Radiation Damage to Materials

All aspects of radiation damage to materials are of interest in our research. While there is an interest in designing radiation hard materials, such as materials that are resistant to embrittlement for use in reactor pressure vessels, there is also an interest in designing radiation soft materials for use as sensitive diagnostics or for use in validation experiments. Radiation damage studies require us to model the evolution of radiation damage beyond the primary defects, i.e., to study defect evolution. In support of this we look at using defect-specific diagnostics. We also look at using radiation exposure at cryogenic and elevated temperatures in order to validate our modeling of material response.

Contact: P.J. Griffin

Publications

Dose Enhancement in Radiation Testing

Dose enhancement effects from material interfaces with different atomic number elements can affect ionization and the resulting material response. Scattered photon contributions with the higher photoelectric cross section from low energy photons can significantly affect the dose at a point near an interface. This can be an important effect in modern semiconductors with high-κ dielectrics, such as hafnium, and metal vias. While we use coupled electron photon radiation transport to model this effect, we also need to validate our modeling. In our experiments we often design test objects so as to minimize the scattered photon content and reduce its influence on our validation measurements.

Contact: Russell DePriest

Publications

Interface of Fission Product Delayed n/γ Environments with Nuclear Effects Codes

Since a research reactor radiation environment has both prompt and delayed fission components for the neutron and photon fields and we are often investigating a transient response of a material in a radiation environment, we are faced with having to use diagnostic measurements to deconvolute the radiation environment based on the mixed neutron/photon response time-dependent response from PCDs and PINs and the time-integrated measurements from activation foils and calorimeters. We need to model the fission product radiation environment in order to support this deconvolution. We also need accurate estimates of the uncertainty in our sensors, and these are only provided though validation experiments and an examination of the consistency of different measurements.

Contact: Matt Sternat

Publications

  • M. Sternat, “Advanced debris neutron and gamma environment characterization using CINDER”, SAND2018-7573J, 2018
  • M. Sternat, et al., “NuGET Debris Neutron and Gamma Environment Characterization Using CINDER”, SAND2018-3485C, 2018
  • M. Sternat, et al., “Debris neutron and gamma emission rates and spectra using CINDER/ENDF-B/VII Data”, SAND2018-3487, 2018
  • A. Salazar, et al., “Comparing debris radiation source term characterization methods between legacy codes and high fidelity, modern calculations using CINDER”, SAND2020-5006J, 2020

Development of Low Neutron Response Dosimeters for Use in Research Reactors, e.g., EPR-based Teflon and Polyethylene Dosimeters

As we investigate the neutron response of the dosimeters we use in the mixed neutron/photon reactor environments that support our radiation testing, we are active in developing new dosimeters that minimize their neutron response. Rather than trying to compensate for the neutron contribution, we have looked at developing a dosimeter that is intrinsically more sensitive to photons than neutrons. Towards this end, we look to measurements that are sensitive to ionizing dose rather than displacement. Alanine and radiochromic film are not good candidates since, due to the hydrogen content, about half of the ionizing energy in a research reactor environment can come from the neutrons. We have been doing research on sensors that are sensitive to ionizing dose but are composed of higher atomic weight materials, such as Teflon, so as to minimize the neutron ionizing dose contribution. Silicon calorimeters are also an important sensor for use in reactor environments. Because of the delayed photon radiation component in a research reactor environment, in calorimeters we look for materials and designs that can provide a high thermal conductivity and where we can attempt to deconvolute the temporal response from prompt and delayed radiation components. One thrust has been to look at the use of calorimeters where the neutron-induced secondary gamma contribution is not significant, e.g. bismuth.

Contact: David Vehar

Publications

Characterization of Neutron Response of Reactor Ionizing Dose Dosimeters

CaF2:Mn TLD thermoluminescence response, alanine EPR signal, and radiochromic film response are all used to characterize the ionizing dose in radiation environments. When these dosimeters are used in mixed neutron/photon environments, it can be hard to capture the difference in response due to the neutrons and photon energy deposition. The literature details the use of track structure calculations to capture the dependence of the response on the density of the ionization, i.e., the electron-hole generation. Sandia is interested in using track structure theory to better characterize the dosimeters used in its reactor fields to characterize the photon exposure, i.e., to apply proper corrections for the neutron response of the dosimeters.

