Primary Gamma Rays Research Articles

Constraints on the fraction of primary gamma rays at ultra-high energies from the muon data of the Yakutsk EAS array

By using data on the total signal and on the muon component of air showers detected at the Yakutsk array, the possible upper limits on the fraction of primary gamma rays at ultra-high energies is analyzed in the framework of the recently suggested event-by-event approach. Upper limits on the photon fraction in the integral flux of primary cosmic rays are derived. At 95% C.L., these limits are 22% for primary energies E 0 > 4 × 1019 eV and 12% for E 0 > 2 × 1019 eV. Despite the presence of muonless events, the data are consistent with the complete absence of photons at least at 95% C.L. The sensitivity of the results to systematic uncertainties, in particular to those of the energy determination for nonphoton primaries, is discussed.

An analytic solution from a spherical gamma emitter

An analytic solution for the fluence rate of primary (non-scattered) gamma rays emerging from a spherical radioactive source is presented. The solution takes self-attenuation into account and is valid at any distance from the source. It is derived using elementary integral calculus and does not require special functions. The key to the relatively simple solution is a shift of the center of the sphere along the z-axis by a distance equal to the radius, so that the surface of the sphere passes through the origin of the coordinate system.

On the background of low threshold imaging Cherenkov telescopes induced by cosmic ray electrons

The next generation large diameter (⩾20m) imaging atmospheric Cherenkov telescopes should allow reduction of the threshold of detection of gamma-rays down to several GeV. At such low energies, the background events can be dominated by cosmic ray electrons. In this paper we estimate the detection energy threshold of a large diameter atmospheric Cherenkov telescope with respect to the diffuse cosmic ray electrons. It is shown that the detection energy threshold of cosmic ray electron induced showers is higher than that for primary gamma showers due to the differences in the development of electromagnetic air showers initiated by these primaries. This can provide a background-free detection region for primary gamma-rays with energy E⩽5–10GeV, provided that the misclassified events induced by primary hadrons of significantly higher energies and muons are effectively suppressed.

Observations of 14 young open star clusters with the HEGRA system of Cherenkov telescopes

Context. A sample of 14 young open star clusters has been observed in the TeV energy regime with the stereoscopic system of the HEGRA (High Energy Gamma-Ray Astronomy) Cherenkov telescopes from 1997 to 2002, resulting in more than 300 h of observation time. Aims. Young open star clusters may contribute to the acceleration of cosmic rays. The detection of γ-rays (from decaying π 0 s produced in hadronic interactions) from these objects could be evidence for such a contribution. The results of our observations are compared to available γ-ray data and to a simple hadronic model in the framework of shock front acceleration of cosmic rays in the stellar winds of the cluster members to test the potential of the presently available data on young open star clusters to constrain this type of model. Methods. The stereoscopic system of HEGRA Cherenkov telescopes makes use of the atmospheric imaging technique. Air showers initiated by primary Gamma-Rays are recorded as elliptical images in the telescope cameras. The images from the different telescopes are then superimposed to reconstruct the parameters of the primary particle. This technique (stereoscopy) was pioneered by the HEGRA experiment. Results. No significant excess has been found in the analysed data set of young open star clusters. The derived upper limit on the TeV gamma-ray flux from Berkeley 87 and the available EGRET data from the same direction do not allow us to fully constrain the simple hadronic model used here. The comparison of the upper limits derived for all 14 objects with the flux detected from TeV J2032+4130 (under the assumption of an association of the TeV-signal with the compact stellar association Cyg OB2) suggests that γ-ray emission from young open star clusters as an object class cannot be ruled out.

Cherenkov light in electron-induced air showers

In this paper, we present the results of a comparative analysis of the Cherenkov light produced by electromagnetic air showers initiated by primary gamma-rays and electrons. It is shown that the lateral distribution functions of the Cherenkov light initiated by these showers are different. This fact is explained by the difference in the development of cascades from gamma-rays and electrons; electron-showers start to develop and produce light earlier than gamma-showers, but they attenuate rapidly. By propagating much farther into the atmosphere, the gamma-initiated shower carries a Cherenkov component much closer to the ground level than electron initiated showers do; this results in a more intense pool of light close to the shower axis. The difference in the altitudes of electron- and gamma-air showers maximums logarithmically decreases with the energy and is more significant for low energy showers with E ⩽ 20 GeV. This can be important for low energy threshold Cherenkov telescopes (below 10 GeV), in which case the background events are due to diffuse cosmic ray electrons only. The minimum energy of primary electrons required for their reliable detection by an imaging atmospheric telescope will be higher than that for primary gamma-rays. For the reasonable value of the geomagnetic field strength of B ⩾ 0.3 G and at the initial energies of E ⩽ 20 GeV, the pulse width of the Cherenkov light generated by electron-showers is considerably larger than that from gamma-showers. At higher energies the shapes of Cherenkov pulse from showers induced by both primaries become almost similar.

