Energy Dispersive Xray Detectors for Spectroscopy Applications J

Energy Dispersive X-ray Detectors for Spectroscopy Applications J Morse, Instrument Support and Development Division, European Synchrotron Radiation Facility, Grenoble. 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 1

Talk Outline Ø what are the (synchrotron) requirements? Ø semiconductor Energy Dispersive X-ray detectors: principle of operation, material limitations Ø energy resolution and Fano statistics Ø preamplifier and electronic noise Ø signal pulse processing and the pile-up limit Ø mutielement arrays: the crosstalk challenges Ø summary 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 2

what are the (detector) requirements ? PEAK Energy range: ‘ 3 rd Generation’ Synchrotrons, X-ray photons ~1 ke. V to >100 ke. V Energy resolution: many measurements concern identification and quantification of elements in sample. Requirement in this case is to resolve individual K, L, (M) fluorescence lines ‘FWHM’ is the usual figure of merit, typically need ΔE ≤ 200 e. V. Gaussian line shape is usually assumed (often wrongly) X-ray Counts Sample X-ray counts Monochromatic Xray beam For trace element analysis -- where we may look for ppm levels in a sample matrix that scatters the incoming beam and itself fluoresces — ‘peak-to-valley’ performance of the detector may be of equal importance 3500 2500 1500 Energy dispersive detector 1000 energy Scattered X rays from incoming X ray beam 3000 2000 500 FWHM Si escape peak from Fe. K α detector fluorescence from sample VALLEY 5000 5500 6000 6500 7000 7500 8000 X-ray photon Energy (e. V) 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 3

Counting rates Energy spectra histograms can only be obtained by analyzing individual photon energies on a ‘count by count’ basis At synchrotons, high beam intensities high total spectrum counting rates are required, 103…>106 counts/sec e. g. for high spatial resolution ‘µ-mapping measurements’: ESRF ID 21 ESRF ID 22 NI Neurite process A Carmona et al JAAS (2008) For analysis of chemical states (e. g. SO 4 n-… XANES, EXAFS studies ), higher resolution may be required. In this case, the incoming synchrotron beam energy crystal monochromator is energy scanned with ΔE ~1 e. V to determine spectral response of sample an energy resolving detector is still required for dilute samples 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 4

Count rates and detection limits For quantitative element analyis, Silicon and Germanium semiconductor detectors are used: - fast photon event counting in parallel, i. e. over all energies in spectrum - good efficiency possible (solid angle covered by detector) -adequate FWHM resolutions of known lineshape (needed for spectrum deconvolution) Bovine liver ‘thick’(200µm) standard 300 ms 300 s beam normal incidence on sample, Vortex silicon drift detector at 75º beam 45 deg incidence, detector at 90º P Cloetens, ESRF-ID 22 N detection limits are set by counting statistics 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 5

Detector and the beamline environment Synchrotrons X-ray beams are focused onto sample emission of sample fluorescence, scatter is from a quasi-point source (~1 … 100µ size) Fluorescence emission is isotropic ideal detector should cover a 4π solid angle for 100% efficiency size of detector is best defined in terms of its solid angle coverage X-r ay a small detector close-up can be as be am effective as a ‘big’ detector further away …but not always! Ω detector Ω sample Sample environment constraints highly variable: Sample environment - high pressure (e. g. diamond anvil cell and press) - cryogenic or high temperature furnace (! infra red background) ESRF-ID 21 - vacuum - available space around sample (microscope, other detectors and instruments… other ‘constant’ problems for detector operation: -vibrations – accoustics -electrical interference from other equipment… Electro- Magnetic Compatibility (EMC) 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 6

Semiconductor detectors: principle of operation Semiconductor material (e. g. crystal of Si or Ge) with X-ray transparent contacts, applied electric field depletes bulk of thermally generated free charge. X-ray semiconductor electrical contacts - photoelectric conversion of an X-ray creates ‘hot’ electrons which rapidly thermalize (~psec), - hole, electron charges drift in applied field towards electrodes - electrical signal develops while the charge drifts in the bulk 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 7

not a new idea… “The crystal counter: a new instrument in nuclear physics”, P. J. Van Heerden, Ph. D Dissertation, Rijksuniversiteit Utrecht, July 1945 but in practice needed development of - materials in which photoelectric charge is not ‘lost in transit’, i. e. by trapping at crystal structure defects or impurity sites ( Ge(Li), Si(Li)… high purity Ge, Si crystals) - development of (surface) electrical contact technologies (problems of time dependent ‘polarization’ effects, charge injection…) 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 8

