Thermal annealing of natural, radiation-damaged pyrochlore

Abstract

Radiation damage in minerals is caused by the α-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400-1000 K) on radiation-damaged pyrochlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1 · 10 18 α-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6 · 10 18 dpg), and Blue River/Canada (10.5 wt% U, 115.4 · 10 18 dpg), are compared with a crystalline reference pyrochlore from Schelingen (Germany). The type of structural recovery depends on the initial degree of radiation damage (Panda Hill 28 %, Blue River 85 % and Miass 100 % according to XRD), as the recrystallization temperature increases with increasing degree of amorphization. Raman spectra indicate reordering on the local scale during annealing-induced recrystallization. As Raman modes around 800 cm −1 are sensitive to radiation damage . The most radiation damaged pyrochlore (Miass) shows an abrupt recovery of both, its short-(Raman) and long-range order (X-ray) between 800 and 850 K, while the weakly damaged pyrochlore (Panda Hill) begins to recover at considerably lower temperatures (near 500 K), extending over a temperature range of ca. 300 K, up to 800 K (Raman). The pyrochlore from Blue River shows in its initial state an amorphous X-ray diffraction pattern superimposed by weak Bragg-maxima that indicates the existence of ordered regions in a damaged matrix. In contrast to the other studied pyrochlores, Raman spectra of the Blue River sample show the appearance of local modes above 560 K between 700 and 800 cm −1 resulting from its high content of U and Ta impurities. DSC measurements confirmed the observed structural recovery upon annealing. While the annealing-induced ordering of Panda Hill begins at a lower temperature (ca. 500 K) the recovery of the highly-damaged pyrochlore from Miass occurs at 800 K. The Blue-River pyrochlore shows a multi-step recovery which is similarly seen by XRD. Thermogravimetry showed a continuous mass loss on heating for all radiation-damaged pyrochlores (Panda Hill ca. 1 %, Blue River ca. 1.5 %, Miass ca. 2.9 %).

Irradiation effects in monazite–(Ce) and zircon: Raman and photoluminescence study of Au-irradiated FIB foils

“…In recent years, confocal spectroscopic techniques with laser excitation, including Raman (e.g., Guenthner et al 2013 ; Wang et al 2014 ; Švecová et al 2016 ; Marillo-Sialer et al 2016 ; Baughman et al 2017 ; Váczi and Nasdala 2017 ; Zietlow et al 2017 ) and photoluminescence (PL; e.g., Panczer et al 2012 ; Nasdala et al 2013b ; Shimizu and Ogasawara 2014 ; Lenz and Nasdala 2015 ), are applied increasingly to estimate the degree of irradiation damage in minerals. The use of these techniques is favoured by a number of analytical advantages, including the opportunity to perform analyses non-destructively and without the need for special sample preparation, and a spatial resolution on the µm 3 range.…”

Mechanical and Structural Response of Radiation-Damaged Pyrochlore to Thermal Annealing

“…The selected and in the following described radiation-damaged pyrochlore samples (referred to as Panda Hill, Blue River and Miass) have been characterized in detail (origin, chemical composition, thermal behavior, suffered radiation dose and amorphous fraction) by Zietlow et al [51] .…”

“…The -decay g − 1 and an amorphous fraction of ~85%. This sample is expected to be initially already partially recrystallized [51] .…”

“…We undertook this study to shed light on the mechanical behavior of pyrochlore to help assess its suitability as a possible nuclear waste matrix. Nanoindentation enables the determination of the hardness and elastic modulus of three natural uranium and/or thorium containing pyrochlore samples, with different degrees of disorder, before and after step-wise annealing up to 900 K (not higher to avoid the increased formation of new phases [51] ). Complementary investigations by photoluminescence and Raman spectroscopy and in-situ annealing transmission electron microscopy enable us to better understand the local structural evolution and its mesoscopic and macroscopic physical consequences.…”

The Quantification of Radiation Damage in Orthophosphates Using Confocal μ-Luminescence Spectroscopy of Nd3+

“…Rutherford backscattering and channeling (RBS/C) experiments give detailed information on the amount of displaced atoms, but are restricted to the characterization of radiation-damage accumulation in ion-irradiation experiments as non-damaged, crystalline starting material is needed for a quantitative interpretation (e.g., Thomé et al, 2012). In recent years, confocal spectroscopic techniques that operate on the micrometer scale, including Raman (e.g., Nasdala et al, 1995, 2010; Geisler et al, 2001; Picot et al, 2008; Shimizu and Ogasawara, 2014; Wang et al, 2014; Marillo-Sialer et al, 2016; Švecová et al, 2016; Baughman et al, 2017; Váczi and Nasdala, 2017; Zietlow et al, 2017) and luminescence spectroscopy (Panczer et al, 2012; Nasdala et al, 2013, 2018; Lenz and Nasdala, 2015) are applied frequently to characterize and estimate the degree of irradiation damage in orthophosphates and other accessory minerals. The latter techniques are based upon a correlation of spectral changes with structural modifications (i.e., band width broadening of narrow peaks) associated with radiation damage accumulation.…”