Calorimeters use telescope for tests
Barbara Warmbein (DESY), 10/03/2017

Calorimeter experts used the AIDA beam telescope to tested layers of hadronic calorimeter. (Image: Adrian Irles, DESY/LAL)

Six layers of active elements of future calorimeters for linear-collider experiments have been tested in connection with an AIDA beam telescope for the first time. A small team from DESY, the University of Sussex and Prague University tested the performance of the calorimeter prototypes to check whether the two could indeed work together – a proof of principle of sorts – and gather more details of the tile structure and spatial resolution of the calorimeter.

While test objects are normally sandwiched between telescope layers this test placed them behind the telescope due to space concerns. The working principle is the same, however: any beam particles coming from the accelerator (DESY’s electron synchrotron in this case) pass through telescope and test object, so anything the telescope sees should be seen at a precisely determined position by the test object as well. In the beginning this proved a challenge for the common EUDAQ data acquisition system because of the configuration of the telescope that currently caters more to LHC detector tests.

Detectors for the linear collider do not need to disentangle events and select which ones to keep and which ones to throw away, like detectors at the LHC. At the linear collider there’s a large gap between particle bunches when colliding and the collisions are less messy because they occur between elementary particles, electrons and positrons, rather than particles with a substructure like the protons at the LHC. That’s why linear-collider detectors don’t need a trigger that would make a synchronisation with the information from the telescope easier.

“We had to figure out which particles seen by the AIDA telescope correspond to which particles seen by us,” explains DESY hadron calorimeter (HCal) expert Katja Krüger. So before they could start taking real data the HCal team had to make a few modifications to make sure they could synchronise their data with the information from the telescope.

The HCal uses scintillator tiles measuring three by three centimetres with a silicon photomultiplier or SiPM detector. The telescope technology is based on monolithic active pixel sensors and has a very high spatial resolution: it tells users with a precision of 2 microns where particles have passed. The calorimeter team is using this precision to check if their tile structure and the spaces between tiles are visible in the data and how their prototype resolves electron showers, which play an important role in the “working” configuration with absorber layers sandwiched between the detector layers. In return, the run with linear collider detector prototypes helps prepare the beam telescope and the EUDAQ framework for future tests for example under the AIDA-2020 project with other prototypes, both individually and as a combined campaign.