Ionizing-Radiation-Induced Color Centers in YAG, Nd:YAG, and Cr:Nd:YAG: Developing and Analyzing a Radiation-Hard Laser Gain Medium
AuthorGlebov, Boris L.
Committee ChairPotter, Kelly S.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThis report presents results from a series of experiments in which YAG samples (undoped, as well as doped with Nd and Cr3+) were exposed to ionizing radiation (gamma rays and UV). These experiments were performed for the purpose of investigating the various phtodarkening processes taking place in these materials in response to the ionizing radiation. The purpose of this investigation was to establish whether and how co-doping YAG with Cr3+ improves the material's resistance to photodarkening due to the ionizing radiation. The experiments tracked time-resolved transmittance of the samples at 1064 nm in immediate response a pulsed exposure to the ionizing radiation, as well as steady-state spectrally-resolved changed in the samples' optical absorption after the irradiation. The investigation revealed a number of photodarkening processes occurring in the samples in response to the ionizing radiation, both transient and permanent. It was further revealed that inclusion of Cr3+ ions in YAG significantly reduces these photodarkening processes, improving the material's radiation resistance. It was observed that materials containing at least 0.5 at% Cr3+ are essentially radiation-hardened, resisting both transient and steady-state changes in transmittance observed in the materials not containing Cr3+.
Degree ProgramOptical Sciences
Degree GrantorUniversity of Arizona
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Investigation of Ionizing-Radiation-Induced Photodarkening in Rare-Earth-Doped Optical Fiber Amplifier MaterialsFox, Brian Philip (The University of Arizona., 2013)Motivated by an increasing demand for functionality and reliability of systems operating in harsh, ionizing-radiation environments, the core of the present research is an investigation of the response of rare-earth-doped, aluminosilicate fibers to ionizing radiation. These rare-earth-doped fibers, consisting of fibers doped with ions of erbium (Er³⁺) and ytterbium (Yb³⁺) designed for use in amplifier systems, reveal average specific losses in response to ⁶⁰Co gamma radiation to be in the range of 0.0285 - 0.193 dB/(m•krad(Si)) at wavelengths from 1300 nm to 1400 nm. An ionizing dose rate dependence was identified in which high dose rates of approximately 40 rad(Si)/s invariably lead to higher induced losses than lower dose rates of approximately 14 rad(Si)/s, indicating the possibility of complex radiation-related phenomena underlying the observed absorption. Data clearly show that Er³⁺-doped fibers are more sensitive to ionizing-radiation in comparison to Yb³⁺-doped fibers, while Er³⁺/Yb³⁺ co-doped fibers are found to be the least sensitive to radiation of all the fibers examined. Evidence of color center formation associated with the dopant aluminum is found in results of visible spectroscopy conducted on gamma-irradiated preform samples and on fibers flown in low-Earth orbit. Near infrared spectroscopic data is consistent with absorption derived from this dopant as well, with the interpretation of band-tailing from the visible portion of the spectrum. Evidence of the formation of a defect intrinsic to the silicate host matrix, the Non-Bridging Oxygen Hole Center (NBOHC), is also found following ionizing radiation of the optical fiber preforms. Since the observed ionizing-radiation-induced absorption is concentrated in the visible portion of the spectrum, the performance of actively operated rare-earth-doped amplifiers is largely impacted by the pump wavelength, which is located at higher energies within the near-infrared portion of the spectrum and therefore closer to the visible portion of the spectrum than the lasing wavelength. Experimental results stemming from rare-earth-doped amplifiers operated under ionizing radiation substantiate the importance of the pumping wavelength, and suggest the presence of cascaded pump photon absorption processes. Based on these results, pumping at longer wavelengths is advised to reduce the effect of color center absorption on this crucial aspect of active fiber amplifier operation.
MOUSE SKIN TUMOR INITIATION BY IONIZING RADIATION AND THE DETECTION OF DOMINANT TRANSFORMING GENE(S).JAFFE, DEBORAH RUTH. (The University of Arizona., 1987)The initiating potential of a range of 4 MeV X-rays was studied using the mouse skin two-stage model of carcinogenesis. A single dose of radiation was followed by promotion with 12-O-tetradecanoyl phorbol-13-acetate (TPA). The effect of TPA on tumor incidence when applied as a single dose 24 hours prior to irradiation was examined. Studies were also designed to investigate the effect of promotion duration on tumor incidence. Animals were promoted with TPA for 10 or 60 weeks. Evidence presented here indicates that ionizing radiation can act as an initiator in this model system. All animals that were promoted with TPA for the same duration had a similar incidence of papillomas (pap) regardless of radiation or TPA pretreatment. However, squamous cell carcinomas (scc) arose only in animals that were initiated with ionizing radiation followed by TPA promotion. Increasing the promotion duration enhanced the incidence of scc at the lower initiation dose. TPA pretreatment at the higher irradiation dose resulted in an overall decrease in tumor incidence. At the lower dose of radiation, TPA pretreatment resulted in an increase in the incidence of scc. The incidence of basal cell carcinomas (bcc) was dose dependent and appeared to be independent of TPA promotion. Although ionizing radiation acts as a weak initiator in mouse skin, the conversion of pap to scc was higher than that reported for chemical initiators. To test this further animals were initiated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) followed by biweekly promotion with TPA. After 20 weeks of promotion, the animals were treated with either acetone, TPA or 8 fractions of 1 MeV electrons. Data indicate that the dose and fractionation protocol used in this study enhanced the progression of pre-existing pap. To examine the role of oncogene activation in radiation induced mouse skin tumors, DNA from various tumors (pap, bcc, scc) were examined for the presence of dominant transforming activity by the NIH3T3 and Rat-2 focus assays. Dominant transforming activity was observed in all tumor types but not in normal or treated epidermis or corresponding liver. The transformed phenotype was further confirmed by growth in soft agar and tumorigenicity in Nude mice. Southern blot hybridization to ras (Ha, Ki, N), raf, neu, erbB and β-lym indicate that these genes are not responsible for the observed transforming activity. These data suggest that the oncogenic sequences activated in these tumors are unique. The work presented here also provides evidence for novel c-myc transcripts and corresponding genomic rearrangements in a few of the tumors studied.