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

2. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling2 Pulse : How does it appear? Direct detection Indirect detection Flow through the processing electronics

3. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling3 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. BaselinePulse height or AmplitudeSignal width Leading edge / Trailing edgeRise time / Fall timeUnipolar / Bipolar

4. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling4 Pulse : How do they look? Fast or slow? Rise time – a few nanoseconds or less Rise time – hundreds of nanoseconds or greater Analog or digital? Amplitude or shape varies continuously Proportionately with the information signal from microphone signal from proportional chamber Quantized information in discrete number of states (practically two) pulse after discriminator

5. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling5 Logic standards O/P must deliver I/P must accept Logic 1 (high) -14 mA to -18 mA -12 mA to -36 mA Logic 0 (low) -1 mA to +1 mA -4 mA to +20 mA O/P must deliver I/P must accept Logic 1 (high) +4 V to +12 V +3 V to +12 V Logic 0 (low) +1 V to -2 V +1.5 V to -2 V Nuclear Instrumentation Module (NIM) Fast negative NIM Slow positive NIMTTLECL Logic 1 (high) 2 – 5 V - 1.75 V Logic 0 (low) 0 – 0.8 V -0.90 V Transistor-Transistor Logic (TTL) and Emitter Coupled Logic (ECL)

6. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling6 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. Typically C ~ 100 pF/m and L ~ few tens of H/m

7. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling7 Signal Transmission (contd.) Characteristic Impedance : Q. All coaxial cables are limited to the range between 50 – 200  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-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

8. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling8 Preamplification Pre-amplifier (Preamp) : (i) Amplify weak signals from the detector (ii) Match the impedance of the detector and next level of electronics. V in V out R1R1 R2R2 V out = -(R 2 /R 1 ) V in Voltage sensitive V in V out CfCf CdCd V out = - Q/C f Charge sensitive

9. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling9 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 Pile up No pile up

10. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling10 Pulse Shaping (contd.) Pulse shaping : How does it work? CR Differentiator : High pass filter RC Integrator : Low pass filter

11. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling11 Pulse Shaping (contd.) CR-RC Shaping Pole zero cancellation

12. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling12 Pulse Shaping (contd.) 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 CR-RC Shaping

13. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling13 Pulse Shaping (contd.) Baseline Shift

14. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling14 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

15. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling15 Pulse Shaping (contd.) More advancement : Semi-Gaussian Shaping

16. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling16 Digitization of pulse height and time Analog to Digital Conversion - ADC Flash ADC V ref Digital output  Input is applied to n comparators in parallel  Switching thresholds are set by resistor chains  2 n comparators for n bits Advantage: Short conversion time (<10 ns) Disadvantages: o limited accuracy o power consumption

17. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling17 ADC (contd.)  Starts with MSB (2 n ).  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 (2 0 ).  n conversion steps for 2 n bit resolution. Pulse stretcher ComparatorControl Logic Register + DAC Successive approximation ADC Advantage: speed is still nice ~ s high resolution can be fabricated on monolithic ICs Disadvantages: o starts with MSB

18. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling18 ADC (contd.) Wilkinson ADC  Charge memory capacitor till the peak  Do the following simultaneously: 1.Disconnect the capacitor from input 2.Switch the current source to linearly discharge the capacitor 3.Start the counter to count the clock pulses till the capacitor is discharged fully (decision comes from comparator) Advantage: excellent linearity – continuous conversion Disadvantage: o slow : T conv = N ch /f clock Typically for f clock ~ 100MHz and N ch = 8192, T conv ~ 10 s N ch is proportional to pulse height

19. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling19 ADC (contd.) Wilkinson ADC Operation Timing diagram

20. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling20 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 s Result: 13 bit conversion in 4 s with excellent linearity

21. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling21 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

22. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling22 Timing circuits Discriminator : Generates digital pulse corresponding to analog pulse Combination of comparator and mono-shot. V th Comparator Monoshot Problem : Time walk

23. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling23 Timing circuits (contd.) Solution 1 : Fast zero crossing Trigger Take the bipolar O/P from shaper/amplifier Trigger at zero crossing point Advantage : The crossing point is independent of amplitude Disadvantage : Works only when the signals are of same shape and rise time

24. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling24 Solution 2: Constant Fraction Trigger Timing circuits (contd.)

25. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling25 Pulse processing - instruments 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 NIM

26. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling26 Pulse processing - instruments Once again 19” wide crate with 25 slots/stations 2U fan tray 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

27. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling27 Pulse processing - instruments VME – Versa Module Eurocard (Europa) Much more compact, high speed bus Fiber optic communication possible Developed in 1981 by Motorola

28. Supriya Das, Bose InstituteWAPP 2009, Mayapuri, Darjeeling28 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/)http://www-physics.lbl.gov/~spieler/Heidelberg_Notes/  ORTEC Documentation - www.ortec-online.com