The ATLAS Pixel Detector & The MonLeak Scan Saúl Rodríguez* Universidad Nacional Autónoma de México *Supervisor: Jed Biesiada, LBNL
The ATLAS Inner Detector A high-resolution tracking sub-detectors closest to the interaction point. ✦ Transition Radiation Tracker: Straw detectors can cope with high ✦ particle rates and high occupancy, 36 space points. Charged particles passing through dielectric constant boundary. Detecting transition- radiation photons. Track resolution 50µm. Semiconductor Tracker: 8 ✦ high-space points per track with Silicon micro-strip detectors. Track resolution of 16µm in R- Φ direction and 580µm in z . Pixel Detector... ✦ Saúl Rodríguez, UNAM Summer Student Sessions 2008 2
The Pixel Detector The innermost tracking sub-detector of ATLAS. It is complex and ✦ highly granular. Composed of 3 layers and 3 disks made of semiconductor detectors closest to the interaction point ⇒ Track resolution of 12µm in in R- Φ direction and 580µm in z . In total, there are ✦ 1744 modules. 16 chips per ✦ module ⇒ 27904 chips. 2880 channels ✦ (pixels) per chip ⇒ 80,363,520 channels! Saúl Rodríguez, UNAM Summer Student Sessions 2008 3
Pixel Modules The pixel sensor is a n+np+. A particle ✦ passing through the sensor ionizes the atoms of the semi-conductor producing pairs of electrons and holes. The high voltage supply (150V) applies an electric field separating the electrons and holes and drawing them to the detector surface. The FEs use charge sensitive amplifying ✦ electronics to collect the signal. The MCC collects the data from the 16 ✦ FEs, performs basic integrity checks and formats the data. Each one has 16 Front-End (FE) chips ✦ The Bump Bonds connected with bump bonds to the pixel sensor. makes possible to place The FEs are connected with wire bonds to a ✦ the FE electronics right flexible circuit board where is the Module next to the pixel Control Chip. sensors, which is a Readout of the FEs by a MCC. ✦ pretty and advanced A module is about 6 × 2 cm2. ✦ technology. Saúl Rodríguez, UNAM Summer Student Sessions 2008 4
ToothPix I This replica provides a test-bed for continuing in the detector and its ✦ long-term behavior, characterization and debugging of know and future problems. The system also serves as platform for development, testing and calibration-analysis software. ToothPix contains 90 modules, approximately 5% of the number of ✦ modules in the Pixel detector: A stave, the active element of a barrel layer, has 13 modules and a length of 80 cm. ✦ Staves are grouped into pairs with a common cooling circuit and referred to as bistaves. In the detector, bistaves are assembled into cylindrical layers, named Layer 0, Layer 1, and Layer 2. The geometry of ToothPix is such to allow for the possibility of cosmic ✦ data-taking, the components were arranged in a vertical slice, so that a single, vertical cosmic ray would pass through as many modules as possible. Saúl Rodríguez, UNAM Summer Student Sessions 2008 5
ToothPix II Saúl Rodríguez, UNAM Summer Student Sessions 2008 6
The MonLeak Scan Since the pixel does not have infinite resistance a leakage current is ✦ drawn when the HV is applied. It is small in non-irradiated sensors. The leakage currents can only be measured in a combination with the ✦ feedback current. The measurable parameter is referred to as MonLeak current. Purpose ✦ Monitoring the radiation damage in details - As the pixel ages with ✦ radiation, the current increases, which degrades the performance of the pixel, increases noise. This needs to be monitored, so that we know which pixels are degrading and how fast. Detecting pixels with high leakage currents - Some pixels are always ✦ hot, drawing lots of current, and this is correlated with noise even before radiation occurs. The scan is one of several ways to detect noisy pixels. ... and without leakage currents - Pixels with disconnected bump bonds will ✦ also not draw leakage current, so a complete absence of this current, especially after some irradiation, is an indication of a broken bump bond on that pixel. A way to test the MonLeak scan? ✦ Saúl Rodríguez, UNAM Summer Student Sessions 2008 7
Illuminating the Pixels ✦ Light shining on the pixel effectively decreases the resistance by liberating charge carriers, which are then free to move through the pixel under the applied voltage and thus increase the leakage current! ✦ Much like the high-energy particles do (at the collision)... except when you shine a light there are many low-energy photons passing through. ✦ The increase in leakage current across the detector maps out the radiation profile versus position and time ⇒ If we see the structural elements of the module on the MonLeak histogram, the scan must be working (at least to first order). Saúl Rodríguez, UNAM Summer Student Sessions 2008 8
The Setup Saúl Rodríguez, UNAM Summer Student Sessions 2008 9
Results I ✦ Pixels NOT illuminated with light. Monleak Histo - I*125pA ✦ Saúl Rodríguez, UNAM Summer Student Sessions 2008 10
Results II ✦ Pixels illuminated with light. Monleak Histo - I*125pA ✦ Saúl Rodríguez, UNAM Summer Student Sessions 2008 11
Like a Digital Camera! HV guard ring MCC Decoupling Capacitors MonLeak Scan of Sketch of one module. Picture of one module. one module. Saúl Rodríguez, UNAM Summer Student Sessions 2008 12
Summary ✦ Monitoring the leakage current is very important as the pixels get irradiated. And with other parameters like the noise, the mapping of hot and dead pixels would be helpful for the diagnostics. ✦ The applied HV might need to be increased as various radiation-dependent processes occur in the pixel, and to do this, we need to know the mapping of the leakage current. ✦ So, more analysis to commission the MonLeak scan! Such as test the stability of the measured current, verifying that hot pixels are hot both with and without illumination, plotting the distribution of the difference between illuminated and non-illuminated currents pixel by pixel... and then in the whole detector before the beam comes! Saúl Rodríguez, UNAM Summer Student Sessions 2008 13
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