Detectors In High Energy Physics Experiments Research Articles

A derivation of the electric field inside MAPS detectors from beam-test data and limited TCAD simulations

Solid semiconductor sensors are used as detectors in high-energy physics experiments, in medical applications, in space missions and elsewhere. Minimal knowledge of the electric field inside the elementary cells of these sensors is highly important for their performance understanding. The field governs the charge propagation processes and ultimately determines the size and quality of the electronic signal of the cell. Hence, the simulation of these sensors as detectors in different analyses relies strongly on the field knowledge. For a certain voltage applied to the cell, the field depends on the specifics of the device's growth and fabrication. The information about these is often commercially protected or otherwise very difficult to encode in state-of-the-art technology computer-aided-design (TCAD) software. In this work, we show that by taking the top-down approach, combining public beam-test data and a very limited public TCAD knowledge, we are able to effectively approximate the 3D electric field function in the pixel cell of one important and widely used example, namely the ALPIDE sensor, for simulating the charge propagation processes. Despite its broad usage worldwide, the ALPIDE field is not available to the community. We provide an effective field function, that adequately describes the sensor behaviour without trying to reconstruct further details about the device or the details behind its processing. We comment on the process by which the effective field function is derived with the help of the Allpix2 software, and on how similar work can be performed for other devices, starting from the same grounds.

Fabrication of Silicon Sensors Based on Low-Gain Avalanche Diodes

Low-Gain Avalanche Diodes are a recently-developed class of silicon sensors. Characterized by an internal moderate gain that enhances the signal amplitude and if built on thin silicon substrates of a few tens of microns, they feature fast signals and exhibit excellent timing performance. Thanks to their fast timing they are planned to be exploited in timing detectors in High-Energy Physics experiments, for example for the upgrades of the ATLAS and CMS detectors at the High Luminosity Large Hadron Collider (HL-LHC) at CERN. However, to achieve a spatially uniform multiplication a large pixel pitch is needed, preventing a fine spatial resolution. To overcome this limitation, the AC-coupled LGAD approach was introduced. In this type of device, metal electrodes are placed over an insulator at a fine pitch, and signals are capacitively induced on these electrodes. The fabrication technology is similar for the two LGAD families, although a fine tuning of a few process parameters needs to be carefully studied. Other R&D efforts towards detectors that can simultaneously provide good time and spatial resolution, based on the LGAD concept, are under way. These efforts aim also to mitigate the loss of performance at high irradiation fluences due to the acceptor removal within the gain layer. In this paper we describe the main points in the fabrication of LGADs and AC-LGADs in a clean-room. We also discuss novel efforts carried on related topics.

The impact and persistence of electrostatic charge on the passivation of silicon strip sensors

Silicon strip sensors as used for tracking detectors in high-energy physics experiments are bare large-area silicon devices without any packaging. To protect them from environmental influences like humidity and mechanical damage, a passivation is deposited as the uppermost layer. The passivation can consist of different materials like silicon oxide, silicon nitride, polyimides or doped glasses. In this study we first demonstrate the impact of static surface charge on the sensor characteristics. This is followed by investigations on how sensitive different passivation layers of silicon strip sensors are against charge-up and how these charges retain. For such purpose, a corona charge-up device has been built and used to charge up the detectors. The surface potential distribution caused by the charge was mapped using an electrostatic voltmeter. The self-discharge of sensors with different passivation layers was investigated by long-term studies.

A novel CMOS sensor with in-pixel auto-zeroed discrimination for charged particle tracking

With the aim of developing fast and granular MonolithicActive Pixels Sensors (MAPS) as new charged particle tracking detectors forhigh energy physics experiments, a new rolling shutter binary pixelarchitecture concept (RSBPix) with in-pixel correlated double sampling,amplification and discrimination is presented. The discriminator featuresauto-zeroing in order to compensate process-related transistor mismatches.In order to validate the pixel, a first monolithic CMOS sensor prototype,including a pixel array of 96 × 64 pixels, has been designed and fabricated inthe Tower-Jazz 0.18 μm CMOS Image Sensor (CIS) process. Results of laboratorytests are presented.

