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Journal articles on the topic 'Gamma photons'

Author: Grafiati

Published: 4 June 2021

Last updated: 10 February 2022

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Tajudin, S. M., Y. Namito, T. Sanami, and H. Hirayama. "PHOTON FIELD OF ~100–200 KEV FOR ENVIRONMENTAL DOSEMETER CALIBRATION." Radiation Protection Dosimetry 188, no. 4 (2020): 486–92. http://dx.doi.org/10.1093/rpd/ncz308.

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Abstract As a reference photon field, several radionuclides have been used frequently, such as 241Am,137Cs and60 Co for calibration. These nuclides provide mono-energy photons for dosemeters covering few tens of keV–MeV. The main energy around 200 keV is important for both environmental and medical fields since the former should consider scattering photons and the later should measure photons from X-ray generator. In our previous work, a backscattered layout can provide a uniform photon field spectra and dose rate with an energy of 190 keV by using an affordable intensity 137 Cs gamma source. Several other quasi-monoenergetic photon fields in the range of 100–200 keV could be obtained by using several available gamma sources. Two calibrated environmental CsI(Tl) survey meters, Horiba PA-1000 and Mr. Gamma A2700, had been measured with the developed backscattered photon field to understand energy-dependent features in order to confirm dosemeter readings. Consequently, both scintillator instruments are sensitive for measurements of the relatively low dose rates at 190 keV.

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Khumalo, Bhekuzulu. "What is Heat; The Photon is Heat." JOURNAL OF ADVANCES IN PHYSICS 15 (January 12, 2019): 6018–38. http://dx.doi.org/10.24297/jap.v15i0.7896.

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All photons will burn you. How do we describe photons as having different amounts of energy? This paper illustrates photons do not have different amounts of energies, rather different types of energy. The experiment of 1800 provides enough data to be analyzed because it has that third point to question the idea that photons carry different amounts of energy. This paper argues that all photons have equal amounts of energy just different types of energy. The composition of the energy within a photon depends on the frequency of a photon, a lower frequency photon like those represented by infrared can boil water faster than higher frequency blue light. A higher frequency photon like a gamma particle is stopped by lead. Given the nature that heat is from photons we can start thinking of sophisticated thermometers that give us the quality of heat not just the quantity of heat. It is the atmosphere that gives more evidence around the nature of photons, we can understand the cycle of the photon/ photonic cycle/ electromagnetic cycle, allowing us to ponder on deep philosophical meanings, intelligent life is there for universe to sustain itself, as well as ask the question why we are not burning given the nature of low frequency electromagnetic radiation. And for those vigorously looking for habitable planets out there, the idea of the circumstellar habitable zone must change to accommodate the proper understanding of heat.

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Abdullah, Nazifah, Inayatullah Shah Syed, and Ahmad Zakaria. "Flat sheet of Zinc material as a filter for gamma photons in Tc-99mm SPECT imaging." International Journal of Engineering & Technology 7, no. 4.15 (2018): 311. http://dx.doi.org/10.14419/ijet.v7i4.15.23018.

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Purpose of the Study: Scattered gamma photons in single photon emission computed tomography (SPECT) is one of the main issues that degrade the image quality. There are several types of scatter correction methods. However, none of the technique is applied in routine clinical Tc-99m SPECT imaging. Therefore, Zinc (Zn) material filter was constructed for pre-filtration of scattered gamma photons in Tc-99m single photon emission computed tomography (SPECT).Materials and Methods: Tc-99m radioactivity was administered into the Carlson’s phantom. Planar and tomographic data were acquired with and without hot regions insert, respectively, with and without material filter. Scatter to non-scatter ratio was measured from the photopeak region of Tc-99m spectra. Tomographic images were generated using filtered back projection method. Contours on images were drawn with ImageJ software. Images obtained with and without material filter were analysed in terms of detectability of hot regions.Results: Remarkable decrease (≈ 21%) in the ratio of scattered to non-scattered gamma photons with material filter was achieved. Detectability of smaller hot regions with material filter was enhanced. In terms of sizes of all hot regions, material filter results appear closer to the original sizes.Conclusion: Material filter technique could be applied in Tc-99m clinical SPECT, while organ phantom studies are needed.

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Coussement, R., S. Gheysen, I. Serdons, et al. "Slowing Down of Gamma Photons." Hyperfine Interactions 151/152, no. 1-4 (2003): 93–104. http://dx.doi.org/10.1023/b:hype.0000020406.76483.05.

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RAZZAQUE, SOEBUR, PETER MÉSZÁROS, and BING ZHANG. "GEV TO PEV ENERGY PHOTON INTERACTIONS IN GAMMA-RAY BURST FIREBALLS AND SURROUNDINGS." International Journal of Modern Physics A 20, no. 14 (2005): 3163–66. http://dx.doi.org/10.1142/s0217751x05026030.

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Shock acceleration mechanism in gamma-ray bursts may produce photons up to a few PeV energy. Photons above 10-100 GeV and below a PeV energy are trapped inside the fireball due to a high opacity of electron-positron pair production with other photons. High energy photons escaping the fireball may interact with cosmic background radiation and provide delayed gamma-ray emission detectable by GLAST. Detection of the prompt and delayed emission may provide useful constraints on the gamma-ray burst model and also on the inter-galactic magnetic field.

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Turşucu, A., D. Demir, and P. Önder. "Effective Atomic Number Determination of Rare Earth Oxides with Scattering Intensity Ratio." Science and Technology of Nuclear Installations 2013 (2013): 1–6. http://dx.doi.org/10.1155/2013/738978.

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Effective atomic numbers () of scientific samples (rare earth) were determined experimentally by scattering of 59.54 keV gamma photons from 5 Ci241Am annular radioactive source. The scattered gamma photons were collected by using a high-resolution HPGe semiconductor detector placed at to the incident beam. This experiment was carried out on several elements in the atomic range for 59.54 keV incident photons. Photopeak efficiency and air and sample absorption corrections were performed on Rayleigh to Compton scattering intensity ratio; then the ratio was plotted as a function of atomic number and a fit curve was constituted. The effective atomic numbers of rare earth oxide samples were determined by this fit curve. Also, related parameters were determined by absorption technique with the same incident photon energy. Obtained values from this fit curve were compared to theoretical values and were found to closely agree with theoretical calculations.

