Pulse and Pulse Processing

1. Pulse and Pulse Processing Supriya Das Centre for Astroparticle Physics and Space Science Bose Institute supriya@bosemain.boseinst.ac.in 4th. Winter School on Astro-Particle Physics (WAPP 2009) Mayapuri, Darjeeling Measure what is measurable, and make measurable what is not so. - Galileo Galilei

2. Indirect detection Direct detection Flow through the processing electronics Pulse : How does it appear? WAPP 2009, Mayapuri, Darjeeling

3. Pulse : Where are the information? Brief surges of current or voltage in which information may be contained in one or more of its characteristics – polarity, amplitude, shape etc. Baseline Pulse height or Amplitude Signal width Leading edge / Trailing edge Rise time / Fall time Unipolar / Bipolar WAPP 2009, Mayapuri, Darjeeling

4. Amplitude or shape varies continuously • Proportionately with the information • signal from microphone • signal from proportional chamber Rise time – a few nanoseconds or less • Quantized information in discrete number • of states (practically two) • pulse after discriminator Rise time – hundreds of nanoseconds or greater Pulse : How do they look? Analog or digital? Fast or slow? WAPP 2009, Mayapuri, Darjeeling

5. Logic standards Nuclear Instrumentation Module (NIM) Slow positive NIM Fast negative NIM Transistor-Transistor Logic (TTL) and Emitter Coupled Logic (ECL) WAPP 2009, Mayapuri, Darjeeling

6. Typically C ~ 100 pF/m and L ~ few tens of mH/m Signal transmission • Signal is produced at the detector – one needs to carry it till the Data • Acquisition system – How? What are the things one needs to keep in mind? • transmission of large range of frequencies uniformly and coherently over the required • distance, typically a few meters. • For transmitting 2-3 ns pulse the transmission line have to be able to transmit signals with • frequency up to several 100 MHz. One solution (the best one), Coaxial cable : Two concentric cylindrical conductors separated by a dielectric material – the outer conductor besides serving as the ground return, serves as a shield to the central one from stray electromagnetic fields. WAPP 2009, Mayapuri, Darjeeling

7. Characteristic Impedance : Signal Transmission (contd.) Q. All coaxial cables are limited to the range between 50 – 200 W. Why? • Reflection, Termination, Impedance matching: • Reflection occurs when a traveling wave encounters a medium where the speed of propagation is • different. • In transmission lines reflections occur when there is a change in characteristic impedance. • Reflection coefficient r = (R-Z)/(R+Z) , where R is the terminating impedance. • if R > Z, the polarity of the reflected signal is the same as the propagating signal and the amplitude of reflected signal is same or less as of that of the propagating signal • in limiting case of infinite load (i.e. open circuit), the amplitude of the reflected signal is the same of the propagating signal • if R < Z, the polarity of the reflected signal is the opposite to the propagating signal and the amplitude of reflected signal is same or less as of that of the propagating signal • in limiting case of zero load (i.e. short circuit), the amplitude of the reflected signal is the same of the propagating signal More on all these during the practical session with Atul Jain WAPP 2009, Mayapuri, Darjeeling

8. R2 Cf R1 Cd Vin Vout Vin Vout Vout = -(R2/R1) Vin Vout = - Q/Cf Preamplification Pre-amplifier (Preamp) : (i) Amplify weak signals from the detector (ii) Match the impedance of the detector and next level of electronics. Voltage sensitive Charge sensitive WAPP 2009, Mayapuri, Darjeeling

9. Pile up No pile up Pulse Shaping Amplifier : Amplifies signal from preamp (or from detector) to a level required for the analysis / recording. When you’re performing pulse height analysis i.e. you’re interested in the energy information – the amplifier should have shaping capabilities. • Pulse shaping: Two conflicting objectives • Improve the signal to noise (S/N) ratio – increase pulse width • Avoid pile up – shorten a long tail WAPP 2009, Mayapuri, Darjeeling

10. CR Differentiator : High pass filter RC Integrator : Low pass filter Pulse Shaping (contd.) Pulse shaping : How does it work? WAPP 2009, Mayapuri, Darjeeling

11. CR-RC Shaping Pulse Shaping (contd.) Pole zero cancellation WAPP 2009, Mayapuri, Darjeeling

12. Fixed differentiator time constant 100ns Integrator time constant 10, 30, 100 ns Fixed integrator time constant 10 ns Differentiator time constant inf, 100, 30, 10 ns Pulse Shaping (contd.) CR-RC Shaping WAPP 2009, Mayapuri, Darjeeling

13. Pulse Shaping (contd.) Baseline Shift WAPP 2009, Mayapuri, Darjeeling

14. Pulse Shaping (contd.) Bipolar pulse : Double differentiation or CR-RC-CR shaping Two advantages : (i) solution to baseline shift (ii) zero-crossing trigger for timing WAPP 2009, Mayapuri, Darjeeling

