TRACE: a New Ancillary Detector for Gamma Ray Spectroscopy. Daniele Mengoni LNL 17 mar. 06

Transcription

1 TRACE: a New Ancillary Detector for Gamma Ray Spectroscopy Daniele Mengoni LNL 17 mar. 06

2 Collaboration Università di Camerino INFN-LNL INFN-Sezione di Perugia INFN-Sezione di Padova IRST-Trento

3 Outlines Introduction Goals Requirements & Specifications Future gamma array & actual ancillaries My future ancillary: simulation, test on ASICs & Si-pad detectors AGATA Cluster Experiment: simulation and data analysis Summary & Outlook

4 Introduction Study of the structure of exotic nuclei using secondary radioactive beams Fusion-evaporation reactions in inverse kinematics to measure Energy and Angle of the recoiling light particle with Ancillary detectors coupled with gamma arrays Direct reactions in inverse kinematics to measure Energy and Angle of the recoiling light particle Ref.: Instrumentation for

5 Detector Specifications Detector: made of Silicon to minimize the absorption of gamma rays, thin junction window (0.1µm). Geometry & Dimensions: solid angle coverage 90% counting rate: 20 khz 40 x 80 mm 2 E: Si-pad det. <150 µm thick, pad 4x4 & 8x8 mm 2 E: Si-pad det. >1.5 mm thick, pad 4x4 & 8x8 mm 2 Angular Resolution: 2 6, 4x4 & 8x8 mm 2 at cm Energy resolution: 50 kev for 5.5 MeV α-particles Wide energy range: 200 kev-20 MeV for p, 80 MeV for α Time Resolution: 500 ps for A=8 & 2 MeV/u Pulse shape analysis: 2 GHz, >10 bits

6 AGATA Ref.: Agata

7 ANCILLARIES Ref.: CUP.

8 TRACE: older prototype Fusion-evaporation -> different from TIARA & MUST (transparent, high granularity forward, low-energy threshold backward) Direct reactions -> similar characteristics like TIARA & MUST

9 My Ancillary: TRACE Good capabilities 1) particle discrimination 2) high detection efficiency 3) good angular and energy resolution 4) transparency for gamma rays electronics mechanics Constraints In doing simulations I m trying to maximize the above features taking into account the various constraints. The result will be a compromise among different requirements.

10 Ref.: E.Farnea. AGATA week contribution. GSI. Feb 2005.

11 TRACE earlier prototypes: barrels with octagonal or hexagonal base

12 TRACE prototype prototype in 2π prototype in the reaction chamber prototype with the demonstrator

13 Telescopes: barrel + end caps ->Barrel: Si strip detector ->End Cap: Si pad (forward) and strip (backward) ->Total channels: ~8000 (high granularity)

14 Transparency Absolute Photopeak Efficiency (%) Peak to Total ratio (%) Rotational cascade : Eγ : (80+nх100)keV,n=0,..,29 ; Mγ: 30 ; β recoil: 0% 10 5 Events gamma ray tracking

15 E- E matrix 32 S(125MeV)+ 40 Ca

16 Doppler Correction Ref.: E.Farnea, F.Recchia. LNL Annual report FHWM 69 kev 7.8 kev

17 Test on Si-pad detectors Standard electronics α-source: 241 Am source FWHM ~ MeV (1.3%) ASIC electronics (VA32_HDR11 chip α-source: 241 Am source in air at 1 cm FWHM ~ MeV (9.1%) The capacitance as a function of the bias voltage in a 1 mm thick Si-pad detector. The leakage current as a function of the bias voltage for a 1 mm thick Si-pad detector. 1 mm thick Sintef detector + ASIC (VA32C2-TA32CG):γ-Sources: 241 Am, 57 Co;energy resolution: ~ 6 60 kev (10%)

18 ENC VA32C2 VA32_HDR11 nominal measured 12e / pf C + 40e 69 e / pf C + 68 e e e / pf C e Energy Resolution pad capacitance 2.4 pf nominal from meas. ENC measured 0.5% 0.8% 3% 9% 10% 10%

19 Si-Pad Detectors SINTEF MICRON thickness 1.0 mm 6 x 21 pads typical pad size:1.8x1.8 mm 2 Bias Voltage: V AC coupled A guard ring to allow a more stable operation at full depletion. thickness ~ 500 µm 5x12 pads pad size 3.75x3.75 mm 2 full depletion voltage: ~ 50V DC coupling

