PROTON AND HEAVY ION ACCELERATOR FACILITIES FOR SPACE RADIATION RESEARCH
Jack Miller
Lawrence Berkeley Laboratory, Berkeley, CA
ABSTRACT
The particles and energies commonly used for medium energy
nuclear physics and heavy charged particle radiobiology and
radiotherapy at particle accelerators are in the charge and energy
range of greatest interest for space radiation health. In this
article we survey some of the particle accelerator facilities in the
United States and around the world that are being used for space
radiation health and related research, and illustrate some of their
capabilities with discussions of selected accelerator experiments
applicable to the human exploration of space.
____________________________________________________
INTRODUCTION
Protons emitted during solar particle events (SPE) and
protons and heavier ions in the galactic cosmic radiation
(GCR), present risks to the health of crewmembers in low
Earth orbit and beyond (National Research Council, 1996;
National Research Council, 2000). The transient but
intense flux of protons in a SPE can produce acute
radiation effects, whereas GCR exposures are chronic and
may produce adverse health effects later in life.
High energy atomic nuclei (HZE) comprise only a
small fraction of the charged particles in the GCR, but,
since the radiation dose per particle is roughly proportional
to the square of the particle charge, even relatively small
numbers of “heavy” ions (charge, Z, greater than 1) can
contribute significantly to the radiation dose and dose
equivalent over time. This is especially the case outside the
protective effects of Earth’s magnetic field and at the high
orbital inclination of the International Space Station, where
the geomagnetic field strength is relatively low.
In recent years high energy heavy charged particles
have become a standard tool for radiation biologists
(Blakely and Kronenberg, 1998), and radiobiology and
biophysics research is being conducted or is planned at a
number of proton and heavy ion accelerators around the
world. The basic science is reviewed in depth elsewhere
(Cucinotta et al., 2002; Nelson, 2003), but some
illustrative examples of the biological and physical
research being done at accelerators and its relevance for
space radiation health are given below. This research is
international in character, with many of the ISS partner
nations represented.
Protons will be considered separately from ions with
Z >1, as they are in many respects distinct from one
another in both the health risks and experimental
challenges they present. However it should be noted that
there is also a good deal of overlap both in the
experimental facilities and methods and in the science.
____________________
This review is not intended to be exhaustive; however the
facilities and research presented here are representative of
the role of charged particle accelerators in space radiation
health research. (For a recent extensive compendium of
space radiation research, see Cirio et al,. 2001.)
ADVANTAGES AND LIMITATIONS OF
ACCELERATORS FOR SPACE RADIATION
RESEARCH
Advantages
The energy range of greatest interest for space radiation
applications (roughly 100–2000 MeV/nucleon) is
fortuitously comparable to what has been available for
many years at proton and heavy ion accelerators, as shown
in Figure 1. Until about the mid-1970’s, accelerator
* Correspondence to: Jack Miller, Ph.D
MS 74-197, Lawrence Berkeley National Laboratory
Berkeley, CA 94720 USA
Email: [email protected]
Phone: 510-486-7130; Fax: 510-486-7934
Figure 1. Flux of selected nuclei in the galactic cosmic radiation
as a function of kinetic energy per nucleon. The dashed lines
denote the approximate energy range of present heavy ion particle
accelerators. (Original plot from Simpson, 1983.)
Gravitational and Space Biology Bulletin 16(2) June 2003
19
J. Miller — Proton and Heavy Ion Accelerator Facilities
experiments were driven primarily by the interests of the
nuclear and high energy physics communities. Heavy
charged particles lose most of their energy in a short
distance near the end of their range, and over the past
several decades, proton and heavy ion irradiation has
become an established treatment for some localized
cancers and for other maladies (Chu, 1999). Existing
accelerators have been adapted for radiotherapy and
radiation biology, and dedicated facilities have been built,
are under construction, or are planned. In at least one
case, the Heavy Ion Medical Accelerator at Chiba, Japan
(HIMAC), the traditional pattern has been reversed, and
an accelerator designed and built specifically for hadron
therapy has been increasingly used for basic science.