Effect of Pre-Irradiation on the Mobility of Positive Ions in Non-Polar Dielectric Liquids

Few data exist on the change of mobility of ions in nonpolar dielectric liquids by pre-irradiation of the liquid sample with ionizing radiation. Such data are especially important for cases where such liquids are used in devices, which operate under exposure of high-energy radiation. Examples are transformers in nuclear power stations or liquid ionization detectors in high-energy physics experiments. In this short communication we summarize data that were obtained during our fundamental studies of ion mobilities in non-polar dielectric liquids. Some data were obtained in the search for warm liquids suitable for ionization detectors in high-energy physics calorimeters.

Preparation for the alignment of the ALICE inner tracking system

Tracking detectors in high-energy physics experiments require an accurate determination of a large number of alignment parameters in order to allow a precise reconstruction of tracks and vertices. In addition to the initial optical survey, the use of tracks in a special software alignment is essential. The methods in use for the alignment of the ALICE Inner Tracking System are presented, starting from residual-based procedure to fitting systems with many thousands of parameters. The two methods are reviewed and the preliminary results showed.

An ultra-light cylindrical GEM detector as inner tracker at KLOE-2

We are developing a low-mass, fully cylindrical and dead-zone-free GEM detector as inner tracker for the KLOE experiment upgrade at the DA Φ NE Φ factory. The inner tracker will be composed of five concentric layers of cylindrical triple-GEM detectors (C-GEM), completely realized with very thin polyimide foils. The final result is a very light detector: only 0.2% of X 0 per layer inside the active area. We successfully built and tested with an X-ray gun a small-size prototype operated in current mode with an Ar/CO 2 = 70 / 30 gas mixture. The very positive results obtained with the prototype open the way for a completely new and competitive category of ultra-light fully sensitive vertex detectors for high-energy physics experiments.

Correlation between radiation processes in silicon and long-time degradation of detectors for high-energy physics experiments

In this contribution, the correlation between fundamental interaction processes induced by radiation in silicon and observable effects which limit the use of silicon detectors in high-energy physics experiments is investigated in the frame of a phenomenological model which includes: generation of primary defects at irradiation, starting from elementary interactions in silicon; kinetics of defects, effects at the p–n junction detector level. The effects due to irradiating particles (pions, protons, neutrons), to their flux, to the anisotropy of the threshold energy in silicon, to the impurity concentrations and resistivity of the starting material are investigated as time, fluence and temperature dependences of detector characteristics. The expected degradation of the electrical parameters of detectors in the complex hadron background fields at LHC & SLHC is predicted.

Software alignment for tracking detectors

Tracking detectors in high-energy physics experiments require an accurate determination of a large number of alignment parameters in order to allow a precise reconstruction of tracks and vertices. In addition to the initial optical survey and corrections for electronics and mechanical effects the use of tracks in a special software alignment is essential. A number of different methods is in use, ranging from simple residual-based procedures to complex fitting systems with many thousands of parameters. The methods are reviewed with respect to their mathematical basis and accuracy, and to aspects of the practical realization.

Serial powering: Proof of principle demonstration of a scheme for the operation of a large pixel detector at the LHC

Large detectors in high-energy physics experiments are mostly built from many identical individual building blocks, called modules, which possess individual parts of the services. The modules are usually also powered by parallel power lines such that they can be individually operated. The main disadvantage of such a parallel powering scheme is the vast amount of necessary power cables which constitutes also a large amount of material in the path of the particles to be detected. For the LHC experiments already now this is a major problem for the optimal performance of the detectors and it has become evident, that for an upgrade programme alternative powering schemes must be investigated. We prove and demonstrate here for the example of the large scale pixel detector of ATLAS that Serial Powering of pixel modules is a viable alternative. A powering scheme using dedicated voltage regulators and modified flex hybrid circuits has been devised and implemented for ATLAS pixel modules. The modules have been intensively tested in the lab and in test beams and have been compared to those powered in parallel with respect to noise and threshold stability performance. Finally, the equivalent of a pixel ladder consisting of six serially powered pixel modules with about 0.3 Mpixels has been built and the performance with respect to operation failures has been studied.

