Additional aperture detectors of gamma-telescope GAMMA-400 calibrations on synchrotron “PAKHRA”: possibility of temporal profiles fractal analysis

Abstract

GAMMA-400 (Gamma Astronomical Multifunctional Modular Apparatus) will be the new generation satellite gamma-observatory. The gamma-ray telescope GAMMA-400 consists of anticoincidence system (top and lateral sections ACtop and AClat), the convertertracker (C), time-of-flight system (two sections S1 and S2), position-sensitive calorimeter CC1, electromagnetic calorimeter CC2, scintillation detectors of the calorimeter (S3 and S4) and lateral detectors of the calorimeter LD.Three apertures provide events registration both from upper and lateral directions. The main aperture provides the best angular (all double (X, Y) tracking coordinate detectors layers information analysis) and energy resolution (energy deposition in the all detectors studying). The main aperture created firstly due to convertertracker (C): gammas converted in tungsten conversion foils are registered. Triggers in the main aperture will be formed using information about particle direction provided by time of flight system and presence of charged particles or backsplash signal formed according to analysis of energy deposition in combination of both layers anticoincidence systems АСtop and AClat individual detectors. Other two apertures used for observation of transient events do not require best angular resolution as gamma-ray bursts and solar flares both from upper and lateral directions. Additional aperture allows particles registering from upper direction, 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 \uf067-quanta (E>100 MeV) recognized using energy deposition analysis in the individual detectors of S3, S4, LD and CC2. Prototype of additional aperture functioning of GAMMA-400 contains two detectors. One of them AC/LD prototype based on BC-408 scintillator with dimensions of 128x10x1 cm 3 . Other is CC1 prototype composed of CsI(Tl) crystal with dimensions of 33x5x2 cm 3 . The positron beam with energies 100-300 MeV was used for calibration of prototypes of GAMMA-400 detectors on synchrotron “PAKHRA”. We calculate fractal dimension of temporal profiles measured during calibrations of AC/LD and CC1 prototypes. Preliminary results are 1.50\uf0b10.05 and 1.48\uf0b10.08 correspondingly. This is similar to Poisson statistics or Erlang one with coefficient up to 10. 26th Extended European Cosmic Ray Symposium IOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012083 IOP Publishing doi:10.1088/1742-6596/1181/1/012083 2 1. GAMMA-400 apertures short description. The gamma-ray telescope GAMMA-400 (Gamma Astronomical Multifunctional Modular Apparatus) consists of three types of detectors: double (X, Y) tracking coordinate detectors, used in the convertertracker (C) and position-sensitive calorimeter CC1, plastic and non-organic scintillators [1, 2]. Following detectors based on BC-408 plastics: time-of-flight system TOF (two sections S1 and S2), top (ACtop) and lateral (AClat) sections of anticoincidence system, scintillation detectors of the calorimeter (S3 and S4) and lateral detectors of the calorimeter (LD) (its installation required for particles registration from lateral directions). All detector systems ACtop, AClat, S1-S4 and LD consist of two sensitive layers of 1 cm thickness each [3]. Two calorimeters made of CsI(Tl): positionsensitive (CC1) and electromagnetic (CC2) ones. CC1 contain of 2 strips layers and 2 scintillation layers. The thickness of CC1 and CC2 is ~2 X0 (~0.1 λ0) and ~20 X0 (~0.9 λ0) respectively (where λ0 is nuclear interaction length). The total calorimeter thickness is ~22 X0 or ~1.0 λ0 for vertical incident particles registration and ~54 X0 or ~2.5 λ0 for laterally incident ones [3]. The silicon photomultipliers (SiPM) are used in all scintillation detectors instead of vacuum PMT for minimization of power consumption. The physical scheme of the under consideration variant of gamma-ray telescope GAMMA-400 construction and its three apertures are shown at figure 1. Figure 1. The physical scheme of the under consideration variant of GAMMA-400 gamma-ray telescope construction and its three apertures. 