X-Ray absorption in various detector materials Beer’s law: I(x) = Ioexp(-µ(E). x) intensity of a photon beam decreases with distance into material, !! the energy of indvidual photons remains the same. At ‘low’ energies, photoelectric effect is dominant: µ(E) ~ E 3… 4 K, L absorption edges but µ is discontinuous at ‘absorption edges’ corresponding to atomic shell structure binding energies 40µm of Ge (or Ga. As) has same total X-ray absorption as ~500µm Si 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 9

Material absorption effects on energy spectrum Useful detector energy range is set by photon absorption range in material (s) -‘window’ transmission cut-off (detector vacuum window - inefficient charge collection for absorption close to front contact of semiconductor Ge - transmission loss at higher energies - incomplete energy absorption (loss by Compton Scattering) Absorption efficiency loss occurs at binding energies of electrons corresponding to shell levels. This is associated with probability of fluorescence emission (K shell fluorescence photon) ‘Escape’ peaks appear in detector energy spectrum at energies (EXray- Efluo), where Efluo transition energy for electron falling from L, M… levels to inner K shell energy level e. g. for Ge Efluo ≈ 9. 9 ke. V (Kα), 1. 2 (Lα 1) for Si ≈ 1. 74 (K α) Escapes complicates spectra with multiple peaks, and information may be ‘lost’ by peak overlaps photoelectron 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 10

Compton Scattering and energy loss Detector Material E = hc/ λ Compton scattered photon escapes detector photoelectric absorption incident photon energy measured (recoil electrononly + Compton photon) X√ all measured energy = Compton recoil electron 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 11

Semiconductor materials for γ and X-ray detection Fano energy resolution, leakage current (noise) Signal development time (max. counting rate) stopping power, X-ray absorption length 4500 3500 monoelemental crystals, excellent charge transport Binary and ternary compounds Stochiometry etc trapping of charge during drift (signal loss) 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 12

statistics and energy resolution Absorbed radiation energy E is shared between crystal lattice excitations (~2/3) and generation of charge carriers(~1/3) this ratio is ~same for many semiconductor materials Lower bandgap materials can offer better resolution due to better Fano statistics NQ is number of generated charge carriers, F is ‘Fano factor’ But low bandgap materials must be cooled to limit noise from thermal generation of carriers ~exp( /k. T) and often suffer from ‘charge trapping’ Cooling below room temperature needed 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 13

Recall, ultimate ‘theoretical’ energy limit set by Fano statistics: Fano factor F ≈ 0. 11 for Si and Ge ε =3. 63 e. V/e-h for Si (F is not a constant) U. Fano, Phys. Rev. 72 (1947) 26 Mn. Kα FWHM = 2. 35 √FεE (Fe 55 source) energy resolution and electronic noise But measured spectral resolution R is quadrature-sum of above Fano statistics and electronic noise : R = √ (Fano)2 + (electronic noise)2 R should have ~Gaussian symmetric shape, but rarely does at ≤ 1% level… multiple causes: • near surface X-ray absorptions with incomplete charge collection • ‘ballistic deficit’ associated with charge collection and pulse filtering time • pulse processor effects (pile-up and baseline degradation at high count rates) Peak-valley performance may be critical 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 14

