B. Wittmer

Investigation of the radiation damage of GaAs detectors by neutrons and photons

Abstract Surface barrier particle detectors, processed in Aachen using SI GaAs from several manufacturers, have been irradiated with neutrons (peak energy ∼1 MeV) up to fluences of 4.0 × 10 14 n/cm 2 and with photons from a 60 Co source with a dose of 100 Mrad. All detectors work well after the irradiation. Detectors biased with 200 V during the neutron irradiation show no significant difference from those not biased. After irradiation with 4.0 × 10 14 n/cm 2 the leakage currents at 200 V are a factor of 4 greater than those before irradiation. At the highest radiation level the signal for minimum ionizing particles corresponds to 7200 electrons (at 200 V bias voltage) independent of peaking times between 40 ns and 2.2 μs. The signals for α-particles (2.2 μs peaking time, 200 V bias voltage) are about 20% of those before irradiation. The exposure to the 100 Mrad photon dose caused little change of the detector performance. The leakage currents were even reduced by about 10%.

Study of radiation damage and substrate resistivity effects from beam test of silicon microstrip detectors using LHC readout electronics

Abstract We present the beam test results of single-sided silicon microstrip detectors, with different substrate resistivities. The effects of radiation damage are studied for a detector irradiated to a fluence of 2.4×10 14 n / cm 2 . The detectors are read out with the APV6 chip, which is compatible with the 40 MHz LHC clock. The performance of different detectors and readout modes are studied in terms of signal-to-noise ratio and efficiency.

Investigation of the radiation damage of GaAs detectors by protons, pions and neutrons☆

Abstract Surface barrier detectors processed in Aachen using SI-GaAs from several manufacturers have been irradiated with high fluences of neutrons (mean energy 1 MeV, fluence up to Φ n ∼ 5 × 10 14 cm −2 ), pions (191 MeV, fluence up to Φ π ∼ 0.6 × 10 14 cm −2 ) and protons (23 GeV, fluence up to Φ p ∼ 2 × 10 14 cm −2 ). The detectors have been characterized in terms of macroscopic quantities like I - V characteristic curves and charge collection efficiencies for incident minimum ionizing- (mip) as well as α-particles. All detectors work well after the exposure. At the highest fluences a sizable degradation concerning the charge collection efficiencies has been observed for all investigated materials. SI-GaAs material with low carbon content (LC material from FCM, Freiberg) seems to be less affected than substrates with a higher carbon concentration. At the highest irradiation level the mip signal from a 250 μm thick detector made of LC material amounts to 8000 electrons (at 400 V bias voltage) independent of peaking times between 40 ns and 2.2 μs. The leakage currents for this material are even reduced after the irradiation.

Energy straggling and multiple scattering in silicon strip detectors

Abstract We present a test beam study of energy straggling and multiple scattering in silicon strip detectors using electrons and pions of momenta up to 50 GeV. Results are compared with GEANT simulation using a simple algorithm to parameterize energy loss distribution. The deflection due to multiple scattering in crystalline structure was investigated by placing a GaAs wafer at various angles.

The CMS tracker front-end and control electronics in an LHC like beam test

A complete prototype of the CMS tracker read-out and control system has been built using components as close as possible to the final design. It is based on an amplifier and analogue pipeline memory chip (APV), analogue optical links transmitting at 40Ms/s and a VME mounted digitisation and data handling board (FED-PMC), supplemented by a control architecture based on FECs which set and monitor the components of the entire system. This system has been successfully operated for the first time under LHC like beam conditions in a 25ns structured beam provided by the SPS at CERN. The objective was primarily to test the synchronisation and pile-up effects in a high trigger rate environment and to examine the many issues involved in operating a complete readout and control system.

