E. Fretwurst

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 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.

Some excitation functions of neutron induced reactions in the energy range 12.6–19.6 MeV

Abstract Excitation functions of neutron induced reactions were measured in the energy range 12.6–19.6 MeV by means of the activation method. Results for the following reactions are reported: (n, 2n) reaction in N14, F19, K39, Ga69 and Sr88, (n, p) reaction in F19 and Zn66 and (n, α) reaction in Mn55. The results are compared with corresponding measurements published by other authors. In the case of the heavier target elements a comparison is also made with the predictions of the statistical theory using compound nucleus cross sections evaluated in the optical model and applying the level density formula given by Lang. Gamma emission as a competing process in the decay of the compound nucleus is also taken into account. The agreement of the theoretical predictions with the experimental results is poor. This, probably, is caused mainly by the level density parameters.

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.

The use of the signal current pulse shape to study the internal electric field profile and trapping effects in neutron damaged silicon detectors

The induced current pulse from ionizing events occurring near contacts on each side of a p+−n−n+ silicon junction detector may be used to map the electric field present in the detector. It is of interest to define the operative effects of fast neutron-induced displacement damage in detectors destined for high radiation environments in SSC or LHC. The hole current shape, in particular, has been useful to determine that the field maximum moves to the “rear” n+ contact as the material apparently changes to p-type at 8 × 1012 1 MeV n/cm2. Trapping times for both holes and electrons have been measured as a function of neutron fluence using the current pulse width to measure charge collection time as well as using calculated charge collection times. A clear linear relationship is found for the trapping probability (l/τ) versus neutron fluence. Current pulse shapes have been calculated for representative detector fields and mobility relationships and comparison with measured shapes is reasonable.

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.