chapter 5

Questions and Answers

What is the main purpose of pulse shaping in signal conditioning?

To get information from the pulse with high precision (correct)

To increase the decay time of pulses

To reduce the amplitude of pulses

To reduce the rise time of pulses

What is the result of pulse overlap at high count rates?

Saturation

Pulse pile-up

Pulse distortion

All of the above (correct)

What is the time constant of a discharging capacitor in a simple RC circuit?

1/RC

R*C

RC (correct)

R/C

What is the time constant of a circuit with a 1 μF capacitor and a 1kΩ resistor?

1 ms Signup and view all the answers

What is the purpose of using a linear amplifier in pulse shaping?

To shape pulses into square signals Signup and view all the answers

According to the 5RC rule, how long does it take for a capacitor to charge to within 1% of its final value?

5RC Signup and view all the answers

What is the main advantage of fast pulses in timing applications?

They have a fast rise time Signup and view all the answers

What type of filter is composed of a series resistor and capacitor?

High-pass filter Signup and view all the answers

What is the effect of stray capacitances, inductances, and resistances on fast pulses?

They cause distortion of pulses Signup and view all the answers

What is the purpose of adjusting the decay time of pulses in preamplifier circuits?

To allow for complete charge collection Signup and view all the answers

What is the purpose of a pulse shaper in linear amplifiers?

To shape the pulse for analysis Signup and view all the answers

What is the primary issue with the output of a single CR differentiating circuit?

The signal has a sharp top and poor signal-to-noise ratio Signup and view all the answers

What is the relationship between the amplitude of pulses and the energy of the original radiation?

The amplitude is directly proportional to the energy Signup and view all the answers

What is the function of an operational amplifier in CR-RC shaping?

To isolate the two individual networks Signup and view all the answers

What is the purpose of pulse shape analysis in radiation detection?

To determine the location of the interaction Signup and view all the answers

What can be inferred about the radiation interaction from the rise time of the pulse?

The location of the interaction Signup and view all the answers

What is the function of the HPGe detector in a typical measuring system?

Sensing element Signup and view all the answers

What is the amplitude of a pulse measured from?

Baseline to its maximum Signup and view all the answers

What is the main cause of overshoot and undershoot in pulses?

Electronic CR-RC networks Signup and view all the answers

What is the usual method of measuring the signal width of a pulse?

Full width at half maximum (FWHM) Signup and view all the answers

What is the purpose of the spectroscopy amplifier in a typical measuring system?

Processing Signup and view all the answers

What is the difference between unipolar and bipolar pulses?

Unipolar pulses have a positive amplitude, while bipolar pulses have both positive and negative amplitudes Signup and view all the answers

What is the typical characteristic of slow pulses?

Slow rise time compared to transit time (t-rise > 100ns) Signup and view all the answers

What is the purpose of the pre-amplifier in a typical measuring system?

Signal conditioning Signup and view all the answers

Study Notes

Circuit Response

A 1 μF capacitor placed across a 1kΩ resistor has a time constant of 1 ms.
After a long time (t » 5RC), the voltage approaches its final (equilibrium) value.
The 5RC rule states that a capacitor charges or decays to within 1% of its final value after about five time constants.

CR Differentiator or High-Pass Filter

The output of a single CR differentiating circuit has several unwelcome features for pulse processing systems.
The sharp top makes any subsequent pulse height analysis difficult.
All high-frequency noise components are passed through by the circuit, implying that the signal-to-noise ratio is usually poor.

CR – RC Shaping

The combination of a CR differentiator and RC integrator is commonly used as a pulse shaper in linear amplifiers.
If an RC integrating stage follows the CR stage, both of the unwelcome features mentioned above can be much improved.
The use of an operational amplifier (op amp) provides impedance isolation, isolating the two individual networks.

Pulse Shape Analysis (PSA)

PSA involves analyzing the shape of the pulse to get information on the interaction position (particle tracking).
Radiation interaction can happen anywhere in the detector sensitive material.
Pulses from interactions near the collecting electrode have shorter rise times.

Pulse Shaping

The shapes of signal pulses from detectors are usually changed or shaped by the signal conditioning or processing elements of the data acquisition system (DAC).
Preamplifier signal has a short rise time and long decay time.
Shaping of the pulse is necessary to get information from the pulse with high precision: pulse height, pulse width, rise time, and fall time.

Pulse Characteristics

A pulse has height (amplitude), width, rise time, and decay time.
Baseline is the voltage (current) level to which the pulse decays (usually set to zero).
Pulse height (amplitude) is the height of the pulse measured from the baseline to its maximum.
Signal width is usually taken as the FWHM.
Leading edge is the flank of the signal that comes first in time.
Trailing edge is the flank of the signal that comes last in time.
Rise time is the time it takes the pulse to rise from 10% to 90% of the full amplitude.
Fall time is as for rise time, but related to the decay of the pulse.

Pulse Defects

Typical pulse defects are overshoot, ringing, tilt, and undershoot.
Overshoot is due to electronic CR-RC networks.
Ringing is due to reflections in the transmission lines.
Tilt is due to attenuation.
Undershoot is due to electronic CR-RC networks.

Pulse Types

Signal pulses can be unipolar or bipolar.
Slow pulses have slow rise time compared to transit time (‘t’rise > 100ns) and are generated by linear devices.
Fast pulses have fast rise time compared to transit time (‘t’rise ~ 1ns) and are used for timing and high count rate applications.

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