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Sabharwal, Arvind D., Bhajan Singh, and B. S. Sandhu. "Energy Dependence of Parameters Characterizing Multiply Backscattering of Gamma Photons." Journal of Nuclear Physics, Material Sciences, Radiation and Applications 1, no. 2 (2014): 239–46. http://dx.doi.org/10.15415/jnp.2014.12020.

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John, R. W. "Brilliance of X rays and gamma rays produced by Compton backscattering of laser light from high-energy electrons." Laser and Particle Beams 16, no. 1 (1998): 115–27. http://dx.doi.org/10.1017/s0263034600011824.

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In the Compton backscattering of laser light from a high-energy electron beam, the scattered photons are, as is well known, much harder than the incident photons. In connection with the inverse Compton effect, the spectral brightness, the brilliance of the backscattered radiation is theoretically investigated. For the brilliance B [photons/(sec × mm2 × sr × 0.1% bandwidth)] of the scattered radiation a defining relation is given. Then, the intensity I0 and the wavelength λ 0 of the incident laser light are assumed such that the intensity parameter η is sufficiently smaller than 1, so that with regard to the scattering process, multiphoton effects need not be considered, and the backscattered photon energy hν and the differential cross section dσ/dω approximately do not depend on η. In this case, the brilliance B linearly scales with I0. Furthermore, it is assumed that the primary electron and the incident laser photon are counterpropagating along a straight line, the head-on incidence of the laser photon. On these assumptions, for the brilliance B of the backscattered radiation, B depending on the back-scattered photon energy hν, an explicit formula is derived; from it, by approximations, a shorter formula for B is obtained.

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Dzhappuev, D. D., I. M. Dzaparova, E. A. Gorbacheva, et al. "Search for astrophysical PeV gamma rays from point sources with Carpet-2." EPJ Web of Conferences 207 (2019): 03004. http://dx.doi.org/10.1051/epjconf/201920703004.

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Early results of the search for Eγ > 1 PeV cosmic photons from point sources with the data of Carpet–2, an air-shower array equipped with a 175 m2 muon detector, are presented. They include 95% CL upper limits on PeV photon fluxes from stacked directions of high-energy IceCube neutrino events and from four predefined sources, Crab, Cyg X-3, Mrk 421 and Mrk 501. An insignificant excess of events from Mrk 421 will be further monitored. Prospects of the use of the upgraded installation, Carpet–3 (410 m2 muon detector), scheduled to start data taking in 2019, for searches of Eγ > 100 TeV photons, are briefly discussed.

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Mastichiadis, A., R. J. Protheroe, and S. A. Stephens. "Cosmic Ray Positron Production by Gamma Ray Interactions on Starlight." Publications of the Astronomical Society of Australia 9, no. 1 (1991): 115–17. http://dx.doi.org/10.1017/s1323358000025133.

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AbstractWe examine the production of cosmic ray positrons by photon-photon pair production of high-energy γ-rays on starlight photons. We start by calculating the production rate as a function of positron energy and distance from the Sun resulting from interactions with sunlight. The results are generalized to production on other types of star. We calculate the average production rate per unit volume averaged over the local region of the galaxy, and we estimate the contribution to the observed intensity from this process.

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CHYLA, W. T. "On generation of collimated high-power gamma beams." Laser and Particle Beams 24, no. 1 (2006): 143–56. http://dx.doi.org/10.1017/s0263034606060216.

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We consider spontaneous thermalization of free photon field (in a vacuum) due to self-interaction, mediated by the virtual e−e+field at ultra-high concentrations of the electromagnetic energy. That nonlinear, attractive, short-range interaction between photons triggers spontaneous evolution of the initial, low-frequency spectrum toward the maximum-entropy spectral distribution peaking in the gamma range of frequencies. Collimation and the total power of the photon beam (pulse) are hardly affected by the process of spontaneous thermalization. We estimate the threshold intensity that triggers spectral evolution of the photon field, the necessary power of the laser beam, the minimum size of the interaction region, parameters of the fully thermalized photon field, and discuss the near-threshold behavior of the electromagnetic field. Possible applications of thermalized photon beams are suggested, for example, they can serve as the pump field to attain gamma-lasing or facilitate ignition in the fusion pellet.

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Derishev, E. V., V. V. Kocharovsky, and Vl V. Kocharovsky. "TeV photons from gamma-ray bursts." Advances in Space Research 27, no. 4 (2001): 813–18. http://dx.doi.org/10.1016/s0273-1177(01)00126-0.

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Zhang, H., I. M. Christie, M. Petropoulou, J. M. Rueda-Becerril, and D. Giannios. "Inverse Compton signatures of gamma-ray burst afterglows." Monthly Notices of the Royal Astronomical Society 496, no. 1 (2020): 974–86. http://dx.doi.org/10.1093/mnras/staa1583.

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ABSTRACT The afterglow emission from gamma-ray bursts (GRBs) is believed to originate from a relativistic blast wave driven into the circumburst medium. Although the afterglow emission from radio up to X-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (HE; ≳GeV) remains uncertain. The recent detection of sub-TeV emission from GRB 190114C by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) raises further debate on what powers the very high energy (VHE; ≳300 GeV) emission. Here, we explore the inverse Compton scenario as a candidate for the HE and VHE emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton or SSC) and isotropic photon fields external to the blast wave (external Compton). For each case, we compute the multiwavelength afterglow spectra and light curves. We find that SSC will dominate particle cooling and the GeV emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. Additionally, considering the extragalactic background light attenuation, we discuss the detectability of VHE afterglows by existing and future gamma-ray instruments for a wide range of model parameters. Studying GRB 190114C, we find that its afterglow emission in the Fermi-Large Area Telescope (LAT) band is synchrotron dominated. The late-time Fermi-LAT measurement (i.e. t ∼ 104 s), and the MAGIC observation also set an upper limit on the energy density of a putative external infrared photon field (i.e. ${\lesssim} 3\times 10^{-9}\, {\rm erg\, cm^{-3}}$), making the inverse Compton dominant in the sub-TeV energies.

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Cristofari, P., M. Renaud, A. Marcowith, V. V. Dwarkadas, and V. Tatischeff. "Time-dependent high-energy gamma-ray signal from accelerated particles in core-collapse supernovae: the case of SN 1993J." Monthly Notices of the Royal Astronomical Society 494, no. 2 (2020): 2760–65. http://dx.doi.org/10.1093/mnras/staa984.

