Low Gain Avalanche Detectors Research Articles

Precision timing with low gain avalanche detectors in the CMS MTD Endcap Timing Layer

The Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC) is undergoing an extensive Phase II upgrade program to prepare for the challenging conditions of the High-Luminosity LHC (HL-LHC). In particular, precision timing can offset the performance degradation due to event pileup at the HL-LHC, recovering the purity of vertices of current LHC conditions. As such, a new timing layer will be introduced to measure minimum ionizing particles (MIPs) with a time resolution of 30 ps. The endcap region of this MIP Timing Detector (MTD) will be instrumented with a hermetic double layer of silicon low gain avalanche detectors (LGADs), covering the high radiation pseudo-rapidity region between |η|=1.6 to 3.0. Radiation tolerance studies of the LGADs indicate excellent performances adequate to cope with the Endcap Timing Layer (ETL) requirements. The LGADs have intrinsic gain, which enhances the MIP signal and provides adequate signal-to-noise for good timing precision. We present the status of the art for the LGADs for the endcap region of the MTD and report on recent beam test results.

Study of silicon sensors for precise timing measurement

Silicon sensors with high timing resolution can help particle identification in the International Linear Collider (ILC). We are studying Low Gain Avalanche Detectors (LGADs) as a sensor with high timing resolution. As a step to develop LGADs we are now characterizing Avalanche Photo Diodes (APDs), which have similar multiplication structure as LGADs. Particles from radioisotopes are used for studies in this paper.

Design and fabrication of Low Gain Avalanche Detectors (LGAD): a TCAD simulation study

Low Gain Avalanche Detectors (LGAD) are silicon sensors with a time resolution better than 20 ps. The ATLAS and CMS experiments are designing LGAD detectors to address the pile-up challenge at the High Luminosity-Large Hadron Collider (HL-LHC) . The Institute of High Energy Physics (IHEP) High-Granularity Timing Detector group has recently developed its first version of LGAD sensors. The LGAD structure was designed using Technology Computer-Aided Design (TCAD) simulations and optimized to obtain a high breakdown voltage and ideal gain. The n-type Junction Termination Extension (N-JTE) zone is a critical structure to guarantee a high breakdown voltage. The gain layer is optimized for an ideal gain factor and hence good time resolution. The optimized LGAD sensor has a gain higher than six and a breakdown voltage higher than 400 V.

Deep diffused APDs for charged particle timing applications: Performance after neutron irradiation

Recent interest in pile-up mitigation through fast timing at the HL-LHC has focused attention on technologies that now achieve minimum ionising particle (MIP) time resolution of 30 picoseconds or less. The constraints of technical maturity and radiation tolerance narrowed the options in this rapidly developing field for the ATLAS and CMS upgrades to low gain avalanche detectors and silicon photomultipliers. In a variety of applications where occupancies and doses are lower, devices with pixel elements of order 1 cm$^2$, nevertheless achieving 30 ps, would be attractive. In this paper, deep diffused Avalanche Photo Diodes (APDs) are examined as candidate timing detectors for HL-LHC applications. Devices with an active area of $8 \times 8$ mm$^2$ are characterised using a pulsed infrared laser and, in some cases, high energy particle beams.The timing performance as well as the uniformity of response are examined. The effects of radiation damage on current, signal amplitude, noise, and timing of the APDs are evaluated using detectors with an active area of $2 \times 2$ mm$^2$. These detectors were irradiated with neutrons up to a a 1-MeV neutrons fluence $\Phi_{eq} = 10^{15}$ cm$^{-2}$. Their timing performance was characterised using a pulsed infrared laser. While a time resolution of $27 \pm 1$ ps was obtained in a beam test using an $8 \times 8$ mm$^2$ sensor, the present study only demonstrates that gain loss can be compensated by increased detector bias up to fluences of $\Phi_{eq} = 6 \cdot 10^{13}$ cm$^{-2}$. So it possibly falls short of the $\Phi_{eq} = 10^{14}$ cm$^{-2}$ requirement for the CMS barrel over the lifetime of the HL-LHC.

