Chunk of Graphite in a Co-60 field and Slab of Paper
in an Electron Beam: Typical Applications of Atomic
Physics
P. M. Bergstrom, Jr.
Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899-8460
Abstract. When the atomic physicist measures or computes, applications of their results may not be immediately
obvious. However, atomic data form the basis of many mature and developing applications. Two of these applications
and their dependence on data are reviewed here. In particular, the national standard for radiation exposure and the
processing of the United States mail with radiation are discussed.
INTRODUCTION
COMPUTATIONAL TOOLS
Radiation physics has traditionally been an area
with extensive needs for data on physical interactions.
Information on photon and electron (or positron)
collisions, particularly at high energies, are the most
useful atomic data in radiation physics. These data
have been applied to typical radiation physics
applications such as shielding, dose deposition and
imaging.
Simulation of particle transport by the Monte Carlo
method is increasingly the tool of choice for radiation
physics applications. This method has been in use in a
variety of fields for a number of decades, often
restricted to the able hands of specialists. Among the
reasons that it has found increasing use in recent years
are the gains in computer power available to the
average researcher. CPU power has progressed enough
to allow such researchers to run the standard transport
codes on desktop computers, without fear of
consuming the expensive clock cycles of
supercomputers while on the learning curve. These
same desktop computers can also be used to provide
precise quantitative results at the end of the learning
curve.
The atomic data used in radiation physics has
evolved substantially over time. Initially, problems
were solved to understand gross details of overall
system behavior and cross sections derived in Born or
simpler approximations were often adequate. As
atomic physicists developed more extensive data sets,
incorporating more details of atomic structure,
radiation physicists were able to quantify systems with
increasing confidence. Now, the direct simulation of
radiation physics problems using Monte Carlo
transport codes has lead not only to the expectation of
better quantitative results in old applications, but also
to an opening of new lines of investigation beyond
those where analytic results are easily achieved. These
modern applications often require the use of more
differential quantities and can serve as a driving force
for the development of better atomic physics data.
Monte Carlo transport is a direct simulation of
particle trajectories as they traverse a system of
interest, sampling the applicable physical laws that
help to determine the particle’s interactions. Figure 1
illustrates the basic features of a Monte Carlo transport
code. In general, there is a source which emits the
particles for the simulation, a description of the system
of interest in terms of its geometry and material
composition and some detectors or tallies which
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determine which interaction should be simulated.
Differential cross sections, used to determine the postcollision properties of particles, need only be known in
a relative sense. That is, the shapes of the cross
sections are more important than their magnitude. In
some cases, even the cross section shapes are not wellknown and simple rules of thumb are applied to
determine secondary particle properties.
provide the results of the simulation. In certain cases,
such as simulations of heating of radioactive waste, all
three portions of the calculation may take place in the
same part of the system.
Assuming that one knows the structure and
composition of the system of interest, the ability to
achieve a satisfactory result from a simulation depends
on the computer time available and on the level of
knowledge of the interactions of particles with matter.
The applications of radiation physics which make
contact with atomic physics are concerned primarily
with the transport of photons, electrons and positrons.
The National Institute of Standards and Technology
(NIST) develops and maintains perhaps the most
widely used databases of interaction properties for
these particles.1,2
Emit
Transport
Simulations of electron and positron transport
differ considerably from those for photon transport.
The main reason is that these particles typically
interact many more times in traversing a medium than
do photons. In order to save computational effort, the
effects of many collisions are condensed into a single
pseudo-collision. The distance to and properties after
this pseudo-collision are determined by collisional and
radiative stopping powers and by multiple-scattering
distributions. These quantities are based on cross
sections for inelastic scattering and bremsstrahlung
and for elastic scattering respectively. Usually, the
stopping powers are applied over a restricted range of
energy transfers. The reason for this is to allow for the
production of energetic secondary particles that do not
deposit their energy locally. Consequently, absolute
total cross sections for the (e,2e), (e+,e+e) and the
bremsstrahlung processes are needed as are relative
differential cross sections to determine the postcollision properties of the particles.
