Physics and Engineering of Radiation Detection

11.5. Microdosimetry 655

B.3 Silicon Microdosimeter

Microdosimeters based on silicon are a relatively newer development. As we will see
they offer some advantages over conventional TEPCs and therefore they are gaining
popularity in the field of hadron therapy.
A silicon microdosimeter is similar to a conventional pn diode detector. However
it has some features that are specific to the requirements of microdosimetry. These
requirements are listed below.

Tissue Equivalence: Tissue equivalence is of course the basic condition in

microdosimetry for radiation therapy. A silicon detector should be constructed

such that it satisfies this condition as closely as possible.

Small Size:The active volume of the detector should be so small that it could

simulated a biological cell.

Defined Collection Volume: We saw earlier that TEPCs suffer from dis-

advantage of either not having a well defined volume or suffering from wall

effects. The condition of well defined volume is important in the case of silicon

microdosimeters as well.

High Sensitivity: The detector should be able to measure very low lineal

energies, of the order of 1keV/μm.

The condition of well defined collection volume is not satisfied in conventional pn
junction diode detectors. The main cause of the uncertainty in active volume is the
diffusion of charges from the bulk silicon. To overcome this problem, a new type
of silicon device, called silicon-on-insulator or SOI, has been introduced. The main
idea is to subtend the depletion region between insulators such that the possibility
of charge transfer from the bulk silicon is minimized. A simplified diagram of such
a device is shown in Fig.11.5.2.

Silicon Substrate

N p−Type Silicon

+

SiO

Al

SiO 2

2

SiO 2

Figure 11.5.2: Structure of a typi-

cal silicon-on-insulator microdosime-

ter. The potential to the substrate

is applied through a P+layer (not

shown) similar to theN+layer. The

connection to the supply is made

by feed through aluminum implant.

A typical mirodosimeter system has

a number of similar structures im-

planted on a single substrate.

There are many advantages of the silicon-on-insulator technique, some of which
are listed below.

Well Defined Volume: The active volume of the dosimeter is defined by

the depletion region. Since the region is surrounded by insulator therefore the

charge diffusion from outside is minimum and hence the volume is well defined.