Many of the experimental and theoretical methods and the
accelerators developed for use in proton and heavy ion
physics are directly applicable to radiobiology and
radiotherapy, and it has become clear that research in
those fields is critical to understanding health risks from
space radiation.
Accelerators can produce well-characterized, almost
mono-energetic beams, so that data can be gathered
rapidly under controlled conditions for specific particles
and energies, and well-defined materials and
configurations. This is useful both for gathering basic
physics and biology data and for calibrating flight
instruments such as dosimeters. Ground-based
experiments have many fewer constraints on size, power
and complexity than do flight experiments, and, while
access to accelerators is limited, it is much easier to
obtain than access to space.
Limitations and the Role of Models
It is not practical at an accelerator to replicate the mixed
radiation fields found in space. Operational spacecraft
have complex internal structures that are difficult to
replicate on the ground, and new structural and shielding
materials are constantly being proposed. These
complications have necessitated the development of
models (see, e.g., Wilson et al., 1991) to simulate the
effects of the space radiation environment under realistic
flight conditions. Models and accelerator-based
measurements therefore complement one another, and
physics measurements and models are being coupled with
biology data and models to make possible comprehensive
theoretical analysis of space radiation risk and risk
mitigation. The goal is for the models to be used routinely
to identify materials with desirable radiation transmission
and radioprotective characteristics and to rapidly and
inexpensively evaluate many different materials and
configurations, the most promising of which can then be
tested at accelerators and, ultimately, in flight.
Protons
Protons have been one of the particles of choice
throughout the more than 70-year history of accelerated
particles as a tool of high energy physics—indeed, “high
energy” has generally been defined by the energy of the
most powerful proton accelerator available at a given
time. The effectiveness of protons as a treatment for
20
Gravitational and Space Biology Bulletin 16(2) June 2003
cancer was recognized many years ago (Wilson, 1946),
and a great deal of data on proton interactions in tissue are
available. Two excellent reviews of modern proton
radiobiology and radiotherapy are Raju, 1995, and
Goitein et al., 2002. Protons are the most common heavy
particle in the trapped radiation belts, the GCR and SPE,
and the energies of greatest interest for space radiation
health are from tens to hundreds of MeV, corresponding
to a range of several centimeters in tissue. These were
high energies in the 1930’s and 1940’s, but today they are
available at accelerators that are small enough to be
located in hospitals, and there are facilities in North
America, Asia, Europe and Africa. Many of these
facilities are suitable for biology, physics and biophysics
research, and some of them are already being used for
space radiation health research.1 Table 1 lists some of the
principal facilities for research with protons exclusively.2
Table 1. Proton-only Accelerator Research Facilities
Facility
Brookhaven National Laboratory Linear Accelerator
Brookhaven, NY USA
Emax
(MeV)
200
Crocker Nuclear Laboratory Cyclotron
University of California
Davis, California, USA
70
Loma Linda Proton Treatment Center
Loma Linda University Medical Center
Loma Linda, California USA)
250
iThemba Laboratory for Accelerator-Based Sciences
Medical Radiation Group
Capetown, South Africa
200
Midwest Proton Radiotherapy Institute
Indiana University Cyclotron Facility
Bloomington, Indiana, USA
210
Northeast Proton Therapy Center
Massachusetts General Hospital
Boston, Massachusetts, USA
230
Paul Scherrer Institut Proton Therapy Facility
Villigen, Switzerland
270
Proton Medical Research Center
University of Tsukuba
Tsukuba, Japan
500
1
Proton accelerators are also commonly used to investigate the
role of space radiation on single event upsets (SEU’s) in
microelectronics, but we will not discuss that here.
