University of Milano

1. 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

2. AGATA: an Advanced GAmma-ray Tracking Array AGATA detector array Beam  40 cm • Proposed for high resolutionγ-ray spectroscopy with exotic beams • Employing highly segmented HPGe detectors, newly developed pulse-shape analysis and tracking methods

3. The new nuclear experiments with exotic beams pose challenging requirements to the front-end electronics HPGe segmented detector γ (1 MeV) charge preamplifier From detector segment pK (50 MeV) Core Second stage Anti-alias ADC Background of energetic particles Segments charge loop  10 cm Individual highly energetic events or bursts of piled-up events could easily cause ADC SATURATION and introduce a significant SYSTEM DEAD TIME Besides having a LOW NOISE, an extremely HIGH DYNAMIC RANGE is required !

4. 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 (50s decay time constant) Output with pulsed-reset A pulsed-reset mechanism could permit a fast recovery of the output quiescent value, so minimizing the system dead time An ADC overflow condition would saturate the system for a long while

5. 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 3rd stage 2nd stage 1st stage 1 Output -1 /Output From detector Charge loop Passive P/Z Amplification Capacitance to be discharged to de-saturate 2nd stage Discharge current De-saturation circuitry Schmitt trigger comparator From ADC OVR (optional) Noise is not at risk as no new path is connected to the input node !

6. 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

7. Triple AGATA segment preamplifier on alumina substrate (Mod. “PB-B1 MI” – Milano) MDR26 connectors Top view PZ trimmers Bottom view Segment preamplifiers Segment preamplifiers Core preamplifier Mechanical dimensions: 57x56x5 mm

8. 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 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 dET / dT = 7.8 MeV/μs Event energy = 100 MeV : Reset time  13μs !

9. Detailed analysis of the reset transient Passive P/Z stage: pole superposition theorem : • large signal: • tail of previous events: • reset current: sum of the three contributions: expression of the reset transient for by equating to zero at t=T, we derive the relationship betweenthe total signal amplitude and the reset time:

10. “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 = 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 : large signal energy Es estimated from the reset time T and the tail contribution Ec

11. Energy estimate of a large individual event from the measurement of the reset time Contribution of the tail of previous events ES = energy of the individual large event T = reset time V1 , V2 = pre- and post-transient baselines b1 , b2 , k1 , E0 = fitting parameters

12. Tests of the large-signal measurement technique performed with a prototype of the circuit and a bulky HPGe detector reset device (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

13. Measurement of large pulses from reset time ES = equivalent energy release T = reset time b1, b2, k1, E0 = fitting parameters V1, V2 = pre- and post-pulse baselines * Rate of 60Co events = 32 kHz Measurement performed at Padova with HPGe detector (courtesy of D. Bazzacco and R. Isocrate) *F. 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

14. 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

15. 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)