AGATA preamplifier performance on large signals from a 241 Am+Be source. F. Zocca, A. Pullia, D. Bazzacco, G. Pascovici

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1 AGATA preamplifier performance on large signals from a 241 Am+Be source F. Zocca, A. Pullia, D. Bazzacco, G. Pascovici AGATA Week - LNL (PD), Italy, November 2007

2 Outline Recalls : Fast reset device of AGATA preamplifiers TOT technique for the estimate of large saturated signals Test measurements with the AGATA capsule at LNL in last July : Test of the TOT technique on large pulser signals from 3 to 50 MeV Calibration procedure and effects of high count rates Spectra acquired from a 241 Am+Be source in reset mode in the energy range from 3 to 10 MeV

3 Needed wide-dynamic-range front-end electronics Exotic nuclei are to be disentangled in a hostile environment of high background radioactivity: (Bremsstrahlung, neutrons, charged particles ) Background of energetic particles γ ( 1-10MeV) p ± K ± ( MeV) HPGe segmented detector Core Segments 10 cm

4 Needed wide-dynamic-range front-end electronics Exotic nuclei are to be disentangled in a hostile environment of high background radioactivity: (Bremsstrahlung, neutrons, charged particles ) Background of energetic particles γ ( 1-10MeV) p ± K ± ( MeV) HPGe segmented detector Core Individual highly energetic events or bursts of piled-up events could easily cause ADC SATURATION and introduce a significant SYSTEM DEAD TIME Segments 10 cm charge preamplifier From detector R F C F Second stage Antialias ADC Besides having low noise and large bandwidth, an extremely WIDE DYNAMIC RANGE is also required! charge loop

5 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 (50µs decay time constant) Output with pulsed-reset An ADC overflow condition would saturate the system for a long while A pulsed-reset mechanism allows a fast recovery of the output quiescent value, so minimizing the system dead time

6 Fast-reset device of AGATA preamplifiers PACAGA5A (GANIL) PB-B1 B1- MI (MILANO) AGATA_ core-pulser (KOELN)

7 Fast-reset device of AGATA preamplifiers PACAGA5A (GANIL) PB-B1 B1- MI (MILANO) AGATA_ core-pulser (KOELN)

8 Fast-reset device of AGATA preamplifiers PACAGA5A (GANIL) PB-B1 B1- MI (MILANO) AGATA_ core-pulser (KOELN)

9 Time-Over-Threshold (TOT) technique

10 Time-Over-Threshold (TOT) technique second-order time-energy relation offset term ( V1 V ) EO 2 E b T + b T k + = E = energy of the large signal T = reset time

11 Time-Over-Threshold (TOT) technique second-order time-energy relation offset term ( V1 V ) EO 2 E b T + b T k + = contribution of the tail due to previous events E = energy of the large signal T = reset time V 1, V 2 = pre-pulse and post-pulse baselines b 1, b 2, k 1, E 0 = fitting parameters

12 Time-Over-Threshold (TOT) technique second-order time-energy relation offset term ( V1 V ) EO 2 E b T + b T k + = contribution of the tail due to previous events E = energy of the large signal T = reset time V 1, V 2 = pre-pulse and post-pulse baselines b 1, b 2, k 1, E 0 = fitting parameters Within ADC range Beyond ADC range standard pulse-height mode spectroscopy new reset mode spectroscopy

13 Experimental setup : AGATA capsule, core preamplifier + built-in pulser Encapsulated AGATA HPGe crystal at LNL HPGe crystal (36+1 segments) 9cm Cold part HV Core preamplifier + pulser Warm part 47 Ω Segment preamplifier 1.8 Ω Pulser signal Cold part Warm part Core preamplifier built-in pulser

14 Calibration procedure (1) 2 ( V1 V2 ) = b1t + b2t EO E + k + 1 Parameters calculated by a fitting procedure Parabolic fitting curve Calibration pulser signals are completely disentangled from the background

