656 Chapter 11. Dosimetry and Radiation Protection
Good Spatial Resolution:A typical silicon microdosimeter system is based
on a number of closely spaced detectors as the one shown in Fig.11.5.2. With
small cell and pitch dimensions, of the order of 10-40μm, high spatial resolution
can be achieved.Low Voltage Operation: The active layer of p-type silicon of a SOI has a
width of about 2μm. This enables the operation of the detector at very low
voltages as opposed to TEPC that requires fairly high voltage to allow charge
multiplication.Another advantage of silicon microdosimeter with respect to TEPC is that its
overall dimensions can be made fairly small by integrating the front-end electronic
components on the same silicon substrate. This makes the handling of the system
easy by avoiding the need to install external electronic components near the detector.
SOI is not the only technology available to build silicon microdosimeters. In fact
the earliest silicon dosimeters were simple pn diode detectors. The SOI technology
has attracted more interest due to its advantages as described above. However it
should not be considered that the microdosimeters based on this technology are per-
fect devices. In general, silicon microdosimeters (or for that matter, microdosimeter
based on any semiconductor material) suffer from some disadvantages as well, some
of which are listed below.
Tissue Equivalence:Semiconductor materials can not be considered equiva-
lent to tissues. Hence the dose measured by them can not be said to represent
dose in tissue. To overcome this problem, some semiconductor microdosimeters
are equipped with a converter material.Geometry: The geometry of a silicon microdosimeters is parallelepiped as
compared to a TEPC that can have spherical geometry. This implies that
the chord length distribution in a silicon microdosimeter is different than in a
TEPC.Radiation Damage: Semiconductors are not very radiation tolerant, which
makes them vulnerable to radiation damage. The lifespan of a silicon detector
is therefore shorter than a TEPC.Field Funneling: The highLETparticles passing through a semiconductor
produce local distortions in the electric field. These distortions may favor
collection of charge pairs produced in the undepleted regions. For newer SOI
technology, this is not of much concern because of the insulator layers.11.6BiologicalEffectsofRadiation......................
All living organisms are made of cells, which themselves are made of atoms. The
radiation can interact with these atoms in many ways, as we have been discussing
throughout this book. At the most basic level, it can either ionize the atom or
interact with its nucleus. In terms of biological damage, both of these interactions
can have serious consequences if they result in weakening the bonds between atoms.
A weekend bond may eventually break up and cause the cell to malfunction.