A High-Granularity Timing Detector (HGTD) for the Phase-II upgrade of the ATLAS detector

The expected increase of the particle flux at the high-luminosity phase of the LHC (HL-LHC) with instantaneous luminosities up to L = 7.5× 1034 cm−1s−1 will have a severe impact on the ATLAS detector performance. The pile-up is expected to increase on average to 200 interactions per bunch crossing. The reconstruction and trigger performance for electrons, photons as well as jets and transverse missing energy will be severely degraded in the end-cap and forward region, where the liquid Argon based electromagnetic calorimeter has coarser granularity and the inner tracker has poorer momentum resolution compared to the central region. A High Granularity Timing Detector (HGTD) is proposed in front of the liquid Argon end-cap calorimeters for pile-up mitigation and for bunch per bunch luminosity measurements. This device should cover the pseudo-rapidity range of 2.4 to about 4.0. Two Silicon sensors double-sided layers are foreseen to provide a precision timing information for minimum ionizing particle with a time resolution better than 50 pico-seconds per hit (i.e. 30 pico-seconds per track) in order to assign the particle to the correct vertex. Each readout cell has a transverse size of 1.3 mm×1.3 mm leading to a highly granular detector with about 3 millions of readout electronics channels. Low-Gain Avalanche Detector (LGAD) technology has been chosen as it provides an internal gain good enough to reach large signal over noise ratio needed for excellent time resolution. Extensive LGAD research and development (R&D) campaigns are carried out to investigate the suitability of this new technology as timing sensors for HGTD. The related readout ASIC is also being studied extensively.

Inverse Low Gain Avalanche Detectors (iLGADs) for precise tracking and timing applications

Low Gain Avalanche Detector (LGAD) is the baseline sensing technology of the recently proposed Minimum Ionizing Particle (MIP) end-cap timing detectors (MTD) at the Atlas and CMS experiments. The current MTD sensor is designed as a multi-pad matrix detector delivering a poor position resolution, due to the relatively large pad area, around 1 mm2; and a good timing resolution, around 20–30 ps. Besides, in his current technological incarnation, the timing resolution of the MTD LGAD sensors is severely degraded once the MIP particle hits the inter-pad region since the signal amplification is missing for this region. This limitation is named as the LGAD fill-factor problem. To overcome the fill factor problem and the poor position resolution of the MTD LGAD sensors, a p-in-p LGAD (iLGAD) was introduced. Contrary to the conventional LGAD, the iLGAD has a non-segmented deep p-well (the multiplication layer). Therefore, iLGADs should ideally present a constant gain value over all the sensitive region of the device without gain drops between the signal collecting electrodes; in other words, iLGADs should have a 100% fill-factor by design. In this paper, tracking and timing performance of the first iLGAD prototypes is presented.

Development of low-energy X-ray detectors using LGAD sensors.

Recent advances in segmented low-gain avalanche detectors (LGADs) make them promising for the position-sensitive detection of low-energy X-ray photons thanks to their internal gain. LGAD microstrip sensors fabricated by Fondazione Bruno Kessler have been investigated using X-rays with both charge-integrating and single-photon-counting readout chips developed at the Paul Scherrer Institut. In this work it is shown that the charge multiplication occurring in the sensor allows the detection of X-rays with improved signal-to-noise ratio in comparison with standard silicon sensors. The application in the tender X-ray energy range is demonstrated by the detection of the sulfur Kα and Kβ lines (2.3 and 2.46 keV) in an energy-dispersive fluorescence spectrometer at the Swiss Light Source. Although further improvements in the segmentation and in the quantum efficiency at low energy are still necessary, this work paves the way for the development of single-photon-counting detectors in the soft X-ray energy range.

A simulation model of front-end electronics for high-precision timing measurements with low-gain avalanche detectors

In this paper we report simulation results of a study aiming to optimize parameters of a detector that uses low-gain avalanche detectors (LGAD) for high-precision timing measurements. The detector is assumed to be composed of a 50 μ m LGAD sensor coupled to front-end readout electronics which is used to measure the time of arrival of minimum ionizing particles. The simulation includes modeling of signal fluctuations in the LGAD sensor, variations of the analog bandwidth and signal-to-noise ratio (SNR) of the front-end electronics, time bin quantization, and radiation damage of the LGAD sensors. Two approaches to measure the timestamp are considered: leading edge and constant fraction. Simulated LGAD pulses before irradiation, and after irradiation with neutron fluences of 5 × 1 0 14 n/cm 2 and 1 × 1 0 15 n/cm 2 , are studied. The time resolution for a 50 μ m LGADs was found to be 35 ps for front-end electronics bandwidths larger than 350 MHz and SNRs larger than 30. The time resolution at SNR of 30 for fluences of 5 × 1 0 14 n/cm 2 and 1 × 1 0 15 n/cm 2 were found to be 31 ps and 37 ps, respectively.