2
The Internet is a good source of up-to-date information on this
rapidly developing field. Three good websites, among many,
are http://sungr3.iss.infn.it/toptera/hadthe.htm (TERA
Foundation, Italy), http://medrad.nac.ac.za (iThemba Labs,
Medical Radiation Group, South Africa) and
http://www.iucf.indiana.edu/MPRI/links_pt.html (Midwest
Proton Radiotherapy Institute, Indiana University)
J. Miller — Proton and Heavy Ion Accelerator Facilities
Of particular interest to NASA investigators is the
Loma Linda University Proton Therapy Center
(LLUPTC), with a synchrotron which accelerates protons
to energies between 40 and 250 MeV, principally for the
treatment of prostate cancer. When not in use for patient
treatments, the synchrotron is available for radiobiology
and biophysics (Nelson et al., 2001). Under an agreement
between the NASA Space Radiation Health Program and
LLUPTC, beam time is provided to NASA-sponsored
investigators at nominal cost.
One of the principal advantages of accelerators for
space radiation research is that physical and biological
measurements can be made under similar, controlled
conditions, making it possible to attempt to identify
biological effects with physical effects. For example,
Robertson and Coutrakon and collaborators (Robertson et
al., 1994; Coutrakon, et al., 1997) have compared the
relative biological effectiveness (RBE)3 of protons for a
specific endpoint with microdosimetric spectra taken
under comparable conditions, and found a correlation
between increased RBE compared to photon irradiation
and increased energy deposition in the sample.
Accelerators are also suitable for tests of specific
materials. Recently at Loma Linda the radiation
transmission properties of U.S. and Russian
extravehicular activity (EVA) suits was measured, as
shown in Figure 2. The U.S. EMU (Extravehicular
Mobility Unit) and Russian Orlan-M suits were
instrumented with a variety of passive and active radiation
detectors placed at several locations in a tissue equivalent
phantom, and exposed to protons and electrons with
energies comparable to what is found in low earth orbit
(LEO), in order to assess the suits’ radioprotective
properties and to identify locations in the body that had
relatively high or low radiation dose (Benton et al., 2001,
Zeitlin et al., 2001). Reductions in dose and dose
equivalent were observed within both helmets and in the
lungs. Little or no dose reduction was observed in more
lightly shielded locations.
3
RBE is a measure of effectiveness per unit dose at producing
a given biological effect of a given radiation modality
compared to some reference standard, such as x-rays or
gamma radiation.
Figure 2. Helmet from U.S. EMU EVA suit instrumented for irradiation in proton beam at Loma Linda University.
The instrumented phantom head is inside the helmet.
Gravitational and Space Biology Bulletin 16(2) June 2003
21
J. Miller — Proton and Heavy Ion Accelerator Facilities
Heavy Ions
Table 2 lists some of the heavy charged particle accelerators
which are being used or which will soon be available for
space radiation research. The NASA Space Radiation
Laboratory (NSRL) at Brookhaven National Laboratory
(BNL) is noteworthy as the first heavy ion accelerator
facility built specifically for space radiation research. Funded
by NASA, it will be commissioned in mid-2003.
Table 2. Heavy Charged Particle Accelerator Research Facilities.
Facility
Zproj
Eproj
Eproj (56Fe)
(MeV/nucleon)
(MeV/nucleon)
Alternating Gradient Synchrotron (AGS)
Brookhaven National Laboratory (BNL)
Brookhaven, New York, USA
1–79
600–30000
600–1000
NASA Space Radiation Laboratory (NSRL)
Brookhaven National Laboratory
Brookhaven, New York, USA
1–79
100–3000
100–1000
Centro Nazionale di Adrotera Oncologica (CNAO)
(planned)
Italy
1,6
250
—
88" Cyclotron
Lawrence Berkeley National Laboratory (LBNL)
Berkeley, California, USA
1–8
55
—
Grand Accelerateur National D’Ions Lourds (GANIL)
Caen, France
6–92
25–95
—
Heavy Ion Medical Accelerator at Chiba (HIMAC)
National Institute for Radiological Sciences
(Chiba, Japan)
1–54
100–800
500
Tandem-ALPI
Laboratori Nazionali di Legnaro (LNL)
Legnaro, Italy
1–8
8–20
—
Superconducting Cyclotron
Laboratori Nazionali del Sud (LNS)
Catania, Italy
1–6
70
—
ETOILE
(2007)
Lyon, France
1,6
50–400
—
National Superconducting Cyclotron Laboratory (NSCL)
Michigan State University
East Lansing, Michigan, USA
1–92
90
—
Nuclotron
Joint Institute for Nuclear Research (JINR)
Dubna, Russia
1–26
6000
6000
6
137
—
SIS-18 Heavy Ion Synchrotron
Gesellschaft für Schwerionenforschung (GSI)
Darmstadt, Germany
1–92
50–2000
1000
Synchrophasotron
Joint Institute for Nuclear Research (JINR)
Dubna, Russia
1–16
4000
—
Ring Cyclotron
Institute for Physical and Chemical Research (RIKEN)
Wako Saitama, Japan
(Wako Saitama, Japan)
1
22
Not all energies are available for all ions. In general, the maximum energy varies inversely with the particle charge. For example, at the
BNL AGS, the maximum energy for protons is 30 GeV; for gold ions the maximum energy is 11 GeV/nucleon.
