Current Science

D. K. Panda ¹, D. Banerjee ¹, S. K. Goyal ¹, A. R. Patel ¹, A. D. Shukla ¹

Affiliations
1 Planetary Sciences and Exploration Group, Physical Research Laboratory, Ahmedabad 380 009, IN

Abstract

We present the development of a cerium-doped lanthanum bromide (LaBr₃ : Ce) gamma-ray spectrometer (GRS) with the primary objective of determining the abundance and distribution of Th, U, K, and other major elements, including Fe on the entire planetary surface by measuring gamma-ray signals produced by radioactive decay, neutron inelastic scattering and neutron capture reactions in the energy region 0.03- 8 MeV. The energy resolution of the LaBr3 : Ce GRS developed in-house using front-end and processing electronics at 511 and 1274 keV is estimated to be 4.1% and 2.5% respectively. The intrinsic activity count rate for our 3ⁿ * 3ⁿ LaBr₃ : Ce GRS is ~61 counts s^-1 (i.e. ~0.18 counts s^-1 cm^-3) for the 40K energy window (1400-1520 keV) and ~3.4 counts s^-1 for the ²³²Th (2550-2700 keV) energy window. Although this large intrinsic activity of the LaBr₃ : Ce crystal inhibits estimation of the concentrations of Th and K, our attempts using a NaI(Tl) GRS (with electronics developed in-house) were more successful. The Th concentration of US-110 was estimated to be ~11.4 ppm and is within 14% of the 13.2 ppm value determined using a HPGe GRS. The K concentration of US-110 was estimated to be 0.87% and is within ~10% of the 0.78% value determined independently using a HPGe GRS.

Keywords

Cerium Radioactive Elements, Gamma Ray Spectroscopy, Lanthanum Bromide, Sodium Iodide.

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J. P. Pabari ¹, D. Banerjee ¹

Affiliations
1 Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, IN

Abstract

The surfaces of airless, non-magnetized bodies like the Moon are directly exposed to solar wind and ultraviolet radiation, causing surface dust grains to be electrically charged and levitated, whenever electric fields exceed the surface forces and gravity. For an improved understanding of the lunar dust environment, we study the surface charging processes using electrostatic modelling and present the results here. We apply Gauss's law to examine the dust levitation and compare the implications with those obtained using freespace capacitance of the particle. Calculating grain charge on surface by assuming its free-space capacitance is erroneous and is therefore inapplicable. The daytime surface potential during high solar activity is estimated to be ˜20 V, while the nighttime potential can be as high as -3.8 kV. The maximum radius of levitating particles is greatly affected by the method used to model the dust levitation. Using Gauss's approach, it comes out to be in the picometre range near the terminator, in contrast to existing calculations which estimate it to be in the nanometre to micrometer range. The LDEX provided no indication of 0.1 μm-sized particles near the terminator, as suggested previously from Apollo observations. This result is not inconsistent with our predictions based on Gauss's law. Hence, it still remains an open question whether dust levitation occurs on the Moon or not, and experiments are necessary on future lunar lander mission which provide direct measurement of surface potential and near-surface charged dust particles to confirm the same.

Keywords

Dust, Levitation, Lunar Environment, Photoemission, Plasma.

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S. S. Hasan ¹, D. Banerjee ¹, B. Ravindra ¹, K. Sankarasubramanian ¹, K. E. Rangarajan ¹

Affiliations
1 Indian Institute of Astrophysics, Bengaluru 560 034, IN

Abstract

The National Large Solar Telescope (NLST) aims primarily to carry out observations of the solar atmosphere with high spatial, spectral and temporal resolution. A comprehensive site characterization programme, that commenced in 2007, has identified an excellent site in the Ladakh region at the Pangong lake, India. With an innovative optical design, NLST is an on-axis Gregorian telescope with a low number of optical elements to reduce the number of reflections and yield a high throughput with low polarization. In addition, it uses high-order adaptive optics to produce close to diffraction limited performance. To control atmospheric and thermal perturbations of the observations, the telescope will function with a fully open dome, to achieve its full potential atop a 25 m tower. The post-focus instruments include broadband and tuneable Fabry-Perot narrow band imaging instruments and a high-resolution spectropolarimeter.

Keywords

Adaptive Optics, High Angular Resolution, Magnetic Fields, Solar Telescope.

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S. Sathiyaseelan ¹, D. K. Panda ², D. Banerjee ², D. Ramesh ¹, A. D. Shukla ²

Affiliations
1 Department of Remote Sensing, Bharathidasan University, Tiruchirappalli 620 023, IN
2 Physical Research Laboratory, Ahmedabad 380 009, IN

Abstract

The Tamil Nadu coastline, India, has extensive dunes which can be used for inferring quaternary palaeoclimates. In the present study, we aim to determine the chronology of the dunes in the Vaigai prodelta region along the southeastern coastline of the country. Optically stimulated luminescence ages for the elevated beach ridge samples SANK-5, SANK-4, SANK-3, SANK-2 and SANK-1 were estimated to be 0.9 ± 0.1, 1.1 ± 0.1, 1.8 ± 0.2, 5.3 ± 0.4 and 27 ± 2.3 ka respectively, from top to bottom of the section. The first four phases of aeolian activation occurred in marine oxygen isotope stage-1, whereas the ~27 ka age suggests that sediment deposition occurred in marine oxygen isotope stage-3. The luminescence ages were in stratigraphic order, and represent the first dates of aeolian activation in the Vaigai region.

Keywords

Aeolian Activation, Chronology, Coastline, Luminescence Ages, Palaeoclimate, Red Dunes.

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