R. L. Bates

The evaporative cooling system for the ATLAS inner detector

This paper describes the evaporative system used to cool the silicon detector structures of the inner detector sub-detectors of the ATLAS experiment at the CERN Large Hadron Collider. The motivation for an evaporative system, its design and construction are discussed. In detail the particular requirements of the ATLAS inner detector, technical choices and the qualification and manufacture of final components are addressed. Finally results of initial operational tests are reported. Although the entire system described, the paper focuses on the on-detector aspects. Details of the evaporative cooling plant will be discussed elsewhere.

Simulation Results From Double-Sided 3-D Detectors

A new ldquodouble sidedrdquo 3-D solid-state detector structure, intended to simplify the 3-D fabrication process, is proposed. In this structure, electrode columns of different doping types are etched from opposite sides of the substrate, with neither set of columns passing through the full substrate thickness. The finite-element simulation package ISE-TCAD is used to determine the performance of this structure. The double-sided detector shows similar electrostatic behavior to a standard 3-D detector, giving a low depletion voltage and fast charge collection. However, unless the electrode column length is very close to the substrate thickness, charge deposited around the front and back surfaces of the device is collected less quickly (though still rapidly compared with a planar geometry device). The breakdown voltage is dominated by high-field regions around the tips of the electrode columns and shows little change when the oxide charge is increased.

Charge collection studies in irradiated HV-CMOS particle detectors

Charge collection properties of particle detectors made in HV-CMOS technology were investigated before and after irradiation with reactor neutrons. Two different sensor types were designed and processed in 180 and 350 nm technology by AMS. Edge-TCT and charge collection measurements with electrons from 90Sr source were employed. Diffusion of generated carriers from undepleted substrate contributes significantly to the charge collection before irradiation, while after irradiation the drift contribution prevails as shown by charge measurements at different shaping times. The depleted region at a given bias voltage was found to grow with irradiation in the fluence range of interest for strip detectors at the HL-LHC. This leads to large gains in the measured charge with respect to the one before irradiation. The increase of the depleted region was attributed to removal of effective acceptors. The evolution of depleted region with fluence was investigated and modeled. Initial studies show a small effect of short term annealing on charge collection.

A combined ultrasonic flow meter and binary vapour mixture analyzer for the ATLAS silicon tracker

An upgrade to the ATLAS silicon tracker cooling control system may require a change from C3F8 (octafluoro-propane) evaporative coolant to a blend containing 10-25% of C2F6 (hexafluoro-ethane). Such a change will reduce the evaporation temperature to assure thermal stability following radiation damage accumulated at full LHC luminosity. Central to this upgrade is a new ultrasonic instrument in which sound transit times are continuously measured in opposite directions in flowing gas at known temperature and pressure to deduce the C3F8/C2F6 flow rate and mixture composition. The instrument and its Supervisory, Control and Data Acquisition (SCADA) software are described in this paper. Several geometries for the instrument are in use or under evaluation. An instrument with a pinched axial geometry intended for analysis and measurement of moderate flow rates has demonstrated a mixture resolution of 3.10-3 for C3F8/C2F6 molar mixtures with 20%C2F6, and a flow resolution of 2% of full scale for mass flows up to 30gs-1. In mixtures of widely-differing molecular weight (mw), higher mixture precision is possible: a sensitivity of <5.10-5 to leaks of C3F8 into part of the ATLAS tracker nitrogen envelope (mw difference 160) has been seen. An instrument with an angled sound path geometry has been developed for use at high fluorocarbon mass flow rates of around 1.2 kgs-1 - corresponding to full flow in a new 60kW thermosiphon recirculator under construction for the ATLAS silicon tracker. Extensive computational fluid dynamics studies were performed to determine the preferred geometry (ultrasonic transducer spacing and placement, together with the sound crossing angle with respect to the vapour flow direction). A prototype with 45deg crossing angle has demonstrated a flow resolution of 1.9% of full scale for linear flow velocities up to 15 ms-1. The instrument has many potential applications.

The effects of radiation on gallium arsenide radiation detectors

Abstract Semi-insulating, undoped, Liquid Encapsulated Czochralski (SI-U LEC) GaAs detectors have been irradiated with 1 MeV neutrons, 24 GeV/ c protons, and 300 MeV/ c pions. The maximum fluences used were 6 × 10 14 , 3 × 10 14 , and 1.8 × 10 14 particles/cm 2 , respectively. For all three types of irradiation, the charge collection efficiencies (cce) of the detector are reduced due to the reduction in the electron and hole mean free paths. Pion and proton irradiations produce a greater reduction in cce than neutron irradiation, with the pions having the greatest effect. The effect of annealing the detectors at room temperature, at 200°C and at 450°C with a flash lamp have been shown to reduce the leakage current and increase the cce of the irradiated detectors. The flash-lamp anneal produced the greatest increase in the cce from 26% to 70% by increasing the mean free path of the electrons. Two indium-doped samples were irradiated with 24 GeV/ c protons and demonstrated no improvement over SI-U GaAs with respect to post-irradiation cce.

