W.F. Sommer

Type inversion in silicon detectors

Abstract Silicon strip detectors and photodiodes were irradiated in an 800 MeV proton beam. The change of the effective doping concentration was monitored by measuring diode C - V curves. Type inversion is observed at a fluence Φ = 1.5 × 10 13 cm −2 . Further evidence for type inversion is obtained from a study of pulses generated by an infrared LED in silicon strip detectors. A two-level parametrization is used to describe donor removal and acceptor state creation during proton irradiation: N eff = N 0 exp(− cφ )− βφ . We measure values of c = (5.5 ± 1.1) × 10 14 cm 2 and β = (0.031 ± 0.006) cm −1 . After type inversion the depletion voltage increases with proton fluence. This may set the limit for the lifetime of silicon detectors at future colliders. However, the occurence of type inversion does not degrade the performance of silicon strip detectors. The effective doping concentration showed a complex post irradiation behaviour. After a short term annealing period the doping concentration increased beyond the value that had been reached immediately after the exposure.

The mechanical properties of 316L/304L stainless steels, Alloy 718 and Mod 9Cr-1Mo after irradiation in a spallation environment

Abstract The Accelerator Production of Tritium (APT) project proposes to use a 1.0 GeV, 100 mA proton beam to produce neutrons via spallation reactions in a tungsten target. The neutrons are multiplied and moderated in a lead/aluminum/water blanket and then captured in 3 He to form tritium. The materials in the target and blanket region are exposed to protons and neutrons with energies into the GeV range. The effect of irradiation on the tensile and fracture toughness properties of candidate APT materials, 316L and 304L stainless steel (annealed), modified (Mod) 9Cr–1Mo steel, and Alloy 718 (precipitation hardened), was measured on tensile and fracture toughness specimens irradiated at the Los Alamos Neutron Science Center accelerator, which operates at an energy of 800 MeV and a current of 1 mA. The irradiation temperatures ranged from 50°C to 164°C, prototypic of those expected in the APT target/blanket. The maximum achieved proton fluence was 4.5×10 21 p / cm 2 for the materials in the center of the beam. This maximum exposure translates to a dpa of 12 and the generation of 10 000 appm H and 1000 appm He for the Type 304L stainless steel tensile specimens. Specimens were tested at the irradiation temperature of 50–164°C. Less than 1 dpa of exposure reduced the uniform elongation of the Alloy 718 (precipitation hardened) and Mod 9Cr–1Mo to less than 2%. This same dose reduced the fracture toughness by 50%. Approximately 4 dpa of exposure was required to reduce the uniform elongation of the austenitic stainless steels (304L and 316L) to less than 2%. The yield stress of the austenitic steels increased to more than twice its non-irradiated value after less than 1 dpa. The fracture toughness reduced significantly by 4 dpa to ∼100 MPa m1/2. These results are discussed and compared with results of similar materials irradiated in fission reactor environments.

Determination of helium and hydrogen yield from measurements on pure metals and alloys irradiated by mixed high energy proton and spallation neutron spectra in LANSCE

The confident design of accelerator-driven spallation neutron devices will require good estimates of the cross-sections for generation of helium and hydrogen in the mixed spectra of high energy protons and neutrons that will be experienced by the structural materials. Improved estimates of these cross-sections were derived from a series of irradiations that were conducted at relatively low temperatures (<100°C) in the Los Alamos Neutron Science Center (LANSCE) as part of the test program supporting the Accelerator Production of Tritium (APT) Program. In this irradiation campaign, a variety of candidate structural alloys and pure metal dosimeter foils were irradiated in various particle spectra, ranging from 800 MeV protons, to mixed energy distributions of both protons and spallation neutrons, and finally to distributions consisting primarily of high energy neutrons. At proton energies on the order of hundreds of MeV, exceptionally high levels of gas atoms are generated in all elemental constituents of typical iron-based and nickel-based structural alloys, with helium typically on the order of ∼150 appm per dpa and hydrogen at approximately a factor of 3–5 higher. Most of the gas production is due to proton and helium recoils from the proton beam interactions with the specimens, although gas and especially damage production from lower-energy spallation neutrons becomes increasingly significant at locations farther from the beam center. The results show that helium production rate per dpa by protons in elements typically found in structural alloys is relatively insensitive to elemental composition. The measured helium concentrations and the derived cross-sections are larger by about a factor of two, however, than those calculated using the LAHET code which was optimized for prediction of neutron/proton ratios in the target tungsten source rods of the APT test. Unlike helium, the retained hydrogen levels are somewhat sensitive to composition, reflecting primarily different levels of diffusional loss, but hydrogen is still retained at rather high concentrations, allowing a lower bound estimate of the hydrogen generation rates.

Temperature effects on radiation damage to silicon detectors

Abstract Motivated by the large particle fluences anticipated for the SSC and LHC, we are performing a systematic study of radiation damage to silicon microstrip detectors. Here we report radiation effects on detectors cooled to 0°C (the proposed operating point for a large SSC silicon tracker) including leakage currents and change in depletion voltage. We also present results on the annealing behavior of the radiation damage. Finally, we report results of charge collection measurements of the damaged detectors made with an 241 Am α source.

