A. de Almeida

Dust release rates and dust-to-gas mass ratios of eight comets

Mass release rates of dust and mass ratios of dust-to-gas release rates of Comets Thiele (1985m), Wilson (1986l), P/Borrelly (1987p), Liller (1988a), Bradfield (1987s), Hartley-Good (1985l), P/Giacobini-Zinner (1984e), and P/Halley (1982i) are estimated from the analysis of continuum flux measurements at optical wavelengths. An attempt is made to estimate the size of each comet nucleus on the basis of water-ice sublimation (vaporization), assuming that the nucleus is spherical and only a fraction of its surface area is active. Where possible, the dust mass release rates are compared with those obtained by other investigators in the optical and IR wavelength regions. Good agreement with results based on IR observations is found.

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.

Photodissociation lifetime of 32S2 molecule in comets

Photodissociation lifetime of 32s2in comets is calculated by absorption of solar photons into the B3Σ− state and velocity distributions of sulphur atoms are determined. Absorption of solar photons of wavelength ~ 280 nm leads to a photodissociation lifetime of about 250 s for 32S2 molecule in comets when sun-comet distance is 1 AU. Forbidden lines corresponding to 1D-3P transitions of neutral sulphur atom may be detectable at about 11 306 and 10 821 Å in comets. The production rate of 32S2 dimer in comet IRAS-Araki-Alcock 1983d compares well with the production rate of CS, observed in comet Bradfield, when compared at the same heliocentric distance. The chemistry of 32S2 dimer formation in the inner coma of a comet is discussed in the framework of some gas phase reactions.

Water release rates, active areas, and minimum nuclear radius derived from visual magnitudes of comets—an application to comet 46P/Wirtanen

Abstract A semi-empirical photometric method is developed to estimate water release rates from visual magnitude measurements of “normal” comets of abundance ratio C 2 :CN ≈ 1.46 ± 0.68 for the heliocentric distance range 2.5-0.5 AU for which the aperture of the instrument, projected on the coma, is larger than the effective scale length of C 2 . The method is applied to comets Austin (1982 VI), Bradfield (1979 X), Bradfield (1980 XV), Bradfield (1987 XXIX), P/Brorsen-Metcalf (1989 X), Levy (1990 XX), Liller (1988 V), and 46P/Wirtanen. The water release rates are compared with those derived from observed OH or OI (6300 A) line fluxes. The H 2 O release rates estimated from visual magnitude measurements may have typical errors of about 2–4, while maximum errors may be as high as a factor of 10. The total active area ( AA ) on the sunlit side of the nucleus has been estimated from derived water release rates combined with theoretical estimates of H 2 O flux rates (molecules cm −2 s −1 ) by vaporization theory for an assumed nuclear visual albedo p v = 0.03. The constraint AA ≤ 2 π ( R N ) 2 has been used for estimation of the lower limit of the nuclear radius of a comet. For comet 46P/Wirtanen, the variation of water release rates with heliocentric distance ( r ), total active area, lower limit of nuclear radius, and fractional active area are: Q (H 2 O) = 1.8 × 10 28 r −7.0±0.4 mols −1 , 6 ± 3 km 2 , 1.0 km, and 0.15, respectively.

Predictions on finding the SiH+ and SiH in interstellar space

Frequencies are computed for transitions in the SiH+ and SiH molecules which are expected to occur in the millimeter region usually covered by radiowave astronomy. These molecules are expected to be in low-temperature sources whose state temperatures are not greater than about 20 K. On the basis of binary gas-phase reactions, the formation and destruction of the SiH+ and the SiH radicals are also discussed.

Quality Assurance of a Two-Dimensional CCD Detector System Applied in Dosimetry

Nowadays, charge-coupled devices (CCDs) are applied for many technological purposes, including areas as astronomy, medical diagnosis and the ones related to light detecting sensors. One of the main advantages of a digital detector is its ability to store and rapidly transfer information to digital equipments, where data can be computed (post-processing). In fact, this is a relevant feature for medical applications, specially for imaging fields, in which the radiation exposure time is highly controlled when different image quality adjustments are necessary. The main goal of this article is to demonstrate a CCD application for optical density-based dosimeters, where high accuracy and spatial resolution are required. An optical prototype was developed considering a visible light source and a CCD camera, allied with a computational processing routine. The system resolution was inferred through two different methodologies, which support a spatial resolution better than 0.5 mm: 1) a radiological test system (line pair gauge) and 2) the modulation transfer function (MTF). Results also indicate that, due to CCD suitable resolution, one and two dimension responses can be acquired. Considering the easiness and fastness of the process, the developed system exhibits potential applicability for radiotherapy dosimetry.