MUONS WITH Eth ≥ 1 GEV AND MASS COMPOSITION IN THE ENERGY RANGE 1018 - 1020EV OBSERVED BY YAKUTSK EAS ARRAY

The ratio of the muon flux density to charged particle flux density at distances of 300 and 600 m from the shower axis (ρμ(300)/ρ s (300) and ρμ(600)/ρ s (600)) is measured. In addition, the energy dependence of ρμ(1000) is analysed for showers with energies above 1018 eV. A comparison between the experimental data and calculations performed with the QGSJET model is given for the cases of primary proton, iron nucleus and gamma-ray. We conclude that the showers with E0 ≥ 3 × 1018 eV can be formed by light nuclei with a pronounced fraction of protons and helium nuclei. It is not excluded however that a small part of showers with energies above 1019 eV could be initiated by primary gamma-rays.

Optimization studies of a Compton suppression spectrometer using experimentally validated Monte Carlo simulations

Recent developments associated with room temperature semiconductor detectors and inorganic scintillators suggest that these detectors may be viable alternatives for the primary detector in a Compton suppression spectrometer (CSS). The room temperature operation of these detectors allows removal of a substantial amount of material from between primary and secondary detectors, if properly designed and should afford substantially better suppression factors than can be achieved by germanium-based spectrometers. We have chosen to study the optimum properties of a CSS with a LaX 3:Ce scintillator (where X is chloride or bromide) as the primary gamma-ray detector. A Monte Carlo photon transport model is used to determine the optimum geometric properties of this spectrometer. To validate the assumptions and basic design of the Monte Carlo simulations, the energy distribution of a 137Cs point source is measured and simulated for two experimental systems. Comparison of the suppression factors for the measured and simulated data validates the model accuracy. A range of CSS physical parameters are studied to determine optimal detector geometry and to maximize the Compton suppression factor. These physical parameters and their optimum values are discussed.

Polarization of Gamma-Ray Bursts via Scattering off a Relativistic Sheath

Gamma-ray burst fireballs could be ensheathed by a relativistic, baryon-enriched outflow that would in selected directions scatter a polarized beam of gamma rays. The strong polarization recently announced by RHESSI is attainable by the scattering mechanism, although not most of the time. The scattering mechanism, when acting in addition to the synchrotron mechanism, could circumvent some of the predictions implied by a purely synchrotron origin for the polarization. The time profile of the scattered component could differ from that of the primary gamma rays.

Gamma-ray and neutron spectrometer for the Dawn mission to 1 Ceres and 4 Vesta

We present the design of the gamma-ray and neutron spectrometer (GR/NS) for Dawn, which is a NASA Discovery-class mission to explore two of the largest main-belt asteroids, 1 Ceres and 4 Vesta, whose accretion is believed to have been interrupted by the early formation of Jupiter. Dawn will determine the composition and structure of these protoplanetary bodies, providing context for a large number of primitive meteorites in our sample collection and a better understanding of processes occurring shortly after the onset of condensation of the solar nebula. The Dawn GR/NS design draws on experience from the successful Lunar Prospector and Mars Odyssey missions to enable accurate mapping of the surface composition and stratigraphy of major elements, radioactive elements, and hydrogen at both asteroids. Here, we describe the overall design of the GR/NS and compare the expected performance of the neutron spectrometer subsystem to the neutron spectrometer on Mars Odyssey. We also describe radiation damage studies carried out on CdZnTe detectors, which will be components of the primary gamma-ray spectrometer on Dawn. We conclude that provisions for annealing at moderate temperatures (40/spl deg/C to 60/spl deg/C) must be made to ensure that the spectrometer will function optimally over the nine-year mission.