Gravitational and Space Biology Bulletin 16(2) June 2003
J. Miller — Proton and Heavy Ion Accelerator Facilities
Space radiation health research at heavy ion
accelerators can be traced back to at least 1972, when
experiments with high energy nitrogen ions at the
Princeton-Penn Particle Accelerator and LBL Bevatron
(Budinger et al., 1972) confirmed that heavy ions
impinging on the retina would be perceived as light
flashes, as shown in Figure 3. This effect had been
predicted by Tobias (Tobias, 1952) and was observed by
the Apollo astronauts. An Italian/Russian/Swedish
collaboration is planning to use this phenomenon as a
probe for possible damage to the central nervous system
by space radiation, in an experiment on the ISS (Narici et
al., 2001). Some of the apparatus to be used in the ISS
experiment is being tested and calibrated with heavy ions
at the BNL AGS, an application for which accelerators
are well-suited. An extended study now in progress is
using accelerated ions at the NIRS HIMAC to do the first
systematic inter-comparison of passive and active
detectors used in space.
Accelerated heavy ions are being used to probe the
fundamental biological processes underlying radiation
damage. Helium ions from the LBNL 88" cyclotron have
been used to produce DNA double strand breaks in human
fibroblasts, and the distribution of sizes of the resulting
DNA fragments has been compared to calculations of a
model that simulates the energy deposition of a heavy
charged particle passing through a chromatin fiber
(Rydberg et al., 2002). The results show promise that
eventually it may be possible to model the microscopic
action of GCR-like heavy ions passing through cell nuclei,
and thereby give the evaluation of some space radiation
health risks a mechanistic underpinning.
Heavy ions (Z >1) present some unique challenges,
since nuclear fragmentation of the incident ions in
materials such as spacecraft or planetary habitat walls or in
crewmembers’ bodies can modify an initially well known
radiation field. This complicates both spacecraft and
habitat shielding design and evaluation of radiation risk.
Figure 3. Dr. Cornelius Tobias, Dr. Edwin McMillan, and Dr. Thomas Budinger act as subjects in an experiment to determine
if high energy heavy ions impinging on the retina are perceived as light flashes, using a beam of high energy nitrogen ions at the
LBL Bevatron.
Gravitational and Space Biology Bulletin 16(2) June 2003
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J. Miller — Proton and Heavy Ion Accelerator Facilities
The effects of materials and structures on the internal
radiation environment will be a constraint on the design
of future spacecraft and habitats—it is already a
consideration on whether to add shielding material to the
ISS (Miller et al., 2003). Consequently, a great deal of
effort has gone into modeling and testing the radiation
transport properties of materials for use in space.
Reducing the uncertainties in the model predictions is
critical: the greater the uncertainty, the greater the weight
penalty in the form of increased shielding thickness
required under the ALARA (As Low As Reasonably
Achievable) principle of radiation protection practice
(Wilson et al., 1993).
