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Max-Planck-Institut für extraterrestrische Physik

- Gamma-Ray Astronomy -

Project:   MEGA

 

  MPEMPE   GroupGamma-Ray Astronomy   GroupProjects   ProjectMEGA

Project MEGA

Overview

Prototype

MEGAlib

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The prototype

link The prototype consists of a tracker with ten layers of double sided silicon strip detectors and 24 calorimeters which surround the tracker.

The Tracker (D1, converter)

The tracker consists of doublesided silicon strip detectors (0.5 mm thick) with analog readout electronics mounted along two sides of each layer. These layers of doublesided Si-stripdetectors are used to track the electron accelerated in a Compton scattering process or electron/positron pairs created in a paircreation event and measure the energy.
The position resolution is 470 Ám (pitch) for events with no charge sharing between strips.
At a distance of 10 mm between the layers this leads to an angular uncertainty due to the measurement accuracy to better than 3°. Much better angular resolution is not necessary because determination of the path of any charged particle (below some 50 MeV energy) is limited in accuracy by small angle scattering.

link

The silicon wafers have a strip pitch of 470 Ám and integrated AC-coupling with punch-through biasing.
The strip pitch is wide enough to collect the full charge in only one strip per side  for most  hits,
while not increasing the capacitance to more than 75 pF.
In each layer , 3x3  6 cm wide silicon wafers are concatenated and connected to 2x384 readout channels.

To show the position resolution of these detectors we projected a Co57 source (122 keV) through a lead mask onto one of the 20cm x 20cm Silicon-layers. The improved position resolution for events with charge sharing between neighbouring strips was not used yet, but will allow to determine the position of the interaction with an accuracy of about 100 micron.The energy resolution of D1 is 15 keV FWHM with a threshold of ~30 keV.

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The Calorimeter (D2)

link In Compton mode the calorimeter absorbs the scattered photon and measures its energy.
The calorimeter is highly segmented. It is divided up in several blocks which are positioned around the tracker.
Each block consists of an array (10 x 12) of CsI(Tl) sticks whose scintillation light is detected  by photodiodes.
The scintillation light generated by the interaction of a photon or charged particle  has to be conducted to the photodiodes.
Therefore the CsI bars are polished and wrapped with highly reflecting paper.
The detection of a signal in one of the photodiode pixels delivers information of two spatial directions. To get the third dimension the bottom blocks have a second array of photodiodes mounted to the opposite side of the crystal bars. From the relative signal in the photodiodes we can determine the z-position (direction along the bars). To see more results on the Calorimeters you can either read one of the thesis works, or you can have a look at our publications.


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