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ABSTRACT Some core-collapse supernovae are likely to be efficient cosmic ray accelerators up to the PeV range, and therefore, to potentially play an important role in the overall Galactic cosmic ray population. The TeV gamma-ray domain can be used to study particle acceleration in the multi-TeV and PeV range. This motivates the study of the detectability of such supernovae by current and future gamma-ray facilities. The gamma-ray emission of core-collapse supernovae strongly depends on the level of the two-photon annihilation process: high-energy gamma-ray photons emitted at the expanding shock wave following the supernova explosion can interact with soft photons from the supernova photosphere through the pair production channel, thereby strongly suppressing the flux of gamma-rays leaving the system. In the case of SN 1993J, whose photospheric and shock-related parameters are well measured, we calculate the temporal evolution of the expected gamma-ray attenuation by accounting for the temporal and geometrical effects. We find the attenuation to be of about 10 orders of magnitude in the first few days after the supernova explosion. The probability of detection of a supernova similar to SN 1993J with the Cherenkov Telescope Array is highest if observations are performed either earlier than 1 d, or later than 10 d after the explosion, when the gamma-ray attenuation decreases to about two orders of magnitude.

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MAVROMATOS, NICK E. "STRING QUANTUM GRAVITY, LORENTZ-INVARIANCE VIOLATION AND GAMMA RAY ASTRONOMY." International Journal of Modern Physics A 25, no. 30 (2010): 5409–85. http://dx.doi.org/10.1142/s0217751x10050792.

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In the first part of the review, I discuss ways of obtaining Lorentz-Invariance Violating (LIV) space–time foam in the modern context of string theory, involving brane world scenarios. The foamy structures are provided by lower-dimensional background brane defects in a D3-brane universe, whose density is a free parameter to be constrained phenomenologically. Such constraining can be provided by high energy gamma ray photon tests, including ultra-high energy/infrared photon–photon scattering. In the second part, I analyze the currently available data from MAGIC and FERMI telescopes on delayed cosmic photon arrivals in this context. It is understood of course that conventional astrophysics source effects, which currently are far from being understood, might be the dominant reason for the observed delayed arrivals. I also discuss how the stringent constraints from studies of synchrotron-radiation from distant Nebulae, absence of cosmic birefringence and nonobservation of ultra-high energy cosmic photons can be accommodated within the aforementioned stringy space–time foam model. I argue that, at least within the currently available sets of astrophysical data, the stringy foam model can avoid all theses constraints in a natural range of the string coupling and mass scale. The key features are: (i) transparency of the foam to electrons and charged probes in general, (ii) absence of birefringence effects and (iii) a breakdown of the local effective Lagrangian formalism. However, in order to accommodate, in this theoretical framework, the data of the FERMI satellite on the delayed arrival of photons from the short intense Gamma Ray Burst GRB 090510, in a way consistent with the findings of the MAGIC telescope, a nonuniform density of brane foam defects must be invoked.

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Kasztelan, M., K. Jȩdrzejczak, and J. Szabelski. "Comparison of MC Geant4 simulation with the measurements of gamma photons produced by neutrons." Modern Physics Letters A 34, no. 06 (2019): 1950046. http://dx.doi.org/10.1142/s0217732319500469.

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In this work, we have focused on results of measurements of the hydrogen line 2223 keV and compared them with the results of Geant4 simulations. The paraffin containing hydrogen was irradiated by neutrons produced by the weak AmBe source. Produced gammas were measured with the germanium detector. The experimental setup was placed inside a carbon chamber which provided the shielding from the external neutrons. The measurements were performed for different amounts of paraffin. The processes playing a role in the description of our measurements are transport and moderation of neutrons, production of gamma rays in neutron-hydrogen interactions, transport and detection of gamma rays. It has been shown that the correctly carried out Monte Carlo simulations reproduced the measured values of the intensity of the observed gamma line 2223 keV from the neutron capture on hydrogen. The absorption of gamma rays is also described correctly. This has been shown in comparing the measurements of gamma line 322 keV from [Formula: see text]Pb with the simulations.

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Alabsy, Mahmoud T., Jamila S. Alzahrani, M. I. Sayyed, et al. "Gamma-Ray Attenuation and Exposure Buildup Factor of Novel Polymers in Shielding Using Geant4 Simulation." Materials 14, no. 17 (2021): 5051. http://dx.doi.org/10.3390/ma14175051.

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Polymers are often used in medical applications, therefore, some novel polymers and their interactions with photons have been studied. The gamma-ray shielding parameters for Polymethylpentene (PMP), Polybutylene terephthalate (PBT), Polyoxymethylene (POM), Polyvinylidenefluoride (PVDF), and Polychlorotrifluoroethylene (PCTFE) polymers were determined using the Geant4 simulation and discussed in the current work. The mass attenuation coefficients (μ/ρ) were simulated at low and high energies between 0.059 and 1.408 MeV using different radionuclides. The accuracy of the Geant4 simulated results were checked with the XCOM software. The two different methods had good agreement with each other. Exposure buildup factor (EBF) was calculated and discussed in terms of polymers under study and photon energy. Effective atomic number (Zeff) and electron density (Neff) were calculated and analyzed at different energies. Additionally, the half-value layer (HVL) of the polymers was evaluated, and the results of this parameter showed that PCTFE had the highest probability of interaction with gamma photons compared to those of the other tested polymers.

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Orellana, M., L. J. Pellizza, and G. E. Romero. "On the reprocessing of gamma-rays produced by jets." Proceedings of the International Astronomical Union 6, S275 (2010): 98–99. http://dx.doi.org/10.1017/s1743921310015723.

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AbstractSystems of two very different sizescales are known to produce very high-energy (VHE) radiation in their jets: AGNs and microquasars. The produced VHE photons (Eγ ~ 1 TeV) can be absorbed by the intense environmental soft photon fields, coming from the companion star (in high mass binaries) or from the accreting material (disk+corona in AGNs), as these are the dominant sources at energies around ~(mec2)2/Eγ. Energetic pairs are created by the photon-photon annihilation, and, depending on how efficient are the competing cooling channels, the absorption can lead to a reprocessing by Inverse Compton pair-cascade development. A self-consistent modeling of these systems as gamma-ray sources should then include, along with the emission and absorption processes, a thorough treatment of the pair cascades. We discuss here on this issue, focusing on our (preliminary) results of numerical simulations devoted to a study case similar to the high-mass microquasar candidate LS 5039.

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Yalçin, P., N. Ekinci, and Y. Kurucu. "Incoherent scattering of 241[Am] gamma photons." Spectrochimica Acta Part B: Atomic Spectroscopy 57, no. 4 (2002): 791–96. http://dx.doi.org/10.1016/s0584-8547(02)00011-3.

