Photons Of Different Energies Research Articles

On the Possible Mechanisms of Activation Changes of Enzymes under Pulsed Irradiation

Classical radiation biology, radiation treatment of patients, and pulsed radiation chemistry usually deal with weak beams of radiation and operate in terms of dose (D). In this case, only the absolute number of ionizing particles or photons interacting with the object independently of one another is important. In this work, we resume our previous investigations of enzyme activation/inactivation produced by pulsed ionizing radiation from a dense plasma focus (DPF) device at a very short and intense irradiation process, when the concentrations of spurs and blobs are sufficiently dense to allow the micro-volumes to overlap each other. The time interval is small compared with the corresponding biochemical process. It is shown that the irradiation of biological objects for a few nanoseconds by beams of x-ray photons with a low dose (D ∼ 10-6–10-3 Gy) but at a high dose power (D/τ∼103–105 Gy/s) may be of a collective nature and result in synergetic effects. In particular, it was found that a remarkable activation/inactivation of enzymes (horseradish peroxidase, angiotensin-converting-enzyme) takes place at low doses in these regimes. The results of irradiating the above-mentioned enzymes in vitro with x-rays of nanosecond pulse duration coming from a dense plasma focus are presented here. Neutrons (2.5 MeV, 103–105 n/cm2 and x-ray photons of different energy ranges (7–9 keV and 20–200 keV) together with dose power factors are analyzed as possible reasons for the activation/inactivation of enzymes in various dose ranges. Our results are compared to those of the same experiments conducted with continuous x-ray sources based on a classical x-ray tube and on a 137Cs-isotope source (D∼10-3–102 Gy).

In-phantom spectra and dose distributions from a high-energy neutron therapy beam

In radiotherapy with external beams, healthy tissues surrounding the target volumes are inevitably irradiated. In the case of neutron therapy, the estimation of dose to the organs surrounding the target volume is particularly challenging, because of the varying contributions from primary and secondary neutrons and photons of different energies. The neutron doses to tissues surrounding the target volume at the Louvain-la-Neuve (LLN) facility were investigated in this work. At LLN, primary neutrons have a broad spectrum with a mean energy of about 30 MeV. The transport of a 10×10 cm 2 beam through a water phantom was simulated by means of the Monte Carlo code MCNPX. Distributions of energy-differential values of neutron fluence, kerma and kerma equivalent were estimated at different locations in a water phantom. The evolution of neutron dose and dose equivalent inside the phantom was deduced. Measurements of absorbed dose and of dose equivalent were then carried out in a water phantom using an ionization chamber and superheated drop detectors (SDDs). On the beam axis, the calculations agreed well with the ionization chamber data, but disagreed significantly from the SDD data due to the detector's under-response to neutrons above 20 MeV. Off the beam axis, the calculated absorbed doses were significantly lower than the ionization chamber readings, since gamma fields were not accounted for. The calculated data are doses from neutron-induced charge particles, and these agreed with the values measured by the photon-insensitive SDDs. When exposed to the degraded spectra off the beam axis, the SDD offered reliable estimates of the neutron dose equivalent.

Planck-scale deformation of Lorentz symmetry as a solution to the ultrahigh energy cosmic ray and the TeV-photon paradoxes

One of the most puzzling current experimental physics paradoxes is the arrival on Earth of Ultra High Energy Cosmic Rays with energies above the GZK threshold. The recent observation of 20TeV photons from Mk 501 is another somewhat similar paradox. Several models have been proposed for the UHECR paradox. No solution has yet been proposed for the TeV-$\gamma$ paradox. Remarkably, the drastic assumption of a violation of ordinary Lorentz invariance would resolve both paradoxes. We present a formalism for the description of the type of Lorentz-invariance deformation (LID) that could be induced by non-trivial short-distance structure of space-time, and we show that this formalism is well suited for comparison of experimental data with LID predictions. We use the UHECR and TeV-$\gamma$ data, as well as bounds on time-of-flight differences between photons of different energies, to constrain the LID parameter space. A model with only two parameters, an energy scale and a dimensionless parameter characterizing the functional dependence on the energy scale, is shown to be sufficient to solve both the UHECR and the TeV-$\gamma$ threshold anomalies while satisfying the time-of-flight bounds. The allowed region of the two-parameter space is relatively small, but, remarkably, it fits perfectly the expectations of the quantum-gravity-motivated space-time models known to support such deformations of Lorentz invariance: integer value of the dimensionless parameter and characteristic energy scale constrained to a narrow interval in the neighborhood of the Planck scale.

