ion beam analysis of gold flecks in a foam lattice f e gauntlett, a s clough
DESCRIPTION
Ion Beam Analysis of Gold Flecks in a Foam Lattice F E Gauntlett, A S Clough Physics Department, University of Surrey, Guildford, GU2 7XH, UK. Ion Beam Characterisation of Gold-Loaded Foam Sample. 1mm. - PowerPoint PPT PresentationTRANSCRIPT
Ion Beam Analysis of Gold Flecks
in a Foam Lattice
F E Gauntlett, A S CloughF E Gauntlett, A S Clough
Physics Department, University of Surrey, Physics Department, University of Surrey,
Guildford, GU2 7XH, UKGuildford, GU2 7XH, UK
Ion Beam Characterisation of Gold-Loaded Foam Sample
At Surrey we have been asked to measure:
Techniques: Scanning Micro-PIXE
Scanning Micro-RBS
Low density foam cylinder (1mm diameter 1mm) loaded with tiny gold spheres (~ 5m diameter).
1mm1mm
1m1mmm
•the total MASS of the gold flecks in the foam,•the average SIZE of the flecks,
•the uniformity of DISTRIBUTION of the flecks.
Gold loaded foam cylinder is encased in a Kapton (polyimide) sleeve.
Surrey Ion Beam Centre
Facility: 2 MV Tandetron
Beamline: Scanning in vacuo proton microbeam
Detectors
Hence we use Amptek CdTe detectors which are ~100% efficient between 10-70keV.
Thus we need to detect both:
Gold K X-rays have energies of 66–78keV, so they are only attenuated by ~1%. However, their yield is very low.
At Ep ~4MeV gold L X-rays (~10keV) are produced in abundance but they are attenuated by ~35% in a 5m diameter gold fleck.
We also detect Backscattered protons with 100mm2 ORTEC ULTRA detectors with a 300m depletion layer.
K X-rays to determine the overall quantity of gold present
L X-rays to determine from the attenuation the average size of the flecks.
and
Detectors in Sample Chamber
We use two detectors in both cases to obviate any instrumental asymmetries.
A Sample in Carbon Blocks and Beam Setting-Up Plate
(View of Vertical Sample Holder)
2m thickm thick Gold Foil
(Comparison)
Foam Sample
Scintillator
Carbon Blocks
Copper Grid
View of sample in the chamber through microscope at 135 to beam direction:
Sample In Chamber
Copper
Sleeve
Carbon
Oxygen
Gold
Energy, keV
Gold L X-rays
Copper K X-rays
Energy, keV
Cou
nts
Cou
nts
Spectra!
The PIXE spectrum, contains gold L X-rays, also a lot of
copper.The Backscatter spectrum,
contains carbon, oxygen and gold.
The microbeam is raster scanned over the scan area, recording the position associated with each event. We can draw 2D maps corresponding with specific features (i.e. counts within a chosen energy window) on the spectra…
Carbon Backscatters, TOP
Carbon Backscatters,
BOTTOM
Gold L X-rays, LEFT
Gold L X-rays, RIGHT
MAPS….Max. Counts
Min. Counts
RBS Detector Spectrum from Foam Containing Gold Flecks
Oxygen Edge
Carbon Edge
Gold Continuum(!)
Energy, keV
Cou
nts
Gold Continuum
Energy, keV
Cou
nts
The gold continuum is sloping, without the sharp edge typical of backscatter spectra from uniform layers:
(Assuming gold flecks are 5m in diameter and have a total mass of ~15% of the foam mass – an upper estimate – the number of gold flecks in a cylinder is ~700 and the areal ratio (sum of cross sectional areas of gold spheres/area of foam cylinder) ~7x10-2 i.e. 93% of the time protons in the beam incident on the foam go through the foam completely missing any gold flecks.)
How Do We Explain the Shape of the Gold Continuum?
165
to detector
protons
It is likely that at most only 1 gold fleck is in the path of a proton.
At the highest backscatter detected energy (4.046 MeV) only the front surface nuclei of any gold fleck at the front of the foam (left on this figure) will contribute.
Gold Continuum Highest Energy
At lower and lower backscatter detected energies, more and more combinations of protons backscattered from within flecks at various depths will contribute.
In Gold Continuum
Au Low E
Au Medium E
Au High E
Energy, keV
Cou
nts
Example
Limiting Depth
The Limiting Depth is when the backscatter energy from the front of a Au fleck at that depth is equal to the backscatter energy from Oxygen at the front of the foam (3.221MeV).