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STECKER, F. W. "High Energy Cosmic Photons: Gamma-Ray Astronomy." Science 237, no. 4813 (1987): 442. http://dx.doi.org/10.1126/science.237.4813.442.

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Katz, J. I. "Delayed hard photons from gamma-ray bursts." Astrophysical Journal 432 (September 1994): L27. http://dx.doi.org/10.1086/187503.

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Khodrog, Osama, Norlaili Kabir, Xue Gong, Qinghai Yuan, and Jianhua Liu. "Scintillation and Luminescence Properties of the Zinc Oxide (ZnO) Doped Frit Crystal Under Gamma Radiation." Journal of Nanoelectronics and Optoelectronics 15, no. 5 (2020): 566–73. http://dx.doi.org/10.1166/jno.2020.2765.

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The main objective of this study is to determine the characteristics of the scintillation and luminescent for Zinc Oxide (ZnO). Two groups of ZnO were prepared: pure (P338) and doped (P338Frit). This study investigates the scintillation and luminescent properties of the prepared ZnO in both powder and crystal forms. The NaI(Tl) scintillator is used as a tool for the control scintillator. It is a crystal that is commercially utilized in many radiation detecting fields. The P338 and P338Frit were used as the scintillator in the experimental setup. The P338 and P338Frit were bombarded with gamma photons of 241Am and 137Cs. The set up was connected to Multichannel Analyzer (MCA) to analyse its gamma spectrum. The scintillation occurrence is followed by the occurrence of luminescence in order for photon interactions to be detected by the MCA system. The photons detected are called count. However, self-absorption of the light is common which prevail the count from being recorded even though the scintillation occurs. The Compton continuum and photoelectric peak of the interaction were not distinguished. In conclusion, the characteristics of the luminescence and scintillation for the prepared ZnO were observed, however, the gamma spectrum peaks recorded were inconclusive.

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Siengsanoh, Kittipong, Pruittipol Limkitjaroenporn, Pichet Limsuwan, Weerapong Chewpraditkul, and Jakrapong Kaewkhao. "Angular Distribution of Multiple Compton Scattered Gamma Rays in Carbon." Advanced Materials Research 770 (September 2013): 346–49. http://dx.doi.org/10.4028/www.scientific.net/amr.770.346.

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This work studied the angular distribution of multiple Compton scattering of gamma energy 662 keV, obtained from 137Cs source. The gamma rays are incident on carbon scatterer of varying the scattering angles. The phenomenon of multiple Compton scattering has been successfully observed using NaI(Tl) scintillation detector. The energy spectra of the detected photons are observed as a long tail to the single-photon Compton line on the lower side of the full-energy peak in the recorded scattered energy spectrum. The full-energy peak corresponding to singly scattered events is reconstructed analytically and observe that the number of multiply scattered events, having same energy as in the singly scattered distribution.

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STERN, B. E., and J. POUTANEN. "GAMMA-RAY EMISSION OF RELATIVISTIC JETS AS A SUPERCRITICAL PROCESS." International Journal of Modern Physics D 17, no. 09 (2008): 1611–17. http://dx.doi.org/10.1142/s0218271808013212.

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Supercriticality of the same kind as that in a nuclear pile can take place in high-energy astrophysical objects producing a number of impressive effects. For example, it could cause an explosive release of the energy of a cloud of ultrarelativistic protons into radiation. More certainly, supercriticality should be responsible for energy dissipation of very energetic relativistic fluids such as ultrarelativistic shocks in gamma-ray bursts and jets in active galactic nuclei (AGNs). In this case, the photon breeding process operates. It is a kind of converter mechanism with the high-energy photons and e+e- pairs converting into each other via pair production and inverse Compton scattering. Under certain conditions, which should be satisfied in powerful AGNs, the photon breeding mechanism becomes supercritical: the high-energy photons breed exponentially until their feedback on the fluid changes its velocity pattern. Then the system comes to a self-adjusting near-critical steady state. Monte-Carlo simulations with detailed treatment of particle propagation and interactions demonstrate that a jet with a Lorentz factor Γ ≈ 20 can radiate away up to a half of its total energy, and for Γ = 40 the radiation efficiency can be up to 80 per cent. Outer layers of the jet decelerate down to a moderate Lorentz factor 2–4, while the spine of the jet has a final Lorentz factor in the range 10–20 independent of the initial Γ. Such sharp deceleration under the impact of radiation must cause a number of interesting phenomena such as formation of internal shocks and an early generation of turbulence.

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Siengsanoh, Kittipong, Weerapong Chewpraditkul, Pichet Limsuwan, Kitipun Boonin, Jakrapong Kaewkhao, and Pruittipol Limkitjaroenporn. "Angular and Intensity Distributions on Multiple Scattering of 662 keV Gamma Photons in Aluminium." Advanced Materials Research 979 (June 2014): 175–79. http://dx.doi.org/10.4028/www.scientific.net/amr.979.175.

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This work studied about the angular and intensity distributions on multiple scattering gamma energy 662 keV, obtained from 137Cs source. The gamma photons are incident on aluminium scatterer of varying thickness is studied experimentally in both the forward and backward hemispheres. The observed spectra recorded by a properly shielded NaI(Tl) scintillation detector. To extract the contribution of multiply scattered photons having same energy as in the singly scattered distribution from the measured spectra, a singly scattered distribution is reconstructed analytically. We observed that the number of multiply scattered photons also increases and saturates at a particular value of the scaterrer thickness and determined at different scattering angles.

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Mehlhaff, J. M., G. R. Werner, D. A. Uzdensky, and M. C. Begelman. "Kinetic beaming in radiative relativistic magnetic reconnection: a mechanism for rapid gamma-ray flares in jets." Monthly Notices of the Royal Astronomical Society 498, no. 1 (2020): 799–820. http://dx.doi.org/10.1093/mnras/staa2346.

