Elimination of external
vibrations plays a key role in maintaining a stable growth
process and maximizing the total volume yield of very large
crystals. Jim McMahon reports.
At Kansas State University, a facility is dedicated 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 country. It focuses
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 miniaturised gas-filled detectors.
The laboratory builds detectors from start to finish in
readily deployable packages for use in better securing borders
from nuclear materials such as plutonium and uranium.
The gamma ray detection aspect of the laboratory's research
is focused on the discovery and development of new dense,
high-Z* semiconductor materials, such as cadmium zinc telluride
(CdZnTe or CZT) and mercuric iodide (HgI2), and several
scintillator materials including lanthanum tribromide (LaBr3)
and cerium tribromide (CeBr3). Research conducted on large
crystal growth with high-Z semiconductor and scintillator
materials has produced large crystal ingot yields.
Gamma rays are electromagnetic radiation of high frequency
(very short wavelength), 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
a 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
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 other to 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 reliable system to produce large crystals at
an economical cost has not been achieved until relatively
Higher ingot yields enable smaller,
faster and more accurate sensors, and allow gamma-ray
detectors to be more economical and field-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° - 1100°C. 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, trimmed to size with a diamond wire
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 destabilize the growth
interface," says Professor Mark Harrison 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," continues Harrison.
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 a 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 0.5Hz, achieve 93 percent isolation efficiency
at 2Hz; 99 percent at 5Hz; and 99.7 percent at 10Hz.
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
"One of the concerns we had was surface waves coming
across the ground, which induced vibration in the crystal
growth system," explains Harrison.
"We are located in a basement," Harrison 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."
Special nuclear materials
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
"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," Harrison says. "This will
increase the possibility of detecting and intercepting shipments
of special nuclear material, should they occur."
The SMART Laboratory serves as a centre for undergraduate
and graduate student education, as well as a facility, to
accommodate funded research projects from various government
and industrial sponsors. Since the opening of the Laboratory
in 1998, eleven patents have been awarded to SMART Laboratory
researchers for various detector designs, with several more
The laboratory's equipment includes an assortment of semiconductor
processing equipment, including a linear-drive diamond cutting
wheel, diamond wire saw, precision slurry saw, wafer dicing
saw, precision lapping and polishing machines, a custom
chemo-mechanical polishing system, a custom six-pocket e-beam
evaporator, a 4-pocket evaporator, two dual filament evaporators,
an ion mill, vacuum annealing chamber, fission chamber plating
station, mask aligners, microscopes, ovens, grinders and
furnaces for annealing, sintering, diffusions and oxidations.
The Lab has numerous crystal growth furnaces used to grow
CdZnTe, LaBr3, and HgI2 crystals for radiation detector
development, which include 40 horizontal and 10 vertical
HgI2 vapour transport furnaces, two high pressure vertical
Bridgman furnaces, two low pressure vertical Bridgman furnaces,
three zone-melt furnaces, a vapor transport purifying furnace,
and a GaAs LPE furnace.
Central to SMART Laboratory is a Class-1000 cleanroom where
radiation detectors are fabricated. The Laboratory is equipped
with a scanning electron microscope, Auger electron analyzing
system, IV and CV tracers, ellipsometer, probers, radiation
sources and Nuclear Instrumentation Module (NIM) electronics
to test and characterise radiation detectors and materials.
* The atomic number which uniquely identifies a chemical
element is represented by the symbol Z. Also known as the
proton number, it is the number of protons found in the
nucleus of an atom and identical to the charge number of
Jim McMahon writes on instrumentation technology. Semiconductor
Materials and Radiological Technologies Laboratory (SMART
Lab) is based at Kansas State University, Kansas, USA. www.k-state.edu.
The Nuclear and Radiological Engineering Department, University
of Florida is based in Gainesville, FL, USA www.ufl.edu.
Minus K Technology Inc is based in Inglewood, CA, USA. www.minusk.com