Study of avalanche fluctuations and energy resolution with an InGrid-TimePix detector

1. Vienna Conference on Instrumentation 2010 Study of avalanche fluctuations and energy resolution with an InGrid-TimePixdetector D. ATTIÉ1), M. CAMPBELL2 ), M. CHEFDEVILLE3) , P. COLAS1), E. DELAGNES1) , K. FUJII4), I.GIOMATARIS1) , H. VAN DER GRAAF5) , X. LLOPART2) , M. LUPBERGER1) , H. SCHINDLER2) ,J. SCHMITZ6), M. TITOV1) 1) CEA/Irfu Saclay, 2) CERN, 3)LAPP Annecy, 4)KEK, 5)Nikhef, 6)Twente U. Avalanche fluctuations are an oldproblem, motivated by recent applications in MPGDs, whichcanbeaddressedwith new tools. Avalanche flutuations with InGrid/TimePix

2. Avalanche statistics, an oldproblem… K. Fujii Wijsman 1949, Yule-Furry : exponential Legler 1955, 1961: it takes a distance UI/E to get ionization energy from the E field Alkhazov 1970 : avalanche process is iterative, moments of the size distribution can be calculated, turnover at low Z Approx. solution for wires: Polya q = 0 : exponential distribution Avalanche flutuations with InGrid/TimePix

3. threshold …with new motivations Motivated by practicalconsequences: • energyresolution of MPGD, contributes 1/√(1+q)N • spatial resolution of a Micromegas TPC, avalanche fluctuations lower by a factor (1+q)/(2+q) the number of effective electrons, D. Arrogancia et al., NIM A 602 (2009) 403 • efficiency for single electron detection Avalanche flutuations with InGrid/TimePix

4. 55 mm m μ 14080 m (pixel array) 55 4 4 16120 m 55 mm 2 2 3 3 1 1 5 5 55 μ m 14111 m New tool : TimePix chip + InGrid CERN-Nikhef-Saclay Collaboration within EUDET Pixel Idea : take a medical imaging chip (Medipix 2), add a clock to each pixel, replace ‘grey levels’ by ‘clock ticks’ (Michael Campbell, XaviLloppart, CERN) 65000 pixels, 14-bit counter, 100 MHz tunable clock frequency -> more voxels than the ALEPH TPC, but tiny! Cover the chip with a deposited grid over SU8 pillars 50 µ high to obtain gas amplification The chip is protected by 7 µ SiN to avoid destruction by sparks. Synchronization Logic Interface Configuration latches Preamp/shaper Counter THL disc. Avalanche flutuations with InGrid/TimePix

5. TimePix Micromegas + TimePix Fe 55 source DRIFT Chamberoperatedwith an Ar+5% isobutane mixture DRIFT SPACE ED ~ 0.7 kV/cm 2.5 cm 50 µm EA ~ 80 kV/cm MICROMESH READOUT InGrid (Nikhef-Twente) Avalanche flutuations with InGrid/TimePix

6. See electrons from an X-ray conversion one by one (55Fe) and count them, study their fluctuations. Measure Time Over Threshold (linear with charge above 5 ke-) for single isolated pixels : direct access to avalanche charge distribution. Avalanche flutuations with InGrid/TimePix

7. Gain fluctuations from Time Over Threshold Select isolated clusters withonly 1 pixel. These are single electron avalanches (~10 µ rms radius). TOT islinearwithnumber of electronsseen by the amplifier above 5 ke- : Ne = 167 TOT – 6700 (redcurve, corresponding to the threshold setting of our data taking) Valid up to 30 000 electrons. This gives a direct access to avalanche size. Number of electrons 10000 20000 30000 TOT (in 28 ns time bins) U in (Volts) Avalanche flutuations with InGrid/TimePix

8. Distributions of avalanche size from TOT atdifferent gains Gain =2900 Z = G/<G> = (167*TOT-6700)/G(V) G(V) from a measurementusing a source. Z=G/<G> Gain =6000 Gain =12600 Z=G/<G> Z=G/<G> Avalanche flutuations with InGrid/TimePix

9. Same Z from TOT distributions but log scale to see the tails Gain =2900 Unfortunately, the tails are dominated by TOT resolutioneffects. Z=G/<G> Gain =6000 Gain =12600 Z=G/<G> Z=G/<G> Avalanche flutuationswithInGrid/TimePix

10. Avalanche size distribution from TOT PolyafitsaboveZmin = 5000/<G> (region of linearity of TOT) are good Howevertheta values are not reliable (verycorrelatedwith the gain measurement and the TOT scale. There is a discrepancybetween the averagenumber of electrons and the gain: thisis a possible effectfrom the protection layer and from the shaping by electronics. We do not regard thesefitted values as measurements of theta. They point to a value of 4.3 but withvery large systematicerrors (factor of 2?) Avalanche flutuations with InGrid/TimePix

11. ELECTRON COUNTING Monte Carlo simulation Gas : Ar+5% isobutane Avalanche flutuations with InGrid/TimePix

12. In our setup, we use the Chromium K-edge to cut the Kb line (Center for X-Ray Optics) Avalanche flutuations with InGrid/TimePix

13. Monte Carlo simulation. Shows thatweneedenough drift distance to separate the clusters. Also shows that the escape peakisbettercontainedthan the photopeak. Avalanche flutuations with InGrid/TimePix

14. Drift distance (z) cutperformedusing the diffusion : sqrt(rmsx2 + rmsy2 ) > 28 pixels (cluster separation) Cloud center within a windowaround the chip center (containment) Gas : Ar+5% isobutane Data Ugrid = 350 V NUMBER OF CLUSTERS Avalanche flutuationswithInGrid/TimePix

15. Use escape peak (only one line, bettercontained) Then correct for collection efficiency (96.5 +- 1 % from MC, in this range of field ratios : 80-90) ConvertU_gridinto gain/threshold (threshold = 1150 e-) Avalanche flutuations with InGrid/TimePix

16. Collection efficiencyfrom simulation : 96.5±1 % Gain measurements (from a 80x80 mm2 coppermeshwith the same gap 50 µm, gas : Ar+5% isobutane 1µ thickness 2µ thickness Predictionfrom R. Veenhof et al., Data (in red) from D. Attié et al. (seealso D. Arrogancia et al. 2009) Avalanche flutuations with InGrid/TimePix

17. ~1 atmoderate gain (few 1000). Maybehigherat gains above 5000 • Exponentialbehaviour (q=0) stronglyexcluded, as well as q>2 Avalanche flutuations with InGrid/TimePix

18. Determination of W and F The background istotallynegligible (time cuttaking 30 time bucketsaround the electroncloudamong 11000) The probability for mergingtwo clusters issmall, with the rmscuts. The probability for loosingelectrons by the containmentcutsissmall. Attachmentalsoisnegligible. The main inefficiencycomesfrom collection : 96.5+-1 % from simulation. Using the escape peak: W= 2897 eV / 120.4 = 24.06 +- 0.25 eV This translates to 245+-3 electrons for the 5.9 keV line, largerthanwhatisusuallyadmitted for pure Ar (227). Photoelectriceffect on the meshis not excluded. This couldalsobe a Penningeffect in the conversion region. The Fano factor couldbederivedfrom the rms of the escape line (6.8 e- ) but needs large corrections frominefficiencies. Gas : Ar+5% isobutane Avalanche flutuations with InGrid/TimePix

19. Energy resolution 55Fe Cr filter 5.9 keVXrays giving N=245 elec. in argon with rmssqrt(F/245) Micromegas detector Peak width: contribution from primary (Fano) fluctuations and gain fluctuations (assuming high detection efficiency) Width=√(F+B)/N Avalanche flutuations with InGrid/TimePix

20. Without Cr filter Resolution measurements with InGrid (Grid integrated on a Si wafer by post-processing technique) 5% rms resolution With Cr filter Avalanche flutuations with InGrid/TimePix

21. Avalanche flutuations with InGrid/TimePix

22. Width=√(F+B)/N With a measured relative width of 0.05 and assuming F=0.2, taking the measurement of N=245, weobtain B=0.41, thusq =1.4 This isroughly consistent with the resultsfrom single electroncountingefficiency. Avalanche flutuations with InGrid/TimePix

23. CONCLUSIONS InGrid, Microbulk, and TimePix are new detectors whichallow to study the conversion and avalanche processeswithunprecedentedaccuracy. Time Over Thresholdmeasurementsgiveaccess to direct measurement of the fluctuations, providedabsolute gain and TOT calibration canbebettercontrolled. The onset of single electronefficiencywith Micromegas gain allows the exponential fluctuations to beexcluded and favoursPolya fluctuations withq close to 1 atmoderate gain and reaching a few unitsat gains of 10 000. To measure Fano fluctuations willrequire an improved setup with a longer drift and bettercontroledfield. Energyresolutionmeasurementsassuming F=0.20 favour a value of q~1. 4. Specialthanks to R. Veenhof, J. Timmermans and Y. Bilevych Avalanche flutuations with InGrid/TimePix