Product News - February 2011
helps large growth crystals
Looking down a quartz-lined furnace at
1100 degrees C
|(Top) Conventional vertical Bridgman
Furnace used to grow the semiconductor material,
cadmium zinc telluride - (CdZnTe).
(Bottom) Experimental 'tilting" Bridgman
furnace used to grow the scintillator material,
cerium tribromide (CeBr3).
At Kansas Slate University, a unique facility
is deducted to the research and development of new and innovative
radiation detector technologies. The Semiconductor Materials
and Radiological Technologies (SMART) Laboratory is the largest
university-based radiation detection laboratory in the United
States It focus on the detection of neutrons and gamma rays,
primarily those from special nuclear material (SNM) for homeland
security applications. SMART Lab investigates and fabricates
a variety of detectors which include compact low-power neutron
detectors, high-resolution room-temperature-operated semiconductor
gamma ray spectrometers, pixelated devices for gamma ray or
neutron imaging, and miniaturized gas-filled detectors. The
laboratory builds detectors from start lo finish In readily
deployable packages for use in detecting nuclear materials
such as plutonium and uranium.
The gamma ray detection aspect of the laboratory's research
h focused on the discovery and development of new dense, high-Z
semiconductor material such as cadmium zinc telluride CdZnTe
or CZT) and mercuric iodide (HgI2), and several scintillator
materials including lanthanum (LaBr3) and cerium tribromide
CeBr3). Research conducted on large crystal growth with high-Z
semiconductor and scintillator materials has producted large
crystal ingot yields.
Gamma rays are electromagnetic radiation of high frequency
(very short wave length), that are produced by sub-atomic
particle interactions such as electron-positron annihilation,
radioactive decay, fusion and fission. Gamma radiation is
highly penetrating photons, extremely energetic. To directly
detect them is very difficult, however -a material with high
Z number is needed, representing a high number of neutrons
and protons in the nucleus. Those nuclei tend to stop gamma
rays much better than other elements such as hydrogen or helium,
for example. A crystal with a high-Z number converts the gamma
rays from electromagnetic waves to excited electrons. The
electrons move through the crystal or create light, one or
the other, and produce something that Is possible to be detected.
If a crystal is very uniform, very homogeneous - it can be
determined that a gamma ray interacted in the crystal by the
effect that is observed in it.
(Top) - Negative-Stiffness Vibration
(Left) Schematic of Negative-Stiffness Isolator
Growing giant crystals
At the SMART Laboratory, crystals of CdZnTe and the scintillator
materials are grown via a vertical Bridgman furnace. In this
process, molten material is directionally solidified from
one end to the produce a large-volume ingot that is a single
crystal. Methods to grow CdZnTe for gamma-ray spectrometers
have been explored since the early 1990s, yet a viaiable system
to produce large crystals at an economical cost has not been
achieved until relatively recently.
Higher ingot yields enable smaller, faster and more accurate
sensors, and allow gamma-ray detectors to be more economical
fleld portable - benefits that can have a significant impact
on national security objectives. Radiation detectors using
CZT can operate in direct-conversion (or photoconductive)
mode at room temperature.
Essentially, SMART Lab researchers encapsulate the material
to be grown inside of a quartz ampoule under vacuum. The quartz
tube is put inside the furnace vertically bringing the material
to a molten state between 500 - 1,100 . Then they very slowly
freeze the material from bottom to top. If the thermal gradients
are correctly performed, a large crystal will develop. Once
the crystal is grown, It is extracted from the tube, trlmmed
to size with a diamond saw and polished to produce a detector.
Critical to maximizing ingot yield is maintaining a stable
crystal growth process through the elimination of external
The general consensus within the crystal growth community
is that uncontrolled vibrations can destabilized the growth
interface. Says Professor Mark Harrision with SMART lab. "As
the material is freezing from bottom to top, there is an interface
between liquid and solid, and it sets up a natural convection
flow that is ideal for growing a big, single crystal. If a
vibration disturbs the liquid directly above the forming crystal
solid, it can change the convection patterns and multiple
crystals will form from the previous single crystal. Which
is contrary to our purpose of growing large, single crystals.
"We looked into various active and air-table vibration
handling systems, and eventually selected negative-stiffness
vibration isolation," he adds.
Negative-stiffness mechanism (NSM) isolators have the flexibility
of custom tailoring resonant frequencies vertically and horizontally.
They employ a completely mechanical concept in low-frequency
vibration isolation. Vertical-motion isolation is provided
by a stiff spring that supports a weight load, combined with
a NSM. The net vertical stiffness is made very low without
affecting the static load-supporting capability of the spring.
Beam -columns connected in series with the vertical-motion
isolator provide horizontal-motion isolation. The horizontal
stiffness of the beam-columns is reduced by the "beam-column"
effect. A beam-column behaves as a spring combined with a
NSM. The result is a compact passive isolator capable of very
low vertical and horizontal natural frequencies and very high
internal structural frequencies.
Vibration transmissibility with negative-stiffness is substantially
improved over air systems, which can make vibration isolation
problems worse since they have resonant frequency that can
match that of floor vibrations. Transmissibility is a measure
of the vibrations that transmit through the isolator relative
to the input vibrations. The NSM isolators, when adjusted
to 0.5 Hz achieve 93% isolation efficiency at 2 Hz; 99% at
5 Hz; and 99.7% at 10 Hz.
|Plots of power spectral density measurements
made with a seismometer at the growth ampoule position
in the two furnances, with and without the negative-stiffness
NSM transmissibility is also improved over active systems. Because
they run on electricity, active systems can be negatively influenced
by problems of electronic dysfunction and power modulations,
which can interrupt crystal growth continuity. Active systems
also have a limited dynamic range - which is easy to exceed
- causing the isolator to go into positive feedback and generate
noise underneath the equipment. Although active isolation systems
have fundamentally no resonance, their transmissibility does
not roll off as fast as negative-stiffness isolators.
"One of the concerns we had was surface waves coming across
the ground, which induce vibration in the crystal growth system,"
explains Prof Harrison.
"We are located in a basement," he says, "Before
we got the NSM system, I could actually see somebody walking
down the stairs through the walls with a seismometer. With the
negative-stiffness system in place, I can't even tell when they
are shelling at the nearby Fort Riley military base."
Gamma ray detectors have been around for years, but they are
either very low efficiency, poor performance or they require
liquid nitrogen cooling, such as those employing germanium.
Imagine the difficulty required to take liquid nitrogen into
a remote desert searching for special nuclear material.
"What we are trying to do at SMART Lab is make it more
feasible, more economical for these detectors to be put in place
at every critical check point, at every airport and shipping
port," says Prof Harrison. "This will increase the
possibility of detecting and intercepting shipments of special
nuclear material, should they occur."
By Jim McMahon, Jim McMahon writes on instrumentation
version of this article