The shielding properties of various materials have
been investigated theoretically using a transport and
fragmentation model weighted according to known
biological effects (Schimmerling et al., 1996). As can be
seen from Figure 4, the self-shielding of an aluminum
spacecraft wall alone is not effective against GCR HZE
particles, but light elements and compounds, especially
hydrogenous ones, are predicted to be effective in
reducing biological effects as measured by both dose
equivalent and cell transformation probability. The
experimental results are not clear cut. For example, in a
study of the induction of chromosomal aberrations by iron
ions in human lymphocytes shielded by polyethylene, the
addition of up to 30 cm of polyethylene was found not to
have a significant effect (Yang et al., 1998). This may be
due to the offsetting effects of nuclear fragmentation,
which tends to decrease average ionization (and therefore
dose) and slowing of the particles, which increases dose.
The effects of the interplay between fragmentation and
energy loss of GCR-like ions in polyethylene have
recently been studied systematically in an accelerator
experiment (Miller et al., 2003). Samples of the
aluminum ISS hull and internal crew quarter wall material
were augmented with 1.2 cm polyethylene and placed in
high energy heavy ion beams. The number of transmitted
particles as a function of charge and energy was
measured, and various dosimetric quantities were
calculated using standard LET-dependent quality factors.
The data show that for these modest thicknesses of
polyethylene there is relatively little ionization energy lost
by the incident ions, but enough nuclear fragmentation to
reduce the average energy loss per incident particle. Since
it is known that some biological effects may occur at very
low doses—even as low as that produced by a single ion
traversing a cell nucleus—these results suggest that it is
advantageous to add relatively modest amounts of
polyethylene as shielding against GCR, as has now been
done in the crew sleeping quarters on the ISS.
Figure 4. Calculated attenuation of dose equivalent and biological effect as a function of depth of various materials,
for one year exposure to GCR at 1977 solar minimum. Left panel: dose equivalent (using quality factor as a function
of LET). Right panel: cell transformation. (From Schimmerling et al., 1996).
24
Gravitational and Space Biology Bulletin 16(2) June 2003
J. Miller — Proton and Heavy Ion Accelerator Facilities
Physics Measurements
The GCR environment in the solar system is well known
and the basic physics underlying the transport properties
of high energy charged particles is well understood (see,
e.g., Wilson et al., 1991). However, accurate and precise
models require accurate and precise measurements of the
nuclear fragmentation cross sections (Townsend, 1993),
as source terms and must be tested against measurements
of the final state radiation fields for given incident
radiation. Similar arguments apply in the case of the selfshielding of critical organs within the human body.
Nuclear reaction products can be divided into three
general classes: projectile fragments (for Z >1); target
fragments and “mid-rapidity” fragments, which are
intermediate in velocity between target and projectile.
Projectile fragments are the fastest and therefore the most
penetrating, and are concentrated in the forward direction.
Mid-rapidity fragments tend to be light fragments emitted
at large angles in the laboratory, and are detected using
the same techniques as projectile fragments, but with the
detector designed to cover angles well away from the
projectile direction. Target fragments are slow and highly
ionizing. Because of their short range they are a challenge
to measure. From an operational standpoint, there are also
distinctions between charged fragments and neutrons and
between light and heavy projectiles. (The latter distinction
is somewhat arbitrary.)
Projectile Fragmentation
Many projectile fragmentation cross sections have been
measured. (See, e.g., Webber et al. 1990; Knott et al.,
1996; Zeitlin et al., 1997; Zeitlin et al., 2001 and
references therein.)
However, since the choice of
projectiles, targets, energies and observables has been
motivated for the most part by basic research in nuclear
physics and astrophysics, the fragmentation cross section
data are still limited in some regions of particular interest
for space radiation health. Similarly, until recently most
of the measurements with thick targets were driven by the
needs of the charged particle radiotherapy community,
and thus have been largely confined to relatively light
ions and tissue-equivalent targets such as water and
polyethylene.
Iron is the heaviest significantly abundant ion in the
galactic cosmic radiation, and as such is the heaviest ion
typically studied with regard to shielding effectiveness.
Figure 5 illustrates how experiment and theory interact. It
can be seen that none of the models successfully
reproduce the data in every respect, indicating that further
model development may be warranted if it is determined
that the resulting uncertainties are unacceptably large.
