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Imaging atmospheric Cherenkov telescopes (IACTs) are ground-based indirect detectors for cosmic gamma rays with energies above tens of GeV. The major backgrounds for gamma-ray observations in IACTs are cosmic-ray charged particles. The capability to reject these backgrounds is the most important factor determining the gamma-ray sensitivity of IACT systems. Monte Carlo simulations are used to estimate the residual background rates and sensitivity of the systems during the design and construction phase. Uncertainties in the modeling of high-energy hadronic interactions of cosmic rays with nuclei in the air propagate into the estimates of residual background rates and subsequently into the estimated instrument sensitivity. We investigate the influence of the difference in the current hadronic interaction models on the estimated gamma-ray sensitivity of the Cherenkov Telescope Array using four interaction models (QGSJET-II-03, QGSJET-II-04, EPOS-LHC, and SIBYLL2.3c) implemented in the air shower simulation tool CORSIKA. Variations in background rates of up to a factor 2 with respect to QGSJET-II-03 are observed between the models, mainly due to differences in the π0 production spectrum. These lead to ∼30% differences in the estimated gamma-ray sensitivity in the 1–30 TeV region, assuming a 50 h observation of a gamma-ray point-like source. The presented results also show that IACTs have a significant capability in the verification of hadronic interaction models. [ABSTRACT FROM AUTHOR]

In recent years, a new generation of space missions has offered great opportunities for discovery in high-energy astrophysics. In this article we focus on the scientific operations of the Gamma-Ray Imaging Detector (GRID) on board the AGILE space mission. AGILE-GRID, sensitive in the energy range of 30 MeV-30 GeV, has detected many γ-ray transients of both galactic and extragalactic origin. This work presents the AGILE innovative approach to fast γ-ray transient detection, which is a challenging task and a crucial part of the AGILE scientific program. The goals are to describe (1) the AGILE Gamma-Ray Alert System, (2) a new algorithm for blind search identification of transients within a short processing time, (3) the AGILE procedure for γ-ray transient alert management, and (4) the likelihood of ratio tests that are necessary to evaluate the post-trial statistical significance of the results. Special algorithms and an optimized sequence of tasks are necessary to reach our goal. Data are automatically analyzed at every orbital downlink by an alert pipeline operating on different timescales. As proper flux thresholds are exceeded, alerts are automatically generated and sent as SMS messages to cellular telephones, via e-mail, and via push notifications from an application for smartphones and tablets. These alerts are crosschecked with the results of two pipelines, and a manual analysis is performed. Being a small scientific-class mission, AGILE is characterized by optimization of both scientific analysis and ground-segment resources. The system is capable of generating alerts within two to three hours of a data downlink, an unprecedented reaction time in γ-ray astrophysics. [ABSTRACT FROM AUTHOR]

In this paper we report on Monte Carlo simulations to investigate real-time monitoring of the track depth profile in particle therapy by measuring prompt gamma ray emissions: a high sensitivity imaging system employing a knifeedge- shaped slit combined with a position-sensitive gamma detector was evaluated. Calculations to test this newconcept were performed for a head-sized software phantom. Clear spatial correlation is shown between the distribution of gamma rays detected with energies above 1.5 MeV and the distribution of prompt gamma rays emitted from the phantom. The number of neutrons originating from nuclear reactions in the phantom that are detected at these high energies is small. Most importantly it is shown that under common therapy conditions enough data may be collected during one spot-step (of the order of 10 ms) to locate the distal dose edge with a 1σ accuracy of better than 1 mm. This indicates that simple slit cameras have high potential for accurate realtime particle therapy adjustment and may become a practical way to improve particle therapy accuracy. [ABSTRACT FROM AUTHOR]

