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New Generation Nuclear Microprobe Systems: A new look at old problems By David N. Jamieson Microanalytical Research Centre School of Physics University.

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Presentation on theme: "New Generation Nuclear Microprobe Systems: A new look at old problems By David N. Jamieson Microanalytical Research Centre School of Physics University."— Presentation transcript:

1 New Generation Nuclear Microprobe Systems: A new look at old problems By David N. Jamieson Microanalytical Research Centre School of Physics University of Melbourne Parkville, 3010 AUSTRALIA 7 th International Conference on Nuclear Microprobe Technology and Applications, Cité Mondiale, Bordeaux, France, September

2 © David N. Jamieson 1999 Electron Emission from Surfaces CVD B-doped diamond films are electrically conductive Diamond has a negative electron affinity Potential applications as a cold cathode electron emitter Measure  : number of electrons emitted from surface per ion impact Measure  =15 to 30 (metals:  = 1.5) H H H electrons Incident ion + – I–I– H H H electrons Incident ion + – I+I+ 50  m Yield max min RBS I–I–

3 © David N. Jamieson 1999 Filiform Corrosion in Aluminium 200  m Yield max min Anticorrosion layer removed Filiform growth C - RBS Al - RBS He Cl - PIXE O - RBS Al - RBS Cl growth head Filiforms grow under breaches in the anticorrosion coating on Al 3 MeV H PIXE data confirms role of Cl in catalysing growth of the filiform

4 © David N. Jamieson 1999 Menke’s Syndrome revisited Menke’s Syndrome is a Cu deficency genetic disorder. The gene responsible for the disorder has now been mapped. Pathways for Cu metabolism within cells can now be controlled and studied with unprecedented precsion. But can the nuclear microprobe cope? Need to resolve Cu distributions within single cells to a spatial resolution of sub-micron. Images here are by indirect immunofluorescence from anti- body labelled Menkes protein. Cells are less than 10 micons in width

5 © David N. Jamieson 1999 Outline The quest for superior spatial resolution in the Nuclear Microprobe: Why has the probe resolution stalled at 1 micron for 2 decades? Some new insights provide possible pathways to future progress Introduction to elementary ion optics –Chromatic aberration - not a problem? –Spherical aberration - not too much of a problem? –Stray magnetic fields - definitely a problem –Demagnification - the way forward –Ion source brightness - small advances to be welcomed A review of the next generation systems Conclusion (Topics not addressed: –High efficiency detectors, fast DAQ’s to handle high intensity beams, –specimen damage,channeling convergence angle)

6 © David N. Jamieson  m wall Chip feature size and NMP resolution Size (micron) Year Moore’s Law <1 pA P5 P6 P7 >100 pA

7 © David N. Jamieson  m wall Spatial Resolution Required: Applications published at the Last Conference 1998 “Pile up”

8 © David N. Jamieson 1999 Image Plane Introductory Ion Optics x i = (x/x)x o + (x/  )  o + (x/  )  o  o + (x/  )  o 3 + (x/   2 )  o  o 2 y i = (y/y)y o + (y/  )  o + (y/  )  o  o + (y/  )  o 3 + (y/  2  )  o 2  o 2 …plus higher order terms Object Plane Aperture Plane (x o,  o, y o,  o,  o ) (x i, y i ) Lens System Magnification (x/x)x o (y/y)y o Focusing (x/  )  o (y/  )  o Chromatic (x/  )  o  o (y/  )  o  o Spherical (x/  )  o 3 + (x/   2 )  o  o 2 (y/  )  o 3 + (y/  2  )  o 2  o 2

9 © David N. Jamieson 1999 Steps to evaluate lens system design: 1. Calculate magnification and coefficients from ion optics computer codes 2. Measure: –Beam Brightness –Chromatic momentum spread from the accelerator (use nuclear resonance) 3. Set object size so that demagnified image is equal to desired probe resolution 4. Set aperture size so that beam current is equal to desired beam current 5. Calculate aberration contribution from maximum divergence and energy spread 6. Add contributions to probe size in quadrature (or similar) 7. Spot size is now greater than desired spot size so go back to 3 and choose a smaller object size Repeat 4-7 until done. d m = 2(x/x)x o | max d c = 2(x/  )  o | max  o | max d s = 2|(x/  )  o 3 | max + |(x/   2 )  o  o 2 | max How to calculate probe resolution? d i 2 = d m 2 +d c 2 +d s 2 Wrong!!

10 © David N. Jamieson 1999 Chromatic Aberration, A closer look Singapore system achieves sub-micron probes with 15 o switcher magnet that has low energy dispersion Yet chromatic aberrations of this system should be large Skilled tuning of system is part of the answer, but not all! Maximum d c depends on getting maximum  and  in the same beam particle d c = 2(x/  )  o | max  o | max High excitation systems

11 © David N. Jamieson 1999 Divergence,  o Energy Spread,  o + high low Chromatic Aberration, A closer look Are  and  correlated? Use MULE* to find out. Here is a slice of object plane phase space taken along  and  System was the HIAF accelerator in Sydney ( From the work of Chris Ryan) Not much beam in the danger zone Beam intensity is peaked in the paraxial zone Ion source Accelerator Magnet Ray used in maximum d c calculation Danger zone Conclusions: Not much beam at edge of phase space Chromatic aberration is not a severe problem *Thank you G.W. Grime

12 © David N. Jamieson 1999 Spherical Aberration, A closer look Traditionally, spherical aberration is computed from the rectangular model (RM) Rectangular model: B(z) = 0 z < 0 B(z) = B 0 0 < z < L B(z) = 0 z > L Results from this model agree with ray tracing codes that use B(r 0, z) measured at r = r 0 Detailed studies have been done by Glenn Moloney –Measured field profiles B(r, z) at several r –Provides 3-D profile of True Fringe Field (TFF) Numerical raytracing from measured B(r, z) reveals different spherical aberration coefficients! L z 0 Coefficient RM TFFM (x/  2 ) (x/  2 ) (y/  3 ) (y/  2  )

13 © David N. Jamieson 1999 Spherical Aberration, A closer look Coefficients calculated from the TFF model give aberration figures of different shapes compared to the rectangular model The figure is more intense in the paraxial region - good!