Score
There are a number of Monte Carlo transport codes
that are available to perform coupled photon-electron
(positron) simulations. The MCNP codes3,4 from Los
Alamos National Laboratory can simulate transport in
fairly arbitrary geometries. Other widely-used codes
with similar capabilities are the EGS code5,6 from
Stanford Linear Accelerator Center and from the
National Research Council of Canada (NRCC) and the
ITS code7 from Sandia National Laboratories.
Another recent code, developed at the University of
Barcelona, is PENELOPE.8 These codes all differ in
some details such as ease of use, databases and
methods for performing charged particle transport.
Figure 1. Overview of Monte Carlo transport calculations.
In the case of photon transport, the only
information about interactions that is needed in an
absolute sense is the sum of the total cross sections for
all photon interactions with the underlying medium at
the energy of the transported particle. This cross
section must be absolute as it is compared with the
physical dimensions of the system of interest in order
to determine where interactions occur. Once it has
been determined that an interaction should occur, the
branching ratios of the possible interactions are used to
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calculations at NIST and are in the process of
incorporating them into the National Standard.
RADIATION PHYSICS APPLICATIONS
We are also in the initial stages of performing
systematic calculations to model our sources. As
mentioned above, the available information on these
sources is incomplete. Unfortunately, the high dose
rate associated with these sources precludes their
detailed examination. However, the information at
hand, in conjunction with sensitivity studies, does
permit us to at least attempt to make realistic models.
National Standards
NIST’s Radiation Interactions and Dosimetry
Group maintains the nation’s standards for the SI Unit
Gray and for radiation dosimetry quantities such as
absorbed dose to water and exposure. These standards
are checked through intercomparisons with similar
groups at national measurement laboratories in other
countries and are transferred to industrial and medical
concerns in the United States.
In Figure 2, we show the apparatus utilized to
realize the national standard for exposure. This
apparatus consists of a Cobalt-60 source and a series
of graphite-walled ionization chambers. The Co-60
source is a several-decades old, modified Theratron
radiotherapy head. The ion chambers were developed
at NIST (then the National Bureau of Standards) in the
1960’s.
Recall that Monte Carlo simulations require
adequate knowledge of the geometric structure of the
system of interest and it’s composition. Unfortunately,
while such details are available for the NIST
ionization chambers, these data are not complete for
the source itself (or indeed for any of the similar
sources that NIST has available). This lack of detail,
coupled with the time-consuming nature of direct
simulations has forced the use of indirect experimental
procedures, idealizations and approximate theories in
order to relate measured quantities to the national
standard.
Figure 2.
Vertical Co-60 beam and graphite-walled
ionization chambers used in standards work at NIST.
Irradiation of the United States Mail
In October of 2001, first class letters containing
anthrax were sent through the United States mail to
members of the media and of Congress. Several deaths
resulted and a number of buildings were closed for
decontamination. A large backlog of potentiallycontaminated mail existed at the Brentwood handling
facility. NIST’s Ionizing Radiation Division (IRD)
was asked to help spearhead an effort, with the Postal
Service (USPS) and with the Armed Forces
Radiobiological Research Institute (AFFRI), to
determine how to decontaminate this mail. The reason
why the IRD was asked to assume this role was it’s
experience in and relationship with the industries that
process materials for sterilization or other purposes by
radiation.