Properties of HPK UFSD after neutron irradiation up to 6e15 n/cm2

In this paper we report results from a neutron irradiation campaign of Ultra-Fast Silicon Detectors (UFSD) with fluences of 1e14, 3e14, 6e14, 1e15, 3e15, 6e15 n/cm2. The UFSD used in this study are circular 50 micro-meter thick Low-Gain Avalanche Detectors (LGAD), with a 1.0 mm diameter active area. They have been produced by Hamamatsu Photonics (HPK), Japan, with pre-radiation internal gain in the range 10-100 depending on the bias voltage. The sensors were tested pre-irradiation and post-irradiation with minimum ionizing particle (MIPs) from a 90Sr based \b{eta}-source. The leakage current, internal gain and the timing resolution were measured as a function of bias voltage at -20C and -30C. The timing resolution was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction method for both. The dependence of the gain upon the irradiation fluence is consistent with the concept of acceptor removal and the gain decreases from about 80 pre-irradiation to 7 after a fluence of 6e15 n/cm2. Consequently, the timing resolution was found to deteriorate from 20 ps to 50 ps. The results indicate that the most accurate time resolution is obtained at a value of the constant fraction discriminator (CFD) threshold used to determine the time of arrival varying with fluence, from 10% pre-radiation to 60% at the highest fluence. Key changes to the pulse shape induced by irradiation, i.e. (i) a reduce sensitivity of the pulse shape on the initial non-uniform charge deposition, (ii) the shortening of the rise time and (iii) the reduced pulse height, were compared with the WF2 simulation program and found to be in agreement.

Silicon detectors for the LHC Phase-II upgrade and beyond RD50 Status report

The inner tracking layers of all LHC experiments were designed and developed to cope with the environment of the present Large Hadron Collider (LHC). At the LHC Phase-II Upgrade foreseen for 2026, the particle densities and radiation levels will increase by roughly an order of magnitude compared to the present LHC conditions. Therefore, the inner tracking layers will need to be replaced. The new inner tracking layers, which will all be based on silicon detectors, must be significantly more radiation hard. Within the RD50 Collaboration, an extensive R&D program has been underway for more than a decade across experimental boundaries to develop silicon sensors with sufficient radiation tolerance for HL-LHC trackers. Critical areas of detectors R&D include HV CMOS sensors, detectors made in the 3D technology and Low Gain Avalanche Detectors (LGADs). We will present the state of the R&D in several silicon detector domains, in particular, 3D and LGAD detectors. We will also comment on the options for detector choices experiments beyond the LHC, using the FCC as an example.

Development of a technology for the fabrication of Low-Gain Avalanche Diodes at BNL

Low-Gain Avalanche Detectors are gathering interest in the High-Energy Physics community thanks to their fast-timing and radiation-hardness properties, which are planned to be exploited, for example, in timing detectors for the upgrades of the ATLAS and CMS detectors at the High Luminosity LHC. This new technology has also raised interest for its possible application for photon detection in medical physics, imaging and photon science. The main characteristic of this type of device is a thin and highly-doped layer that provides internal and moderate gain, in the order of 10-20, that enhances the signal amplitude. Furthermore, the thin substrate of a few tens of microns allows fast carrier collection. This paper details on the fabrication technology, specifically developed at Brookhaven National Laboratory for the detection of minimum ionizing particles. The static electrical characterization and the gain measurements on prototypes will be reported, too.

Development of the CMS MIP timing detector

The CMS detector at the CERN Large Hadron Collider is undergoing an extensive Phase II upgrade programme to prepare for the challenging conditions of the High-Luminosity LHC. In particular, a new timing detector, the MTD, will measure minimum ionizing particles with a time resolution of 30–50 ps and hermetic coverage up to a pseudo-rapidity of |η|=3. The MTD will consist of a central barrel region based on L(Y)SO:Ce crystals read out with SiPMs and two end-caps instrumented with radiation-tolerant low gain avalanche detectors. The precision time information from the MTD will reduce the effects of the high levels of pile-up expected at the High-Luminosity LHC and will bring new and unique capabilities to the CMS detector. The time information assigned to each track will enable the use of 4D reconstruction algorithms and will further discriminate interaction vertices within the same bunch crossing to recover the track purity of vertices in current LHC conditions. We present the motivations for precision timing at the High-Luminosity LHC, the overall detector design and technological choices including ongoing R&D studies targeting enhanced timing performance and radiation tolerance.