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ABSTRACT Rapid gamma-ray flares pose an astrophysical puzzle, requiring mechanisms both to accelerate energetic particles and to produce fast observed variability. These dual requirements may be satisfied by collisionless relativistic magnetic reconnection. On the one hand, relativistic reconnection can energize gamma-ray emitting electrons. On the other hand, as previous kinetic simulations have shown, the reconnection acceleration mechanism preferentially focuses high energy particles – and their emitted photons – into beams, which may create rapid blips in flux as they cross a telescope’s line of sight. Using a series of 2D pair-plasma particle-in-cell simulations, we explicitly demonstrate the critical role played by radiative (specifically inverse Compton) cooling in mediating the observable signatures of this ‘kinetic beaming’ effect. Only in our efficiently cooled simulations do we measure kinetic beaming beyond one light crossing time of the reconnection layer. We find a correlation between the cooling strength and the photon energy range across which persistent kinetic beaming occurs: stronger cooling coincides with a wider range of beamed photon energies. We also apply our results to rapid gamma-ray flares in flat-spectrum radio quasars, suggesting that a paradigm of radiatively efficient kinetic beaming constrains relevant emission models. In particular, beaming-produced variability may be more easily realized in two-zone (e.g. spine-sheath) set-ups, with Compton seed photons originating in the jet itself, rather than in one-zone external Compton scenarios.

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Kachelrieß, M., S. Ostapchenko, and R. Tomàs. "TeV Gamma Rays from Ultrahigh Energy Cosmic Ray Interactions in the Cores of Active Galactic Nuclei: Lessons from Centaurus A." Publications of the Astronomical Society of Australia 27, no. 4 (2010): 482–89. http://dx.doi.org/10.1071/as09072.

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AbstractTeV gamma rays have been observed from blazars as well as from radio galaxies like M 87 and Centaurus A. In leptonic models, gamma rays above the pair production threshold can escape from the ultrarelativistic jet, because large Lorentz factors reduce the background photon densities compared to those required for isotropic emission. Here we discuss an alternative scenario, where very high energy photons are generated as secondaries from ultrahigh energy cosmic rays interactions in the cores of active galactic nuclei. We show that TeV gamma-rays can escape from the core despite large infrared and ultraviolet backgrounds. For the special case of Centaurus A, we study whether the various existing observations from the far infrared to the ultrahigh energy range can be reconciled within this picture.

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Raiteri, C. M., G. Ghisellini, M. Villata, et al. "Optical Monitoring of Gamma-Ray Loud Blazars." Symposium - International Astronomical Union 175 (1996): 287–88. http://dx.doi.org/10.1017/s0074180900080840.

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The observations by the Compton Gamma Ray Observatory (CGRO) have shown that highly variable and radio-loud quasars emit a significant fraction of their energy in the γ band. According to the Inverse Compton model, the γ-ray emission is due to upscattering of soft (IR-optical-UV) photons by high energy particles. Optical monitoring is thus of great value in providing information on the mechanisms that rule the production of the seed photons for the γ-ray radiation and on the γ-ray emission itself. In particular, detection of variability correlations between optical and γ-ray emissions would be a crucial test for the theoretical predictions.

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Eungwanichayapant, A., W. Maithong, and D. Ruffolo. "Synchrotron radiation from giant e± pair halos." Proceedings of the International Astronomical Union 7, S284 (2011): 417–19. http://dx.doi.org/10.1017/s1743921312009556.

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AbstractA giant e± pair halo is formed by electromagnetic cascades developing around an AGN under the intergalactic magnetic field (1nG - 1μG). Many studies have been focussed on the pair halos in the gamma band because it has been predicted that the e±s in the pair halos up-scatter the Cosmic Microwave Background (CMB) to be gamma-rays. However, the pair halos do not emit only gamma photons but also X-ray photons via synchrotron radiation. In this paper, the Spectral Energy Distributions (SEDs) and the angular distributions of the synchrotron radiation of the pair halos from the Monte Carlo simulations will be discussed.

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Bernardini, M. G., G. Ghirlanda, S. Campana, et al. "Limits on quantum gravity effects from Swift short gamma-ray bursts." Astronomy & Astrophysics 607 (November 2017): A121. http://dx.doi.org/10.1051/0004-6361/201731332.

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The delay in arrival times between high and low energy photons from cosmic sources can be used to test the violation of the Lorentz invariance (LIV), predicted by some quantum gravity theories, and to constrain its characteristic energy scale EQG that is of the order of the Planck energy. Gamma-ray bursts (GRBs) and blazars are ideal for this purpose thanks to their broad spectral energy distribution and cosmological distances: at first order approximation, the constraints on EQG are proportional to the photon energy separation and the distance of the source. However, the LIV tiny contribution to the total time delay can be dominated by intrinsic delays related to the physics of the sources: long GRBs typically show a delay between high and low energy photons related to their spectral evolution (spectral lag). Short GRBs have null intrinsic spectral lags and are therefore an ideal tool to measure any LIV effect. We considered a sample of 15 short GRBs with known redshift observed by Swift and we estimate a limit on EQG ≳ 1.5 × 1016 GeV. Our estimate represents an improvement with respect to the limit obtained with a larger (double) sample of long GRBs and is more robust than the estimates on single events because it accounts for the intrinsic delay in a statistical sense.

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Al-Alawy, Iman Tarik, Alaa B. Kadhim, and Sura Salim Ahmed. "Simulate and Study the Effect of Compton Scattering Angle on the Radiography Inspection." International Letters of Chemistry, Physics and Astronomy 64 (February 2016): 11–18. http://dx.doi.org/10.18052/www.scipress.com/ilcpa.64.11.

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Radiography inspection have been used in this work to detects and image the corrosion in the outside surface of the pipe. Gamma ray with 0.662 MeV from127Cs source have been chosen for this inspection while NaI(Tl) detector is use to detect and collect the scatter photons with radius 6cm. A simulation follow the history of photon by using Fortran Language and concerned on the effect of Compton scattering angle on the count rate of scatter photons and on the quality of the image. The Compton scattering angles that study are in the range from (90°-100°) to (170°-180°) with step 10°. The results show that the greater count rate of scattered photons is improved in the angle range (90°-100°). Therefore, the count rate decreases with increasing the angle range especially within the range (170°-180°) which has a minimum count rate and affect on the image resolution, then it could not distinguish between the corrosion and non-corrosion region.

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WANG, XIANG-YU, HAO-NING HE, and ZHUO LI. "KLEIN–NISHINA EFFECTS IN THE PROMPT AND EXTENDED HIGH-ENERGY GAMMA-RAY EMISSION OF GAMMA-RAY BURSTS." International Journal of Modern Physics D 20, no. 10 (2011): 2023–27. http://dx.doi.org/10.1142/s0218271811020111.