High-voltage planar Si detectors for high-energy physics experiments: comparison between metal-overhang and field-limiting ring techniques

The application of Si detectors in the high-energy physics experiments requires a reliable performance in adverse radiation conditions, which is the main test for these detectors. Devices requiring high voltages face the problem of high peak electric field in the vicinity of junction termination. Various contours and design principles have evolved in the effort to reduce these peak fields. Out of many proposed solutions, metal-overhang and guard ring techniques are of general interest for improving the breakdown performance of the Si detectors (in the vertical devices). In the present work, the breakdown performance of metal-overhang and field-limiting ring techniques is compared for various values of junction depth, substrate resistivity, relative permittivity of the dielectric passivant and fixed oxide charge under similar conditions. The results demonstrate the superiority of metal-overhang technique over field-limiting ring technique for planar shallow-junction high-voltage Si detectors used in high-energy physics experiments.

High-voltage planar Si detectors for high-energy physics experiments: comparison between metal-overhang and field-limiting ring techniques

The application of Si detectors in the high-energy physics experiments requires a reliable performance in adverse radiation conditions, which is the main test for these detectors. Devices requiring high voltages face the problem of high peak electric field in the vicinity of junction termination. Various contours and design principles have evolved in the effort to reduce these peak fields. Out of many proposed solutions, metal-overhang and guard ring techniques are of general interest for improving the breakdown performance of the Si detectors (in the vertical devices). In the present work, the breakdown performance of metal-overhang and field-limiting ring techniques is compared for various values of junction depth, substrate resistivity, relative permittivity of the dielectric passivant and fixed oxide charge under similar conditions. The results demonstrate the superiority of metal-overhang technique over field-limiting ring technique for planar shallow-junction high-voltage Si detectors used in high-energy physics experiments.

Radiation hardness perspectives for the design of analog detector readout circuits in the 0.18-/spl mu/m CMOS generation

This paper presents a study of the ionizing radiation tolerance of analog parameters of 0.18-/spl mu/m CMOS transistors, in view of the application to the design of front-end integrated circuits for detectors in high-energy physics experiments. Static, signal, and noise performances of devices with various gate dimensions were monitored before and after irradiation up to a 300-kGy(Si) total dose of /sup 60/Co /spl gamma/-rays. Different device biasing conditions under irradiation were used, and the relevant results are discussed. A comparison with previous CMOS generations is carried out to evaluate the impact of device scaling on the radiation sensitivity.

Analysis and comparison of the breakdown performance of semi-insulator and dielectric passivated Si strip detectors

Abstract The harsh radiation environment in future high-energy physics (HEP) experiments like LHC provides a challenging task to the performance of Si microstrip detectors. Normal operating condition for silicon detectors in HEP experiments are in most cases not as favourable as for experiments in nuclear physics. In HEP experiments the detector may be exposed to moisture and other adverse atmospheric environment. It is therefore utmost important to protect the sensitive surfaces against such poisonous effects. These instabilities can be nearly eliminated and the performance of Si detectors can be improved by implementing suitably passivated metal-overhang structures. This paper presents the influence of the relative permittivity of the passivant on the breakdown performance of the Si detectors using computer simulations. The semi-insulator and the dielectric passivated metal-overhang structures are compared under optimal conditions. The influence of various parameters such as passivation layer thickness, junction depth, metal-overhang width, device depth, substrate resistivity and fixed oxide charge on the junction breakdown voltage of these structures is extensively studied. The results presented in this work clearly demonstrate the superiority of the metal-overhang structure design employing semi-insulator passivated structures over dielectric passivated ones in realising a given breakdown voltage. The effect of bulk damage caused by hadron environment in the passivated Si detectors is simulated, to a first order approximation, by varying effective carrier concentration (calculated using Hamburg Model) and minority carrier lifetime. This approach allows getting an insight of the device behaviour after radiation damage by evaluating the electric field distribution, and thus proves helpful in predicting some interesting results.

Radiation damage in silicon detectors for high-energy physics experiments

Radiation effects in silicon detectors are discussed in view of their application in future high-energy physics experiments. An overview is given of the major changes in the operational parameters due to radiation damage and their origin in the radiation-induced microscopic disorder in the silicon bulk. The principal radiation hardening technologies are described that have been adopted by the high-energy physics community to face the hostile radiation environment where silicon pixel and microstrip detectors will operate in the Large Hadron Collider.

Present status of Geant4

Geant4 is a software toolkit for both full and fast Monte Carlo simulation of detectors in high-energy physics experiments. It has been developed under a worldwide collaboration of about 100 scientists, coming from over 40 institutes and HEP experiment groups in 15 countries. The first production version of Geant4 was released in December 1998. In this report, the present status of the toolkit is summarized.

Investigation on the improved radiation hardness of silicon detectors with high oxygen concentration

We present an investigation on the influence of the oxygen concentration on radiation-induced changes in the effective doping concentration of silicon detectors. Diodes fabricated from silicon with interstitial oxygen content ranging from below 2×10 14 to 9×10 17 cm −3 have been irradiated with fast neutrons up to a fluence of 2×10 15 cm −2. Our main interest focused on the so-called stable damage component in the change of the effective doping concentration being of prime importance for the application of silicon detectors in high-energy physics experiments. We demonstrate, that with a high oxygen enrichment the donor removal is appreciably reduced, reaching a value of only 10% of the initial doping concentration for [O i]=9×10 17 cm −3, while for normal detector grade material with [O i] below 5×10 16 cm −3 that value is 60–90%. Furthermore, we show that the fluence proportional introduction of stable acceptors is independent of the oxygen concentration with an averaged introduction rate of (1.49±0.03)×10 −2 cm −1. Only one material was found exhibiting a significantly smaller value of about 0.6×10 −2 cm −1 and thus indicating the possibility to suppress the radiation-induced acceptor creation by material modification. Finally, we show that the experimental findings disagree in several important aspects with predictions made by microscopic defect kinetics models, leaving the physical background of some of the measured data as an open question.

Charge production in thin gap multi-wire chambers

Resistive cathode thin gap chambers (TGCs) have been used as particle detectors in high-energy physics experiments for more than a decade. A quantitative understanding of charge production mechanisms in TGCs has been developed and a simulation program produced to accurately describe the expected response from a chamber as a function of its design and operating parameters. This simulation is based upon a description of the processes of electron cluster production, drift and avalanche effects and space charge contributions from residual ions. Improved measurements of chamber performance are presented, and the parameters of the simulation have been fitted to these data yielding values consistent with estimates based on the physical mechanisms involved. Sensitivity of chamber timing and amplification properties to operating parameters is discussed.

New problems in nuclear microprobe analysis of materials

Advanced materials are being evaluated for use as novel radiation detectors and microelectronic devices, including, potentially, synthetic diamond radiation-hard detectors for high-energy physics experiments and tissue equivalent dosimeters. Use of a nuclear microprobe has allowed spatially resolved electrical properties of the detector material to be measured. However quantitative analysis requires good models for charge collection mechanisms by ion beam induced charge (IBIC). In fact, nuclear microprobe analysis is playing an increasingly prominent role in the analysis of detector materials and devices by IBIC, with secondary roles also being played by ionoluminescence (IL) and the traditional techniques of Rutherford backscattering and particle induced X-ray emission. In this paper, many recent applications are reviewed and some examples of applications of the nuclear microprobe to the study of new materials and devices are presented. Some of these applications involve wide band gap materials, such as GaN, as well as novel detectors for radiation dosimetry in cancer therapy, photovoltaic devices and other microelectronic devices.

Characterization of Charge-Coupled Analog Memories for Nuclear Data Acquisition

Analog memory charge-coupled devices (CCDs) have potential use in instrumentation for nuclear data acquisition, particularly for large detectors for high-energy physics experiments. Some measurements aimed at characterizing the Fairchild 321 for this application have been made. This includes measurements on linearity, noise, dark current and transfer inefficiency. The results are given together with the measurement techniques used.