Primary Gamma Rays Research Articles

Performance of the HAWC Observatory and TeV Gamma-Ray Measurements of the Crab Nebula with Improved Extensive Air Shower Reconstruction Algorithms

The High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, located on the side of the Sierra Negra volcano in Mexico, has been fully operational since 2015. The HAWC collaboration has recently significantly improved their extensive air shower reconstruction algorithms, which has notably advanced the observatory performance. The energy resolution for primary gamma rays with energies below 1 TeV was improved by including a noise-suppression algorithm. Corrections have also been made to systematic errors in direction fitting related to the detector and shower plane inclinations, O(0.°1) biases in highly inclined showers, and enhancements to the core reconstruction. The angular resolution for gamma rays approaching the HAWC array from large zenith angles (>37°) has improved by a factor of 4 at the highest energies (>70 TeV) as compared to previous reconstructions. The inclusion of a lateral distribution function fit to the extensive air shower footprint on the array to separate gamma-ray primaries from cosmic-ray ones based on the resulting χ 2 values improved the background rejection performance at all inclinations. At large zenith angles, the improvement in significance is a factor of 4 compared to previous HAWC publications. These enhancements have been verified by observing the Crab Nebula, which is an overhead source for the HAWC Observatory. We show that the sensitivity to Crab-like point sources (E −2.63) with locations overhead to 30° zenith is comparable to or less than 10% of the Crab Nebula’s flux between 2 and 50 TeV. Thanks to these improvements, HAWC can now detect more sources, including the Galactic center.

Constraining decaying very heavy dark matter from galaxy clusters with 14 year Fermi-LAT data

Galaxy clusters are promising targets for indirect detection of dark matter thanks to the large dark matter content. Using 14 years of Fermi-LAT data from seven nearby galaxy clusters, we obtain constraints on the lifetime of decaying very heavy dark matter particles with masses ranging from 103 GeV to 1016 GeV. We consider a variety of decaying channels and calculate prompt gamma rays and electrons/positrons from the dark matter. Furthermore, we take into account electromagnetic cascades induced by the primary gamma rays and electrons/positrons, and search for the resulting gamma-ray signals from the directions of the galaxy clusters. We adopt a Navarro-Frenk-White profile of the dark matter halos, and use the profile likelihood method to set lower limits on the dark matter lifetime at a 95% confidence level. Our results are competitive with those obtained through other gamma-ray observations of galaxy clusters and provide complementary constraints to existing indirect searches for decaying very heavy dark matter.

Photon strength functions from (p,γ) reactions

The 50Cr(p,γ)51Mn proton capture reaction has been used to study the photon strength functions by utilizing primary gamma ray transitions from the entry states to discrete states of known spins and parities. The reaction was conducted with the 3 MV Tandetron accelerator at iThemba LABS which delivered proton beams of 2.5 to 2.740, 2.760 to 3.0 MeV and 3.675 to 4.498 MeV in intervals of 20-25 keV with beam currents of up to 5 μA. In this work the proton capture reaction was employed together with the Average Resonance Capture method to extract the shape of the PSF of 51Mn.

Modeling the propagation of very-high-energy γ-rays with the CRbeam code: Comparison with CRPropa and ELMAG codes

Context. Very-high-energy γ-rays produce electron positron pairs in interactions with low-energy photons of extragalactic background light during propagation through the intergalactic medium. The electron-positron pairs generate secondary γ-rays detectable by γ-ray telescopes. This secondary emission can be used to detect intergalactic magnetic fields (IGMF) in the voids of large-scale structure. Aims. A new γ-ray observatory, namely, Cherenkov Telescope Array (CTA), will provide an increase in sensitivity for detections of these secondary γ-ray emission and enable the measurement of its properties for sources at cosmological distances. The interpretation of the CTA data, including the detection of IGMF and study of its properties and origins, will require precision modeling of the primary and secondary γ-ray fluxes. Methods. We assess the precision of the modeling of the secondary γ-ray emission using model calculations with publicly available Monte Carlo codes CRPropa and ELMAG and compare their predictions with theoretical expectations and with model calculations of a newly developed CRbeam code. Results. We find that model predictions of different codes differ by up to 50% for low-redshift sources, with discrepancies increasing up to order-of-magnitude level with the increasing source redshifts. We identify the origin of these discrepancies and demonstrate that after eliminating the inaccuracies found, the discrepancies between the three codes are reduced to 10% when modeling nearby sources with z ~ 0.1. We argue that the new CRbeam code provides reliable predictions for the spectral, timing, and imaging properties of the secondary γ-ray signal for both nearby and distant sources with z ~ 1. Thus, it can be used to study gamma-ray sources and IGMF with a level of precision that is appropriate for the prospective CTA study of the effects of γ-ray propagation through the intergalactic medium.