Contact: David Vehar

Publications

Characterization of Neutron Benchmark Fields Used to Validate Nuclear Data

We use nuclear data, e.g., dosimetry cross sections, to support the characterization of the neutron environment in our radiation test facilities. We also use neutron benchmark fields to validate the dosimetry cross sections. We have provided detailed characterization data for the neutron environments in various fields produced, using spectrum-modifying “buckets”, at our Annular Core Research Reactor (ACRR). We also work on extending spectrum unfolding and adjustment codes, such as the SAND-II iterative unfold code and the LSL least squares code, to interface with the latest dosimetry cross sections and to produce high fidelity uncertainties in the resulting neutron spectrum. We have research efforts that are exploring new spectrum determination methods, such as the use of genetic algorithm, and to identify and validate addition cross sections that are useful in dosimetry applications. Because of the lack of high-fidelity dosimetry cross sections with differentiating structure in the 0.1 – 2.5 MeV energy region, we have explored the use of silicon transistors and GaAs LEDs as dosimetry sensors and interfaced these responses with our spectrum determination codes. Due to the current lack of high intensity 252Cf(s.f.) fields, we are exploring new ways to calibrate dosimetry reactions, such as 32S(n,p)32P, where the primary measurement is of a beta activity rather than a gamma spectrum.

Contact: Patrick Griffin

Websites

Publications

Defect Evolution and Neutron Fluence Rate Effects from Neutron Exposure

Our interest in nuclear data extends from the nuclear interaction itself into the results from the subsequent electronic interactions of the recoil products. Thus, we have research efforts that investigate the evolution of more complex defects from the initial recoil atoms using molecular dynamics (MD) and density functional theory (DFT). Much of our research focuses on electronic materials, such as silicon, GaAs, and GaN, but we also look at radiation embrittlement of structure materials, such as the iron in light water reactor pressure vessels. We examine, from a theoretical and experimental perspective, the equivalence of ion irradiations and neutron irradiations and support standards development by ASTM E10.08 – Procedures for Neutron Radiation Damage Simulation. Our interest extends beyond the time regimes explored by MD into modeling that used kinetic Monte Carlo kMC) approaches and mean field rate theory (MFRT).

Contact: Edward Bielejec

Publications

Validation of Thermal Neutron Silicon Displacement Damage Metrics

We try to validate our modeling using experiments under a wide range of neutron/photon radiation conditions. One aspect of particular interest is material damage from thermal neutrons. There have been several reports of silicon displacement damage from thermal neutrons not following the normal damage metrics, i.e., large capture kerma values in many materials and less displacement damage for thermal neutrons environments than predicted by the Frenkel pair production metric. In response to these observed discrepancies, research efforts are underway to: a) improve the modeling of the capture kerma calculations based on nuclear data models; b) validate, quantify, and document the thermal neutron displacement discrepancy in large volume sensitive silicon transistors; and c) use defect-specific DLTS techniques to look for aid in understanding the source of the model discrepancy.

Contact: William Charlton

Combined Environment Radiation Effects (Ionizing Dose and Displacement Damage)

Our previous research has look at radiation damage to semiconductors caused by both ionization and by displacement. Since some devices can be sensitive to both damage processes, we are actively looking at combined radiation effects and establishing models to quantify any synergistic effects. We have developed a set of definitions to help characterize and quantify the combined effects and we are gathering damage data in lateral bipolar transistors, optocouplers, and operational amplifiers.

Contact: Michael Gregson

Improved/Expanded Nuclear Data Processing with NJOY-2016

Improved/Expanded Nuclear Data Processing with NJOY-2016 In support of our application of nuclear data to mission areas, we use and build upon nuclear data processing codes such as NJOY-2016, FRENDY, and LISTEF. As one example of this, Sandia has tailored a version of the LANL-developed NJOY-2016 code to: a) permit user control of the form of the damage partition function used in determining the displacement damage; b) enable use of a user-specified recoil energy-dependent efficiency function; c) permit user control of the displacement threshold formalism, e.g., implementation of the Norgett-Robinson-Torrens (NRT) threshold treatment rather than the default sharp threshold Kinchin-Pease treatment; d) enable use of an arcdpa recoil energy-dependent efficiency function; e) suppress, for analysis purposes, the contribution of low atomic weight particles, e.g., protons and alpha particles, to the displacement damage; f) streamline the conversion of covariance data from the ERRORR module into the LSL or BOXER format; g) interface with codes to provide the burst generation rate (BGR) and LET spectra used to support nSEE damage metrics; and h) make corrections to how nuclear data is used within NJOY-2016 in producing the (n,γ) capture kerma.

Contact: Patrick Griffin

Project Scope:

  • Add NRT damage energy to complement the baseline sharp threshold Kinchin-Pease damage energy (complete)
  • Correct issues with the treatment of the capture reaction damage metrics (in process)
  • Add arcdpa efficiency function for damage metrics (in process)
  • Add user-defined damage partition function to complement the baseline Robinson analytic expression for the damage partition function (in process)