ISPA-Tubes with YAP:Ce Active Windows for X and Gamma Ray Imaging.

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1 PIXEL 2000 International Workshop on Semiconductor Pixel Detectors for Particles and X-Rays Genova - Porto Antico - Magazzini del Cotone (Sala Libeccio) June 5-8, 2000 ISPA-Tubes with YAP:Ce Active Windows for X and Gamma Ray Imaging. C. D'Ambrosio 1, F. de Notaristefani 2, H. Leutz 1, D. Puertolas 2, E. Rosso 1 1 CERN, 2 INFN Section of Rome, Italy

2 CERN INFN-Roma III (HIRESPET* Collaboration) Institute of Physics, Academy of Sciences-Prague Alice and LHCb exp. at CERN EP-TA2 and EP-MIC groups at CERN Industrial partners: D.E.P. (NL) Crytur Ltd. (CZ) Edgetek (FR) *

3 OUTLINE Short introduction to the ISPA-tube Conventional designs of gamma cameras based on ISPA-tubes and results Present developments : scintillating windows (YAP:Ce) Conclusions and future outlook

4 Position-sensitive photon detection with an ISPA-tube Optical input window Photon Photocathode Photoelectron VACUUM H. V. (typ kv) Signal out (global analog readout) Solder bump Pixel chip developed by CERN/EP-MIC Pixel detector 500µm x 50µm Pixel electronics 500µm x 50µm Electron-hole pairs (typ ) Signal out (pixel binary readout) Bias voltage (typ V)

5 The self-triggering principle detector chip electronics chip Fast, analog and global information Trigger for strobe Immediate calibration in photoelectron or energy Selection of a window in energy possible Pixel signals out (with present chip, pixel response is binary Precise space information 2-D imaging

6 Detection of γ-rays with an ISPA-tube YAP-detector Input window Photocathode (-25 kv) Rear contact Fast (10 ns) global analog information 57 Co source 122 kev γ's Lead phantom Photoelectrons VACUUM Binary pixel pattern read out in 10 µs Photons Silicon pixel array (1024 elements) Bump bonds Electronics pixel array (1024 elements) 30 lead-throughs Ω chip assembly developed by RD19 collaboration Pixel size 75mmx500mm

7 γ-imaging with an ISPA-tube coupled to YAP:Ce crystal detectors * Properties of pure YAP Chemical formula YAlO 3 (inert, non hygroscopic) Crystal structure Orthorhombic (no cleavage) Density g.cm Molecular weight Z eff 34 Hardness Moh 8.6 Refractive index n at 400 nm 1.97 at 500 nm 1.95 Transparency nm 240 to >1000 Additional properties of YAP doped with Ce Light emission peak nm 365 Light decay (1/e) ns 27 Radiation length cm 2.7 Avr. K X-ray energy of Yttrium kev 15.2 Refractive index n at 400 nm 1.92 at 500 nm 1.91 Due to its properties YAP can be easily machined and optically polished. Arrays of small individual elements (1mm 2 down to 300µm 2 can be assembled) * Our YAP crystal detectors are produced by Crytur Ltd, Turnov, Czech Republic

8 Performances of ISPA-tubes in imaging The overall performances of the ISPA-tube rely on both The input window arrangement: the goal is to collect as many photoelectrons as possible while preserving the localisation of the gamma event The anode pixel chip: the goal is to detect as many photoelectrons N pe as possible on the detector plane (energy resolution considerations) and to get a binary pattern reproducing the light spot on the photocathode with a number of firing pixels N hit (<N pe ) allowing a precise c.o.g calculation (analysis event per event)

9 Different possible configurations X- or γ-ray YAP:Ce array X- or γ-ray Photons Fibre optic window YAP:Ce plate Photons Photoelectron cluster Photoelectron cluster (FWHM)~crystal elements (FWHM)~2mm X- or γ-ray YAP:Ce array X- or γ-ray Photons YAP:Ce plate Photons Quartz window Photoelectron cluster Photoelectron cluster (FWHM)~2-2.5 mm (FWHM)>3.5 mm

10 Result summary Fibre window ISPA-tube: ++ excellent spatial resolution from 100 µm (array) to 300 µm (plate) -- poor E-resolution (only a few photoelectrons) Quartz window ISPA-tube: acceptable spatial resolution from 500 µm (array) to 700 µm (plate) + good E-resolution at 122 kev from 20% FWHM (plate) to 40% FWHM (array) 200 p.e. 80p.e. see IEEE TNS, vol. 42, no6, p and vol. 44, no5, p.1747 ISPA-tube with larger active surface (40 mm diameter) + The demagnification (~4) principle has been also successfully applied for gamma imaging applications, with sub-millimeter spatial resolution see NIM, A442, (2000), p.279