Radiation hardness of MSM biasing structures for GaAs microstrip detectors

Abstract In order to achieve a high signal-to-noise ratio and a low-space consumption it is necessary to use integrated resistors for the biasing of GaAs microstrip detectors used in high-energy physics experiments (e.g. CMS, H1). Metal-semiconductor-metal (MSM) structures operating in the reach through regime seem to be a promising solution for that purpose. According to the requirements on radiation hardness of strip detectors to be used in the CMS experiment, we have irradiated MSM biasing structures with up to 4.6 × 10 14 neutrons cm 2 . The radiation effects on the dc and noise characteristics of the devices are investigated. Furthermore, the effects of an annealing procedure are examined.

Time structure of charge signals and noise studies of GaAs detectors irradiated by neutrons and protons

SummarySemiconductor detectors processed in Aachen using SI GaAs from different manufacturers have been irradiated with neutrons (peak energy ≈1 MeV) up to 4.0·1014 n/cm2 and protons (energy 24 GeV) up to 8.2·1013 p/cm2. All detectors work well after the exposure. The leakage current density at 200 V of detectors made of AXT material increases by a factor of four after the highest neutron fluence and a factor of three after the maximal proton fluence. For the FEW-LC material the leakage current decreases significantly after irradiation. No significant difference can be observed between biased and non-biased detectors during the exposure to neutrons. The equivalent noise charge (ENC) calculated from the noise density spectra agrees well with that extracted from the pedestal width for the different neutron and proton fluences. Before irradiation the charge signals for minimum ionizing particles (MIPs) increase with the peaking time in the range of 40 ns to 2 μs, while this dependence cannot be observed after the exposure to neutrons or protons. The reason for this behaviour is the different time structure of the charge signals before and after irradiation. For AXT material the charge signals for MIPs correspond to 7100 electrons at 200 V before irradiation for a peaking time of 40ns, while signals of 7200 electrons after 4.0·1014 n/cm2 and of 4300 electrons after 8.2·1013 p/cm2 are obtained. For FEW-LC material the signals at 200 V are reduced from 15 700 electrons before irradiation to 6000 electrons after 8.2·1013 p/cm2 for this peaking time.

Charge collection efficiencies and reverse current densities of GaAs detectors

Abstract GaAs surface barrier diodes with different areas have been fabricated and tested as particle detectors. It is shown that the reverse current is affected both by the area of the Schottky contact and by its circumference. The charge collection properties can be explained on the basis of a model that takes into account trapping effects and the distribution of the electric field within the detector. From this model (several) rules for optimizing GaAs detectors are derived.

Measurements on GaAs strip and pixel detectors in a 50 GeV pion beam

Abstract GaAs strip and pixel detectors constructed in Aachen have been tested at CERN in a 50 GeV pion beam in September 1995 in collaboration with the CMS Tracker group. The strip detectors had a pitch of 100 μm and were made of a 250 μm thick Freiberger SI-GaAs wafer [1]. The three strip detectors had a strip width of 25, 50 and 75 μm, respectively. Using the fast PreMux128 preamplifier multiplexer chip ( τ p = 50 ns) a signal to noise ratio of 15 was obtained for the widest strips at normal beam incidence for a bias voltage of 170V. The 8 × 8 pixel arrays with a pixel size of 1 × 1 mm 2 and 0.5 × 0.5 mm 2 , respectively, were read out with the PreMux128 as well. Here a signal of 12500e − was obtained for both detectors, leading to a maximum signal to noise ratio of 20 at perpendicular beam incidence and 170 V bias voltage.

Beam test results on Si and GaAs microstrip detectors with length up to 64 cm

Abstract This study is aimed at exploring the details of various aspects of building long microstrip ladders for future particle physics collider experiments. Several configurations of silicon microstrip detectors (up to 64 cm in length) and two GaAs detectors were constructed and beam tested using the Viking front end chip and a PC-based data acquisition system allowing easy comparison and understanding of the results. Results on cluster charge, signal-to-noise, resolutions, and efficiencies for different configurations are presented and compared here.