15. Pulse Shaping (contd.) More advancement : Semi-Gaussian Shaping WAPP 2009, Mayapuri, Darjeeling

16. Vref Digital output Flash ADC Digitization of pulse height and time Analog to Digital Conversion - ADC • Input is applied to n comparators in parallel • Switching thresholds are set by resistor chains • 2n comparators for n bits • Advantage: • Short conversion time (<10 ns) • Disadvantages: • limited accuracy • power consumption WAPP 2009, Mayapuri, Darjeeling

17. Comparator Control Logic Register + DAC Pulse stretcher Successive approximation ADC ADC (contd.) • Advantage: • speed is still nice ~ ms • high resolution • can be fabricated on monolithic ICs • Disadvantages: • starts with MSB • Starts with MSB (2n). • Compares the input with analog correspondent of that bit (from DAC) ands sets the MSB to 0 or 1. • Successively adds the next bits till the LSB (20). • n conversion steps for 2nbit resolution. WAPP 2009, Mayapuri, Darjeeling

18. WilkinsonADC ADC (contd.) • Advantage: • excellent linearity – continuous conversion • Disadvantage: • slow : Tconv = Nch/fclock • Typically for fclock ~ 100MHz • andNch = 8192, Tconv ~ 10 ms • Nch is proportional to pulse height • Charge memory capacitor till the peak • Do the following simultaneously: • Disconnect the capacitor from input • Switch the current source to linearly discharge the capacitor • Start the counter to count the clock pulses till the capacitor is discharged fully (decision comes from comparator) WAPP 2009, Mayapuri, Darjeeling

19. ADC (contd.) Wilkinson ADC Timing diagram Operation WAPP 2009, Mayapuri, Darjeeling

20. ADC (contd.) Analog to Digital Conversion – Hybrid technology • Use Flash ADC for coarse conversion : 8 out of 13 bits • Successive approximation or Wilkinson type ADC for fine resolution Limited range, short conversion time 256 channels with 100 MHz clock – 2.6 ms Result: 13 bit conversion in 4 ms with excellent linearity WAPP 2009, Mayapuri, Darjeeling

21. Digitization of time (contd.) Time Digitization : TAC, TDC • Counter: • Very simple : count clock pulses between START and STOP. • Limitation : speed of counter, currently possible 1 GHz • - time resolution ~ 1 ns • Analog Ramp: charge a capacitor through current source START : turn on current source , STOP : turn off current source use Wilkinson ADC to digitize the storage charge/voltage Time resolution ~ 10 ps WAPP 2009, Mayapuri, Darjeeling

22. Vth Timing circuits Discriminator : Generates digital pulse corresponding to analog pulse Combination of comparator and mono-shot. Monoshot Comparator Problem : Time walk WAPP 2009, Mayapuri, Darjeeling

23. Solution 1 : Fast zero crossing Trigger Timing circuits (contd.) Take the bipolar O/P from shaper/amplifier Trigger at zero crossing point Advantage : The crossing point is independent of amplitude Disadvantage : Works onlywhen the signals are of same shape and rise time WAPP 2009, Mayapuri, Darjeeling

24. Solution 2: Constant Fraction Trigger Timing circuits (contd.) WAPP 2009, Mayapuri, Darjeeling

25. connector Pulse processing - instruments NIM • Physical/mechanical parameters : • width – 19” (full crate) • width of the slot – 1.35” • height – 8.75” Electrical parameters : +/- 24 V, +/- 12 V, +/- 6 V, +/- 3 V (sometimes) connector WAPP 2009, Mayapuri, Darjeeling

26. Pulse processing - instruments CAMAC – Computer Automated Measurement and Control Main difference with NIM – computer interface Back plane contains power bus as well as data bus Station 24 & 25 reserved for the controller Once again 19” wide crate with 25 slots/stations 2U fan tray WAPP 2009, Mayapuri, Darjeeling

27. Pulse processing - instruments VME – Versa Module Eurocard (Europa) Developed in 1981 by Motorola Much more compact, high speed bus Fiber optic communication possible WAPP 2009, Mayapuri, Darjeeling

28. References • Many of the diagrams you’ve seen here are from • Radiation Detection and Measurement – G.F. Knoll • Techniques for Nuclear and Particle Physics Experiments – W.R. Leo • Nuclear Electronics – P.W. Nicholson • Radiation Detection and Signal processing (lecture notes) – H. Spieler (http://www-physics.lbl.gov/~spieler/Heidelberg_Notes/) • ORTEC Documentation - www.ortec-online.com WAPP 2009, Mayapuri, Darjeeling