Microcontrolled pyro-electric instrument for measuring X-ray intensity in mammography

A novel instrument for measurement of X-ray intensity from mammography consists of a sensitive pyro-electric detector, a high-sensitivity, low-noise current-to-voltage converter, a microcontroller and a digital display. The heart of this device, and what makes it unique is the pyro-electric detector, which measures radiation by converting heat from absorbed incident X-rays into an electric current. This current is then converted to a voltage and digitised. The detector consists of a ferro-electric crystal; two types were tested; lithium tantalate and lithium niobate. X-ray measurement in mammography is challenging because of its relatively low photon energy range, from 11 keV to 15 keV equivalent mean energy, corresponding to a peak tube potential from 22 to 36 kV. Consequently, energy fluence rate or intensity is low compared with that of common diagnostic X-ray. The instrument is capable of measuring intensities as low as 0.25 mWm−2 with precision greater than 99%. Not only was the instrument capable of performing in the clinical environment, with high background electromagnetic interference and vibration, but its performance was not degraded after being subjected to 140 roentgen (3.6×10−2 C kg−2 air) as measured by piezo-electric (d33) or pyro-electric coefficients.

Haser model CN, C2 and C3 production rates in some comets

Emission fluxes of CN, C2 and C3 species observed in the coma of some comets are analysed in the framework of Haser model. CN, C2 and C3 production rates are determined using recently derived fluorescence efficiencies and dependence of CN, C2 and C3 production rates on the heliocentric distance is studied. Evidence for a burst type activity around January 15, 1974 in the post-perihelion period of comet Kohoutek (1973f) is observed.

8 and 10 MeV Electron Beams Small Field-Size Dosimetric Parameters Through the Fricke Xylenol Gel Dosimeter

When small field sizes are recommended in radiotherapy, the dosimeter must have an adequate spatial resolution in order to determine the absorbed dose at the region of interest. The study of electron small field size is important since its dosimetry is not commonly performed in the clinical routine. It was verified that the Fricke Xylenol Gel (FXG) chemical dosimeter, with an effective atomic number of 7.75 and density of 1.05 g/cm3, presents adequate spatial resolution for absorbed dose distribution measurements, when small field sizes (square and circular) for 8 and 10 MeV electron beams are considered. The absorbed dose values are proportional to the absorbance spectrophotometric measurements that are proportional to the concentration of Fe+3 and the xylenol orange (XO) dye complex produced in the gel. The FXG behavior, for small field sizes irradiated with electron beams, was compared with those obtained using a small ionization chamber (IC). In this study, dosimetric parameters, such as beam profile, output factor, and percentage depth dose were evaluated. Since the dosimeter results showed no significant differences and the IC is considered the standard reference dosimeter by radiotherapy protocols, the FXG was validated for dosimetric parameter measurements to small field-size electron-beam irradiations.

FXG mass attenuation coefficient evaluation for radiotherapy routine

The knowledge of a radioactive beam energy or quality is important in radiotherapy once it is correlated with the type, size, and localization of the tumor. One indicative of the radiation quality is the half-value-layer (HVL), the material thickness which reduces the beam intensity to half. The analysis of a treatment beam spectrum can be inferred through its homogeneity coefficient (HC, ratio between the first and the second HVL) that for values ≥ 0.7 has the indication to be adequate for treatments. Another important indicator of radiation quality is the mass absorption coefficient (cm2/g), related to the photons energies absorbed in a particular exposed material. Once that several materials can be used as radiation detectors for X and γ dosimetry, this work has the purpose to verify the ferrous Xylenol gelatin (FXG) material performance, through its μ/ρ behavior and compare it with the μ/ρ behavior for soft tissue. The X and γ energies where selected, in the energies normally used in radiotherapy and their spectra were evaluated using the HC coefficient. The μ/ρ, for the FXG material, were obtained experimentally and from simulation with X-COM and a developed routine using the GEANT4 Library. From the results from all μ/ρ values obtained for the FXG material, when compared to those from water, one can see similar behaviors, when one considers measurements for energies greater than 78.0 keV. These results indicate that, once the human body is composed with ±80 % of water, the FXG for the energies used, could also be used as soft tissue simulator.