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Determination of Light Elements by Rutherford Backscattering Technique on Teflon Filters. Presented by Jaspinder P. Singh University of California Crocker Nuclear Laboratory Davis, California. Contents. What Is Rutherford Backscattering? RBS & PESA Analysis Spectrums

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determination of light elements by rutherford backscattering technique on teflon filters

Determination of Light Elements by Rutherford Backscattering Technique on Teflon Filters

Presented by

Jaspinder P. Singh

University of California

Crocker Nuclear Laboratory

Davis, California

contents
Contents
  • What Is Rutherford Backscattering?
  • RBS & PESA Analysis Spectrums
  • Peak Integration Methods
  • What Can the Integrated Counts Tell Us?
  • Relationships in RBS Analysis
  • Experimental Setup
  • Procedure for Blank Subtraction
  • Results
  • Conclusions & Works In Progress
what is rutherford backscattering
What Is Rutherford Backscattering?

Rutherford Backscattering (RBS) is an analytical technique in which a high energy beam of low mass ions are directed at a sample. Particles which scatter from the sample are collected in a detector placed at close to a 180 degree angle.

Because the amount of energy transferred to the sample atom depends on the ratio of masses between the ion and the sample atom, the chemical composition of the sample can be determined from measuring the energy of scattered ions.

what is rutherford backscattering4
What Is Rutherford Backscattering?

RBS can also be used to perform a depth profile of the sample if the incident ion doesn’t hit a sample atom at the surface, but one further in.

Proton Elastic Scattering (PESA) is an analytical technique based on the same principles as RBS, except with the detector placed for non-backscattered particles.

rbs analysis spectrums
RBS Analysis Spectrums

+

t

2000

X

θ1

θ

θ2

1000

Counts

Channel

Ω

Detector

peak integration methods
Peak Integration Methods

For clearly separated peaks, summing the counts in a range of channels can by done simply with a spreadsheet software.

However, overlapping peaks or high background noise can make summing unreliable. A different method of integration must then be used.

peak integration methods10
Peak Integration Methods

This Hydrogen peak barely escapes the background noise.

Thus, a summation of counts in its channel range would yield inaccurate data for its true area.

peak integration methods11
Peak Integration Methods

The Hydrogen peak can be interpreted without interference from the background using, in this case, a Gaussian curve.

What was once noisy and chaotic is translated into a smooth and distinct peak.

what can the integrated counts tell us
What Can The Integrated Counts Tell Us?

In thin films, the integrated counts in the peak comes from scattering from the ith element, and reveal the amount of atoms of i in the film.

Because the amount of energy transferred to the sample atom depends on the ratio of masses between the ion and the sample atom, the chemical composition of the sample can be determined from measuring the energy of scattered ions.

relationships in rbs analysis
Relationships in RBS Analysis

From the detector solid angle Ω, the integrated peak count Ai for Q incident ions, and the cross section σi(E,θ), the areal density (Nt)i can be determined.

relationships in rbs analysis14
Relationships in RBS Analysis

The Rutherford cross-section σR(E,θ) is calculable as well, with the knowledge of the atomic numbers of the incident and target ions (Z1 and Z2, respectively).

relationships in rbs analysis15
Relationships in RBS Analysis

The integrated counts can be used in conjunction with the cross-section and other known values to determine, for example, the amount of protons in the run or the thickness of the film.

relationships in rbs analysis16
Relationships in RBS Analysis

The ratio between the integrated counts of two elements in a multi-element film can reveal the stoichiometric relationship of the elements. The constants cancel out.

experimental setup
Experimental Setup

For the determination of light elements in Teflon filters, we used:

  • A 4.5 MeV proton beam from 76-inch Crocker Nuclear Laboratory (CNL) cyclotron at the University of California, Davis.
  • A scattering chamber from ORTEC series 2800.
  • A surface barrier detector (Si) placed at 150 degree to the beam axis and 170mm from the center of the scattering chamber.
  • A detector thickness that will stop the 4.5 MeV proton beam with a resolution of 25 KeV.
procedure for blank subtraction
Procedure for Blank Subtraction

Consider an unloaded Teflon Filter (CF2).

procedure for blank subtraction20
Procedure for Blank Subtraction

C12(p,p)C12

F19(p,p)F19

There is a distinct carbon peak and three fluorine energy states

procedure for blank subtraction21
Procedure for Blank Subtraction

C12(p,p)C12

F19(p,p)F19

C12(p,p)C12

F19(p,p)F19

That Teflon filter is then loaded with elemental carbon (graphite) in a re-suspension chamber.

procedure for blank subtraction22
Procedure for Blank Subtraction

C12(p,p)C12

Af,l

F19(p,p)F19

Af,ul

C12(p,p)C12

F19(p,p)F19

Although both filters are the same, there could be differences in the time length of an experimental run. Therefore, the loaded filter is normalized to the unloaded filter by the amplitude of the fluorine ground state.

procedure for blank subtraction23
Procedure for Blank Subtraction

C12(p,p)C12

F19(p,p)F19

There is a little more counts in the loaded filter near the fluorine ground state. This is due to the presence of oxygen.

procedure for blank subtraction24
Procedure for Blank Subtraction

C12(p,p)C12

O16(p,p)O16

By taking the difference between the loaded Teflon and the unloaded Teflon, we are left with distinct carbon and oxygen peaks. Using the methods previously described, the areas of these peaks can yield the amount of the element loaded.

results
Results

Direct Method

Another example of a Teflon filter before and after loaded elemental carbon (graphite) using a re-suspension chamber. The difference in weight and the area of the deposit gave an areal density of 74.84 µg/cm². The value extracted from RBS was 69±6 µg/cm².

slide26

Simulated Method

Mesa Verda spectrum with the calculation is made using SIMNRA for a Teflon sample and normalized to the fluorine peak.

slide27

Indirect Method

Spectrum of a Mesa Verde Teflon filter compared with the

spectrum of the blank Teflon filter. To extract the oxygen is straightforward.

Fluorine has two excited states that can be excited with 4.5 MeV protons.

slide28

The oxygen implied from IMPROVE measurement is calculated from the mass concentration of sulfur and soil elements (Si;Al;Ca;Ti;Fe) in the oxides form. SOIL=2.2*[Al]+2.49*[Si]+1.63*[Ca]+2.42*[Fe]+1.94[Ti]

slide29

Tallgrass Prairie National Preserve, Kansas

Everglades National Park, Florida (Saturday)

Lava Beds National Park, California

Everglades National Park, Florida (Monday)

conclusions
Conclusions
  • Proton Rutherford Back Scattering analysis has been used to measure carbon and oxygen on Teflon filters.
  • We cannot measure nitrogen at this time because we do not have a suitable standard.
  • The method shows promise to extend the Crocker Nuclear laboratory’s analytical techniques.
  • Further development is needed and will be pursued in the coming months.