Calorimetry in particle physics experiments

Transcription

1 Calorimetry in particle physics experiments Unit n. 7 Front End and Trigger electronics Roberta Arcidiacono

2 Lecture overview Signal processing Some info on calorimeter FE Pre-amplifiers Charge sensitive Current sensitive Readout Shaper Readout & ADCs Digital filtering L1 Calorimeter Trigger NA48 example R. Arcidiacono Calorimetria a LHC 2

3 Signal processing Signal Processing is a way of converting an obscure signal into useful information Signal processing includes signal amplification, signal shaping (filtering) and readout The basic goal is to extract the desired information (Amplitude, time of the signal) from the obscuring factors (e.g. noise, pile-up) R. Arcidiacono Calorimetria a LHC 3

4 Signal processing Most detectors provide a certain amount of (induced) charge onto an output electrode (deriving from moving ionization/excitation charge). The electrode represents a certain capacitance For signal-processing point of view, they behave like capacitive charge sources charge generator with capacitance in parallel Differences between detectors: The typical charge at the detector output can differ by six orders of magnitude The output capacitances can differ by the same factor Signal dynamics Time available for the measurement R. Arcidiacono Calorimetria a LHC 4

5 Typical signal processing chain Digital Signal Processors Very small charge (fc) from detector need amplification and shaping to match the converter input characteristics Meas. of Amplitude (ADC) and/or Time (TDC) R. Arcidiacono Calorimetria a LHC 5

6 Ex of Signal induced by a moving charge d Parallel Plate Ion Chamber Anode (A) E i V b Q A,el = -q Q A,ion = q V (P) A = -q V A V (P) A = q V A x d x d Applying Green s Theorem x A constant induced current flows in the external circuit Cathode (C) I = dq A, el dt = q d dx dt R. Arcidiacono Calorimetria a LHC 6

7 Noise Spectra in the frequency domain white noise pink, 1/f noise R. Arcidiacono Calorimetria a LHC 7

8 Noise sources Source of serial noise related to amplification technique Source of parallel noise due to imperfections in the amplifier or in the detector (current losses) and to parassitic resistances (R p ) of the input stage Noise important only if it contributes to the output of the filter. It is fundamental to know the transfer function of the filter. R. Arcidiacono Calorimetria a LHC 8

9 Signal processing series white noise e 2 W =a series 1/f noise e 2 f =c/ f noiseless preamplifier A signal processor Q s(t) C d i 2 W =b i 2 f =d f C i parallel parallel white noise f noise The detector signal is modeled as a current source delivering a current pulse with time profile s(t) and charge Q, across the parallel of the detector capacitance C d and the preamplifier input capacitance C i. R. Arcidiacono Calorimetria a LHC 9

10 Dynamic range It is defined as: Maximum signal/minimum signal (or noise) Typical values: Often specified in db (20 log Vmax/Vmin) = db Also in bits : 2 n = Vmax/Vmin= bits The large dynamic range is a key parameter for the design of the calorimeter FE electronics R. Arcidiacono Calorimetria a LHC 10

11 Calorimeter FE electronics Calorimeter readouts requirements: Response linear over a large dynamic range (18 bit) Noise ( ENC equivalent noise charge ) should not dominate the energy resolution; low coherent noise Read-out rate capability** adapted to observed interaction rate Sensitivity to magnetic field, radiation, temperature to be considered! ** occupation time, integration time, time resolution R. Arcidiacono Calorimetria a LHC 11

12 Introduction on calorimeter FE electronics Calorimeter readouts nowadays are characterized by: A large dynamic range ~ bits Low noise Large number of channels (hundred-thousands) High speed: the shaping times are now in the ns region R. Arcidiacono Calorimetria a LHC 12

13 Pre-amplifi ers Main function: Receive weak signal from a detector, amplify it and pass it on via cables to the heart of the electronic processing system. Mounted as close as possible to the signal source, to reduce extra noise (which will be amplified as well), and to reduce as well signal attenuation (along cables) Impedance matching (between input stage and preamplifier) must also be achieved, to avoid pulse distortion. Trend: integrate the whole processing chain in FE electronics Peaking time: time required for a shaped pulse to go from the baseline to the peak R. Arcidiacono Calorimetria a LHC 13

14 Pre-Amplifi ers Overview The power per channel is relatively high mw : price to pay to obtain low noise figures. R. Arcidiacono Calorimetria a LHC 14