14 © David N. Jamieson 1999 Ion Source Brightness: Flux Peaking Legge et al (1993) showed a 1 order of magnitude decrease in probe size required a 5 orders of magnitude increase in brightness for uniform model True situation more complicated: 1 order of magnitude decrease in probe size requires 2 orders of magnitude increase in brightness Uniform phase space Set 5 nA For 5 nA divergence is 2.5 times less than uniform model so spherical aberration is reduced by a factor of  m 200  m 75  m 2 MeV He + Current (pA)

15 © David N. Jamieson 1999 shadow 130mm525mm grid Without magnet With Magnet Stray DC Magnetic Fields: Parasitic aberration Non-uniform stray DC fields are a problem Shadows of a line focus on a fine grid should be straight line Small bar magnet has severe effect See large sextupole field component aberrations Sources of stray DC fields in the MARC laboratory: –Iron gantry and stairway over the beam line –Steel equipment racks –Gas bottles –Stainless steel beam tube itself!

16 © David N. Jamieson 1999 shadow 130mm525mm grid Deflect here beam BEAM PIPE Stray DC Magnetic Fields: Aberrations of a beam pipe Type 316 stainless steel beam pipe through quadrupole lenses 10 mm internal diameter Beam diameter 6 mm Grid shadow pattern reveals aberrations See strong effect from different deflections of the beam pipe! Effect here produced by a few cm length What effect does 8 m have?

17 © David N. Jamieson 1999 Stray AC Magnetic Fields: Beam spot jitter x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x B stray (t) object virtual object Stray AC field causes a shift in the virtual object position The beam spot is scanned by the stray field in a complex fashion image shift h Mh lens

18 © David N. Jamieson 1999 Stray AC fields cause virtual movement of the object collimator Used a 2-D scan with y-coils disconnected Gives position as a function of time in map of Cu x-rays B y (nT) Stray AC Magnetic Fields: Beam spot jitter 3  m

19 © David N. Jamieson 1999 Stray AC Magnetic Fields Where: M = Magnification = 1/Demagnification q = beam particle charge L = Length of beam line E = beam energy m = beam particle mass It is good to have: High demagnification systems Short systems On the Melbourne system it is required that: B stray < 20 nT for x i < 0.1  m

20 © David N. Jamieson 1999 Stray AC fields in MARC laboratory: Where from? Field as a function of time tells the story Start: 6pm April Place: MP2 beam line, MARC laboratory To MARC lab 50 m

21 © David N. Jamieson 1999 Modify RF Ion Source Beam from ion source emerges with low energy Gas leakage from ion source canal fills low energy end of accelerator Gas scattering degrades ion source brightness Solution: Add recirculating turbopump gas T.p. old new From the work of Roland Szymanski

22 © David N. Jamieson 1999 Modify Accelerator Column Remove corona needles and replace with resistors (Have now increased brightness by a factor of 10) So need to design a system optimised for a flux peaked beam… High demagnification!

23 © David N. Jamieson 1999 Selected new quadrupole systems 1970 Russian quadruplet D x =D y = Leipzig separated quadruplet D x =80 D y = CSIRO/MARC high excitation quintuplet D x =67 D y = Oxford high excitation triplet D x =25 D y = Oxford separated triplet D x =240 D y = New system D x =D y =200 ?

24 © David N. Jamieson Strong demagnification in a long system CSIRO quintuplet system Leipzig two stage system Strong demagnification in a short system, 80 mm WD Very intense beam spot into 1  m

25 © David N. Jamieson  m at 20 nA Resolution Versus Beam Current: CSIRO/MARC quintuplet system 1.3  m at 0.5 nA Accelerator brightness = 1.2 pA.  m -2.mrad -2.MeV  m 1.2  m x 0.9  m at 0.1 nA CSIRO-GEMOC Nuclear Microprobe 2.0  m at 10 nA 3100 pA/  m 2 ! 1.8  m at 8 nA 12.7  m From the work of Chris Ryan

26 © David N. Jamieson 1999 Future Developments Conclusion: To break through the 1 micron wall Install heavier magnetic shielding! But be sure to clean off DC fields (10 nT). Don’t worry about chromatic and spherical aberration, they are not a severe as first though because of flux peaking (<0.1  m) Make brighter ion sources by small tweaks, even a factor of 10 is helpful (x1/3) Install an optimised system for a strongly flux peaked accelerator, this will have a large demagnification (of necessity a high excitation system) (M -1 > 200) Need more radical lens design to reduce working distance and increase fields (40 mm) Apply the new system to some interesting problems! (< 0.1  m resolution) MP2 Bochum Leipzig Oxford triplet CSIRO 5 New Ox

27 © David N. Jamieson 1999

28 Spherical Aberration: A closer look The TFF model also revealed the need for careful attention to the field overlap between adjacent lenses Must have a linear field gradient as a function of beam direction to minimise aberrations Need to shape pole ends to achieve this z Pole tip ? N SN S z N N S S


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