First ultraviolet high-gain harmonic-generation free-electron laser.

We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti:sapphire laser has been used to produce saturated amplified radiation at the 266 nm third harmonic. The results confirm the predictions for HGHG FEL operation: stable central wavelength, narrow bandwidth, and small pulse-energy fluctuation.

Autonomic reactivity and hormonal secretion in lactate-induced panic attacks.

To compare autonomic and neuroendocrine responses during lactate-induced panic attacks, heart rate variability and cortisol and atrial natriuretic hormone (ANH) levels were measured in patients with panic attacks and in healthy control subjects. In a randomized double-blind design, all subjects received either 10 ml/kg body weight of 0.5 M racemic sodium lactate or normal saline from 1100 to 1120. Spectral analysis of the R-R interval of analog electrocardiograms was performed, and total (0.001-0.45 Hz), low-frequency (0.01-0.05 Hz), midfrequency (0.05-0.15 Hz), and high-frequency power (0.15-0.45 Hz) were computed. Cortisol was measured 12 times in the period from 0900 to 1300, and ANH was measured at 1100, 1120, and 1200 by radioimmunoassay. In both panickers (n = 6) and nonpanickers (n = 8), an infusion of lactate resulted in an acceleration of heart rate, a reduction in total spectral power, and a decrease in the high- and low-frequency components of spectral power. Panickers showed a significant enhancement of the high-frequency power, whereas in nonpanickers, a shift from the mid- and high-frequency toward the low-frequency power emerged. ANH plasma concentrations during lactate infusion in panickers showed a significant increase (115 and 131% at 1120 and 1200, respectively, over concentrations at 1100) in contrast to nonpanickers (20 and 74%, respectively). No group or treatment effects on cortisol secretion emerged, which is in line with former reports. Our study supports preliminary observations that lactate-induced panic attacks enhance the release of ANH, a vasodilatator and inhibitor of sympathetic activity. Hence this hormone not only could inhibit the secretion of the stress hormone cortisol but, in parallel, could also attenuate the sympathetic stimulation to the heart. These inhibitory effects of ANH could explain the so-far-unresolved dissociation between psychopathological alterations and autonomic and endocrine responses of panic attacks.

Approximations to saturation curves in gas-filled parallel-plate ionization chambers.

Saturation curves were measured for air and nitrogen in the exposure rate range from approximately 1 to approximately 500 R/min. Because of the differences between the ion-ion versus electron-ion recombination processes, the air and nitrogen saturation curves differ considerably even when measured under identical chamber and exposure rate conditions. A comparison between our experimental data and various approximations to saturation curves in air, developed by others, is given. It is then shown that the air saturation curve can be approximated by a sum of three simple exponential functions.

Calculation of surface dose in rotational total skin electron irradiation.

A single-field rotational total skin electron irradiation technique has recently been developed at the McGill University for treatment of skin malignancies. The dose received by a given surface point during rotation in a uniform large electron field depends on the radius of rotation of the surface point, on the local radius of curvature of the contour in the vicinity of the point of interest, and on the shadows cast by limbs (arms upon trunk or head and neck, and legs upon each other). A method for calculating the surface dose distribution on a patient is presented accounting for the various parameters affecting the dose. A series of measurements were performed with polystyrene and a humanoid phantom, and an excellent agreement between measured and calculated dose distributions was obtained.

Physical aspects of a rotational total skin electron irradiation.

A technique for rotational total skin electron irradiation is presented in which the patient stands on a slowly rotating platform (SSD = 285 cm) in a large uniform linear accelerator electron field (Eo = 3.5 MeV). The beam is scattered by the transmission ionization chamber and by a special lead/aluminum scattering filter, and then degraded by a sheet of Lucite. A Farmer chamber is used as a patient dose monitor and a method for absolute dose calibration is presented. The field is uniform to within +/- 5% for dimensions of 180 X 40 cm2. The surface dose for rotational therapy is equal to 45% of the maximum dose in a stationary beam. The rotating beam exhibits a dose maximum on the surface, falls to 80% at 0.5 cm and has an x-ray contamination of approximately 4%. The surface dose rate is about 25 cGy/min for the rotating beam. The rotational beam percentage depth dose distributions, calculated using stationary beam information, agree well with measured data. The stationary beam exhibits a dose maximum at 4 mm in tissue, a surface dose of 93%, 80% dose at a depth of 1 cm, a practical range of 1.75 cm, and an x-ray contamination of 2.5%. The rotational total skin electron irradiation significantly reduces the patient treatment and setup time and solves the problem of beam matching, when compared to standard multiple-beam techniques.