A novel dual-emitter vacuum Compton detector (D-VCD) with higher gamma ray detecting efficiency is proposed. The emitters are made of Ta-Al clad metal. The gamma ray sensitivity is studied by Monte Carlo simulation using the MCNP code. A comparison between calculations and results measured by using the 1.25 MeV gamma ray of Co-60 is also performed. Experimental sensitivities for two sample D-VCDs with the same materials and structures are 1.92 x 10[?]20 and 2.02x10[?]20 C * cm2/MeV separately, which are consistent with the simulation result of 1.98x10[?]20 C * cm2/MeV and are 4 times higher than that of VCD with a single Fe emitter. According to the simulation results, in a gamma energy range from 0.5 to 3 MeV, the maximum sensitivity variance for the D-VCD is less than 15%, and less than 5% in a range from 1 to 2 MeV in particular. The novel D-VCD is applicable to the detection of intense pulse gamma rays. [ABSTRACT FROM AUTHOR]

Cadmium telluride (CdTe) and its compounds are the materials of choice for producing industrial quality hard x-ray and gamma ray detectors with high spectral resolution and signal-to-noise ratio. However, optimization of the growth process still proves challenging as the yield is small due to inhomogeneities in the material parameters. Here we investigated the influence of inhomogeneous resistivity on charge collection efficiency of CdZnTe radiation detectors operating at high photon fluxes of incoming radiation. We applied a complex of experimental methods—contactless resistivity and photoconductivity mapping, photoluminescence and laser-induced transient current technique. We observed that the charge collection efficiency at low fluxes is nearly independent of resistivity, while at high fluxes the performance of high resistivity part substantially decreases when compared to the lower resistivity part. This behavior is explained by characteristic evolution of defect structure attaining shallow defect self-compensation during the cooling of the solidified crystal. Instabilities at impurity segregation and temperature gradients at the crystal growth cause different concentration of defects that manifest themselves as deep energy levels within the material bandgap. The defect self-compensation is successfully simulated by theoretical model considering defect reactions in tellurium-saturated CdZnTe. [ABSTRACT FROM AUTHOR]

Protons with modern pencil-beam scanning delivery are widely used in state-of-the-art radiotherapy. To reduce the unwanted effect of proton range uncertainties, prompt gamma (PG) monitoring is investigated and considered one of the most promising methods for real-time, in vivo range verification. Despite good correlation between the penetration depth of the PG signal and proton range in most cases, mismatch can occur especially because of tissue heterogeneities. Moreover, detectability and reproducibility of the prompt gamma signal critically depends on counting statistics. Nowadays, conventional treatment planning systems do not account for the degree of correlation between dose and PG signal nor the expected PG signal counting statistics, which considerably influences the possibility of a reliable verification of the intended beam range. Hence, in this project, we investigate a new treatment planning approach, in which the spot-by-spot conformities between PG and dose profiles (PG-dose correlation) as well as PG signal detectability and precision are taken into account based on a TPS optimizer. To investigate the feasibility of this idea, a research computational platform, combining Monte Carlo (MC, Geant4) pre-calculated pencil beams with the analytical Matlab-based TPS engine CERR, is used for treatment planning. Geant4 is employed for realistic simulation of the dose delivery and PG generation of all spots in the heterogeneous patient anatomy given by CT images. First of all, a treatment plan is created using a charged particle extension of CERR. Secondly, the PG fall-off positions of all individual pencil beams are evaluated and compared to the 80% distal dose fall-off positions. Thirdly, the PG-dose correlations of all spots are quantified. A new plan, in which a few spots with the best PG-dose correlation are boosted to ensure PG detectability with good precision, is then made. Finally, the optimized plan is fully recalculated on the same patient CT using Geant4, and the result is evaluated considering both plan quality and beam range monitorability. The evaluation shows that the re-optimized treatment plan is comparable to the initial plan in terms of dose distribution, dose averaged LET distribution and robustness, while fulfilling the set statistical conditions for reliable PG monitoring of the few automatically or manually selected spots. The method could thus complement, and for the selected pencil beams even overcome limitations of, alternative suggested approach such as pencil beam aggregation to provide sufficient counting statistics for precise PG range retrieval with good correlation to the treatment dose. [ABSTRACT FROM AUTHOR]

