Ionizing Radiation Division

Division Overview   |   Program Directions   |   Major Technical Highlights

Program Directions

• Brachytherapy Source Dosimetry. Brachytherapy (treatment with sealed radioactive sources) has seen a tremendous increase in the use of low-energy photon emitting seeds for prostate cancer and the introduction of intravascular brachytherapy, using beta-particle- (and photon-) emitting sources to inhibit arterial restenosis (re-closing) following balloon angioplasty. In both cases, NIST responded to the needs of the manufacturers, regulators and clinical physicists by developing new standards and measurement methods to calibrate the quantities needed to ensure accurate dosimetry for the wide variety of sources introduced, and disseminating these standards through a network of secondary calibration laboratories.

• Diagnostic X-Ray Beam Dosimetry. Dosimetric measurement standards, in terms of air-kerma (or exposure) for x-ray beams from 10 kVp (x-ray source accelerating potential in kilovolts) to 300 kVp are developed and maintained at NIST, and 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 mammography (Mo- and Rh-anode) x-ray beams.

• Standards and Calibrations for Radiation Therapy Dosimetry. More than 600,000 cancer patients per year are treated in the US with radiation beams, mainly from high-energy electron accelerators (either directly with the electrons or converting 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, the basis for calibrating instruments used to measure the absorbed dose delivered in therapy beams.

• Standard Reference Data on Radiation Interactions. NIST has nearly five decades in the development of critically evaluated, comprehensive databases of cross-section information for ionizing photons (x and gamma rays), electrons, and heavy charged particles. These data are often adopted by national and international standards organizations for use in radiation protection, medical therapy, and industrial applications. This work continues in the Division's Photon and Charged-Particle Data Center.

• Theoretical Dosimetry and Radiation Transport Calculations. 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 design, optimization, and insight often inaccessible to measurement.

• Radiation Processing Dosimetry. NIST has been a world leader in the dosimetry for the high levels of absorbed dose used in the industrial radiation processing of materials (e.g., polymer curing, 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 developed at NIST, and a new NIST system is near completion for on-demand, internet-based e-calibrations for industry, based on alanine/EPR dosimetry.

• Radiation Protection Dosimetry. NIST exposure standards for x-ray beams and gamma-ray sources are the basis for the radiation dosimetry monitoring of workers in the U.S. We are currently developing an instrument calibration service for low exposure rates of ¹³⁷Cs (down to tens or hundreds of µR/h). Our program in retrospective tooth-tissue/EPR dosimetry is now supporting a number of epidemiological studies.

• Radiation Source Facilities and Characterization. NIST maintains a 7 MeV to 32 MeV electron linac in its Medical Industrial Radiation Facility (MIRF), along with 4 MV Van de Graff and 500 keV electrostatic accelerators. These are used in a variety of radiation applications, including the current development at MIRF of a High-Energy Computed Tomography (HECT) facility. Plans are to acquire a clinical medical linac to support the development of therapy-level dosimetry calibrations.

• Symmetries of the Weak Nuclear Force. The end station on cold neutron guide NG-6 is operated as a national user facility for investigation of the symmetries and parameters of the nuclear weak interaction. High-accuracy measurement of the neutron lifetime, a search for time-reversal asymmetry in neutron beta decay, and measurements of parity non-conserving spin rotation are among the experiments competing for beam time.

• Magnetically Trapping Ultra Cold Neutrons. In collaboration with physicists at Harvard University, superthermal production of ultracold neutrons (UCN) in superfluid helium and magnetic trapping of these neutrons have been successfully demonstrated. Several kinds of upgrades to the trap are in progress to increase the number of trapped neutrons and to reduce background events which mask the observation of neutron beta decay in the trap. The initial application of this UCN source will be a neutron lifetime measurement with a potential improvement in accuracy of better than a factor of 10 compared to the present best value.

• Neutron Fields for Materials Dosimetry and Personnel Dosimetry. A diverse array of well-characterized and documented neutron fields is maintained for calibrations and for development of methods for materials dosimetry and personnel dosimetry. A new generation of staff has taken over these activities with continuing guidance from emeritus staff members, who are serving on contract or as guest researchers. The application of calorimetry to absolute neutron counting is being pursued to validate and improve on traditional methods of fluence measurement and neutron source calibration.

• Neutron Interferometry and Optics. The Neutron Interferometer and Optics Facility is now in full operation as a national user facility with a busy schedule of experiments. Large new interferometer crystals of NIST design can operate over a wavelength range of roughly 0.2 nm to 0.45 nm, with fringe visibility as high as 88 % at the shorter wavelengths. Experiments include applications for materials science as well as fundamental physics measurements. Very substantial advances are being made in neutron scattering length measurements. Neutron optics developments include phase contrast imaging.