15 Calibration procedure (2) Pulser energies within the 10MeV ADC range

16 Calibration procedure (2) Pulser energies within the 10MeV ADC range Main issue : energy calibration of pulser lines beyond ADC range (from 10 MeV on) as no γ-rays of known energies from a portable calibration source are available at these higher energies

17 TOT technique applied to overthreshold pulser signals (1) Pulser 5.97 MeV 60 Co background rate = 1.3 khz 60 Co background rate = 14.5 khz

18 TOT technique applied to overthreshold pulser signals (2) 60 Co background rate = 1.3 khz 60 Co background rate = 14.5 khz 5.97 MeV = 10.5 kev (0.18 %) 5.97 MeV= 15.2 kev (0.25 %)

19 TOT technique applied to overthreshold pulser signals (3) Background event rate = 800 Hz Pulser line energy E 1 = MeV E 2 = MeV E 3 = MeV E 4 = MeV E 5 = MeV E 6 = MeV kev kev kev kev kev kev Resolution (fwhm) 0.35 % 0.19 % 0.11 % % % % Less than 0.4% over the full range Pulser energy = 5.97 MeV Event rate Resolution (fwhm) 1.3 khz kev 0.18 % 2.3 khz kev 0.20 % 4.2 khz kev 0.21 % 8.2 khz kev 0.22 % Pulser energy = MeV Event rate 1.2 khz 2.4 khz 4.2 khz 8.2 khz Resolution (fwhm) kev kev kev kev 0.11 % 0.12 % 0.13 % 0.17 % 14.5 khz kev 0.25 % 14.5 khz kev 0.21 % Pulser energy = 18.8 MeV Event rate Resolution (fwhm) 1.3 khz kev % 2.3 khz kev % 4.2 khz kev 0.10 % 8.2 khz ~ 30 kev 0.16 % 14.2 khz ~ 40 kev 0.21 %

20 TOT technique applied to overthreshold pulser signals (3) Background event rate = 800 Hz Pulser line energy E 1 = MeV E 2 = MeV E 3 = MeV E 4 = MeV E 5 = MeV E 6 = MeV kev kev kev kev kev kev Resolution (fwhm) 0.35 % 0.19 % 0.11 % % % % Less than 0.4% over the full range Pulser energy = 5.97 MeV Event rate Resolution (fwhm) 1.3 khz kev 0.18 % 2.3 khz kev 0.20 % 4.2 khz kev 0.21 % 8.2 khz kev 0.22 % 14.5 khz kev 0.25 % Pulser energy = MeV Event rate Resolution (fwhm) 1.2 khz kev 0.11 % 2.4 khz kev 0.12 % 4.2 khz kev 0.13 % 8.2 khz kev 0.17 % 14.5 khz kev 0.21 % Pulser energy = 18.8 MeV Event rate Resolution (fwhm) 1.3 khz kev % 2.3 khz kev % 4.2 khz kev 0.10 % 8.2 khz ~ 30 kev 0.16 % 14.2 khz ~ 40 kev 0.21 % Obtained resolutions less than 0.25% for all the tested count rates

21 TOT technique applied to overthreshold pulser signals (4)

22 Peak shift at increasing count rates 1.3 khz count rate 14.5 khz count rate Time (µs) Time (µs) The baseline shifts downwards owing to the AC-coupling between the preamplifier and the core electrode

23 Peak shift at increasing count rates 1.3 khz count rate 14.5 khz count rate Time (µs) Time (µs) The baseline shifts downwards owing to the AC-coupling between the preamplifier and the core electrode Estimate of the energy peak shift according to Campbell s theorem : E shift = λ < E > T λ = event rate < E > = mean event energy T = reset time

24 Experimental setup: 241 Am+Be source AGATA capsule at LNL 241 Am+Be source with Ni target Fast neutrons thermalized in paraffin and captured by natural metallic nickel γ-photons produced in the 4 to 9 MeV range

25 241 Am+Be spectrum in pulse-height mode Energy Resolution (fwhm) in pulse-height mode MeV ( 60 Co) 2.99 kev 0.25 % MeV ( 60 Co) 3.24 kev 0.24 % MeV (H) 4.51 kev 0.20 % MeV ( 12 C) 104 kev 2.34 % MeV (Fe) 11 kev 0.14 % MeV (Fe) 11 kev 0.14 % MeV (Ni) 15 kev 0.17 %