We report the Fermi Large Area Telescope (LAT) detections of high-energy (>100 MeV) γ-ray emission from two recent optically bright classical novae, V1369 Centauri 2013 and V5668 Sagittarii 2015. At early times, Fermi target-of-opportunity observations prompted by their optical discoveries provided enhanced LAT exposure that enabled the detections of γ-ray onsets beginning ∼2 days after their first optical peaks. Significant γ-ray emission was found extending to 39–55 days after their initial LAT detections, with systematically fainter and longer-duration emission compared to previous γ-ray-detected classical novae. These novae were distinguished by multiple bright optical peaks that encompassed the time spans of the observed γ-rays. The γ-ray light curves and spectra of the two novae are presented along with representative hadronic and leptonic models, and comparisons with other novae detected by the LAT are discussed. [ABSTRACT FROM AUTHOR]

Gamma-ray detectors based on thick monolithic scintillator crystals can achieve spatial resolutions  <2 mm full-width-at-half-maximum (FWHM) and coincidence resolving times (CRTs) better than 200 ps FWHM. Moreover, they provide high sensitivity and depth-of-interaction (DOI) information. While these are excellent characteristics for clinical time-of-flight (TOF) positron emission tomography (PET), the application of monolithic scintillators has so far been hampered by the lengthy and complex procedures needed for position- and time-of-interaction estimation. Here, the algorithms previously developed in our group are revised to make the calibration and operation of a large number of monolithic scintillator detectors in a TOF-PET system practical. In particular, the k-nearest neighbor (k-NN) classification method for x,y-position estimation is accelerated with an algorithm that quickly preselects only the most useful reference events, reducing the computation time for position estimation by a factor of ~200 compared to the previously published k-NN 1D method. Also, the procedures for estimating the DOI and time of interaction are revised to enable full detector calibration by means of fan-beam or flood irradiations only. Moreover, a new technique is presented to allow the use of events in which some of the photosensor pixel values and/or timestamps are missing (e.g. due to dead time), so as to further increase system sensitivity. The accelerated methods were tested on a monolithic scintillator detector specifically developed for clinical PET applications, consisting of a 32 mm  ×  32 mm  ×  22 mm LYSO : Ce crystal coupled to a digital photon counter (DPC) array. This resulted in a spatial resolution of 1.7 mm FWHM, an average DOI resolution of 3.7 mm FWHM, and a CRT of 214 ps. Moreover, the possibility of using events missing the information of up to 16 out of 64 photosensor pixels is shown. This results in only a small deterioration of the detector performance. [ABSTRACT FROM AUTHOR]

We present a study of the angular resolution of the AGILE gamma-ray imaging detector (GRID) that has been operational in space since 2007 April. The AGILE instrument is made of an array of 12 planes that are each equipped with a tungsten converter and silicon microstrip detectors, and is sensitive in the energy range 50 MeV–10 GeV. Among the space instruments devoted to gamma-ray astrophysics, AGILE uniquely exploit an analog readout system with dedicated electronics coupled with silicon detectors. We show the results of Monte Carlo simulations carried out to reproduce the gamma-ray detection by the GRID and we compare them to in-flight data. We use the Crab (pulsar + Nebula) system for discussion of real data performance, since its energy spectrum is representative of the majority of gamma-ray sources. For Crab-like spectrum sources, the GRID angular resolution (FWHM of at 100 MeV; at 1 GeV; integrating the full energy band from 100 MeV to tens of GeV) is stable across a large field of view, characterized by a flat response up to off-axis. A comparison of the angular resolution obtained by the two operational gamma-ray instruments, AGILE/GRID and Fermi/LAT (Large Area Telescope), is interesting in view of future gamma-ray missions, which are currently under study. The two instruments exploit different detector configurations that affect the angular resolution: the former is optimized in the readout and track reconstruction, especially in the low-energy band, the latter is optimized in terms of converter thickness and power consumption. We show that despite these differences, the angular resolution of both instruments is very similar, between 100 MeV and a few GeV. [ABSTRACT FROM AUTHOR]