A study of the use of flat‐panel imagers for radiotherapy verification (in English)

This thesis investigates the feasibility of the application of flat‐panel imagers (FPIs) based on amorphous silicon to radiotherapy verification. Intensity modulated radiotherapy imposes new demands on verification procedures. A contrast resolution of 1% may be required from portal imaging systems to resolve the target structure and the organs at risk (OAR). Furthermore a spatial resolution of 1 mm is desired. The imaging characteristics, spatial resolution, efficiency, and noise characteristics of the FPI for megavoltage imaging were investigated regarding these demands. Possible improvements from scatter rejection by a modified detector design were investigated by simulating the spectral response of the converter plates of these detectors to photons of different energies and the response to scattered photons. Since low contrast objects like the target structure and the OAR can be detected in radiographic transmission images only for some rare cases, tomographical imaging was tested for position verification. This was also done using an FPI and the megavoltage (MV) source of the treatment accelerator. A different approach to position verification is the integration of kilovoltage (kV) computed tomography (CT) into a clinical linear accelerator. Although this approach has higher demands on the hardware side, it will deliver higher contrast images at a lower dose. A comparison of MVCT and kVCT and an outlook to new possibilities using integrated CT therefore completes this thesis.

Modelling the electro-thermal response of superconducting transition-edge X-ray sensors

A two-temperature model for simulating the electrothermal response of superconducting transition-edge sensor (TES) X-ray detectors is presented. The model considers that the absorber and the superconducting layer (SL) of the detector are at different temperatures. The time constants within the absorber and the SL are assumed to be much smaller than all remaining time constants of the TES. The time dependences of the current through the SL, the temperature of the absorber, and the temperature of the SL after the absorption of single photons of different energy are found by numerically solving a set of nonlinear differential equations. In order to verify the model, a calculated current transient is compared with a previously measured pulse. Exploring the parameter space defined by appropriate ratios of electro-thermal time constants, we derive constraints for the asymptotically stable, oscillation-free operation of a TES.

Oscillation Waveforms and Amplitudes from Hot Spots on Neutron Stars

The discovery of high-amplitude brightness oscillations during type I X-ray bursts from six low-mass X-ray binaries has provided a powerful new tool to study the properties of matter at supranuclear densities, the effects of strong gravity, and the propagation of thermonuclear burning. There is substantial evidence that these brightness oscillations are produced by spin modulation of one or two localized hot spots confined to the stellar surface. It is therefore important to calculate the expected light curves produced by such hot spots under various physical assumptions, so that comparison with the observed light curves may most sensitively yield information about the underlying physical quantities. In this paper we make general relativistic calculations of the light curves and oscillation amplitudes produced by a rotating neutron star with one or two hot spots as a function of spot size, stellar compactness, rotational velocity at the stellar surface, spot location, orientation of the line of sight of the observer, and the angular dependence of the surface specific intensity. For the case of two emitting spots we also investigate the effects of having spot separations less than 180° and the effects of having asymmetries in the brightness of the two spots. We find that stellar rotation and beaming of the emission tend to increase the observed oscillation amplitudes whereas greater compactness and larger spot size tend to decrease them. We also show that when two emitting spots are either nonantipodal or asymmetric in brightness, significant power at the first harmonic is generated. By applying these results to 4U 1636-536, the two emitting spots of which produce power at the first harmonic, we place strong constraints on the neutron star's magnetic field geometry. We also show that the data on the phase lags between photons of different energies in the persistent pulsations in SAX J1808-58 can be fitted well with a model in which the observed hard leads are due to Doppler beaming.

