M.A. Parada

Ferrous Xylenol Gel Measurements for 6 and 10 MV Photons in Small Field Sizes

The Fricke dosimeter is a ferrous sulfate aqueous solution that, when irradiated, oxidizes the Fe2 + ions to Fe3 + . This new concentration, generally determined through spectrophotometry, is directly proportional to the ionizing radiation absorbed energy. The Fricke Xylenol Gel dosimeter (FXG) was developed through the incorporation of swine skin gelatin and xylenol orange. These modifications provided better signal stability and sensitivity for lower absorbed dose measurements, such as those used in radiotherapy. In this work FXG samples were irradiated with absorbed doses of 2 Gy, from 6 MV and 10 MV photons, using small field sizes geometry for dosimetric parameters determination. All the FXG dosimeter readings were accomplished with our specially developed spectrophotometer, using a narrow light beam at the wavelength of 585 nm, where the highest absorbance sensitivity occurs. From our results, we can confirm not only that the FXG dosimetric system (FXG plus a high lateral spatial resolution spectrophotometer) can be used for general dosimetry, but as well for small field size dosimetry of interest in radiosurgery.

Teflon electret radiation dosimeter

Abstract Electrets are insulating materials with a quasi-permanent electric polarization. Since charge compensation occurs when subjected to ionizing radiation, electrets may be used for α, β, γ, X, e− and neutron radiation dosimetry. The compensating charge may either be produced in the electret material itself or by interaction by the radiation field with surrounding insulating material. We report the results of investigations of electrets produced from Teflon® polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexa-fluoropropylene (FEP) and tetrafluoroethylene-per-fluoromethoxyethylene (PFA) films. The electret state was produced uniformly on one surface of the films by a corona method with a negative 15 kV potential applied to multiple pointed electrodes facing the film on a grounded plate. After polarization, the films were exposed to a known X radiation and the uncompensated charge was nondestructively measured with capacitive probes. The area of the probe was designed in accordance with the spatial resolution desired. The resolution and sensitivity of such probe is ultimately limited by the Paschen discharge between the electret and the probe. Response curves compared the ratio of uncompensated charge density after and before exposures to the radiation as a function of exposure. The linearity of these curves shows that the films may be used as a radiation dosimeter. The slope of the response curves indicates the sensitivity to the ionizing radiation. The PFA film displays two linear regions which correspond to two electron trap levels. We demonstrate that the image forming nature of these planar electret dosimeters has a millimeter of spatial resolution. We also report the development of innovative electret geometry for measurements of the directional dependence of the radiation and by choice of the surrounding insulating materials, an almost complete selectivity in mixed radiation fields.

Fluoropolymer studies for radiation dosymetry

The polymers Tetrafluoroethylene- hexa- fluoropropylene (FEP) and Tetrafluoroethylene- per- fluoromethoxyethelene (PFA) are normally used as anti-adherent coatings and can also be applied for several applications in research. For example, they can be used as radiation dosimeters for X-ray and gamma photons, electrons, protons and other ionazing particles. In order to determine radiation induced damage, that can compromise applications in dosimetry, FEP and PFA films were bombarded with protons of 1 MeV at fluences from 1 × 1011 protons/cm2 to 1 × 1016 protons/cm2. During the bombardment, the chemical species emission was monitored with a Residual Gas Analyzer (RGA), and results show that the CF3 radical is the specie preferentially emitted. The bombarded films were also analyzed with Optical Absorption Photospectrometry (OAP) which shows quantitative chemically specific evidence of the damage caused by the proton bombardment. Our results show that damage to polymers is detectable for all fluences used in this work, but damage that can compromise applications in dosimetry occurs only for fluences greater than 1 × 1014 protons/cm2.

Digital Image Analysis to Standardize a Photometric Method in Colorimetric Quantification

Abstract Techniques applying digital images increasingly have been used in biology, medicine, physics, and other research areas. The image coordinates can represent light intensities values to be detected by a CCD. Based on this concept, a photometric system composed of a LED source and a digital camera as a detector was used for optical density measurements. Standards for permanganate, glucose, and protein solutions were determined by colorimetric methods using our device. Samples of protein of Pasteurella mutocida bacteria membrane and, also, fractions of rabbit kidney membrane, rich in Na, K‐ATPase, with unknown concentrations were dosed through the Hartree method using our photometric system.

