Características de la cuenca hidrográfica:

The Large Area Telescope (LAT) is a pair-conversion imaging γ-ray telescope on-board the Fermi satellite. It was launched on 11 June 20084with science observations beginning on 4 August 2008 (236). Fermi-LAT has a wide field-of-view5_{with a large effective area.}

The instrument operates in its primary ’scanning’ mode approximately 95% of the time (153). While in this mode, Fermi-LAT alternates pointing above and below the orbital plane providing complete sky coverage once the instrument has completed two orbits approximately every three hours.

Fermi-LAT is equipped with the following components, each designed to perform

a particular task. Details of each Fermi-LAT component are described in Atwood et al. (236).

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http://fermi.gsfc.nasa.gov/ssc/observations/ (last viewed:13/12/2015)

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Figure 4.3: Schematic of the Fermi-LAT detector on-board Fermi and its various compo- nents. The incoming photon interacts with the converter tracker and is converted into an electron positron pair. The tracker (TKR) array is surrounded by an anti-coincidence detector (ACD). The schematic also shows the location of the calorimeter (CAL) and the data acquisition (DAQ) electronics. Heat produced by the TKR, CAL and DAQ is transferred to the thermal blanket, made up of radiators, through heat pipes in the grid. Image of the LAT was obtained from Atwood et al. (236).

Converter-Tracker (TKR)

The converter-tracker (TKR) is made of 16 tracker modules, each consisting of 18 XY6alternating tungsten foils and silicon strip detector planes, which promote the pair-conversion of an incident γ-ray photon into an electron-positron pair and then measures the direction of the particles resulting from pair conversion (see Figure 4.4). Of the 18 XY planes, 16 planes have tungsten converter plates of two different thickness. Of the 16 XY planes, 12 planes, which are furthest from the calorimeter, have thinner tungsten converter plates compared to the 4 XY planes closest to the calorimeter (see Figure 4.4). The different thickness in the plates serves to balance

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between optimising the point spread function (PSF)7at low energies and maximis- ing the effective area8 which is important at higher energies. The trade off with the thicker converter plates results in a decrease in angular resolution of less than a factor of two at 1 GeV, for photons converting in that region. The aggregate of the thick layers also functions to limit the number of back-scattered particles (i.e. backsplash effect) from the calorimeter (CAL) returning into the TKR and anti- coincidence detector (ACD) in high energy events9. In addition to this, the thicker planes also serve to reduce background contamination in front-converting events caused by tails of showers from events entering back into the CAL. Finally, the last two of the 18 planes immediately before the CAL do not have any tungsten converter foils. This is because the TKR trigger requires hits in three adjacent XY planes making it insensitive to γ-rays which convert in the last to layers. A detailed description of this can be found in (236) and (153).

Calorimeter (CAL)

The CAL is located at the bottom of the TKR and has a mass of∼ 1800 kg (Figures 4.4 and 4.5). The CAL, like the TKR is also made of 16 modules, each consisting of 96 CsI(T1) crystals, arranged horizontally in eight layers of 12 crystals each. The CAL functions to measure the energy deposition from the electromagnetic particle shower produced by the electron-positron pair and images the shower develop- ment profile. The crystal elements are read out by photodiodes, mounted on both ends of the crystal. These photodiodes measure the scintillation light transmitted to each end of the CsI crystal. The position resolution of the energy deposition along the crystal is given by the difference in the light levels measured at each end. This position resolution scales with the deposited energy, ranging from a few mil- limetres (low energy deposition of∼ 10 MeV) to a fraction of a millimetre (large energy deposition of > 1 GeV).

As such, the CAL is a 3-dimensional imaging calorimeter such that each of the CsI

7_{using the thin converter plates in the first 12 XY planes (often referred to as the front section of the TKR)} 8

using four XY planes with converter plates six times thicker than the first 12 XY converter plates (often referred to as the back section)

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The backsplash effect refers to the Compton scattering (in the ACD) of isotropically distributed sec- ondary particles produced in the electromagnetic shower created by an incident high-energy photon. This may then create false veto signals from the recoil electrons. (236).

crystals produces three spatial coordinates for the energy deposited within it. Two of the coordinates specify the physical location of the crystal in the array. The third is obtained by measuring the light yield asymmetry at the ends of the crystal along its long dimension. These three coordinates provide spatial imaging of the shower along with an accurate reconstruction of its direction (236; 153).