Sinergy of extra-terrestrial particle accelerators and accelerators operated in the terrestrial atmosphere

We performed simulations of the EAS particle transport initiated by ultra-high-energy gamma rays entering the terrestrial atmosphere. During thunderstorms, the electric fields above the LHAASO array efficiently modulate EAS particles. Because of this modulation, the possible bias on the energy estimation for primary energies up to 100 TeV can be up to ten-fold. Moreover, for the higher primary energies (≈1 PeV), the electric field modulation enlarges “genuine” primary gamma ray energy 2.5-3 times. This result is very important because observation of PEVatrons by LHAASO is based on rather small statistics, and the highest energy events can occasionally coincide with thunderstorms. We also demonstrate the existence of a threshold effect on the intracloud E-field strength needed to trigger the startup of a runaway process that exponentially multiplies the free electrons entering a strong atmospheric electric field.

A method to indicate heavy minerals in logging-while-drilling

The low-energy region of the gamma ray spectrum, less than 200 keV, often is either discarded or not analyzed in borehole logging or airborne radiometric surveys. There is much useful information about earth properties to be extracted from the low-energy region because it is sensitive to photoelectric absorption within the rock, a process that is highly sensitive to the atomic number of the elements within the rock. In addition, the rate of photoelectric absorption is proportional to the density of the rock. At higher energies, the spectrum is primarily sensitive to density variations only. The ratio of counts in the two energy regions may be used as a heavy mineral indicator (HMI). This ratio normalizes density effects and is primarily sensitive to variations in the average (or effective) atomic number ([Formula: see text]) of the rock. Previous work by other authors has focused on the application of this idea for wireline logging with an isotopic source and multiple scattering of copious primary gamma rays, which allows for good counting statistics in the recorded spectrum. Recent advances in gamma ray detectors and logging-while-drilling (LWD) techniques open the possibility of applying the HMI measurement to the natural gamma ray LWD logs. Drilling speeds are much slower than the running speeds of wireline tools, resulting in improved counting statistics. Using computer simulations, laboratory experiments, and field measurements, we find that the natural gamma ray spectrum, recorded while (diamond) drilling, provides sufficient counting statistics to be used for robustly calculating HMI. As iron is the most abundant and influential element in photoelectric gamma ray scattering processes in rock fabrics, we postulate that we can, in most cases, use HMI to indicate the iron content of the rocks. The HMI calculated under these conditions provides a good measure of changes in [Formula: see text] of rocks and correlates well with iron-rich sections. Our field and laboratory tests demonstrate that the HMI measures can be used as a proxy to separate many metalliferous ores from waste based on the differences in [Formula: see text].

A rapid and direct method for half value layer calculations for nuclear safety studies using MCNPX Monte Carlo code

Half Value Layer calculations theoretically need prior specification of linear attenuation calculations, since the HVL value is derived by dividing ln(2) by the linear attenuation coefficient. The purpose of this study was to establish a direct computational model for determining HVL, a vital parameter in nuclear radiation safety studies and shielding material design. Accordingly, a typical gamma-ray transmission setup has been modeled using MCNPX (version 2.4.0) general-purpose Monte Carlo code. The MCNPX code's INPUT file was designed with two detection locations for primary and secondary gamma-rays, as well as attenuator material between those detectors. Next, Half Value Layer values of some well-known gamma-ray shielding materials such as lead and ordinary concrete have been calculated throughout a broad gamma-ray energy range. The outcomes were then compared to data from the National Institute of Standards and Technology. The Half Value Layer values obtained from MCNPX were reported to be highly compatible with the HVL values obtained from the NIST standard database. Our results indicate that the developed INPUT file may be utilized for direct computations of Half Value Layer values for nuclear safety assessments as well as medical radiation applications. In conclusion, advanced simulation methods such as the Monte Carlo code are very powerful and useful instruments that should be considered for daily radiation safety measures. The modeled MCNPX input file will be provided to the scientific community upon reasonable request.

