Thermoluminescence Peak Temperature Research Articles

Dy3+ doped Zn0.66Mg0.3Al2O4 nanophosphor for high dose radiation thermoluminescence dosimetric applications

Zinc Magnesium Aluminate doped with Dysprosium (Zn (1-x-y) Mg(y)Al2O4:x Dy (x=0.04 and y=0.3 mol%)) was prepared using the solution combustion synthesis method. The prepared phosphor was characterized using energy-dispersive X-ray spectroscopy, scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and Fourier transform infrared, respectively, to study crystal structure, surface morphology, and vibrational properties. The thermoluminescence peak temperatures were 250°C, 248°C, and 246°C at irradiating doses of 600–1000 Gy, respectively. Through the computation of the activation energy (E), order of kinetics (b), and frequency factor (s−1), the kinetic parameters were assessed using the TL glow curve. It is observed from the TL dose-response curve that the intensity rises nearly linearly for all doses between 600 and 1000 Gy. Nanophosphor Zn0.66Mg0.3Al2O4:0.04Dy shows almost linear dose-response for a dose range of 600–1000 Gy and an optimum activation energy (E) of 0.93–0.97 eV, which makes it suitable for high-dose radiation thermoluminescence dosimetric applications.

Exploring beta irradiation responses in Sr2+-Doped hydroxyapatite: A thermoluminescence study

This research investigates the structural and thermoluminescence (TL) properties of Sr2+-doped Ca5(PO4)3OH (HAp) phosphors. X-ray diffraction (XRD) analysis confirms that HAp has a hexagonal structure with a P63/m space group. The TL behaviour of HAp:Sr2+ phosphors is examined under beta irradiation in the temperature range of 25–500 °C. The study reveals three TL glow peaks at temperatures of 75, 212, and 296 °C. Investigation of TL responses across heating rates (HR) ranging from 0.1 to 5 °C/s shows that higher HRs result in increased maximum TL peak temperatures and reduced TL intensities. This trend suggests an augmented contribution from non-radiative transitions as the heating rate increases. Importantly, no evidence of thermal quenching is found in the HAp samples. Two different methods, Hoogenstraaten's and Booth-Bohun-Porfianovitch, yield consistent activation energy values of 0.50, 1.22, 1.67 eV, and 0.48, 1.19, 1.63 eV, respectively. Reusability assessments indicate a standard deviation of less than 5 %, confirming the reliability of the materials. Tm−Tstop analysis reveals a continuous distribution of trap levels. TL properties in HAp samples have been comprehensively characterized, contributing not only to enhancing our understanding of their behaviour, but also highlighting their potential applications in various areas. The findings of this study have provided valuable insight into the development of efficient catalysts that are suitable for a variety of applications.

Defect characterization in Bi12GeO20 single crystals by thermoluminescence

Bi12GeO20 single crystal grown by Czochralski method was investigated in terms of thermoluminescence (TL) properties. TL experiments were performed for various heating rates between 1 and 6 K/s in the temperature region of 300–675 K. One TL peak with peak maximum temperature of 557 K was observed in the TL spectrum as constant heating rate of 1 K/s was employed. Curve fitting, initial rise and variable heating rate methods were applied to calculate the activation energy of trap level corresponding to this TL peak. Analyses resulted in a presence of one trap center having mean activation energy of 0.78 eV. Heating rate characteristics of revealed trap center was also explored and theoretically well-known behavior that TL intensity decreases and peak maximum temperature increases with heating rates was observed for the trap level. Distribution of trapping levels was studied by thermally cleaning process for different Tstop between 425 and 525 K. Quasi-continuously distributed trapping levels were revealed with mean activation energies ranging from 0.78 to 1.26 eV. Moreover, absorption analysis revealed an optical transition taking place between a defect level and conduction band with an energy difference of 2.51 eV. These results are in good agreement for the presence of intrinsic defects above valence band in Bi12GeO20 crystals.