On the origin of highest energy gamma-rays from Mkn 501

The spectra of very high energy gamma-radiation from distant extragalactic objects suffer significant deformations during the passage of primary gamma-rays through the intergalactic medium. The recently reported fluxes of diffuse infrared background radiation indicate that we detect, most probably, heavily absorbed TeV radiation from Mkn 421 and Mkn 501. This implies that the absorption-corrected spectrum of Mkn 501 may contain a sharp pile-up which contradicts to the predictions of the conventional models of TeV blazars, and thus may leads to the so-called IR background-TeV gamma-ray crisis. To overcome this difficulty, in this paper we propose two independent hypotheses assuming that (i) the TeV radiation from Mkn 501 has a secondary origin, i.e. it is formed during the development of electron-photon cascades in the intergalactic medium initiated by primary gamma-rays; (ii) the pile-up in the source spectrum is a result of (in deep Klein-Nishina regime) of ambient optical radiation by an ultrarelativistic conical cold outflow (jet) with motion Lorentz factor Gamma_0 >= 3 10^7. Within the uncertainties caused by the limited energy resolution of spectral measurements, the observed TeV radiation of Mkn 501 formally can be explained by the intergalactic cascade gamma-rays, assuming however an extremely low intergalactic magnetic field in the direction to the source at the level of < 10^{-18} G. We also demonstrate that the bulk motion comptonization scenario can quite naturally reproduce the unusual spectral features in the absorption-corrected TeV spectrum of Mkn 501, and briefly discuss the astrophysical implications of this hypothesis.

Distributions of secondary muons at sea level from cosmic gamma rays below 10 TeV

The fluka Monte Carlo program is used to predict the distributions of the muons which originate from primary cosmic gamma rays and reach sea level. The main result is the angular distribution of muons produced by vertical gamma rays which is necessary to predict the inherent angular resolution of any instrument utilizing muons to infer properties of gamma ray primaries. Furthermore, various physical effects are discussed which affect these distributions in differing proportions.

High-resolution gamma-ray spectroscopy

To obtain the highest possible resolution in a measurement has always been one of the major challenges for experimental physicists because increased resolution generally results in progress. At the Institut Laue-Langevin, gamma rays emitted after neutron capture can be recorded with parts-permillion resolution. This is achieved by diffracting the gamma rays on highly perfect Sior Ge crystals. Precise measurement of the Bragg angles and the crystal lattice spacings permits the determination of wavelengths or energies. This outstanding resolving power allows the measurement of extremely small Doppler effects caused by the emission of primary gamma rays. These so-called gamma-ray-induced Doppler broadening measurements have given rise to applications in both nuclear and condensed matter physics.

Thermal and electromagnetic properties of166Erand167Er

The primary gamma-ray spectra of 166-Er and 167-Er are deduced from the (3-He,alpha gamma) and (3-He,3-He' gamma) reaction, respectively, enabling a simultaneous extraction of the level density and the gamma-ray strength function. Entropy, temperature and heat capacity are deduced from the level density within the micro-canonical and the canonical ensemble, displaying signals of a phase-like transition from the pair-correlated ground state to an uncorrelated state at Tc=0.5 MeV. The gamma-ray strength function displays a bump around E-gamma=3 MeV, interpreted as the pygmy resonance.

Critical temperature for quenching of pair correlations

The level density at low spin in the 161,162-Dy and 171,172-Yb nuclei has been extracted from primary gamma rays. The nuclear heat capacity is deduced within the framework of the canonical ensemble. The heat capacity exhibits an S-formed shape as a function of temperature, which is interpreted as a fingerprint of the phase transition from a strongly correlated to an uncorrelated phase. The critical temperature for the quenching of pair correlations is found at Tc=0.50(4) MeV.

Spatial and energy distribution of muons inγ-induced air showers

The FLUKA Monte Carlo program is used to calculate the effects of hadroproduction by primary gamma rays incident upon the Earth's atmosphere; for the results presented in this paper, only primary angles at $0\ifmmode^\circ\else\textdegree\fi{}$ from zenith are considered. The FLUKA code is believed to be quite accurate in reproducing experimental photon hadroproduction data in the 1 GeV to 10 TeV energy range studied. The charged pions which are so produced can decay to muons with sufficient energy to reach ground level. The number of these muons and their radial and energy distribution are studied for incident gamma ray energies from 1 GeV to 10 TeV. The number of these muons is not negligible; they can, in certain circumstances, be used to study potential sources of gamma rays such as gamma ray bursts. It is found, for example, that a 10 TeV incident primary gamma ray produces, on average, 3.4 muons which reach ground level; the gamma ray energy which produces the maximum number of muons at ground level depends on the spectral index of the primary gamma spectrum, a constant which describes how the primary gamma flux rises with decreasing primary energy. For example, for a differential spectral index of 2.7, there is a broad maximum number of muons coming from $\ensuremath{\approx}30$ GeV primary gamma ray energy.

