G. Lindström

Radiation hardness of silicon detectors — a challenge from high-energy physics

An overview of the radiation-damage-induced problems connected with the application of silicon particle detectors in future high-energy physics experiments is given. Problems arising from the expected hadron fluences are summarized and the use of the nonionizing energy loss for normalization of bulk damage is explained. The present knowledge on the deterioration e⁄ects caused by irradiation is described leading to an appropriate modeling. Examples are given for a correlation between the change in the macroscopic performance parameters and e⁄ects to be seen on the microscopic level by defect analysis. Finally possible ways are out-lined for improving the radiation tolerance of silicon detectors either by operational conditions, process technology or defect engineering. ( 1999 Elsevier Science B.V. All rights reserved.

Radiation damage in silicon detectors

Radiation damage effects in silicon detectors under severe hadron and gamma-irradiation are surveyed, focusing on bulk effects. Both macroscopic detector properties (reverse current, depletion voltage and charge collection) as also the underlying microscopic defect generation are covered. Basic results are taken from the work done in the CERN-RD48 (ROSE) collaboration updated by results of recent work. Preliminary studies on the use of dimerized float zone and Czochralski silicon as detector material show possible benefits. An essential progress in the understanding of the radiation-induced detector deterioration had recently been achieved in gamma irradiation, directly correlating defect analysis data with the macroscopic detector performance.

Relation between microscopic defects and macroscopic changes in silicon detector properties after hadron irradiation

Abstract Silicon detectors produced from materials with different resistivities and oxygen concentrations have been irradiated with energetic neutrons, protons and pions. Isothermal annealing studies have shown correlation between microscopic defect evolution and the macroscopic detector performance. It was found that the annealing behavior of the electron traps attributed to the single and double charged divacancy is strongly related to the current related damage parameter α. In both cases the isothermal evolution is independent of the oxygen and doping concentration in the material under investigation ( 2×10 14 O ] 18 cm −3 and 10 12 P ] 13 cm −3 ) and the absolute values do not depend on the particles used for the irradiation provided the fluence is properly normalized by the nonionizing energy loss (NIEL). In contrast to this result the introduction rates of the observed point defects VOi and CiCs were however found to depend on the particle type. Thus clear indication is given that the generation of point defects does not scale with NIEL. Compared to neutron irradiated samples the introduction rate after irradiation with charged hadrons was found to be higher by a factor around 2.

Radiation studies and operational projections for silicon in the ATLAS inner detector

Abstract The current models for bulk damage in high resistivity silicon have been reviewed. The damage constants were obtained from a global data survey. On this basis the degradation of the silicon counters in the ATLAS Inner Detector during a 10 year LHC operation is forecasted.

Reverse annealing of the effective impurity concentration and long term operational scenario for silicon detectors in future collider experiments

Abstract Systematic investigations of the reverse annealing effect after radiation damage have been performed in particular for the change of the effective impurity concentration. The pronounced long term anneal observed at room temperature was further investigated by isochronal and isothermal studies. In order to demonstrate the radiation hardness of silicon detectors during 10 years of LHC operation an experiment was started at a higher temperature which compressed the real operational scenario to about 10 months.

Results on radiation hardness of silicon detectors up to neutron fluences of 1015 n/cm2☆

Abstract Our ongoing investigations of the radiation hardness for silicon detectors have been extended to neutron fluences up to 10 15 n/cm 2 . Emphasis was put on the damage induced change of the effective impurity concentration and its related room temperature annealing. The consequences for measurements of the bulk generation current are studied. While most results have been obtained for detectors irradiated without bias first damage experiments under operating conditions exhibit an additional effect attributed to the SiO 2 Si interface.

COMPARISON OF DEFECTS PRODUCED BY FAST NEUTRONS AND 60CO-GAMMAS IN HIGH-RESISTIVITY SILICON DETECTORS USING DEEP-LEVEL TRANSIENT SPECTROSCOPY

Measurements of radiation-induced defects in high-resistivity silicon detectors irradiated with 14.1 MeV neutrons and 60Co-gammas have been performed using the Deep-Level Transient Spectroscopy technique (DLTS). Five electron traps and one hole trap were found in both neutron- and gamma-irradiated samples differing only in the relative defect concentrations. Furthermore, two additional levels were only detected in the neutron irradiated detectors. The observed defects are identified by comparing the measured energy levels, capture cross sections, and introduction rates to those known from literature. In addition, these assignments are supported by the annealing behaviour observed in two neutron-irradiated samples during a short-term annealing at room temperature and an isochronal heat treatment. The differences found between the defect production by fast neutrons and 60Co-gammas are discussed.

Silicon detector developments for calorimetry: Technology and radiation damage

Large area silicon detectors with a sensitive area of 5 × 5 cm2 and a thickness of 400 μm have been developed for the instrumentation of the plug calorimeter at H1/HERA. The novel method, a combination of surface barrier and planar processes, has resulted in detectors with excellent quality and long term operation stability. A current density of down to 1 nA/cm2 was reached. Studies of radiation damage produced by 14 MeV neutrons, 25 MeV protons and 20 keV X-rays were performed and the resulting current increase, impurity removal and charge collection deficiency investigated. Long term storage and short term heat treatments showed appreciable annealing effects by up to 94%.

Radiation hardness of silicon detectors for future colliders

Abstract The radiation hardness of silicon pad detectors, especially developed for the PLUG-calorimeter of the H1 experiment at HERA was investigated with respect to neutron and electron irradiation. Be(d,n)-neutrons with an average energy of 6.2 MeV up to a fluence of 10 15 n/cm 2 and 1.8 MeV electrons up to a dose of 1 MGy (10 16 e/cm 2 ) were used. Degradation effects of the diode properties regarding the reverse current, depletion voltage and charge collection efficiency are studied at room temperature and with no bias applied during irradiation. Special emphasis is put on the separation of the respective damage generation and its subsequent self annealing. The observed effects are discussed with respect to radiation levels to be envisioned for experiments with future colliding beam machines.

Second-order generation of point defects in gamma-irradiated float-zone silicon, an explanation for “type inversion”

Radiation-induced defects in silicon diodes were investigated after exposure to high doses of Co60-gamma irradiation using the thermally stimulated current method. We have found that, for high irradiation doses, a second-order defect can be detected. This defect is largely suppressed in oxygen-enriched material while it is the main cause for the space charge sign inversion effect observed in standard float-zone material.