to develop and provide standards for radioactivity
based on the SI unit, the becquerel, for homeland security, environmental,
medical, and radiation protection applications.
INTENDED OUTCOME AND
BACKGROUND
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Figure 1. Calibration of 90Y in a syringe
geometry for nuclear medicine applications. |
The Radioactivity Group is responsible
for developing metrological techniques
to standardize new radionuclides for
research, and for exploring applications
in health care, worker protection,
environmental protection, and national defense.
A vigorous program is underway to
develop microcalorimetric methods for
measuring power from radioactive
sources. This work is proceeding in
parallel with use of liquid scintillation
detector methods based on triple-to-double-coincidence
ratio (TDCR) counting. Accurate activity measurements
over a broad dynamic range are
obtained using these two complementary
techniques. For ultralow-level atom
counting, we are continuing development
of resonance ionization mass
spectrometry (RIMS), which has applications
in nuclear forensics as well as
in environmental radioactivity.
The Radioactivity Group provides leadership
in a national program for radiopharmaceutical
standards. We provide
the national standards for nuclear medicine
radionuclides used in 13 million
diagnostic procedures and 200,000
therapeutic procedures annually in the
U.S. A major effort in this area is to
provide traceability not only to the
manufacturer of the radiochemicals, but
also to the clinic. This has been realized
for the recently approved drug Zevalin,
used in the treatment of refractory
non-Hodgkin's lymphoma (Fig. 1).
Because of heightened concern in the
U.S. about possible terrorist attacks
using radiological or nuclear weapons,
we are leading a national effort to develop
standards and protocols for radiation
instrumentation for first responders.
Expanding efforts are directed at four
goals: 1) validation of equipment standards
and test methods for handheld
detectors for first responders; 2) a testbed
for cargo container and truck
inspections for radiological and nuclear
materials, including the coordination of
instrument testing within the National
laboratories; 3) radionuclidic forensic
methods based on mass spectrometry;
and 4) a national quality assurance
system that supports homeland security needs.
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 metrology community recognized a
fundamental flaw--that there was often
a lack of direct linkage to national
radioactivity standards. This situation
is being addressed by the publication of
three ANSI standards. These consensus
standards call for a traceability-testing
program that links the quality of
operational measurements to national standards.
Accomplishments
Testing of Radiation
Detection Portable and Portal Instruments
Characterizations of commercially
available instruments for measurement
and identification of unknown radionuclides
were carried out in support of the
development and testing of American
National Standards Institute (ANSI)
standards N42.32, N42.33, N42.34,
and N42.35. Measurements were based
on the performance of the devices, i.e.,
the capability of the detectors to ensure
a correct radionuclide identification and
to determine exposure or dose-rate values,
in a given time interval, for various
radioactive sources and accuracies.
Thirty portable instruments and four
portal monitors were tested. The
portable instruments' responses, as a
function of energy, were tested using
two 60Co and two 137Cs ranges, as well
as the x-ray range operating at effective
energies of 47 keV, 80 keV, and 120 keV.
For portal monitors, a new testing
facility was installed, allowing the testing
of these monitors using several radioactive
sources transported through the
monitors at different speeds and distances.
The monitors were field-tested in
a parking lot and at the NIST C Gate.
The availability of this new testing facility
and our access to a wide variety of
radioactive sources allow us to test portal
monitors presently being developed
for detection and identification of radionuclides.
Gamma-Ray Emitting Test
Sources for Portal Monitors used for Homeland Security
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Figure 2. Radioactive test sources developed at
NIST for the testing and evaluation of radiation
detectors to be used by first responders.
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Basing our research on the new ANSI N42.35 standard,
"Evaluation and Performance of Radiation Detection Portal
Monitors for Use in Homeland Security," as well as the
related Test and Evaluation Protocol for Homeland Security,
we developed a set of test sources for the testing of portal
monitors. (See Fig. 2.)
