Radon 222 concentrations in the air in Carlsbad Caverns have been measured at different times of year in order to define certain features of the natural circulation of the cave atmosphere and to estimate the internal radiation exposure to visitors and Park Service personnel. Concentrations average 48 pCi/l. in the summer when the temperature lapse rate provides a stable cave atmosphere. In the winter months air from outside having a "' Rn concentration of only about 0.2 pCi/l. mixes with the cave air reducing radon levels to about 15 pCi/l. A simple model obtained by equating the input of "' Rn atoms from the rocks and soils of the cave's interior surfaces to the sum of terms for radioactive decay and dilution of the cave atmosphere by outdoor air enables one to predict, for steady-state conditions, the approximate levels of activity to be expected for all months of the year. If it is assumed that the short-lived daughter products of 222Rn are present in equilibrium amounts, Park Service personnel spending most of their time underground approach the 4 WM/yr (working level months per year) exposure limit established tor the uranium industry. A visitor on a 3-hr trip through the Caverns would receive only a few per cent of the exposure guides for the general population.
We have demonstrated that the phases of the stimulated Brillouin scattering reflections of two separately focused pump beams can be locked to a common phase by a backward seed of pump laser light.
Absfmct-The threshold for instability in Brillouin-enhanced fourpump =pump wave mixing has been experimentally determined as a function of both the phase mismatch and the ratio of the pump beam intensities, and is shown to agree with theoretical modeling. The effective input noise intensity for four-wave mixing in the unstable regime is compared to the noise in a stimulated Brillouin scattering amplifier and is found to be higher by a factor of three in the forward direction. Competition between two input signals has been investigated and it is shown that the signal which arrives first dominates the interaction in the unstable regime. w i u m E4(wa Stokes m n e i 1-An(CStOk08 P h W COnJUgOtO I z s o 2.L o3 = w, +U*-0 4 Fig. l. Diagram of four-wave mixing interaction. In our experiments, the angle between the pump beams and the signal beam was < 3 mrad, So the interaction was essentially colinear.
We report the first known observation of amplified reflection through degenerate four-wave mixing at 10.6 microm. Reflectivities of over 100% are reported in both n-type and p-type germanium at pump intensities of about 100 MW/cm(2). The maximum reflectivity reported is 800% in p-type Ge.
The nonlinear optical properties of a new class of strained-layer superlattices (intrinsic Stark effect superlattices) have been investigated. Specifically, we have compared the nonlinear transmission of Ga1−xInxAs-GaAs strained-layer superlattices grown along the (211) axis to identical superlattices grown along the (100) axis, and found that the optical nonlinearity in the (211) sample is about one order of magnitude greater than in the (100) sample. A blue shift of the exciton resonance and an increase in the exciton absorption strength in the (211) sample with increasing light intensity was observed (attributed to screening of the intrinsic Stark effect fields by photogenerated carriers), resulting in the stronger optical nonlinearity. The maximum of the nonlinear absorption index, ‖α2‖, in the (211) sample was 54 cm/W (‖Im χ3‖=0.33 esu) whereas in the (100) sample the maximum of ‖α2‖ was 6.9 cm/W (‖Im χ3‖=0.042 esu). The measured carrier recovery time in both samples was 2 ns.
The optical and magneto-optical properties of crystals of Hg1−xCdxTe (x=0.37) have been measured over the CO2 laser wavelength region. The absorption edge of this composition occurs at about 3.2 μm, and the Faraday rotation consists of interband and intraband components. The latter is very large in n-type material due to the small electron effective mass (0.052). The figure of merit of n-type material is about 1.0 deg/kOe at 10.6 μm. Optical-damage studies give a damage threshold of 160 MW/cm2, indicating application to high-power laser isolator devices.
We describe the first reported direct determination of the 10 -Am nonlinear susceptibility in intrinsic Ge through time-resolved ellipse rotation. We found that X(3)(1111). = 2.5 X 10(-11) esu and X(3)(1221)= 1.3 X 10(-11) esu, significantly below previous estimates.
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