High Intensity Photon Research Articles

Measurements of 3914-Å light production and electron scattering from electron beams artificially injected into the ionosphere

Measurements of electron backscatter and photon fluxes at 3914 A excited by a 40-keV artificial electron beam were made on an ionospheric sounding rocket. Echo 3 launched from Fairbands, Alaska, on April 17, 1974. The backscattered electron fluxes were found to have a constant value independent of altitude, indicating no dependence on the atmospheric density and being several orders of magnitude higher than that predicted by Coulomb scattering. This result confirms measurements on the Echo 1 and 2 experiments. The origin of this anomalous scattering has not been identified. In contrast, photon emission from the ambient atmosphere excited by the beam at 3914 A varied directly with the change in the density of N$sub 2$ molecules. A rocket 'halo' of high-intensity backscattering and photon emission accompanied beam injections close to 90degree local magnetic pitch angle and was probably caused by beam scattering from the rocket.

A beam monitor system for high-intensity photon beam in the multi-GeV range

A system of instruments used to monitor the high-power SLAC photon beam is described. Operational experience in a series of photoproduction experiments shows an overall accuracy of ±1% with beam powers from 100 W to 3.5 kW at bremsstrahlung end point energies from 5 to 18 GeV.

Photoproduction of Charged Pion Pairs from Hydrogen with Gamma Energies up to 1500 MeV

Charged-pion-pair photoproduction has been investigated up to a $\ensuremath{\gamma}$ energy of 1500 MeV, using the Caltech 12-in. heavy-liquid bubble chamber with a small-diameter, high-intensity photon beam passing through a central beam-tube gaseous hydrogen target surrounded by the sensitive Freon. Scanning, analysis, and data-reduction techniques have been developed to deal with the problems of two-view stereo, hidden-event origins, absence of magnetic field, and the range-energy and multiple-scattering relationships that occur in the heavy materials. Roughly 5700 pictures have been scanned and analyzed, yielding 754 acceptable events. Cross-section and parameter distributions are generally consistent with the results of previous experiments. The total cross section rises sharply from near zero at 500 MeV to order 80 \ensuremath{\mu}b at 650 MeV, then falls slowly to 40 \ensuremath{\mu}b at 1500 MeV. The presence of the ${\ensuremath{\rho}}^{0}$ is clearly seen in the pion pair production at higher energies.

The cadmium photoactivation dosimeter

Of the sixteen elements whose nuclei can be excited to metastable states by 3 MeV bremsstrahlung, cadmium has potentially the most value for dosimetry. The metastable state, Cd 111 m , has a half-life of 49 min and emits 149 and 247 keV photons to return to the ground state. Irradiation of 1 g of cadmium by enough 3 MeV bremsstrahlung to give a dose of 1 Mrad in water gives sufficient cadmium nuclei in the metastable state to register about 3000 cpm. This activity is enough to make cadmium very useful as a small volume dosimeter in very inhomogenous, high-intensity photon fields. Its performance is rather sensitive to various factors that change the photon energy distribution.

Positive Photopion Production from Hydrogen Near Threshold

Differential cross sections for photopion production from protons were measured for 32 photon energies from 154 to 185 MeV with the Alvarez 4-in. liquid-hydrogen bubble chamber in the bremsstrahlung beam of the Berkeley electron synchrotron. High photon intensities of 1.5\ifmmode\times\else\texttimes\fi{}${10}^{7}$ MeV/pulse were used by collimating the beam down to a narrow "ribbon" which was viewed edge on by the camera. Although the pion origins were obscured by the heavy electron background, the remainder of the $\ensuremath{\pi}$ track was visible for most events. Of 5000 $\ensuremath{\pi}\ensuremath{-}\ensuremath{\mu}$ decays seen, 3400 were deemed suitable for calculations. The results are in excellent agreement with the theoretical calculations by Ball, who applied the Mandelstam representation to the process. The measured values of the square of the matrix element, averaged over c.m. angles, are: All data are subject to an additional correlated error of 4.1% because of the uncertainty in the beam normalization. The cross section for the process is the product of the square of the matrix element times the kinematic factor $\frac{{p}^{*}}{{k}^{*}}$, where ${p}^{*}$ and ${k}^{*}$ are the c.m. momenta of the pion and the photon. Also a value of $\ensuremath{\Lambda}=(+0.931\ifmmode\pm\else\textpm\fi{}0.59)e$ was obtained, where $\ensuremath{\Lambda}$ is the multiplicative factor associated with the matrix element for photopion production from pions as calculated by Wong.

The Compton effect on relativistic electrons and the possibility of obtaining high energy beams

When light photons are scattered on extremely relativistic electrons, the energies of the scattered photons are of the same order of magnitude as those of the electrons. The possibility of exploiting this feature for obtaining high energy gamma beams in electron accelerators is considered. To obtain gamma beams by this method high photon fluxes are required. Ruby lasers are discussed as a high intensity photon source. (C.E.S.)

Optical Methods for Negative Ion Studies

A high vacuum crossed-beam apparatus for the study of photodetachment of electrons from negative ions is described, with emphasis on (1) the optical system which transmits a filtered high intensity photon beam, (2) a high transmission mass selector and associated ion optics, and (3) the sensitive ac preamplifier, amplifier, and phase sensitive detector used for measuring the photodetached electron current. The methods used for calibrating and operating the apparatus are discussed.