1. Radiation Measurements - Concepts and Misconceptions
Valentin T. Jordanov
Yantel, LLC · Los Alamos, NM · USA

2. The Art of Lecturing
Glenn Knoll was an excellent teacher with an impeccable lecturing style
This is what I have learned from Glenn Knoll about being a good lecturer
Prepare ahead, know the subject well
Organize the talk to meet time requirements
Speak with deeper, clear voice
Be confident of what you say, there is no room for doubts
Don't get distracted, especially by your own thoughts
Use proper language
I have never mastered anything from above except the last one because
“Never make fun of someone who speaks broken English. It means they know another language.” H. Jackson Brown Jr.

3. Concepts and Misconceptions
an abstract or generic idea generalized from particular instances
something conceived in the mind
Concept - an idea of what something is or how it works
Misconceptions - a false idea or belief
Concepts-Misconceptions are dynamic
Concepts may be forgotten and rediscovered
Concepts may turn into misconceptions and vice versa
Concepts-Misconceptions can be a driving factor

4. Misconceptions as Beliefs
visual BELIEF
influential BELIEF
AC COUPLING
(high-pass RC filter)
NaI(Tl) LIGHT OUTPUT
TEMPERATURE DEPENDENCE
Misconception – belief that the above curve represents the temperature dependence of the NaI(Tl) light output
In reality this curve represents the temperature dependence of combined effects of the light output and the pulse shaper response
Sources of influence – repetitive, wide spread citations in publications, text books, commercial literature, presentations etc.
Misconception – belief that the response to unipolar signal is unipolar signal when RC is very, very large compared to the pulse width PW as seen on the oscilloscope screen
In reality the output signal of the AC coupling network is always bipolar
The output is perfectly zero balanced
Charge Conservation Law

5. The Concept of Radiation Measurement System
PULSE MODE
The system response to radiation are counts per unit time (counting rate).
The input (physical) and the output (abstract) signals are stochastic - random in nature.
Counting rate unit - counts-per-second (cps).
The output relates to the radioactivity which is measured in SI units Bq (Becquerel).
The pulse processor optimizes signal-to-noise ratio and applies various criteria to measured pulses in order to increment a specific counter at the system output.
Minimal configuration - one counter and one timer. Multichannel analyzers, for example, have thousands of counters and more than one timer.

6. The Misconception of Measurement Units
Particle radiation is emitted randomly in time
Detector interactions are also random in time
Burst radiation from accelerators or other generators is also randomly emitted within the duration of the burst
Poisson Statistics
Pulse mode measurements – the output is counting rate, unit [cps]
Wrongly the SI unit for frequency [Hz] is used as unit for the output of the pulse mode system – the measurement unit misconception.
Frequency is a characteristic of deterministic signals and there is absolutely no justification to use it for stochastic signals.
Is it appropriate to use Hz for radioactivity (stochastic process) – e.g. Cs 137 with 1.5GHz activity? What about using the Becquerel (Bq) to measure frequency or time – e.g. set the wake-up alarm clock at 2x104 Bq-1

7. Counting Throughput - Deterministic and Stochastic Pulse Signals
Periodic Pulse Signal
Random Radiation Signal
Counting Throughput
PW = 1ms – Pulse Resolving Time
Counting random signals produce throughput at any incoming rate and any finite pulse width

8. Spectral Measurements – Potential for Confusion
Radiation Spectrum
Natural Uranium
70keV to 200keV
Radiation Measurement - Spectroscopy
HORIZONTAL AXIS – Energy or Amplitude
VERTICAL AXIS – Counting Rate [cps]
Using Hz may cause confusion
Hz makes counting rates look like frequency
counting rate [cps]
energy or amplitude
Electrical Measurement - Spectroscopy
HORIZONTAL AXIS – Frequency [Hz]
VERTICAL AXIS – Energy or Amplitude
Peaks may look similar to the radiation peaks
Confusion is potential when Hz is used as a unit for counting rates
Frequency Spectrum
Trumpet at a Concert
0 Hz to 6000 Hz
energy or amplitude
frequency [Hz]

9. Wrong Measurement Units – Potential for Disaster
Mars Climate Orbiter
1999
Disintegrated in the atmosphere of Mars
WHY?
Measurement Units Discrepancy
The Orbiter flight control system was designed by NASA to accept trust instructions in Newtons (N) – a SI Unit for force
Lockheed Martin provided ground based control software that generated instructions for trust control in pound- force (lbf) – an Imperial Unit for force
1 lbf = 4.45 N
The misspowered thrusters pushed the Orbiter into the Martian atmosphere where it was lost
Cost of the mission $328 million

10. Input Rate, Throughput and Counting Statistics in Radiation Spectroscopy
Statistical fluctuations in the number of counts – Poisson Probability Distribution
Total Number of Counts = Time * Throughput Rate
Spectra represent an approximation of the Probability Distribution Function (PDF) of the detector/shaper pulse heights – area sampled PDF
Pulse pile-up causes spectral distortion
It is impossible to reject double, triple etc. coincidence pulse pile-up as they are generated by the detector as a single event
Fast pile-up rejectors may reduce the pile-up effect to a certain level. There is a limitation on their resolving time
The Incoming counting rate determines the coincidence pulse pile-up for a given set of a pulse shaper and pile-up rejector
To reduce spectral distortion due to the coincidence pulse pile-up the incoming counting rate should be kept as low as practical.

