Radiation hardness of thin Low Gain Avalanche Detectors

Abstract

Low Gain Avalanche Detectors (LGAD) are based on a n ++ -p + -p-p ++ structure where an appropriate doping of the multiplication layer (p + ) leads to high enough electric fields for impact ionization. Gain factors of few tens in charge significantly improve the resolution of timing measurements, particularly for thin detectors, where the timing performance was shown to be limited by Landau fluctuations. The main obstacle for their operation is the decrease of gain with irradiation, attributed to effective acceptor removal in the gain layer. Sets of thin sensors were produced by two different producers on different substrates, with different gain layer doping profiles and thicknesses (45, 50 and 80 µm). Their performance in terms of gain/collected charge and leakage current was compared before and after irradiation with neutrons and pions up to the equivalent fluences of 5 · 10 15 cm −2 . Transient Current Technique and charge collection measurements with LHC speed electronics were employed to characterize the detectors. The thin LGAD sensors were shown to perform much better than sensors of standard thickness (∼300 µm) and offer larger charge collection with respect to detectors without gain layer for fluences < 2 · 10 15 cm −2 . Larger initial gain prolongs the beneficial performance of LGADs. Pions were found to be more damaging than neutrons at the same equivalent fluence, while no Work performed in the framework of the CERN-RD50 collaboration.

Timing performance of small cell 3D silicon detectors

“…The fit of the convoluted Landau and Gaussian distributions to the data is also shown. The difference of factor ∼ 10 was observed in the most probable signal (parameter p1 of the fit) which is also expected from the measured gain [7] and the difference in the detector thickness. As the noise level is 20% larger for LGAD, due to higher capacitance (see Fig.…”

“…The first stage of amplification uses fast transimpedance amplifier designed by UCSC [15] followed by a second amplifier which gives signals large enough to be relatively easily recorded by a 40 GS/s digitizing oscilloscope with 2.5 GHz bandwidth. The reference time required for measurement of the time resolution was provided from a non-irradiated LGAD detector produced by HPK [7,15]. It is 50 µm thick, has a diameter of 0.8 mm and high gain of ∼ 60 at 330 V and room temperature.…”

“…The time resolution of HPK-50D sensors was measured (in the same setup with a method described below) by using two such detectors and was determined to be 26 ps. More details on timing and charge collection measurements with these LGADs can be found in [7,15]. The electrons were collimated to an angle of < 1 • by a combination of collimator, small cell size and the circular opening in the PCB boards hosting the sensors.…”

“…However, the gain degrades with irradiation [6,7]. The high gain of LGADs can be maintained at equivalent fluences below 10 15 cm −2 , where their performance has been demonstrated to fulfill the HL-LHC requirements [8].…”

4D tracking with ultra-fast silicon detectors

“…This is possible only in thin sensors (50-micron thick or less) where the external voltage can create an electric field of the order of 250-300 kV/cm without causing electrical breakdown in other areas of the detector. Figure 53 shows the bias voltage necessary to maintain a gain G = 10 as a function of irradiation for 50-micron thick CNM sensors implanted with a shallow gain layer, using the irradiation results from [45] (squares), the initial acceptor removal parameterisation of equation (5-4) with the value of the parameters shown in Figure 27, and the Massey model of impact ionization [27], The very good agreement between the WF2 predicted values and the measured points indicates the correctness of the initial acceptor removal model and the appropriate gain parameterisation of the Massey equations in this range of the electric field. The thick solid line indicates the bulk contribution to the total gain value (right y-axis): it starts to be important at ~1e15 n eq /cm 2 , and it becomes ~ 50% at ~2e15 n eq /cm 2 .…”