Showing papers on "Fluence published in 1968"

Some observations on charge buildup and release in silicon dioxide irradiated with low energy electrons

Abstract: The positive charge buildup produced in silicon dioxide by low energy electrons (0 to 30 keV) has been investigated as a function of beam energy and oxide thickness. The induced charge, as evidenced by displacement of capacitance versus voltage plots, was found to be a function of the beam energy dissipated within the oxide in the vicinity of the oxide-silicon interface. The charge induced at a particular fluence level in an oxide of given thickness increases with energy up to some level E max beyond which the charge buildup rate falls off as the energy is increased further. Continued falloff in the buildup rate was observed in several samples irradiated at energies of 200 keV and 1 MeV. E max has been found to correspond to the beam energy which, according to predicted range-energy data, produces maximum energy dissipation per unit path length in the oxide near the silicon interface. Constant temperature annealing of irradiated MOS samples has indicated that the annealed charge is linearly dependent on the logarithm of elapsed time over a finite time interval. This is particularly evident at room temperature where a linear dependence on In ( t ) has been observed out to 105seconds. Such a time dependence of released charge can result either from thermal activation of trapped carriers from a uniform trap distribution or from thermal emission of recombination electrons over a Schottky barrier from the silicon into the oxide; however, both of these models predict the released charge to be a linear function of absolute temperature. A much stronger temperature dependence has been observed during these experiments.

The fluence of low energy electrons established within and emitted from irradiated conducting materials.

Abstract: A parallel-plate vacuum chamber has been used to measure the low energy electron fluence emitted from and established within irradiated materials. The variation of the electron fluence with photon energy and atomic number of the material has been studied and implications for dosimetry discussed. The low energy electron fluence within soft tissues irradiated by photons is estimated as 4 × 106 electrons cm−2 sec−1.

Thermoluminescence response of CaF2:Mn in polytetrafluoroethylene to electrons.

Abstract: Foils of powdered CaF2:Mn in polytetrafluorethylene, with thicknesses from 15 to 150 μm, were exposed to monoenergetic electrons having energies in the region between 20 and 400 keV. The thermoluminescence response of the foil was studied as a function of the energy and the fluence of the incident electrons, the foil thickness, and the electron energy absorbed in the foils. The dependence on electron energy of the response per unit fluence was found to change considerably with foil thickness, the maximum reponse occurring at approximately the energy of the incident electrons whose ranges are equal to the thickness of the foil layers. From the lowest incident-electron energy employed (20 keV) up to this energy (200 keV for the 150 μm layer), response per unit fluence increased linearly with electron energy. Therefore, response per unit energy fluence (in this energy range approximately equal to total energy absorbed in the foils) was independent of the energy of the incident electrons, at least within the large error of the experiment. The response per unit of electron energy absorbed also agreed, within the limits of experimental error, with that obtained for foils exposed to 60Co gamma radiation.