In-situ diffusivity measurement technique

Abstract We have developed a technique for the measurement of diffusivities in liquids over a wide temperature range. A radiotracer, initially located at one end of the cylindrical diffusion sample, is used as the diffusant. The sample is positioned in a concentric isothermal radiation shield with collimation bores located at defined positions along its axis. The intensity of the radiation emitted through the collimators is measured vs. time with solid state detectors and associated energy discrimination electronics. Diffusivities are calculated from the signal difference between pairs of collimation bores. Self-diffusivities obtained with In/In 114m in space and on Earth illustrate the high precision obtainable with this technique. The In/In 114m space data were close to the ground results, however, the data scatter was much less. By employing a tracer that emits photons of different energy, and thus, different self-absorption, transport in the bulk of the sample can be distinguished from that in the proximity of the wall. No difference was found. In support of this work 2-D numerical modelling of the effect of various blockages on the concentration profile and the resulting apparent diffusivity were conducted. For the methodology that we use very little effect was seen except in the case of extreme blockages.

Selective Metal Deposition on Silicon Substrates

In order to delineate n/p junctions at (100) or cleaved Si faces, different plating processes (electroless in the dark or under illumination with photons of different energy) have been investigated. The effects of metal cation concentration, of illumination upon the formation of deposits onto n‐ or p‐type silicon substrates, and p/n Si junctions have been studied. The role of an interfacial oxide layer has been emphasized. It is shown that platinum or palladium electroless deposition can be carried out on p‐Si under ultraviolet illumination, in solutions free of . By contrast, in presence of a Si p/n junction, platinum, and palladium are only deposited on n‐type areas. Models are proposed to explain the observed selective deposition.

A comparison of semiempirical models for generating tungsten target x-ray spectra.

Models for the computer generation of tungsten target x-ray spectra proposed by Birch and Marshall [Phys. Med. Biol. 24, 505-517 (1979)] and recently by Tucker, Barnes, and Chakraborty [Med. Phys. 18 211-218 (1991)] are compared. Some basic differences in the equations for the number of bremsstrahlung photons of different energies in the spectra are discussed. The models are compared in terms of their ability to characterize x-ray spectra from constant potential clinical units using three parameter equivalent spectra (EQSPEC) determined from the fit of model generated transmission curves through aluminum to measured data. The Kramers model for x-ray generation is included for completeness. It is shown that two of the models generate very similar x-ray spectra from given transmission curves although the fitting parameters in the EQSPEC characterization differ.

Two-photon transitions to excitons in quantum wells.

We calculate two-photon transition rates of exciton states in GaAs quantum wells for varying well widths. The exciton states are described in the effective-mass approximation and are obtained variationally assuming wave functions that are separable in the r coordinates. The two-photon transition rate is calculated taking explicitly into account the sum over all the intermediate states, using a variational procedure. We calculate transition rates of excitons of s symmetry as well as of p symmetry, which are allowed for polarization of the radiation beam along the growth axis and in the layer planes, respectively. From our calculations we find that transitions to s-type excitons have sufficiently large rates to be observable, whereas transitions to p-type excitons seem to have rates too small to be observable; these results are in agreement with experiment. The transition rate of s excitons increases significantly for decreasing well width, whereas the transition rate of p excitons is found to be almost independent of the well width. Moreover, our model allows us to calculate transition rates for photons of different energy. We have shown the resonant enhancement of the transition rate of s-type excitons when one of the photon energies approaches the energy of an intermediate state.

