AGATA preamplifier performance on large signals from a Am+Be …agata.pd.infn.it/LLP_Carrier/AGATA_Week_2007_pdf_private... · 2008-02-06 · Outline Recalls : zFast reset device

AGATA preamplifier performance on large signals from a 241Am+Be

source

F. Zocca, A. Pullia, D. Bazzacco, G. Pascovici

AGATA Week - LNL (PD), Italy, 12-15 November 2007

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 241Am+Be source in “reset” mode in the energy range from 3 to 10 MeV

HPGe segmented detectorHPGe segmented detectorγ (≈ 1-10MeV)

p± K±(≈ 10-100 MeV)

Background of energetic particles

Core

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…)

Segments

≈ 10 cm

HPGe segmented detectorHPGe segmented detectorγ (≈ 1-10MeV)

p± K±(≈ 10-100 MeV)

Background of energetic particles

Segments

Core

≈ 10 cm

Needed wide-dynamic-range front-end electronics

Besides having low noise and large bandwidth, an extremely WIDE DYNAMIC RANGE is

also required !

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

FR

FCSecond stage

Anti-alias ADC

Exotic nuclei are to be disentangled in a hostile environment of high background radioactivity: (Bremsstrahlung, neutrons, charged particles…)

Mixed reset technique: continuous + pulsed

ADC overflow voltage level

Saturated output without pulsed-resetIdeal non-saturated

output without pulsed-reset

Preamplifier output with continuous-reset (50µs decay time constant)

Output with pulsed-reset

A pulsed-reset mechanism allows 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

Fast-reset device of AGATA preamplifiers

PACAGA5A PACAGA5A (GANIL)(GANIL)

PBPB--B1B1-- MI MI (MILANO)(MILANO)

AGATA_AGATA_corecore--pulser pulser (KOELN)(KOELN)









Time-Over-Threshold (TOT) technique

Time-Over-Threshold (TOT) techniquesecond-order time-energy

relation offset term

( ) OEVVkTbTbE +−−+= 2112

21

E = energy of the large signal

T = reset time




21

contribution of the tail due to previous events


T = reset time

V1 , V2 = pre-pulse and post-pulse baselines

b1 , b2 , k1 , E0 = fitting parameters




21

contribution of the tail due to previous events


T = reset time

V1 , V2 = pre-pulse and post-pulse baselines

b1 , b2 , k1 , E0 = fitting parameters

Within ADC range standard “pulse-height mode” spectroscopy

Beyond ADC range new “reset mode” spectroscopy

Experimental setup : AGATA capsule, core preamplifier + built-in pulser

Encapsulated AGATA HPGe crystal at LNL

1.8 Ω

HV

Pulser signal

Warm partCold part

Core preamplifier + pulser Core preamplifier + pulser

47 Ω

Cold part Warm part

Segment preamplifier Segment preamplifier

HPGe crystal (36+1 segments) HPGe crystal (36+1 segments) 9 cm

built-in pulser

Core preamplifier

Calibration procedure (1)

( ) OETbTbVVkE ++=−+ 221211

Parameters calculated by a fitting procedure

Parabolic fitting curve

Calibration pulser signals are completely disentangled from

the background


Pulser energies within the 10MeV

ADC range


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

TOT technique applied to over-threshold pulser signals (1)

Pulser signal @ 5.97 MeV60Co background rate = 1.3 kHz 60Co background rate = 14.5 kHz


60Co background rate = 1.3 kHz 60Co background rate = 14.5 kHz

Resolution @ 5.97 MeV = 10.5 keV (0.18 %)

Resolution @ 5.97 MeV= 15.2 keV (0.25 %)


Background event rate = 800 Hz

0.039 %19.16 keVE6 = 49.434 MeV

0.043 %14.33 keVE5 = 33.369 MeV

0.067 %12.55 keVE4 = 18.797 MeV

0.11 %11.28 keVE3 = 10.656 MeV

0.19 %11.42 keVE2 = 5.9720 MeV

0.35 %11.68 keVE1 = 3.3501 MeV

Resolution (fwhm) Pulser line energy

Less than 0.4% over the full range

Event rate Resolution (fwhm)

1.3 kHz 10.50 keV 0.18 %

2.3 kHz 11.79 keV 0.20 %

4.2 kHz 12.57 keV 0.21 %

8.2 kHz 13.23 keV 0.22 %

14.5 kHz 15.18 keV 0.25 % 0.21 %22.56 keV14.5 kHz

0.17 %17.87 keV8.2 kHz

0.13 %14.02 keV4.2 kHz

0.12 %12.97 keV2.4 kHz

0.11 %12.07 keV1.2 kHz

Resolution (fwhm)Event rate

Pulser energy = 10.65 MeV

0.21 %~ 40 keV14.2 kHz

0.16 %~ 30 keV8.2 kHz

0.10 %18.64 keV4.2 kHz

0.083 %15.56 keV2.3 kHz

0.069 %12.94 keV1.3 kHz


Pulser energy = 18.8 MeVPulser energy = 5.97 MeV


Background event rate = 800 Hz

0.039 %19.16 keVE6 = 49.434 MeV

0.043 %14.33 keVE5 = 33.369 MeV

0.067 %12.55 keVE4 = 18.797 MeV

0.11 %11.28 keVE3 = 10.656 MeV

0.19 %11.42 keVE2 = 5.9720 MeV

0.35 %11.68 keVE1 = 3.3501 MeV

Resolution (fwhm) Pulser line energy

Less than 0.4% over the full range

Pulser energy = 10.65 MeV Pulser energy = 18.8 MeVEvent rate Resolution (fwhm)

