T.H. Blewitt

Low-temperature fast-neutron radiation damage studies in superconducting materials

Abstract The residual resistivity increase rate, dpi/dt ,as a function of induced resistivity for fast-neutron irradiated Cu, Nb, Nb doped with oxygen, V, Ta, Pb, Cd and NbTi has been studied after 18 K fast neutron irradiation, dpi/dt as a function of irradiation induced resistivity was approximately linear for all samples. Saturation values of the resistivity and defect concentrations have been determined and are comparable with previous values for Cu and Nb. The isochronal recovery has been studied up to 579 K. Large effects of oxygen on the annealing behavior are observed in niobium and vanadium and annealing peaks are identified with oxygen migration near 460 K.

Saturation behavior of cascade damage production using fission fragment and ion irradiations

Abstract The saturation of electrical-resistivity changes, which were induced by various types of irradiations at liquid helium temperatures, was studied as a function of the primary recoil energy in copper and silver. For both copper and silver, the saturation resistivity was observed to decrease as the primary recoil spectrum was shifted to higher energies. For copper, the saturation resistivity varied from 6.5 nΩ · m for a 150-keV H + irradiation to 4.4 nΩ · m for a 600-keV Bi ++ irradiation. For silver, the saturation resistivity varied from 6.1 nΩ · m for a 150-keV H + irradiation to 4.7 nΩ · m for a 235 U fission-fragment irradiation. The saturation resistivity was also determined for 235 U fragment damage in aluminum, nickel, gold and vanadium.

Resistivity and Tc measurements in low temperature irradiated Nb3Sn and Nb3Ge

The electrical resistivity and superconducting transition temperature were measured in films of Nb3Sn and Nb3Ge after fast neutron irradiation at 20 K to 1.3 × 1018n/cm2 (E > 0.1 MeV) and fission fragment irradiation at 65 K up to an equivalent fast neutron dose of 5 × 1020n/cm2. The rate of change of ρ up to the saturated state fits a model for which the radiation induced damage is in the form of disorder. The changes in ρ have an exponential dependence with dose that agrees with earlier models of disordering in which the rate of change of disordering (proportional to ρ) is proportional to the degree of order. Annealing up to 425 K after the high dose irradiation did not result in a change in ρ, and Tc remained below the measurable temperature of 4 K. However, simultaneous recovery in Tc and ρ occurred between 280 and 500 K after the low dose experiment, indicating defect mobility at this temperature.

Neutron irradiation facilities at the intense pulsed neutron source

Abstract There are facilities for irradiation down to 4.2K with fast neutrons at Argonnes recently constructed Intense Pulsed Neutron Source (IPNS-I). The large irradiation volume, the neutron spectrum and flux, the ability to transfer samples without warm up, and the dedication of the facilities during the irradiation make this ideally suited for radiation damage studies on components for superconducting fusion magnets. Possible experiments are discussed on cyclic irradiation and annealing of stabilizers in a high magnetic field, mechanical tests on organic insulation irradiated at 4K, and superconductors measured in high fields after irradiation.

Damage saturation effects on volume and resistivity changes induced by fission-fragment irradiation of copper☆

Abstract Damage production and saturation has been monitored in copper by simultaneous electrical resistivity- and length-change measurements. Damage was introduced by 235U fission fragments at either 7 or 85 K. At both temperatures, the resistivity and length changes were linearly related to each other for resistivity changes less than 80% of saturation resistivity. The linear relationship was the same for both irradiation temperatures and was the same as that observed previously for 10B fission fragment irradiations at 4 K. These results are interpreted to show that the resistivity change per defect is unaffected by irradiation under conditions which lead to interstitial clustering.

