These photodiodes are coated with a crystalline cesium iodide scintillator or a rare-earth scintillator Terbium-doped Gadolinium dioxide sulfide. When these scintillators are struck by X-rays, visible light is emitted proportional to the incident X-ray energy. The light photons are then converted into an electric charge by the photodiode arrays.
Unlike the selenium-based system, this type of indirect-conversion detector technology requires a two-step process for X-ray detection, i. Company Profile. Company News. Industry News. Focus News. MRI Systems. DR Systems. Ultrasound Systems. Contact Us. Direct vs. Direct-conversion flat panel detectors based on amorphous selenium The selenium-based technology uses an amorphous selenium-coated thin-film-transistor TFT array to capture and convert X-ray energy directly into digital signals. Indirect-conversion flat panel detectors based on amorphous silicon A second type of digital sensor is based on amorphous silicon.
These arrays record light created by the impact of X-rays on a scintillation material, such as photostimulable phosphors. The most salient characteristic of CCDs is that they are relatively small - typically cm2, which is much smaller than typical projected X-ray areas.
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Because of this, CCD-based systems require optical coupling to reduce the size of the projected visible light image and transfer the image to the face of one or more CCDs. Although CCD-based detectors require optical coupling and image demagnification, they are attractive because they are both widely available and relatively low in cost. CCD Technology. DR vs.
CCD vs. Production of pairs of elementary stable doubly charged heavy leptons is characterized by a number of distinct experimental signatures that would provide effective search for them at the experiments at the LHC and test the atom-like structure of the cosmological dark matter. Here we discuss following Bulekov et al. Searches for such kind of particles were carried out in many cosmic ray and collider experiments see for instance review in [ 39 ].
In a tree approximation, such particles cannot decay to pair of quarks due to electric charge conservation and only decays to the same sign leptons are possible. The latter implies lepton number nonconservation, being a profound signature of new physics. In general, it makes such states sufficiently long-living in a cosmological scale.
Since the kinematics and cross sections for double charged particle production processes cannot be reliably predicted, search results at collider experiments are usually quoted as cross section limits for a range of charges and masses for well-defined kinematic models. In these early experiments, the mass limit was set at the level of up to GeV see for review [ 39 ].
Significant increase of beam energy at the Large Hadron Collider LHC opens a new era in the high energy physics and allows accessing uncharted territories of particle masses. In Run 1 — , the ATLAS [ 40 ] collaboration at LHC performed two searches for long-lived multi-charged particles, including the double charged particles: one search with 4. Both these searches feature particles with large transverse momentum values, traversing the entire ATLAS detector. Such particles will leave a very characteristic signature of high ionization in the detector.
More specifically, the searches look for particles with anomalously high ionization on their tracks in three independent detector subsystems: silicon pixel detector Pixel and transition radiation tracker TRT in the ATLAS inner detector, and monitoring drift tubes MDT in the muon system. The estimate of the expected background originating from the SM processes and providing input into the signal region D was calculated to be 0. The comparison of observed cross-section upper limits and theoretically predicted cross-sections is shown in Figure 4. Figure 4. Comparison between observed upper cross section limits and theoretically predicted cross section values for the 8 TeV is shown in Figure 5.
Figure 5. For the 8 TeV search, the mass limit of GeV was obtained. A combination of the results of two experiments for 8 TeV would mean an increase of statistics by a factor of 2. Having said that, one can conclude that the mass limit based on the results of both experiment for double charged particles can be set at the level of about GeV. Several assumptions were made in these speculations [ 17 ]:.
If these two experiments combined their independently gathered statistics together for this kind of search, the limits would go as high as up to 1.
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Dark Matter that is considered to be electrically neutral, potentially can be formed by stable heavy charged particles bound in neutral atom-like states by Coulomb attraction. Analysis of the cosmological data and atomic composition of the Universe gives the constrains on the particle charge showing that only -2 charged constituents, being trapped by primordial helium in neutral O-helium states, can avoid the problem of overproduction of the anomalous isotopes of chemical elements, which are severely constrained by observations. Cosmological model of O-helium dark matter can even explain puzzles of direct dark matter searches.
The Efficiency of Production of Characteristic X-radiation in Thick Targets of a Pure Element
This explanation involves minimal number of parameters of new physics, being based on known laws of quantum mechanics and atomic and nuclear physics. However, we have seen that the nontrivial features of OHe-nucleus interaction still leave open the crucial question on the existence and effect of dipole potential barrier in this interaction. Such barrier provides dominance of elastic processes in OHe interaction with matter and existence of shallow potential well in OHe interaction with intermediate mass nuclei. Search for such anomalous form of nuclear matter extends the set of probes for dark-atom hypothesis.
The crucial problem of OHe scenario is the nuclear physics of OHe. Though it involves only known atomic and nuclear physics the proper quantum mechanical treatment is very complicated in the lack of small parameters in the problem of OHe-nucleus interaction. Such treatment should also take into proper account simultaneous action of Coulomb and nuclear forces on He from the approaching nucleus. There is a hint to solve this problem and the development of the quantitative analysis is under way.
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As soon as this problem is solved all the richness of the astrophysical features of OHe scenario will find proper physical basis see e. OHe hypothesis can also explain observed excess of positron annihilation line in the galactic bulge and high energy positron fraction of cosmic rays. Such explanation implies upper limits on the mass of a double charged constituents within 1 TeV range, challenging their searches at the LHC.
The latter, composed of techniquarks, reveal their structure at much higher energy scale and should be produced at colliders and accelerators as elementary species. In the context of composite dark matter scenario accelerator search for stable doubly charged leptons acquires the meaning of direct critical test for existence of charged constituents of cosmological dark matter. The signature for AC leptons and techniparticles is unique and distinctive what allows to separate them from other hypothetical exotic particles.
The most stringent results achieved so far exclude the existence of such particles up to their mass of GeV. It is expected that if these two collaborations combine their independently gathered statistics of LHC Run 2 — , the lower mass limit of double charged particles could reach the level of about 1.
It can provide severe test for composite dark matter origin of possible indirect effects of dark matter. One can conclude that signatures of composite dark matter provide an extensive set of direct and indirect probes challenging their physical and astrophysical test. The author confirms being the sole contributor of this work and has approved it for publication. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
I express my gratitude to K. Belotsky, O.
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Bulekov, J. Cudell, C. Kouvaris, M. Laletin, A. Romaniouk, and Yu. Smirnov for their collaboration in obtaining original results on which this review is based. The paper involves with proper references the results published by author in Khlopov [ 1 ], Bulekov et al. The work was supported by grant of Russian Science Foundation project N Khlopov MY.
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