1.3 kHz 10.50 keV 0.18 %

2.3 kHz 11.79 keV 0.20 %

4.2 kHz 12.57 keV 0.21 %

8.2 kHz 13.23 keV 0.22 %

14.5 kHz 15.18 keV 0.25 %

Pulser energy = 5.97 MeV

0.21 %22.56 keV14.5 kHz

0.17 %17.87 keV8.2 kHz

0.13 %14.02 keV4.2 kHz

0.12 %12.97 keV2.4 kHz

0.11 %12.07 keV1.2 kHz


0.21 %~ 40 keV14.2 kHz

0.16 %~ 30 keV8.2 kHz

0.10 %18.64 keV4.2 kHz

0.083 %15.56 keV2.3 kHz

0.069 %12.94 keV1.3 kHz


Obtained resolutions less than 0.25% for all the tested count rates


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

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 : λ = event rate

< E > = mean event energy

T = reset time

TEEshift ><=∆ λ

Experimental setup: 241Am+Be source

AGATA capsule at LNL

241Am+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

Resolution (fwhm) in “pulse-height” mode Energy

1.1732 MeV (60Co) 2.99 keV 0.25 %

1.3325 MeV (60Co) 3.24 keV 0.24 %

2.2233 MeV (H) 4.51 keV 0.20 %

4.440 MeV (12C) 104 keV 2.34 %

7.6312 MeV (Fe) 11 keV 0.14 %

7.6456 MeV (Fe) 11 keV 0.14 %

8.9984 MeV (Ni) 15 keV 0.17 %

241Am+Be spectrum in pulse-height mode


1.1732 MeV (60Co) 2.99 keV 0.25 %

1.3325 MeV (60Co) 3.24 keV 0.24 %

2.2233 MeV (H) 4.51 keV 0.20 %

4.440 MeV (12C) 104 keV 2.34 %

7.6312 MeV (Fe) 11 keV 0.14 %

7.6456 MeV (Fe) 11 keV 0.14 %

8.9984 MeV (Ni) 15 keV 0.17 %



1.1732 MeV (60Co) 2.99 keV 0.25 %

1.3325 MeV (60Co) 3.24 keV 0.24 %

2.2233 MeV (H) 4.51 keV 0.20 %

4.440 MeV (12C) 104 keV 2.34 %

7.6312 MeV (Fe) 11 keV 0.14 %

7.6456 MeV (Fe) 11 keV 0.14 %

8.9984 MeV (Ni) 15 keV 0.17 %


241Am+Be spectrum in reset mode

241Am+Be spectrum

“reset” mode (by TOT technique)

“pulse-height” mode (by ADC)

Energy Resolution (fwhm)in pulse-height mode

Resolution (fwhm)in reset mode

4.440 MeV (12C) 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 %

7.6312 MeV (Fe) 11 keV 0.14 %

7.6456 MeV (Fe) 11 keV 0.14 %

8.9984 MeV (Ni) 15 keV 0.17 % 18.9 keV 0.21 %

18.8 keV(29.4 keV

for the double-peak)

0.25 %(0.38 % for

the double-peak)

Comparison on the double-peak Fe line (7.6312-7.6456 MeV)

“reset” mode“pulse-height” modeFWHM = 18.8 keV ( 0.25 % )FWHM = 11 keV ( 0.14 % )

Comparison on the 8.99 MeV Ni line

“reset” mode“pulse-height” modeFWHM = 19 keV ( 0.21 % )FWHM = 15 keV ( 0.17 % )

At high energies the performance in reset mode approaches the performance in pulse-height mode

The ideal acquisition chain: “dual-channel” core preamplifier

1st channel

2nd channel

~ 5 MeV

~ 20 MeV

Reset threshold ~ 10 MeV


1st channel

2nd channel

~ 5 MeV

~ 20 MeV


Pulse-height mode (ADC ~ 5 MeV)


Reset mode (from ~ 20 MeV on)

Result :


1st channel

2nd channel

~ 5 MeV

~ 20 MeV




Reset mode (from ~ 20 MeV on)

Result :

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.

ConclusionsThe 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 8.998 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

AGATA preamplifier performance on large signals from a Am+Be …agata.pd.infn.it/LLP_Carrier/AGATA_Week_2007_pdf_private... · 2008-02-06 · Outline Recalls : zFast reset device

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