Physics and Engineering of Radiation Detection

7.2. Position Sensitive Detection 449

strips. In digital mode of operation, where only binary information is recorded, the
resolution is given by

R≈

p

√

12

. (7.2.2)

This implies that the resolution can be improved by decreasing the pitch. However
the pitch can not be decreased to very low values due to engineering constraints and
therefore a compromise has to be made. Most silicon detectors are built with a pitch
of 40μm. The resolution can also be increased by operating the detector in analog
mode. The reason is that the relative distribution of charges amongst neighboring
strips can be used to reconstruct the spatial profile of the charge with the peak that
determines the actual position of particle interaction.
Reading out each and every strip of a microstrip detector is not always possible
or even desired. The reader can visualize how difficult it would be to make connec-
tions on pads that are only 40μmapart. Of course the soldering is done through
specialized machines but even then the task is extremely difficult. Also diagnostics
and repair of such a structure would be very labor intensive and involved. Therefore,
in general, the SMSD based systems have intermediate strips that are not read out.

Example:

Estimate the improvement in spatial resolution of a microstrip detector

operating in digital mode if the pitch is decreased from 100μmto 40μm.

Solution:

The resolution with the pitch of 100μmis given by

R 1 =

p 1

√

12

=

100

√

12

=28. 8 μm.

Similarly the resolution with 40μmpitch is

R 2 =

40

√

12

=11. 5 μm.

The improvement in resolution is then given by

δR =

R 1 −R 2

R 1

× 100

=

28. 8 − 11. 5

28. 8

× 100

= 60%. (7.2.3)

Perhaps the biggest advantage of semiconductor microstrip detectors over multi-
wire proportional chambers is that they can be constructed as double sided detectors.
In such a detector the strip implantation is done on both sides. Fig.7.2.8 shows a