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Prompt and extended high-energy (> 100 MeV) gamma-ray emission has been observed from more than ten gamma-ray bursts by Fermi Large Area Telescope (LAT). Such emission is likely to be produced by synchrotron radiation of electrons accelerated in internal or external shocks. We show that IC scattering of these electrons with synchrotron photons are typically in the Klein–Nishina (KN) regime. For the prompt emission, the KN effect can suppress the IC component and as a result, one single component is seen in some strong bursts. The KN inverse-Compton cooling may also affect the low-energy electron number distribution and hence result in a hard low-energy synchrotron photon spectrum. During the afterglow, KN effect makes the Compton-Y parameter generally less than 1 in the first seconds for a wide range of parameter space. Furthermore, we suggest that the KN effect can explain the somewhat faster-than-expected decay of the early-time high-energy emission observed in GRB090510 and GRB090902B.

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LUONGO, ORLANDO, and DAMIANO TOMMASINI. "COSMOLOGICAL MEASUREMENTS OF LORENTZ INVARIANCE VIOLATION AT THE LIFŠHITZ POINT." International Journal of Modern Physics D 21, no. 08 (2012): 1250070. http://dx.doi.org/10.1142/s0218271812500708.

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We consider the neutrino fluxes and photon lensing from Gamma Ray Bursts and galaxy shocks, in order to predict measurable quantum corrections at late times. In particular, changes of the Einstein shell condition are expected to modify the time delay of neutrinos and photons at the Planck scale. Hence, we study the consequences of the Lorentz invariance violation through the use of such photons and neutrinos. The Lorentz invariance violation is expected to depend on the cosmological model, considered a priori in the Einstein equations; therefore, we focus on the so-called Hořava model which has recently attracted great interest. In addition, we compare our results with the ΛCDM and modified Chaplygin gas models, respectively. We infer several theoretical constraints which could reveal such quantum gravity effects.

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MILOTTI, E., F. DELLA VALLE, G. ZAVATTINI, et al. "EXPLORING QUANTUM VACUUM WITH LOW-ENERGY PHOTONS." International Journal of Quantum Information 10, no. 08 (2012): 1241002. http://dx.doi.org/10.1142/s021974991241002x.

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Although quantum mechanics (QM) and quantum field theory (QFT) are highly successful, the seemingly simplest state — vacuum — remains mysterious. While the LHC experiments are expected to clarify basic questions on the structure of QFT vacuum, much can still be done at lower energies as well. For instance, experiments like PVLAS try to reach extremely high sensitivities, in their attempt to observe the effects of the interaction of visible or near-visible photons with intense magnetic fields — a process which becomes possible in quantum electrodynamics (QED) thanks to the vacuum fluctuations of the electronic field, and which is akin to photon–photon scattering. PVLAS is now close to data-taking and if it reaches the required sensitivity, it could provide important information on QED vacuum. PVLAS and other similar experiments face great challenges as they try to measure an extremely minute effect. However, raising the photon energy greatly increases the photon–photon cross section, and gamma rays could help extract much more information from the observed light–light scattering. Here we discuss an experimental design to measure photon–photon scattering close to the peak of the photon–photon cross section, that could fit in the proposed construction of an FEL facility at the Cabibbo Lab near Frascati (Rome, Italy).

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Levinson, Amir, and Benoît Cerutti. "Particle-in-cell simulations of pair discharges in a starved magnetosphere of a Kerr black hole." Astronomy & Astrophysics 616 (August 2018): A184. http://dx.doi.org/10.1051/0004-6361/201832915.

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We investigate the dynamics and emission of a starved magnetospheric region (gap) formed in the vicinity of a Kerr black hole horizon, using a new, fully general relativistic particle-in-cell code that implements Monte Carlo methods to compute gamma-ray emission and pair production through the interaction of pairs and gamma rays with soft photons emitted by the accretion flow. It is found that when the Thomson length for collision with disk photons exceeds the gap width, screening of the gap occurs through low-amplitude, rapid plasma oscillations that produce self-sustained pair cascades, with quasi-stationary pair and gamma-ray spectra, and with a pair multiplicity that increases in proportion to the pair production opacity. The gamma-ray spectrum emitted from the gap peaks in the TeV band, with a total luminosity that constitutes a fraction of about 10−5 of the corresponding Blandford−Znajek power. This stage is preceded by a prompt discharge phase of duration ∼rg/c, during which the potential energy initially stored in the gap is released as a flare of curvature TeV photons. We speculate that the TeV emission observed in M87 may be produced by pair discharges in a spark gap.

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Sari, Adrien, Sara Garti, Frédéric Lainé, et al. "The Potential of Photon Activation and Neutron Activation Techniques for Fast Soil Characterization." EPJ Web of Conferences 225 (2020): 09001. http://dx.doi.org/10.1051/epjconf/202022509001.

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In the frame of a partnership between CEA and VINCI, various measurement techniques are applied to soil analysis and tested in different laboratories located at CEA Saclay (France). This paper deals with two nuclear measurement techniques assessed in this project. More specifically, this paper presents the feasibility study carried out for two non-destructive active methods: photon activation and neutron activation. First, some atomic nuclides are activated either by photons or neutrons. Secondly, gamma-rays of specific energies are emitted by activated nuclides and gamma-ray spectrometry enables to identify these activated nuclides. Calibration of the full measurement system with reference samples would enable to quantify the mass of activated nuclides. Irradiations performed for photon activation measurements were conducted using a linear electron accelerator (linac) as the latter enables to generate high-energy photons by Bremsstrahlung thanks to its conversion target. Furthermore, irradiations performed for neutron activation measurements were also conducted with a linac. Indeed, photons may be converted to neutrons by photonuclear reactions using a secondary target. In the frame of this project, experiments were carried out at the SAPHIR platform (CEA Saclay) with a Linatron-M9 VARIAN linac. The electron energy was either 6 or 9 MeV. For neutron activation measurements, a secondary target made of heavy water has been used as neutron source and a polyethylene cell enabled to thermalize neutrons and increase the number of reactions of interest. In this paper, we present the different experimental setups and the measurement protocols established for this feasibility study. We show experimental results obtained with raw material samples coming from three construction sites.

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Tang, S., B. King, and H. Hu. "Highly polarised gamma photons from electron-laser collisions." Physics Letters B 809 (October 2020): 135701. http://dx.doi.org/10.1016/j.physletb.2020.135701.

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Popov, A. N., D. P. Barsukov, and A. V. Ivanchik. "Absorption of Photons from Distant Gamma-Ray Sources." Astronomy Letters 44, no. 10 (2018): 579–87. http://dx.doi.org/10.1134/s1063773718100067.

