1. Kolympari, Crete, June 16, 20091 Study of avalanche fluctuations and energy resolution with an InGrid-TimePix detector P. Colas Progress report, based on PC, IEEE Dresden 2008, Max Chefdeville’s thesis 2009, and more recent analysis Paul Colas, CEA/Irfu Saclay

2. Kolympari, Crete, June 16, 2009 2 Gain fluctuations An old problem : Wejsman 1949, Legler 1955, 1961, Riegler 2003,… With a renewed interest: space resolution of an MPGD tracker (gain fluctuations lower by a factor (1+  )/(2+  ) the number of effective electrons, D. Arrogancia et al., NIM A 602 (2009) 403 threshold Efficiency for single-electron detection And with new means of investigation : Microbulk, InGrid + TimePix G/ Avalanche size distribution P. Colas

3. Kolympari, Crete, June 16, 20093 Gain fluctuations Though there is no clear justification for this, we use Polya to parameterize the gain distribution. For  =0, the distribution is an exponential (Furry model) Alternative convention is parameter m=1+  P. Colas

4. Kolympari, Crete, June 16, 20094 New experimental handles P. Colas Many measurements have been carried out (see T. Zerguerras’s talk). New detectors provide new handles: Electron counting with InGrid on TimePix provides a direct measurement of Fano fluctuations, giving access to the contribution of gain fluctuations to the width of the observed 55 Fe peak (itself measured by InGrids or Microbulks). Electron counting with InGrid on TimePix provides a direct measurement of Fano fluctuations, giving access to the contribution of gain fluctuations to the width of the observed 55 Fe peak (itself measured by InGrids or Microbulks). Time-over-threshold on single pixels give the charge distribution of single electron avalanches Time-over-threshold on single pixels give the charge distribution of single electron avalanches Study of electron counting vs gain gives a sensitivity to  Study of electron counting vs gain gives a sensitivity to 

5. Kolympari, Crete, June 16, 20095 TimePix chip Idea : take a medical imaging chip (Medipix 2), add a clock to each pixel, replace ‘grey levels’ by ‘clock ticks’ (Michael Campbell, Xavi Lloppart, CERN) 65000 pixels, 14-bit counter, 100 MHz tunable clock frequency -> more voxels than the ALEPH TPC, but tiny! 55  m Pixel 14111  m 16120  m 14080  m (pixel array) 11 2233 44 55 55 μ m μ m Preamp/shaper THL disc. Configuration latches Interface Counter Synchronization Logic P. Colas

6. Kolympari, Crete, June 16, 20096 See electrons from an X- ray conversion one by one and count them, study their fluctuations (Nikhef-Saclay) P. Colas

7. Kolympari, Crete, June 16, 20097 Micromegas + TimePix TimePix READOUT MICROMESH DRIFT DRIFTSPACE E D ~ 1 kV/cm E D ~ 1 kV/cm E A ~ 80 kV/cm InGrid (Nikhef-Twente) 55 Fe Cr filter Micromegas detector 5.9 keV Xrays giving 220 elec. in argon with rms sqrt(F/220) Peak width: contribution from primary (Fano) fluctuations and gain fluctuations (assuming high detection efficiency) Width=√(F+B)/N P. Colas

8. Kolympari, Crete, June 16, 20098 Chromium K-edge (Center for X-Ray Optics) Microbulk detector KEK, january 2007 P. Colas

9. Kolympari, Crete, June 16, 20099 Result : 5.6% r.m.s. resolution (Broken record) Noise very small thanks to adequate filter on the mesh P. Colas

10. Kolympari, Crete, June 16, 200910 5% rms resolution InGrid measurements P. Colas

11. Kolympari, Crete, June 16, 200911P. Colas

12. Kolympari, Crete, June 16, 200912  = 2 For F=0.20 Resolution (rms) Theta parameter P. Colas

13. Kolympari, Crete, June 16, 200913 In the gain fluctuations, the avalanche statistics and the effect of the field configuration in the hole cannot be disentangled. A full simulation shows how the resolution depends on the detector geometry. P. Colas

