CdTe Detectors for Quantitative X-Ray Fluorescence
25Pages

Amptek, Inc14 DeAngelo Dr, Bedford MA 01730 > CrossRoadsScientific414 Av. Portola, El Granada, CA > 2007 Denver X-ray ConferencePage 1of 25 size="-1">

֕Compound semiconductor with wide bandgap(4.4 eV), high density (6.2 g/cm > 3 ), and high atomic number (48,52)Charge transport better than most alternatives > -4 2 h =2x10 cm /VՕStudied and used for -ray spectroscopy since late 1960s > ֕Schottky (blocking) contacts to reduce leakage currentI > 2 dark 5 nA/cm at 500V and 300K > ՕM nstructure from Acrorad, Inc

+VLR Thermoelectric Cooler > d Output >

All are complete spectroscopy systems֖All share core technologies > Digital pulse processor for pulse shaping, selection logic, and multichannelanalyzerՕPower supplies, including 1.5 kV bias supply and closed loop temperature controlUSB interface, +5V power input ՖTargeted at different applications > PX4 and XR100 for benchtop& laboratoryDP4 and PA210 for embedding in instruments X123 for compact, packaged system 2007 Denver X-ray ConferencePage 7of 25 size="-3">

Gaussian convolved with ideal tailing Ideal GaussianIdeal Hole Tailing Charge collection Q(x)֖Photoelectric absorption > Exponential attenuation Hecht equation N(x)Combination N(Q)֖Convolve with GaussianENC and FanoFairly accurate representation֖Estimate performance Not in analytical form, so difficult to use for spectra fitting > Counts 110115120125130 Energy (keV) ModelMeasured Model Counts Measured Counts 110115120125130 Energy (keV) 2007 Denver X-ray ConferencePage 10of 25 size="-1">

Counts Counts Counts 05101520Energy (keV) 5456586062Energy (keV) 110115120125130Energy (keV) <20 keV: All interactions near cathode, no tailing, Gaussian 50 keV: Small asymmetric correction to Gaussian 100 keV: Interactions uniform, tail important, shape complexCan model using Van Espentype tail but truncate at Q > anode 2007 Denver X-ray ConferencePage 11of 25 size="-1">

Carried out by Paul Bennett of RMD, Inc. Modeled Cd K Measured Cd K Escape peak / photopeak ratio Te K Equations of the general form: Esc = a > 2 3 4 5 + a > 1 e + a > 2 ?e + a > 3 ?e + a > 4 ?e + a > 5 ?e where Esc=escape events, e=parent energy, and the a terms are coefficients. > 2007 Denver X-ray ConferencePage 13of 25 size="-1">

2007 Denver X-ray ConferencePage 14of 25 >

2007 Denver X-ray ConferencePage 15of 25 >

2007 Denver X-ray ConferencePage 16of 25 >

CdTe spectra, at higher energy, have more scattering into detector֖Plot shows result of applying Si parameters to CdTeYields residual false peaks 2007 Denver X-ray ConferencePage 17of 25 size="-2">

Adjust parameters to give high curvature background continuum > ֕First, Cd-Te escape peaks are removed (partially at least)Second, automatic background function applied to spectrum ՖVery little residual continuum > 2007 Denver X-ray ConferencePage 18of 25 size="-2">

CdTe dead layer much more significant than Si > ֕Metal contact (200nm Pt)CdTe higher Z and density ՖSecondary electrons deposit more energy while escapingAt low energies, peak to background ratio lower for CdTe > 2007 Denver X-ray ConferencePage 19of 25 size="-2">

Cathode (-) Anode (+) Cathode (-) Anode (+) Data taken at 300K, 200V Start2 hours 4 hours6 hours8 hours + + > 16 hours 18 hours 20 hours22 hours24 hours h + + > h +

Cd-Te escape֖Background Gaussian peak fits > ֕Raw SpectrumProcessed Spectrum ՕGaussian Peak Fit Spectrum Ֆ68% Sn for Sn-Pb thickness = 1 mm63% Sn for Sn-Pb thickness = 0.5 mm֖(Nominal Sn = 63%) Even though Pb-K peaks have significant low-energy tails, quantitative analysis is possible. > 2007 Denver X-ray ConferencePage 24of 25 size="-2">

CdTe is a powerful tool for measuring metals via XRF > ֕It has high sensitivity for K lines, with fewer interferencesOne can carry out quantitative analysis ՖSpectral characteristics require changes to algorithms > Hole tailing shape is differentՕEscape peaks more significant Continuum more significant and shape differentՕAmpteks XRS-FP will implement these corrections ҖFor more information, go to www.amptek.com > 2007 Denver X-ray ConferencePage 25of 25 size="-2">