2,476 research outputs found
Unfolding X-ray spectra using a flat panel detector. Determination of the accuracy of the method with the Monte Carlo method
[EN] The primary X-ray spectrum depends on different parameters such as high voltage, filament current, high voltage ripple, anode angle and thickness of filter material.
The objective of this work is to determine whether the unfolding technique based on the Tikhonov regularization method is accurate enough to estimate the X-ray
spectrum when slight changes in the operation variables are considered. In this frame, several X-ray spectra are considered (extracted from the IPEM78 Catalogue
Report) varying the main operation variables of the X-ray tube (high voltage, voltage ripple, filter thickness and filter material). With those spectra, the corresponding
absorbed dose curves are obtained by simulation with a MCNP5 model reproducing a flat panel detector and a PMMA wedge. Once the absorbed dose
curves are simulated and applying the unfolding Tikhonov regularization method, the unfolded spectrum is obtained, which is finally compared with the theoretical
one (IPEM78 Catalogue Report). Discrepancies between unfolded and primary X-ray spectra can be attributed to the fact that this is an ill-posed problem, and the
unfolding of the spectrum is strongly affected by the method used to improve the conditioning of the response function (response matrix).Gallardo Bermell, S.; Ródenas Diago, J.; Verdú Martín, GJ. (2019). Unfolding X-ray spectra using a flat panel detector. Determination of the accuracy of the method with the Monte Carlo method. Radiation Physics and Chemistry. 155:233-238. https://doi.org/10.1016/j.radphyschem.2018.09.014S23323815
Application of the Monte Carlo method to analyze materials used in flat panel detectors to obtain X-ray spectra
An accurate knowledge of the photon spectra emitted by X-ray tubes in radiodiagnostic is essential to
better estimate the imparted dose to patients and to improve the quality image obtained with these
devices. In this work, it is proposed the use of a flat panel detector together with a PMMA wedge to estimate
the actual X-ray spectrum using the Monte Carlo method and unfolding techniques. The MCNP5
code has been used to model different flat panels (based on indirect and direct methods to produce
charge carriers from absorbed X-rays) and to obtain the dose curves and system response functions. Most
of the actual flat panel devices use scintillator materials that present K-edge discontinuities in the mass
energy-absorption coefficient, which strongly affect the response matrix. In this paper, the applicability
of different flat panels for reconstructing X-ray spectra is studied. The effect of the mass energyabsorption
coefficient of the scintillator material has been studied on the response matrix and consequently,
in the reconstructed spectra. Different unfolding methods are tested to reconstruct the actual
X-ray spectrum knowing the dose curve and the response function. It has been concluded that the regularization
method Modified Truncated Singular Value Decomposition (MTSVD) is appropriate to unfold
X-ray spectra in all the scintillators studied.Gallardo Bermell, S.; Pozuelo, F.; Querol Vives, A.; Ródenas Diago, J.; Verdú Martín, GJ. (2015). Application of the Monte Carlo method to analyze materials used in flat panel detectors to obtain X-ray spectra. Annals of Nuclear Energy. 82:240-251. doi:10.1016/j.anucene.2014.08.065S2402518
The spectrum of high-energy cosmic rays measured with KASCADE-Grande
The energy spectrum of cosmic rays between 10**16 eV and 10**18 eV, derived
from measurements of the shower size (total number of charged particles) and
the total muon number of extensive air showers by the KASCADE-Grande
experiment, is described. The resulting all-particle energy spectrum exhibits
strong hints for a hardening of the spectrum at approximately 2x10**16 eV and a
significant steepening at c. 8x10**16 eV. These observations challenge the view
that the spectrum is a single power law between knee and ankle. Possible
scenarios generating such features are discussed in terms of astrophysical
processes that may explain the transition region from galactic to extragalactic
origin of cosmic rays.Comment: accepted by Astroparticle Physics June 201
Rigidity-dependent cosmic ray energy spectra in the knee region obtained with the GAMMA experiment
On the basis of the extensive air shower (EAS) data obtained by the GAMMA
experiment, the energy spectra and elemental composition of the primary cosmic
rays are derived in the 1-100 PeV energy range. The reconstruction of the
primary energy spectra is carried out using an EAS inverse approach in the
framework of the SIBYLL2.1 and QGSJET01 interaction models and the hypothesis
of power-law primary energy spectra with rigidity-dependent knees. The energy
spectra of primary H, He, O-like and Fe-like nuclei obtained with the SIBYLL
interaction model agree with corresponding extrapolations of the balloon and
satellite data to ~1 PeV energies. The energy spectra obtained from the QGSJET
model show a predominantly proton composition in the knee region. The
rigidity-dependent knee feature of the primary energy spectra for each
interaction model is displayed at the following rigidities: ~2.5+/-0.2 PV
(SIBYLL) and ~3.1-4.2 PV (QGSJET). All the results presented are derived taking
into account the detector response, the reconstruction uncertainties of the EAS
