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

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.

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

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

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.

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

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

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]

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

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.

Input Rate and Pile-Up Rejection b 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 – 1200 ±4% c – 4800 ±8% b – 200 ±2% d – 1000 ±4% double rate ~ (peak rate)2 double rate ~ PUR resolving time c d

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

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

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

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

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

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

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

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

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. 92-96, August 1995.

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. 92-96, August 1995.

Experimental Tests ca. 1993

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

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

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

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

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)

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