Structural Changes in Polymer Films by Fast Ion Implantation

In applications from food wrapping to solar sails, polymers films can be subjected to intense charged panicle bombardment and implantation. ETFE (ethylenetetrafluoroethylene) with high impact resistance is used for pumps, valves, tie wraps, and electrical components. PFA (tetrafluoroethylene‐per‐fluoromethoxyethylene) and FEP (tetrafluoroethylene‐hexa‐fluoropropylene) are sufficiently biocompatible to be used as transcutaneous implants since they resist damage from the ionizing space radiation, they can be used in aerospace engineering applications. PVDC (polyvinyllidene‐chloride) is used for food packaging, and combined with others plastics, improves the oxygen barrier responsible for the food preservation. Fluoropolymers are also known for their radiation dosimetry applications, dependent on the type and energy of the radiation, as well as of the beam intensity.In this work ETFE, PFA, FEP and PVDC were irradiated with ions of keV and MeV energies at several fluences and were analyzed through techniques ...

Charge density limits in polymer film electrets

Corona charging process is widely used to produce the electret state in materials that include polymers. Polymer films may be charged in air by sweeping the surface with a linear array of corona triode electrodes. We report the development of a constant current total area charging system and our production of electrets on polymers films of ETFE (Ethylenetetrafluoroethylene), PFA (Tetrafluoropropylene-per-fluoromethoxethylene) and FEP (Tetrafluoroethylene-hexa-fluoropropylene). The corona charging and charge reading experimental setups are described. We have also produced electrets in these materials with MeV ion beams and consider for the first time important phenomena that occur in the high fields that are produced near surface of charged films especially near the rear surface that may be close to a ground plane.

Electret Pattern Fornation in Teflon by Pyroeletric and Photogalvanic Electron Emission From LiNbO 3

High field emission of electrons in the c direction of pyroelectric LiNbO3 has recently been reported [Ref.1]. With modest temperature variations, electron energies are shown to be sufficient to produce the characteristic X-rays from niobium in the crystal and from an adjacent copper foil. We report using these energetic electrons to create the patterned electret state in ETFE (Ethylenetetrafluoroethylene) and in FEP (Tetrafluoroethylene-hexa-fluoropropylene). Temperature sufficient to cause electron emission was established in a LiNbO3 crystal near or in contact with the polymer films. The persistent negative charge near the surface of the films could then be measured nondestructively with a capacitive probe, which we have developed for other electret applications. Photogalvanic electron emission produced by light illumination of the doped photosensitive LiNbO3 crystal was also tested for the electret pattern formation. The charge density injected into the polymer inferred from our measurements is limited by the maximum energy of the pyroelectrically and photogalvanically emitted electrons since the electret field opposes the pyroelectric and photogalvanic field. The density and spatial distribution of charges injected by the pyroelectricity of LiNbO3 into TEFLON electrets are consistent with a selffocusing effect observed in a vacuum by the authors of reference

New electret charger and charge reader systems

Electrets are candidate materials for radiation dosimetry with applications in medical physics. Dosimeters produced with electret materials are able to detect /spl gamma/ - and X-rays, /spl alpha/, /spl beta/, electrons and other charged particles and, with appropriate converters, fast and slow neutrons. To produce and use an electret dosimeter one must have a system to inject an initial charge into the electret material, and another system to read and compare the initial charge with that remaining after the radiation exposure. We have developed a charging device to produce large area thin film electrets using corona discharge with an array of 130 electrodes. Also we have developed one reader and improved another that accurately measure the electret charge density. The first is a plane plate reader connected to an electrometer that measures an induced charge from which the average charge density on the electret can be inferred. The second is a coaxial probe that, together with a scanning system can record 2D images of non uniform exposures on an electret dosimeter film with a spatial resolution determined by the dimensions of the probe and its distance from the film. We report results from both of these charge readers and compare electrets produced from 25 /spl mu/m thick ETFE (Ethylenetetrafluoroethylene),PFA (Tetrafluoropropylene-per-fluoromethoxyethylene) and FEP (Tetrafluoroethylene-hexa-fluoropropylene) polymer films. We demonstrate that the corona charging technique and both reading systems are adequate for electret film dosimeter measurements. We present the relationship between the responses from the two reading systems and we show that the planar probe is more sensitive and rapidly measures average exposure.