The local-filament pattern in the anomalous transparency of the Universe for energetic gamma rays

The propagation length of high-energy photons through the Universe is limited by e^{+}e^{-} pair production on the extragalactic background radiation. Previous studies reported discrepancies between predicted and observed attenuation, suggesting explanations in terms of new physics. However, these effects are dominated by a limited number of observed sources, while many do not show any discrepancy. Here, we consider the distribution in the sky of these apparently anomalous objects, selected in two very different approaches: the study of unphysical hardenings at distance-dependent energies in deabsorbed spectra of TeV blazars, and the observation of ultra-high-energy air showers from the directions of BL Lac type objects. In both cases, directions to the anomalous sources follow the projected local distribution of galaxies: all the distant sources, contributing to the anomalies, are seen through the local filament. This matches the prediction of the proposed earlier explanation of the anomalies based on mixing of photons with axion-like particles in the filament’s magnetic field. For ultra-high energies, this axion interpretation may be tested by the search of primary gamma rays.

Mass composition of cosmic rays determined by the muon fraction with ϵthr≥1 GeV in air showers with energy greater than 5 EeV

The ratio of the number of muons with a threshold of 1 GeV and charged particles at a distance of 600 m from the axis is analyzed. Air showers with energies above 5 EeV and zenith angles with less than 60 degrees are considered. Comparison of experimental data with calculations by the QGSJETII-04 model for various primary nuclei, including a gamma ray, showed that the mass composition of cosmic rays up to energies of $\sim$ 10 EeV mainly consists of protons and helium nuclei, with a small number of heavy nuclei. Data include air showers with a very low muon content, which are assumed to be produced by primary gamma rays.

Shielding composites for neutron and gamma-radiation with Gd2O3@W core-shell structured particles

Particles of micron gadolinium oxide core and nano tungsten shell were introduced into aluminum matrix to prepare neutron and gamma ray co shielding materials. The neutron and gamma ray simulation results show that the absorption efficiency for neutron and the half value layers for primary and secondary gamma ray meet the shielding application. Morphology of core-shell structures and composites have been analyzed using scanning electron microscopy. Compared with the original particles reinforced aluminum matrix composite, tensile strength is improved by the core-shell structure particles. The tensile fracture morphology shows that the main reason for the transgranular fracture of composite is that the grain boundary Gd2O3@W particles prevent the crack growth.

A new approach to monitor $$^{13}\hbox {C}$$-targets degradation in situ for $$^{13}\hbox {C}(\alpha ,\hbox {n})^{16}\hbox {O}$$ cross-section measurements at LUNA

Direct measurements of reaction cross-sections at astrophysical energies often require the use of solid targets able to withstand high ion beam currents for extended periods of time. Thus, monitoring target thickness, isotopic composition, and target stoichiometry during data taking is critical to account for possible target modifications and to reduce uncertainties in the final cross-section results. A common technique used for these purposes is the Nuclear Resonant Reaction Analysis (NRRA), which however requires that a narrow resonance be available inside the dynamic range of the accelerator used. In cases when this is not possible, as for example the 13C(alpha,n)16O reaction recently studied at low energies at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy, alternative approaches must be found. Here, we present a new application of the shape analysis of primary gamma rays emitted by the 13C(p,g)14N radiative capture reaction. This approach was used to monitor 13C target degradation {\em in situ} during the 13C(alpha,n)16O data taking campaign. The results obtained are in agreement with evaluations subsequently performed at Atomki (Hungary) using the NRRA method. A preliminary application for the extraction of the 13C(alpha,n)16O reaction cross-section at one beam energy is also reported.

ДОСЛІДЖЕННЯ ПРОЦЕСІВ ДЕТЕКТОРА ГАММА-ВИПРОМІНЮВАННЯ ДЛЯ РОЗРОБКИ ПОРТАТИВНОГО ЦИФРОВОГО СПЕКТРОМЕТРА

When considering methods of combating the illicit circulation of nuclear materials, it is necessary to detect trace amounts of materials, and in many cases not to seize them immediately, but to establish the place of storage, processing, routes of movement, etc. As a result, there is a new demand for isotope identification measurements to meet a wide range of different requirements. Measurements should be carried out in the field in a short time, when results need to be obtained within tens of seconds. The devices with which the personnel work should be small and low-background. Such requirements appear when working to identify cases of illegal trade in nuclear materials and radioactive sources, as well as when solving radiation protection problems and when handling radioactive devices and waste. In this work, new generation radiation sensors and measuring systems based on them have been created, which open up previously unknown possibilities in solving problems of nuclear fuel analysis, increasing the accuracy and efficiency of monitoring technological parameters and the state of protective barriers in nuclear power plants, and creating means for IAEA inspections. For the first time a portable digital gamma-ray spectrometer for radiation reconnaissance in the field was developed and created. Distinctive features of such devices are: The analysis showed that the required value of error due to energy dependence of the sensitivity can be achieved using, for example, Analog Devices 10-bit AD9411 ADCs with a sampling rate of 170 MHz. The number of quantization levels is determined by the requirement to measure the dose rate of gamma radiation with an energy of at least 10 keV. This minimum energy corresponds to the use of 10-bit ADCs. On the basis of the developed model, an ionizing radiation detector for dosimetry was created. Its fundamental difference from known devices is the use of CdZnTe crystals as a primary gamma-ray converter (sensor). The advantages of such a solution, proved by previous studies, made it possible to create a detector with: high resolution, no more than 40 keV; a wider dynamic range of values of the recorded radiation dose rate - from background to emergency operating modes of the reactor; lower value of the energy equivalent of noise.