Signal: time development Silicon detector, 300µm thick, Vdepletion. V = 60 V, Vbias= 200 V Vbias T= 300ºK - - + + X-ray photoelectric absorption: High energy physics: MIP particle track different interaction depths for each photon x for photons variation in signal-time development according to photon interaction point In spectroscopy measurements, problem is avoided by use of charge sensitive preamplifier which integrates the i(xabsorption, t) signal current assuming no charge trapping! 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 15

the charge preamplifier RP ID C semiconductor crystal Signal amplitude Pulse restore preamp Cf gm FET-charge preamp time Signal output from preamplifier charge preamp, signal out (volts) = charge in (from X-ray photo-conversion) preamp feedback capacitance Cf Charge preamplifier Signal amplitude is proportional to collected photoelectric charge (i. e. to X-ray energy) …and independent of detector bias, interaction point (charge drift time variations) 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 16

preamplifier noise: theory Charge q created by X-ray absorption is q = 1. 6 x 10 -19 Exray(e. V)/ (Coulombs) = 3. 63 e. V / electron-hole pair for Si , 2. 9 e. V / electron-hole pair for Ge for germanium detector preamp’ with typical feedback Cf=0. 1 p. F, a 10 ke. V Xray gives a voltage step signal of only 0. 5 m. V and we need to measure this with a precision of <1% ! NOISE contribution of electronics (preamplifier) must be minimized For charge preamplifier, ‘Equivalent Noise Charge’ (expressed in e- r. m. s) analysis gives total noise ‘parallel’ parallel noise series noise ‘series’ 1/f noise τ is signal ‘shaping time’ 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 17

noise: practice To reduce noise: Detetctor cooling duc -minimize detector ‘leakage current’ ID reduce temperature, detector material bulk and surface, design tricks C re -maximize RP OK, RP ∞ by using a ‘pulse restore’ ing silicon detector array, LBL -minimize k. T (OK, cool detector, but limits to this…) -minimize C (crystal geometry, ‘drift diode’ designs, FET type/integration) -optimize choice of τ pulse shaping (or peaking) time can be varied ‘online’ as needed by experiment… …but need to count at high rates (≥ 1/10τ) limits the maximum τ values, i. e. problems of pulse pile-up… 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 18

signal pulse processing Pulse processor (for spectroscopy, now almost always ‘digital’ systems) has several tasks: minimize preamp noise contribution to resolution (filter peaking time and shape ) detect and reject pulse pile-up events (and detector preamp pulse restores…) and record corresponding detector ‘dead time’: X-ray events Fa (tim st ch e i ann nfo el ’) peaking time ‘slow’ channel (energy) PUR spectrum 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 19

signal pulse ‘pile-up’ Spectroscopy pulse processors are ‘paralysable’: TP TP -‘dead-time’ TP for processing each event - any second event coccurring within time TP must be rejected to avoid false ‘pileup’ peak in spectrum TP time between pulses increasing For Poisson time-distributed X-ray events, measured spectrum output count rate can be obtained from d lo ead ss t es im e = OCR O C R TP ≈ 5 x pulse ‘shaping time’ or ≈ 2 x ‘peaking time’ IC R ICR = OCR exp(-ICR x TP) Limits to OCR usually: TP ) 1 -OCR R/ T P( Low counting rates: insufficient detector size (solid angle) C ICR=O 1/e. TP ICR = O CR exp(- High counting rates: TP cannot be reduced (energy resolution degrades!) ICR x T P) ICR 1/ TP multielement detector systems 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 20

silicon drift diode (SDD) Charge carriers collected by low capacity anode electrode contact preamp’ first stage may be integrated into detector (C <100 f. F) SDDs exploit the complex processing technologies available for planar processing of silicon: multielectrodes establish transverse drift field charge collected over large surface area (≤ 1 cm 2) without increasing anode capacity X ray X-rays -high resistivity (i. e. low impurity) silicon low bulk leakage current - thermoelectric Peltier cooling -10ºC…-70ºC is sufficient for spectroscopy with pulse processor peaking times ~0. 2 … ~10 µsec compact and lightweight (~kgm) systems, insensitive to accoustics-vibrations 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 21

SDD: a very clever trick Gatti et al. IEEE Trans. Nucl. Sci. NS-32 (1985) 1204) 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 22

practical SDD detector for high count rates SII ‘Vortex’ drift diode discrete JFET preamp with pulse restore operation no energy peak shifts with counting rate ~2 00 ESRFmm ID 22 NI ~47 mm 2 active detection area P/V Throughput count rates to ~500 kcps possible (0. 25µs peaking, 230 e. V Mn. K FWHM ) m a cf. Si(Li) detector be~25 kcps y a (5µs peaking X-r 160 e. V Mn. K FWHM) but peak / valley 700 ~ 1000 sample cf. 10 000 for Schottky Si(Li) or Schottky Ge 0 5000 10000 15000 20000 test data (Mn foil fluo’), ID 21 ESRF 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 23