Radiation Induced Defects and Thermoluminescence Characteristics in Eu, Dy and Eu/Dy Doped-Quartz Sol-Gel by 2 Gy Beta and 2 MeV ⁴He⁺ Irradiations

Thermoluminescence (TL) of pure and Eu3+ and Dy3+ doped synthesis quartz was synthesized and their ion beam and thermoluminescence properties were investigated. The as prepared, doped and co-doped quartz and the effects of imparting 2 Gy beta dose and 2MeV 4He+ ion beam irradiation is investigated. The basic model proposed and can explain our observations is that, the dominant signals from the as prepared material arise from the incorporation of the transitions within the RE dopants enhanced by the intensity from the intrinsic or host defect sites within the synthesis quartz network. The complex shape TL glow curves indicate that irradiation causes major distortions to the lattice with the incorporation of extrinsic impurities and RE doping processes, induce perturbations and alter the energy levels pattern of the free ions and assigned transitions probabilities in a manner that that depends on the dopants, their concentrations and the host material. The larger Eu ions stabilize the emission more than that of the Dy ions. The TL peak temperatures are commonly correlated via charge transfer processes and scale with the ions size, in such a manner that the close proximity (or shallow traps) allows lower temperature electron release, whereas the more distant variants (deep traps) are less distorted, but are still able to couple to the higher energy orbitals of the Eu ions.

A method for routinely monitoring the reproducibility of thermal pretreatment prior to optically stimulated luminescence measurements

Thermal pretreatments are used prior to optically stimulated luminescence (OSL) measurements in methods of dose determination, but the reproducibility of such heating appears to be poor within laboratories, and between laboratories. Monitoring the position of the 110 °C thermoluminescence (TL) peak in quartz can be used to routinely assess the reproducibility of heating within a repeated set of measurements on an aliquot (e.g. during the single aliquot regenerative dose protocol) providing feedback on whether changes in sample preparation can improve reproducibility. Poor reproducibility between measurements from a single aliquot are likely to arise from grains between the hotplate and sample holder. Variability between aliquots may also be affected by deformed sample holders. Differences between readers may also be caused by differences in the calibration of the hotplate. Reproducibility of heating can be improved within runs, and between instruments, by taking care to avoid loose grains, by removing deformed sample holders, and if necessary by calibrating hotplates. It is suggested that individual aliquots should only be accepted if the apparent 110 °C TL peak temperature within a sequence of measurements varies by 5 °C or less. Improving the reproducibility of thermal treatment is important in reducing scatter in equivalent dose determinations based on OSL from quartz, but also for signals such as TT-OSL from quartz, and the various feldspar luminescence signals. Temperature control is also critical for accurate kinetic analyses.

A study of the kinetics of a high temperature thermoluminescence peak in annealed natural quartz

Thermoluminescence of a high temperature secondary glow-peak in natural quartz annealed at 900 °C is reported. The glow-curve of a sample irradiated to 10 Gy and measured at 1 °C/s shows three peaks; the main peak at 71 °C and two other weaker-intensity peaks at 125 °C and 177 °C. For reference, the peaks are labelled as I, II and III. This study is concerned with the secondary peak at 177 °C (peak III). The electron trap responsible for peak III is stable at ambient temperature as determined by monitoring the peak intensity after various delays between irradiation and measurement. The activation energy and frequency factor of the peak were estimated as ~1.24 eV and ~1012 s−1 respectively. The dose response of the peak in the range 1–300 Gy is sublinear. The influence of either partial heating or irradiation dose on the peak position suggest that the peak follows non-first-order kinetics.

Thermoluminescence study of beta irradiated Sr2CeO4:Eu3+ phosphor synthesized using solution-combustion process

Eu3+ doped strontium cerate (Sr2CeO4:Eu3+) phosphor was synthesized by solution - combustion technique using urea as a reducer at initiating temperature of 600 °C. The morphological, structural and thermoluminescence (TL) properties of Sr2CeO4:Eu3+ were investigated. X-ray diffraction patterns revealed the polycrystalline nature of Sr2CeO4:Eu3+ with single phase. The scanning electron microscopy photographs shows that the particle size distributions of all phosphor powders are rather broad with particle diameters in the range from ~ 50 nm to ~ 2 µm. The TL glow curve was found which composed of two well defined peaks at around 323 and 372 K. The electron-trapping properties in terms of TL glow curves are discussed in detail. The TL intensity was recorded for different beta doses and it is seen to increase with increasing doses. A linear behaviour of the TL intensity and peak temperature versus irradiation dose was found for dose up to 185 Gy. The effect of different heating rates was also studied and is seen to cause slight peak temperature shifts. Repeated measurements on the same sample seem not to have any effect on peak temperature positions and TL intensity indicating the sample was stable. Different kinetic parameters like activation energy (E), frequency factor (s) and geometrical factor (μg) were calculated for different TL glow curves. Trap depths were also calculated by different methods including initial rise, variable heating rate and peak shape methods.

