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Silicon
chip built by NIST researchers with 16 tiny gamma ray
detectors that may help nuclear inspectors improve analysis
of plutonium and other radioactive materials. Each detector
is one millimeter square.
View
a high resolution version of this image.
Image
credit: NIST
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Boulder,
Colo.—Scientists at the Commerce Department’s
National Institute of Standards and Technology (NIST) have
designed and demonstrated the world’s most accurate
gamma ray detector, which is expected to be useful eventually
in verifying inventories of nuclear materials and detecting
radioactive contamination in the environment.
The
tiny prototype detector, described today at the American Physical
Society national meeting in Baltimore, can pinpoint gamma
ray emission signatures of specific atoms with 10 times the
precision of the best conventional sensors used to examine
stockpiles of nuclear materials. The NIST tests, performed
with different forms of plutonium at Los Alamos National Laboratory,*
also show the prototype greatly clarifies the complex X-ray
and gamma-ray emissions profile of plutonium.
Emissions
from radioactive materials such as uranium or plutonium provide
unique signatures that, if accurately measured, can indicate
the age and enrichment of the material and sometimes its intended
purpose or origin.
The
1-square-millimeter (mm) prototype collects only a small amount
of radiation, but NIST and Los Alamos researchers are collaborating
to make a 100-sensor array that could be deployed in the field,
perhaps mounted on a cart or in a vehicle.
“The
system isn't planned as a primary detection tool,” says
NIST physicist Joel Ullom. “Rather, it is intended for
detailed analysis of material flagged by other detectors that
have larger collection areas but less measurement accuracy.”
An array could be used by inspectors to determine, for example,
whether plutonium is of a dangerous variety, whether nuclear
fuel was made for energy reactors or weapons, or whether what
appears to be radium found naturally in the environment is
actually explosive uranium.
“People
at Los Alamos are very excited about this work,” says
Michael Rabin, a former NIST postdoc who now leads a collaborating
team at Los Alamos. The Los Alamos National Laboratory operates
and improves the capability to handle nuclear materials and
sends scientists to participate in United Nations nuclear
inspection teams.
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The
data plots above show detection of gamma rays with specific
energies. Arrows point to energies identified with the
new detector that are difficult to detect in the red
plot made with a conventional detector.
View
a high resolution version of this image.
Image
credit: NIST, National Nuclear Security Agency, Los
Alamos National Laboratory |
An array
of the new sensors might give inspectors new capabilities,
such as enabling them to determine the plutonium content of
spent reactor fuel without handling the fuel or receiving
reliable information from the reactor's operators. Plutonium
content can indicate whether a reactor is being used to produce
weapons or electrical power.
The
gamma ray detector is a variation on superconducting “transition
edge” sensor technology pioneered at NIST laboratories
in Boulder, Colo., for analysis of X-rays (for astronomy and
semiconductor analysis applications) and infrared light (for
astronomy and quantum communications). The cryogenic sensors
absorb individual photons (the smallest particles of light)
and measure the energy based on the resulting rise in temperature.
The temperature is measured with a bilayer of normal metal
(copper) and superconducting metal (molybdenum) that changes
its resistance to electricity in response to the heat from
the radiation.
To stop
gamma rays, which have higher energy than infrared light and
X-rays, the sensors need to be topped with an absorbent material.
A layer of tin, 0.25 mm thick, is glued on top of each sensor
to stop the gamma rays. The radiation is converted to heat,
or vibrations in the lattice of tin atoms, and the heat drains
into the sensor, where the temperature change is measured.
NIST researchers have developed microfabrication techniques
to attach absorbers across an array.
Researchers
expect the 100-detector array to measure 1 square centimeter
in size. The NIST team already has developed multiplexed readout
systems to measure the signals from large sensor arrays, and
recent advances in commercial refrigeration technology are
expected to allow pushbutton operation of the system without
liquid cryogens.
The
ongoing research is funded by NIST and by the U.S. Department
of Energy.
* J.N.
Ullom, B.L. Zink, J.A. Beall, W.B. Doriese, W.D. Duncan, L.
Ferreira, G.C. Hilton, K.D. Irwin, C.D. Reintsema, L.R. Vale,
M.W. Rabin, A. Hoover, C.R. Rudy, M.K. Smith, D.M. Tournear,
and D.T. Vo. 2005. Development of large arrays of microcalorimeters
for precision gamma-ray spectroscopy. Published in The
Conference Record of the IEEE Nuclear Science Symposium,
Puerto Rico, Oct. 23-29, 2005.
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