J. N. Goswami

Ionization states of the anomalous cosmic rays

Ionization states of individual N, O, and Ne ions in the low energy (10-30 MeV/n) anomalous cosmic rays (ACR) are determined in the Anuradha experiment conducted onboard Space Shuttle Spacelab-3 mission during 1985 April-May. The geomagnetic field is utilized as a momentum filter in this experiment to obtain upper limits on the ionization states of individual ACR ions. A combination of an extremely sensitive passive plastic detector and an active electromechanical system allow us to obtain accurate information on the arrival locations and directions of individual ACR ions and hence the threshold rigidity needed by each of these ions to reach the point of their detection in the Spacelab-3 orbit.

High energy X-γ ray spectrometer on the Chandrayaan-1 mission to the Moon

The Chandrayaan-1 mission to the Moon scheduled for launch in late 2007 will include a high energy X-ray spectrometer (HEX) for detection of naturally occurring emissions from the lunar surface due to radioactive decay of the238U and232Th series nuclides in the energy region 20–250 keV. The primary science objective is to study the transport of volatiles on the lunar surface by detection of the 46.5 keV line from radioactive210Pb, a decay product of the gaseous222Rn, both of which are members of the238U decay series. Mapping of U and Th concentration over the lunar surface, particularly in the polar and U-Th rich regions will also be attempted through detection of prominent lines from the U and Th decay series in the above energy range. The low signal strengths of these emissions require a detector with high sensitivity and good energy resolution. Pixelated Cadmium-Zinc-Telluride (CZT) array detectors having these characteristics will be used in this experiment. Here we describe the science considerations that led to this experiment, anticipated flux and background (lunar continuum), the choice of detectors, the proposed payload configuration and plans for its realization

Ionization states of low-energy cosmic rays - Results from Spacelab 3 cosmic-ray experiment

The Indian cosmic ray experiment Anuradha, conducted onboard Spacelab 3 during April 29-May 6, 1985 was designed to obtain information on the ionization states of low-energy cosmic rays, using the geomagnetic field as a rigidity filter to place an upper limit on the ionization state of individual cosmic ray particles. This paper presents data confirming the presence of three distinct groups of energetic particles in the near-earth space: (1) low-energy (15-25 MeV/nucleon) anomalous cosmic rays that are either singly ionized or consistent with their being in singly ionized state, (2) fully ionized galactic cosmic ray ions, and (3) partially ionized iron and sub-iron group ions (which account for about 20 percent of all the iron and sub-iron group ions detected at the Spacelab 3 orbit within the magnetosphere in the energy interval 25-125 MeV/nucleon). It is argued that these partially ionized heavy ions are indeed a part of the low-energy galactic cosmic rays present in the interplanetary space.

26Al as a heat source for early melting of planetesimals: Results from isotopic studies of meteorites

Ion microprobe studies of magnesium isotopic composition in igneous components from several chondritic meteorites have been carried out to look for26Mg excess that may be attributed to the presence of the now-extinct radionuclide26Al(τ ∼ 1 Ma) at the time of formation of these objects. A positive evidence for the presence of26Al in the analysed objects will strengthen its case as the primary heat source for the early thermal metamorphism/melting of meteorite parent bodies. Based on calculated temperature profiles inside chondritic objects of different sizes and initial26Al/27Al ratios, we have estimated the initial abundances of26Al needed to provide the heat necessary for the wide range of thermal processing seen in various types of meteorites. The magnesium isotopic data obtained by us do not provide definitive evidence for the presence of26Al at the time of formation of the analysed igneous phases in different chondritic meteorites. Experimental evidence for a planetary scale distribution of26Al in the early solar system to serve as a significant heat source for the thermal metamorphism and melting of meteorite parent bodies (planetesimals) remains elusive.

Isotopic analysis of early solar system objects by an ion microprobe: Parametric studies and initial results

Magnesium, potassium and calcium isotope compositions in terrestrial samples and refractory phases from primitive meteorites are determined using an ion microprobe. A thorough investigation of the different instrument parameters is carried out to ensure that conditions necessary for high mass resolution and high precision isotopic studies are adequately satisfied. The instrument can be tuned to achieve mass resolution (M/ΔM) of up to 10,000 (M≤60); it has a very good dynamic stability (ΔB/B≤10 ppm over durations of ≤40 minutes) and the counting system has an effective dead-time of ≤25 nsec and a dynamic background of ≤0·01 c/s. Reproducibility and precision of isotopic measurements are checked by analyzing magnesium and titanium isotopic compositions in terrestrial standards and isotopically doped silicate glasses. A precision of 2‰ (2σm) was achieved during magnesium isotopic analysis in samples with low Mg content (200 ppm). Results from studies of magnesium and potassium isotopic compositions in several Ca−Al-rich refractory inclusions (CAIs) from the primitive meteorites Efremovka and Grosnaja, representing some of the early solar system objects, are presented. The well-behaved Mg−Al isotopic systematics confirm the pristine nature of the Efremovka CAIs inferred earlier from petrographic and trace element studies. The Grosnaja CAIs that have experienced secondary alterations show disturbed magnesium isotopic systematics. Observation of excess26Mg in several of the analyzed CAIs confirms the presence of the now extinct26Al (t1/2=7×105 years) in the solar nebula at the time of CAI formation. Our data also suggest a relatively uniform distribution of26Al in the solar nebula. Several Efremovka CAIs with excess26Mg also have excess41K resulting from the decay of41Ca (t1/2≃105 years). This observation constrains the time interval between cessation of nucleosynthetic input to the solar nebula and the formation of some of the first solar system solids (CAIs) to less than a million years.

