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
INTENDED OUTCOME AND
The Radiation Interactions and Dosimetry Group advances the measurement of quantities important in the radiological sciences through programs in the
dosimetry of x rays, gamma rays, electrons, and other charged particles. Its mission is to develop, maintain, and disseminate the national measurement
standards for these radiations, and to engage in research on radiation interactions and effects to meet requirements for new standards and to address
the needs of industry, medicine, and government.
We maintain the national standards for the gray (Gy), the Système International (SI) unit for radiation dosimetry, and develop, maintain, and
disseminate high-quality data on fundamental radiation interactions. These data are used extensively in radiation transport calculations and
simulations, with algorithms and codes often developed by our staff, to solve a wide range of problems in radiation science and applications.
Standards are disseminated both directly to the customer and through networks of secondary calibration laboratories. We work closely with such networks
to maintain measurement-quality assurance and traceability, by means of calibrations and proficiency testing services.
Our staff continues to make important contributions to the work of national and international standards and scientific organizations, and we are
central to measurement-quality assurance of dosimetry in the many application areas of ionizing radiation. For example, the radiation doses in cancer
treatment, radiosurgery, diagnostic radiology, and public or worker exposures in the U.S. are traceable to our standards and programs.
Our focus on homeland security continues, while significant progress was made in our more traditional radiation-dosimetry programs.
Figure 1. Angular “slice” of HECT test object obtained using 19 MV x rays from the MIRF accelerator. A drill bit is clearly observed on the
left side of the image; aluminum support hardware and mounting holes are apparent in the center portion of the image.
The Division’s accelerator facilities continue to support research efforts in industrial and medical dosimetry, homeland security, and radiation-hardness
and materials-effects studies. Substantial progress has been made in the development of the High-Energy Computed Tomography (HECT) facility. A number
of hardware and software improvements were made as a result of collaboration with engineers from Savannah River National Laboratory, including the
installation of an object manipulation system consisting of a rotating turntable and a two-dimensional linear stage. A number of test objects are
being assembled, which should allow us to evaluate the performance of the HECT imaging hardware and gain expertise in the use of tomographic
Quality-assurance testing continues on the Clinac 2100C medical accelerator, typical of those used in cancer therapy. Using a recently acquired
radiation scanner, depth-dose measurements in water were conducted in order to validate machine performance. Software controls have been implemented
to allow the beam to be cycled on and off for a fixed period of time and a preset number of cycles. This technique will allow frequency-domain
analysis of the response of a second-generation Domen-type water calorimeter, which will become the primary standard for high-energy absorbed
dose-to-water. Plans are to make use of the Clinac in a high-energy calibration laboratory based on this primary dosimetry standard.
X-Ray Calibration Range Measurements
Figure 2. 320 kV replacement x-ray tube.
Figure 3. NIST 300 kV calibration range, showing the Wyckoff-Attix free-air chamber (white assembly), the national standard for
medium-energy x-ray beam air kerma.
The calibration and irradiation of instruments that measure x rays are performed in the NIST x-ray calibration facilities in terms of the physical
quantity air kerma. Calibrations are performed by comparing the instrument to a NIST primary standard, which includes four free air chambers.
Air-kerma-measurement comparisons with the BIPM and other primary standards laboratories are conducted for quality assurance. One important comparison
began in 2006. The comparison involved a series of measurements at PTB in Germany and at NIST using the air-kerma standards and two NIST
reference-class transfer ionization chamber standards. Tungsten and molybdenum reference beam qualities in the range from 10 kV to 50 kV (low energy)
were used. This comparison with PTB will be the third comparison for mammography energies, but the first for NIST using the recently developed
BIPM/CCRI reference beam qualities. Prior to the next direct BIPM comparison, the results of the CCRI beam comparison will verify the new correction
factors as applied to the NIST standard for these beam qualities.
