Long-term Fading Research Articles

Long-Term Fading of Peak 5 in TLD-100 Using Computerised Glow Curve Deconvolution (CGCD) and Thermal Peak Isolation Techniques

Abstract The long-term fading of peak 5 has been studied using four different annealing regimes. Following 400 oC for one hour annealing and 'natural cooling', peaks 4 and 5 (separated via (CGCD) appear to have an exquisitely complex time-dependent response, possibly due to clustering of dipole trapping centres associated with peaks 2 and 3. Peak 4 grows over the first nine months of storage and only then begins to decay: peak 5, on the other hand, decays rapidly in the first six months and then stabilises or even begins to glow. Peaks (4+5) together, however, fade in a montonically decreasing manner at a rate of approximately 8% per annum. Using thermal cleaning procedures to isolate the behaviour of peak 5 from its low temperature satellites, it has been established that the 20 oC mean-life of peak 5 is 5.5±1 year (1 SD). This mean-life is many orders of magnitude smaller than predicted by any of the kinetic measurements of s and E and suggests that precipitation of Mg2+ may be the major contributor to the decay of peak 5 over long storage intervals.

Thermoluminescence dating of the holsteinian marine formation of Herzeele, northern France

AbstractThermoluminescence dating of potassium feldspar coarse grains from the fossiliferous Holsteinian marine formation of Herzeele, at the southern limit of the North Sea Basin, yields a minimum age of 228 £ 30 ka. A tentative correction for the long‐term fading of the TL signal in potassium feldspar is proposed, assuming a lifetime (τ) of 711 ka. This correction would yield a preliminary age estimate of 271 £ 36 ka. These results support a minimum Oxygen Isotope Stage 7 age, but do not preclude, at this stage of our research, a possible correlation with late Oxygen Isotope Stage 9. These results are in agreement, within the error of the method, with independent absolute dates (U‐Th and ESR) obtained on in situ marine shells, which range from 225 ka to 348 ka. This study provides evidence of the reliability of TL age estimates on potassium feldspar coarse grains from shallow‐marine sediments older than the Last Interglacial.

Long-Term Fading Study of the Cooled Optically Stimulated Luminescence in CaF2:Mn

Long-Term Fading Study of the Cooled Optically Stimulated Luminescence in CaF₂:Mn

Neutron Dosimetry with CR-39 Track Detectors using Electrochemical Etching: Recent Improvements, Dosimetric Characteristics and Aspects of Routine Application

Recent progress in improving the dosimetric properties of CR-39 etched track detectors is reviewed. With respect to the two-step electrochemical etching (ECE) technique the effects of electrical field strength and various radiator/absorber combinations on the neutron energy response were investigated in the energy range 144 keV to 14 MeV, showing agreement between experimental and theoretical results for the neutron response, if appropriate field strength dependent critical proton angles are used for calculation. Results of international intercomparison experiments show that with respect to energy response the ECE technique has already reached a certain level of optimisation. The energy response, angular dependence, response of CR-39 in contact with (n,a) radiators, dose range and lowest detectable dose are discussed. The results of long-term fading and changes in response and background are presented for commercial CR-39 materials manufactured by American Acrylics and Pershore Mouldings, which were stored over more than one year in air or nitrogen. The high scatter in response and background within the sheet and batch production, and thus the lowest detectable dose and the relatively small dose range, still limit the application of CR-39 in personnel monitoring.

The problem of long-term fading of absorbed palaeodose on ESR-dating of quaternary mollusc shells

Due to relatively low thermal stability of radiation-induced centres in biogenic carbonate of subfossil molluscs (mean life θ = 3.7 × 10 5 y at 10°C, g = 2.0012) correct dating of the latter by means of ESR-spectroscopy without considering natural relaxation of absorbed palaeodose my prove rather problematic. The long-term fading of palaeodose complicates correlation between absorbed natural radiation energy, and the value of the laboratory effect observed restricts the upper dating limit and may result in considerably younger ages on dating of the most ancient Quaternary samples. The present paper deals with the connection between the absorbed palaeodose value and the age of the sample, its dosimetric properties, as well as with the effect of different factors on the upper dating limit. Energetic parameters of CO 3- 3 centres (g = 2.0012) have been determined for the shell's carbonate: trap depth E = 1.52 ± 0.01 eV, frequency factor s = 8 × 10 13 s -1, mean life θ = 1.14 × 10 6y at 5°C. Mathematical models of the palaeodose accumulation in nature are discussed. The author suggests palaeodosimetric equations for calculating the age of shells and some other materials. The equations may be applied both to ESR- and TL-dating.