Ordered alloys reveal fundamental mechanisms of neutron damage production

Abstract Ordered alloys have historically played an important role in investigating basic mechanisms of neutron irradiation damage. We have been particularly active in this field at Argonne National Laboratory, and in this paper we will review four different areas of our experimentation using ordered alloys. We will briefly review and critically re-examine our magnetic saturation measurements performed on Ni 3 Mn during neutron irradiations. Disordering during thermal-neutron bombardment at 5 K uniquely revealed the existence, number and average length of replacement collision sequences. Confirmation of our analysis by others will be presented; some refinements will be introduced; comparisons with recent molecular-dynamic calculations will be drawn; and the (n, γ) recoil spectrum in Ni 3 Mn will be reanalyzed. Overall, we conclude that 112 to 150 〈110〉 replacements occur, within one or two sequences, per (n, γ) recoil with an average energy of 492 eV. Similar measuremens on Ni 3 Mn during fast-neutron bombardment at 5 K yielded the total number of replacements per typical 30 keV cascade and demonstrated that very few interstitials were produced at large distances from the cascade by 〈110〉 replacement sequences. We will discuss our very recent results on high-resolution TEM measurements of disordered zones in fast-neutron irradiated Cu 3 Au. These results have been compared with damage calculations for the accurately measured fast-neutron flux and energy spectrum in CP5 and reveal the spatial characteristics of the deposited energy in individual cascades. Resistivity measurements of Cu 3 Au during fast-neutron bombardment at 150°C have revealed both the disordering process within cascades and ordering by irradiation-enhanced vacancy diffusion. These experiments provide a means of comparing different neutron energy spectra and studying irradiation-enhanced diffusion in a system amenable to theoretical treatments.

Measurements of neutron spectra and fluxes at spallation-neutron sources and their application to radiation effects research

Abstract We have measured the neutron spectra, fluxes, and flux distributions produced by nuclear spaliation resulting from 478 MeV proton bombardment of tantalum and depleted uranium targets surrounded by a thick lead neutron reflector. The configuration was chosen to simulate a radiation effects facility at a spallation-neutron source. The method of multiple foil activation with spectrum unfolding by the STAYSL computer code was used to measure the neutron spectra. The experimental results are compared in detail with the results of computer calculations on the same configuration of targets and reflector. The neutron production and transport codes HETC and VIM were employed in these calculations. The neutron spectra, and characteristic parameters for studies of radiation effects, are compared with those of fission-neutron sources. Based on the above measurements, the predicted performance and capabilities of the Radiation Effects Facility at the Intense Pulses Neutron Source at Argonne National Laboratory are discussed.

Critical current density changes in irradiated Nb3Sn

Soll recently modeled changes in the current-carrying capacity of superconducting Nb3Sn after irradiation. As the dose increases, the critical current density (Jc) generally increases, reaches a maximum, and decreases. The model relates the maximum Jc for different types of irradiations to the integrated damage energy (ED) that the irradiating particles transfer to the lattice. Earlier, Soll et al. related cirtical-temperature (Tc) decreases in irradiated Nb3Sn to Ed, which appears more reasonable since Tc is a measure of disorder (or replacements) accompanying defect production, and the final defect configuration (or displacements) are less important. The annealing temperature for the disorder (~700°C) exceeds any of the irradiation temperatures (TIRR); therefore, TIRR is unimportant in the Tc experiments. However, different defect structures exhibit considerably different flux pinning and the Jc model does not consider different spatial variations of the defects during production or migration and agglomeration of the defects during high-TIRR experiments, both of which affect flux pinning. The lack of the model to take into account the physics of the damage is the subject of this paper. Arguments are presented why the defect configurations should be considered, and recently published data is presented that conflict with the conclusions of this damage-energy model.

Cascade damage in nickel: Production, saturation and recovery☆

Abstract Changes in the electrical resistivity and length were simultaneously measured during the bombardment of nickel at 70 K with 235 U fission fragments and during a subsequent isothermal annealing program. The resistivity changes and length changes were linearly related during the damage production for damage concentrations less than 80% of the defect saturation limit. The initial ratio of the length change to the resistivity change was 8.4 × 10 4 /Ω m . At high damage concentrations, the length change increased at a slightly higher rate than the resistivity change. During isothermal annealing of the near-saturation defect concentration, the resistivity and the length changes had identical recoveries through stage I and most of stage II. During stage III annealing, the ratio of the length change to the resistivity change showed a small increase. At temperatures above 475 K, this ratio showed a marked increase as the resistivity change exhibited recovery without a corresponding recovery of the length change. These differences, which are attributed to the direct formation and growth of three-dimensional vacancy agglomerates, and results of transmission electron microscopy suggest that the sign of the third derivative of the resistivity with respect to fast-neutron fluence (the sense of curvature in the damage rate) is positive for materials in which cascades collapse to loops, and negative for materials in which three-dimensional vacancy agglomerates are stable.

Saturation volume changes and resistivity changes in nickel

Saturation defect concentrations generated by thermal neutron irradiation of /sup 235/U doped nickel at liquid helium temperature were measured by changes in electrical resistivity and volume. The experimental procedure is described. (GHT)