Background fractal dimension of event temporal profiles in additional aperture of gamma-telescope GAMMA-400 by results of calibrations on synchrotron "PAKHRA".
Abstract
Gamma-telescope GAMMA-400 (Gamma Astronomical Multifunctional Modular Apparatus) consists of the converter-tracker (C), time-of-flight system TOF (2 sections S1 and S2), top (ACtop) and lateral (AClat) sections of anticoincidence system, position-sensitive calorimeter CC1 makes of 2 strips layers and 2 layers of CsI(Tl) detectors, electromagnetic calorimeter CC2 composed of CsI(Tl) crystals, scintillation detectors of the calorimeter (S3 and S4). Installation of lateral detectors of the calorimeter (LD) required for particles registration from lateral directions. All detector systems ACtop, AClat, S1-S4, LD consist of two BC-408 based sensitive layers of 1 cm thickness each. The thickness of CC1 and CC2 is ∼ 2 X_{0} (∼ 0.1 λ_{0}) and ∼ 20 X_{0} (∼ 0.9 λ_{0}) respectively (where λ_{0} is nuclear interaction length). The total calorimeter thickness is ∼ 22 X_{0} (∼ 1.0 λ_{0}) for vertical incident particles registration and ∼ 56 X_{0} (∼ 2.5 λ_{0}) for laterally incident ones.Three apertures provide events registration both from upper and lateral directions. Gamma-telescope GAMMA-400 is optimized for the γ-quanta and charged particles with energy 100 GeV detection with the best parameters in the main aperture. Other apertures used for studying of transient events do not require best angular resolution as solar flares and gamma-ray bursts. The main aperture created firstly due to converter-tracker (C): gammas converted in tungsten conversion foils are registered. Triggers in the main aperture will be formed using information about particle direction provided by TOF system and about presence of charged particle or backsplash obtained from ACtop and AClat anticoincidence detectors in energy band 20 MeV - 1.0 TeV for gammas and E > 100 MeV for electrons. Additional aperture allows to registered particles from upper directions which don't interact with converter-tracker and don't formed TOF signal. Particles detection in additional aperture starts with signal of CC1 fast discriminators in anticoincidence with TOF. Energy band for gammas registration in this aperture is similar to the main one. In the lateral aperture low energy (0.2 - 100 MeV) photons classified by using simple anticoincidence signals from the individual detectors of LD and CC2. Higher energies γ-quanta (E > 100 MeV) recognized using energy deposition analysis in the individual detectors of S3, S4, LD and CC2.High-energy γ-quanta registered in scintillator mostly after formation of electron-positron pairs. Thus electron and positron beams also used to gamma-detectors calibration. The positron beam with energies 100-300 MeV was used for calibrations of prototypes of GAMMA-400 detectors on synchrotron "PAKHRA". GAMMA-400 prototype of additional aperture functioning consist of two detectors. One of them is BC-408 based with dimensions of 128× 10× 1 cm^{3} (AC/LD prototype) and other composed of CsI(Tl) crystal with dimensions of 33× 5× 2 cm^{3} (CC1 prototype). A fractal analysis often used for investigation of time series corresponding to transitive events (solar flares, gamma-ray bursts and so on). The fractal dimensions must be different for the temporal profiles of events caused by different physical processes. It has some features that allow it to be used to study sets with characteristics varying over a wide range: scaling (two events with similar temporal profiles but with different durations have a similar fractal dimension) and the possibility to process simultaneously the fractal dimension distributions obtained by using data from different detectors if the background fractal dimensions for these detectors are the same. Thus background fractal dimension is useful characteristic of detector. We calculate fractal dimension of AC/LD and CC1 prototypes temporal profiles measured during calibrations and obtain preliminary results of 1.50 ± 0.05 and 1.48 ± 0.08 correspondingly. This is similar to Poisson statistics with coefficient of error in counting up to 10. Analogous results were previously obtained for other detectors used for GRBs registration such as BATSE/CGRO and Swift/BAT.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E.117A