15 Dictionary ASIC = application-specific integrated circuit is an integrated circuit (IC) customized for a particular use Hybrid = a componenti discreti miniaturizzati BiCMOS = Bipolar Complementary Metal Oxide Semiconductor, tecnologia mista che integra CMOS e BJT sullo stesso chip semiconduttore. CMOS (Complementary MOS)= tecnologia utilizzata in elettronica per la progettazione di componenti digitali utilizzando transistor. JFET= junction field effect transistor, tipo di transistor ad effetto di campo, via di mezzo tra i transistor a giunzione bipolare (BJT) e i MESFET, a basso rumore. GaAs = compound di gallium and arsenic. Semiconduttore usato per microwave frequency integrated circuits infrared light-emitting diodes, laser diodes and solar cells. R. Arcidiacono Calorimetria a LHC 15

16 Shapers Shapers: The goal is to optimize the signal to noise ratio, adapting preamps output to ADC input window. In the past complex architectures to optimize series and parallel noise contribution. In fast calorimetry parallel noise is no longer a concern. More the physics noise ( pileup of minimum bias events) is what usually determines the optimum shaping time. The earlier digitization and the progress in DSPs has boosted the use of digital filtering R. Arcidiacono Calorimetria a LHC 16

17 Shapers: optimum shaping time Shaper has to minimize the quadratic sum of electronics noise (which increases at fast shaping) and pileup noise (which increases at slow shaping) As the pileup noise is proportional to the collider luminosity the optimum shaping time should be varied as the luminosity evolves performed by further digital filtering R. Arcidiacono Calorimetria a LHC 17

18 Read-out technique Experiments rely on multilinear handling of the large dynamic range of calorimeters (multi gain ADC converter stage) Most experiments now digitize very early often just after preamplifier/shaper The data storage until LV1 arrives is more often digital although analog pipelines reach excellent performance with up to 13 bits dynamic range and simultaneous readwrite operation. R. Arcidiacono Calorimetria a LHC 18

19 Readout overview R. Arcidiacono Calorimetria a LHC 19

20 R. Arcidiacono Calorimetria a LHC 20

21 On Trigger Systems... Trigger system has to identify interesting events and reject all unwanted interactions Nowadays, rejection factor is orders of magnitude Cannot do it at beam crossing rate Algorithms are too sophisticated. Accelerator related backgrounds can contribute to the problem (e + e - vs pp) Multi-Level trigger Algorithms can be implemented in Hardware, typically custom boards, often matched to geometry of detector Algorithms can be implemented in Software Farm Or be a mix of the two R. Arcidiacono Calorimetria a LHC 21

22 L1 Calorimeter Trigger Calorimeters can provide fast informations and be used L1 Luminosity, Calo Pattern recognition much easier than tracker Typical budget time ~ 2-3 s (signal transfer time included) Pipelined synchronous trigger, no dead-time, fixed latency wrt event time R. Arcidiacono Calorimetria a LHC 22

23 L1 Calorimeter Trigger L1 Calo triggers at colliders (detector has projective geometry) computes: Electron/Photon objects Jet/Tau objects Missing E T /Total E T R. Arcidiacono Calorimetria a LHC 23

24 NA48 ECAL Trigger Built for the selection of K 0 2π 0 4γ Large reduction and high trigger efficiency 40 MHz dead-time free pipeline Computes every 25 ns: Total Energy Energy Centre of gravity Kaon lifetime Number of peaks in calorimeter projections R. Arcidiacono Calorimetria a LHC 24

25 NA48 Trigger Requirements Neutral Trigger = NUT Particle rate in detector = 1 MHz Data digitized and stored in 200 μs ring buffers Neutral trigger decision every 25 ns with latency of 3.2μs: Select 2π 0 Suppress background 3-body decay Small loss through accidental activity R. Arcidiacono Calorimetria a LHC 25

26 NUT pipeline schema R. Arcidiacono Calorimetria a LHC 26

27 NUT chain in detail Bandbass Filter for noise reduction Schema of the neutral trigger signal flow in CPD R. Arcidiacono Fine-time reconstruction of a peak Calorimetria a LHC Look-up-Tables System 27

28 NUT performance Very high efficiency > 99.9% R. Arcidiacono Calorimetria a LHC 28