Zonation due to alpha-decay damage in a natural single crystal of zircon from Sri Lanka is discussed. The zones vary in thickness on a scale from one to hundreds of microns. The uranium and thorium concentrations vary from zone to zone such that the alpha decay dose is between 0.2 x 10 to the 16th and 0.8 x 10 to the 16th alpha-events per milligram. The transition from the crystalline to the aperiodic metamict state occurs over this dose range. At doses greater than 0.8 x 10 to the 16th alpha events/mg there is no evidence for long-range order. This type of damage will accumulate in actinide-bearing, ceramic nuclear waste forms. The systematic pattern of fractures would occur in crystalline phases that are zoned with respect to actinide radionuclides.

https://science.sciencemag.org/content/sci/236/4808/1556.full.pdf

Alpha-decay--induced fracturing in zircon: the transition from the crystalline to the metamict state.

We discuss molecular-dynamics simulations of ion damage in silicon, with emphasis on the effects of ion mass and energy. We employ the Stillinger-Weber potential for silicon, suitably modified to account for high-energy collisions between dopant-silicon and silicon-silicon pairs. The computational cells contain up to ${10}^{6}$ atoms and these are bombarded by B and As atoms at incident energies from 1 keV up to 15 keV. We show that the displacement cascade results in the production of amorphous pockets as well as isolated point defects and small clusters with populations which have a strong dependence on ion mass and a weaker relationship to the ion energy. We show that the total number of displaced atoms agrees with the predictions of binary collision calculations for low-mass ions, but is a factor of 2 larger for heavy-ion masses. We compare the simulations to experiments and show that our results provide a clear and consistent physical picture of damage production in silicon under ion bombardment. We studied the stability of the damage produced by heavy ions at different temperatures and the nature of the recrystallization mechanism. The inhomogeneous nature of the damage makes the characterization of the process through a single activation energy very difficult. An effective activation energy is found depending on the pocket size. We discuss our results considering the Spaepen-Turnbull recrystallization model for an amorphous-crystalline planar interface. \textcopyright{} 1996 The American Physical Society.

https://www.ele.uva.es/~mmm/papers/1996_Caturla_PRB_54.pdf

Ion-beam processing of silicon at keV energies: A molecular-dynamics study.

Pre-amorphization damage in ion-implanted silicon

Recently, the words “metamict” or “metamictization” have been increasingly used instead of “amorphous” or “amorphization” in acknowledgment of the fact that the first recognized example of the transition from the crystalline to the aperiodic state was in natural materials—minerals. The term (originally “metamikte” from Greek for “mix otherwise” because of their complex compositions) was first defined by Broegger in a Danish encyclopedia as one of three classes of amorphous substances: porodine (= colloidal), hyaline, and metamikte. Minerals considered to be metamict were judged amorphous because of their conchoidal fracture and isotropic optical properties; however, well-developed crystal faces evidenced the prior crystalline state.Hamberg was the first to suggest that metamictization is a radiation-induced, periodic-to-aperiodic phase transition caused by alpha particles which originate from constituent radionuclides in the uranium and thorium decay series. Rinne and Vegard confirmed by x-ray diffraction studies that metamict minerals were either amorphous or finely crystalline. Later work supported the idea of a radiation-induced transformation. The work of Stackelberg and Rottenbach established that the decrease in density, refractive indices, and birefringence correlated with the breakdown of the structure with increasing alpha-decay dose. Stackelberg and Rottenback tried to test this hypothesis directly by bombarding a thin slab of zircon with alpha particles. The results were inconclusive because the slab fractured, but this must have been one of the first experiments in which an “ion beam” was used to “modify” a material.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400067865

The Metamict State

The dependence of the damage produced by self‐ion implantation in Si on dose is determined and is shown to exhibit two distinct behaviors: an initial sublinear increase of damage with dose, followed by a period of greatly accelerated growth. Ion backscattering analysis using both single‐ and double‐alignment channeling measurements is used to determine the distribution of damage in the samples. The nature of the damage is determined from its thermal annealing behavior and differences in the spectra recorded in the two channeling configurations. Damage is found to consist predominantly of two components: simple defects, such as divacancies, and regions of amorphous Si. The behavior of these components is shown to be divergent at the fluence which separates the two different growth regimes. A model is proposed which considers the amorphization process in Si as a critical‐point phenomenon, one in which the onset of amorphization leads to a cooperative behavior among the various types of damage resulting in a greatly accelerated transition.

New model for damage accumulation in Si during self‐ion irradiation

Atomic structure of collision cascades in ion-implanted silicon and channeling effects

The dependence of near-surface crystallinity on substrate heating during high dose O/sup +/ implantation into Si and post implantation annealing was studied. It is shown that the damage morphology in the crystalline region above the SiO/sub 2/ layer is very sensitive to substrate temperature and that, under optimum implant conditions which minimize damage conditons in the near surface, annealing does not improve the near surface crystallinity. Rutherford backscattering/channeling spectroscopy and cross-sectional transmission electron microscopy were used to analyze the microstructure of the samples. 8 references, 5 figures.

Influence of substrate heating and thermal annealing on the surface crystallinity in single Si crystals implanted with high doses of 16O

Mechanisms for damage nucleation in Si during heavy ion irradiation are discussed. The influence of dose rate on the accumulation of damage is shown to be dependent upon the mass of the implanted ions. For a constant fluence, it was found that damage increases with dose rate for all ions studied, although the effect was consistently smaller as the mass of the ion was increased. This systematic change is interpreted to indicate the increasing importance of cascade effects on damage nucleation as the mass of the ion is increased. However, the large influence of dose rate for lighter ions is an indication of a different mechanism for damage nucleation, one which does not depend on cascade effects and is more homogeneous in nature.

O.W. Holland

Papers

Atomic structure of collision cascades in ion-implanted silicon and channeling effects

Influence of substrate heating and thermal annealing on the surface crystallinity in single Si crystals implanted with high doses of 16O

Dose rate effects in silicon during heavy ion irradiation