Analysis of the Impact of Cross-sectional Data Discrepancies on the Effectiveness of Radiation Shielding Design Using Monte Carlo Codes

The effect of cross-sectional data discrepancies on the effectiveness of radiation shielding design has been investigated in this paper. The MCNP code with cross-sections from ENDF/B-V and ENDF/B-VII has been used to determine the gamma ray dose equivalent, H*(10), behind a lead glass shield enclosing a slow neutron source. It is observed that the radiative capture gamma ray dose behind the shield is higher when ENDF/B-V cross-sections are used compared to that produced by ENDF/B-VII cross-sections. The discrepancy is due to absence of energetic primary gamma rays when ENDF/B-VII is used. The results show that shielding design using ENDF/B-VII cross-sections could underestimate the shield by a fair margin and compromise safety. It is therefore necessary to consider more than one release of ENDF/B when using lead glass for shielding slow neutron capture gamma rays. The discrepancies need to be addressed in the next releases.

The Upper Limit on Isotropic Flux of Diffuse Gamma Rays with Energy Above 700 TeV as Measured by the Carpet-2 Air Shower Array

An experiment for measuring the flux of cosmic diffuse gamma rays with energies higher than 100 TeV (the Carpet-3 project) is under preparation at the Baksan Neutrino Observatory. The experiment suggests to increase areas of both the muon detector and the surface shower detectors. The experiment’s sensitivity to the showers initiated by primary gamma rays is estimated in this paper for different configurations of the array. In addition, preliminary estimates of the upper limit on diffuse gamma rays with energy above 700 TeV are presented. They are derived from experimental data of the Carpet-2 array (previous version of the experiment) for the net exposure of 9.2 years of observation.

Importance of Voxel Size in Defect Localization Using Gamma-Ray Scattering

The voxel, defined as the volume of the intersection between incident (primary) and scattered beams, plays an important role in the localization of defects in samples having several interests. In this work, the gamma rays emitted from a 137Cs radioactive source (having the strength of 222 GBq) are scattered from various regions of a wood sample. The scattered gamma flux is detected by an NaI(Tl) scintillation detector placed at 110 deg to the primary gamma-ray beam. Defect (decay) in the wood is simulated by drilling two collinear cylindrical flaws (having diameters of 0.8 and 1.2 cm) in the wood sample and then filling it with a mixture of sawdust and glue. Three sets of collimators with diameters of 6, 7, and 8 mm for the source and detector are used to vary the voxel size (volume). It has been found that better contrast (29.43% for a 1.2-cm defect and 16.37% for an 0.8-cm defect) is achieved for the smallest voxel (16.13 cm3) in comparison to the other two voxels (25.65 and 38.36 cm3). Further, better contrast for the smallest voxel is confirmed by comparing gray images obtained using MATLAB for all three voxel sizes at different scan positions. It has been concluded that for a given experimental setup, the accuracy of defect (decay) detection demands reduced voxel size.

REDUCTION IN AMBIENT GAMMA DOSE RATE FROM RADIOCESIUM DUE TO SNOW COVER.

In the present study, variations in ambient gamma dose rate associated with snow cover were examined in a radioactive-contaminated site in Fukushima Prefecture, Japan. The ambient gamma dose rates decreased with increasing snow depth. The reduction trends were different between fresh snow (0.1-0.2 g/cm3) and granular snow (0.3-0.4 g/cm3) depending on snow density. Snow cover water content (snow water equivalent) calculated from snow depth and density was a key parameter governing the reduction in the ambient gamma dose rate. The ambient gamma dose rates reduced to 0.6 and 0.5 at 4 g/cm2 and 8 g/cm2 of snow water equivalent, respectively. Based on gamma-ray flux density distributions, the ambient gamma dose rates from the primary gamma rays decreased more compared to those from scattered gamma rays due to snow cover.

