Development of novel silicon sensors with high time and spatial resolution
Taikan Suehara, Yuto Deguchi, Yuto Uesugi, Yu Kato, Ryo Yonamine
aa r X i v : . [ phy s i c s . i n s - d e t ] J a n (ELPH Experiment : Development of novel silicon sensors with high time andspatial resolution
Taikan Suehara , Yuto Deguchi , Yuto Uesugi , Yu Kato , and Ryo Yonamine Department of Physics, Faculty of Science, Kyushu University, Fukuoka, 819-0395 Department of Physics, Graduate School of Science, Kyushu University, Fukuoka, 819-0395 Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo, 113-0033 Department of Physics, Graduate School of Science, Tohoku University, Sendai, 980-8578
Silicon pad sensors with novel functions of higher timing resolution (LGAD: Low Gain AvalancheDetector) and higher position resolution (PSD: Position Sensitive Detector) are studied for an applicationto Silicon-Tungsten electromagnetic calorimeter for a detector of the International Linear Collider (ILC).Prototype sensors are fabricated, equipped with dedicated ASICs (Application-Specific Integrated Cir-cuits) and tested with a positron beam as well as a radioisotope. The first results of the measurementsof timing resolution with LGADs and position reconstruction with PSDs are reported. §§§ Introduction
The International Linear Collider (ILC) is a future electron-positron collider for precise measure-ments of Higgs bosons and various BSM searches. Silicon-tungsten electromagnetic calorimeter (SiW-ECAL) is one of the candidates to be used in the International Large Detector (ILD) [1], one of thedetector concepts for the ILC. We are proposing two types of new silicon sensors, Low Gain AvalancheDetectors (LGADs) and Position Sensitive Detectors (PSDs) to be replaced to the standard silicon padsof SiW-ECAL. LGADs are possible to obtain precise timing information and PSDs are sensitive to hitpositions within a cell. With LGADs and PSDs position and timing resolution of the SiW-ECAL canbe significantly improved, which should proceed further reach of Higgs and BSMs in the ILC physicsprogram.LGADs are silicon sensors with the internal avalanche amplification, which have already beenproved to realize the timing resolution down to 30 psec [2]. The precise timing information can beprimarily utilized to perform particle identification of hadrons by Time-of-Flight (ToF) method, whichcan be combined with dE/dx information obtained in the Time Projection Chamber, which is the maintracking detector in the ILD.PSDs are silicon sensors with each cell having an electrode at each corner instead of a simple padspread over the cell. When the signal charge reaches P + pad, the charge is resistively split to electrodesvia a resistive layer on the surface. The hit position is reconstructed as the gravity center of signalstrengths of the electrodes at the four corners. In contrast to using smaller cells, the position resolutioncan be improved with minimal increase of the readout channels if we replace the silicon pads with PSDsn SiW-ECAL.We prepared samples for both LGADs and PSDs to demonstrate the possibilities to be used forSiW-ECAL and to measure characteristics of the sensors for the optimization. We have conducted mea-surements with the positron beam provided from ELPH as well as particles from radioisotopes. §§§ Setup of the test beam
Testboard(LGAD)Activecollimator Daughterboards forLGAD sensorsbeam Testboard(PSD)
Fig.1. Overview of the setup of LGAD/PSD measurement. The positron beam penetrates up to6 LGADs and a PSD to obtain concurrent signal.Figure 1 shows the overview of the setup. The LGADs and PSDs are equipped with testboards ofSkiroc2-CMS ASIC (Application-Specific Integrated Circuit). Skiroc2-CMS has 64-channel readout forsilicon sensors with precise charge and timing information. The timing jitter is ∼
30 psec with charge of >
150 fC with dynamic range of ADC > §§§ Results with the LGADs
The sensors we have tested at the beam is listed in Table 1 [3]. Some sensors are designed asAvalanche Photo-Diodes (APDs) which are used for optical photon measurements with avalanche gain.We recognized those as prototype of LGADs since the basic structure is the same. There are also proto-types of LGADs developed in Hamamatsu (pkg-10 and pkg-20) which were also tested with beam. Thereare two types of LGADs/APDs, reach-through type and inverse type. The reach-through type has provenperformance of timing resolution, but since the amplification region is confined below the readout pads,there is significant inactive area. In contrast, the inverse type has amplification on the bottom of theable 1. List of LGADs and APDs tested with beam at ELPH, Tohoku University.Spec No. Type Size Count φ φ φ φ × φ φ >
100 fC (right).Timing resolution of reach-through APDs (S2384) were measured with the ToA data of the coinci-dence events. After the correction of non-linearity and time-walk of the ToA, we see clear correlationbetween two sensors as shown in Figure 2 (left). Figure 2 (right) shows the correlation of events having >
100 fC of charge to avoid jitter of the electronics. The timing resolution of single sensor was calculatedas 385 ±
94 psec. The result is worse than expected. Possible reason is imperfect correction of ToA sincethe correction coefficients are obtained with measurements at different places. We will investigate thiswith further measurements.ig.3. Picture of a PSD (left) and magnified view of resistive P + surface (top-right, left-half ofthe PSD) and resistive layer surface (bottom-right, right-half of the PSD). §§§ Results with the PSDs
We fabricated a few types of PSDs having 4 × + surface, which forms meshes to increase the resistivity. The other is a dedicated resistive layer connectedto a dotted P + layer, which gives 10-30 times more resistance [4]. The resistivity of the edges of the PSDsis set lower to reduce position distortion. The cell size is 5 . × . , and the sensor thickness is 650 µ m for all the sensors.Fig.4. Reconstructed position of each cell with a PSD, with beta radiation from Sr.Figure 4 shows the response of the PSD sensors with a Sr radioactive source. The position isalculated by X rec = ( Q + Q ) − ( Q + Q ) Q + Q + Q + Q Y rec = ( Q + Q ) − ( Q + Q ) Q + Q + Q + Q where X rec and Y rec are the reconstruction position in X and Y axes and Q i is measured charge at eachelectrode on the corner. The difference between maximum and minimum X rec ( Y rec ), called “dynamicrange” should be − X rec = Y rec = ± §§§ Summary
LGADs/APDs and PSDs sensors have been studied for the application to ILD SiW-ECAL. The firstmeasurement of the timing resolution of the APDs with positron beam is obtained as 385 ±
94 psec,which we will try to improve with updated setup. We successfully reconstructed the hit positions onPSDs with good dynamic range using a Sr radioactive source. Further studies with positron beam areplanned.
Acknowledgments
The test beam experiment was conducted with the support of ELPH, Tohoku University. We ap-preciate Omega group for the support on the operation of the Skiroc2-CMS chip. This work is partiallysupported by JSPS KAKENHI Grant Number JP17H05407.
References [1] T. Behnke, James E. Brau, Philip N. Burrows, M. Peskin, et al. , The International Linear ColliderTechnical Design Report - Volume 4: Detectors, arXiv:1306.6329 (2013).[2] McCarthy, Thomas G., Upgrade of the ATLAS Liquid Argon Calorimeters for the High-LuminosityLHC, arXiv:1612.07102 (2016).[3] Y. Deguchi, K. Kawagoe, E. Mestre, R. Mori, T. Suehara, T. Yoshioka, Study of silicon sensors forprecise timing measurement, 2020 JINST C05051.[4] Y. Uesugi, R. Mori, H. Yamashiro, T. Suehara, T. Yoshioka, K. Kawagoe, Study of Position SensitiveSilicon Detector (PSD) for SiW-ECAL at ILC, 2020 JINST15