parameters, and fluctuations in the EAS development.Comment: 28 pages, 18 figures, accepted for publication in Astroparticle
Physic
The Spectral Energy Distribution of Fermi bright blazars
(Abridged) We have conducted a detailed investigation of the broad-band
spectral properties of the \gamma-ray selected blazars of the Fermi LAT Bright
AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray
spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray
data, collected within three months of the LBAS data taking period, we were
able to assemble high-quality and quasi-simultaneous Spectral Energy
Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is
similar to that of blazars discovered at other wavelengths, clearly showing, in
the usual Log - Log F representation, the typical broad-band
spectral signatures normally attributed to a combination of low-energy
synchrotron radiation followed by inverse Compton emission of one or more
components. We have used these SEDs to characterize the peak intensity of both
the low and the high-energy components. The results have been used to derive
empirical relationships that estimate the position of the two peaks from the
broad-band colors (i.e. the radio to optical and optical to X-ray spectral
slopes) and from the gamma-ray spectral index. Our data show that the
synchrotron peak frequency is positioned between 10 and
10 Hz in broad-lined FSRQs and between and Hz in
featureless BL Lacertae objects.We find that the gamma-ray spectral slope is
strongly correlated with the synchrotron peak energy and with the X-ray
spectral index, as expected at first order in synchrotron - inverse Compton
scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton
(SSC) models cannot explain most of our SEDs, especially in the case of FSRQs
and low energy peaked (LBL) BL Lacs. (...)Comment: 85 pages, 38 figures, submitted to Ap
Direct measurement of neutrons induced in lead by cosmic muons at a shallow underground site
Neutron production in lead by cosmic muons has been studied with a Gadolinium
doped liquid scintillator detector. The detector was installed next to the
Muon-Induced Neutron Indirect Detection EXperiment (MINIDEX), permanently
located in the T\"ubingen shallow underground laboratory where the mean muon
energy is approximately 7 GeV. The MINIDEX plastic scintillators were used to
tag muons; the neutrons were detected through neutron capture and
neutron-induced nuclear recoil signals in the liquid scintillator detector.
Results on the rates of observed neutron captures and nuclear recoils are
presented and compared to predictions from GEANT4-9.6 and GEANT4-10.3. The
predicted rates are significantly too low for both versions of GEANT4. For
neutron capture events, the observation exceeds the predictions by factors of and for GEANT4-9.6
and GEANT4-10.3, respectively. For neutron nuclear recoil events, which require
neutron energies above approximately 5 MeV, the factors are even larger, and , respectively.
Also presented is the first statistically significant measurement of the
spectrum of neutrons induced by cosmic muons in lead between 5 and 40 MeV. It
was obtained by unfolding the nuclear recoil spectrum. The observed neutron
spectrum is harder than predicted by GEANT4. An investigation of the
distribution of the time difference between muon tags and nuclear recoil
signals confirms the validity of the unfolding procedure and shows that GEANT4
cannot properly describe the time distribution of nuclear recoil events. In
general, the description of the data is worse for GEANT4-10.3 than for
GEANT4-9.6.Comment: 29 pages, 22 figures, 4 table
The Spectral Energy Distribution of Fermi Bright Blazars
We have conducted a detailed investigation of the broadband spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi γ-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/γ-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these γ-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual log ν-log ν F _ν representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low- and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, α_(ro), and optical to X-ray, α_(ox), spectral slopes) and from the γ-ray spectral index. Our data show that the synchrotron peak frequency (ν^S _(peak)) is positioned between 10^(12.5) and 10^(14.5) Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 10^(13) and 10^(17) Hz in featureless BL Lacertae objects. We find that the γ-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter γ-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum γ-ray sources, the correlation between ν ^S _(peak) and γ-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars
Measurement of boron and carbon fluxes in cosmic rays with the PAMELA experiment
The propagation of cosmic rays inside our galaxy plays a fundamental role in
shaping their injection spectra into those observed at Earth. One of the best
tools to investigate this issue is the ratio of fluxes for secondary and
primary species. The boron-to-carbon (B/C) ratio, in particular, is a sensitive
probe to investigate propagation mechanisms. This paper presents new
measurements of the absolute fluxes of boron and carbon nuclei, as well as the
B/C ratio, from the PAMELA space experiment. The results span the range 0.44 -
129 GeV/n in kinetic energy for data taken in the period July 2006 - March
2008
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