SDDs with higher peak-valley performance - very shallow, abrupt dopant-profile implant for front contact - Zr collimator ring (avoids partial charge collection from X-rays at detector periphery ~ 20 mm grid supported ultrathin Peltier thermoelectric (~0. 5µ) polymer window cooler -20ºC data from pndetector. de (2µS pulse processor peaking time) 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 24

‘teardrop’ SDD design integrated FET structure near Fano-limited resolution at low count rates (peaking times >1µs) pn. Sensor Gmb. H Collimating mask e. g. Zr FET teardrop geometry + metal collimator teardrop SDD peak / valley of 7000 standard SDD radiation protection of FET (hole-accumulation in surface oxide and trapping at Si- Si. O 2 interface) but SDD Si thickness limit ~0. 4 mm events near FET peripheral events cf. 3 mm for ‘conventional’ Si(Li) structure and >10 mm for Ge 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 25

multielement detectors and beam polarization Multielement detectors can offer higher overall count rates: e. g. for N independent counting channels and uniform angular distribution of counts we can expect a total count rate capability increased N-fold multielement detectors (e. g. germanium 13 – 100 elements) are now commercialized Synchrotron undulator beams focused on sample are typically ~ 99% linear polarized angular dependence of both Rayleigh (elastic) Compton (inelastic) scattering but an EDX detector measures total count rate (i. e. fluorescence and scatter) in practice, effective count rate gain from an N channel detector is <N or <<N, and dependent on: - the experiment-detector geometry - the sample under investigation (concentration, matrix Z, crystallinity…) - energy of excitation beam 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 26

Scattering of linear polarized radiation Polarization dependent elastic scattering cross section: Compton scattering ignored here (‘low energy’ case) max min R. E. van Grieken, A. A. Markowicz, Handbook of X-ray Spectrometry (2002). horizontally polarized x-ray beam max min max non isotropic process Cross section for fluorescence radiation is isotropic (independent of ) 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 27

central element of multiement detector Photon density distributions projected on central element of detector Sample Fluorescence 30 mm Elastic scatter 20 mm mm 20 Spectrum recorded by central element Fe in low-Z matrix Monte-Carlo calculations: L. Vincze et al. , Spectrochimca Acta B, 48, 553 (1993), 50, 127 and 1481 (1995), 54 1711 (1999). 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 28

detection limit with multielements Application: fluorescence from ‘dilute’ samples Sample 8 18 Elastic 19 scatter 9 17 30 mm 20 mm 16 mm 20 7 6 15 2 1 5 14 3 4 13 10 11 12 importance of multielement ‘packing factor’ i. e. inter-element dead spaces 8 usualcase Ideal for ‘discrete elements’ 19 18 9 19 18 7 8 2 2 7 17 17 11 4 6 Si escape 5 14 15 11 4 12 5 15 16 310 3 1 16 6 10 9 13 12 13 14 J Slazchetko, ESRF 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 29

monolithic multielement detectors a semiconductor can be electrically segmented by lithographic mask doping of contacts to create an x, y matrix of individual sensing areas. This gives a 100% sensitive area but there are problems: After an X-ray is absorbed, diffusion creates a ‘cloud’ of electric charge which may be split the signal between bordering sensing areas. Alternatively, a fluorescence photon may be emitted and absorbed in a neighbour sensing area These physical crosstalk effects clearly become more serious as the individual detector areas are reduced in size. Solution is use of a grid collimator covering border areas. As well as its shunt capacity to a common rear electrode contact, each sensing area is capacitively coupled to its neighbours. Electronic crosstalk from individual fet preamplifier restore switching generates false spectral peaks. Problem is worst for short pulse processor peaking times. Solution is ‘synchronous’ FET restore or signal veto. 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 30

multielement detectors: use of collimator mask Molybdenum mask on planar silicon detector developed at NSLS-BNL 384 detector elements each 1 mm 2 area, 400µm n-type silicon thick No mask peak-valley 200 Mo mask, peak-valley 1000 C. G. Ryan et al. /Nucl. Instr. and Meth. Phys. B 260 (2007) 1– 7 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 31