Dual frequency observations of flares with the VLA

We describe observations of three flares made at 5 and 15 GHz with the VLA, two subflares near the limb on 1981 November 21 and 22, and an M7.7 flare on 1981 May 8. Even though the time histories of the November flares indicated simple impulsive bursts, the VLA observed no 5 GHz radiation at all from one flare, and from the other, the 15 GHz radiation emanated from a source which was smaller, lower and displaced from the 5 GHz source. Without the spatial information, we would have derived incorrect results from the assumption that photons of different energy (both at X-ray and radio wavelengths) arose from one homogeneous volume.

A method for the correction of self-absorption of low energy photons for use in routine INAA

Practical application of oow energy gamma rays and X-rays in I.N.A.A. was restricted because of the complexity of the X-ray spectrum and sample self-absorption. This paper describes a method for the calculation of sample self-absorption on the basis of the actual sample spectra only, as measured with a high resolution semiconductor X-ray detector. In the 20–400 keV energy range, the attenuation coefficient can be represented by a three parameter function of photon energy. This was verified by measuring the transmission of photons of different energies through a range of materials. Experiments with neutron irradiated U.S.G.S. standard reference materials with known major oxide composition showed that self-absorption thus calculated from the observed spectra is in good agreement with the results of theoretical calculations based on known attenuation coefficients.

Post-Collision Interaction in the XenonN4,5−OOAuger Spectrum Excited by Photon Impact

The ${N}_{5}\ensuremath{-}{O}_{2,3}{O}_{2,3}$ $^{1}S_{0}$ Auger peak of xenon following ionization in the ${N}_{5}$ shell by photons of different energy has been investigated. When the photon energy is lowered close to threshold a shift of the position of the peak maximum to higher energies and a characteristic asymmetric intensity distribution of the Auger line are observed. The observed shifts agree well with values calculated with the classical Berker-Berry model modified because of the velocity change of the slow photoelectron in the Coulomb field of the inner-shell ionized atom.

Spectral distribution of photoionization cross sections by photoconductivity measurements

A new technique for measuring the spectral distribution of photoionization cross sections in photoconductors is presented. The method makes use of the fact that the occupancy of an impurity level is not changed during illumination with photons of different energy if the photocurrent is kept constant. A constant photocurrent is achieved by adjusting the light intensity. The spectral distribution of the photoionization cross section is then given by the inverse of the photon flux as a function of the photon energy. To demonstrate the applicability of this measuring technique, measurements were performed on oxygen-doped GaAs samples. Three deep energy levels (two 0.46 and 0.79 eV below the conduction band and one 0.48 eV above the valence band) were investigated and the spectral distribution of five photoionization cross sections were measured. All of them are in good agreement with calculated values, and thus an accurate determination of the threshold energies is achieved. For comparison, similar investigations were performed with p+n junctions giving results which are in good agreement with the photoconductivity measurements.

Excited State Symmetry Assignment Through Polarized Two-Photon Absorption Studies of Fluids

We consider the effect of molecular symmetry on the two-photon absorption constants of a randomly oriented sample of molecules. It is shown that a complete polarization study of an allowed two-photon transition of a fluid generally yields enough information to permit an unequivocal identification of the symmetry species of the excited state. Ambiguities occur in only a few cases. Thus, oriented molecule studies for symmetry assignment will be largely unnecessary in two-photon molecular spectroscopy. Symmetry assignment rules are given for all of the 32 crystallographic point groups and for the two groups of linear molecules. It is shown that certain transitions, which are allowed for two photons of different energies, are forbidden for two photons of the same energy. The application of this formalism to Raman scattering problems is discussed.

Insensitive zones in the intrinsic region of Ge(Li) coaxial detectors

Insensitive regions in the intrinsic volume of a double open ended coaxial Ge(Li) detector have been put in evidence and evaluated by using the gamma ray scanning technique with photons of different energy. Insensitive zones have been observed at both open ends of the diode, having circular geometry with a thickness increasing from the p core to the n + electrode. Their size is sensitive to the detector bias and can be reduced by a suitable warm up and by redrift cycles. Their existence is probably related to local imperfect compensation of the Li drifted germanium.