No abstract

No abstract

Novel silicon detectors with charge gain were designed (Low Gain Avalanche Detectors - LGAD) to be used in particle physics experiments, medical and timing applications. They are based on a n++-p+-p structure where appropriate doping of multiplication layer (p^+) is needed to achieve high fields and impact ionization. Several wafers were processed with different junction parameters resulting in gains of up to 16 at high voltages. In order to study radiation hardness of LGAD, which is one of key requirements for future high energy experiments, several sets of diodes were irradiated with reactor neutrons, 192 MeV pions and 800 MeV protons to the equivalent fluences of up to Φeq=1016 cm−2. Transient Current Technique and charge collection measurements with LHC speed electronics were employed to characterize the detectors. It was found that the gain decreases with irradiation, which was attributed to effective acceptor removal in the multiplication layer. Other important aspects of operation of irradiated detectors such as leakage current and noise in the presence of charge multiplication were also investigated.

No abstract

We performed low temperature shot noise measurements in superconductor (TiN) -strongly disordered normal metal (heavily doped Si) weakly transparent junctions. We show that the conductance has a maximum due to coherent multiple Andreev reflections at low energy and that the shot noise is then twice the Poisson noise (S = 4eI). When the subgap conductance reaches its minimum at finite voltage the shot noise changes to the normal value (S = 2eI) due to a large quasiparticle contribution.PACS numbers: 74.40.+k, 74.50.+r, 73.23.-b We know, from early measurements, that the full shot noise in an electronic device S P oisson = 2qI (first measured by W. Schottky in a vacuum diode) is proportional to the mean value of the current I and to the charge q of the carriers [1]. This result holds for N-I-N tunnel junctions where N is a normal metal and I an insulating barrier [2] with q = e the electronic charge. In S-I-N, due to electron pairing in the superconductor (S), the shot noise is expected to be twice the full shot noise : S = 4eI. However, in such junctions the subgap current is very small and the shot noise is not measurable. The subgap current can be restored if the quasiparticles of the normal metal are coherently backscattered towards the interface. This reflectionless tunneling regime can be achieved by adding a second barrier in the normal part of the junction (S-I-N-I-N) or when the normal metal is disordered enough [3,4,5,6,7,8,9,10,11]. The enhancement of the subgap current is only seen at low energy when the electron-hole coherence time in the normal metal is longer than the time it takes for a quasiparticles to return to the interface. Then, the coherent addition of two (or more) Andreev reflections, each of them with a very small probability Γ 2 (Γ is the transparency of the barrier), yields to an increase of the Andreev current through the interface. This effect can be large, leading to an Andreev current proportional to Γ instead of Γ 2 ≪ Γ ≪ 1 and can be comparable to the normal current above the gap (also proportional to Γ). Another way to increase the subgap current is to use highly transparent S-N junctions. In this case doubled shot noise is predicted and has been observed experimentally at various temperatures [12,13]. However, the noise level was always three times smaller than the doubled full shot noise because of the diffusive nature of the normal metal used in these experiments. Moreover, as shown experimentally and reproduced theoretically [15], the doubling of the shot noise occurs at any energies below the superconducting gap and electronhole coherence is not required.In this letter, we report shot noise measurements in a junction where a superconductor (TiN) is in contact with heavily doped silicon. We show that the shot noise is twice the full shot noise at low energy and equals the Poisson value at bias much smaller than the superconducting gap. This behavior evidences a crossover from a low bias Andreev dominated to a large bias quasiparticle dominated subgap conductance.The sample...

Abstract-We report on measurements on Ultra-Fast Silicon Detectors (UFSD) which are based on Low-Gain Avalanche Detectors (LGAD). They are n-on-p sensors with internal charge multiplication due to the presence of a thin, low-resistivity diffusion layer below the junction, obtained with a highly doped implant. We have performed several beam tests with LGAD of different gain and report the measured timing resolution, comparing it with laser injection and simulations. For the 300μm thick LGAD, the timing resolution measured at test beams is 120ps while it is 57ps for IR laser, in agreement with simulations using Weightfield2. For the development of thin sensors and their readout electronics, we focused on the understanding of the pulse shapes and point out the pivotal role the sensor capacitance plays.

No 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.