Abstract. Fast and accurate relative positioning for baselines less than 20 km in length is possible using dual-frequency Global Positioning System (GPS) receivers. By measuring orthometric heights of a few GPS stations by dierential levelling techniques, the geoid undulation can be modelled, which enables GPS to be used for orthometric height determination in a much faster and more economical way than terrestrial methods. The geoid undulation anomaly can be very useful for studying tectonic structure. GPS, levelling and gravity measurements were carried out along a 200-km-long highly undulating pro®le, at an average elevation of 4000 m, in the Ladak region of NW Himalaya, India. The geoid undulation and gravity anomaly were measured at 28 common GPS-levelling and 67 GPSgravity stations. A regional geoid low of nearly A4 m coincident with a steep negative gravity gradient is compatible with very recent ®ndings from other geophysical studies of a low-velocity layer 20±30 km thick to the north of the India±Tibet plate boundary, within the Tibetan plate. Topographic, gravity and geoid data possibly indicate that the actual plate boundary is situated further north of what is geologically known as the Indus Tsangpo Suture Zone, the traditionally supposed location of the plate boundary. Comparison of the measured geoid with that computed from OSU91 and EGM96 gravity models indicates that GPS alone can be used for orthometric height determination over the Higher Himalaya with 1±2 m accuracy.

No abstract

No abstract

The Internal Bremsstrahlung (IB) spectra emitted in the beta decays of t4C and 36C1 are measured with a HPGe detector and a multichannel analyser along with a standard geometrical set up. The experimental results are compared with the corresponding theoretical distributions due to Knipp and Uhlenbeck and Bloch (KUB), Lewis and Ford, Nilsson and also due to Chang and Falkoff. The total IB yields of intensity and of energy from each of the isotopes in the investigated energy regions are also determined and compared with the corresponding theoretical values. The present results of IB from 14C are found to be in excess over all the corresponding theoretical distributions throughout the investigated photon energy region from 60 keV to 130 keV. It may also be pointed out that the IB from this isotope has not been studied earlier and as such the results in this case are considered new and important. In the case of 36C1 the experimental results are found to be above KUB theory but corresponding closely to the theory of Chang and Falkoff, except below 150 keV where there is experimental excess over theory.