Search

Background: Many dose calculation algorithms for radiotherapy planning need to be configured for each clinical beam using pre-defined measurements. An optimization process adjusts the physical parameters able to estimate the energy released in the medium in any geometrical condition. This work investigates the impact of measured input data quality on the configuration of the type "c" Acuros-XB dose calculation algorithm in the Eclipse (Varian Medical Systems) treatment planning system.Methods: Different datasets were acquired with the BeamScan water phantom (PTW) to configure 6 MV beams, for both flattened (6X) and flattening filter free mode (6FFF) for a Varian TrueBeam: (i) a correct dataset measured using a Semiflex-3D ion chamber, (ii) a set in missing lateral scatter conditions (MLS), (iii) a set with incorrect effective point of measurement (EPoM), (iv) sets acquired with PinPoint-3D chamber, DiodeP, microDiamond detectors. The Acuros-XB dose calculation algorithm (version 15.6) was configured using the reference dataset, the sets measured with the different detectors, with intentional errors, and using the representative beam data (RBD) made available by the vendor. The physical parameters obtained from each optimization process (spectrum, mean radial energy, electron contamination), were analyzed and compared. Calculated data were finally compared against the input and reference measurements.Results: Concerning the physical parameters, the configurations presenting the largest differences were the MLS conditions (mean radial energy) and the incorrect EPoM (electron contamination). The calculation doses relative to the input data present low accuracy, with mean differences > 2% in some conditions. The PinPoint-3D ion chamber presented lower accuracy for the 6FFF beam. Regarding the RBD, calculations compared well with the input data used for the configuration, but not with the reference data.Conclusion: The MLS conditions and the incorrect setting of the EPoM lead to erroneous configurations and should be avoided. The choice of an appropriate detector is important. Whenever the representative beam data is used, a careful check under more clinical geometrical conditions is advised. [ABSTRACT FROM AUTHOR]

Purpose: Spectral information of the output of x-ray tubes in (dual source) computer tomography (CT) scanners can be used to improve the conversion of CT numbers to proton stopping power and can be used to advantage in CT scanner quality assurance. The purpose of this study is to design, validate, and apply a compact portable Compton spectrometer that was constructed to accurately measure x-ray spectra of CT scanners. Methods: In the design of the Compton spectrometer, the shielding materials were carefully chosen and positioned to reduce background by x-ray fluorescence from the materials used. The spectrum of Compton scattered x-rays alters from the original source spectrum due to various physical processes. Reconstruction of the original x-ray spectrum from the Compton scattered spectrum is based on Monte Carlo simulations of the processes involved. This reconstruction is validated by comparing directly and indirectly measured spectra of a mobile x-ray tube. The Compton spectrometer is assessed in a clinical setting by measuring x-ray spectra at various tube voltages of three different medical CT scanner x-ray tubes. Results: The directly and indirectly measured spectra are in good agreement (their ratio being 0.99) thereby validating the reconstruction method. The measured spectra of the medical CT scanners are consistent with theoretical spectra and spectra obtained from the x-ray tube manufacturer. Conclusions: A Compton spectrometer has been successfully designed, constructed, validated, and applied in the measurement of x-ray spectra of CT scanners. These measurements show that our compact Compton spectrometer can be rapidly set-up using the alignment lasers of the CT scanner, thereby enabling its use in commissioning, troubleshooting, and, e.g., annual performance check-ups of CT scanners. [ABSTRACT FROM AUTHOR]

The effect of postweld heat treatments (PWHT) and hydrogen charge on disbonding susceptibility of type 347 weld overlays on 2.25Cr-lMo base material was studied. Following PWHTs at 690° C (ASME code requirements) high disbonding susceptibilities were found. Martensitic bands were formed during welding at the cladding interface. During PWHT at 690° C, these bands have been reaustenitized and massive carbide precipitation took place in regions closest to the parent metal. Carbide precipitation involved austenite destabilization during cooling from PWHT and local formation of virgin martensite. The presence of high hydrogen contents and of tensile residual stresses in interfacial regions of the clad resulted in a heavy local loss of ductility and disbonding occurrence. In order to reduce the disbonding susceptibility, temperatures not exceeding 600° C should be reached during PWHT, so that the reaustenitization of cladding interfacial regions can be avoided and more favorable stress and hydrogen distributions be obtained locally. A reduced disbonding susceptibility can be obtained on samples heat treated at 690° C only if a final heat treatment at 590° C is superimposed, confirming the importance of structural evolution during PWHT in influencing resulting residual stresses and hydrogen distributions. [ABSTRACT FROM AUTHOR]