Limiting Depth
For 4MeV protons DL~0.7mm.
Thus to detect all the gold flecks in the full length of the foam cylinder - 1mm - with 4MeV protons we must use X-rays.
Carbon
Oxygen Gold
Energy, keV
Cou
nts
Can we get an estimate of the gold mass?
• Re-run spectra offline, screening out the wire
• Normalisation
• Calculations
• Validation of Technique
Carbon (Backscatters)
Gold (Backscatters)
Top Bottom
Gold and Carbon Maps from Backscatter Spectra
Gold K X-rays
Gold L X-rays
Left Right
Gold Maps from PIXE Spectra
Energy, keV
Cou
nts
New SpectraC
ou
nts
Energy, keV
Au L X-rays
Energy, keV60 70 80Energy, keV0
90
Cou
nts
10Au K X-rays
Can We Get an Estimate of the Gold Mass?
• Re-run spectra offline, screening out the wire
• Normalisation
• Calculations
• Validation of Technique
Normalisation with Protons Backscattered from Carbon
NC/ACGold Foil
Sample
Can we get an estimate of the gold mass?
• Re-run spectra offline, screening out the wire
• Normalisation
• Calculations
• Validation of Technique
We can use this to relate the gold K X-ray scatters from the gold flecks to those from the foil:
We find MAu = 3.2 0.4 g
where MAu is the mass of gold in the sample, MAuF is the mass of gold foil included in the scan and the differential cross-sections are evaluated at the mean proton energies ES , EF in the sample and foil respectively.
K X-raysWe then get an excellent measure of the beam charge ratio between the foam sample run and the foil run:
QS
QF
NCS
ACS
NCF
ACF
NKS
NKF
QS
QF
MAu
MAuF
d
d
K ES
d
d
K EF
NLS
NLF
QS
QF
MAu
MAuF
d
d
L ES
d
d
L EF
e
S
Au l
e
S
AuTF
2
From this equation, using the gold mass MAu from the K X-ray measurements, we can find a characteristic attenuation length l, for the L X-rays in the gold.
And for the L X-rays:
We find D = 5.5 0.6 m
L X-rays
This can be related to the gold diameter D, using an expression derived by Dirac, for the mean chord length in any one direction in a sphere D = 3 l:
Can We Get an Estimate of the Gold Mass?
• Re-run spectra offline, screening out the wire
• Normalisation
• Calculations
• Validation of Technique
Experimental Validation Using a Complementary Technique
• An alternative technique to measure the mass of gold flecks in foam is XRF (rhodium target x-ray tube). However this can only produce L X-rays from gold, eliminating the technique as a candidate for measuring 5 m diameter flecks.
• So, to check our technique, we did measurements on a foam sample containing a similar mass of gold but having fleck diameters of ~0.5 m.
• In these sub-micron flecks L X-ray attenuation is of order 1%!
From XRF measurements the gold mass is:
MAu = 4.1 0.8 g
From our K X-ray measurements we find:
MAu = 3.8 0.7 g
From our L X-ray measurements we find:
MAu = 4.4 0.5 gAll three measurements are
compatible.
Another interesting feature of the measurement on the 0.5 m flecks is the Backscatter spectrum – which, at the high backscatter end, looks like one you would expect from a low density gold film.
Backscatter from Sub-Micron Gold Flecks
Energy, keV
Cou
nts
Estimate of mass from RBS Measurements
Because of energy loss in the polyimide sleeve the gold backscatter spectrum above the Oxygen edge is representative of only a fraction, an elliptic cylinder, of the foam volume.
From this though we can calculate the gold density. Assuming a uniform distribution of the gold we can then infer the mass by multiplying by the full foam cylinder volume. We find D = 5.0 0.5 gBUT: This probably over-estimates the mass due to slight deformations of the cylindrical shape.
Conclusions
We have successfully:
This work is continuing using an updated version of one of the CdTe detectors. This will eliminate much of the background electronic noise on the spectra of the older detector and improve the statistical accuracy.
• measured the mass of the gold flecks loaded within the foam,
• devised a novel technique for characterising gold flecks in foam,
• observed their spatial distribution in 2D maps,
• validated the technique.
• measured the average fleck size,
Any Questions?
Thank you for listening!