In addition to participating in measurement comparisons, the NIST x-ray facilities are used to conduct proficiency tests for various customers.
These include secondary calibration facilities, chamber manufacturers, the nuclear industry, the Department of Energy, the Department of Defense,
private calibration facilities, medical facilities, and the Food and Drug Administration. Calibrations are provided in terms of gray/coulomb for
various reference-quality ionization chambers to achieve traceability to NIST.
Updates made to our website (http://physics.nist.gov/Divisions/Div846/Gp2/gp2.html) explain the proficiency test policy.
A new free-air ionization chamber has been designed to realize air kerma for x-ray beams of 50 kV to 300 kV, replacing the Wyckoff-Attix chamber used
at NIST as a primary x-ray standard (medium energy) for more than fifty years. The dimensions and the parallel-plate design of the new chamber are
identical to the Wyckoff-Attix chamber, but the materials are different. The chamber incorporates a unique guard bar and insulator design, and
precision slides facilitate alignment and the direct measurement of the air-attenuation correction. Once the new standard is constructed,
it will be fully evaluated in a parallel measurement arrangement until the correction factors are established and tested.
X-Ray Security-Screening Standards for Homeland Security
With support from the Department of Homeland Security (DHS), we have been developing technical-performance standards for four classes of x-ray security
screening systems: checkpoint cabinet, computed tomography, cargo and vehicle, and human subject. Four ANSI working groups have been organized to
develop national standards for technical performance, focusing particularly on image quality.
Figure 4.Objects being considered to test penetration, contrast sensitivity, and resolution for standards being developed for x-ray and
gamma-ray noninvasive inspection systems for cargo containers and vehicles.
Each standard will include both test methods and x-ray phantoms appropriate for the application. Examples of artifacts that are being designed and
tested are shown in Fig. 4. In the upper portion, to test penetration and contrast sensitivity of high-energy inspection systems used to inspect cargo,
the orientation of an arrow must be determined through increasing thicknesses of steel. The lower portion of the figure is a test piece proposed to
test the spatial resolution of CT imaging systems.
In related work, we have established a testbed for assessing the image quality of portable x-ray and imaging systems used by bomb squads for explosives and
ordinance detection and disarmament.
The results of testing will be used to establish minimum image-quality standards and to update a National Institute of Justice standard covering these systems.
Testing a Method for Establishing e-Traceability to NIST High-Dose Measurement Standards
Figure 5.Alanine dosimeters used in radiation processing allow for accurate transfer dosimetry.
To meet rapid turn-around time needs, yet maintain high accuracy and minimal uncertainties, an Internet-based dosimetry calibration system is being
developed for remotely certifying the calibration of industrial radiation sources. Using alanine dosimeters and a Bruker e-scan electron paramagnetic
resonance (EPR) spectrometer, subscribers’ dosimetry systems will be calibrated to the NIST reference dosimetry system. A computational method for
determining the measurement conversion factor (MCF) relating the NIST reference system to the remote customer system has been established.
The uncertainty in applying the MCF to the dosimeter response has been evaluated and used to establish an overall uncertainty budget.
The Internet-based system promises to deliver immediate certification results to customers, on-demand at lower cost. A working alpha version was
assembled on lab computers and a server within the Division. Recently, several system design changes were required to adapt to changes in security
requirements, changes that actually led to a simpler and more user-friendly system. The next stage will be to transfer the software to Bruker to
begin beta testing the system.
A testbed consisting of two separate e-scan spectrometers at NIST is being used to test the MCF calculation method. Calibrated alanine dosimeters are
assembled into a “customer” e-scan dosimeter insert set, as would be used for an actual calibration event, and the results are used to test the MCF
computation. Testing the electronic certification process is conducted by measuring a series of unknown/test dosimeters, applying the MCF, and
calculating dose from the NIST standard reference curve. These tests will be performed for all of the commercially available inserts and will serve
as guidelines for the operational aspects of this future service.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents