Ion Beam Analysis of Gold Flecks in a Foam Lattice F E Gauntlett, A S Clough

transcript

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.

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

Energy, keV

Gold L X-rays

Copper K X-rays

Energy, keV

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

Gold Continuum

Energy, keV

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?

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

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

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

New SpectraC

Energy, keV

Au L X-rays

Energy, keV60 70 80Energy, keV0

10Au K X-rays

Can We Get an Estimate of the Gold Mass?

• Normalisation

• Calculations

Normalisation with Protons Backscattered from Carbon

NC/ACGold Foil

Sample

Can we get an estimate of the gold mass?

• Normalisation

• Calculations

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:

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?

• Normalisation

• Calculations

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

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!

Ion Beam Analysis of Gold Flecks in a Foam Lattice F E Gauntlett, A S Clough

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