1. Properties of Instrumentation
Nuclear Engineering 304

2. Objectives Learn how to use an oscilloscope
Significance of different modes of operation of oscilloscope
Measure the gain of an amplifier with an oscilloscope
Observe the output pulses from a
Linear amplifier
Single channel analyzer (SCA) [discriminator]
Detector
Operate the counters with the window of the SCA at several positions

3. Objectives (cont)
Obtain a 60Co energy spectrum with the multichannel analyzer (MCA)
Operate the MCA in pulse height and time modes
Understand the function and operation of each component used in this experiment
Understand the limitations of each component and the interactions between various components
Become competent in the operation of selected counting equipment so that future experiments can be conducted in a timely manner

4. Precautions
Be careful with charge sensitive preamplifiers. Turn off the voltage before disconnecting. This preamplifier is easy to damage and expensive to repair.
Verify that the input voltage to a particular device is within specifications.
Use an oscilloscope to review your output signals from each NIM device.
Improper termination and mismatched impedances (either 50  or 1M ) will cause signal echoes – known as ringing – which lead to false counts.

5. Equipment Oscilloscope Scaler and timer Multichannel analyzer (MCA)
Single channel analyzer (discriminator)
Precision pulser
Scintillation detector
Photomultiplier tube
Preamplifier
Linear amplifier
Cables and connectors

6. Oscilloscopes
Oscilloscope is analogous to a camera that captures signal images that we can measure and interpret
Is it an accurate picture of what actually happened?
Is the picture clear or fuzzy?
How many pictures can we take per second?
Characteristics of oscilloscopes
Number of channels
Bandwidth
Digital vs. Analog
Sample rate
Rise time

7. Characteristics of Oscilloscopes
Most scopes have either 2 or 4 channels (inputs)
Can use 2nd channel as trigger for 1st channel
View one or more waveforms at once
Bandwidth
Typical range 100 – 400 MHz affordable scopes (~ 50 GHz max)
Determines fundamental ability to measure a signal and resolve high-frequency changes
Note that limiting the bandwidth reduces noise and provides a cleaner signal

8. Characteristics of Oscilloscopes
Sample rate
How frequently a digital oscilloscope takes a snapshot or sample of the signal.

9. Oscilloscope Fundamentals
Front panel of scope divided into four main sections
Vertical (volts/div)
Horizontal (sec/div)
Trigger
Display
Three basic adjustments
Attenuation or amplification of the signal. Use the volts/div control to adjust the amplitude of the signal
Time base. Use the sec/div control to set the amount of time per division.
Use the trigger level to stabilize a repeating signal, or to trigger on a single event.

10. Oscilloscope Fundamentals
Vertical System Controls
Termination (1M or 50 ohm)
Position
Allows you to move waveform up and down on screen
Volts/Division
Scale factor. The maximum voltage you can display on the screen is the volts/div setting multiplied by the number of vertical divisions.
Coupling (AC, DC, Ground)
DC coupling shows all of an input signal
AC coupling blocks the DC component of a signal so waveform is centered around zero
Ground disconnects the input signal from the vertical, which lets you see where zero volts is located on the screen. Switching from DC to ground is a handy way of measuring signal voltage with respect to ground.

11. Oscilloscope Fundamentals
AC and DC Input Coupling

12. Oscilloscope Fundamentals
Horizontal System Controls
Trigger Position
Allows you to move waveform left and right on screen
Seconds/Division
Scale factor. Selects the rate at which the waveform is drawn across the screen
XY mode lets you display an input signal, rather than the time base, on the horizontal axis
This mode of operation allows for phase shift measurement techniques

13. Oscilloscope Fundamentals
Trigger Controls
Crucial for clear signal characterization because it synchronizes the horizontal sweep at the correct point of the signal, thus
Trigger Sources
Any input channel
The power source signal
An external source other than the signal applied to an input channel
A signal internally defined by the oscilloscope, from one or more input channels
allowing you to stabilize repetitive waveforms.

14. Oscilloscope Fundamentals
Trigger Modes
In normal mode the oscilloscope only sweeps if the input signal reaches the set trigger point; otherwise (on an analog oscilloscope) the screen is blank or (on a digital oscilloscope) frozen on the last acquired waveform. Can be disorienting since you may not see the signal at first if the level control is not adjusted correctly.
In auto mode the oscilloscope will sweep, even without a trigger. If no signal is present, a timer in the oscilloscope triggers the sweep. Ensures that the display will not disappear if the signal does not cause a trigger.

