Notes on APV Detectors

Warning

This page is incomplete and contains references to deprecated C++ API. Must be revised.

In NA64 an APV chip is used within tracking detectors. To the moment, the two types of APV-based detectors are used: Micromegas and GEMs.

APV chip itself plays a role of “analog multiplexer” – it concatenates voltage pulses coming from metalized read-out planes of the Micromegas and GEMs, transmitting multiple inputs (64) into a single output.

Note, that technically, this output is then usually digitized by means of SADC chip, but in DaqDataDecoding this fact is not reflected in any software abstraction so it is not important from software point of view.

So, following the fundamental difference of typical SADC-only detector and APV ones, we distinguish their hits as two individual hit classes calling them a SADCHit and APVHit correspondingly. SADC-only detectors are typically such classes like calorimeters, beam counters, veto detectors and so on.

APV-based detectors (Micromegas and GEMs) are typically used for beam tracking purposes due to their outstanding spatial resolution.

A particle passing through the detector volume produces ionization of the gas media filling the working volume that then propagates towards the electrodes. Electrodes are usually a sensitive elements of the tracker detectors – a charged avalanche, once hitting the electrode, produces a current pulse which is then being measured by detector’s DAQ.

Data acquizition software represents measurement on APV detector as a series of individual _digits_. Each _digit_ corresponds to single triggered wire on the detector plane and bears information of voltage amplitude on the wire.

The main subject of this note is the clusterization technique applied to set of digits in order to group the into a cluster that corresponds particle coordinates.

Read-out Planes Layout

In the DAQ software the _digits_ contains a numeric field called channel (returned by CS::ChipAPV::GetChannel() getter). Basically, this _channel_ number corresponds to some physical _wire_ which has fixed spatial position and orientation, and thus being useful to determine hit coordinates in one projection.

Note: by wire we mean any sensitive metallized element of the read-out plane. Technically, for Micromegas and GEMs they are actually thin metalized stripes.

However, the mapping of the channel into wire may not be trivial subject. Sometimes detetector design imply galvanic connection of several wires into one channel, so hit present at one channel can physically match multiple physical wires.

Up to the moment all GEM detectors used in NA64 has on-to-one identity match, while Micromegas APV channels are usually connected to 5 wires.

We implement both mappings in the src/NA64DPStrips/Layout.c file as hardcoded mapping function, available as C callbacks to encompassing C++ code.

Note, that originally GEMs mapping is not identical. For reference purposes the GEMs mapping was implemented as a function in src/NA64DPStrips/Layout.c, but it is not used. Instead, the identity function is used for GEMs (in current NA64 DAQ GEMs channel-to-wire mapping is truly identical).

The mapping function callback type is defined as a function that receives a channel number and pointer as an input argument and returns pointer to an array with corresponding wire numbers and number of arrays elements as an output. Since C does not support multiple return values, we had to provide pointer to variable that has to contain the number of elements in the array as second argument to a function.

In the include/NA64DPStrips/Layout.h C-header the following types are defined: * NA64DP_APVStripNo_t to be a _wire_ number, * NA64DP_APVWireNo_t to be a _channel_ number * NA64DP_APVNWires_t to be a number-of-elements types * NA64DP_DemultiplexingMapping is a mapping callback type

Following the C++ type-traits technique, the choice of a particular callback function is implemented within a EvFieldTraits<APVHit>::mapping_for() function.

Clustering Algorithms

We (a bit misleadingly) call the DDD APV digit a hit meaning the single wire measurement. Having multiple hits is not enough to derive a track point for the track reconstruction – one have to unify it into a weighted cluster first to assure a real intersection.

It might be done (especially, concerning Micromegas multiple wire routing) in few ways.

The most straightforward one is to look up for adjacent hits assuming that charged plasma avalanche will cover multiple wires at once, without gaps.

Todo

Faced a puzzling results with straightforward solution, need to check, re-think, communicate with experts