26th Extended European Cosmic Ray Symposium IOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012083 IOP Publishing doi:10.1088/1742-6596/1181/1/012083 3 The gamma-ray telescope is optimized for registration of \uf067-quanta and charged particles with energy above 100 GeV with the best parameters in the main aperture from upper direction. The main aperture is 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 particles or backsplash obtained from ACtop and AClat anticoincidence detectors in energy band of 20 MeV-1.0 TeV for gammas and E>100 MeV for electrons [4]. Gamma-telescope operated in event by event registration mode in this aperture. Other two apertures are used for observation of transient events as gamma-ray bursts and solar flares both from upper and lateral directions do not require best angular resolution. 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 [4]. 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 \uf067quanta (E>100 MeV) are recognized using energy deposition analysis in the individual detectors of S3, S4, LD and CC2. The angular resolution is provided by double (X, Y) tracking coordinate detectors layers in the CC1. Electromagnetic shower starting point position id defined due to methods analogues to using in accelerator technique (so-called «center-of-gravity technique») allow accuracy ~1 mm for electrons (positrons) with Е~8 GeV – for example, in experiments BTeV [5] and PANDA [6]. 2. Prototype of additional aperture of GAMMA-400 calibration on synchrotron “PAKHRA” High-energy \uf067-quanta are registered in GAMMA-400 mostly by formation of electron-positron pairs in converter-tracker C. The positron beam with energies 100-300 MeV [7] was used for calibrations of prototypes of GAMMA-400 detectors on synchrotron “PAKHRA”. Scheme of beam forming and apparatus installation on synchrotron С-25Р “PAKHRA” is presented at figure 2. Figure 2. The scheme of beam forming and apparatus installation on synchrotron С-25Р “PAKHRA”. 26th Extended European Cosmic Ray Symposium IOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012083 IOP Publishing doi:10.1088/1742-6596/1181/1/012083 4 GAMMA-400 prototype of additional aperture functioning consist of two detectors. One of them is BC-408 based with dimensions of 128\uf0b410\uf0b41 cm 3 (one detecting strip from AC prototype) and other composed of CsI(Tl) crystal with dimensions of 33\uf0b45\uf0b42 cm 3 (one block of CC1 prototype). 3. Fractal dimension of temporal profiles measured during calibrations of AC and CC1 prototypes For investigation of time series corresponding to solar flares, gamma-ray bursts and other transient events fractal analysis is often applied. 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 fractal dimensions) 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. Moreover, the fractal dimensions must be different for the temporal profiles of events caused by different physical processes [8, 9]. Thus background fractal dimension is useful characteristic of detector. Fluctuations of count rate registered in scintillation detector during satellite experiments caused due to three reasons. First is fluctuations background caused by gammas and charged particles cosmic and magnetosphere origin. Statistical fluctuations of such background are described by Poisson or Gauss statistics outside the radiation belts and other disturbed regions of magnetosphere. Second reason is fluctuations of produced scintillation photons and photoelectron number in photomultiplier. Corresponding statistical fluctuations are poissonian or gaussian in the first approximation in the linear region of SiPM. Other reason is transient processes in electronic system. Figure 3. The example of trend of CC1 prototype, Epositrons=300 MeV, run#5, December 6, 2017. There are some methods for time profile fractal dimension definition. In the main, these methods based on dissection of a time profile on bins and analysis of count rate statistical fluctuations in each bin [8, 9]. If amount of experimental points in bin k is not enough for statistical analysis (usually if k\uf0a320) then cell algorithm of fractal dimension definition is used [8, 9]: the part of plane in which analyzable curve is locate covers by cells with side \uf064. Let N(\uf064)-amount of cells, which has one generic point with this curve even if. Then we define certain gauge for this curve: ( ) D L N \uf064 \uf064 \uf03d \uf0b4 (1) 26th Extended European Cosmic Ray Symposium IOP Conf. Series: Journal of Physics: Conf. Series 1181 (2019) 012083 IOP Publishing doi:10.1088/1742-6596/1181/1/012083 5 For usual (non fractal) curve L=0 for \uf064\uf0ae\uf030 but for fractal curve gauge (1) is nonzero for some D\uf0b91. Fo