Estimation of band gap of muscovite mineral using thermoluminescence (TL) analysis

In this article thermoluminescence (TL) mechanism in muscovite mineral has been reported in detail. The trap spectroscopy of the mineral has been studied from the TL glow curve of X-ray irradiated sample. A stable TL peak has been observed at around 347K in the glow curve, however after annealing the sample above 573K an additional peak is observed at around 408K. In the TL emission spectra an emission peak has been observed at around 447nm at TL peak maximum temperatures. The glow curves are analyzed by Tm−Tstop analysis, peak shape method and fractional glow technique. Analyses showed that there is a trap center and a radiative recombination center at depth around 0.71 and 2.78eV from the conduction band. These two localized centers do not affected by annealing up to 773K. However, annealing at 573K (or above) a new electron trap center was found to generate at depth around 1.23eV. This significant result has been confirmed with the help of phototransfer phenomena observed under UV irradiation. Due to UV irradiation on the excited sample, the transfer of trapped charges from the deeper trap level (1.23eV) to the shallow level (0.71eV) has been observed. Based on the analysis a schematic band diagram of muscovite crystal has been proposed. With the help of the band model, radiative as well as non-radiative recombination mechanisms have been discussed. The band gap of the material has been estimated to be around 5.09eV and verified.

Ion size effects on thermoluminescence of terbium and europium doped magnesium orthosilicate

Abstract

Effect of optical bleaching on the dosimetric TL peak in Al2O3:C for blue and UV emissions

Al2O3:C is now a well established thermoluminescence (TL) and optically stimulated luminescence (OSL) dosimeter for personnel, environment, space and medical applications. Its principle emissions are in blue (360–480 nm) and in UV (315–370 nm) regions arising from the F and F+ centers respectively. The present work is aimed to examine the effect of blue (470 nm) illumination on TL emission from main dosimetric trap (MDT) in both blue (384–445 nm) and UV (340–372 nm) spectral windows. On optical bleaching with blue light the TL peak temperature (Tm) is shifted to higher temperature side and full width at half maximum (FWHM) also increases, implying the MDT is not a single trap, rather it is composed of several discrete traps or distribution of traps. This accords with earlier published works. The Tm – Tstop method, and distribution of activation energies, measured using initial rise method suggest that the MDT is composed of distribution of traps or close overlapping of discrete traps. The low temperature part of the TL glow peak bleaches faster than the high temperature part of the TL peak, suggesting that the traps responsible for the low temperature part of the TL glow curve have higher photo-ionization cross-section for OSL. Both illumination and phototransferred TL (PTTL) experiments suggest a distribution of traps and increase of FWHM with Tm. Partial annealing experiments on continuous wave OSL (CW-OSL) signal suggests that the traps responsible for the lower temperature part of the TL peak are responsible for the fast component (higher photo-ionization cross-section). Quantitative analysis of components with different photo-ionization cross-section of the partially annealed OSL decay curves suggest that 220 °C preheating reduces the faster component by 100%, medium component by 60%, and the slower component remains practically unaffected. Similar behaviours are observed in both the emission windows.

Synthesis and thermoluminescence characterization of MgB4O7:Gd,Li

Magnesium tetraborate (MTB) doped with rare earth elements were synthesized by solid state sintering technique. Among the different rare earth dopants studied in this phosphor, gadolinium doped phosphors resulted in a single intense dosimetric peak at 250 °C and this is the first report in rare earth-doped MgB4O7 with a glow peak above 200 °C Photoluminescence (PL) and thermoluminescence (TL) studies were performed with this phosphor after exposing the powder samples to ionizing radiation. Monovalent dopants, including Na, Li and Ag, were found to increase the TL sensitivity of the MgB4O7:Gd phosphor without a shift in the TL peak temperature. The TL emission spectra showed characteristic emission of the host lattice, which showed an increase on doping with rare earth or monovalent codopants. The TL sensitivity, dose response curve, and post-irradiation storage stability were studied for the possible use of this material in radiation dosimetry applications. The TL parameters, such as the activation energy, the frequency factor, and the order of kinetics were determined for the Gd-doped MgB4O7 phosphor. The phosphor was found to be reusable after a few cycles of irradiation and annealing. The post-irradiation storage stability studies showed that this near tissue-equivalent phosphor, which has a gamma sensitivity five times that of TLD-100, is suitable for medical dosimetry applications.