Observation of low-energy (30-100 MeV/nucleon) partially ionized heavy ions in Galactic cosmic rays

Experimental evidence is presented that shows the existence of low-energy (30-100 MeV/nucleon) partially ionized heavy ions in Galactic cosmic rays. The measurements were made by a CR-39 plastic nuclear detector module aboard the Spacelab-3 mission during the cosmic-ray experiment Anuradha (April 29 - May 6, 1985), in which the ionization states of low-energy cosmic-ray ions of elements O to Fe were determined by utilizing the earths magnetic field as a momentum analyzer. It was found that, while most of the ions of the Fe group in this energy range were fully ionized, Ti, Cr, and Fe were observed in partially ionized states equal to or lower than 6, 8, and 20, respectively. The possible sources of these particles are discussed. 24 refs.

Studies of anomalous cosmic ray oxygen ions in space and their ionization states in ANURADHA experiment in spacelab-3

Abstract During April 29 to May 6, 1985, the Spacelab-3 mission of NASA was in orbit carrying among others, the Indian cosmic ray experiment ANURADHA. The major objectives of the experiment were the measurements of the fluxes and ionization states of anomalous cosmic rays by means of a time resolution device which in turn provided information of the arrival location and direction of the particles. This report presents the results of the measurements of ionization states of anomalous cosmic ray heavy ions by trajectory computation method. In the energy interval 15–20 MeV/N, we obtained ionization states of oxygen as +1. Also the orbit-average flux of oxygen ion indicates the charge state of +1. Both the methods show consistent results. Trajectory computations for few other ions of nitrogen and neon was also carried out which shows their ionization states as +1 or ≦ +2.

Short-lived nuclides in the early solar system

Isotopic records in meteorites provide evidence for the presence of several short-lived nuclides in the early solar system with half-lives varying from 105 to ∼8x107 years. Most of the nuclides with longer half-life (> 107 years) are considered to be products of stellar nucleosynthesis taking place over long time scales in our galaxy. However, for the relatively shorter-lived nuclides, two possibilities exist; they could be products of energetic particle interactions taking place in a presolar or early solar environment, or, they could have been produced in a stellar source and injected into the protosolar molecular cloud just prior to its collapse. The presently available data appear to support the latter case and put a stringent constraint of less than a million years for the time scale for the collapse of the protosolar molecular cloud to form the Sun and some of the first solar system solids. This short time scale also suggests the possibility of a triggered origin for the solar system with the very process of injection of the short-lived nuclides acting as the trigger for the collapse of the protosolar molecular cloud. Fossil records of the short-lived nuclides in meteorites also provide very useful chronological information on the early solar system processes like the time scale for nebular processing, the time scales for differentiation and for metal/silicate fractionation within planetesimals. The currently available data suggest a time scale of a few million years for nebular processing and a relatively short time scale of about ten million years within which differentiation, melting and recrystallization in some of the planetesimals took place.

Windows to early solar system processes: Refractory inclusions in the CV and CM chondrites

The refractory element-enriched inclusions found in the carbonaceous meteorites give cosmochemists a fascinating glimpse at processes which occurred near the birth of the solar system. Although many complications must still be unravelled, the weight of the available evidence indicates that many of these objects condensed directly from the solar nebula, and have remained relatively unaltered up to the present. Their mineralogical and chemical compositions therefore reflect conditions at the time of their formation. The most thoroughly studied of the inclusions are those from the Allende CV meteorite. These, in general, have mineral assemblages similar to those which would be predicted for nebular condensation. The mineralogical agreement is not strict, however, and also the bulk chemical compositions sometimes deviate markedly from expected trends. More work is required to understand these differences. A range of isotopic anomalies in many elements has been found, in these inclusions. Some of these suggest an extra-solar system origin for a part of the material in the inclusions. Although much less work has been done on the inclusions in the CM meteorites, current data indicate that they will prove to be at least as valuable as those from Allende. Chemical data show that some inclusions in the Murchison meteorite are more refractory than the most refractory Allende inclusions. Isotopic anomalies, including25Mg excesses and oxygen-16 enriched oxygen, indicate that, in spite of chemical and mineralogical differences, the Murchison and Allende inclusions contain common isotopic components, and are probably contemporaneous.

Observation of enhanced sub-iron (ScCr) to iron ration in low energy cosmic rays of 50–100 MeV/N in spacelab-3

Abstract Relative abundances of sub-iron (Sc-Cr) to iron nuclei in low energy (50–100 MeV/N) galactic cosmic rays have been determined from an analysis of about 100 events of heavy ions (Z = 10−28) recorded in a detector assembly flown in the Anuradha cosmic ray experiment in the Spacelab-3 on a six day mission in April–May 1985. The measured abundance ratio of (Sc-Cr)/Fe nuclei in 50–100 MeV/N energy range is 1.1 ± 0.3, and the present result of enhanced ratio of sub-iron to iron nuclei is in agreement with other experimental results in 200–800 MeV/N range. The over-abundance of iron secondaries at these low energies cannot be explained in the conventional models for propagation of cosmic rays. Available experimental data indicate a very different time history for the low energy iron-group, as compared to those of lighter nuclei in galactic cosmic rays.