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

 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

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 !

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

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 !

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

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

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

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

“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

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

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

Measurement of large pulses from reset time Rate of 60 Co background events 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 * 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

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

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 MeV range)