the Ga+. Calculation of the recoil cascades of the substrate atoms are also presented. The nanogap electrodes fabricated in this study were found to have empty gap resistances exceeding several hundred MΩ. A comparison of the gap length versus electrical resistance on glass substrates is presented. The results thus outline some important issues in low-conductance measurements. The proposed nanogap fabrication method can be extended to various sensor applications, such as chemical sensing, that employ the nanogap platform. This method may be used as a prototype technique for large-scale fabrication due to its simple, fast and reliable features.

Structural changes in zirconolite under α-decay

We report the simulation of alpha recoil cascades in zirconolite (CaZrTi2O7) using the molecular dynamics method under a variety of simulation conditions. Interatomic potentials have been developed through fitting to crystalline and melted structures and their energy surfaces obtained from ab initio calculations. Single and double cascades have been modelled, showing that damage recovery in damaged crystalline material is greatly impaired when there are multiple events. The calcium ions are significantly easier to displace than other ions, which is consistent with a recent ab initio study of defect energies.

Topological connectivity analysis of accumulated radiation damage from multiple molecular dynamics recoil cascades

ABSTRACTThe results of sequential large-scale molecular dynamics (MD) simulations of radiation damage cascades in Gd2Ti2O7 and Gd2Zr2O7 are presented. Twelve alpha recoil cascades, each due to a recoil atom with 40 keV of kinetic energy, are performed in both materials and a stark contrast in behaviour observed. Topological connectivity analysis is used to analyse the structural evolution of the two systems. Our results provide important insight into accumulation of disorder in pyrochlore-structured ceramics.

Model of defect reactions and the influence of clustering in pulse-neutron-irradiated Si

Transient reactions among irradiation defects, dopants, impurities, and carriers in pulse-neutron-irradiated Si were modeled taking into account the clustering of the primal defects in recoil cascades. Continuum equations describing the diffusion, field drift, and reactions of relevant species were numerically solved for a submicrometer spherical volume, within which the starting radial distributions of defects could be varied in accord with the degree of clustering. The radial profiles corresponding to neutron irradiation were chosen through pair-correlation-function analysis of vacancy and interstitial distributions obtained from the binary-collision code MARLOWE, using a spectrum of primary recoil energies computed for a fast-burst fission reactor. Model predictions of transient behavior were compared with a variety of experimental results from irradiated bulk Si, solar cells, and bipolar-junction transistors. The influence of defect clustering during neutron bombardment was further distinguished through contrast with electron irradiation, where the primal point defects are more uniformly dispersed.

Low-energy primary knock on atom damage distributions near MeV proton beams focused to nanometre dimensions

In this preliminary study, the spatial extent of the defects introduced in Si by a normally incident 1 MeV H + nanobeam was investigated using a hybrid approach. First, the standard SRIM code was employed to calculate the angular and energy distributions of Primary Knock-on Atoms (PKA) using the Binary Collision Approximation (BCA). The long mean free path and the kinematics of scattering in a screened Coulomb potential resulted in an anisotropic PKA distribution that was mainly directed perpendicular to the primary ion trajectories. The PKA energy E dependence was close to 1/ E n with 1.5 < n < 2 such that the vast majority of PKAs had low energies (⩽40 eV) close to the threshold displacement energy. Subsequently, the low energy PKA data was used as input to a Molecular Dynamics (MD) code that was used to follow the recoil cascades. The force field resulting from Si–Si atom interactions was represented in the MD calculations by the Environmental-Dependant Interaction Potential (EDIP). Finally, the spatial distribution of Self Interstitial Atoms (SIA) and vacancies was characterised using the Pixel Mapping (PM) method. The results revealed that the low energy PKA’s resulted in localised damaged regions with Si Self-Interstitial Atoms (SIA) and associated vacancies concentrated within a few nm of the ion track. Although some clustering occurred, the majority of SIA were present as monomers. This is in agreement with previous work on low energy B + implanted Si. The contribution from the spatial distribution of displacements from high energy (>40 eV) PKAs was investigated using SRIM. It was found that within the maximum region of the SIA concentration (1–2 nm radius of the ion track) the contribution from high energy PKA’s is relatively unimportant (∼5%).

Monte Carlo simulation of two-dimensional implanted dopant distributions at mask edges

Two-dimensional implanted dopant distributions at mask edges are studied using the Monte Carlo code IMSIL. The models implemented in the program are reviewed. An empirical model of electronic stopping describes correctly the range of channeled B, P, and As ions in a wide energy range. The damage model takes defect recombination into account but does not require the simulation of recoil cascades. Two-dimensional dopant distributions are calculated by randomly selecting the starting points of the ions between two positions defining a mask opening. The simulation results show that the penetration below the mask is larger than expected and that a Gaussian function is inappropriate to describe the lateral distribution function. The discrepancy increases with decreasing implantation energy. The dependence of the two-dimensional profiles on mask edge orientation, tilt angle, and ion species, and the influence of a screening oxide are investigated.