monolithic multielement devices multielement silicon drift diode arrays ‘near wafer-scale’ lithographic processing large, tightly-packed arrays possible 39 cell detector with on-chip FETs, total active area 195 mm 2 (after L Strüder, MPI-Garching) other practical challenges of large cell counts: yield issues (bad cell and cell-to-cell variability, especially on-chip FET parameters) power dissipation (cooling!) need for multi channel pulse processors overall system fabrication complexity / cost 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 32

high energy spectroscopy at ESRF magnetic Compton scattering spectra (fixed, monochromatic beam energy ~50 … 150 ke. V) Slit selection of Compton backscatter angle monochromatic beam sample Compton profiles Ge detector Compton scattered photon peak elastic scattered photon peak (beam energy) At present, only Germanium detectors are adequate for this application: - high Z for adequate absorption (large detector volumes (>cm 3) with negligible charge carrier trapping) -high energy resolution (ΔE/E ~0. 5% at 100 ke. V) and clean Gaussian line shape 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 33

high energy spectroscopy Strain scanning of materials with white beam (E ~100… 250 ke. V) e. g. Ti-alloy aircraft turbofan blade or Ge ct dete diffraction peak intensity (counts) Spatial resolution ~0. 1 x 0. 1 mm 2 Ehkl d sinθ =hc / 2 Bragg’s Law ESRF ID 15 Energy (ke. V) sample is stressed in-situ shifts in diffraction peak energies give Δd strain values, or fixed strain patterns in a sample may be mapped by x, y scanning in the beam Two detectors give simultaneous measurements in orthogonal scattering planes strain tensor measurement 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 34

even higher energies (> Me. V…) At high energies, Compton scattering is dominant interaction large volume Germanium detectors required spectroscopy possible by reconstructing photon interactions using information from multiple detectors partially absorbed events can be vetoed Φ= 5 cm, L = 7 cm ‘Clover’ detectors shaped for close-packed geometry and near 4π solid angles in nuclear pyhsics experiments eurogam 2 P. Jones et al. , Nucl. Instr. and Meth. A 362 (1995) 556 Detectors can also be electrically ‘segmented’ to give better tracking granularity 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 35

Summary At low count rates (<< 1/peaking time) Silicon and Germanium approach theoretical performance limits (Fano statistics) over the large range of X-ray energies used at 3 rd generation synchrotrons. Higher ΔE/E resolutions or better X-ray absorption may be theoretically be obtained with compound semiconductor materials. For precise, quantitative spectroscopy, there are no competitors to Siilicon and Germanium due to lack of large, pure, and perfect crystals of binary or ternary compounds. For low energies (>20 ke. V), Silicon is a near-ideal detector material offering advanced processing technologies including the fabrication of on-detector low-noise electronics. Higher energies require Germanium, but detectors made from this material are unlikely to ever reach the sophistication of silicon devices due to the lack of a large scale market (i. e. electronics) to support the needed developments. Pulse processor pile-up effects degrade spectral quality and counting efficiency at high count rates (≥ 1/peaking time). Multi-element detectors may attain higher count rates but the gain is limited by geometric considerations and the practical challenges of making individual detector channels operate in a truly independent manner. 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 36

some bibliography Radiation Detectors in general: G Knoll ‘Radiation Detection and Measurement’, Wiley , 2000 C Delaney, E Finch ‘Radiation Detectors: Physical Principles and Applications’, OUP 1992 Semiconductor Detectors, detailed physics: H Spieler, ‘Semiconductor Detector Systems’, OUP, 2005 G Lutz, ‘Semiconductor Radiation Detectors: Device Physics’, Springer Berlin 1999 EDX detectors, application issues (n. b. mainly for analytical electron microscopy): J Goldstein et al, ‘Scanning Electron Microscopy and X-Ray Microanalysis’, Springer 2003 R Van Grieken et al, Handbook of X-Ray Spectrometry, Dekker 2002 1 st EIROforum School on Instrumentation, Cern 11 -15 May 2009 37