Very high energy astrophysics with the SHALON Cherenkov telescopes

Abstract The base of experiments using the imaging atmospheric Cherenkov telescopes is that the Cherenkov light emitted from the particles of extensive air shower created by the primary gamma-ray is collected by a mirror reflector and then detected by a pixelized PMT camera. SHALON are the imaging atmospheric Cherenkov telescopes created for the gamma-ray astronomy at the energies of 800 GeV to 100 TeV. Telescopes are located in the Tien-Shan mountains at the altitude of 3340 m a.s.l. The telescope systems have characteristics sufficient to record precise information about the shower structure in the energy range under consideration. SHALON experiment has been operating since 1992 and covers the wide astroparticle physics topics including an acceleration and origin of cosmic rays in supernova remnants , the physics of relativistic flaring objects like a black holes and active galactic nuclei as well as the long-term studies of the different type objects.

A Liquid Scintillator Detector for the Carpet-3 Experiment

A new liquid scintillator detector created at the Carpet-2 air shower array of the Baksan Neutrino Observatory of the Institute for Nuclear Research is described. The detectors of this type will be placed in the modules of the ground-surface part of the new Carpet-3 array, which will be used to study galactic diffuse γ rays with energies of primary gamma rays Eγ > 100 TeV. For this detector the most probable amplitude of the anode signal from extensive air showers (EAS) is approximately 13 mV, while the energy resolution is ~42%. The inhomogeneity of the light-collection coefficient is 0.26. The number of photoelectrons at the photocathode is approximately 360. The detector is easy to operate and costs less than the traditional scintillation detectors used in our air-shower array.

Application of New Approximations of the Lateral Distribution of EAS Cherenkov Light in the Atmosphere

A new knee-like approximation of the lateral distribution function (LDF) of EAS Cherenkov light in the 30–3000 TeV energy range was proposed and tested with simulated showers in our earlier studies. This approximation fits the LDFs of individual showers accurately for all types of primary particles gamma-rays, protons, and nuclei) and is suitable for reconstructing the shower core, determining the energy, and separating gamma-induced showers from hadron-induced ones. In the present study, the knee-like fitting function is used to determine the parameters of real showers detected by TAIGA-HiSCORE. It is demonstrated that this approximation characterizes properly all types of individual LDFs of experimental events in the 300–1000 TeV range. The accuracy of fit is governed by fluctuations intrinsic to the process of measurement of the Cherenkov photon density. The probability density function of these fluctuations was reconstructed and introduced into simulations. Certain useful methodical applications of the knee-like approximation are con-sidered, and the possibility of shower sorting into nuclei groups is examined. The extensive statistical coverage and detailed LDF measurement data of HiSCORE have provided the first opportunity to examine in depth the LDF of Cherenkov radiation in the 300–1000 TeV range.

Compton camera based on Timepix3 technology

The Compton camera concept is based on the reconstruction of recorded Compton scattering events of incoming gamma rays. The scattering of primary gamma ray occurs in the first detector (called scattering detector—usually thin) recording the position and energy of the recoiled electron. The scattered gamma quantum continues towards the second detector (called absorption detector - usually thick) where it is absorbed. The second detector records the energy and the position of this scattered gamma. Using the Compton scattering equation it is possible to determine the scattering angle, and estimate possible directions of the original gamma ray as a surface of a cone. When the Compton camera records the number of such events, the location and the shape of the gamma source can be reconstructed. Timepix3, a hybrid single photon counting imaging pixel detector, is a perfect device for creation of a compact Compton camera. Timepix3 is an event based readout chip (every hit pixel is immediately sent to a readout) and can record the time-of-arrival (ToA) and energy of an incident gamma simultaneously in each pixel. The chip offers high energy resolution (1 keV at 60 keV, 7 keV at 356 keV), as well as time resolution (1.6 ns). The Timepix3 readout chip can be combined with different sensor materials (Si, CdTe, CZT). In this contribution, we present a very compact detector system for imaging with gamma-rays using the Compton camera principle. The system consists of at least two layers of hybrid pixel detectors Timepix3 with the sensors being optimized for gamma-ray tracking. The front detector layer (scattering) is made of silicon of 1 mm thickness, while the last layer (absorbing) is equipped with thick CdTe or CZT sensors up to 2 mm in thickness. The total absorption of the whole detector can be very high if several CdTe or CZT layers are used. The maximal number of layers is not limited, but the practical evaluation was performed with 2 layers. Thanks to Timepix3 simultaneous measurement of ToA and energy, it is possible to precisely detect coincidence events in the detector layers. Based on the energy and position of these events, it is possible to estimate the possible direction of the original gamma. The angular resolution of the presented Compton camera depends on the detected energy, and it is in the order of 1 degree.