Timing performance of small cell 3D silicon detectors

A silicon 3D detector with a single cell of 50 × 50 μm2 was produced and evaluated for timing applications. The measurements of time resolution were performed for 90Sr electrons with dedicated electronics used also for determining time resolution of Low Gain Avalanche Detectors (LGADs). The measurements were compared to those with LGADs and also simulations. The studies showed that the dominant contribution to the timing resolution comes from the time walk originating from different induced current shapes for hits over the cell area. This contribution decreases with higher bias voltages, lower temperatures and smaller cell sizes. It is around 30 ps for a 3D detector of 50 × 50 μm2 cell at 150 V and −20 °C, which is comparable to the time walk due to Landau fluctuations in LGADs. It even improves for inclined tracks and larger pads composed of multiple cells. A good agreement between measurements and simulations was obtained, thus validating the simulation results.

Precision Timing with the CMS MIP Timing Detector

The Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC) is undergoing an extensive Phase II upgrade program to prepare for the challenging conditions of the High Luminosity LHC (HL-LHC). A new timing layer is designed to measure minimum ionizing particles (MIPs) with a time resolution of 30 ps and hermetic coverage up to a pseudo-rapidity of |η|=3. This MIP Timing Detector (MTD) will consist of a central barrel region based on LYSO:Ce crystals read out with SiPMs and two end-caps instrumented with radiation-tolerant Low Gain Avalanche Detectors (LGADs). The precision time information from the MTD will reduce the effects of the high levels of pile-up expected at the HL-LHC and will bring new and unique capabilities to the CMS detector. The time information assigned to each track will enable the use of 4D-vertexing which will render a 5-fold pile-up reduction thus recovering the current conditions. Precision timing will also enable new time-based isolations and improved b-tagging algorithms. All of this translates into a 20% gain in effective luminosity when looking at di-Higgs boson events decaying to a pair of b-quarks and two photons. We present the current status and ongoing R&D of the MTD, including implications on the physics reach at the HL-LHC and test beam results.

Evaluation of characteristics of Hamamatsu low-gain avalanche detectors

Low-gain avalanche detectors (LGADs) are attractive because of their fast response to realize a 4-dimensional (4D) tracker in future experiments in high energy physics and other applications. We fabricated LGAD diodes and strip sensors. Their responses before and after irradiation to gamma rays or neutrons were evaluated with light-emitting diodes (LEDs) of various wave lengths and with an infrared laser. The sensors showed a gain of more than 10 before irradiation. Little reduction of gain was observed with gamma irradiation. A substantial reduction of gain was observed after neutron irradiation. A gain increase observed in the interstrip region after neutron irradiation, whereas the gain was equal to one before irradiation.

Reasons for high charge collection efficiency of silicon detectors at HL-LHC fluences

Position sensitive silicon detectors are more resilient to radiation then predicted from extensive measurements over the R&D phase for Large Hadron Collider. It was demonstrated over the past few years that they can be successfully operated also at the upgraded LHC (HL-LHC) and even beyond. A review of silicon properties affecting charge collection after irradiation will be given with special emphasis on their changes in very harsh radiation environments such as HL-LHC. Mainly three reasons: charge multiplication, high electric fields in undepleted bulk and saturation of trapping probabilities lead to better than expected performance of the silicon detectors. Initial acceptor removal, important for novel detector technologies such as Low Gain Avalanche Detectors and depleted CMOS sensors, will be also addressed.

Radiation Tolerance of Proton-Irradiated LGADs

Low gain avalanche detectors (LGADs), silicon sensors with intrinsic charge amplification, are being considered as a possible technology for tracking and timing in the high luminosity upgrade of the CERN Large Hadron Collider. In order to work in such an environment, LGADs must be sufficiently radiation hard. The characterisation before and after irradiation of properties, such as gain, charge collection, spatial homogeneity, space charge, and leakage current, is vital for assessing the performance and viability of LGADs. This paper presents the results obtained from the study of LGADs irradiated with 24-GeV/c protons up to a maximum fluence of $10^{15}~\text{n}_{\text {eq}}/\text{cm}^2$ . The characterisation was performed mainly by means of the transient current technique with red and infrared laser pulses. It was found that the gain decreases with increasing fluence. At a fluence of $10^{15}~\text{n}_{\text {eq}}/\text{cm}^2$ , the charge collected is similar to that of a normal p-i-n diode. Whilst this might be explained by an effective acceptor removal, it was also found that there are clear signs of a double junction in these devices, after irradiation. In addition, the spatial charge collection homogeneity before and after irradiation was evaluated.

