Internal Dosimetry
The internal dosimetry program conducts analyses and
consultation for the study and management of internal radiation exposure. The
analyses include collection of information on work and residence history, past
and current radiation exposure, bioassays to measure the presence of
radionuclides within body tissues (in vivo) or body fluids and excretions
(in vitro), and calculation of dose associated with observed uptakes.
Consultation includes interpretation of bioassay results and can extend to
collaboration with health care professionals and workplace supervisors. The
internal dosimetry program includes a documented quality assurance program for
in vivo bioassays and a comprehensive technical basis
for the assessment of internal exposure. The program meets the requirements and
recommendations of the DOE Implementation Guide for Internal Dosimetry Programs
(10 CFR 835) and the American National Standards Institute Performance Criteria
for Radiobioassay (N13.30). The Center is also involved in the DOE Laboratory
Accreditation Program for internal dosimetry and radiobioassay.
The internal dosimetry program is provided as an outreach
service to the public to support education about the Center's environmental
studies and naturally occurring radiation, and to provide assessment of
potential exposure to radioactive contaminants of concern. The program also
provides support to the WIPP by conducting bioassays for radiation workers on a
routine basis. Full-spectrum dosimetry services are available to evaluate
internal radiation exposure to radiation workers and members of the public in
the case of an accident at the WIPP. In addition, internal dosimetry services
can be provided to other entities that employ the use of radioactive materials.
 The Center's fixed in vivo bioassay facility occupies
approximately 966 square feet, and provides the primary analytical
infrastructure for the internal dosimetry program. The facility includes a large
shielded counting chamber, dedicated instrument control workstation, two change
rooms with showers and toilets, and a reception area. The counting chamber
measuring 8 feet x 8 feet x 8 feet and is constructed of 10 inch thick cast
iron, with a full graded-Z liner consisting of lead, tin and stainless steel.
The cast iron composing the chamber was produced for industrial use prior to
1945, and re-cast for the chamber using a specially selected foundry, resulting
in very low background radiation from anthropogenic and naturally occurring
constituents. The instrument control workstation includes a video display
terminal and intercom that are used to monitor subjects during the examination.
Signal processing electronics are located outside the counting shield next to
the instrument control workstation. The in vivo bioassay facility was
commissioned in July 1997.
The counting chamber is equipped with a lung and whole body
counting system using technologically advanced, hyperpure germanium detectors.
Lung and whole body counts are simultaneously performed with the counting
subject lying horizontally on a specially designed counting bed. Two,
four-detector germanium arrays designed for lung and whole body counting are
positioned over the chest and under the bed, respectively. Under routine
operation, photon interactions in the lung counting detectors are integrated
using two independent signal processing chains: one chain for low photon
energies (5 to 250 keV) and one chain for high photon energies (200 to 2000 keV).
The high-energy chain is analyzed independently and then added to the output of
the whole body counting detectors to increase sensitivity. This combination of
technology allows for sensitive monitoring for internally deposited transuranics,
naturally occurring radioactive materials, and mixed fission / activation
products. A dedicated computer serves the integrated electronic system for the
acquisition, storage and analysis of gamma-ray spectra collected by the
instrument. Resolution of the system is 450, 750, and 2100 eV at photon energies
of 5.9, 122, and 1332 keV, respectively. Routine sensitivities for 238Pu,
239Pu and 241Am in lungs are approximately 930, 2400 and 4 Bq,
respectively. Routine sensitivities for 137Cs, 60Co and
152Eu in the whole body are approximately 10, 10, and 60 Bq, respectively.
Ultrasound techniques are used to measure subjects' chest wall thickness and
composition to account for photon attenuation for positive lung burdens. The
Center's staff schedule subjects participating in in vivo bioassays, with
a total of 60 minutes allocated for each appointment. Prior to undergoing the
lung and whole body count, subjects view a videotape that explains the
procedure, and further explanation is provided by internal dosimetry staff.
In addition to providing services in bioassay, staff of the
internal dosimetry program carry out basic research in radiation detection
technology and novel applications of in vivo bioassay techniques to
environmental studies. The staff of the internal dosimetry program are also
responsible for the Center's radiation protection program to ensure compliance
with the Center's Radioactive Material License, granted under the authority of
the New Mexico Environment Department.
View photos of the counting chamber - Opens in a new window
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