Long-term stability of the TL signal in alkali feldspars

The long-term stability of the TL signal in alkali feldspars was investigated by studying infinitely old samples extracted from Tertiary sands. Mean lives in the range 800–5400 ka were obtained, indicating that long-term fading has to be taken into account. Fading corrections were calculated for a range of TL ages. The study indicated that the stability of the TL signal in alkali feldspars is less than that predicted from kinetic studies, and suggested a practical dating limit of 500 ka.

Editorial

This paper presents TL age estimates of Pleistocene raised marine terraces from southern Calabria (southern Italy). It focuses on the dating of last interglacial shallow-marine sand deposits, including several Tyrrhenian key reference deposits whose stratigraphic assignment is based on biostratigraphic evidence (presence of a Strombus bubonius fauna) and geochronometric data (amino-acid data calibrated with UTh dates on coral). The TL age estimates are obtained on alkali feldspar coarse grains using their blue TL emission. They are in good agreement with the IR-OSL age estimates obtained on the same feldspar samples. Our feldspar TL method is tested on two Tyrrhenian key reference sites, Ravagnese and Bovetto, in the Reggio di Calabria area. Their measured TL age estimates (97 ± 11 ka and 100 ± 10 ka) are lower than their estimated geological age (i.e. Oxygen Isotope Substage 5e). The correction for long-term fading suggested by Mejdahl (1988), is tested on these two samples using a lifetime of 446 ka as estimated from both Calabrian and Miocene sand collected within the same area. This correction would yield preliminary TL ages (116 ± 13 ka and 116 ± 12 ka) that are in better agreement with the expected geological age. The application of our TL method to paleoshorelines lying below the Tyrrhenian reference shoreline of Ravagnese-Bovetto, yields stratigraphically consistent TL age estimates. The TL method is also applied to two supposedly pre-last interglacial paleoshorelines which are geomorphologically older than the Tyrrhenian reference shorelines. They remain however chronologically undifferentiated, both by the TL and the amino-acid methods. Finally, the preliminary feldspar TL results obtained on the Miocene and the Calabrian sand deposits (> 1 Ma), suggest that our feldspar TL method is limited, in the SW Calabrian area, to about 200 ka.

Close-Spaced Diversity Antenna for HF Transmission

The advantages of ionospheric communications over satellite communications for the military are reduced cost and lower vulnerability to physical attack. However, the long haul ionospheric signal is not as reliable as the satellite communications signal, and experiences both long-term and short-term fading. Frequency management shows good promise in the reduction of long-term fading. Short-term fading, caused by waves reflecting from multiple layers changing in real time, can be reduced by space diversity schemes. However, a spacing of 10 wavelengths is generally needed for HF space diversity and is not practical for the highly mobile military requirements of today. This paper reports a field component [ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> (electric) and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</tex> (magnetic) fields] diversity scheme in which two antennas are spread much less than one wavelength (λ/16) resulting in typical correlation between the two signals of less than 0.7. A theoretical analysis was made and experiments verified the concept of this new scheme.

Antenna scanning technique to improve troposcatter system performance

This is a new technique proposed to improve the performance of a troposcatter system. In this technique, the transmitting or receiving antenna, or both, are continuously scanning the space to find the scattering volume that produces the optimal signal strength. The overall effect is to reduce both short-term and long-term fadings. It is equivalent to a combination of using selection angle diversity for short-term fast fading and increasing system margin for long-term slow fading, by picking up the best signal. An extensive experimental study is suggested.

Cochannel interference rejection in a mobile radio space diversity system

Diversity is often used in radio systems to alleviate the problem of fading. It is shown here that the use of diversity in a mobile radio system also will result in better utilization of the frequency spectrum. In a system which reuses channels in physically separated areas, probability distributions of signal-to-cochannel interference ratios in 1-, 2-, and 4-branch diversity systems are developed by Monte Carlo methods. We consider two cases: first, we assume signal strength to be Rayleigh distributed, which is a valid model for reception within a small localized area; second, we take shadow losses into account by assuming a more realistic long-term fading model. The effect of using diversity in a mobile radio system is considerable with either model. Assuming a hexagonal cell pattern of frequency reuse, it is shown that frequencies may be reassigned more often in a diversity system than in a system which does not use diversity. The conclusion, therefore, is that a diversity system utilizes the frequency spectrum more efficiently than an equivalent nondiversity system.

