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University of Milano Department of Physics and INFN HIGH DYNAMIC RANGE LOW-NOISE PREAMPLIFICATION OF NUCLEAR SIGNALS A. Pullia, F. Zocca, C. Boiano, R. Bassini, S. Riboldi, D. Maiocchi Department Conference “Highlights in Physics 2005” October 14, 2005 AGATA: an Advanced GAmma-ray Tracking Array AGATA detector array 40 cm Proposed for high resolution γ-ray spectroscopy with exotic beams Employing highly segmented HPGe detectors, newly developed pulse-shape analysis and tracking methods The new nuclear experiments with exotic beams pose challenging requirements to the front-end electronics HPGe segmented detector charge preamplifier RF Core Background of energetic particles From detector segment CF Second stage Antialias ADC Segments 10 cm Individual highly energetic events or bursts of piled-up events could easily cause ADC SATURATION and introduce a significant SYSTEM DEAD TIME charge loop Besides having a LOW NOISE, an extremely HIGH DYNAMIC RANGE is required ! New mixed reset technique: continuous + pulsed Ideal non-saturated output without pulsed-reset Saturated output without pulsed-reset ADC overflow voltage level Preamplifier output with continuous-reset (50s decay time constant) An ADC overflow condition would saturate the system for a long while Output with pulsed-reset A pulsed-reset mechanism could permit a fast recovery of the output quiescent value, so minimizing the system dead time Implemented mixed reset technique: a time-variant charge preamplifier Circuit architecture: fast de-saturation of the 2nd stage Cold part of preamplifier Warm part of preamplifier 2nd stage 1st stage From detector Charge loop Passive P/Z Amplification Discharge current Schmitt trigger comparator 3rd stage 1 Output -1 /Output Capacitance to be discharged to de-saturate 2nd stage De-saturation circuitry From ADC OVR (optional) Noise is not at risk as no new path is connected to the input node ! 1st stage output voltage swing The realized pulsed-reset technique does not act on the 1st stage and so can’t “protect” it against saturation The architecture of the 1st stage has been studied to provide a large output voltage swing ( 10 V) and so to a prevent a risk of an overflow condition Signal acquired at 1st stage output… …and at preamplifier output Triple AGATA segment preamplifier on alumina substrate (Mod. “PB-B1 MI” – Milano) MDR26 connectors Top view PZ trimmers Bottom view Segment preamplifiers Mechanical dimensions: 57x56x5 mm Core preamplifier Segment preamplifiers Action of pulsed-reset device In a first approximation, a directly proportional relationship exists between the pulsed-reset time T and the event energy ET Curve (1)-(10) = from 5 to 50 MeV Curve (11) = 100 MeV 4 11 dET I CF I dT C q C I = reset current = 55 mV/MeV (1st stage conversion gain) C = 2nd stage capacitance (to be discharged) CF = feedback capacitance Ψ = 2.92 eV/pair (for HPGe) Es: CF=1pF, C=4.7nF, I=2mA Amplitude [V] 3 2 3 4 5 6 7 8 9 10 2 1 1 0 0 2 4 6 8 Time [µs] dET / dT = 7.8 MeV/μs Event energy = 100 MeV : Reset time 13μs ! 10 12 Detailed analysis of the reset transient Passive P/Z stage: pole P C R1 R2 superposition theorem : 1) large signal: v01 ( t ) H e t P 2) tail of previous events: v02 ( t ) h e t P 3) reset current: t v03 ( t ) I R1 R2 1 e P sum of the three contributions: expression of the reset transient VPZ ( t ) h H I R1 R2 e for t 0, T t P I R1 R2 by equating to zero at t=T, we derive the relationship between the total signal amplitude and the reset time : T H h I R1 R2 e P 1 “Reset time-energy” relationship If we convert the voltage amplitudes H and h in the equivalent energies Es and Ec (by using the conversion gain ), we obtain the relationship T P ET E S EC R1 R2 e 1 I T = reset time ET = equivalent total energy subjected to reset ES = energy of the large signal EC = equivalent energy of the tail of previous signals We can expand the exponential term with no loss of accuracy since T<<τP : ET E S EC ES I I T T 2 ... C 2 C P I I T T 2 ... EC C 2 C P large signal energy Es estimated from the reset time T and the tail contribution Ec Energy estimate of a large individual event from the measurement of the reset time ES I I T T 2 ... EC C 2 C P Contribution of the tail of previous events E S b1T b2T 2 k1 V1 V2 EO ES = energy of the individual large event T = reset time V1 , V2 = pre- and post-transient baselines b1 , b2 , k1 , E0 = fitting parameters Tests of the large-signal measurement technique performed with a prototype of the circuit and a bulky HPGe detector (Padova, July 2004) A spectroscopy-grade pulser injects a large pulse at the preamplifier input A 60Co source provides a background of lower events which destroys the large signal resolution if no correction is made reset device Measurement of large pulses from reset time ES b1T b2T k1 V1 V2 EO 2 * ES = equivalent energy release T = reset time b1, b2, k1, E0 = fitting parameters Rate of 60Co events = 32 kHz V1, V2 = pre- and post-pulse baselines Rate of 60Co Resolution @ 10 MeV background in Ge (FWHM) events 1 kHz 0.26 % 2 kHz 0.32 % 4 kHz 0.30 % 8 kHz 0.37 % 16 kHz 0.57 % 32 kHz 0.56 % Measurement performed at Padova with HPGe detector (courtesy of D. Bazzacco and R. Isocrate) Zocca, ”A new low-noise preamplifier for g-ray sensors with smart device for large signal management”, Laurea Degree Thesis, University of Milano, October 2004 (in Italian). See http://topserver.mi.infn.it/mies/labelet_iii/download_file/capitolo6.doc *F. Extending the energy range by reconstruction of the large signals from reset time 122 keV 344 keV + pulser 1408 keV 2.02 keV fwhm Extended range Energy range in normal mode ~ 2MeV Future developments Tests of the pulsed-reset device with a triple AGATA preamplifier coupled to an AGATA HPGe segmented detector Tests of the large-signal measurement technique when applied to measure the energy of real highly energetic events (photons or energetic particles in the 10-50 MeV range)