For purposes of improving models, nuclear physics
effects which might be obscured in the complicated final
states of heavier ion collisions may be easier to sort out in
light ion collisions. Carbon ions are now being used for
radiotherapy at GSI and NIRS, which makes information
on carbon fragmentation in tissue-like materials of
interest. For shielding applications, relatively light ions
such as helium, carbon, neon and silicon are worth
Figure 5. Charge changing cross sections for ∆Z = -1 to -14
for 1.05 GeV/nucleon 56Fe ions incident on H, C, Al and Cu
targets. The lines represent the predictions of different nuclear
fragmentation models: NUCFRG2 (Wilson et al., 1994),
QMSFRG (Cucinotta et al., 1998) and OPTFRAG (Townsend et
al., 1999).
studying both in their own right and because they are
produced as secondary fragments by interactions of
heavier primary ions in spacecraft shielding and in the
human body. Measurements with lighter ions are being
made at SIS-18 (Schall et al., 1996a, 1996b) and HIMAC
(Fukumura et al., 1996; Zeitlin et al., 2001).
Target Fragmentation
Target fragmentation is difficult to measure, due to the
short range of the target fragments. Fragments produced
by a 1044 MeV deuteron beam in a gold target have been
measured at the Nuclotron in Dubna, Russia (Malakhov,
2001). Charged fragments as heavy as nitrogen were
detected. Typical fragment energies were up to 20 MeV
for protons, 10 MeV for α particles and 2–5 MeV for the
heavier fragments. Although these measurements were
with a heavy target, the number of light ions in the GCR
and produced in secondary collisions makes them relevant
and argues for further measurements, perhaps with
shielding and tissue equivalent targets.
An effective, albeit labor intensive method for
measuring target fragments is with plastic nuclear track
detectors (PNTDs). The measurements of Benton et al.
(2001) of radiation inside the U.S. and Russian EVA
produced by 232 MeV proton beams were made using
PNTDs.
Neutrons
Neutrons are not strictly within the scope of this article,
but they are an important component of radiation in space
and are amenable to studies at accelerators. A recent
workshop on neutron production (Benton and Badhwar,
2001) concluded that high energy secondary neutrons
Gravitational and Space Biology Bulletin 16(2) June 2003
25
J. Miller — Proton and Heavy Ion Accelerator Facilities
(>10 MeV) will contribute up to 20% of the total dose
equivalent to personnel on the International Space Station.
These neutrons will be produced in roughly equal
measure by cascades initiated by trapped protons and
GCR heavy ions and by GCR projectile fragmentation.
Until fairly recently there was a paucity of measurements
of neutron production at beam energies greater than tens
of MeV, but that situation has been changing, with a
number of recent measurements at beam energies above
100 MeV/nucleon. Heilbronn et al. (1998, 1999),
Kurosawa et al. (1999a) and Kurosawa et al. (1999b)
have measured neutron angular distributions with a
number of different projectile/target/energy combinations
and thick targets. As is the case with charged particle
production, none of the models of neutron production
accurately reproduce the data in all cases. Direct
measurements with neutron beams are being done at Los
Alamos and CERN.
CONCLUDING REMARKS
Particle accelerators have proven to be a powerful tool for
space radiation research: facilitating basic research in
biology and physics; making possible focused and
systematic ground-based measurements that would be
impractical to make in space; and serving as a proving
ground for materials, radiation protection concepts and
instrumentation. In recent years it has become
increasingly clear that the mitigation of space radiation
health risks will require collaboration among
experimenters and theorists, biologists and physicists.
Accelerator experiments are a critical part of these efforts.
ACKNOWLEDGEMENTS
I thank the many colleagues who contributed information
and data for this review, in some cases prior to
publication: E.R. Benton, B. Bonhomme, F.A. Cucinotta,
L. Heilbronn, Y. Iwata, E. Krasavin, A.I. Malakhov, A.
Moroni, D. Schardt, L.W. Townsend, J.W. Wilson and C.
Zeitlin. Financial support for the author from the NASA
Space Radiation Health Program is gratefully
acknowledged.
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