E. Brannen

The Response of Organic Scintillators to Electron Energy Deposited in Them

The response of organic scintillators to electron energy deposited in the medium was investigated doing work concerned with the validity of results obtained with a scintillation dosimeter. The fluorescent yield of such scintillators was shown to be proportional to energy deposition in the medium down to energies of 20 kev by using an iron-free beta -ray spectrometer and a well-type scintillator. Nonlinear effects can be expected to occur only near the surface of the scintillator.

Excitation processes and relaxation rates in the pulsed water vapor laser.

This paper presents an experimental study of time-resolved gain in H(2)O, H(2)O-He, and H(2)O-H(2) mixtures as a function of gas composition and excitation current. Utilizing the fast rising (~70 nsec) pulse from H(2)O-He laser as a probe, the amplifier gain was measured with a time resolution of about 100 nsec. The gain was observed to follow the excitation current pulse rather closely indicating that population inversion was established in times less than 100 nsec. This suggested that excitation was most likely by means of rapid cascading from higher levels and/or by direct electron impact. The gain was found to be describable by a two-level rate equation model containing one dominant relaxation rate and assuming immediate excitation of the levels involved by inelastic collisions with electrons. With pure H(2)O, the relaxation rate was proportional to pressure to within 10%, indicating that the upper level was de-excited primarily by c llisions with other H(2)O molecules. At a pressure of 1 Torr the relaxation rate in pure H(2)O was 0.35 +/- 0.05 for the 28-microm transition. The addition of small amounts of foreign gases was observed to increase this relaxation rate, consistent with the measured decrease in the amplifier gain. By subsequently increasing the water vapor pressure it was found possible to optimize the gain at an enhanced level over the pure H(2)O case. The peak gain obtained in water vapor at 1000 A was 0.34 m(-1). Under foreign gas addition this increased to 0.68 m(-l) for the same peak current. In this case the relaxation rate, as a function of the foreign gas (He or H(2)) pressure, remained constant to within 10%, suggesting that these gases at higher concentrations may enhance the system gain by altering the discharge conditions without appreciably collisionally de-exciting the upper laser level.

Absorption Line Parameter Measurements Using Laser Spectroscopy

An analytical method is described for obtaining the precise location of an absorption line and the pressure broadening coefficients due to self-broadening or foreign gas broadening from experimental measurements using a laser operating on a single line near the absorption line. These absorption line characteristics are obtained from the pressure dependence of the transmittance of the laser radiation for the gas of interest, the analysis involving a least squares fit to a family of Lorentz curves. The method includes a computer search for the region of best fit to the Lorentz profile and provides both the values of and errors in the above coefficients. The pressure broadening coefficient obtained is the same as for the more general Voigt curve. The method is applied to the absorption line of vinyl chloride near 27.972 microm and of methane near 3.392 microm and the results compared with a graphical fit to a Voigt profile. The self-broadening coefficients obtained were alpha = 0.15 x 10(-3) +/- 0.97 x 10(-4) cm(-1) Torr(-1) for vinyl chloride and alpha = 0.14 x 10(-3) +/- 0.46 x 10(-4) cm(-1) Torr(-1) for methane. The separation between the helium-neon laser line at 2947.903 cm(-1) and the methane absorption line at 2947.888 +/- 0.015 cm(-1) was found to be 0.21 x 10(-2) +/- 0.15 x 10(-2) cm(-1).

TIME DEPENDENCE OF THE POWER OUTPUT AT 337 μ IN A CN LASER

The time dependence of the laser output pulse at 337 μ in an acetone and nitrogen gas mixture has been studied using a fast indium antimonide detector. Current pulses from a line‐type modulator were varied in length from 2 to 20 μsec with amplitudes from 50 to 180 A. The start of the laser output pulse occurred after the current pulse in the discharge was over, from 6 to 8 μsec after the end of the current pulse. These results indicate that, for the currents used, population inversion and the achievement of conditions allowing laser action, take from 6 to 8 μsec after the discharge ends to become established.

Carbon dioxide absorption of He–Ne laser radiation at 4.2 μm: characteristics of self and nitrogen broadened cases

A laser resonance absorption spectrometer is used to investigate the characteristics of both self and nitrogen collision broadened carbon dioxide in resonance with He-Ne laser radiation at 4.2 microm. The absorption coefficient in these broadening conditions has contributions from the R(28) to R(34) absorption lines of the nu(3) CO(2) spectrum. The Fletcher-Powell optimization method is used to reduce the raw absorption data and to find the best value average collision broadening coefficient and laser emission frequency for a Lorentzian line shape model of the contributing lines. Pure carbon dioxide absorption in a pressure range of from 0.0016 atm (1.25 Torr) to 0.33 atm (250 Torr) is described well by the model with an average self broadening coefficient of 0.084 +/- 0.008 cm(-1) atm(-1) for laser frequencies located at either 2370.591 +/- 0.020 cm(-1) or 2371.135 +/- 0.019 cm(-1). Nitrogen broadened carbon dioxide in the total pressure range of from 0.13 atm (100 Torr) to 1.18 atm (900 Torr) is characterized by the same model with the laser frequency at 2371.102 +/- 0.007 cm(-1) atm(-1). The average absorption coefficient for low concentrations of carbon dioxide in a 1-atm total pressure nitrogen environment has been determined experimentally as 9.90 +/- 1.49 cm(-1) atm(-1). All the listed results are at 296 K.

Four-Sector Racetrack Microtrons

Racetrack geometries of the guide field are capable of freeing the microtron type of electron accelerator from its inherent limitations. Two completed racetrack microtrons of 2.5 and 6.3 MeV energy are described. A possible application of the design features of these accelerators to microtrons of up to 50 MeV is discussed. Preliminary results of a design study for a 200 MeV racetrack microtron are presented.

Three‐Dimensional Scintillation Dosimeter

A dosimeter for electron beams has been constructed which utilizes the luminescence of plastic phosphors under electron bombardment. A phosphor, in sheet form, was placed in a Lucite medium and the luminescence of a small volume of the phosphor was focused on a photomultiplier. By scanning the phosphor, the three‐dimensional dose distribution within the medium was determined. Special attention was paid to obtaining the dose close to the surface of the medium by which the electron beam entered.

Beam Extraction in the Microtron

This paper describes the utilization of a simple universal vacuum joint, as the support for a magnetic shield type of deflector in the form of a cylindrical soft iron pipe. Variable insertion of this pipe and change of direction in azimuth permit the extraction, with up to 70% efficiency, of all the microtron orbits, making available, in this case, electrons between 1 and 4 Mev energy, in steps of 0.5 Mev. Radiation intensity, or electron current, from any particular orbit, up to the limit determined by the peak for that orbit, may be set at any desired level by small changes in the insertion of the deflector.

Time dependence of the power output at 119 µ in a water vapor laser

The power output pulse at 119 µ in a water vapor laser was observed for discharge current durations up to 6 µs. Laser action did not occur until the end of the discharge current pulse, indicating that laser action is inhibited during the discharge for the pulse lengths and currents used.

Operating characteristics of a fixed alignment CO2 waveguide laser

A simple CO2 waveguide laser with fixed alignment is described and the operating characteristics for various gas mixtures given. Maximum power obtained was 3 W at a working pressure of 120 Torr. Alignment was still maintained after three years of operation. With a fresh filling of gas, power at the 1‐W level was routinely obtained over a day’s running time.