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Hoerbe, Mario R., Paul J. Morris, Garret Cotter, and Julia Becker Tjus. "On the relative importance of hadronic emission processes along the jet axis of active galactic nuclei." Monthly Notices of the Royal Astronomical Society 496, no. 3 (2020): 2885–901. http://dx.doi.org/10.1093/mnras/staa1650.

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ABSTRACT With the coincident detection of a gamma-ray flare and a neutrino from the blazar TXS 0506+056, active galactic nuclei (AGNs) have been put into focus as possible sources of the diffuse neutrino flux. We present a space and time-resolved model of the high-energy particle emission of a plasmoid assumed to travel along the axis of an AGN jet at relativistic speed. This was achieved by modifying the publicly available crpropa (version 3.1+) propagation framework that in our work is capable of being applied to source physics on sub-kpc scales. The propagation of a population of primary protons is modelled in a purely turbulent magnetic field and we take into account interactions of these protons with photons scattered from the accretion disc, synchrotron radiation emitted by ambient relativistic electrons, as well with themselves and with other ambient matter. Our model produces a PeV-neutrino flare caused mainly by photohadronic interactions of primaries with the accretion disc field. Secondary high-energy gamma-rays partly attenuate with the ambient photon fields whose combined optical depths achieve their minimal opacity for photons of around 10 TeV. Thus, our model is well capable of producing neutrino flares with a significantly reduced emission of gamma-rays in jets with a hadronic jet component that in the future can be fit to specific AGN flare scenarios.

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Khue, Pham Duc, and Nguyen Van Do. "Study of the $^{nat}$Sm$(\gamma ,xn)^{143m,g}$Sm Reactions Induced by Bremsstrahlung with End-point Energies Just above GDR Region." Communications in Physics 24, no. 3S2 (2014): 38–44. http://dx.doi.org/10.15625/0868-3166/24/3s2/5050.

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We investigate the energy dependence of the isomeric yield ratios for the $^{nat}$Sm$(\gamma ,xn)^{143m,g}$Sm reactions with bremsstrahlung energies of 40-, 45-, and 50-MeV, just above the giant dipole resonance (GDR) region by the off-line $\gamma $-ray spectrometric method. The bremsstrahlung photons were produced from the 100 MeV electron linear accelerator (linac) at the Pohang accelerator laboratory (PAL), Pohang, Korea. In order to improve the accuracy of the experimental results the necessary corrections were made. The present results are compared with similar literature data of the $^{nat}$Sm($\gamma $,xn)$^{143m,g}$Sm reactions measured at various photon energies and of the $^{nat}$Sm(n,2n)$^{143m,g}$Sm reaction induced by 14 MeV neutrons in order to examine the effects of an excitation energy and an input angular momentum.

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Kawakubo, Yuta, Takanori Sakamoto, Atsumasa Yoshida, and Demos Kazanas. "Systematic Spectral Lag Analysis of Swift Known-zGRBs." Advances in Astronomy 2015 (2015): 1–12. http://dx.doi.org/10.1155/2015/341018.

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The difference of photon arrival time, which is known as spectral lag, is well known characteristics of gamma-ray bursts (GRBs). In particular, long duration GRBs show a soft lag which means that high energy photons arrive earlier than soft photons. The lag-luminosity relation is the empirical relationship between the isotropic peak luminosity and the spectral lag. We calculated the spectral lags for 40 known redshift GRBs observed bySwiftaddition to the previous 31 GRB samples. We confirmed that most of our samples follow the lag-luminosity relation. However, we noticed that there are some GRBs which show a significant scatter from the relation. We also confirm that the relationship between the break time and the luminosity of the X-ray afterglow (so-called Dainotti relation) extends up to the lag-luminosity relation.

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Edwards, P. G., G. A. Moellenbrock, S. C. Unwin, A. E. Wehrle, and T. C. Weekes. "Markarian 421: Te V Gamma-ray Flares and VLBI Structure." International Astronomical Union Colloquium 164 (1998): 45–46. http://dx.doi.org/10.1017/s0252921100044456.

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AbstractPhotons spanning over 17 decades in energy have been detected from Markarian 421. The discovery of >100 MeV photons by the EGRET instrument on board the Compton Gamma Ray Observatory led to the discovery of >300 GeV photons by the Whipple group using the air Čerenkov method. In May 1996 the source underwent an unprecedented outburst at these energies, inspiring observations with the VLBA to determine whether such high energy flares are associated with changes in the milliarcsecond structure of this BL Lac object.

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Thomas, Brian C., Dimitra Atri, and Adrian L. Melott. "Gamma-ray bursts: not so much deadlier than we thought." Monthly Notices of the Royal Astronomical Society 500, no. 2 (2020): 1970–73. http://dx.doi.org/10.1093/mnras/staa3364.

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ABSTRACT We analyse the additional effect on planetary atmospheres of recently detected gamma-ray burst afterglow photons in the range up to 1 TeV. For an Earth-like atmosphere, we find that there is a small additional depletion in ozone versus that modeled for only prompt emission. We also find a small enhancement of muon flux at the planet surface. Overall, we conclude that the additional afterglow emission, even with TeV photons, does not result in a significantly larger impact over that found in past studies.

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Alamoudi, Mohammed, Mohammad Amir Sattari, Mohammed Balubaid, et al. "Application of Gamma Attenuation Technique and Artificial Intelligence to Detect Scale Thickness in Pipelines in Which Two-Phase Flows with Different Flow Regimes and Void Fractions Exist." Symmetry 13, no. 7 (2021): 1198. http://dx.doi.org/10.3390/sym13071198.

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Scale deposits can reduce equipment efficiency in the oil and petrochemical industry. The gamma attenuation technique can be used as a non-invasive effective tool for detecting scale deposits in petroleum pipelines. The goal of this study is to propose a dual-energy gamma attenuation method with radial basis function neural network (RBFNN) to determine scale thickness in petroleum pipelines in which two-phase flows with different symmetrical flow regimes and void fractions exist. The detection system consists of a dual-energy gamma source, with Ba-133 and Cs-137 radioisotopes and two 2.54-cm × 2.54-cm sodium iodide (NaI) detectors to record photons. The first detector related to transmitted photons, and the second one to scattered photons. The transmission detector recorded two signals, which were the counts under photopeak of Ba-133 and Cs-137 with the energy of 356 keV and 662 keV, respectively. The one signal recorded in the scattering detector, total counts, was applied to RBFNN as the inputs, and scale thickness was assigned as the output.

