CMS Bulletin

TRACKER

The strip tracker took data very efficiently during 2010 with system availabilities of above 97% in the pp running and close to 100% during the heavy-ion running. The number of active channels in the readout is largely stable around 98%.

The maintenance and development during the extended technical stop have been focussed on improving the operating conditions of the main silicon strip cooling plants SS1 and SS2, which have been items of concern (see last Bulletin). In order to stabilise and smooth the operation of SS1 and SS2, larger bypass valves and variable frequency drivers (VFDs) have been introduced. Possible noise induced by operation of the VFDs on other parts of CMS has been evaluated and no increased noise has been reported so far. The leak rate of every single line on SS2 was measured with the precise test-rig. Besides the known leaky lines, ten other SS2 lines were measured to leak between 120 g/day and 1200 g/day under the given test conditions, establishing a good reference for future measurements. Remote-filling lines are being installed to allow refilling of tanks without access to the experimental cavern. After the Tracker was restarted, a new leaky circuit was found and closed (SS2 line 30 on TID minus). This is under investigation and makes a total of 5 closed lines out of 180. Of 48 modules on line 30, 42 can be operated for the time being without direct cooling. In addition, the operating pressure for SS2 circuits was reduced by 0.5 bars to increase the safety margin for operation, whilst maintaining the same operating temperature at the front-end.

The strip tracker DAQ started very smoothly after the technical stop, taking only one shift (8 hours) from the first power switch-on to the first successful global running. In addition, the online software was upgraded to the latest release of the XDAQ software framework (version 10), and a full re-installation of all PCs involved in data-taking to confirm the possibility of fast recovery has been successfully completed. Also, a significant reduction of the time needed to stop and start runs (from 30 s to 10-15 s) was achieved.  During a week of continuous global running, a total of 1.2 million tracks from cosmic muons was recorded, following a request from the Tracker Alignment group. Work is ongoing to enable a fast switch from collision to cosmic data-taking in inter-fill periods. Tracker interlocks have been fully checked-out.

Monitoring of the various operational parameters of the system has received ever-increasing attention. Efforts have started to improve the calibration of the detector control units (DCUs) providing in-situ measurements of temperatures, currents and voltages on the detector modules. An important step to consolidate standard tools was the commissioning of the Tracker Web Based monitoring (TkWBM) which is now available from the central CMS monitoring page.

The dry gas system was largely overhauled; a new switch-over panel from nitrogen to dry-air has been installed to allow for easy maintenance by the DT group and fixing some standing monitoring issues. New types of humidity sensors (fibre and ceramic type) are under investigation, in preparation of Tracker cold operation after the first long shutdown.

The first data-taking with pp collisions has been very successful. The first, low luminosity runs have been efficiently used to confirm the ideal sampling point and to measure in-situ the depletion voltage (with bias voltage scans) of each individual sensor. Radiation-induced current is still below our measurement precision. Radiation damage projections indicate that the present running conditions of the silicon strips, with coolant at 4 °C, will not result in significant impact on the strip Tracker before the first long shutdown.