20 AGATA Trigger Rate Estimates single total Demonstrator 14 khz 83 khz AGATA M g =1 15 khz 3 MHz M g =30 50 khz 300 khz AGATA has a fully digital pipelined electronics. All detectors are time stamped. It can be considered a trigger-less system. Ref.: www-dapnia.cea.fr/sphn/agata/ancillary/index.php

21 TRACE Trigger Rate Estimates Extreme conditions: 1000 mb reaction cross section 1 mg/cm 2 target thickness 100 pna beam current 6 evaporated particles 80% solid angle coverage 20 MHz rate ---> 20 khz/segment ~5 reduction of current intensity compatible with Agata 20pnA 4MHz (maximum global supported counting rate) 4 khz/segment

22 How to read out all the channels? Charged particles prompt detector un-triggered rate similar to AGATA un-triggered rates. Reduction of a factor >10 is expected after setting trigger conditions actual ancillaries would lead to 3MHz x 0.1 rate of the system future ancillary need to share Agata counter clock leading to ~3MHz maximum counting rate TDR acquisition mode Ref.: www-dapnia.cea.fr/sphn/agata/ancillary/index.php

23 ANY SOLUTION? 64 pads/detector -> 64 signals to be digitalized Actually(serial read-out) VA32C: 32,1MS/s=>2 x 32 x 1µs=64 µs!! Future (hopefully sparse read out) xxxxx : 128, 10MS/s =>10 µs serial < 1 µs sparse (2 3 ch max) AGATA : 3MHz--->.3 µs

24 Test on Read-out ASIC Electronics working principle VATAGP3 Read out mode : sparse mode. The sparse read out provides only the channels with trigger to be read to increase the read out speed. 128 channels; 4 chips addressable. Fast shaper 150 ns. VA32C2-TA32CG VA32_HDR_11-TA32 VADAQ

25 Summary Exhaustive simulations are going on: efficiency, Doppler correction.. but more realistic event generators are needed. Outlook Still waiting for thicker Si detector prototype to be used in the first TRACE unit. A new ASIC has to be provided! An in-beam test is going to be planned.

26 First AGATA (cluster) Experiment -Sept IKP KOLN- Goal: checking the feasibility of tracking with the first cluster prototype under (future) AGATA working conditions.

27 Experimental Data d( 48 Ti, 49 Ti)p kev FWHM 35 kev (single crystal)

28 Evidence For Various Reactions 48 Ti(d,d) 48 Ti* FWHM = 300 kev SUM UP of red and purple?

29 after Ge time windows Inelastic scattering

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31 Kinematics windows Q -1/2 M c v MeV -2 MeV h/2πl= M c v 2 R T l = 0.47 but l GS =3, l I =1 Miss-matched transfer reaction Ref.: Teoria delle collisioni tra ioni pesanti F. ZARDI Introduction to nuclear reactions G.R. SATCHLER

32 Simulated System inside outside (d,p) Inverse Kinematics Event Generator: 1.Cross section produced with DWBA and loaded in the C code. 2.Energy lost of the beam in the target before interaction 3.Proton energetic and directional straggling 4.Recoil energetic straggling after interaction 5.Gammas loaded from an input file in the generator Ref.:DWBA.

33 Simulated Results sector rings E sectors sector E rings rings sector rings

34 Simulated Doppler correction FHWM 15.6 kev (single) 4.3 kev

35 Summary More realistic transfer event generators are needed to make exhaustive simulations with TRACE. The analysis of the experiment is still going on, looking for the actual value of the resolution achievable in working conditions. Outlook Still waiting for thicker Si detector prototype to be used in the first TRACE unit. An in-beam test is going to be planned.

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37 Motivation Our work is devoted to the design of a new instrument for particle spectroscopy, to be used also as ancillary detector for large gamma arrays. The detector is designed to mainly fulfill the requirements coming from the study of nuclei using direct and fusion-evaporation reactions with RNB.

38 Introduction The availability of the RNB will enable the study of the nuclear structure far from the stability valley through variuos kind of nuclear reactions, among which the direct reactions are powerfull tools to investigate the first excited nuclear states in unknown nuclei. Elastic and inelastic scattering provide informations on the nuclear density, while transfer, knock-out and break-up reactions allow us to study the microscopic shell structure of the nucleus. These reactions, which with RNB can be performed only in inverse kinematics, require the detection of the light charge particles and neutrons, often with a spectrometer for the heavy project-like particle. Fusion-evaporation reactions, on the other hand, are valid tools to approach the proton drip-line, accelerating proton rich nuclei towards the Coulomb barrier energies, and enable the study of high-spin states in the exotic nuclei. Ref.: Instrumentation for Goals Our work is devoted to the design of a new instrument for particle spectroscopy, to be used also as ancillary detector for large gamma arrays. The detector is designed to mainly fulfill the requirements coming from the study of nuclei using direct and fusionevaporation reactions with RNB.

39 Main specifications of a detector for light charged particles Detector specifications: Efficiency Geometry Position resolution Energy resolution Time resolution Energy range Particle discrimination: E-E technique & PSA Electronics ASICS DSS

40 Main requirements High detection efficiency: it has to cover as much as possible the solid angle, with a high granularity in order to minimize multiple hits probability. Transparent to gamma rays for the coupling with a gamma spectrometer. Fine discrimination among the various particles: protons, alphas and heavier ions. Good position resolution: for Doppler correction and good energy resolution. Good energy resolution: for detailed spectroscopy. Good time resolution: for TOF discrimination of light ions. Wide energy range: measurement of various reactions. Pulse shape analysis: fast DSS to achieve very low thresholds.

41 Why? -> Prominent technical merits : High efficiency + high granularity Ref.: Agata Proposal@

42 Ancillaries TRANSFER REACTIONS The energy-angle measurement of the light ejectiles are sufficient to provide mass identification. The measurement of the energies of coincident γ-rays can assist the ejectile identification and provide a greatly enhanced energy resolution of individual quantum states. This new capability with respect to the first generation of transfer studies, is mainly due to the greatly improved sensitivity that is now achievable with germanium detector arrays. The direct reactions allow to determine the angular momentum transferred and the excitation energy of the state populated in the residual nucleus. In addition, the probability for transfer to a particular excited state provides information on the single-particle components in the nuclear wave function. FUSION-EVAPORATION REACTIONS Particle gamma coincidences to deduce the charge and mass of the residual nuclei. Minimal requirements: 1) capability of discriminating between the different types of light particles (p, alpha); 2) high detection efficiency to reduce the number of high-multiplicity events misinterpreted as low-multiplicity events; 3) high granularity to reduce the multiple-hit probability.

43 Simulations

44 1. Single Nucleon Transfer Reaction (d,t),(d,p): TRACE Physics - Study of the shell gap N=20: inverse reactions on the neutron rich F isotopes - Shell gap N=40,50 per 68Ni,78Ni: inverse reactions on isotopes in that mass region - N=Z line: reationsi on Kr isotopes 2. Two nucleons Transfer Reaction (t,p), (p,t), (Be-9,Be-7),(d,alpha),(Li-6,alpha): - inverse reactions on Ni isotopes and even-even isotopes wih N=Z 3. Study of similar transitions in mirror nuclei: inverse reactions with Z>N beams (as Kr-72, Zn- 60) on deuteron target (d in Pd or Pt). Ref.: Eurisol. Appendix A, E. Detector Thick Si-Pad detector AC coupled, double metallization. Electronics ASICs: parrallel reading of each channel (Agata-like) or sparse readout reading (if quickly enough). Still studying, but the sparse read-out appear to be the only feasible solution for a massive number of channels (10000).

45 Simulations: prototypes Fully segmented array of Si-pad telescopes (and/or strips). High solid angle coverage.

46 SINTEF thickness 1.0 mm 6 x 21 pads typical pad size:1.8x1.8 mm 2 Bias Voltage: V AC coupled A guard ring to allow a more stable operation at full depletion. The pads are connected via strips to the bond pads located on one side of the detector, suitable for wire bonding to a PCB read-out board.

47 MICRON Device Type: IMAGE PIXEL ARRAY 500 thickness ~ 500 µm 5x12 pads pad size 3.75x3.75 mm 2 full depletion voltage: ~ 50V DC coupling As the detector was delivered completely naked, a self-made AC circuitry was used for the coupling either with the ASIC and the standard DAQ system.