Anti-Coincidence Detector (ACD)

This surrounds the LAT and is composed of 89 plastic scintillator tiles (25 covering the top of the instrument while 16 tiles cover each of the four sides respectively) (153). This element functions to reject background signals produced by cosmic ray particles incident on the LAT. While incoming γ-ray photons pass freely through the ACD tiles, charged particles (cosmic rays) cause a flash of light. The ACD also suppresses the backsplash effect caused by the heavy (∼ 1800 kg) calorimeter (236). This is achieved by the segmented design of the ACD which significantly reduces the area of the ACD which may contribute to the backsplash effect from an incident candidate photon (236; 32). The ACD is designed and tested to detect charged particles efficiently.

Data Acquisition System (DAQ)

The data acquisition system (DAQ) performs on-board preliminary filtering of background events by first collecting data from the other components (TKR, CAL and ACD). The DAQ implements a multilevel event trigger system (see Section 4.5.1) before data are sent to the ground for further processing. The filters reduce the number of downlinked events by removing the number charged-particle back- ground events. It also maximises the rate of the events triggered by γ-ray photons. The DAQ is equipped with an on-board science analysis platform which searches for transients (see Section 4.5.2) (236).

Instrument Methodology

In this section, the basic methodology of the instrument is presented. This can be de- scribed using the path taken by a single γ-ray photon as it moves through the LAT de- tector components.

Figure 4.4: Schematic of the precision conversion-tracker (TKR) and calorimeter (CAL) on-board LAT which function to promote pair-conversion of incoming γ-ray photons as well as measuring the directions of the resulting pair-converted electrons and positrons. The TKR front section (located furthest from the CAL) is made up of 12 thinner XY planes. This is followed by the back section of the TKR which is made up of four thicker XY planes. The final two XY planes closest to the CAL do not have tungsten converter planes. Image of the LAT was obtained from Ackermann et al. (153).

When a γ-ray photon is incident on the LAT detector, it begins by travelling through the converter-tracker modules. Here, there is a high possibility that the photon will interact with the field of one of the heavy tungsten atoms in the converter planes and produce an electron-positron pair (see Section 2.3). The incoming γ-ray has energies much larger than the rest mass energies of the resulting electron-positron pair. This ensures that the charged particles continue predominantly in the direction of the incident

γ-ray photon, while allowing the tracker to reconstruct the direction of that photon10. As a result, the reconstruction is heavily dependent on the multiple scattering of the charged

10_{The initial directions of the converted electron and positron pair are reconstructed from the conversion}

Figure 4.5: Schematic of the CAL module located below the TKR (see Figure 4.4) depict- ing the 96 CsI crystal detector modules arranged in eight layers and the position of the readout electronics. Image of the CAL was obtained from Ackermann et al. (153).

particles within the tracker and the spatial resolution of the tracker. Once the pairs travel through the planes of the converter-tracker modules, they reach the CAL11. The CAL then measures the total energy deposited onto it through the amount of scintillation light measured by the photodiodes located at both ends of each CsI crystal (see Section 4.5.1).

The output from the CAL is then passed through to the DAQ system, which reads input from the TKR, CAL and ACD to estimate the energy and direction of the incident

γ-ray photon. Finally, information is transmitted to the ground for further processing.

11_{It must be noted that not all converted electron positron pairs reach the CAL, particularly for the cases}

Figure 4.6: Schematic of the ACD surrounding the TKR and CAL modules with its 89 plastic scintillator tiles. In order to minimise gaps between tiles, the tiles overlap in one dimension and scintillating fibre ribbons are used to cover the remaining gaps. This is done to improve the efficiency of event triggering and filtering processes (see Section 4.5.1). Image of the ACD was obtained from Ackermann et al. (153).