Gamma-ray detectors based on bright monolithic scintillation crystals coupled to pixelated photodetectors are currently being considered for several applications in the medical imaging field. In a typical monolithic detector, both the light intensity and the time of arrival of the earliest scintillation photons can be recorded by each of the photosensor pixels every time a gamma interaction occurs. Generally, the time stamps are used to determine the gamma interaction time while the light intensities are used to estimate the 3D position of the interaction point. In this work we show that the spatio-temporal distribution of the time stamps also carries information on the location of the gamma interaction point and thus the time stamps can be used as explanatory variables for position estimation. We present a model for the spatial resolution obtainable when the interaction position is estimated using exclusively the time stamp of the first photon detected on each of the photosensor pixels. The model is shown to be in agreement with experimental measurements on a 16 mm  ×  16 mm  ×  10 mm LSO : Ce,0.2%Ca crystal coupled to a digital photon counter (DPC) array where a spatial resolution of 3 mm (root mean squared error) is obtained. Finally we discuss the effects of the main parameters such as scintillator rise and decay time, light output and photosensor single photon time resolution and pixel size. [ABSTRACT FROM AUTHOR]

The merger of a binary of neutron stars provides natural explanations for many of the features of short gamma-ray bursts (SGRBs), such as the generation of a hot torus orbiting a rapidly rotating black hole, which can then build a magnetic jet and provide the energy reservoir to launch a relativistic outflow. However, this scenario has problems explaining the recently discovered long-term and sustained X-ray emission associated with the afterglows of a subclass of SGRBs. We propose a new model that explains how an X-ray afterglow can be sustained by the product of the merger and how the X-ray emission is produced before the corresponding emission in the gamma-band, though it is observed to follow it. Overall, our paradigm combines in a novel manner a number of well-established features of the emission in SGRBs and results from simulations. Because it involves the propagation of an ultra-relativistic outflow and its interaction with a confining medium, the paradigm also highlights a unifying phenomenology between short and long GRBs. [ABSTRACT FROM AUTHOR]

One of the key open question in the study of jets, in general, and jets in gamma-ray bursts (GRBs), in particular, is the magnetization of the outflow. Here we consider the photospheric emission of Poynting-flux-dominated outflows, when the dynamics is mediated by magnetic reconnection. We show that thermal three-particle processes, responsible for the thermalization of the plasma, become inefficient at a radius of cm, far below the photosphere, at cm. Conservation of the total photon number above combined with Compton scattering below the photosphere enforces kinetic equilibrium between electrons and photons. This, in turn, leads to an increase in the observed photon temperature, which reaches MeV (observed energy) when decoupling the plasma at the photosphere. This result is weakly dependent on the free model parameters. We show that in this case, the expected thermal luminosity is a few percent of the total luminosity, and could therefore be detected. The predicted peak energy is more than an order of magnitude higher than the observed peak energy of most GRBs, which puts strong constraints on the magnetization of these outflows. [ABSTRACT FROM AUTHOR]

One of the most important unresolved issues in gamma-ray burst (GRB) physics is the origin of the prompt gamma-ray spectrum. Its general non-thermal character and the softness in the X-ray band remain unexplained. We tackle these issues by performing Monte Carlo simulations of radiation–matter interactions in a scattering dominated photon–lepton plasma. The plasma—initially in equilibrium—is driven to non-equilibrium conditions by a sudden energy injection in the lepton population, mimicking the effect of a shock wave or the dissipation of magnetic energy. Equilibrium restoration occurs due to an energy exchange between the photons and leptons. While the initial and final equilibrium spectra are thermal, the transitional photon spectra are characterized by non-thermal features such as power-law tails, high energy bumps, and multiple components. Such non-thermal features are observed at infinity if the dissipation occurs at small to moderate optical depths, and the spectrum is released before thermalization is complete. We model the synthetic spectra with a Band function and show that the resulting spectral parameters are similar to observations for a frequency range of 2–3 orders of magnitude around the peak. In addition, our model predicts correlations between the low-frequency photon index and the peak frequency as well as between the low- and high-frequency indices. We explore baryon and pair-dominated fireballs and reach the conclusion that baryonic fireballs are a better model for explaining the observed features of GRB spectra. [ABSTRACT FROM AUTHOR]