15. Oscilloscope Fundamentals
Trigger Level and Slope
The slope control determines whether the trigger point is on the rising or the falling edge of a signal. A rising edge is a positive slope and a falling edge is a negative slope
The level control determines where on the edge the trigger point occurs

16. Nuclear Instrumentation
Scaler / Timers
Adjustable timer used to determine count window
selectable resolution of 0.01 seconds or 0.01 minutes
3 levels of multipliers (NM * 10P)
Scaler is the basic NIM counting system
Usually at least 100 Mhz (100 million counts per sec)
Accept negative or positive pulses (jumper selectable)
Pulse pair resolution ~ 10 ns
Min. pulse width to be counted ~ 4 ns
Input range ~ +100 mv to + 10V
Must “filter” input using SCA to prevent low level noise from appearing to be actual counts

17. Nuclear Instrumentation
Single Channel Analyzer (Discriminator)
Prepares amplifier output pulse for input into counting devices
Without discrimination, system noise causes false counts
Gives standardized (+) output or logic signals (fast or slow)
Improper settings results in lost counts or measuring noise
Discrimination of pulses above and below a certain amplitude
Only pulses corresponding to a energy window are passed
Lower threshold set at E, upper threshold at ∆E
All voltage pulses between the given energy range are counted

18. Nuclear Instrumentation
Linear Amplifier
Amplification is minor role, pulse shaping is true purpose
Complex active filters, Gaussian
Convert preamplifier output into suitable signal
Generally used for semiconductor detectors, proportional counters, and scintillation detectors
Usually do not use Linear Amp and SCA in same signal path
Major features
Coarse gain, fine gain, selectable input polarity
Selectable pulse shaping time (cutoff frequency)
Front panel BNC inputs or rear pre-amp inputs

19. Nuclear Instrumentation
Precision Pulser
Simulates output from scintillation detector/preamplifier
Method to test and calibrate counting system
Major features
Repetition rates up to 2 kHz
Selectable square or tail pulse
Three decay constants for tail pulse
Selectable pulse height 0 to +10V (0 to +5V at 50 termination)
SYNC provides trigger signal pulse for oscilloscope
Attenuators to change between various signal levels

20. Detectors, PMT’s & PreAmps
The general types of detectors
Gas-filled detectors (ion chambers, GM, proportional detectors)
Semiconductor detectors (HPGe, CdTe, Si)
Scintillation detectors (NaI, fast plastics)
Good timing and poor energy resolution (fast plastics)
Good energy resolution (NaI) – but not as good as semiconductors
 has no mass or charge, it cannot be detected directly
 interaction produces cascade of secondary charged particles
Photoelectric effect
Compton scattering
Pair Production
Electrons interactions produce pulses of light (fluoresce)
Fluorescence converted to electrical pulse by PMT

21. Detectors, PMT’s & PreAmps
Photomultiplier tube converts extremely weak light output from crystal into electrical signal
Only a few hundred light pulses in typical interaction
PMT amplification creates 107 – 1010 electrons per pulse
Have fairly linear response, and maintain most of the timing information of original event

22. Detectors, PMT’s & PreAmps
Three essential functions
Conversion of charge to voltage pulse
Signal amplification
Pulse shaping
Place close to detector to minimize noise & transmission losses in cables
Output pulse with an amplitude proportional to the integrated charge from the detector
Input from the anode signal
Positive polarity pulse output
May have separate anode and dynode outputs
Used with HPGe, NaI, plastics scintillators and proportional detectors

23. Radioactive Sources Co-60 sources
60Co  60Ni* + β- + anti- [ MeV total for beta & antineutrino] 60Ni* 1.17 MeV MeV 
T1/2 = 5.27 years
Don’t assume 3 months time doesn’t make difference in activity, ~ 4%
Always express in mCi or some equivalent
1 dps = 1 Bq
1 mCi = 3.7 E 10 dps = 2.22 E 12 dpm
1 measured count ≠ 1 source disintegration due to detector efficiencies
Need efficiencies or coincidence counting techniques to determine measured activity

24. MultiChannel Analyzer (MCA)
Two primary modes
PHA = Pulse Height Analysis (Counts vs. Energy)
MCS = Multichannel Scaling (Counts vs. Time)
In PHA mode can be thought of as a series of SCA’s
Collects pulses in all voltage ranges at once
Have height or amplitude proportional to energy
Count the number of occurrences at each energy and form histogram
Displays information in real time
In MCS modes can be though of a a series of scalers
Each channel corresponds to a time “bin”
Step through channels based on internal clock or external pulse (trigger)
Applications include half-life measurements

25. References Tektronix Canberra Ortec Ludlum Instruments
Canberra
Ortec
Ludlum Instruments