The variations of residual stresses and microstructures obtained by surface hardening with a 15-kw, continuous-wave CO laser on samples of AISI 1045 (C43) and AISI 4140 (40CD4) have been studied. The ranges for laser operating parameters were 10 to 38 W/mm s for the power and 0.024 to 1.12 seconds for the beammetal interaction time. From residual stress measurements, it is found that the stresses in the more superficial layers may vary, with changing laser operating conditions, from compressive stress values (>200 MPa) to strong tensile stress values (up to approximately +800 MPa). The maximum temperatures reached during laser treatment, the depth of heating, and the boundary conditions imposed influence the residual stress state. It seems possible to limit the onset of tensile stress states at the surface or even to obtain a compressive stress state by heating the material for relatively long periods of time (0.5 to 1.5 seconds) and by avoiding high surface temperatures. These operating procedures, however, do not provide a particularly finegrained martensitic structure. It has, therefore, not proved possible to optimize treatment from both a microstructural and residual stress point of view simultaneously by means of single laser treatment. Some preliminary tests involving suitably programmed repeated treatments of samples seem to indicate that improved combinations of the depth of hardening, the characteristics of the martensite, and the state of the residual stress can be obtained. To account for the experimental results, hypotheses have been advanced to explain the stress evolution during laser heating cycles. [ABSTRACT FROM AUTHOR]

Residual stresses originating from laser hardening of localized areas on disk-shaped samples of steels AISI 1045 (C45), AISI 4140 (40CD4) and nodular iron (UNI G90-55-5) were studied. When the measured residual stresses are compared with the stress results obtained in a previous work, in which the whole surface of analogous samples was treated, the influence of the constraint exerted by the material surrounding the treated area became evident: the residual stresses values measured in the present work are significantly higher for the same operative parameters. The role played by the maximum temperature reached on the surface during heating was made evident: the resulting surface stresses tend to approach higher tensile values with increasing surface temperatures. From the data obtained it is therefore evident that the control of this variable is essential to avoid the development of unfavourable surface stress states. Furthermore, in the case of cast iron, the maximun temperature reached influences the retained austenite content which, at high levels, modifies significantly the resulting stress state. Repeated thermal treatments were also carried out. By this means a compromise is achieved between the characteristics of the martensite obtained and the resulting surface stresses. [ABSTRACT FROM AUTHOR]

We report production rates, rotational temperatures, and related parameters for gases in C/2013 R1 (Lovejoy) using the Near InfraRed SPECtrometer at the Keck Observatory, on six UT dates spanning heliocentric distances (Rh) that decreased from 1.35 AU to 1.16 AU (pre-perihelion). We quantified nine gaseous species (H2O, OH*, CO, CH4, HCN, C2H6, CH3OH, NH3, and NH2) and obtained upper limits for two others (C2H2 and H2CO). Compared with organics-normal comets, our results reveal highly enriched CO, (at most) slightly enriched CH3OH, C2H6, and HCN, and CH4 consistent with “normal”, yet depleted, NH3, C2H2, and H2CO. Rotational temperatures increased from ∼50 K to ∼70 K with decreasing Rh, following a power law in Rh of –2.0 ± 0.2, while the water production rate increased from 1.0 to 3.9 × 1028 molecules s–1, following a power law in Rh of –4.7 ± 0.9. The ortho-para ratio for H2O was 3.01 ± 0.49, corresponding to spin temperatures (Tspin) ⩾ 29 K (at the 1σ level). The observed spatial profiles for these emissions showed complex structures, possibly tied to nucleus rotation, although the cadence of our observations limits any definitive conclusions. The retrieved CO abundance in Lovejoy is more than twice the median value for comets in our IR survey, suggesting this comet is enriched in CO. We discuss the enriched value for CO in comet C/2013 R1 in terms of the variability of CO among Oort Cloud comets. [ABSTRACT FROM AUTHOR]