UCRL-PROP-220207

**Radiation Effects**

**March 24, 2006**

**Abstract**

The assessment of radiation effects for system survivability supports
NNSA's Stockpile Stewardship Program. Regimes of interest include
ultra-high total dose and dose rate environments as well as ultra-low
dose rate environments due to nuclear bursts, natural and enhanced space
environments, or proximity to special nuclear materials. Multi-coupled
physics must be simulated, including particle radiation transport, and
the material, mechanical, and electrical response to radiation.

**Introduction**

For radiation effects, the ASC Program needs robust computational
methods, suitable for massively parallel computing platforms, to predict
radiation environments, as well as the material, mechanical and
electrical response to radiation. The physics that must be simulated
includes particle radiation transport; electromagnetic effects from
x-rays that generate pulsed electron currents internal to systems;
electronic radiation effects from x-rays, electrons, neutrons (including
neutron activation and gamma-rays from activated materials), and
high-energy heavy ions; effect of radiation on optoelectronics,
nanoelectronics, and MicroElectroMechanical Systems (MEMS); and
mechanical shock due to rapid thermal deposition, impulse from
vaporization (blow-off), and blast, and material damage by reactive
plasma interactions with particulates and surfaces, polymer
decomposition, and fissile morphological changes.

**Radiation Transport**

Modeling of particle radiation transport is fundamental for all types of
radiation effects predictions. Steady-state and time-dependent methods,
both deterministic and Monte Carlo, are needed for the transport of
neutrons, gamma-rays, electrons, and ions \[1\]-\[4\]. For radiation
effects analysis, various output is needed from a transport calculation,
including dose and dose enhancement \[5\], charge deposition, and
electron emission currents. Both linear transport and non-linear
transport (e.g. radiation/hydrodynamics) and radiation/electromagnetics)
are required.

Further research in radiation transport for radiation effects is needed
in many areas. Some examples include:

-   Improved deterministic transport algorithms that are very efficient
    for massively parallel machines, particularly for very large
    unstructured meshes;

-   Improved automatic biasing techniques for coupled electron-photon
    Monte Carlo transport, particularly for deep penetration and/or
    secondary particle production;

-   Improved Monte Carlo transport algorithms for faster computational
    analysis of geometries constructed by Computer Aided Design (CAD)
    software;

-   Radiation transport techniques suitable for use at small feature
    sizes (nanometers to microns), and validated physical models for
    x-ray and electron interactions at lower energies of 100 eV to 10
    keV.

**Electromagnetics**

Nuclear bursts are known to give rise to a variety of electromagnetic
pulse effects (EMP) that can damage electrical components. Of interest
to the survivability of NNSA systems is system-generated electromagnetic
pulse or SGEMP. This phenomenon is induced by the electron currents that
are internally generated within a system by the pulse of x-rays from a
nuclear detonation. To simulate SGEMP, advanced modeling techniques are
needed for full-wave, time-domain electromagnetics (EM) using hybrid
meshes; charged particle-in-cell transport fully coupled to EM; magneto-
and electrostatics on unstructured grids; and full-wave,
frequency-domain EM using hybrid boundary-element (BE) and finite
element methods. For SGEMP within system cavities, validated models have
been developed for the high (atmospheric) and low pressure regimes,
where the mean free path of electrons is significantly smaller or larger
than the physical dimensions of the problem, and the model
approximations work reasonably well. Further development of advanced
computational electromagnetic models are needed for:

-   SGEMP within cavities at intermediate pressure gas pressures;

-   High voltage breakdown induced by the changes in conductivity
    induced by radiation, a phenomenon that is becoming increasingly
    important as electrical components are miniaturized.

**Electronics and Sensors**

The performance of transistors can be degraded by a variety of
environmental effects, including radiation damage from both x-rays and
neutrons. The radiation effects themselves also consist of a variety of
mechanisms including displacement damage, single event effects, and
total ionizing dose (TID). In some instances, the exposed system is
intended for a significant lifetime within a mixed radiation field. For
instance, electrical designs may introduce intrinsic (onboard) sensing
capabilities into the systems and devices themselves. These additional
circuit elements and sensors can allow the system to react to the
encountered radiation environment \[11\]-\[14\]. Advanced computational
models are needed to predict the effect of radiation on optoelectronics,
nanoelectronics, MEMS, and sensors.

Initial research must concentrate on the fundamental circuit elements
(e.g., transistors, memory cells, etc). Once the proof-of-concept is
demonstrated for the diagnostic modules of the basic constructs, more
advanced configurations should be investigated. In particular, the
complex devices will require novel approaches to monitoring and
diagnostics, and represent a greater challenge. For instance, although
most researches would acknowledge that the mechanical portion of a MEMS
device is typically less susceptible to TID, studies have shown that the
electric field of some capacitive-type MEMS accelerometers can be
altered by radiation-induced charge buildup, which in turn affected the
movable sensor mass.

