Induced Track Density Research Articles

Chronological and chemical approaches to obsidians from Bakla Tepe and Liman Tepe, Western Anatolia

In this study, the provenance of 42 obsidians from the Chalcolithic and Early Bronze Age levels of two settlements – Bakla Tepe and Liman Tepe – located on the Aegean coast of Anatolia were investigated with an interdisciplinary approach using fission-track (FT) dating, Instrumental Neutron Activation Analysis (INAA) and Epithermal Neutron Activation Analysis (ENAA). Some artefacts showed FT ages of a few thousand years. Apparent FT ages of the remaining samples are distributed over a wide range, from 0.53 ± 0.03 to 1.43 ± 0.20 Ma. After application of the size-correction method, most artefacts were distributed in a homogeneous group characterized by FT ages varying from 1.48 ± 0.47 to 1.80 ± 0.20 Ma, with a mean value of 1.65 ± 0.05 Ma and low induced track density corresponding to low U content. The remaining 3 samples showed relatively high induced track densities. One of them has an apparent age of 0.53 ± 0.03 Ma and a size-corrected age of 1.02 ± 0.07 Ma. The probable potential sources for the studied samples were identified as the island of Melos in the Aegean, and the central Anatolian sources – particularly the Göllüdağ complex – through comparison of the FT data. INAA and ENAA studies have been carried out on 34 artefacts at the TRIGA Mark II research reactor of the University of Pavia. The identification of the sources was attained through cluster analysis of the chemical data. These results agree fully with those obtained by FT dating: most artefacts originated from the Melos–Dhemenegaki flow, and only 3 samples from central Anatolia. The current study provides a contribution to a better understanding of the circulation of obsidians in Anatolia.

Standardless fission-track dating of the Durango apatite age standard

Five dating strategies were used for determining the standardless fission-track age of the Durango apatite. These use the same fossil-track densities but differ in the manner in which the induced-track densities are determined. A conventional age calculation, without correction for experimental factors, gives inconsistent ages with method-related differences >15%. Correcting for these factors brings the ages in line with each other and with the reference age but leaves no room for a partial-annealing correction based on the confined-track lengths. Three further reasons suggest that a length correction is not appropriate. (1) The evidence for length-based corrections is inconclusive. (2) The plateau age of the Durango apatite is consistent with its apparent fission-track age to within 1%. (3) The calculated effective etchable length of the fossil tracks agrees within error with that of the induced tracks; both are further consistent with the measured mean length of confined induced tracks. The circumstance that the (U,Th)/He ages of the accepted and proposed apatite age standards are consistent with their reference ages leaves no margin for a lowered fission-track age resulting from partial annealing, although the case of the Durango apatite itself is inconclusive because of its exceptional crystal size. It is conjectured that the shortening of the fossil tacks in the Durango apatite is due to a lowering of the track etch rate over time. In this case, annealing equations fitted to induced-track data underestimate the extent of confined-track-length reduction in geological samples.

ジルコンＦＴ年代測定における試料評価と測定値の解釈

How to deal with and to evaluate fission track data of zircons from the point of view of users were presented. Analytical diagrams including a grain-age histogram, a correlation between spontaneous and induced track counts, a correlation between spontaneous and induced track densities, an age spectra, a radial plot and a uranium-concentration histogram, were explained to read grain data. Grain-age distributions were simulated for a sample with an identical age but various conditions of track counts per grain. Data from about 2, 500 samples previously determined were presented and categorized for assessment of unknown samples. These are important as a guide to evaluate fission track data with relatively large uncertainties. Fission track dating of zircon is performed as follows: 1) assessment of a sample, 2) dating and analysis of grain data, and 3) interpretation of ages. For sample assessment, it is important that grouping of zircons in the sample based on their appearances such as color, morphological features and fission track densities on external surfaces. For interpretation of fission track ages, thermal history analysis with track length distributions is useful. Crosscheck of fission track ages between external and internal surfaces of zircon is also important. For examples, three samples from deep boreholes of the Niigata sedimentary basin were investigated and the meaning of zircon ages was discussed.

Neutron dosimetry and fission-track age calibration: insights from intercalibration of uranium and thorium glass dosimeters

Three glass dosimeters possessing various U isotopic and Th/U ratios, i.e. NBS-SRM612 (612), Corning® 1 (CN1) and Corning® 3 (CN3), were intercalibrated at three reactor facilities of different neutron thermalisation, i.e. KUR-TCPn, KUR-Pn2 and MITR-IP facilities, to assess the influence of fast neutron-induced fissions of 238U and 232Th on fission-track (FT) age calibration. The induced track density ratios for two glass couples, ϱ CN3 ϱ CN1 and ϱ 612 ϱ CN1 , show systematic, negative correlation with increasing thermal to fast neutron flux ratios, Φ th Φ f . These facts demonstrate the significant contribution of 238U and 232Th fissions to observed induced track density at less thermalised facilities wherein Φ th Φ f is ∼ 10. For the quantitative assessment of 238U and 232Th fissions, the intercalibration concept was examined mathematically and thus their generalised descriptions were established. On the basis of the ideal ϱ CN3 ϱ CN1 ratio determined at a highly thermalised facility, the magnitude of influence of 232Th fission (or that of 238U fission) is accordingly given by the measured ϱ CN3 ϱ CN1 at each facility and Th/U (or 238 U 235 U ) ratio of an unknown sample. These results elucidate that in the context of FT age calibration, the criterion of “well-thermalised” neutron flux is controlled by the fluence monitor used, Th/U ratio of a sample to be dated and the minimum error allowed for analysis. Various constraints, implications and recommendations on neutron dosimetry and FT age calibration are discussed.

スフェーンを用いたフィッショントラック法による年代測定 塙深成岩体の例

Fission track age of sphene of five samples from the Hanawa pluton near the Tanakura Shear Zone, northeastern Honsyu, Japan was able to be measured by grain-by-grain method, although the number of spontaneous track (density) of sphenes is variable among rock samples and sphene crystals capable of measurement are a little among mineral grains of a sample. The ages were determined comparing with age standard sample (M. D. C., 98.7 Ma). Mineral grains were burned in a tephron resin was etched by 50M(1 M=1 mol dm-3) NaOH(130°C) after. The spontaneous track density was measured on the polished surface and the induced track density on the mica detecter after the thermal neutron irradiation. Since the tracks induced by 232Th and 238U occupy considerable amounts on the irradiation at a reacter with a low Cd ratio, age data were corrected by Th/U ratio. Th and U contents in sphene measured by NAA range from several ten to several hundred ppm corresponding to 0.089-4.0 in their ratio (Th/U). As a results of the correction by Th/U, the age data became 2% younger than before. The ages of the mylonite-like subfacies, large sphene bearing subfacies, and fine-grained quartz diorite-tonalite facies are 67.4, 91.9 and 92.5, 92.5 and 93.1 Ma respectively. The K-Ar ages of hornblendes in large sphene-bearing subfacies are 10% younger than those of the fine-grained quartz diorite-tonalite facies (Shibata and Uchiumi, 1983), while F. T. age of sphene of both facies are consistent. This is caused by the gap of the closure temperature in both methods. More younger age (67 Ma) of the mylonite-like subfacies is probably due to shear heating effect along the Tanakura Shear Zone.

Intercalibration of thermal neutron dosimeter glasses NBS-SRM612 and corning 1 in some irradiation facilities: A comparison

Abstract Thermal neutron dosimeter glasses NBS-SRM612 (SRM612) and Corning 1 (CN1) were intercalibrated using three irradiation facilities to reveal the relationship between the intercalibration factor of CN1 and SRM612 and the degree of thermalisation of neutron flux. Three facilities utilized for the intercalibration are Pneumatic Tube-2 (Pn-2) and Graphite Pneumatic Tube (Gr) of Kyoto University Research Reactor KUR-1 and Irradiation Pit (IP) of Musashi Institute of Technology TRIGA II reactor. Induced track density ratios ρ cn1 /ρ srm612 were repeatedly determined in each facility and the weighted mean values of ratios were calculated as 2.88 ± 0.06 (2σ) for Pn-2, 3.08 ± 0.06 for Gr, and 2.98± 0.05 for IP, showing a significant discrepancy of values between Pn-2 and Gr. In conjunction with previously reported data, ρ cn1 /ρ srm612 ratio is found to be closely related to the Cd ratio, an indicator of the degree of thermalisation. The facility of a higher Cd ratio yields a higher ρ cn1 /ρ srm612 ratio, which in fact suggests the significant effect of the 238 U and 232 Th fission induced by epithermal and fast neutrons on induced track density in less thermalised facilities.

Contribution to statistics in fission track counting

Abstract In order to test the new statistical model proposed in two papers by McGee, Johnson and Naeser for calculating the standard error in fission track dating, spontaneous and induced track counts from external detector method—EDM—or similar were simulated by random numbers, assuming that, for a given uranium content, fission tracks were spatially distributed according to Poisson distributions. By the results of such a simulation it can be concluded that: 1. (1) In EDM, (1/nS + 1/nI) 1 2 represents a reliable evaluation of the relative standard error of ρS/ ρI — ratio of spontaneous to induced track densities in a sample in which nS spontaneous and nI induced tracks were counted. 2. (2) The new model confirms the validity of the above conclusion, by applying it to the spontaneous and induced track counts whose relative standard deviations of the means were evaluated by normal sampling statistics. Population method—PM—data were also simulated; ( σ '2S + σ '2I) 1 2 , where σ ' S and σ 'I are the above relative standard deviations of the mean, offers a reliable evaluation of the uncertainty of ρS/ρI ratio for the simple cases analyzed in the present work.

A practical method of estimating standard error of age in the fission track dating method

A first-order approximation formula for the propagation of error in the fission track age equation is given by P A = C[P 2 s+P 2 i+P 2 φ−2rP sP i] 1 2 , where P A , P s , P i and P φ are the percentage error of age, of spontaneous track density, of induced track density, and of neutron dose, respectively, and C is a constant. The correlation, r, between spontaneous are induced track densities is a crucial element in the error analysis, acting generally to improve the standard error of age. In addition, the correlation parameter r is instrumental is specifying the level of neutron dose, a controlled variable, which will minimize the standard error of age. The results from the approximation equation agree closely with the results from an independent statistical model for the propagation of errors in the fission-track dating method.

Fission Track Dating of Archaeological Materials from Japan

THE method of dating by fission tracks1 has been used by several workers to date rock minerals and baked relics2. The method depends on the spontaneous fission of 238U atoms in mineral or glass taking place at a constant rate, and leaving fission tracks. Once formed, the fission tracks disappear if the material is heated above a critical temperature. The fission track age, T (yr), can be represented by the following equation3 where ρs is the fossil fission track density (cm−2), ρi is the induced track density by bombardment with thermal neutrons (cm−2), λ, is the total decay constant for uranium (yr−1), λf is the fission decay constant for 238U (we use 6.85 × 10−17 yr−1 (ref. 4), Φ is the thermal neutron dose (cm−2), σ is the thermal neutron cross section for fission 235U (cm−2), η) is the isotope ratio 235U/238U, and Rs and Rt are the mean values of the etchable length of the fragments produced by a spontaneous fission and that produced by the induced fission (cm).