Beach Sand Exploitation Centre at Cox's Bazar, Bangladesh, produces commercial grade concentrations of magnetite, ilmenite, zircon, etc., from the high-grade accumulations available along the beach and foredune of Cox's Bazar. Solid state nuclear track detectors (CR-39 foils) were used to determine indoor radon concentration of radioactive mineral sands and the technologically enhanced radiation level inside the pilot plant of the Centre. It is found that the concentrations at processed mineral stock areas are high, and the maximum concentration was found to be 2,103 +/- 331 Bq m(-3) (0.23 +/- 0.03 WL). The indoor concentration of radon and its decay products in the raw sand stock area and at other locations was in the range of 116 +/- 27 Bq m(-3) (0.03 +/- 0.003 WL) to 2,042 +/- 233 Bq m(-3) (0.22 +/- 0.03 WL).

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III-N deep ultraviolet 0 emitters with AlGaN multiple quantum wells (MQW) active regions have been demonstrated on sapphire, Sic and bulk GaN substrates [1,2]. Sapphire is a good substrate choice for deep UV LEDs as it allows for efficient light extraction through the back side [2]. We now for the first time report milliwatt power 340 nm UV LEDs over sapphire with quaternary AlInGaN MQW active region on sapphire substrate. We also for the first time show a stable room and cryogenic temperature operation for these AlInGaN based UV light emitters.For this study, the LED structure was grown over basal plane sapphire substrate and consisted of high quality bottom n+-AlGaN clad layer followed with three pairs of AIInGahVAlGaN MQW active region. For simplicity we refer to it as AlInGaN LED. It was then caped with p-AlGaN and p+-GaN layers for hole injection and ohmic contact formation. Square geometry 100 pm and 200 pm devices were fabricated and characterized. Figure 1 shows the room temperature current-voltage (I-V) characteristics for the AlInGaN LED. The wafers were then diced into individual chips and packaged as reported earlier [3]. Figure 2 shows the GI characteristic under dc pumping for packaged AlInGaN LED. The optical power was measured in a calibrated integrating sphere with UV enhanced silicon photodetector. A record optical power of 0.405 mW at 50 mA of dc bias was measured for the packaged LED, which translates to an external quantum efficiency of 0.22%.The electrical and optical characteristics of the AlInGaN LED were then measured over a temperature range from 10K-300K. Pulse bias was employed to minimize self-heating at low temperatures. Figure 3 shows the EL spectra at 20 mA of bias current as a h c t i o n of device temperatures. As seen, the emission intensity increases with decrease in temperature from 300K down to 10K. Pulse optical power increased from 3 mW at room temperature to about 9.1 mW at 10K (pump current of 200 mA). Details of these measurements along with comparative study with AlGaN MQW LEDs will be presented to elucidate the effect and advantage of indium incorporation in the active region. The cryogenic characteristics will be used to determine the most significant growth parameters affecting the quantum efficiency. REFERENCES:[I] T.