In some cases, it is now understood that these
procedures lead to errors that, while fairly small in an
absolute sense, can be remedied by relative results of
different Monte Carlo runs. One of these problem
areas is the determination of the so-called wall
correction for the ionization chambers. The
relationship between the measured current and
exposure (or air kerma) is determined by use of
Spencer and Attix’s version9 of Bragg-Gray cavity
theory.10 The theory makes a number of idealizations
which do not hold in practice and which are modified
by small correction factors. The wall correction
accounts for attenuation and scatter in the wall of the
ionization chamber and has usually been determined
by an experimental extrapolation to zero wall
thickness.11 Some time ago, researchers at the NRCC
pointed out that this procedure lead to flawed (at the
sub 1% level) results and recommended certain Monte
Carlo simulations that could produce more accurate
values for the correction.12 We have performed such
There are a number of possibilities for processing
materials by radiation. Electrons and photons from
accelerators and photons from radioactive sources are
routinely used for such tasks. The latter alternative was
ruled out by the USPS in the event that routine
processing would need to be implemented at USPS
mail-processing centers. NIST’s IRD then applied a
coupled computational-experimental approach to
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evaluate the potential ways to process the
contaminated mail at the commercial facilities which
had been leased for that purpose.
REFERENCES
1. Berger, M. J. and Hubbell, J. H., XCOM: Photon Cross
Sections on a Personal Computer, National Bureau of
Standards Report NSBIR 87-3597, Gaithersburg, MD
20899 (1987).
In the case of mail processing, the configuration of
the source is well-known and is readily described in a
direct simulation. The unknowns in the process are
the contents of the mail. Hence, initial considerations
focused on how to sort and package the mail in order
to assure proper delivery of radiation. Monte Carlo
simulations were used to determine, in conjunction
with measurement, when to use photons and when to
use electrons, the energy of the incident particles, the
shielding effects of various objects, the packaging
density and even the conveyor belt speed.
2. Berger, M. J., Inokuti, M., Anderson, H. H., Bichsel, H.,
Dennis, J. A., Powers, D. Seltzer, S. M. and Turner, J. E.,
Stopping Powers for Electrons and Positrons,
International Commission on Radiation Units and
Measurements Report 37, Bethesda, MD, (1984).
3. Briesmeister, J. F., MCNPTM- A General Monte Carlo NParticle Transport Code, Los Alamos National
Laboratory Report LA-13709-M, Los Alamos, NM
(2000).
Figure 3 shows the result of one such model study.
In this figure, the dose deposited in a uniform stack of
letters by two opposing beams of electrons is
simulated. The object was to flatten the dose profile
enough to assure that the mail stayed within the basic
guidelines for the process. These guidelines were that
the dose had to be high enough to decontaminate the
mail and be low enough to remain below the threshold
where the mail would suffer damage, e.g. ignition.
4. Waters, Laurie S., MCNPXTMUser’s Manual, Los
Alamos National Laboratory Report LA-UR-02-2607,
Los Alamos, NM (2002).
5. Nelson, Walter R., Hirayama, Hideo and Rogers. David
W. O., The EGS4 Code System, SLAC-Report-265,
Stanford, CA (1985).
6. Kawrakow, I., Med. Phys. 27, 485-498 (2000).
7. Halbleib, John A., Kensek, Ronald P., Valdez, Greg D.,
Seltzer, Stephen M., and Berger, Martin J., IEEE Trans.
Nucl. Sci. 39, 1025-1030 (1992).
Deposited Dose
8. Baro, J., Sempau, J. Fernandez-Varea, J. M. and Salvat,
F., Nucl. Inst. Meth. B 100, 31-46 (1995).
0.4
0.35
9. Spencer, L. V. and Attix, F. H., Rad. Res. 3, 239-254
(1955).
0.3
Dose
0.25
10. Gray, L. H., Proc. Roy. Soc. A 156, 578-596 (1936).
0.2
11. Loftus, T. P. and Weaver, J. T., J. Res. NBS. A 78, 465476 (1974).
0.15
0.1
12. Bielajew, A. F., Med. Phys. 17, 583-587 (1990).
0.05
0
0
2
4
6
8
10
12
Depth [cm]
Figure 3. Calculation of relative dose versus depth in paper
for for opposing beams of 7 MeV electrons.
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