Trace element analysis of steel samples by particle induced X-ray emission

690

Nuclear

Trace element analysis X-ray emission D.K. Wilson, J.L. Duggan, Department

Instruments

and Methods

in Physics Research

of steel samples by particle

D.L. Weathers,

F.D. McDaniel

B56/57

(1991) 690-693 North-Holland

induced

and S. Matteson

of Physics, University of North Texas, Denton, TX 76203, USA

T. Thompson Texas Utilities Electric, Dallas, TX 75201, USA

I.L. Morgan IDM Inc., Austin,

TX 78758, USA

Absolute concentrations of trace elements in steel samples obtained from Texas Utilities power plants have been determined by computer analysis of PIXE spectra produced by 1-2 MeV protons. The X-ray spectra were measured with a windowless Si(Li) detector that is mounted on the 55 o beam line of the 3 MV NEC 9SDH-2 tandem accelerator at UNT. The peak areas were obtained by fitting a theoretical spectrum of known elemental concentration to the filtered data using the Guelph PIXE Analysis Code [J.A. Maxwell et al., Nucl. Instr. and Meth. B43 (1989) 2181. Absolute concentrations were derived from the peak areas by comparison to spectra of steel standards of known composition from NIST and secondary standards. Thin samples approximately 10 ug/cm* thick were also prepared by evaporation onto thin carbon foils. The purpose of the thin sample preparation was to determine which elements evaporated stoichiometrically from the thick samples. In addition, the PIXE results for percentage Fe were checked with the results of Rutherford backscattering. These two thin sample comparisons gave results that were in agreement to +5% which gives additional support to the value quoted from the thick target measurements. Concentrations as low as 54 ppm were observed and, for statistically significant peaks, fitting errors were under 5%. These measurements are being made in an attempt to understand the role of impurities in failed electrical power plant components,

1. Introduction Particle a mature

induced

X-Ray

technique

for

wide variety metallic

obtained

workers

mately samples

types

[2-41.

electrical

which

analysis

has been

The present

power

onto

elements

a

of thick

work details

a

in a variety

The samples

generation

Co. Thin

were also prepared

of

documented

concentrations

Electric

by evaporation

has become

identification

[l], and PIXE

element

Utilities

10 ug/cm2

to determine

(PIXE)

using thick target PIXE.

from

Texas

emission elemental

of various

of the trace

steel samples from

of samples

targets

by numerous study

power

were

Class VII

SRM

1270,

calibrate

approxi-

from some of the

the entire

structural

too large

Hence, region

slabs

of material

varied

in thickness

in., their other

geometry.

hand-polished

grit

were analyzed

were steeple

rotors

as

from

0 1991 - Elsevier Science Publishers

grinder

down

material

in order

to 600 beam

they were cleaned the grinding

B.V. (North-Holland)

a well-

They were then

in order

(about

to remove

surface

over the

1 mm X 2 mm).

using acetone

and methanol

material.

were prepared

pieces of the steel onto thin carbon the sample

to assure

and form a uniform

of the proton

The thin targets

by thick sample PIXE, and analyzed

belt

were being

were first flattened

known beam-target-detector

and three of those were also prepared

at the

but

dimensions

stoichiometri-

Finally,

chamber.

the sample

evaporated

to remove

of, or

and as such

were made.

than 1.25 in.. The samples

a coarse

obtained,

portions

in the PIXE

through

ASTM

Standard

and used to

in question

using

2. Sample preparation

0168-583X/91/$03.50

target

foils in order

dimension

All of the samples

slices

NBS

The samples

were either

component

always less than 0.125 no greater

as a thick

to be mounted

of interest

and were

sample,

results.

standard,

cross-sectional

unwanted

thin targets.

was prepared

the thick target the NBS

were

assemblies

thin carbon

tally.

Twelve samples

turbine

steel. One other

excepting

The

equipment

targets

generating

A470

by evaporating

small

foils. Small pieces of

were cut from the thick targets.

