Radiation Dosimetry Standards:
to develop dosimetric standards for x rays,
gamma rays, and electrons based on the SI unit, the gray, for homeland
security, medical, radiation processing, and radiation protection
applications.
INTENDED OUTCOME AND
BACKGROUND
The Radiation Interactions and Dosimetry Group promotes accurate
and meaningful measurements of dosimetric quantities pertaining to ionizing
radiation: x rays, gamma rays, electrons, and energetic, positively
charged particles. We maintain the national measurement standards for the
System International (SI) unit for radiation dosimetry, the gray.
NIST is a world leader in the measurement of high levels of absorbed dose, as
required in the industrial radiation processing of materials (e.g.,
sterilization of single-use medical devices, food irradiation, and destruction
of biological weapons). Accurate transfer dosimetry is increasingly done on the
basis of alanine/EPR dosimetry, rather than the radiochromic film dosimetry
originally developed at NIST and offered here for many years as a calibration
service. A new NIST system is near completion for on-demand, Internet-based
e-calibrations for industry, based on alanine/EPR dosimetry.
Brachytherapy (treatment with sealed radioactive sources) has seen a tremendous
increase in the use of low-energy, photon-emitting seeds to treat prostate
cancer and in the use of beta-particle- (and photon-) emitting sources to
inhibit arterial restenosis (re-closing) following balloon angioplasty. In both
cases, NIST has responded to the needs of the manufacturers, regulators, and
clinical physicists. We develop new standards and measurement methods to
calibrate the quantities needed to ensure accurate dosimetry for the wide
variety of sources introduced, and we disseminate these standards through a
network of secondary calibration laboratories.
More than 600,000 cancer patients per year are treated in the U.S. with
radiation beams, mainly from high-energy electron accelerators (either directly
with the electrons or by converting them to high-energy x rays). NIST
maintains and disseminates the standards for air kerma (exposure) and for
absorbed dose to water from 60Co gamma-ray beams. These provide the
basis for calibrating instruments used to measure the absorbed dose delivered
in therapy beams. Standards for diagnostic radiology are developed and
maintained at NIST in terms of air-kerma, for x-ray beams from 10 kVp to
300 kVp (x-ray source accelerating potential in kilovolts). These are
disseminated to manufacturers and the medical physics community in North
America through a network of secondary calibration laboratories. NIST maintains
more than 75 beam qualities for conventional, W-anode, x-ray beams, and
17 beam qualities for mammographic, Mo- and Rh-anode, x-ray beams.
The radiation-transport and Monte Carlo methods pioneered and developed at NIST
to calculate the penetration of electrons and photons in matter are used in
most of the major codes today. Monte Carlo simulation is increasingly applied
to problems in radiation metrology, protection, therapy, and processing as an
accurate tool for designing and optimizing radiation systems and for providing
important insight into processes inaccessible to measurement.
Accomplishments
Radiation-Sources for Medical and Industrial Applications
In addition to a dozen gamma-ray (60Co and 137Cs) sources
and five x-ray ranges, NIST maintains the Medical Industrial Radiation Facility
(MIRF), along with a 4 MeV Van de Graff and a 500 keV
electrostatic accelerator. These are used in a variety of radiation
applications, such as material modification, radiation-hardness testing,
electron- and bremsstrahlung-beam dosimetry, and high-energy computed
tomography development. Design and construction are underway on two new
accelerator facilities. Installation is being completed of a 6 MeV to
20 MeV electron-beam (6 MV and 18 MV bremsstrahlung-beam) linear
accelerator to support the development of direct, therapy-level dosimetry
calibrations. Planning is also underway for a 10 MeV, 17 kW electron
linac to support the standards and calibrations program for industrial
radiation processing and the study of radiation effects in materials.
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Figure 5. NIST test packages to validate radiation doses needed to
decontaminate parcels for U.S. Postal Service. |
Assistance to U.S. Postal Service in Decontamination of Mail
After anthrax-laced mail was delivered to media and government offices,
resulting in five deaths, numerous illnesses, and enormous disruption and
economic loss, we responded rapidly to identify industrial irradiation of the
mail as an effective and readily-available process to kill anthrax spores.
Leading a task force established by the White House Office of Science and
Technology Policy, we worked with the Armed Forces Radiobiology Research
Institute, the U.S. Postal Service (USPS), and industrial irradiation
facilities to provide critical dosimetry measurements and to validate the
process.
Figure 6. Dose distribution of sample of irradiated mail. | |
Based on an extensive program of Monte Carlo radiation-transport calculations
and accurate dosimetry measurements in a variety of mail configurations, NIST
provided advice for optimizing the process parameters and developing a national
strategy to effectively handle the highly-variable mail and parcel stream.
This collaboration continues, expanded to include qualitative measurements of
radiolytic products produced in the mail during the irradiation, quantitative
measurements of the effects on the archival properties of paper due to
irradiation, and the design of a dedicated USPS mail-irradiation facility.
High-Energy Computed Tomography (HECT) Facility
The 7 MeV to 32 MeV Saggataire linac in the NIST Medical Industrial
Radiation Facility (MIRF) offers unique possibilities as an x-ray source for a
high-energy computed tomography facility. A beamline and camera are under
development at MIRF to study the x-ray inspection of cargo containers, trucks,
and other large objects. We are also carrying out theoretical and experimental
investigations into neutron production in high-energy x-ray beams. It may prove
possible to also use photoneutrons, produced at high photon energies, as an
active probe to interrogate containers and screen for explosives and other
terrorist materials.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2002" - Table of Contents |