• Neutron Imaging for Fuel Cell Research. Neutron tomography and real-time radiography are being applied to the observation of hydrogen and water transport in operating fuel cells and to other industrial applications. Recent improvements in CCD imaging systems and the widespread availability of computed tomography (CT) and 3D image reconstruction software have made it possible to set up a neutron CT imaging system with only modest resources. Neutron CT imaging can complement x-ray CT scans, by providing higher sensitivity to hydrogen, boron, lithium and certain other elements and isotopes in many important industrial applications. New developments in phase-contrast imaging in large beams without the need for an interferometer are also being pursued.

• Polarized ³He-based Neutron Spin Filters. We are currently applying neutron spin filters based on polarized ³He to materials science and fundamental physics with neutrons. We produce the polarized gas by either of two optical pumping methods, known as spin-exchange and metastability-exchange. Both continue to be improved for the needs of neutron applications. We have employed spin filters for experiments on the small angle neutron scattering spectrometer (SANS) and we are also pursuing application to neutron reflectometry at the NCNR and Argonne National Laboratory. For fundamental physics, we produce unique cells for a parity violation experiment at Los Alamos.

• Radionuclide Standards for Nuclear Medicine. The development of a standard for the -emitting radiotherapy nuclide ²¹¹At will be the highest-priority project for the radiopharmaceutical standards program during the present fiscal year. Other important projects include the investigation of geometrical effects in measuring nuclides such as ¹⁷⁷Lu and ¹⁶⁶Ho in vials and syringes.

• Radionuclide Metrology Development. A pulse recording technique has been developed which will permit a given data set to be analyzed ex post facto. Intercomparison of the results of various types of analytical reductions on the same set of data will be possible and become routine, which will lead to reduced systematic uncertainties and very much faster measurements.

• Traceability for Low-level Radiochemistry Metrology. Many tens of thousands of low-level radiochemical measurements are made annually to support environmental remediation and occupational health programs. The credibility of these measurements has been based on participation in regulation driven performance evaluation programs of limited scope. The fundamental flaw that the metrology community recognizes is that there is a lack of direct linkage to the national radioactivity standards. This situation is being addressed in the publication of three ANSI Standards. These three consensus standards call for a traceability testing program that links the quality of operational measurements to the national standards. The Radioactivity Group has established such a traceability testing program for low-level radiochemistry laboratories such as: Westinghouse Carlsbad, University of New Mexico at Carlsbad, Sandia National Laboratory, and EPA Montgomery.

• Criteria for Production of QC Materials. NIST, in collaborations with DOE/EM, NRC, NIST, FDA, universities, utilities, national laboratories, instrument manufacturers, commercial radioactivity standards companies, and commercial proficiency evaluation companies initiated the process of establishing consensus criteria for the production of QC materials as a sub-issue to its traceability priority. This issue is of major importance because material quality is critical to the credibility of PE programs/analytical results, and is fundamental to other issues.

• Virtual Radiobioassay Standard Phantom. Exposure of occupational (weapons production, environmental clean-up, nuclear power generation, and waste management) personnel and the public to radioactive sources is non-invasively evaluated by external gamma-ray measurement. NIST is being asked to develop standard phantoms containing radionuclides as a calibration reference for the gamma-ray measuring instruments.

• Standards, Calibrations and Instrumentation for Environmental Monitoring. The measurement of environmental surface contamination, particularly around nuclear sites and in environmental remediation, has posed an important and difficult problem. Three systems that are under study and evaluation are (i) imaging plate technology, (ii) glow-discharge resonance ionization mass spectrometry, and (iii) thermal ionization mass spectrometry.

• Environmental Management and Nuclear Site Remediation. Resonance Ionization Mass Spectrometry continues to be developed with improvements in sensitivity and selectivity based on several factors such as lowering of the background, careful choice of excitation scheme, development of a graphite furnace source, and incorporation of a non-axial-beam geometry. The Nuclear Regulatory Commission is moving toward increasing sensitivity requirements for in situ measurements.

• Radionuclide Speciation in Soils and Sediments. While regulators and the public are interested in assuring that radionuclide decontamination in the environment is cost-effective and thorough, the underlying basis for soil and sediment decontamination is the speciation of the radionuclides. This project addresses the identification of radionuclide partitioning in soils and sediments. The approach involves the development of the NIST Standard Extraction Protocol for identifying the distribution of radioactive elements in soils and sediments.