Thermoluminescence in gamma irradiated iPP-VGVGCNF

Nanocomposites have been obtained by dispersing various amounts of vapor grown carbon nanofibers within isotactic polypropylene via melt mixing. The as obtained nanocomposites were gamma irradiated at various integral doses by using a 60Co source. The irradiation was performed in air, at room temperature, and at a dose rate of about 1kGy/h up to an integral dose of 28kGy. Thermoluminescence investigations revealed the role of carbon nanotubes in the radiation-induced modification of polymer-based nanocomposites. Almost all samples (pristine polymer and polymer-based nanocomposites irradiated and not irradiated) showed two overlapping thermoluminescence signals. It is concluded that the dispersion of carbon nanotubes decreases the amount of trapped electrons improving eventually the radiation stability of the polymeric matrix. The low temperature thermoluminescence peak was tentatively associated to melting phenomena. The origin of the high temperature thermoluminescence peak is more complex. Nevertheless, the experimental data suggests that this peak is sensitive to the polymer-filler interface.

Luminescence characteristics of minerals separated from irradiated onions during storage under different light conditions

The aim of this study was to determine the effects of light conditions during 2 years of storage on the luminescence characteristics of contaminating minerals, isolated from irradiated onions of 2 different origins. The potential use of photostimulated luminescence (PSL) as a screening and thermoluminescence (TL) as a confirmatory identification method was investigated during post-irradiation periods. Nonirradiated onions had 1,612 photon counts (PCs), However, the irradiated onions had much higher PCs (45,672–469,696, positive). The PCs of the irradiated onions decreased with storage time. However, all the irradiated onions had PCs with positive values (>5,000) even after 2 years of storage except onions stored under natural light. The decline in PCs because of light conditions during storage was in the order of sunlight, artificial light, and a darkroom, respectively. Minerals extracted from the nonirradiated samples exhibited TL glow curves of low intensities with maximum peak after 300°C. However, all irradiated samples had TL glow peaks in the temperature ranges of 185–225°C. The TL intensity and TL ratio of the irradiated samples decreased during storage with a slight shift in the TL peak temperature towards higher temperatures. The TL characteristics were most promising for samples stored under natural light conditions, however all the irradiated onions could be identified even after 2 years of storage.

Characterization of Antarctic micrometeorites by thermoluminescence

Abstract— In order to explore the nature and history of micrometeorites, we have measured the thermoluminescence (TL) properties of four micrometeorites, three cosmic spherules, and one irregular scoriaceous particle, that we found in a survey of 17 micrometeorites. These micrometeorites have TL sensitivities ranging from 0.017 ± 0.002 to 0.087 ± 0.009 (on a scale normalized to 4 mg of the H3.9 chondrite Dhajala). The four micrometeorites have very similar TL peak temperatures and TL peak widths, and these distinguish them from CI, most CM, CV, CO, and ordinary chondrites. However, the TL properties of these micrometeorites closely resemble those of the unusual CM chondrite MacAlpine Hills (MAC) 87300 and terrestrial forsterites. Heating experiments on submillimeter chips of a CM chondrite and a H5 chondrite suggest that these TL properties are have not been significantly affected by atmospheric passage. Thus we suggest that there is no simple linkage between these micrometeorites and the established meteorite classes, and that forsterite is an important component of these micrometeorites, as it is in many primitive solar system materials.

Thermoluminescence sensitivity and thermal history of type 3 ordinary chondrites: Eleven new type 3.0–3.1 chondrites and possible explanations for differences among H, L, and LL chondrites

Abstract— We review induced thermoluminescence (TL) data for 102 unequilibrated ordinary chondrites (UOCs), many data just published in abstracts, in order to identify particularly primitive UOCs and further explore TL systematics that may have implications for the history of the chondrites and their parent body. We have identified 11 UOCs of petrologic types 3.0–3.1: Adrar 003, Elephant Moraine (EET) 90066, EET 90161, Grosvenor Mountains (GRO) 95502, Lewis Cliff (LEW) 88477, Meteorite Hills (MET) 96503, Yamato (Y)‐790787, Y‐791324, Y‐791558, Y‐793565, and Y‐793596. These samples represent an important new resource for researchers interested in the nature of primitive solar system materials. Previously reported trends in which TL sensitivity increases with TL peak temperature and TL peak width, which we interpret in terms of crystallization of feldspar in the ordered or disordered forms during metamorphism, are confirmed by the new data. Importantly, the present data strengthen the trend described earlier in which the mean level of metamorphism experienced by UOCs increases along the series LL, L and H. This suggests either different burial depths for the UOCs from each class, or formation at similar depths in regoliths of different thickness.

Ca 2+ Function in Photosynthetic Oxygen Evolution Studied by Alkali Metal Cations Substitution

Effects of adding monovalent alkali metal cations to Ca 2+-depleted photosystem (PS)II membranes on the biochemical and spectroscopic properties of the oxygen-evolving complex were studied. The Ca 2+-dependent oxygen evolution was competitively inhibited by K +, Rb +, and Cs +, the ionic radii of which are larger than the radius of Ca 2+ but not inhibited significantly by Li + and Na +, the ionic radii of which are smaller than that of Ca 2+. Ca 2+-depleted membranes without metal cation supplementation showed normal S 2 multiline electron paramagnetic resonance (EPR) signal and an S 2Q A − thermoluminescence (TL) band with a normal peak temperature after illumination under conditions for single turnover of PSII. Membranes supplemented with Li + or Na + showed properties similar to those of the Ca 2+-depleted membranes, except for a small difference in the TL peak temperatures. The peak temperature of the TL band of membranes supplemented with K +, Rb +, or Cs + was elevated to ∼38°C which coincided with that of Y D +Q A − TL band, and no S 2 EPR signals were detected. The K +-induced high-temperature TL band and the S 2Q A − TL band were interconvertible by the addition of K + or Ca 2+ in the dark. Both the Ca 2+-depleted and the K +-substituted membranes showed the narrow EPR signal corresponding to the S 2Y Z + state at g = 2 by illuminating the membranes under multiple turnover conditions. These results indicate that the ionic radii of the cations occupying Ca 2+-binding site crucially affect the properties of the manganese cluster.

The induced thermoluminescence and thermal history of plagioclase feldspars

Feldspars are a common component in igneous and metamorphic rocks. Most feldspars exhibit luminescence, and this has proved useful in a number of mineralogical applications. In this paper, we concentrate on the thermoluminescence (TL) properties of feldspar, or the luminescence produced when a sample of feldspar is heated. We determined the induced TL properties of four feldspars of various compositions in their natural states, and after heating, and we compared the TL data with structural changes as determined by X-ray diffraction. The major TL peak at 120–240 °C in the TL glow curve, a plot of light intensity against temperature, varies significantly among feldspar-bearing samples. Meteorites and lunar samples with slow cooling histories of ~10 °C/My as determined by independent methods, have induced TL peak temperatures of ~120 °C, while samples with fast cooling histories (~100 °C/My) have induced TL peak temperatures of ~220 °C. This variation in TL peak temperature can be reproduced by heating the present feldspar samples, meteorites and lunar samples prior to the TL measurement. Most of the present samples in their natural state had TL peak temperatures of ~120 °C. Heating below 750 °C in the laboratory caused no change in TL peak temperatures or the structural disorder of the feldspar, while heating > 750 °C caused TL peak temperatures to move to ~220 °C and disordered the feldspar structure. We suggest that induced TL peak temperature in feldspar is influenced by the degree of Al-Si ordering in the feldspar. Thus, induced TL peak temperature can be used as an indicator of cooling rate for igneous and metamorphic rocks.

INFLUENCE OF Gd3+ AND Y3+-DOPING ON LOW TEMPERATURE THERMOLUMINESCENCE OF Pb WO_4

The influence of Gd3+ and Y3+ doping on the low temperature thermoluminescence (TL) of PbWO4(PWO) is investigated. In the case of polycrystal powder, the TL above 180K is eliminated by doping Y3+ or Gd3+ ions. Meanwhile, a new TL peak at lower temperature appeare. For the PWO single crystal there are two TL peaks at about 130 and 253K, but only one single peak at about 130K for PWO:Y crystals. The common effect of doping trivalent ions is to decrease or eliminate the TL above 180K and create a new TL peak at lower temperature. It indicates that the traps responsible for TL above 180K are due to the intrinsic defects which probably are Pb3+ and O-. During the preparation of PWO, the evaporation of the PbO component is easier than that of WO3, which will lead to the deficiency of PbO. The deficiency of O ions can be compensated by annealing in air while the deficiency of Pb is permanent. Therefore in the volume surrounding the Pb vacancy, there is a lack of positive charge. So the charge deficiency in the Pb-sublattic can be most easily balanced by an increase of concentration of Pb3+ and O- Doping Gd3+ or Y3+ can decrease the concentration of Pb3+ and O-. Gd3+ or Y3+ can also play a role of electron traps, which create new TL peaks at 150 and 125K, respectively. The trap depth of Gd3+ and Y3+ are 0.32 and 0.29eV, respectively. The difference between doping Gd3+ and Y3+ can be explained by the difference between their coulomb potentials.

Determination of trapping parameters of the high temperature thermoluminescence peak in equilibrated ordinary chondrites

Most meteorites exhibit thermoluminescence (TL) that can be used to constrain their recent thermal and irradiation history, but quantitative conclusions require a knowledge of the detailed TL peak structure of the TL glow curve. We have determined TL peak parameters for the high temperature portion of the glow curve for six ordinary chondrites: Chicora (LL6); Innisfree (L5); Lost City (H5); Paragould (LL6); Pribram (H5); and Tilden (L6). The saturation dose for all these meteorites is approximately 3600 Gy. Published procedures were used to determine the number and temperatures of peaks in the high temperature (>570 K) portion of the glow curve and peak fitting was used to estimate TL trap parameters for each peak. These data were then tested and adjusted, if necessary, by comparing calculated decay results with TL glow curves for samples heated at ≈420 K for various times. We find evidence for four TL peaks in the high temperature portion of the glow curve, where trapping parameters vary slightly from meteorite to meteorite. For the Lost City meteorite, the TL peak temperatures (K), activation energies (E, eV), and Arrhenius factors ( s, × 10 −9 s −1) are: 325, 1.26, 4.8; 360, 1.33, 3.88; 401, 1.44, 5.8; and 455, 1.5, 2.25, respectively. These data could be used to estimate dose rates for meteorites; however, the albedo values required for the calculation are not yet sufficiently known. However, terrestrial ages, or surface exposure ages, for meteorite finds from hot deserts like those in Australia or North Africa, can be estimated from these data.

The metamorphic history of eucrites and eucrite‐related meteorites and the case for late metamorphism

Abstract— We report induced thermoluminescence (TL) data for separates from three howardite, eucrite and diogenite (HED) meteorites and the Vaca Muerta mesosiderite. The results of thermal modeling of the surface of their parent body are also described. The TL sensitivities for matrix samples from the LEW 85300, 302 and 303 paired eucrites and the Bholghati howardite are lower than the TL sensitivities for the clasts, which is consistent with regolith working of the matrix in fairly mature regoliths. Within an isochemical series of HED meteorites, TL sensitivity reflects metamorphic intensity, but clast‐to‐clast variations in the TL sensitivities of the Vaca Muerta mesosiderite and clasts in the EET 87509, 513 and 531 paired howardite primarily reflect differences in mineralogy and petrology. Thermoluminescence peak temperatures indicate that all the components from the LEW 85300, 302 and 303 paired eucrites experienced a reheating event involving temperatures >800 °C, which is thought to have been due to impact heating, and therefore that the event was concurrent with or postdated brecciation. The Vaca Muerta clasts are essentially unmetamorphosed, but the induced TL data indicate that the remaining howardite, eucrite, dioenite and mesosiderite (HEDM) meteorites experienced metamorphism to a variety of intensities but involving temperatures <800 °C. Laboratory heating experiments show that temperatures >800 °C cause a change in TL peak temperature. Feldspars from a variety of terrestrial and extraterrestrial sources show this behavior, and x‐ray diffraction and kinetic studies suggest that it is indirectly related to Al, Si disordering.Cooling rates are not consistent with autometamorphism following the initial igneous event or with heating by subsequent eruptions of lava onto the surface of the HED parent body. Instead, our thermal models suggest that the metamorphism occurred within a regolith ejecta blanket of up to a few kilometers thick, with different levels of metamorphism corresponding to different thicknesses of blanket, between essentially 0 and ∼2 km, rather than different burial depths in a regolith of uniform thickness. We argue that metamorphism occurred 3.9 Ga ago and was associated with the resetting of the Ar‐Ar system for the HED meteorites.