Public Safety
Portable Radiation Detectors
An advancement in more portable radiation detectors was made possible
by satisfying a NASA need for a non -clogging Joule-Thomson (J-T) cryostat
to provide very low temperature cooling for various sensors.
Through a Small Business Innovative Research (SBIR) contract from Kennedy
Space Center, General Pneumatics Corporation's Western Research Center
(GP WRC) in Phoenix, Arizona was tasked to overcome reliability problems
and other shortfalls in conventional J-T cryostats. Later, GP WRC worked
with EG&G ORTEC in Oak Ridge, Tennessee, to apply the technology to
cooling high-resolution gamma-ray spectrometers, under a contract to Bechtel
Nevada. The U.S. Department of Energy's Remote Sensing Laboratory in Nevada,
operated by Bechtel Nevada, directed the development.
J-T cryostats produce cryorefrigeration by expanding a gas from high
pressure through a nozzle. In many uses, the cryostat automatically regulates
the flow to provide rapid cooldown and then conform to match the heat load.
A common problem in conventional cryostats is that the very small diameter
orifice of the nozzle can easily become clogged by contaminants in the
flow.
| General Pneumatics cryostat nozzle work
was pivotal in building more portable gamma-ray detectors. A prototype
J-T cryostat-cooled HPGe high-resolution gamma-ray detector is designed
to monitor nuclear materials. |
In the NASA SBIR project, GP WRC developed an anti-clogging, flow-regulating
J-T cryostat. The GP WRC cryostat design has a tapered annular expansion
nozzle which is highly resistant to clogging due to a large ratio of circumference
to flow area. Grooves in the nozzle increase flow friction to allow wider,
less clog -susceptible passage. Turbulent voids continuously break up and
clear contaminants from the flow. The GP WRC J-T cryostat design allows
several hours of continuous operation with gas contamination levels that
would clog conventional cryostats within six minutes.
GP WRC's J-T cryostat also solved flow regulation difficulties in cooling
the gamma-ray detectors. Flow regulation is induced by the heat content
of the return flow gas, instead of by the temperature near the nozzle as
in conventional cryostats. This provides a large flow for fast cooldown,
while conserving gas according to the heat load at operating temperatures.
This NASA-supported cryostat development played a key part in the development
of more portable high -purity germanium gamma-ray detectors. Such are necessary
to discern between radionuclides in medical, fuel, weapon, and waste materials.
Despite the abatement of the Cold War, the ability to monitor nuclear materials,
verify possible hazards, and develop counter proliferation tactics has
become increasingly crucial to global security.
| J-T cryostats produce cryorefrigeration
by expanding a gas from high pressure through a nozzle. |
A high-purity germanium gamma-ray detector is typically cooled with
liquid nitrogen, which may take10 hours or more because of the relatively
large, thermally-isolated mass. The dependence on liquid nitrogen severely
limits the mobility and continuous operating time of such detectors. Electric-powered
mechanical cryocoolers induce unwanted vibration or are too bulky and power-consuming
to be very portable. Thermoelectric coolers, while vibrationless, cannot
achieve the necessary temperature or power efficiency.
Essential to developing a more portable gamma-ray detector unit was
the J-T cryostat expertise gained by General Pneumatics as a result of
its NASA SBIR project. The outcome was a cryostat that can cool gamma-ray
detectors, without vibration, using compressed gas that can be stored compactly
and indefinitely in a standby mode.
In addition to the gamma-ray detector application, General Pneumatics
has parlayed its NASA-backed research to produce custom J-T cryostats for
other government, commercial, and medical applications.
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