• Volume 90, Issue 4

      December 1981,   pages  337-460

    • On the origin of the low energy cosmic rays from stellar sources

      S Biswas N Durgaprasad Sushma S Trivedi

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      The model of stellar origin of the anomalous component in the low energy cosmic rays for He to Fe ions observed in space vehicles is studied in the light of recent results. The model of heliospheric origin by Fisket al which has several attractive features cannot explain the long-term variations of intensity observed during 1974 to 1978 as pointed out by Nagashima and Morishita. The stellar origin model of Durgaprasad and Biswas, on the other hand, can easily account for the sudden appearance of the anomalous component in 1972 and its large decrease in intensity in 1978 on the basis of polarity reversal of the solar magnetic field as discussed by Nagashima and Morishita (1980). In this work, we show that in the stellar model energetic ions of He, C, N, O, etc. could originate in O-type stars which manifest very strong stellar wind with mass loss rate of 3·10−8 M per year. These have terminal velocities of about 1200 to 4000 km/sec and aretypically a few times their escape velocity. These velocities correspond to ion energies of 10 to 100 keV/amu. These ions are in partly ionised state and are accelerated in the interstellar shock fronts to about 1 to 50 MeV/amu and thus account for the observed anomalous component of low energy cosmic rays.

    • Irradiation history and atmospheric ablation of meteorites: Results from cosmic ray track studies

      J N Goswami

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      The dominant component of nuclear tracks observed in meteoritic minerals poor in uranium is produced by cosmic ray very heavy (vh:Z>20) nuclei. Studies of cosmic ray tracks and other cosmogenic effects in meteorites give us information on the irradiation history of these meteorites and enable us to estimate the extent of ablation during their atmospheric transit, and hence their pre-atmospheric masses. In a specific type of meteorite, known asgas-rich meteorite, one finds individual grains and xenoliths that have received solar flare and galactic cosmic ray irradiation prior to the formation of these meteorites. Detailed studies of these exotic components give insight into the accretionary processes occurring in the early history of the solar system. Some of the important results obtained from such studies and their implications to meteoritics are summarized.

    • Records of ancient cosmic radiation in extraterrestrial rocks

      Narendra Bhandari

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      Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of26A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (Js,Ro)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×105 to 2×106 years.

      Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (53Mn,26Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and22Ne/21Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.22Ne/21Ne ratio <1.06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from60Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid.

      Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid.

      Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.

    • Fission tracks and cooling rates of meteorites

      Ghislaine Crozaz

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      Recent work on fission track studies of meteorite samples to obtain cooling rates of meteorite parent bodies is reviewed. The cooling rates of chondrites are in excess of 1°K/106 yr. Fission track studies of phosphate grains in mesosiderites do not support the extremely slow cooling rates of 0·1°K/106 yr for these meteorites, inferred from metallographic studies. The accumulating evidence from fission track studies indicates a gross underestimation of the cooling rates of meteorites as determined by the metallographic techniques.

    • Fission track geochronology of India

      K K Nagpaul

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      The fission track ages of cogenetic/co-existing minerals namely garnet, muscovite and apatite from three mica beltsi.e., Bihar, Rajasthan, Nellore of peninsular India and Himalayan region, coupled with the corresponding closing temperatures of the minerals have been used to reveal the thermal and uplift histories of these regions. The data show that the extra-peninsular part of the subcontinent during Himalayan orogenic cycle (upper cretaceous-tertiary) witnessed the highest cooling and uplift rates in comparison to the older cycles in peninsular India.

    • Precision, accuracy and meaning of fission track ages

      G Poupeau

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      The precision, meaning, and accuracy of the fission track (ft) dating method are reviewed from an examination of the recent literature as well as previously unpublished data from the author's laboratory.

      It is concluded that forapparentft ages (i.e. ages derived from the canonical age equation) a precision (2σ level) of the order of ±4% to ±5% can be reached provided that (i) uranium is sufficiently homogeneously distributed in the dated samples, at least locally; and (ii) a large enough number of tracks can be counted.

      Modelft ages,i.e. ages, for which partial geological track annealing is taken into account, have variable degrees of precision. While model ages obtained with the track-size method seem, as evaluated from the literature, to have usually a limited precision of the order of ±30% (2σ), plateau ages usually have a precision better than ±5% at a 2σ confidence level. Because it provides an objective test on the accuracy of track identification, as well as some insight of the variability of closing temperatures between various samples of a given mineral phase, the Isochronal Plateau (icp) method, when applicable, will be preferred (Poupeauet al 1980a). However, for phases which could be damaged by heating at relatively high temperatures, as for example hydrated glass shards from tephra, an Isothermal Plateau (itp) approach is to be preferred.

      Due to uncertainties about the value of the238U spontaneous fission decay constant λƒ, as well as difficulties inherent in the dosimetry of thermal neutrons in nuclear reactors, theft method of dating is not an independent one. Presently, it relies on the existence of geological standards (volcanic rocks) of known age, allowing, to determine anoperational ‘λƒ’ value (Naeseret al 1980). Accordingly, the accuracy of anft age is limited by the accuracy on the age of the standard. It should be better than ≈5%.

      For volcanic, hypovolcanic rocks, and shallow intrusives, theft method dates the time of formation, provided they were not further reheated. More generally, the track method providescooling ages. Closing temperatures calculated from laboratory experiments vary from ≲300°C to 100°C, according to minerals, for slow cooling rates (≈1°C/m.y.). For apatites, recent geological calibrations (Naeseret al 1980; Gleadow and Duddy 1980) confirmed laboratory extrapolations. The association of theft method with other geochronometers is therefore critical to the study of the cooling history of old cratons as well as to the evaluation of uplift/erosion rates in recent belts.

    • Application of solid state nuclear track detectors in fission studies

      R H Iyer

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      Solid state nuclear track detectors (ssntd) were introduced as an important research tool in nuclear science and technology in the early 1960s. In this paper an attempt is made to give an overview of some of the important applications ofssntd in the study of fission-related phenomena. The areas covered are: (a) spontaneous fission half-lives, (b) compound nuclear life-time measurements, (c) fission cross-section, excitation functions and fission fragment angular distributions, (d) fission isomers, (e) search for superheavy elements and (f) absolute fission yield measurements. In each case a few examples of experimental work carried out in various laboratories including the Bhabha Atomic Research Centre (barc) Bombay are discussed to highlight the significant contributions these studies have made to our understanding of nuclei and nuclear fission. The important role played byssntd in each of the above areas of fission studies is illustrated. Some specific cases are cited where the innovative use ofssntd has lead to results of profound significance in fission physics. A general review of the lead to results of profound significance in fission physics. A general review of the impact of these studies on our present understanding of nuclei and nuclear fission as well as a brief outline of the problems and future prospects are also given in the paper.

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