**Material Effects: Electronic Excitations**

Research on radiation effects in materials have in the past mainly dealt
with the damage produced by nuclear or atomic collisions in which energy
is directly imparted on atoms resulting in their displacement from
regular crystal lattice positions. This displacement damage leaves
behind crystal lattice defects that subsequently diffuse and aggregate
into larger defects, causing changes in the properties of the irradiated
materials. Much less attention has been paid to radiation effects in
which the primary or initial radiation damage is in electronic
excitations.

Radiation effects associated with electronic excitations can be
separated according to two categories of excitations: widespread, but
singular electronic excitations in dilute systems, and concentrated,
collective excitations in condensed matter. In the first category fall
low radiation environments as encountered in atmospheric and in
chemically reactive plasmas. The second category involves passage of
very high energy, heavy ions that create showers of secondary electrons
and tracks of charged and electronically excited ions in the material.

We want to solicit research on radiation damage to materials exposed to
such radiation environments utilizing advanced computational methods.
Two possible areas of research are mentioned below as examples to
inspire other ideas and proposals.

1)  Reactive plasma interactions with particulates and surfaces
    \[6\],\[7\]

> Molecular ions and radicals produced in irradiated gases and in
> plasmas undergo many chemical reactions both in the gas phase and on
> surfaces. In this manner, new materials can form as particulates and
> as surface films. In addition, air-borne particulates and pathogens
> can be processed in reactive plasmas for diagnostic purposes or for
> removal. The entire chain of events and reactions, from the generation
> of the molecular ions or radicals, to the formation and growth of
> surface deposits, requires innovative approaches to combine
> computational methods for the various time and length scales.

2)  Electronic Radiation Effects produced by High-Energy, Heavy Ions
    \[8\],\[9\],\[10\]

> The passage of high-energy ions through matter creates an ion track
> and an intense shower of secondary electrons. Subsequent charge
> recombination and dissipation of deposited energy can result in
> material ejection and in structural changes within the ion track. Even
> large macroscopic deformation can be induced in amorphous materials
> while exposed to penetrating beams of heavy ions. These radiation
> effects are collective phenomena of the exposed material driven
> temporarily into extreme states of non-equilibrium. New theoretical
> approaches and computational methods must be developed to describe
> such extreme states of matter.

**Material Effects: Polymers and Fissile Materials**

When basic material degradation is at issue, such as polymers and
fissile material exposed to radiation and other stress fields, we must
take a modified approach in which the material, or a sample of it, is
subjected to the operational stresses and its characteristics monitored
for changes \[15\]. This requirement alone necessitates the use of MEMS,
whose performance could be degraded by radiation. A systematic method to
understanding the radiation effects on polymers and fissile materials
would require:

-   Develop understanding of rate controlling processes for different
    radiation exposures;

-   Create mathematical models of processes since standard chemical
    kinetics analysis methods are insufficient.

-   Develop decomposition kinetics and gas generation models and
    equations of state;

-   Improve understanding of fundamental decomposition chemistry, and ab
    initio quantum chemistry calculations of transition states;

-   Develop methods to [rapidly]{.underline} postulate and test
    hypothetical models, in particular for transport and structural
    integrity model and computation of material response; and

-   Perform validation experiments.

To assure success, we need a combined theory and experimental approach
to study the material response to various scenarios causing the
decomposition of polymer and fissile components. This approach will be
based on a combination of theory, constitutive model development,
analysis, and experimentation. The state of knowledge in these areas can
only improve by this combined approach, which can affect a predictive
capability and, if done correctly, with the capability to quantify
uncertainties that pervade every step of these fields, from formation of
theory, constitutive model development, analysis, design and execution
of experiments, and interpretation and reduction of data.

Len Lorence, Sandia National Laboratories, ljloren@sandia.gov

Sharif Heger, Los Alamos National Laboratory, heger@lanl.gov

Bill Wolfer, Lawrence Livermore National Laboratory, wolfer1@llnl.gov

**Acknowledgement**

This work was, in part, performed by the Lawrence Livermore National
Laboratory, Los Alamos National Laboratory, and Sandia National
Laboratories under auspices of the U.S. Department of Energy.

***References***

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\[10\] Papers in Nucl. Instr. and Meth. B **209** (2003)

\[11\] K.E. Holbert, "Predictive Sensors for Radiation Effects," White
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\[12\] K.E. Holbert, S. Sankaranarayanan, S.S. McCready, D.R. Spearing,
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\[15\] R. Behrens, "Thermal Decomposition Processes in PBX9501, HMX,
LX04 & RDX," Sandia National Laboratories, Combustion Research Facility,
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This work was performed under the auspices of the U.S. Department of
Energy by University of California, Lawrence Livermore National
Laboratory under Contract W-7405-Eng-48.