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One possible mechanism by which diet may reduce cancer risk is through enhancement of metabolic systems that prevent activation of carcinogens or accelerate carcinogen inactivation. We studied the effects of diet and 7,12-dimethylbenz-(a)anthracene (DMBA) on hepatic and mammary gland CYP1A1, CYP1A2 and CYP1B1 enzymes in female Sprague-Dawley rats. Diets (AIN-93G) were fed from conception to adulthood, and DMBA was given by oral gavage at age 48-50 d. The protein sources of diets were casein (CAS), soy protein isolate (SPI) or whey protein hydrolysate (WPH). The DMBA-induced hepatic ethoxyresorufin-O-deethylase and methoxyresorufin-O-demethylase activities and CYP1A1 protein and mRNA expression were lower (P < 0.05) in SPI-fed rats compared with those fed casein. Differences in mammary gland CYP1 expression were also observed with decreased DMBA induction (P < 0.05) of all three CYP1 proteins and mRNAs in rats fed either SPI or WPH compared with those fed CAS. Most notable were the decreased constitutive and DMBA-induced mammary gland expression of CYP1B1 protein of 93 and 96%, respectively, in the SPI-fed rats relative to the CAS-fed controls. The diet-induced changes in CYP1 enzyme expression were consistent with changes in the AhR and ARNT transcription factors that regulate them. Decreased (P < 0.05) mammary constitutive AhR and ARNT proteins were measured in SPI-fed rats. There was also a 100% increase in constitutive AhR protein in the WPH-fed rats that paralleled a 100% increase in constitutive CYP1B1 protein in the mammary gland. These results demonstrate the importance of diet in regulation of phase I metabolism in liver and mammary gland, and suggest a potential mechanism by which soy or whey proteins reduce DMBA-induced mammary tumor incidence. KEY WORDS: * cytochrome P450 * rats * soy protein * whey protein * casein.

The spatial distribution of x-ray induced net bulk space charge in stabilized amorphous selenium (a-Se) x-ray photoconductors is studied using the time-of-flight photoconductivity technique. It is shown that x-ray irradiation resulted in a net negative space charge distribution in a-Se photoconductors. The spatial variation of the space charge density also decayed exponentially from the irradiation surface.

The use of high resistivity amorphous solids as photodetectors, especially amorphous selenium, is currently of great interest because they are readily produced over large area at substantially lower cost compared to grown crystalline solids. However, amorphous solids have been ruled out as viable radiation detection media for high frame-rate applications, such as single-photon-counting imaging, because of low carrier mobilities, transit-time-limited photoresponse, and consequently, poor time resolution. To circumvent the problem of poor charge transport in amorphous solids, we propose unipolar time-differential charge sensing by establishing a strong near-field effect using an electrostatic shield within the material. For the first time, we have fabricated a true Frisch grid inside a solid-state detector by evaporating amorphous selenium over photolithographically prepared multi-well substrates. The fabricated devices are characterized with optical, x-ray, and gamma-ray impulse-like excitations. Results prove the proposed unipolar time-differential property and show that time resolution in non-dispersive amorphous solids can be improved substantially to reach the theoretical limit set by spatial spreading of the collected Gaussian carrier cloud. [ABSTRACT FROM AUTHOR]

Although the effect of the impact ionization and the consequent avalanche multiplication in amorphous selenium (a-Se) was established long ago and has led to the development and commercialization of ultrasensitive video tubes, the underlying physics of these phenomena in amorphous semiconductors has not yet been fully understood. In particular, it is puzzling why this effect has been evidenced at practical electric fields only in a-Se among all amorphous materials. For instance, impact ionization seems much more feasible in hydrogenated amorphous silicon (a-Si:H) since the charge carrier mobility in a-Si:H is much higher than that in a-Se and also the amount of energy needed for ionization of secondary carriers in a-Si:H is lower than that in a-Se. Using the description of the avalanche effect based on the lucky-drift model recently developed for amorphous semiconductors we show how this intriguing question can be answered. It is the higher phonon energy in a-Si:H than that in a-Se, which is responsible for the shift of the avalanche threshold in a-Si:H to essentially higher fields as compared to a-Se. [ABSTRACT FROM AUTHOR]