14. Kolympari, Crete, June 16, 200914 Energy resolution & collection The collection efficiency varies with the field ratio The collection efficiency varies with the field ratio Record 55 Fe spectra at various field ratios Record 55 Fe spectra at various field ratios Peak position VS field ratio Peak position VS field ratio Assume peak maximum is full collection

15. Kolympari, Crete, June 16, 200915 Study efficiency vs gain SiProt on chips: spreads the charge over 2-3 pads: count clusters Use the rms size of the x-ray spot to select contained events Use the time distribution to remove noise P. Colas

16. Kolympari, Crete, June 16, 2009 16 Measured spectra at -330 V Timepix #1 Timepix #1 Timepix #2 Timepix #2 5.9 and 6.5 keV escape events (event ratio ~ 7:1) 5.9 and 6.5 keV escape events (event ratio ~ 50:1)

17. Kolympari, Crete, June 16, 2009P. Colas17 Single electron detection efficiency and gain Trend depends on: Trend depends on: the threshold t, the gas gain G and m the threshold t, the gas gain G and m (TimePix) threshold

18. Single electron detection efficiency and gain Number of detected electrons at given voltage determined by Number of detected electrons at given voltage determined by Adjusting 2 gaussians on escape peak Adjusting 2 gaussians on escape peak K beta parameters constrained by K alpha ones K beta parameters constrained by K alpha ones 3 free parameters 3 free parameters Number of detected electrons and voltage Number of detected electrons and voltage Use common gain parametrization Use common gain parametrization Fix p 2 (slope of the gain curve) Fix p 2 (slope of the gain curve) 2 free parameters: t/A and ηN 2 free parameters: t/A and ηN

19. Single electron detection efficiency and gain m rms (%)   1 100 39.43 2 71 3.08 3 58 1.16 4 50 1.41 5 45 1.60 Best fit for m = 3 (  =2) Yields √ b = 1/√m ~ 58 % Also, ηN = 115 e- Upper limit on W(Ar5iso) < 25 eV Correcting for un-efficiency: Upper limit on F(Ar5iso) < 0.3

20. Kolympari, Crete, June 16, 2009P. Colas20  m=  +1 RESULT OF THE FIT  = 2.2 -0.6 +1.3

21. Kolympari, Crete, June 16, 200921 W and F in Ar/iso 95/5 at 2.9 keV Assume full collection efficiency of detector #1 N p = N c = 115 ± 2 e- W = 25.2 ± 0.5 eV Peak width measured with detector #2 corrected for detection and collection eff. (87 %) RMS(N p ) ~ 4.3 % F = 0.21 ± 0.06 Extrapolation to 5.9 keV photo-peak straightforward N p = 230 ± 4 e- Consistent with, and more precise than previous measurements Consistent with measured values and theoretical estimate 0.17 for pure Ar P. Colas

22. Kolympari, Crete, June 16, 200922 Conclusions New ‘almost perfect’ detectors give gain fluctuations wich can be parametrized by polya with  ~ 2. New ‘almost perfect’ detectors give gain fluctuations wich can be parametrized by polya with  ~ 2. from e-counting vs V mesh :  =2.2 +1.5 -0.6 from e-counting vs V mesh :  =2.2 +1.5 -0.6 Fano fluctuations are now accessible by electron counting. Fano fluctuations are now accessible by electron counting. Best resolution understood as sqrt((F+B)/N ), with F=0.2 and B=0.3 for Micromegas Best resolution understood as sqrt((F+B)/N ), with F=0.2 and B=0.3 for Micromegas More systematic measurements with best possible InGrids+TimePix to be made More systematic measurements with best possible InGrids+TimePix to be made P. Colas

23. Thanks to Kolympari, Crete, June 16, 200923 D. ATTIÉ 1), M. CAMPBELL 2 ), M. CHEFDEVILLE 3), E. DELAGNES 1), K. FUJII 3), Y.GIOMATARIS 1), H. VAN DER GRAAF 4), X. LLOPART 2), J. SCHMITZ 5), J. TIMMERMANS 4) 1) Irfu,CEA Saclay; 2) CERN ; 3) KEK, 4) Nikhef; 5) Twente Most of the work presented here was carried out by Max Chefdeville for his PhD thesis. P. Colas