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.  40 cm Beam AGATA detector array AGATA : an A dvanced GA mma-ray T racking A rray Proposed for high resolution γ -ray spectroscopy with exotic beams Employing highly segmented HPGe detectors, newly developed pulse-shape analysis and tracking methods

3. HPGe segmented detector γ (  1 MeV) p  K  (  50 MeV) Background of energetic particles Segments Core  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 charge preamplifier From detector segment Second stage Anti- alias ADC The new nuclear experiments with exotic beams pose challenging requirements to the front-end electronics Besides having a LOW NOISE, an extremely HIGH DYNAMIC RANGE is required !

4. ADC overflow voltage level New mixed reset technique: continuous + pulsed Saturated output without pulsed-reset Ideal non-saturated output without pulsed-reset Preamplifier output with continuous-reset (50  s decay time constant) Output with pulsed-reset An ADC overflow condition would saturate the system for a long while A pulsed-reset mechanism could permit a fast recovery of the output quiescent value, so minimizing the system dead time

5. 1st stage 2nd stage Charge loopPassive P/ZAmplification From detector Cold part of preamplifier Warm part of preamplifier Schmitt trigger comparator Discharge current From ADC OVR (optional) Output Capacitance to be discharged to de-saturate 2nd stage De-saturation circuitry /Output 1 3rd stage Implemented mixed reset technique: a time-variant charge preamplifier Circuit architecture: fast de-saturation of the 2 nd stage Noise is not at risk as no new path is connected to the input node !

6. Signal acquired at 1 st stage output……and at preamplifier output 1 st stage output voltage swing The realized pulsed-reset technique does not act on the 1 st 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

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

8. Action of pulsed-reset device Curve (1)-(10) = from 5 to 50 MeV Curve (11) = 100 MeV dE T / dT = 7.8 MeV/μs Event energy = 100 MeV : Reset time  13μs ! In a first approximation, a directly proportional relationship exists between the pulsed-reset time T and the event energy E T I = reset current  = 55 mV/MeV (1 st stage conversion gain) C = 2 nd stage capacitance (to be discharged) C F = feedback capacitance Ψ = 2.92 eV/pair (for HPGe) Es: C F =1pF, C=4.7nF, I=2mA

9. Passive P/Z stage: pole superposition theorem : 1)large signal: 2)tail of previous events: 3)reset current: sum of the three contributions: expression of the reset transient for by equating to zero at t=T, we derive the relationship between the total signal amplitude and the reset time : Detailed analysis of the reset transient

10. “Reset time-energy” relationship If we convert the voltage amplitudes H and h in the equivalent energies E s and E c (by using the conversion gain  ), we obtain the relationship E S = energy of the large signal E C = equivalent energy of the tail of previous signals We can expand the exponential term with no loss of accuracy since T<<τ P : large signal energy E s estimated from the reset time T and the tail contribution E c E T = equivalent total energy subjected to reset T = reset time

11. Energy estimate of a large individual event from the measurement of the reset time Contribution of the tail of previous events E S = energy of the individual large event T = reset time V 1, V 2 = pre- and post-transient baselines b 1, b 2, k 1, E 0 = fitting parameters

12. Tests of the large-signal measurement technique performed with a prototype of the circuit and a bulky HPGe detector (Padova, July 2004) reset device A spectroscopy-grade pulser injects a large pulse at the preamplifier input A 60 Co 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 Rate of 60 Co background events Resolution @ 10 MeV in Ge (FWHM) 1 kHz0.26 % 2 kHz0.32 % 4 kHz0.30 % 8 kHz0.37 % 16 kHz0.57 % 32 kHz0.56 % Rate of 60 Co 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  -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 * E S = equivalent energy release T = reset time b 1, b 2, k 1, E 0 = fitting parameters V 1, V 2 = pre- and post-pulse baselines

14. Energy range in normal mode 2MeV Energy range in normal mode ~ 2MeV 1408 keV 2.02 keV fwhm Extending the energy range by reconstruction of the large signals from reset time Extended range + pulser 122 keV 344 keV

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)