Precision scans of the Pixel cell response of double sided 3D Pixel detectors to pion and X-ray beams

Three-dimensional (3D) silicon sensors offer potential advantages over standard planar sensors for radiation hardness in future high energy physics experiments and reduced charge-sharing for X-ray applications, but may introduce inefficiencies due to the columnar electrodes. These inefficiencies are probed by studying variations in response across a unit pixel cell in a 55μm pitch double-sided 3D pixel sensor bump bonded to TimePix and Medipix2 readout ASICs. Two complementary characterisation techniques are discussed: the first uses a custom built telescope and a 120GeV pion beam from the Super Proton Synchrotron (SPS) at CERN; the second employs a novel technique to illuminate the sensor with a micro-focused synchrotron X-ray beam at the Diamond Light Source, UK. For a pion beam incident perpendicular to the sensor plane an overall pixel efficiency of 93.0±0.5% is measured. After a 10o rotation of the device the effect of the columnar region becomes negligible and the overall efficiency rises to 99.8±0.5%. The double-sided 3D sensor shows significantly reduced charge sharing to neighbouring pixels compared to the planar device. The charge sharing results obtained from the X-ray beam study of the 3D sensor are shown to agree with a simple simulation in which charge diffusion is neglected. The devices tested are found to be compatible with having a region in which no charge is collected centred on the electrode columns and of radius 7.6±0.6μm. Charge collection above and below the columnar electrodes in the double-sided 3D sensor is observed.

First tests of a novel radiation hard CMOS sensor process for Depleted Monolithic Active Pixel Sensors

The upgrade of the ATLAS [1] tracking detector for the High-Luminosity Large Hadron Collider (LHC) at CERN requires novel radiation hard silicon sensor technologies. Significant effort has been put into the development of monolithic CMOS sensors but it has been a challenge to combine a low capacitance of the sensing node with full depletion of the sensitive layer. Low capacitance brings low analog power. Depletion of the sensitive layer causes the signal charge to be collected by drift sufficiently fast to separate hits from consecutive bunch crossings (25 ns at the LHC) and to avoid losing the charge by trapping. This paper focuses on the characterization of charge collection properties and detection efficiency of prototype sensors originally designed in the framework of the ALICE Inner Tracking System (ITS) upgrade [2]. The prototypes are fabricated both in the standard TowerJazz 180nm CMOS imager process [3] and in an innovative modification of this process developed in collaboration with the foundry, aimed to fully deplete the sensitive epitaxial layer and enhance the tolerance to non-ionizing energy loss. Sensors fabricated in standard and modified process variants were characterized using radioactive sources, focused X-ray beam and test beams before and after irradiation. Contrary to sensors manufactured in the standard process, sensors from the modified process remain fully functional even after a dose of 1015neq/cm2, which is the the expected NIEL radiation fluence for the outer pixel layers in the future ATLAS Inner Tracker (ITk) [4].

The cooling capabilities of C2F6/C3F8 saturated fluorocarbon blends for the ATLAS silicon tracker

We investigate and address the performance limitations of the ATLAS silicon tracker fluorocarbon evaporative cooling system operation in the cooling circuits of the barrel silicon microstrip (SCT) sub-detector. In these circuits the minimum achievable evaporation temperatures with C3F8 were higher than the original specification, and were thought to allow an insufficient safety margin against thermal runaway in detector modules subject to a radiation dose initially foreseen for 10 years operation at LHC. We have investigated the cooling capabilities of blends of C3F8 with molar admixtures of up to 25% C2F6, since the addition of the more volatile C2F6 component was expected to allow a lower evaporation temperature for the same evaporation pressure.A custom built recirculator allowed the in-situ preparation of C2F6/C3F8 blends. These were circulated through a representative mechanical and thermal setup reproducing an as-installed ATLAS SCT barrel tracker cooling circuit. Blend molar compositions were verified to a precision of 3.10−3 in a custom ultrasonic instrument.Thermal measurements in a range of C2F6/C3F8 blends were compared with measurements in pure C3F8. These indicated that a blend with 25% C2F6 would allow a reduction in evaporation temperature of around 9oC to below -15oC, even at the highest module power dissipations envisioned after 10 years operation at LHC. Such a reduction would allow more than a factor two in safety margin against temperature dependant leakage power induced thermal runaway.Furthermore, a blend containing up to 25% C2F6 could be circulated without changes to the on-detector elements of the existing ATLAS inner detector evaporative cooling system.

A double-sided, shield-less stave prototype for the ATLAS Upgrade strip tracker for the High Luminosity LHC

A detailed description of the integration structures for the barrel region of the silicon strips tracker of the ATLAS Phase-II upgrade for the upgrade of the Large Hadron Collider, the so-called High Luminosity LHC (HL-LHC), is presented. This paper focuses on one of the latest demonstrator prototypes recently assembled, with numerous unique features. It consists of a shortened, shield-less, and double sided stave, with two candidate power distributions implemented. Thermal and electrical performances of the prototype are presented, as well as a description of the assembly procedures and tools.

The detector control system of the ATLAS SemiConductor Tracker during macro-assembly and integration

The ATLAS SemiConductor Tracker (SCT) is one of the largest existing semiconductor detectors. It is situated between the Pixel detector and the Transition Radiation Tracker at one of the four interaction points of the Large Hadron Collider (LHC). During 2006-2007 the detector was lowered into the ATLAS cavern and installed in its final position. For the assembly, integration and commissioning phase, a complete Detector Control System (DCS) was developed to ensure the safe operation of the tracker. This included control of the individual powering of the silicon modules, a bi-phase cooling system and various types of sensors monitoring the SCT environment and the surrounding test enclosure. The DCS software architecture, performance and operational experience will be presented in the view of a validation of the DCS for the final SCT installation and operation phase.