Finding the statistical properties of the median values of a fading signal directly from its decibel values

A method is introduced by which the statistical properties of the long-term fading of a signal can be found from a section of decibel-scale data. First the median of a subsection of signal fading in decibels is found directly from the average of its decibel values over each given time interval; then the long-term fading can be plotted. From such fading data, the statistical properties can be found directly and quickly from the measured level-crossing-rate (LCR) curve.

Overwater propagation of millimeter waves

A field test program conducted to determine the short-and long-term fading characteristics and atmospheric absorption effects on an overwater millimeter-wave transmission path is described. A 6.1-nautical-mile overwater link was established using a 53-GHz transmitting frequency. Low grazing angles were used with antenna heights of 15 to 30 feet above sea level. The results of the program indicate 1) that short-term fading is Rayleigh distributed in the severest cases and is caused by surface reflections, and 2) that long-term fading variations occur more frequently and with greater intensity during warm weather periods.

Studies of within-the-horizon propagation at 9300 Mc

Propagation characteristics of radio signals in the 9300-Mc frequency range were investigated over a 113-km tropospheric within-the-horizon path in Eastern Colorado. Special attention was given to the short-term fading characteristics of the received carrier envelopes and to the bandwidth capability of the medium in this frequency range, which was studied by the comparison of amplitude variations of two CW carriers separated by 100 Mc in frequency. For this purpose, correlation coefficients between carrier envelopes as well as the distributions of carrier amplitude ratios were analyzed. Although these parameters are related, the amplitude ratio has some advantages as an indicator of selective fading phenomena for within-the-horizon paths. By sampling the 9250 and 9350 Mc instantaneous carrier amplitudes at the rate of one per second, an over-all value of 0.91 was obtained for their cross-correlation coefficient. The standard deviation of the amplitude ratios expressed in db at 9250 and 9350 Mc averaged 0.76 db, with a maximum hourly value of 1.81 db. These results include the effect of space diversity, as separate antennas were used for transmission and reception of the two carriers, but they support the feasibility of wide-band modulation techniques for within-the-horizon paths if judged by the statistics of amplitude variations at discrete frequencies at the limits of the band considered. Short-term and long-term fading characteristics at 9300 Mc are similar to the ones previously observed on lower frequencies over this path. An analysis of prolonged space-wave fadeouts in this frequency range resulted in fadeout depths up to 25 db below weekly transmission loss medians, approximately log-normal distributions of fadeout durations, and the expected diurnal variations of fadeout occurrence typical of a continental climate.

VHF and UHF signal characteristics observed on a long knife-edge diffraction path

During 1959 and 1960 long-term transmission loss measurements were performed over a 223 kilometer path in Eastern Colorado using frequencies of 100 and 751 Mc. This path intersects Pikes Peak which forms a knife-edge type obstacle visible from both terminals. The transmission loss measurements have been analyzed in terms of diurnal and seasonal variations in hourly medians and in instantaneous levels. As expected, results show that the long-term fading range is substantially less than expected for tropospheric scatter paths of comparable length. Transmission loss levels were in general agreement with predicted knife-edge diffraction propagation when allowance is made for rounding of the knife edge. A technique for estimating long-term fading ranges is presented and the results are in good agreement with observations. Short-term variations in some cases resemble the space-wave fadeouts commonly observed on within-the-horizon paths, although other phenomena may contribute to the fading. Since the foreground terrain was rough, there was no evidence of direct and ground-reflected lobe structure. In most cases comparatively high correlation exists between signals received simultaneously on two antennas with 8.3 and 14 meters vertical separation. These separations were chosen as being representative for practical space diversity systems designed for eliminating the effects of fading arising from direct and ground reflected phase interference phenomena. The comparatively high correlation observed suggests that space diversity will be relatively less successful in mountain obstacle paths with rough terrain near the terminals than on tropospheric scatter paths or on line-of-sight paths over smooth terrain. The enhancement of field strength associated with propagation over mountain ridges may cause concern in applications where mountains are being counted on to shield unwanted radio waves. Some radio astronomy installations have been located in mountain valleys for this reason, and it is possible that obstacle-gain effects may aggravate rather than alleviate interference.