Theoretical Quantum Efficiency Research Articles

Studies on some α-Fe 2O 3 photoelectrodes

p- and n-type α-Fe 2O 3 samples were prepared and the doping limits of Mg 2+, Cu 2+ and Ca 2+ ions ascertained from X-ray diffraction studies and electrical resistivity measurements. The doping limits are 0.2 and 0.1 at % for Mg 2+ and Cu 2+, respectively. α-Fe 2O 3 is not doped by Ca 2+ and forms a solid-solution to give CaFe 2O 4. Temperature variation of resistivity and thermoelectric power between 300 and 700 K showed that d-level conduction in α-Fe 2O 3 remains unchanged by Mg 2+ doping, but is changed by Cu 2+ doping. Optical constants (n, k) were determined by a Krämers-Kronig analysis from the reflectance spectra recorded in the 450–800 nm wavelength region following a procedure involving a straight line approximation of the reflection attenuation. Pure α-Fe 2O 3 shows an indirect band gap of 2.135 eV and Mg 2+ doped samples show an indirect band gap of 2̃.5 eV and a direct band gap of 1̃.6 eV. The calculated theoretical quantum efficiencies of the samples indicate Cu 2+-doped α-Fe 2O 3 to be better photoanodes in a photoelectrochemical cell for water decomposition than pure α-Fe 2O 3 or Mg 2+-doped α-Fe 2O 3. Experimentally, however, none of these samples show good photoresponse which is attributed to the indirect nature of the band gaps in these samples and their d-level conduction.

X-ray sensitivity of a charge-coupled-device array

The quantitative response of a charge-coupled device (CCD) to x rays in the 5–14-keV range was measured and computed. Comparison of experimental and theoretical quantum efficiencies provides a method to determine electron diffusion lengths in these devices. Compared to a common x-ray film, the CCD amplifier combination tested in this work has a similar threshold and significantly smaller range of response. The CCD offers somewhat poorer spatial resolution than the x-ray film. Comparisons of the CCD with other active x-ray detectors are also presented.

Multiphonon, Raman, and Two-Photon Excitation of Rare-Earth Ions in Crystals: Photon and Phonon Noise in 10-μ Quantum Counters

Noise arising from strong laser pumping and 77-K internal phonon noise upconversion in the proposed supposedly noiseless 10-μ quantum counters using rare-earth-doped crystals is described. The discussions are specialized to the crystals states of the Eu3+ ion. It is shown that at the pump power (∼50 kW cm−2) necessary for background-limited 10-μ quantum counter upconversion detection at 77 K, all the three major processes of noise generation discussed are insignificant compared to the background thermal (300 K) radiation considered previously in arriving at realistic theoretical quantum efficiencies of the proposed 10-μ quantum counters.

The Detection of Modulated Optical Signals by Photoelectric Emission

ABSTRACT The probability of electron emission from a photocathode is related to the probability of photon arrival by defining a theoretical quantum efficiency. Taking dark current into account, the measured and theoretical quantum efficiencies are identical. From this relationship, expressions can be obtained for the mean square photoelectron fluctuation and the uncertainty in photon counting. The power spectrum of the photocurrent fluctuations can be derived in terms of shot noise and the photon fluctuation spectrum (or energy fluctuation spectrum). This result is applied to the detection of beat frequencies and amplitude modulation of quasi-monochromatic light signals. The signal-to-noise ratio for detection is shown to depend on measurable parameters of the detector (quantum efficiency dark current and area) and of the light sources (total intensity, coherence area, line width and polarization).

Compton Backscattered Photons in Precision Proportional Counter Spectrometry

The importance of Compton backscattering in gas proportional spectrometry in the energy range 10 to 100 kev is pointed out. If the backscattered photons are unresolved from the main peak, a large error may be introduced under certain conditions in absolute and relative intensity measurements based on theoretical quantum efficiency (geometry×absorption); if the backscattered peak is resolved, failure to recognize its origin may result in the ``discovery'' of a new gamma ray. The effect is especially pronounced when a collimator is used to establish geometry; for example, the intensity of backscattered photons can amount to as much as half the intensity of the main peak at 46.5 kev with a ¼-in. diameter collimator.