Decision-making Methodology of Optimal Shielding Materials by Using Fuzzy Linear Programming

The main purpose of our studies are to select materials and determine the ratio of constituent materials as the first stage of optimum shielding design to suit the individual requirements of nuclear reactors, reprocessing facilities, casks for shipping spent fuel, etc. The parameters of the shield optimization are cost, space, weight and some shielding properties such as activation rates of individual irradiation and cooling time, and total dose rate for neutrons (including secondary gamma ray) and for primary gamma ray. Using conventional two-valued logic (i.e. crisp) approaches, huge combination calculations are needed to identify suitable materials for optimum shielding design. Also, re-computation is required for minor changes, as the approach does not react sensitively to the computation result. Present approach using a fuzzy linear programming method is much of the decisionmaking toward the satisfying solution might take place in fuzzy environment. And it can quickly and easily provide a guiding principle of optimal selection of shielding materials under the above-mentioned conditions. The possibility of reducing radiation effects by optimizing the ratio of constituent materials is investigated.

Evidence for an Early High-Energy Afterglow Observed with BATSE from GRB 980923

In this letter, we present for the first time evidence in the BATSE data for a prompt high-energy (25-300 keV) afterglow component from a Gamma-Ray Burst (GRB), GRB980923. The event ranks third highest in fluence (>25 keV) in the BATSE catalog and consists of a period of rapid variability lasting about 40 s followed by a smooth power law emission tail lasting about 400 s beyond the trigger time. An abrupt change in spectral shape is found when the tail becomes noticeable. Our analysis reveals that the spectral evolution in the tail of the burst mimics that of a cooling synchrotron spectrum, similar to the spectral evolution of the low-energy afterglows for GRBS. This evidence for a separate emission component is consistent with the internal-external shock scenario in the relativistic fireball picture. In particular, it illustrates that the external shocks can be generated during the primary gamma-ray emission phase, as in the case of GRB990123.

Primary gamma-rays from 53 keV neutron capture reaction in23Na

The nucleus23Na has been investigated by studying the primary γ-rays emitted from 53 keV neutron capture in it using a high resolution and high efficiency (100%) HPGe detector and NaI(T1) detector for anti-Compton. 24 primary γ-rays were placed in the24Na, in which 3 primary γ-rays were new ones from a (n, γ) reaction, and reported for the first time. In order to obtain an exact energy calibration within 7 MeV,56Fe(n,γ)57Fe reaction was used at thermal neutron energy. Intensity calibration was obtained from the27Al(p,γ)28Si reaction atEp=2046 keV. The neutron binding energy of24Na was determined to be 6959.75 keV.

γray astronomy with muons

Although gamma ray showers are muon-poor, they still produce a number of muons sufficient to make the sources observed by GeV and TeV telescopes observable also in muons. For sources with hard gamma ray spectra there is a relative `enhancement' of muons from gamma ray primaries as compared to that from nucleon primaries. All shower gamma rays above the photoproduction threshold contribute to the number of muons $N_\mu$, which is thus proportional to the primary gamma ray energy. With gamma ray energy 50 times higher than the muon energy and a probability of muon production by the gammas of about 1\%, muon detectors can match the detection efficiency of a GeV satellite detector if their effective area is larger by $10^4$. The muons must have enough energy for sufficiently accurate reconstruction of their direction for doing astronomy. These conditions are satisfied by relatively shallow neutrino detectors such as AMANDA and Lake Baikal and by gamma ray detectors like MILAGRO. TeV muons from gamma ray primaries, on the other hand, are rare because they are only produced by higher energy gamma rays whose flux is suppressed by the decreasing flux at the source and by absorption on interstellar light. We show that there is a window of opportunity for muon astronomy with the AMANDA, Lake Baikal and MILAGRO detectors.

Nature of the highest energy cosmic rays

Ultra high energy gamma rays produce electron--positron pairs in interactions on the geomagnetic field. The pair electrons suffer magnetic bremsstrahlung and the energy of the primary gamma ray is shared by a bunch of lower energy secondaries. These processes reflect the structure of the geomagnetic field and cause experimentally observable effects. The study of these effects with future giant air shower arrays can identify the nature of the highest energy cosmic rays as either gamma-rays or nuclei.