The standard specifies that the test sources should be
point sources (relative to the size of the portal monitors),
single radionuclides, and encapsulated in 0.25 mm thick
stainless steel disks. The radionuclides required for portal
monitor testing are 57Co, 60Co,
133Ba, 137Cs, 228Th, and
241Am. These sources are useable not only for testing
and evaluating instrumentation, but also for calibrating test
sources and for training instrument users.
We are aware of five companies thatcurrently produce or
distribute radioactive sources in the USA. AEA Technology
showed an interest in commercializing our test sources, and
they are getting ready to produce and distribute them more widely.
AEA Technology particates in a Measurement Assurance Program
with NIST to assure traceability to U.S. national standards
for sources used in nuclear power and medical applications.
A similar and more extended program is being implemented for
homeland security applications.
Microcalorimetry for Absolute Radioactivity
Standardizations
A commercial "isothermal microcalorimeter"
has been adapted for use in performing classical, calorimetric-based
calibrations of radioactive sources.
Microcalorimetry is one of the best nondestructive
methods for the standardization
of small, high-activity (GBq-range),
ß and x-ray sources, a category under
which almost all brachytherapy sources fall.
This dual-cell, near-isothermal (heat
flow) microcalorimeter operates at nearambient
temperatures, utilizing specially
fabricated, source-holder measurement
cells that are used to maximize the energy
absorption of the ionizing radiation.
It incorporates resistance heaters within
the measurement cells to very accurately
determine power calibrations.
In 2004, to improve the temperature
stability of the operating environment,
the isothermal microcalorimeter was
moved into the new AML building. The
instrument is now located in a laboratory
with ambient temperature stabilized
to 0.2 ºC, which has increased the stability
of the instrument baseline, and
thus its sensitivity, by a factor of two.
The isothermal microcalorimeter was
used for the calibration of an 55Fe low-energy
x-ray source. The low energies of
the emitted x rays and Auger electrons
(less than 6.5 keV) made use of conventional
counters to measure this radionuclide
extremely difficult. On the other
hand, the low energy of the decay particles
allowed for the primary calibration
of 55Fe by microcalorimetry without the
need for an additional absorber within
the cell, thus increasing sensitivity.
Preliminary results suggest that the
standardization of 55Fe sources by
isothermal microcalorimetry can be
performed with an expected accuracy
of better than 1 %.
Measurement Standards for Recently Approved
Radiotherapy Nuclide
The American Cancer Society estimates
that there will be 64,000 new cases of
lymphoma in the U.S. this year, with an
expected five-year survival rate (for non-Hodgkin's
lymphoma) of 52 %. One of
the most promising new drugs for treatment
of this disease is Zevalin, which is
a monoclonal antibody labeled with the
radionuclide 90Y. While 90Y has many
properties that make it suitable for use
in radiotherapy, those same properties
present a number of challenges regarding its measurement.
FDA approval of Zevalin brought about
an exploding need for standards and
measurement quality assurance for
radiopharmaceuticals using 90Y. A workshop
held at NIST in December 2001 identified the measurement issues
involved. Invited speakers included representatives
from NIH, radiopharmaceutical manufacturers, radiopharmacies,
isotope producers, government regulators, and NIST.
As a result, NIST examined the measurement
of 90Y solution standards in
clinically useful geometries, to develop
calibration factors and an estimation of
the uncertainties involved in those measurements.
NIST worked with radiopharmacies, the FDA, the Society of
Nuclear Medicine, the American
Pharmaceutical Association, and others
to organize and implement a protocol to
establish measurement traceability for
the estimated 450 radiopharmacies.
In related work, NIST assisted in the
development of a long-term standard for
the calibration of detectors used in 90Y
measurements. The short half-life
(64.053 h) of 90Y does not allow for
long-term use of a single standard
source. However, AEA Technology QSA
(Burlington, Massachusetts) manufactures
a mock 90Y syringe using a solid
source of the long-lived parent radionuclide
90Sr. NIST examined the characteristics
of these sources in a detector, as
compared to those of 90Y solution
sources. With the proper execution of
an initial calibration exercise at a clinic,
these standards may be used to check
calibration throughout the year.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2004" - Table of Contents |