OA055] Preliminary test of innovative strip silicon detectors for therapeutic proton beam monitoring

Purpose Unlike currently used gas ionization chambers, solid state detectors offer large granularity and sensitivity to single incoming particles, therefore being ideally suited to improve the present technology for beam monitoring in particle therapy. However, several drawbacks, such as radiation damage, prevented their use so far on high flux therapeutic beams. Two prototype devices for monitoring particle beams are under development, based on innovative silicon low-gain avalanche detectors optimized for time resolution (Ultra Fast Silicon Detectors – UFSDs). Preliminary results with proton beams are presented. Methods UFSDs are low-gain avalanche detectors optimized for time resolution, where sensors as thin as 50 μ m provide signals of ∼ 1 ns time duration with time resolutions of tenths of ps, and large enough signal-to-noise ratio to efficiently discriminate proton signal. One prototype device is being developed to directly count individual protons at high rates, while a second one is under investigation to measure the beam energy with time-of-flight techniques. This requires the design of custom UFSD sensors as well VLSI readout electronics. From simulations’ results and first beam tests with UFSD pads, strip detectors were produced, with two geometries (30 mm and 15 mm length) and different doping modalities to improve radiation hardness. In parallel, prototypes of a new readout chip have been submitted to the foundry. Results Strips sensors were characterized in the laboratory through laser test and I(V) curve studies, and a test with a therapeutic proton beam, with energy ranging from 62 to 227 MeV, was done. Results were obtained via offline analysis of the collected waveforms for Boron and Gallium doped sensors. Time resolution of 35 ps, signal duration of ns, and good S/N separation were found. A gain degradation of 20% was founded in single pad sensors after 10 12 protons/cm2 irradiation. Pile-up effects are under investigation. Conclusions Based on the preliminary results, UFSDs are found to be a promising improvement of monitor chamber. The aim of this contribution is to review the advancement of the project and to report on the results of the test of UFSD strip sensors with a therapeutic proton beam.

Beam test measurements of Low Gain Avalanche Detector single pads and arrays for the ATLAS High Granularity Timing Detector

For the high luminosity upgrade of the LHC at CERN, ATLAS is considering the addition of a High Granularity Timing Detector (HGTD) in front of the end cap and forward calorimeters at |z|= 3.5 m and covering the region 2.4 <|η|< 4 to help reducing the effect of pile-up. The chosen sensors are arrays of 50 μm thin Low Gain Avalanche Detectors (LGAD). This paper presents results on single LGAD sensors with a surface area of 1.3×1.3 mm2 and arrays with 2×2 pads with a surface area of 2×2 mm2 or 3×3 mm2 each and different implant doses of the p+ multiplication layer. They are obtained from data collected during a beam test campaign in autumn 2016 with a pion beam of 120 GeV energy at the CERN SPS. In addition to several quantities measured inclusively for each pad, the gain, efficiency and time resolution have been estimated as a function of the position of the incident particle inside the pad by using a beam telescope with a position resolution of few μm. Different methods to measure the time resolution are compared, yielding consistent results. The sensors with a surface area of 1.3×1.3 mm2 have a time resolution of about 40 ps for a gain of 20 and of about 27 ps for a gain of 50 and fulfil the HGTD requirements. Larger sensors have, as expected, a degraded time resolution. All sensors show very good efficiency and time resolution uniformity.

Radiation hardness of gallium doped low gain avalanche detectors

Low Gain Avalanche Detectors (LGADs) are based on a n++-p+-p-p++ structure where appropriate doping of multiplication layer (p+) leads to high enough electric fields for impact ionization. Operation of these detectors in harsh radiation environments leads to decrease of gain attributed to the effective acceptor removal in the multiplication layer. In order to cope with that devices were produced where boron was replaced by gallium. The initial radiation hardness studies show a smaller degradation of gain with neutron fluence indicating that gallium is more difficult to displace/deactivate from the lattice site than boron.