26 241 Am+Be spectrum in pulse-height mode Energy Resolution (fwhm) in pulse-height mode MeV ( 60 Co) 2.99 kev 0.25 % MeV ( 60 Co) 3.24 kev 0.24 % MeV (H) 4.51 kev 0.20 % MeV ( 12 C) 104 kev 2.34 % MeV (Fe) 11 kev 0.14 % MeV (Fe) 11 kev 0.14 % MeV (Ni) 15 kev 0.17 %

27 241 Am+Be spectrum in pulse-height mode Energy Resolution (fwhm) in pulse-height mode MeV ( 60 Co) 2.99 kev 0.25 % MeV ( 60 Co) 3.24 kev 0.24 % MeV (H) 4.51 kev 0.20 % MeV ( 12 C) 104 kev 2.34 % MeV (Fe) 11 kev 0.14 % MeV (Fe) 11 kev 0.14 % MeV (Ni) 15 kev 0.17 %

28 241 Am+Be spectrum in reset mode

29 241 Am+Be spectrum reset mode (by TOT technique) Energy MeV ( 12 C) Resolution (fwhm) in pulse-height mode Resolution (fwhm) in reset mode 104 kev 2.34 % 104 kev 2.34 % ~5.6 MeV 10.5 kev 0.14 % 18.8 kev 0.34 % ~6.1 MeV 15.1 kev 0.17 % 17.1 kev 0.28 % MeV (Fe) MeV (Fe) MeV (Ni) 11 kev 0.14 % 11 kev 0.14 % 18.8 kev (29.4 kev for the doublepeak) 0.25 % (0.38 % for the doublepeak) 15 kev 0.17 % 18.9 kev 0.21 % pulse-height mode (by ADC)

30 Comparison on the double-peak Fe line ( MeV) pulse-height mode FWHM = 11 kev ( 0.14 % ) reset mode FWHM = 18.8 kev ( 0.25 % )

31 Comparison on the 8.99 MeV Ni line pulse-height mode FWHM = 15 kev ( 0.17 % ) reset mode FWHM = 19 kev ( 0.21 % ) At high energies the performance in reset mode approaches the performance in pulse-height mode

32 The ideal acquisition chain: dual-channel core preamplifier 1 st channel ~ 5 MeV Reset threshold ~ 10 MeV 2 nd channel ~ 20 MeV

33 The ideal acquisition chain: dual-channel core preamplifier 1 st channel ~ 5 MeV Reset threshold ~ 10 MeV 2 nd channel ~ 20 MeV Result : Pulse-height mode (ADC ~ 5 MeV) Pulse-height mode (ADC ~ 20 MeV) Reset mode (from ~ 20 MeV on)

34 The ideal acquisition chain: dual-channel core preamplifier 1 st channel ~ 5 MeV Reset threshold ~ 10 MeV 2 nd channel ~ 20 MeV Result : Pulse-height mode (ADC ~ 5 MeV) Pulse-height mode (ADC ~ 20 MeV) Reset mode (from ~ 20 MeV on) A prototype of the dual core board has already been realized and will be tested in these days here in Legnaro with the AGATA capsule.

35 Conclusions The potentiality of the TOT technique for γ-ray spectroscopy has been proved. The obtained resolution in reset mode was of < 0.4% in all the tested range from 3 MeV to 50 MeV. A remarkable resolution of 0.21% was obtained on the Ni spectrum line at the energy of MeV. The purpose of the TOT technique is not that of replacing the standard pulse-height analysis : reset-mode spectroscopy is to be applied BEYOND the range of the ADC in order to extend the energy measurement range. Future tests and developments are foreseen to address the discussed issues regarding the calibration procedure at high energies and the energy peak shift at increasing count rates. Acknowledgements to B. Million, A. Bracco and Milano nuclear-physics group for strongly supporting this work and for valuable suggestions and hints