Physics and Engineering of Radiation Detection

8.6. Discrimination 495

Vin

Vout

Vh

Vl

Time

Time

Logic High

Figure 8.6.1: Pulse discrimina-

tion by a two-level discrimina-

tor. Only pulses that are be-

tween the two preset thresholds

are counted by the subsequent

circuity.

developers could be integrated into the system, interconversion of logic levels could
pose an engineering problem. To avoid this problem, certain logic standards have
been developed. It is highly recommended that the electronics designers try their
best to use one of these standards in their circuitry. The following are the most
commonly used logic standards.

NIM Logic:NIM stands for Nuclear Instrumentation Modules, a set of mod-

ules that were developed for nuclear instrumentation. A logic was developed

specifically for these modules. In this logic 0V is defined as logic 0. Logic 1,

however, is defined in terms of current and not voltage. A current of -16mA

into a 50 Ω resistor corresponds to logic high, which is equivalent to about -800

mV.

TTL Logic:TTL is an acronym of Transistor Transistor Logic. It is perhaps

the most widely used logic standard. As opposed to NIM, it is defined solely

in terms of voltage: a voltage of 0V corresponds to logic 0 while the logic is

said to be high if the level is between 2V and 5V.

ECL Logic:The Emitter Coupled Logic or ECL is based on differential am-

plification of the digital signals and adjustment of the dc voltage levels through

emitter followers. The ECL logic levels are conventionally defined as -1.6Vfor

low and -0.75Vfor high.

8.6.A PulseCounting..........................

In the previous chapters we saw that in most detection systems the height of the
output pulse is proportional to the energy delivered by the incident radiation. This
implies that the energy spectrum of the radiation can be determined by counting
pulses of different amplitudes. This is essentially what is done in spectroscopic
systems. However counting pulses is not limited to radiation spectroscopy since one
might even desire to simply count all the pulses to determine the total radiation