Long-duration gamma-ray bursts (GRBs) are powerful probes of the star formation history of the universe, but the correlation between the two depends on the highly debated presence and strength of a metallicity bias. To investigate this correlation, we use a phenomenological model that successfully describes star formation rates, luminosities, and stellar masses of star-forming galaxies and apply it to GRB production. We predict the luminosities, stellar masses, and metallicities of host galaxies depending on the presence (or absence) of a metallicity bias. Our best-fitting model includes a moderate metallicity bias, broadly consistent with the large majority of the long-duration GRBs in metal-poor environments originating from a collapsar (probability , with range at 90% confidence level), but with a secondary contribution () from a metal-independent production channel, such as binary evolution. Because of the mass–metallicity relation of galaxies, the maximum likelihood model predicts that the metal-independent channel becomes dominant at , where hosts have higher metallicities and collapsars are suppressed. This possibly explains why some studies find no clear evidence of a metal bias based on low-z samples. However, while metallicity predictions match observations well at high redshift (), there is tension with low-redshift observations, since a significant fraction of GRB hosts are predicted to have (near) solar metallicity. This is in contrast to observations, unless obscured, metal-rich hosts are preferentially missed in current data sets, and suggests that lower efficiencies of the metal-independent GRB channel might be preferred following a comprehensive fit that includes metallicity of GRB hosts from complete samples. Overall, we are able to clearly establish the presence of a metallicity bias for GRB production, but continued characterization of GRB host galaxies is needed to quantify its strength. Tabulated model predictions are available in electronic format. [ABSTRACT FROM AUTHOR]

We present the statistics of the ratio, , between the prompt and afterglow “plateau” fluxes of gamma-ray bursts (GRBs). We define this as the ratio of the mean prompt energy flux in Swift BAT and the Swift XRT one, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the “plateau”. Like the distribution of many other GRB observables, the histogram of is log-normal with maximum at a value , FWHM of about two decades, and with the entire distribution spanning about five decades in the value of . We note that the peak of the distribution is close to the proton-to-electron mass ratio , as proposed to be the case in an earlier publication, on the basis of a specific model of the GRB dissipation process. It therefore appears that, in addition to the values of the energy of peak luminosity , GRBs present us with one more quantity with an apparent characteristic value. The fact that the values of both these quantities ( and ) are consistent with the same specific model invoked to account for the efficient conversion of their relativistic proton energies to electrons argues favorably for its underlying assumptions. [ABSTRACT FROM AUTHOR]

We performed an experimental study to verify the range of passively scattered proton beams by detecting prompt gamma-rays emitted from proton-nuclear interactions. A method is proposed using a single scintillation detector positioned near the distal end of the irradiated target. Lead shielding was used to attenuate gamma-rays emitted along most of the entrance path of the beam. By synchronizing the prompt gamma-ray detector to the rotation of the range modulation wheel, the relation between the gamma emission from the distal part of the target and the range of the incident proton beam was determined. In experiments with a water phantom and an anthropomorphic head phantom, this relation was found to be sensitive to range shifts that were introduced. The wide opening angle of the detector enabled a sufficient signal-to-background ratio to be achieved in the presence of neutron-induced background from the scattering and collimating devices. Uniform range shifts were detected with a standard deviation of 0.1 mm to 0.2 mm at a dose level of 30 cGy to 50 cGy (RBE). The detectable magnitude of a range shift limited to a part of the treatment field area was approximately proportional to the ratio between the field area and the area affected by the range shift. We conclude that it is feasible to detect changes in the range of passively scattered proton beams using a relatively simple prompt gamma-ray detection system. The method can be employed for in vivo verification of the consistency of the delivered range in fractionated treatments. [ABSTRACT FROM AUTHOR]

