Design of portable-transportable units: Comparison of possible - PowerPoint PPT Presentation

Design of portable-transportable units: Comparison of possible choices Román Padilla Alvarez Maria Liz Crespo International Atomic Energy Agency MLAB, ICTP

Outline:  Analytical needs Bulk analysis  Spatially resolved measurements   Excitation Radioisotopes  X-ray tubes   Modifying excitation spectrum Filters  Optical elements   Detectors  Geometry arrangements  Concluding remarks

Analytical needs: Bulk analysis (average composition) • Large area needs to be illuminated • Spatially resolved measurements • (identifying changes in elemental distribution) Suitable collimation / focusing device is • needed

Hardware for excitation Sources  • Radioisotopes ( a , g , x-rays) • X-Ray Tubes • Electrons (SEM) • Charged particles (accelerators) • Synchrotron radiation

Hardware for excitation Sources  • Radioisotopes ( a , g , x-rays) • X-Ray Tubes • Electrons (SEM) • Charged particles (accelerators) • Synchrotron radiation

Radioisotopes

Radioisotopes Isotope 55 Fe 244 Cm 109 Cd 241 Am 57 Co Energy (keV) 5.9 14.3, 22.1, 59.5 122 18.3 88 Elements Al – V Ti-Br Fe-Mo Ru-Er Ba - U (K-lines) Elements Br-I I- Pb Yb-Pu None none (L-lines)  While isotopes have fallen out of favor they are still useful for many portable applications.

Radioisotopes: Advantages and limitations • Pro´s Compact, simple construction o Portability o Monochromatic excitation o Low cost o • Con´s Change in flux due to radioactive decay o Constant radiation exposure o Non-tunable energy o

End Window X-Ray Tube  X-ray Tubes  Voltage determines which elements can be excited.  More power = larger sensitivity  Anode selection determines optimal source excitation (application specific).

Side Window X-Ray Tube Be Window Glass Envelope HV Lead Target (Ti, Ag, Rh, etc.) Electron beam Copper Anode Filament Silicone Insulation

X-ray production in an x-ray tube  Characteristic Lines Intensity Continuum Distribution Breaking radiation   E     0  N ( E ) E kiZ E  E 1   E 0 Energy

Tunable energy distribution 

X-ray tubes: Advantages and limitations • Pro´s Different anode materials available o Tunable energy by selecting HV o Low power tubes can be even portable o Not constant radiation exposure (on/off) o Possibility to use modifyiing devices o • Con´s Require of power generator o For power 600 w cooling system is required o Limited life time (~ 3000 hrs) o

Hardware for excitation  Modifiers  • Energy selection: • Spatial: Collimators Filters o o Monochromators x-ray optics devices o o Secondary targets o

Hardware for excitation  Modifiers  • Energy selection: • Spatial: Collimators Filters o o Monochromators x-ray optics devices o o Secondary targets o

Absorption filters  Titanium Filter transmission curve % Absorption T Edge R Low energy x-rays A Very high energy N are absorbed S x-rays are transmitted M I T T X-rays above the E absorption edge energy are D absorbed ENERGY Ti Cr The transmission curve shows the parts of the source spectrum are transmitted and those that are absorbed

Absorption filters 

Absorption filters (Ag tube) 

Absorption filters (Ag tube) 

Absorption filters 

Hardware for excitation  Modifiers  • Energy selection: • Spatial: Collimators Filters o o Monochromators x-ray optics devices o o Secondary targets o

Secondary targets  Improved Fluorescence and lower background The characteristic fluorescence of the anode source is used to excite the sample, with the lowest possible background intensity. It requires almost 100x the flux of filter methods but gives superior results. For lower power tube (50 w) still possible with optimized geometry designs Radiation travel path Average distance (mm) 23 x-ray tube exit window – secondary target Secondary target – sample 17 Sample – detector window 23

Secondary targets 

Comparison ST vs Direct or filtered 

Hardware for excitation  Modifiers  • Energy selection: • Spatial: Collimators Filters o o Monochromators x-ray optics devices o o Secondary targets o

Policapillary lens 

Policapillary lens vs Pinhole  Spot size ~ 15 - 20 μm Gain in intensity x 300

Detectors  Proportional Counters  Poor energy resolution Scintillation Detectors WDXRF  Si(Li)  LEGe  PIN Diode  Improved energy resolution EDXRF SDD  CCD cameras  CZT, other 

Main features of detectors Efficiency  o How many photons produce a signal Energy resolution  o Capability to differentiate close by amplitude (energy) signals Charge collection time  o Time required to collect charge

Intrinsic Efficiency T : Fraction that is transmitted through the entrance layers • D : Fraction that is detected in the sensitive volume • � � � � � �� � � � �� � �� � �� � �� � � �� � � � �� � �� �� �� � � � �� � �� �� �� � � � �� � �� �

Efficiency for various semiconductor detectors

Efficiency for various semiconductor detectors

Energy resolution

PIN Energy resolution ~ 180 – 190 eV (Mn-K a ) • Charge collection ~ 10 m s • Input capability ~ 10 5 photons/sec •

Silicon Drift (SDD) Energy resolution ~ 140 – 160 eV (Mn-K a ) • Charge collection ~ 1 m s • Input capability ~ 10 6 photons/sec •

Digital signal processing (DSP) Total time for processing one pulse ~ 15-20 n s •

Geometry arrangement: Excitation and  detection angles  Maximize the detection of x-ray emission while minimizing the detection of the primary radiation scattered at the sample Diff Coherent Scat sections (E0=17.443) Diff Incoherent Scat sections (E0=17.443) 0.9 0.02 Si Si 0.018 0.8 Fe Fe 0.016 0.7 Zr Zr 0.014 0.6 0.012 0.5 0.01 0.4 0.008 0.3 0.006 0.2 0.004 0.1 0.002 0 0 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 Scatter angle Scatter angle

Geometry arrangement: Effective Solid  angles

Geometry arrangement: Effective Solid  angles

Removal of spurious peaks Reducing instrumental background  ST: Ag, HV: 50 kV, DSP, Si(Li) (30 mm 2 , 4 mm, 190 eV), t meas = 2000 s Scatter from primary tube radiation Unshielded Duralumin ST sample After Ag coating, holder, Sample SUPRAPUR H 3 BO 3 , No sample, I=5 mA I= 20 mA

Concluding remarks Design of XRF spectrometers requires of a thorough selection of options, based on • Pursued features of analytical performance. • Cost/benefit analysis

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