Comparison of Damage Accumulation Models for Boron Implantation in Silicon

ABSTRACTTwo models of damage accumulation during boron implantation are compared: The first one analyzes the full collision cascades and lets vacancies and interstitials recombine, if they are located within some capture radius of each other. The second one uses a constant fraction of defects surviving damage annihilation within a recoil cascade and a damage saturation density to take into account recombination with defects generated in previous cascades. While the first model is more fundamental, the second one is computationally more efficient. By comparing model predictions with 20 keV boron implantations at various doses, performed into (100) and (110) silicon with 7° and 0°, respectively, we conclude that different capture radii have to be used for damage annihilation within the recoil cascades and with defects generated in previous cascades. Moreover, we show that the two models are almost equivalent, if appropriate parameters are chosen. Recombination factors determined from simulations using a capture radius are almost independent of depth and implantation energy.

Surface sputtering from cold dark matter interactions: proposed search for its diurnal modulation

Abstract Nuclear recoil cascades induced by Cold Dark Matter (CDM) elastic scattering can produce the ejection of target atoms from solid surfaces. We calculate the yield and energy distribution of these sputtered atoms in a variety of materials. These parameters would suffer a large diurnal modulation induced by the rotation of the Earth and its motion through the galactic halo. Schemes for the detection of this unique CDM signature are proposed.

Computer simulations of sputtering

Monte Carlo simulations have become a useful tool for studying ion radiation effects at or near surfaces or interfaces, such as sputtering, reflection, mixing, etc. The principal advantage of Monte Carlo calculations is that any physical process can be included directly. Also multielement and multilayer targets, even complex geometries, can be treated exactly in order to simulate realistic cases. The present paper will concentrate mainly on sputtering calculations with the Monte Carlo code TRIM, which treats ion and recoil transport in amorphous matter. It is based — as are analytic theories and most other Monte Carlo codes — on binary collisions with target atoms initially at rest. Over the past few years, the basic physical input has been greatly improved. Both the interatomic potentials and the electronic stopping powers proved to be of crucial importance even for the lowest energies occurring in recoil cascades. With the Kr-C or universal potential and the recent ZBL electronic stopping, which includes the Z oscillations, and a planar surface binding energy set equal to the heat of sublimation, realistic sputtering predictions could be obtained for most metals — without the use of any adjustable parameters.

Radiation re-solution of fission gas in uranium dioxide and carbide

Abstract Calculations have been performed to estimate the removal rate of fission gas atoms from bubbles due to collisions with energetic fission fragments and recoil cascades. The efficiency of this process was found to be higher than estimated earlier, but is still too low to be responsible for the experimental observations of fission gas bubble destruction during irradiation of oxide fuel. An irradiation experiment to investigate the interaction of fission spikes with free surfaces has enabled a simple theory to be developed which can explain the shrinkage of bubbles and pores by the surface relaxation of a shock wave produced by the passage of a fission fragment. This mechanism occurs in oxides but not carbides because of the faster dispersion of the fission fragment energy and provides the major reason for the difference in gas bubble distributions in oxide and carbide fuel. This process, however, does not remove gas atoms from the bubbles. Since high levels of apparently diffusive fission gas release are observed in oxides, the “effective solubility” of the fission gases required for this release must be sought in phenomena other than the fission spike.

Channeling measurement of irradiation-induced lattice-site change of oxygen atoms in niobium at 10 and 293 K.

We report the effects of ion irradiation on the lattice site of oxygen atoms dissolved in single-crystal niobium as observed by in situ 10- and 293-K angular-scan channeling-technique measurements, the irradiations at each temperature being of 750-keV ${\mathrm{H}}^{+}$, 2.0-MeV ${\mathrm{He}}^{+}$, and 2.0-MeV ${\mathrm{N}}^{+}$ ions to doses giving the same theoretical number of displacements per atom in the niobium depth region studied. The results show that the oxygen lattice site can be changed at 10 K by interaction between the oxygen atoms and the vacancy-containing regions of energetic recoil cascades, and at 293 K also by the trapping at the oxygen atoms of thermally migrating vacancies.

A theoretical treatment of cascade mixing in depth profiling by sputtering

An analytic expression for mixing in recoil cascades is discussed with particular emphasis on in-depth concentration profiling. Examples presented show that an asymmetric distortion is not necessarily a consequence of direct recoil implantation or an anisotropic velocity distribution.