Microstructure of both as-irradiated and deformed 304L stainless steel irradiated with 800 MeV protons

A 304L stainless steel water degrader was irradiated with 800 MeV protons at Los Alamos Neutron Science Centre (LANSCE) up to 8.5 dpa at temperatures up to 250 degreesC. Tensile tests showed that hardening and embrittlement were induced in the material. In order to understand the irradiation hardening and embrittlement mechanism, the microstructure in both as-irradiated and deformed material has been studied. The results of TEM investigations show that in the as-irradiated material the main features are: (a) very dense small defect clusters, part of them can be resolved as stacking fault tetrahedra (SFT), with a mean size of about 1.6 run independent of irradiation dose; (b) large Frank loops. whose size increases with dose but whose density varies little with dose; (c) amorphization of precipitates; and (d) no observable helium bubbles or cavities. The main feature in the deformed material is the formation of twin lamellae and bundles of twin lamellae. In all of the four samples (0, 0.7, 3.4 and 6.8 dpa) studied, dense twin lamellae have been observed. The twin planes are {111}. Similar to channels observed in irradiated and deformed fcc pure metals, the original microstructures inside the twin lamellae, namely small clusters and Frank loops, have been removed. The width of the twin lamellae and their bundles varies from a few nanometers to more than 100 nm. The structure outside the twin lamellae is little changed

Production of helium by medium energy (600 and 800 MeV) proton

Abstract There are no neutron sources that can produce simultaneously displacement cascade damage, gases and other transmutation product impurities, at levels significant to a fusion reactor such as the Next European Torus (NET). However, this same combination of damage is produced in materials bombarded by medium energy protons. Therefore, proton irradiation facilities have been added to the worlds most intense medium energy proton accelerators, SIN in Switzerland and LAMPF in the USA, to study the effects of these damage forms in combination. To confirm the accuracy of the computer codes used to model the nuclear reactions, especially regarding helium production, Fe, Cu, Mo, W, Ni, Al and Au were bombarded in the LAMPF proton beam. The buildup of 4 He and 3 He per proton was determined by vaporization of the samples and analysis of the gases released by isotope dilution mass spectrometry.

Mechanical properties and microstructure in low-activation martensitic steels F82H and Optimax after 800-MeV proton irradiation

Low-activation martensitic steels, F82H (mod.) and Optimax-A, have been irradiated with 800-MeV protons up to 5.9 dpa. The tensile properties and microstructure have been studied. The results show that radiation hardening increases continuously with irradiation dose. F82H has lesser irradiation hardening as compared to Optimax-A in the present work and DIN1.4926 from a previous study. The irradiation embrittlement effects are evident in the materials since the uniform elongation is reduced sharply to less than 2%. However, all the irradiated samples ruptured in a ductile-fracture mode. Defect clusters have been observed. The size and the density of defect clusters increase with the irradiation dose. Precipitates are amorphous after irradiation.

Defect microstructure in copper alloys irradiated with 750 MeV protons

Abstract Transmission electron microscopy (TEM) disks of pure copper and solid solution copper alloys containing 5 at% of Al, Mn, or Ni were irradiated with 750 MeV protons to damage levels between 0.4 and 2 displacements per atom (dpa) at irradiation temperatures between 60 and 200°C. The defect cluster density in copper was observed to be constant for irradiation temperatures below about 130°C, and to decrease with increasing temperature above 150°C. About 60% of the defect clusters in copper were resolvable as stacking fault tetrahedra (SFT). Cavity formation was observed for irradiation temperatures above about 150°C. The dislocation loop and network densities were relatively low in all of the irradiated pure copper specimens. Contrary to expectations, the loop density and size both decreased with increasing irradiation temperature. Solute additions did not have any significant effect on the total density of small defect clusters, but they did cause a significant decrease in the fraction of defect clusters resolvable as SFT to ~ 20 to 25%. In addition, the dislocation loop density (> 5 nm diameter) was more than an order of magnitude higher in the alloys compared to pure copper.

Tests of the radiation hardness of VLSI integrated circuits and silicon strip detectors for the SSC under neutron, proton, and gamma irradiation

As part of a program to develop a silicon strip central tracking detector system for the Superconducting Super Collider (SSC), the effects of radiation damage in silicon detectors and their associated front-end readout electronics are being studied. The authors report on the results of neutron and proton irradiations at the Los Alamos National Laboratory and gamma -ray irradiations at UC Santa Cruz. Individual components on single-sided AC-coupled silicon strip detectors and on test structures were tested. Circuits fabricated in a radiation-hard CMOS process and individual transistors fabricated using dielectric isolation bipolar technology were also studied. Bulk damage to the silicon itself is seen as the limiting factor in the lifetime of a detector system. In particular, it is the acceptor site creation in the active volume of the silicon detector that will limit the lifetime to approximately 10 yr for the innermost detectors. >

Summary of the results from post-irradiation examination of spent targets at the FZ-Juelich

Abstract The lifetime of structural components of spallation targets (beam window, liquid metal container, return hull) is determined by the irradiation-induced changes of the mechanical properties of their materials. An extensive test program was initiated using specimens obtained from spent target components from operating spallation facilities (Los Alamos Neutron Science Center, LANSCE and the Spallation Neutron Source at Rutherford–Appleton Laboratory, ISIS). The investigated materials include a nickel-based alloy (IN 718), an austenitic stainless steel (AISI 304L), a martensitic stainless steel (DIN 1.4926) and a refractory metal (tantalum). The materials experienced 800 MeV proton irradiation to maximum fluences of >10 25 p/m 2 . The mechanical property changes were investigated by microhardness measurements, three-point bending tests and tensile tests at temperatures ranging from room temperature (RT) to 250 °C. Subsequent scanning electron microscopy was employed to investigate the fracture surfaces. Generally, irradiation hardening and a decrease in ductility with increasing proton fluence was observed. Nevertheless, all materials except IN 718 tested at RT, retained some ductility up to the maximum doses explored. The transmission electron microscopy investigation showed that a high density of ‘black dots’ and dislocation loops appeared in all materials. No effect of long-range radiation-induced segregation at grain boundaries was detected by energy dispersive X-ray investigation on AISI 304L and IN718 which failed by intergranular fracture.