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KATAOKA, JUN. "SUZAKU VIEW OF POWERFUL GAMMA-RAY QUASARS." International Journal of Modern Physics D 17, no. 09 (2008): 1483–89. http://dx.doi.org/10.1142/s0218271808013066.

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We present the results from multiwavelength campaigns of three powerful gamma-ray quasars, PKS 1510-089, RBS 315 and Swift J0746.3+2548, recently organized with Suzaku. The Suzaku observation provided one of the highest S/N X-ray spectra ever reported between 0.3 and 50 keV. For these quasars, the X-ray spectrum is well represented by an extremely hard power-law with photon index Γ ≃ 1.2, but is augmented by an additional soft component apparently below 1 keV for PKS 1510-089, whereas a strong deficit of soft photons is observed in RBS 315. We model the broadband spectra of these powerful quasars and argue that the power of the jet is dominated by protons but with the number of electrons/positrons exceeding the number of protons by a factor ≃ 10. We also argue that an extremely hard X-ray spectra may result from a double power-law form of the injected electrons, with the break energy γ br ≃ 1000 corresponding to the anticipated threshold of diffusive shock acceleration.

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Privas, Edwin, and Laurent Chabert. "Reflector features and physics consideration issued from the Jules Horowitz Reactor design analyses." EPJ Nuclear Sciences & Technologies 4 (2018): 18. http://dx.doi.org/10.1051/epjn/2018040.

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Mechanic solicitations induced by neutron and photon interactions have to be featured for components lifespan determination. TechnicAtome is in charge of both the design and building on behalf of CEA of the 100 MW Jules Horowitz Reactor (JHR). This modular Material Testing Reactor is under construction in southern France, with radioisotope production and material testing capabilities. Inner core components have been designed based on mechanical and thermohydraulic considerations. Both studies require neutronic physical quantities like the neutron flux and deposited energies. The JHR reflector is outside the primary loop and is composed of beryllium. Gamma shields are partially positioned between the reflector and the core to reduce photon heating on aluminum structures. The design is completed and this paper deals with the neutronic and photonic impacts on the reflector. A Monte Carlo methodology based on the MCNP code was developed to model the reactor and enhance fluxes and energy deposited maps. MCNPs mesh options are used over the detailed geometry model. The convolution with mechanical meshes enables to determine neutronic parameters on local structures, material by material. Time required for such modeling is very long if one requires results on every mesh with a maximum uncertainty of 2% (1σ). To reduce time calculation by a factor 3.5 on refined meshes, MCNP biasing methods have been used. Spatial distribution of the gamma heating shows the importance of the interface with the surrounding area. For example, photon and neutron interactions close to the gamma shield create numerous photons with lower energy adding heating at the shield interfaces. In order to keep high flux in the experimental part of the reflector, gamma shields are not continuously set around the reactor vessel. Consequently, some photon leakage arises in the reflector area, with limited impact on aluminum structures. The overall thermal flux map shows local effects and gradients that have to be taken into account by the physics studies. Material swellings are deduced from the fluxes on all reflector structures.

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Mao, Jirong, and Jiancheng Wang. "Jitter radiation: towards TeV-photons of gamma-ray bursts." Monthly Notices of the Royal Astronomical Society 505, no. 3 (2021): 4608–15. http://dx.doi.org/10.1093/mnras/stab1644.

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ABSTRACT The synchrotron mechanism has the radiation limit of about 160 MeV, and it is not possible to explain the very high energy (VHE) photons that are emitted by high-energy objects. Inverse Compton scattering as a traditional process is applied for the explanation of the VHE emission. In this paper, jitter radiation, the relativistic electron radiation in the random and small-scale magnetic field, is proposed to be a possible mechanism to produce VHE photons. The jitter radiation frequency is associated with the perturbation field. The spectral index of the jitter radiation is dominated by the kinetic turbulence. We utilize the jitter radiation to explain the gamma-ray burst (GRB 190114C and GRB 180720B) VHE emissions that were recently detected by the Imaging Atmospheric Cherenkov Telescopes. We suggest that this mechanism can be applied to other kinds of VHE sources.

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Gómez-Ávila, Selim, Lao-Tse López-Lozano, and José Carlos Olvera-Meneses. "Light signals from spin one tensor dark matter." Pädi Boletín Científico de Ciencias Básicas e Ingenierías del ICBI 7, no. 13 (2019): 72–75. http://dx.doi.org/10.29057/icbi.v7i13.3634.

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In this work we report on some observables associated with the annihilation of dark matter, modeled as a spin one real field in the antisymmetric tensor representation. This formulation has the interesting feature that stability is achieved without the introductionof ad hoc symmetries, as reported by Cata and Ibarra in 2014. In this model, the coupling to photons ocurrs through a Higgs portal term. The expression for the cross section in the photon scattering is calculated as well as their energy and the gamma ray flux.

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Ping, Y. L., X. T. He, H. Zhang, B. Qiao, H. B. Cai, and S. Y. Chen. "Gamma-ray source through inverse Compton scattering in a thermal hohlraum." Laser and Particle Beams 31, no. 4 (2013): 607–11. http://dx.doi.org/10.1017/s0263034613000463.

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AbstractA new inverse Compton scattering scheme for production of high-energy Gamma-ray sources is proposed in which a Giga-electronvolt (GeV) electron beam is injected into a thermal hohlraum. It is found that by increasing the hohlraum background temperature, the scattered photons experience kinematic pileup, resulting in more monochromatic spectrum and smaller scattering angle. When a relativistic electron beam with energy 1 GeV and charge 10nC is injected into a 0.5 keV hohlraum, 80% of the scattered photons have energy above 0.5 GeV.

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DE ANGELIS, ALESSANDRO, and REYNALD PAIN. "IMPROVED LIMITS ON PHOTON VELOCITY OSCILLATIONS." Modern Physics Letters A 17, no. 38 (2002): 2491–96. http://dx.doi.org/10.1142/s021773230200926x.

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The mixing of the photon with a hypothetical sterile paraphotonic state would have consequences on the cosmological propagation of photons. The absence of distortions in the optical spectrum of distant Type Ia supernovae allows to extend by two orders of magnitude the previous limit on the Lorentz-violating parameter δ associated to the photon–paraphoton transition, extracted from the absence of distortions in the spectrum of the cosmic microwave background. The new limit is consistent with the interpretation of the dimming of distant Type Ia supernovae as a consequence of a nonzero cosmological constant. Observations of gamma-rays from active galactic nuclei allow to further extend the limit on δ.

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