Laser energy partitioning in nanosecond pulsed laser-induced air breakdown: effect of incident laser energy.

Air breakdown is generated by a 1064 nm nanosecond pulsed laser beam, and laser energy deposited in the breakdown (E d), transmitted through the plasma region (E t) and carried away by the shock wave (E s) is estimated for the incident laser energy (E i) range of 60-273 mJ. The E d is approximately 85% of E i at 60 mJ, rapidly increasing to 92% at 102 mJ. The shock wave front velocity and radius are measured as a function of E i and propagation distance. The shock wave velocity nicely follows the v∝E i0.3 trend predicted by the laser-supported detonation wave model. The Sedov-Taylor theory is used to estimate E s, which rapidly increases with E i, but E i to E s conversion linearly decreases from 83% to 48%. At lower values of E i, most of the laser energy is carried away by the shock wave, whereas the laser energy used in plasma heating or released in the form of electromagnetic and thermal radiation becomes important at higher laser energies. This implies that laser energy partitioning is highly dependent on the value of incident laser energy. These findings provide important insights into the fundamental physics of air breakdown and will be useful in a variety of applications such as laser-induced breakdown spectroscopy, laser ignition, and laser propulsion.

Excitation Wavelength and Colloids Concentration-Dependent Nonlinear Optical Properties of Silver Nanoparticles Synthesized by Laser Ablation.

We reported experimental results from investigations that employed the Z-scan method to explore the dependence of silver nanoparticles' (AgNPs) nonlinear optical properties on the excitation wavelength, AgNP concentration, and size. Using a 532 nm Nd: YAG laser beam at 100 mJ/pulse for different ablation times, AgNPs were synthesized from a silver target immersed in distilled water. UV-Vis spectroscopy and an atomic absorption spectrometer are used to characterize the optical properties of laser-synthesized AgNPs as well as their concentrations. The AgNPs' size and shape are determined using a transmission electron microscope (TEM). The laser-synthesized AgNPs are spherical, with an average particle size of 12 to 13.2 nm. Whatever the ablation time, the AgNP colloids exhibit reversed saturable absorption and a negative nonlinear refractive index (n2). Both n2 and the nonlinear absorption coefficient (α3) increase as the AgNP concentration increases. As the excitation wavelength and average size of the AgNPs increase, n2 and α3 decrease.

A study on natural radioactivity in Khewra Salt Mines, Pakistan.

The Khewra Salt Mines, the second largest salt mines in the world, are located 160 km south of Islamabad, the capital of Pakistan. Around 1000 workers are involved in the removal of salt from these mines. More than 40,000 visitors come annually to see the mines. The visitors and workers are directly exposed to the internal and external radiological hazards of radon and gamma rays in these mines. The general public is affected by the intake of the salt containing the naturally occurring radionuclides. Therefore the concentration of radon (²²²Rn) in the Khewra Salt Mines and activity concentrations of the naturally occurring radionuclides in the salt samples from these mines were measured. Both active and passive techniques were employed for the measurement of radon with Radon Alpha Detector (RAD-7) and SSNTD respectively. The concentration of ²²²Rn was 26 ± 4 Bq m⁻³ measured by the active method while 43 ± 8 Bq m⁻³ was measured by the passive method. The activity concentration of the radionuclides was measured using gamma ray spectrometry with HPGe detector. The mean activity of 4°K in salt samples was found to be 36 ± 20 Bq kg⁻¹ and the concentration of ²²6Ra and ²³²Th in the salt samples was below the detection limits. Gamma radiation hazard was assessed in terms of the external gamma dose from salt slabs and the rooms made of salt and the annual effective dose due to gamma radiation. The exposure to radon daughters, annual effective dose and excessive lifetime cancer risk due to radon in the mines were estimated. The mean annual effective dose due to an intake of 4°K from the salt was calculated as 20.0 ± 11.1 µSv, which is lower than the average annual effective dose rate of 0.29 mSv, received by the ingestion of natural radionuclides. Due to the low concentration values of primordial radionuclides in the salt and radon ²²²Rn) in the mines, a 'low level activity measurement laboratory' is suggested to be established in these mines.

Assessment of radiological hazard of NORM in Margalla Hills limestone, Pakistan.

Studies on naturally occurring radioactive material (NORM) in the limestone from the Margalla Hills have been carried out by measuring gamma activity and to access its radiological implications if any. For data acquisition, a High-Purity Germanium detector was employed. The activity concentrations of (226)Ra, (232)Th, and (40)K were found to be 14.32 ± 0.24, 2.05 ± 0.04, and 13.80 ± 0.20 Bq kg(-1), respectively. These values are relatively lower as compared to that in the limestone of other countries and much lower than the values reported for the natural building stones. The average specific activities due to (226)Ra were found to be higher when compared with (40)K and (232)Th. Indices of radium equivalent activity (Ra(eq)), internal hazard (H(in)), indoor absorbed gamma dose rate (D(R,)), and corresponding annual effective dose (E(eff)) were also determined for the limestone-made rooms. All of these indices were found to be in the lower ranges. The Margalla Hills limestone does not pose any excessive radiological health hazard as a building material and in industrial uses for a common man.