The kinetics of the photodarkening effect has been studied experimentally for amorphous selenium (a-Se) layers at room temperature and at an elevated temperature (35 °C) close to the glass transition. By switching an intense pumping light on and off with a period of 100 s, we have studied the kinetics of both the buildup of photodarkening and its relaxation (recovery). It was found that at 35 °C, only a reversible component of photodarkening has been observed. This result has been interpreted within the framework of a phenomenological model assuming that photodarkening is caused by light-induced transitions of structural units from their ground states into metastable states. Our estimate for the energy barrier EB between these states obtained for the photodarkening process (EB∼0.8 eV) coincides with that obtained from the analysis of the relaxation process. At room temperature, an irreversible component of photodarkening has been observed along with the reversible one. The energy barrier responsible for the relaxation of the reversible component at room temperature appears the same as at 35 °C. This suggests that the energy barrier identified represents a fundamental feature of the photoinduced structural metastability in amorphous selenium. [ABSTRACT FROM AUTHOR]

The review of avalanche multiplication experiments clearly confirms the existence of the impact ionization effect in this class of semiconductors. The semilogarithmic plot of the impact ionization coefficient (α) versus the reciprocal field (1/F) for holes in a-Se and electrons in a-Se and a-Si:H places the avalanche multiplication phenomena in amorphous semiconductors at much higher fields than those typically reported for crystalline semiconductors with comparable bandgaps. Furthermore, in contrast to well established concepts for crystalline semiconductors, the impact ionization coefficient in a-Se increases with increasing temperature. The McKenzie and Burt [ S. McKenzie and M. G. Burt, J. Phys. C 19, 1959 (1986) ] version of Ridley’s lucky drift (LD) model [ B. K. Ridley, J. Phys. C 16, 3373 (1988) ] has been applied to impact ionization coefficient versus field data for holes and electrons in a-Se and electrons in a-Si:H. We have extracted the electron impact ionization coefficient versus field (αe vs F) data for a-Si:H from the multiplication versus F and photocurrent versus F data recently reported by M. Akiyama, M. Hanada, H. Takao, K. Sawada, and M. Ishida, Jpn. J. Appl. Phys.41, 2552 (2002) . Provided that one accepts the basic assumption of the Ridley LD model that the momentum relaxation rate is faster than the energy relaxation rate, the model can satisfactorily account for impact ionization in amorphous semiconductors even with ionizing excitation across the bandgap, EI=Eg. If λ is the mean free path associated with momentum relaxing collisions and λE is the energy relaxation length associated with energy relaxing collisions, than the LD model requires λE>λ. The application of the LD model with energy and field independent λE to a-Se leads to ionization threshold energies EI that are quite small, less than Eg/2, and requires the possible but improbable ionization of localized states. By making λE=λE(E,F) energy and field dependent, we were able to obtain excellent fits to α vs 1/F data for both holes and electrons in a-Se for both EI=Eg/2 and EI=Eg. In the former case, one expects occupied localized states at EF (=Eg/2) to be ionized and in the second case, one expects excitation across the bandgap. We propose that ionization excitation to localized tail states very close to the transport band can explain the thermally activated α since the release time for the observed activation energies is much shorter than the typical transit times at avalanche fields. For the a-Se case, EI=Eg≈2 eV leads to the following conclusions: (a) For holes, λE has negligibly little field dependence but increases with energy. At the ionization threshold energy λE∼4 nm. (b) For electrons, λE increases with energy and the field with λE∼2 nm at the ionization threshold and at impact ionization fields. For electron impact ionization in a-Si:H, the data can be readily interpreted in terms of near bandgap ionization EI=Eg and a λE that decreases with increasing field, and having very little energy dependence. The energy relaxation length has opposite tendencies in a-Se and a-Si:H, which probably reflects the distinctly different types of behavior of hot carriers in the transport band in these two amorphous semiconductors. [ABSTRACT FROM AUTHOR]