Evolution of silicon sensors characteristics of the current CMS tracker
Introduction
Silicon detector properties are changing with irradiation, annealing and aging. The main quantities which are influenced by these effects are the leakage current and the full depletion voltage. The silicon strips tracker of the Compact Muon Solenoid (CMS) includes sensors of many different geometries and a large range of resistivities. This leads to a large spread of full depletion voltage values. Taking also the difference in fluence through the sensors into account, it is important to monitor the detector properties on the granularity of sensor modules. The leakage current for each module is measured at regular intervals by its Detector Control Unit (DCU). A detailed description can be found in Ref. [1]. The full depletion voltage has to be derived out of bias voltage scans. We investigated two different scanning methods, one based on noise measurements (see Section 2.1) the other based on signal of particle tracks (see Section 2.2).
The aim is to collect an integral dataset of the leakage current, the full depletion voltage as well as the fluence and the temperature history for all modules, and compare our data with the current estimations based on the Hamburg model [2].
According to the Hamburg model the leakage current after irradiation evolutes according to:with the temperature dependent current related damage rate for a given fluence and volume V.
The evolution of the effective space charge is given by:with the stable damage part NC, the two annealing parts NA (short term, beneficial) and NY (long term, reverse). The number of effective charge carriers is proportional to the full depletion Voltage. The prediction for the evolution of the full depletion voltage based on the current estimations of irradiation within the next 10 years at CMS is shown in Fig. 1.
With the fluence to which the detector was exposed until the end of 2010 (up to fast hadrons), we do not expect a significant change in any of the detector properties, based on the Hamburg model. In order to verify this assumption, and to ensure that no aging effects beyond the Hamburg model occurred, we investigated leakage current measurements from 2009 and from 2010. A comparison showed that, within the accuracy of the DCU readout (about ), the leakage currents are very stable and no irradiation damage or aging effects are visible so far (see Fig. 2).
Section snippets
The bias voltage scans
The two different approaches we developed differ in many aspects. Since the noise based bias voltage scan (Section 2.1) does not use the particle beam, it can be performed during any maintenance period when the tracker bias voltage for the sensors can be turned on. The measurement takes about 8 h and is planed to be performed every month.
The bias voltage scan with particle tracks (Section 2.2) can only be performed during stable collisions. However it is more accurate than the noise approach.
Comparison of the different results
The reference value of full depletion voltage of every sensor used in CMS has either been determined directly by the producing companies or by one of the CMS quality test centers. All measurements are done with a approach, further information can be found in Ref. [7]. In Fig. 8 a correlation plot between our measurements with particle tracks and the reference values is shown for the inner barrel partition of the tracker. Within this partition, modules with one thick sensor each are
Conclusion
Within the accuracy of our measurements we did not detect any change in the detector properties from irradiation or aging effects. The CMS strip tracker is perfectly stable. For future monitoring we established two different full depletion voltage measurement approaches. With these methods a regular observation of the full depletion voltage with a very high granularity is possible. In combination with the leakage current, measured with the DCUs, the temperature, measured with DCUs and hardwired
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Cited by (6)
Evolution of silicon sensor characteristics of the CMS silicon strip tracker
2013, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentCitation Excerpt :The leakage current values obtained with the different methods are in a good agreement to each other (see Fig. 1). A description of the signal and the noise bias voltage scan, performed at the CMS strip tracker, can be found in Ref. [2]. The two different approaches we developed differ in many aspects.
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