11 Current ISPA prototype YAP:Ce scintillating window X- or γ-ray X- or γ-ray Photons YAP:Ce window Photons YAP:Ce plate Photoelectron cluster quartz window Photoelectron cluster LHC1 * chip implementation new electronics amplifier 100 ns peaking time, globally adjustable threshold, adjustable delay line, coincidence logic and memory smaller pixel size (50x500µm) Electrical tests ~7.5% (150) pixels are masked (noisy) test input ~4900e - γ = 1710 (~85%) pixels respond with an efficiency of ~95% * The LHC1 chip has been developed at CERN by the RD19 and the EP/MIC group

12 Quantum efficiency of S20 photocathode on YAP:Ce scintillating window Q.E YAP-window HPMT Q.E Quartz-window HPMT Q.E. YAP-window ISPA Q.E. Quartz-window ISPA Quantum Efficiency [%] Wavelength [nm] Perfect stability observed over 2 years

13 Energy spectra of some different sources Emissions converted in the YAP:Ce window of the ISPA-tube Pulse height distributions measured on the silicon chip rear side Co 122 kev (FWHM) ~ 22% 3000 counts [a.u.] 2000 Compton edge (39 kev) Pb ~ 80 kev + b. sc Number of photoelectrons Am 60 kev (FWHM) ~ 26.5% 3000 counts [a.u.] Compton edge (11keV) Number of photoelectrons

14 Energy spectra of some different sources Emissions converted in the YAP:Ce window of the ISPA-tube Pulse height distributions measured on the silicon chip rear side kev (FWHM) ~ 40% 109 Cd counts [a.u.] Y escape (6.3 kev) 88 kev 1000 (x20) Number of photoelectrons kev counts [a.u.] Fe Number of photoelectrons

15 Energy spectra of some different sources Emissions converted in the YAP:Ce window of the ISPA-tube Pulse height distributions measured on the silicon chip rear side two K lines (72.19 kev) 203 Hg counts [a.u.] kev Number of photoelectrons

16 Photoelectron numbers versus the energies of total absorption peaks for several gamma sources measured with the YAP-window ISPA-tube y = x R 2 = Hg γ Number of photoelectrons Cd γ 57 Co γ Hg K 241 Am γ Cd Ag X Fe Mn X 203 Hg L Energy [kev]

17 Image of a 60 kev γ-source ( 241 Am) through a 2-holes (0.35 mm φ) lead collimator (5 mm thick) 20 k-events ~1 mm

18 Intensity profile of the two holes along the X-direction distance of the two holes = 0.90 mm on chip FWHMx meas. = mm mean1 = 2.80 mm sigma1 = mm mean2 = 3.70 mm sigma2 = mm c.o.g. coordinate projection x(mm)

19 Average number of firing pixel per gamma event: N hit mean at ~50 hits Number of hits per event Note: N hit ~50 is < to N pe ~100. The greatest part of the difference is due to overlap effect

20 Center-of-gravity residual projection along the X-direction =(FWHM) resx = 1.50 mm on chip c.o.g. residuals x(mm)

21 The estimation of the spatial resolution is simply given by: 2 FWHM φ + Nhit 2 res = (FWHM ) 2 (FWHM) resx = 1.50 mm; (FWHM) resy = 1.46 mm; N hit ~50 x = <===> FWHMx meas. = mm y = <===> FWHMy meas. = mm The difference between the estimated values and the measured ones can be related to the tails in the residual distributions, which worsens the precision in the c.o.g. calculation.

22 CONCLUSIONS First results with the YAP:Ce window ISPAtube are very encouraging (cluster size /2, N pe x1.2). They can be used to detect a wide range of energies (window thickness can be adjusted). Better matching of refractive index if coupled to other standard crystals. The performances can be further improved with those of future pixel chip anode.

23 FUTURE OUTLOOK Developments on heavier Ce-doped scintillators and of larger dimensions. Implementation of ALICE chip. Possible use of thinned detector unit.

24 Attenuation coefficients of Cerium-doped RE 3+ perovskyte scintillators 100 LuAP Lu 0.1 Y 0.9 AP Linear attenuation coefficient (cm -1 ) 10 1 YAP Lu 0.3 Y 0.7 AP 140 kev 0.1 ( 99m Tc) Energy (MeV)

25 Some properties of Cerium-doped RE 3+ perovskyte scintillators Rel. L. Y. Density Peak emission Light decay %NaI(Tl) (g.cm -3 ) (nm) (ns) NaI(Tl) BGO YAP:Ce Lu 0.1 Y 0.9 AP:Ce Lu 0.3 Y 0.7 AP:Ce LuAP:Ce kev with a 3-mm thick scintillating window: YAP;Ce Lu 0.1 Y 0.9 AP:Ce Lu 0.3 Y 0.7 AP:Ce LuAP:Ce ~30% efficiency (~55% total absorption) ~45% efficiency (~70% total absorption) ~65% efficiency (~80% total absorption) ~95% efficiency (~95% total absorption)