ILC NewsLine
FCAL’S Best Friends

Mechanical setup for irradiating the diamond sensors (no sensor equipped). To the left the exit window of the accelerator is visible. Behind a copper collimator (middle) a sensor can be placed for irradiation. The copper block (right) catches the beam after passing the sensor and lets us measure the beam current and thus the absorbed dose.

The testbeam crew in front of the concrete door to the accelerator hall (from left to right): C. Grah (DESY), A. Ignatenko (DESY), R. Schmidt (BTU Cottbus), W. Lohmann (DESY), E. Kouznetsova (DESY), M. Ohlerich (BTU Cottbus), H. Henschel (DESY), K. Afanaciev (INTAS, NCPHEP Minsk). Not on the photo: W. Lange (DESY).

The forward calorimeters in the future ILC detectors will be an extremely hostile environment, and to be able to make measurements, the detectors have to be able to take a lot without giving in. That's why the Forward Calorimeter Collaboration is testing all its options and has just come back from Darmstadt University's S-DALINAC - for Superconducting Darmstadt Linear Accelerator - with results on the radiation hardness of diamond sensors for the detector.

Forward calorimeters measure the energy of particles crossing them in the direction of the beams. The FCal Collaboration with its 10 institutes from around the world works on a calorimeter system consisting of a luminosity and a beam calorimeter, all of which can be used in any future ILC detector, independent of the concept from which they were born. Their performance depends on their radiation hardness: at the ILC we have to tackle a new phenomenon - beamstrahlung. Photons are radiated when the electron and positron bunches feel the others' electromagnetic fields. They generate a shower of electron-positron pairs, more photons and a lot of other stuff that gets trapped in the calorimeter. The beam calorimeter that hugs the beam line will be irradiated with a dose of up to several million gray (Gy) per year. The stuff they are made of better be good, and the three candidates that might be able to take all that and still work are special silicon sensors, sensors made of gallium arsenide, and, most promising so far, pCVD diamonds.

"We have irradiated the diamonds with more than five million Gy, and the sensors still work," says Christian Grah, responsible for testbeam preparation and coordination from DESY's Zeuthen laboratory, who has just returned from the test in Darmstadt and is evaluating the results with his colleagues. They used the university's accelerator for a week, irradiating the diamond samples for one hour, then taking reference spectra, then irradiating the sample some more. The diamonds seem to have no problem being shot at for these intense periods and are prime candidates for the innermost calorimeter. The problem is: they are expensive, there aren't many producers, and it's not an industrial product - and at least one square metre of diamond sensors would be needed. "We are considering a hybrid structure, made up of diamonds near the beampipe and silicon sensors on the outside, which will save cost," explains Grah.

Many collaboration members, including for the first time students from the technical university Cottbus, came for the test, in part funded by the INTAS programme to support scientific cooperation between the EU and the states of the former Soviet Union. "And the team from Darmstadt's technical university gave us great support, too," says Grah. The electron injector used for the measurements has just the right energy to simulate the conditions expected at the ILC.

-- Barbara Warmbein