We present an experimental study of a novel method to verify the range of proton therapy beams. Differential cross sections were measured for 15 prompt gamma-ray lines from proton-nuclear interactions with 12C and 16O at proton energies up to 150 MeV. These cross sections were used to model discrete prompt gamma-ray emissions along proton pencil-beams. By fitting detected prompt gamma-ray counts to these models, we simultaneously determined the beam range and the oxygen and carbon concentration of the irradiated matter. The performance of the method was assessed in two phantoms with different elemental concentrations, using a small scale prototype detector. Based on five pencil-beams with different ranges delivering 5 × 108 protons and without prior knowledge of the elemental composition at the measurement point, the absolute range was determined with a standard deviation of 1.0–1.4 mm. Relative range shifts at the same dose level were detected with a standard deviation of 0.3–0.5 mm. The determined oxygen and carbon concentrations also agreed well with the actual values. These results show that quantitative prompt gamma-ray measurements enable knowledge of nuclear reaction cross sections to be used for precise proton range verification in the presence of tissue with an unknown composition. [ABSTRACT FROM AUTHOR]

Observations with the Cherenkov telescopes are in principle limited to clear sky conditions due to significant absorption of Cherenkov light by clouds. If the cloud level is high enough or the atmospheric transmission of the cloud is high, then high energy showers (with TeV energies) can still produce enough Cherenkov photons allowing detection by telescopes with large sizes and cameras with large field of view (FOV). In this paper, we study the possibility of observations of showers, induced by high-energy particles in the atmosphere, in the presence of clouds that are completely or partially opaque for Cherenkov radiation. We show how the image parameters of the Cherenkov light distribution on the telescope camera are influenced for different opacity and altitude of the cloud. By applying the Monte Carlo simulations, we calculate the scaled LENGTH and WIDTH parameters with the purpose to separate γ-ray and proton initiated showers in real data. We show, that the high level of the night sky background effects the selection efficiency of the γ-ray initiated showers. However, application of the higher image-cleaning level significantly improves expected quality factors. The estimated γ-ray selection efficiency for the detector with the camera field of view (FOV) limited to 8 is slightly better than for the camera with an unlimited FOV, although the number of identified γ-ray events is lower. We conclude that large Cherenkov telescopes with large FOV cameras can be used for observations of very high energy γ-rays in the presence of clouds. Consequently, the amount of useful data can be significantly enlarged. [ABSTRACT FROM AUTHOR]

The high atomic number of bismuth iodide and the high-sensitivity photoluminescence quenching of conjugated polymers are leveraged in their nanocomposites for the detection of high-energy photons. With the introduction of oleylamine, the composites can be processed from solutions in organic solvent into high-quality films containing as much as 50 wt% BiI3. BiI3 in the resulting ternary composites is dispersed on a nanoscale or smaller, allowing the formation of transparent composites. Oleylamine was found to form an interfacial layer between the polymer matrix and BiI3 nanodomains to block charge transfer between BiI3 and the polymer. Upon gamma ray exposure, the luminescent intensity of the ternary composites decreased linearly with the radiation dosage. [ABSTRACT FROM AUTHOR]

Recent advances in ground-based gamma ray astronomy have led to the discovery of more than 70 sources of very high energy (Eg [?] 100 GeV) gamma rays, falling into a number of source populations including pulsar wind nebulae, shell type supernova remnants, Wolf-Rayet stars, giant molecular clouds, binary systems, the Galactic Center, active galactic nuclei and 'dark' (yet unidentified) galactic objects. We summarize the history of TeV gamma ray astronomy up to the current status of the field including a description of experimental techniques and highlight recent astrophysical results. We also discuss the potential of ground-based gamma ray astronomy for future discoveries and describe possible directions for future instrumental developments. [ABSTRACT FROM AUTHOR]