1. Planar Edgeless Silicon Detectors for the TOTEM Experiment
To measure the total cross-section of the LHC the TOTEM experiment will detect leading protons coming from the Interaction Point 5 (IP5)
The best choice for such detectors are silicon microstrips planes placed orthogonally to the beam axis. Nevertheless in standard fabrication these devices present an insensitive edge of 1mm where termination structures are placed to improve their breakdown performance. To reduce this dead region the TOTEM collaboration has developed a new approach for the realisation of devices with insensitive edges of only 50m!!!
Roman Pot Stations
The leading proton detectors will be installed in special beam insertion, the Roman Pots and will approach the 10 of the high intensity beam as close as possible.
The idea…
New Development:
Current Terminating Structure
The conceptual idea of the new approach is to apply the full detector bias across the detector chip cut and collect the resulting leakage current on an outer ring, which surrounds the active area and which is biased at the same potential as the detecting strips. This ring is separated from the detector biasing electrode. Separating this ring from the bias ring strongly reduces the influence of the current generated at detector edge on the active detector area. In contrast with the other ring structures which provides voltage termination, this structure terminates the current, and therefore we have called it Current Terminating Structure.
Terminating structure 40 m wide
Thermo-Electric Characterization
First silicon detectors with this CTS produced in August 2003 (joined effort between the TOTEM-CERN and Ioffe PTI- St.Petersburg/ RIMST Moscow).
1.E-09
1.E-06
1.E-03
3.4
3.9
4.4
4.9
1/T, 1e3/K
Current, A
(294K)
(256K)
(227K)
(204K) I1 I2
reverse bias of 100V
40 m I2 I1 + -
bias ring Al p+ n+
cut edge
SiO2
n-type bulk
current terminating ring
Detail of the edge of a microstrip silicon detector with Current Terminating Structure. With this type of terminating structure the cut of the dice can be even just 40m away from the end of the strips.
Bulk Leakage Current not influenced by the Edge Surface Current (I2 << I1) at 300K
I1 does not decrease exponentially with the Temperature
Excellent detector stability for biases higher than 200V at room temperature.
Cross-section of a Silicon detector with Current Terminating Structure and its biasing scheme.
…the proof!!!
Test Beam Results (at 300 K)
(Sept. ‘03, muon beam in X5 at CERN) RD TD
The signal-to-noise performance of the Test Detectors as a function of the x-position as recorded by the Reference Detector have shown a uniform value around 22 until 50m away from the strips end. This suggests full efficiency up to this position. However, the S/N distributions at the edges show a slight decrease in the pulse height, indicating a loss in efficiency of %.
Self-referencing module with detectors and readout electronics
The sensitivity at the edge of detectors with CTS has been measured with a muon beam by checking coincident hits in two Test Detectors with CTS with the cut edges, facing each other and being parallel, and in a Reference Detector placed on their back with the strip direction parallel to the sensitive edges of the two TD’s.
Due to the high spatial resolution of the RD (strip pitch of 50 microns), the insensitive distance between the two TD’s can be measured precisely and can be compared with the mechanical distance enabling a precise determination of the efficiency drop at the edges of the test detectors
Signal-to-noise distribution of the test detector at the edge as recorded by the reference detector at the end of the strip (position 74) and 50 m away (position 75 )
Metrology:
(1209±10)m TD TD
Data from Test Beam: (1229±8)m
Gennaro Ruggiero, CERN, PH Department, CH-1211 Geneva 23, Switzerland
On behalf of the
TOTEM Collaboration
Self-referencing module made of a board in G10 laminated with kapton hosting a reference detector (RD) and two test detectors (TD). All the detectors readout with APV25 chips packed in the CMS TOB hybrid
Distribution of hits in the reference detector in coincidence with hits in the two test detectors. Their fit is compared to the beginning of the sensitive area of the two TD’s (dashed red line).