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 The ACTAR detection principle

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Figure 1 : Arrangement of an active target detector

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Figure 2 : Working principle of a time-projection chamber

The ACTAR TPC uses a time-projection chamber (TPC). It is a gaseous detector capable of tracking in three dimensions the charged particles traversing its volume as illustrated in Fig 1.
The working principle of a Time Projection Chamber is illustrated in Fig. 2. In a TPC electrons are produced by the ionization of the atoms of the gas, induced by the energetic charged particles. An electric field causes the electrons to drift to an amplification zone, where their number is multiplied (with gain factors ranging from 103 to 106). Different technologies can be used to achieve the amplification. The electrons cause a signal (either by induction, or by direct collection) on a segmented plane, creating a two-dimensional projection of the ionization track. The third dimension is reconstructed by measuring the drift time of the electrons through the gas volume.


Design and construction of the Demonstrator

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Figure 3 :CAD view of the demonstrator

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Figure 4 : Exploded view of the mechanical elements of the Demonstrator


The ACTAR collaboration has decided to build a prototype of the full detector during the research and development phase of the ACTAR TPC project. The ACTAR TPC “Demonstrator” validated a scaled-down version of the GET electronics and allowed the mechanical design of the chamber and micro-pattern gaseous detectors (MPGD) connections to be fully defined. The Demonstrator is equipped with 2048 electronic channels. The pads are arranged in a 64x32 configuration. Connection solutions have been designed in order to test the feasibility of a 2x2-mm2 pads.

Figure 3 shows a CAD view of the Demonstrator. Its shape is cubic and the internal volume dimensions are 240x190 mm2 in the horizontal plane and 210 mm in height. The active volume is 128x64 mm2 of pad readout with a 170-mm vertical drift height . The mechanical ensemble weights around 40 Kg and is supported by a simple frame which allows to locate the GET electronics directly underneath the readout plane.

Design of the pads plane bulked with a micromegas plane

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Figure 5 : Arrangement of a Micromégas electron amplification device

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Figure 6 : Vacuum interface concepte

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Figure 7 : PCB microvia

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Figure 8 : Connector


The ACTAR TPC will employ micro-pattern gaseous detectors (MPGD) for the amplification. Its principle is illustrated in fig 5.
The present design of ACTAR TPC is based on 2x2 mm2 pads. The detection plane is composed of 64x32 pads. The challenge of this detector is the high density of pads in vacuum, read out by electronics placed outside.
In this case the vacuum seal is between the pad plane and the bottom plate of the chamber. Figure 6 shows a scheme of the concept where we see the direct link
between the cables and the pad plane. The pad plane is a thick multilayer printed circuit board (PCB) of 4.5 mm glued on the bottom grid. The technology used for this PCB permits to keep a vacuum tightness. The grid has undergone a series of tests over several months ; the gluing of this PCB is sufficient to sustain a 3.5-bar pressure, the minimum needed by the CODAP rules. The PCB uses a special technology with microvias in order to offset the connecting vias between subsequent layers. The vias and lines which are very dense. The design, made by in collaboration with the electronic department of IPN Orsay, is quite complex and must fit in a basic cell of 16x6 pads in order to ease the extrapolation to larger planes. Figure 7 shows a picture of this design.
The connector used is the SAMTECH LSHM-150 (Fig. 8) having a very small pitch of 0.5 mm which will require a precise soldering process. The choice and size of this connector is critical as it must be small enough not to reduce the grid strength and get a pitch based on a standard, reliable and cheap connector.
The cabling method between connector and GET electronics uses mini-coaxial cables with a low capacitance in order to get a good signal-to-noise ratio. The integration of this type of cable requires a PCB adaptor made by the electronic department which does the interface with the PCB at the extremities.


Contact : Bernard Génolini


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Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE)
UMR 8608 - CNRS/IN2P3

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