11. Input Rate and Pile-Up Rejectionb
Pulse Shaping
Triangular
2.5ms Peaking Time
Pile-Up Resolving Time
a,c – 88ns
b,d – 500ns
Counting Rate
a,b – 8000 cps
c,d – 1600 cps
Peak to Double Peak Ratio
a – ±4%
c – ±8%
b – ±2%
d – ±4%
double rate ~ (peak rate)2
double rate ~ PUR resolving time c d

12. High Throughput System – Single Detector Multiple Shapers
Adaptive System
Single Detector
N Different Shapers
Only one pulse height from the shapers is used to increment the spectrum
If there is a pile-up condition then the pulse height of the shaper with the longest peaking time and free of pile-up is used
Requires perfect gain matching
May exhibit ballistic deficit sensitivity as the shapers have different impulse response
Major drawback – resolution is function of the incoming rate
High throughput determined by the shortest pulse shape

13. Single Detector – Multiple Shapers Analytical Performance
Single Detector – significant coincidence pile-up at high counting rates
Resolution is a function of the input rate
Reduces the measurement time due to the high throughput
A single shaper with the shortest shaping time may provide similar results

14. Alternative High Throughput System
Parallel System
N Identical Detector
N Identical Shapers
All pulse heights from the shapers are used to increment the spectrum
Pile-up rejection in each shaper
High throughput by adding the counts from all detectors
Matching resolution of all detector channels
Less sensitive to coincidence pile-up as the input rate is distributed among N detectors
Drawback – requires N detectors

15. Which Approach to Choose?
Example Cs-137 / Eu-154 (662keV, 1274keV)
Cs-137 peak (662keV) incoming rate RCS, double peak (1324keV) rate DCS
PUR resolving time - TPUR
𝐷𝐶𝑆 𝐵 =𝑁 𝑅𝐶𝑆 𝑁 (1− 𝑒 − 𝑅𝐶𝑆 𝑁 ∗𝑇𝑃𝑈𝑅 )
𝐷𝐶𝑆 𝐵 ≈ 𝐷𝐶𝑆 𝐴 𝑵
Double Peak Intensity
Reduced by ~ N!
𝐷𝐶𝑆 𝐴 =𝑅𝐶𝑆∗(1− 𝑒 −𝑇𝑃𝑈𝑅∗𝑅𝐶𝑆 )
FWHM = f(incoming rate)
FWHM = constant

16. The Concepts of Analog and Digital Pulse Processing
Analog Systems
Accept and process physical signals
Continuous time domain
process all signal information
excellent timing
Arithmetic
true integration
easy addition/subtraction
difficult multiplication/division
Signal can be delayed - distortions, no waveform storage
Digital Systems
Accept and process abstract signals
Discrete time domain
digitized signals – loss of information carried by the digitized physical signals
sampling and quantization effects
Arithmetic
accumulation instead of integration
easy arithmetic operations
Storage and/or delay of signals without any distortions – BIGGEST ADVANTAGE OVER THE ANALOG SYSTEMS

17. Is Analog Dead?
In my opinion digitization of the raw detector signals without proper and/or specialized analog conditioning is a misconception
Radiation measurement signals are physical signals which always will require analog signal processing. This could be in its simplest implementation using a single resistor network
Fast signals in the nanosecond range require either analog signal processing or digitization at GHz sampling rates. Such sampling rates limit greatly the practicality and the real time operation of the radiation measurement systems
Pulse-shape discrimination of signals from fast scintillators is a typical example

18. Sampling Fast Signals (PMT Anode)
ADC sampling frequency critical for extracting signal information - GHz range
Real time digital pulse processing - Data Processing Frequency ≥ ADC Sampling Frequency – limitation at GHz frequencies, especially for complex math
Limitations at ADC lower sampling frequencies - clock-event synchronization and poor signal representation

19. Sampling Integrated Signals
The integrated signal contains all of the information delivered by the fast anode signal
Can be sampled at lower frequency
Clock-event synchronization is still needed

20. Synchronous Sampling of a PMT Integrated Signal
Signals
a – Constant Fraction Timing Signal
b – PMT Anode Signal
c – Integrated Anode Signal
d – Synchronous ADC Sampling Clock FC TC
Charge Comparison Method
d (75MHz ADC Sampling frequency) is synchronized with a (constant fraction discriminator)
V. T. Jordanov, G. F. Knoll, "Digital Pulse-Shape Analyzer Based on Fast Sampling of an Integrated Charge Pulse", IEEE Trans Nucl Sci, Vol. 42, No 4, pp , August 1995.

21. Figure of Merit – Integrated Signal PSD
V. T. Jordanov, G. F. Knoll, "Digital Pulse-Shape Analyzer Based on Fast Sampling of an Integrated Charge Pulse", IEEE Trans Nucl Sci, Vol. 42, No 4, pp , August 1995.

22. Experimental Tests ca. 1993

23. A New Concept for Pulse Shape Analysis – Front-End
HARDWARE
Simple but dedicated analog front-end electronics
Clock-event synchronization is not needed
Works with fast scintillators with pulse shape discrimination capability
Can be used with PMT or other light readout devices
Can be implemented in a miniature, low power ( < 1W) portable device

24. Principle of the New PSD Concept*
Ballistic
Deficit
Based on the gamma-neutron difference due to ballistic deficit in pulse heights
Measurement of time intervals between constant fraction points of the pulses
Amplitude independent measurement of multiple time intervals for a single pulse
*Patent Pending

25. Time Interval Linear Filter
Constant
Fraction
Discriminators
Pulse Shape Identifier
Constant Fraction Thresholds

26. Neutron-Gamma Experimental Pulse Shapes 125MHz ADC
1.25MeVee
350keVee
125keVee
33keVee

27. Radiation Measurement Instrumentation vs Consumer Technology
IBM370
1966
2016
It is easier to denounce new concepts than expressing doubt in believed old misconceptions (research community)
Old, good concepts or misconceptions are constantly abandoned in search for something new (market economy)

28. The Real World Proof of Concepts
What may look a bad idea or misconception for someone can be a perfect concept for others, especially when the outcome of its real world application is REWARDING