Design of a readout system Design of a readout system for RPCsfor RPCs

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The LODEN GroupThe LODEN Group

The group is an association of Fermilab scientists who teamed up to build a cosmic ray detector from RPCs.

I was assigned as intern to this group and I helped in building a readout for RPCs

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Resistive Plate Resistive Plate Chamber(RPC)Chamber(RPC)

An RPC is a glass chamber filled with a mixture of gases,an electric field supplied by an High Voltage source. It is used to detect charged particles through their interactions with the molecules of the gases.

The particles strip off electrons from the molecules of the gases, which results in a spark inside the chamber.

The spark induces a voltage on an adjoining readout system.

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SchematicsSchematics

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How it worksHow it works

Glass electrodes are used to apply an electric field of ~4kV/mm across a 2mm gap.The gap has a mixture of argon,isobutane and HFC123a gas. An ionizing particle initiates a discharge which capacitively induces a signal on external pickup strips.

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Building a Readout SystemBuilding a Readout System

Factors taken into consideration:CostEfficiency AvailabilityConstruction.

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Inductive Readout SystemInductive Readout System

Is a capacitor like setupWith two surfaces, one for signal

transmission and the other for signal protection.

A dielectric material between the surfaces

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Two Methods of ConstructionTwo Methods of Construction

Strip readout Method: A board is stripped

vertically and another one horizontally.

A signal is picked by a a vertical and horizontal strip

Both strips locates position.

Padded r/out Method: Board is partitioned

into square segments. A signal is picked up a

square segment Square segment

locates position

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Diagram of Readout SystemsDiagram of Readout Systems

Strip Readout System Padded Readout System

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The strips method was chosen over the padded method due to the following reasons. 1) It is easier to make strips on a board than to make square segments. 2) For a large area of RPC it will cost much more to attach signal cable to each square than to each strip. 3) For a a board of dimension a by b, we need a*b for square segments, while we need a+b for strips.

Strips over PadsStrips over Pads

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Material for constructionMaterial for construction

Copper strips on mylar foil is normally used for a strip readout board.

Copper and mylar are every expensive.An home insulating board from home depot

was used as a substitute for the “copper strips on mylar”.

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The BoardThe Board

As one side made of aluminum.

Another side made of aluminum coated with insulating material.

A dielectric made of foam.

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What next ?What next ?

Make strips of different width on the board.Setup experiment to:

– Study signal transmission in each strip.– Determine the right strip for experiment– Study reflection.

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Pre-ExperimentPre-Experiment

Strips were made on the board by a saw.

The ends of the ground plane were cleaned with ethyl , for cable attachments.

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Purpose of Experiment:Purpose of Experiment:To DetermineTo Determine

The width of the strip that will give the least backend reflection

The terminating impedance at the back of the board for each strip

The strip that gives the least front end reflection, after termination .

 

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Experimental SetupExperimental Setup

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Reflections in CablesReflections in CablesWhen signals travel trough two cables of

different impedances, reflections are setup at the boundary of the two cables.

If two cables of impedances R and Z are connected to each other, ρ gives the value of reflection for both cables.

 

ZR

ZR

R Z

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ProblemProblem

A pulser was required to generate pulses.A pulser output feeds a 50 ohm coaxial

cable.120 ohm twisted-flat cable was to receive

pulses from the pulser.Problem: Interconnecting the two cables

directly will pose a problem.Why?

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Avoiding ReflectionsAvoiding Reflections

When two cables of different impedances are to be connected to each other.

The principle of termination is employed.Termination is the addition of

supplementary impedance(s) to the impedances of two cables.

Termination can be done either in series or in parallel or a combination of both.

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ApplicationApplication

The circuit represented by the circuit diagram was designed to connect the 120 ohms and 50 ohms cable.

It helped to avoid reflections at the interface of the two cables.

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ProceduresProcedures The diagram shows a typical

display on the oscilloscope Termination was done on a

trial and error basis. Resistors attached in turn, to

flatten out backend The values and displays were

recorded for further analysis.

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ResultsResultsDimension of board: 2m x 0.9m x1.25m

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Analysis of ResultsAnalysis of Results

The result was tested for experimental accuracy by comparing with a theoretical equation.

The resistors vs width/thickness ratio was plotted on the same graph with the theoretical equation

Impedance Equation

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ComparisonComparison

The values obtained from the experiment are accurate to a certain degree.

The experimental values were well distributed around the equation line.

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Conclusions from experiment.Conclusions from experiment.

All strips widths are okay for further experimental purposes

Strip of width 1.8 cm gave the least front-end reflection

Reflections at the front end are negligible. Strip of width 3cm was selected for further

experimental purposes