48 Equivalent noise charge (ENC) of the VA32_HDR11 chip, in units of electron charge, as a function of the input capacitance has been measured. A linear fit of the experimental data is also shown, which is given by the relation: ENC = e / pf C + e The linearity of VA32C2 chip extends up to 220 kev, and the ENC curve is described by the relation: ENC 68 = 69e / pf C + e The statistical fluctuation for 60 kev photons in Silicon is 130 e -, which gives an energy resolution of 0.6%. A capacitance of 2.4 pf gives an ENC of 230 e -, which leads to an energy resolution worse than 3% for the VA32C2 chip. Similar arguments lead to an energy resolution 9% for the VA32_HDR11 chip.

49 Electronics Number of channels: ~ 8000 Dynamic range for Light Charged Particles MeV Si = 56x10 3-6x10 6 e - = 9 fc 1 pc Ranges in in silicon silicon for for protons: MeV MeV mm mm MeV MeV mm mm Ranges in in silicon silicon for for alphas: alphas: MeV MeV mm mm MeV MeV mm mm Pulse shape analysis: DSS 2 GHz, 10 bits Multiplexer based system for reducing the number of ADCs and feed-through by a factor of ~ 100 R&D for ASIC with PSA

50 Detector Material: Silicon The density of Silicon and its small ionization energy result in adequate signals even with very thin active layers, and the produced signals are fast, typically tens of ns. Thickness: de stage is 150 um thick, E slice 1500 um thick. available energetic range: 15 MeV for p, 60 MeV for 4He Pad (or strip?): the electronic noise is mainly produced by the preamplifier and its loaded capacitance, which is due to the intrinsic detector capacitance and the connecting cables in between. Energy Resolutoin needed: 1% over the whole dynamic range ---> (2x2) mm pad with a 1.5 thickness would be a solution ---> massive huge amount of channels ---> a compromise is needed!!

51 New Si-pad Detectors from IRST - Trento PAD2 strip1 PAD1 STRIP: Thickness 64 strips/side; 1.5 mm strips Thin orthogonal junction on window the two sides strip (up pitch to µmnm) Two High different resistivity strip width: 300 µm (STRIP1) (>30 kω cm) 200µm (STRIP2). Bias voltage: V (multi guard rings) One single metal layer Near edge bonding contacts strip2 PAD3 PAD1=AC, 6x5 pads, 4x4 mm 2 PAD2=AC, 8x32 pads, 1x1 mm 2 PAD3=DC, 8x8 pads, 2x2 mm 2

52 Simulated results Particle Discrimination a) E-dE matrices (mainly fusion-ev.) b) E vs Theta matrices (transfer) c) Si pulse shape analysis (..not yet done) Doppler Correction d) Doppler correction Transparency e)absolute Photopeak eff; P/T ratio

53 Simulated Results: hitpatterns sectors Ge rings

54 Simulated Results E E sectors rings E slice Ge E

55 Experimental apparatus Ancillary device: DSSD 32 rings 64 sectors Ge detector: first AGATA symmetric cluster (3 detectors)

56 Estimated Cross Sections FUSION (PACE calc.) EL. SCAT. ~100 mb typical INEL. SCAT. ~10 mb typical Barrier ~95 MeV TRANSFER ~1 mb typical, depending on the matching

57 (Inverse) Kinematics Ref.: W.Catford. Acta Physica Polonica B. Vol.32 (2001)

58 Development of a Compton Camera with Si-pad and Segmented Germanium Detectors A gamma ray emitted from a radionuclide is scattered by the first detector (scatterer) inducing a signal which is recorded in coincidence with the signal induced by the absorption of the scattered photon in the second detector (absorber). The principle of the method is based on the Compton formula, which in the approximation of scattering on free electrons at rest reads: E E1 = cosθ = 1+ m e c 2 1+ E / m c 1 cosθ E0 E1 ( ) ( ) 0 e

59 Detectors and electronics The experimental setup is based on a position sensitive detector and its read-out electronics. The recommended material for the scatterer is Silicon, which has good spectroscopic response and a high ratio of Compton to photoelectric cross section for low-energy photons. As absorber the segmented Germanium detector is chosen. The Ge-segmented detector MARS and its DAQ are shown.

60 Simulation Results The system performances regarding the optimal geometry and efficiency were evaluated. Article to be revised for publication on IEEE TNS

61 ... in progress Exploiting the gamma tracking algorithms developed for the next generation of γ- ray spectrometers, which have yet to be tested experimentally, and using algorithms for source localization through cone intersection we can imagine various applications in gamma ray imaging. Main recipe for γ-ray tracking The main configurations that will be studied through simulations and tested in the near future are: The source position is located using a cone intersection code, developed by our group.

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