Vacuum Studies of LHCb Vertex Locator Sensors

Gwenaëlle Lefeuvre, Ray Mountain, Marina Artuso Department of Physics, Syracuse University

Abstract : The stability of operation of silicon strip sensors to be used in the Vertex Locator (VELO) of the LHCb experiment is investigated. The IV curves of three such sensors (two fabricated on n-type substrates and one fabricated on p-type substrate) have been measured under varying pressure from 10-5 and 10-2 mbar and at 21°C and -9.5°C. Their stability over time at nominal operating conditions has been verified. Possible micro-discharges and performance of irradiated sensors are part of this study.

Our Measurements 1. IV characterization at 21°C and -9.5 °C Pressure varying from ~10-5 mbar to 10-3 mbar

n-type sensor p-type sensor

Basics of the VELO design• 42 stations, 21 of each side of the beam. • each station has 2 silicon sensors measuring r

and φ.• small overlap region for alignment.

Silicon sensors• n+ in n-bulk sensors• n+ in p-bulk sensor (as a possible upgrade)• ~300 μm thick• cooling to minimize radiation damage (-7°C)• 2048 strips / sensor (over 170 000 channels in total)• 40 to 100 μm pitch• resolution ~8 to 20 μm, function of track angle and pitch • 3 years of operation in a radiation environment• previously qualified at room conditions (humid air), and in vacuum

Non uniform radiation environment• 1.3 * 1014 neq/cm2/year at r = 8 mm (inner radius)• 5.0 * 1012 neq/cm2/year at r = 42 mm (outer radius)

VELO Sensors

Vacuum system• Varian Turbo Pumping Station • 4.5 inch vacuum line connects to chamber for sufficient throughput• cylindrical stainless steel chamber, 40 x 35 cm, SU design• Conflat ports for feedthroughs• reaches 10-5 mbar in ~1/2 h

Intermediate pressures• dry air to avoid condensation when cooling • stainless steel backflow line• small intermediate tank and precision valve to tune a controlled leak rate to the chamber• pressure stable for several hours to several days

Cooling• bi-phase liquid nitrogen • aluminum line coiled in copper blocks• thermal contact with aluminum sensor holder• stable at -10°C for ~1h with no heating effects

Probes• 3 PT100 RTDs around the sensor• ionization gauge for pressure inside the chamber

Connections• bias : copper foil glued to bias pad, guard rings floating• bulk and inner guard ring wire bonded to ceramic pads• kapton lead wires make the connections to feedthrough

Future Work• Studies with IR camera to identify local micro-discharges • Repeat measurements on irradiated sensors

Reference• Malcolm JOHN, CERN, International Symposium on Detector Development (SNIC), 2006

Sensors under test• 2 n-on-n and 1 n-on-p• comparison with official LHCb database (for n-type sensors): IV curve and depletion voltage.

TechniqueSimultaneous separate measurement of both bulk and guard ring currents

Measurement• each point is an average over 5 readings of the current• ΔP < 0.1 unit• ΔT < 0.2 °C at 21 °C• ΔT < 0.5 °C at -9.5 °C• pressure variation controlled by backflow of dry air• typical uncertainties :

statistical < 1 % systematic ~ 5 %

Results • Pressure variation – No anomalous increase of the current at intermediate pressure.• Bulk current always higher than guard ring current.• Apparent variation with pressure could be due to remaining humidity and charge trapping on the surface of the sensor.

• Time stability – checked for several hours to 2 days : drift of 0.1 μA for n-type sensor and 0.2 μA for p-type sensor. Over the same period, ~1 to 3% variation for n-types sensors and 8% for p-type sensor.

• Temperature dependence – Current increases by a factor 2 over 30°C for n-type sensor, over 50°C for p-type sensor.

2. Stability of the current vs time

n-type sensor100 V

p-type sensor100 V

n-type sensor100 V

p-type sensor100 V

Typical operating conditions of VELO sensors

P = 10-4 mbar T = -7°C V ~ 100 V

SU Vacuum SetupStudy the stability of operation under a wide range of well-controlled conditions.

3. Adiabatic rise of the temperature

Room T: 21.5°C Low T: -9.5°C Room T: 21.5°C Low T: -9.5°C

r-sensors

-sensors

0.1μA 0.1μA

0.2μA

0.2μA

0.1μA

5 nA

0.1μA

5 nA