This is what the raw material of the sensor looks like. Image: SINTEF |
At the Micro and Nano Laboratory in Gaustadbekkdalen in Oslo,
scientists have created one of the most advanced radiation sensors in
the world: an X-ray detector that can reveal the composition of
materials in a fraction of a second.
The
sensor has been developed by SINTEF nanotechnologists, and is already
an exclusive component in great demand by industries that supply
advanced analytical instruments for materials science.
This
type of sensor is known as a silicon drift diode (SDD), and it is the
basic component of a number of instruments that are used in everything
from medical X-ray systems to monitoring experiments at CERN, where
scientists are searching for the most basic building blocks of matter.
Another application is in art and archaeology, where the detector can
identify which materials have been used and just what they consist of.
Cleanliness an essential element
“The
sensor consists of a double-sided microstructure that is fabricated on
silicon wafers. Such structures are complex, and difficult to produce.
Today, we are one of only two or three suppliers of such sensors in the
whole world,” says research scientist Niaz Ahmed.
Although
the tiny device measures no more than 8 x 8 mm it takes eight weeks to
produce, and the entire fabrication needs to take place in a super-clean
environment.
“Even
a single grain of dust is capable of destroying the whole process by
short-circuiting the equipment, or damaging its nano-scale structures.
This is why the advanced laboratory is equipped with vibration-reducing
foundations and air-filtration systems that remove particles as small as
100 nm from the laboratory.
The
sensor is produced by oxidizing the silicon wafer in several stages,
creating a physical structure on nanometre scale. Once this has been
done, the scientists dope it with charged atoms at various levels. The
result is an incredibly light-sensitive diode which, once it has been
connected to the appropriate electronics, can reveal changes in the
physical structure of most materials.
Brilliantly efficient
The
sensor uses spectroscopy, which is based on sending light through a
transparent object. When the light beam emerges from the other side of
the object, the sensor read off changes in its characteristics.
“To
put it simply, we can say that the sensor sorts the light into its
individual energy levels by counting the photons and calculating their
energy,” says Ahmed.
Unlike
standard silicon-based sensors, the way the SDDs work requires them to
have structures on both surfaces of the sensor chip.
“This
can only be done with the help of advanced equipment and extremely high
levels of accuracy,” says the SINTEF scientist, and explains how.
One
side of the sensor is called the “window side”, and is turned towards
the source of radiation. It absorbs the X-ray beam almost without loss.
The other side is known as the “ring side” and has a concentric annular
structure; i.e. the rings have the same centre but increase in radius,
something like a parabolic aerial in microformat. This means that the
electrons generated by the radiation source are captured by the central
electrode, which in turn enables the X-ray sensor to discard all the
irrelevant electronic signals, or “noise”.
“Because
it easily distinguishes between different materials by registering
differences in the absorption energy of their component elements, the
chip can be used to identity forbidden materials such as lead, cadmium
and mercury,” explains Ahmed.
Due
to its unique sensitivity, the Norwegian-developed sensor is in very
high demand on the world market. The researchers have also managed to
make it so efficient that it uses very little energy, which is important
when the sensor is connected to other electronics.
