Physics and Engineering of Radiation Detection

720 Chapter 13. Data Acquisition Systems

requires proper calibration since the relationship between time and charge may not
be perfectly linear. Apart from a TAC, one also needs atiming discriminatorto
build a time spectroscopy system. A timing discriminator can simply be a fast
single channel analyzer. Recall that a single channel analyzer produces a logic pulse
whenever the input pulse amplitude is between a preset amplitude window. The
logic pulse thus produced also has a definite relationship to the arrival time of the
linear pulse. That is the leading edge of the logic pulse can be used to determine
the arrival time of the linear pulse, provided the internal electronic delay is precisely
known. Most single channel analyzers also provide the user the with possibility of
adding additional delay if required.
The time resolution required for time spectroscopy depends on the particular
requirements of the experiment. However for typical applications it ranges from a
few nanoseconds to a few picoseconds. Obtaining such a high resolution requires
fairly fast electronic components with fast response and settling times. Fig.13.1.4
shows the block diagram of a typical time spectroscopy system. All the components
in the chain must have fast timing to achieve the required overall timing resolution.

Detector Fast Preamplifier Fast Amplifier DiscriminatorFast Timing

Time−to−AmplitudeConverter PC

Figure 13.1.4: Block diagram of a simple time spectroscopy system.

Sometimes it is desired to perform both timing and energy spectroscopy at the
same time. In such a case one can simply split the preamplifier output into two
pulses such that one goes into the time spectroscopy system while the other passes
through the energy spectroscopy system.

13.1.DCoincidenceSpectroscopy

Time coincidence units are used to determine the coincidence between time of ar-
rival of particles. For example, in a PET scanner two back-to-backγ-rays in time
coincidence represent an electron-positron annihilation event. Additionally in some
experiments one might also want to obtain the energy spectrum seen by one of the
detectors.
Fig.13.1.5 shows a simple but complete time coincidence system. The particles
are detected by two separate detectors that are usually of same type. The pulse
is first amplified and shaped before being transported to the timing single channel
analyzer. Whenever the height of a pulse is within the acceptable SCA window set
by the user, a logic pulse gets sent to the coincidence module. If two such pulses
arrive at the module within a time window set by the user, it generates a logic
pulse. This logic pulse can be used by a multi channel analyzer to retain the pulse
height information of one of the pulses related to that particular event. Note that