The samples

D.K. Wilson et al. / Trace element analysis of steelsamples were then etched for approximately one hour in a solution of distilled water and HCl (37%) in order to remove material, especially contaminants from the cutting process. The postetch sample weights, ranged from 24.0 to 54.6 mg, are atso shown in table 2. The resulting samples were evaporated onto 4 or 5 ultraclean [5] thin carbon foils (15 f 4 l_tg/crn’) placed at varying distances from the evaporation source to produce a range of target thicknesses for each sample and to allow for target breakage. Care was taken to evaporate all of the sample material both by evaporating for long times (between 60-90 min) and at very high temperatures (at or near the point of thermal fatigue of the tungsten boat), the purpose being to minimize the problem of non-stoichiometric evaporation of the complex sample; however, a white, powdery residue in the boat was noted in every case.

3. Experimental

Ion

To

Current

To

Lock

and

Integrator

Pulse

aeam

Processor

MCA

Fig. 1. Multipurpose target chamber for performing PIXE, and nuclear reaction analysis.

RBS,

ber for these measurements were 1-3 nA. Targets were mounted in an alu~num target holder at the center of the chamber. The holder was fitted with thick carbon shielding to minimize background Al X-rays. When analyzing the thick targets, the charge integration was measured directly from the target holder. For the thin target case it was measured from a Faraday cup behind the target. Two solid state surface barrier particle detectors for both forward angle (6D = 0.00544 sr) and backscattering (652 = 0.00073 sr) were located at 135” and 45 o with respect to the beam, both being used for RBS measurements of the thin target thicknesses simultaneously with the X-ray measurements. A Link Analytical Windowless Si(Li) detector was used for the thin target X-ray measurements and a similar Ge(Li) detector was used for the thick target measurements. For both thick and thin targets, the X-ray spectra had to be collected and the total beam current in-

technique

The PIXE measurements were made at the University of North Texas’ Ion Beam Modification and Analysis Laboratory [6]. The proton beam was obtained from a standard Cs sputtering negative ion source (SNICS) fitted with a TiH, cathode. The proton beam was accelerated by the 3 MV NEC 9SDH-2 tandem accelerator to I and 2 MeV for the thick and thin targets respectively. After the analyzing magnet the beam was transmitted down the 55” beamline to the target chamber. The target chamber was designed to be able to perform nuclear reaction analysis, Rutherford backscattering analysis, and X-ray measurement in UHV (see fig. 1). The base pressure of the chamber was - lo-* Torr. An X-Y pair of collimating slits defined the beam to be - 1 mm square. Typical beam currents into the cham-

Table 1 Trace element concentrations material specification

691

in weight percent found for thick steel targets, sample series 12 and 15, and ASTM A470 Class VII

Sample

Si

S

V

Cr

Mn

co

Ni

MO

12A 12D 12F 12L 12N 12P 12s 12u 15A 15B 15c 15D ASTM min A470 max

0.02 0.02 0.2 0.02 0.02 0.02 0.03 0.025 0.035 0.07 0.05 0.06

0.10 0.10 0.01 0.065 0.11 0.10 0.045 0.090 < 0.1 -=z0.1 c 0.1 < 0.1

0.1 0.09 0.1 0.08 0.11 0.09 0.08 0.08 0.10 0.12 0.17 0.15

2.09 2.05 2.13 2.13 2.10 2.00 2.08 2.06 2.30 2.10 2.25 2.26 3..25 2.00

0.17 0.15 0.20 0.15 0.16 0.16 0.15 0.24 0.22 < 0.2 0.19 0.31 0.20 0.60

0.4 0.3 < 0.2 0.1 0.2 rc 0.4 i 0.4 c 0.1 0.3 ( 0.4 c: 0.4 ci 0.4

3.19 3.25 3.30 3.30 3.12 3.25 3.30 3.50 3.44 3.90 3.45 3.41 3.25 4.00

0.05 < 0.2 0.25 0.10 < 0.3 <: 0.1 0.15 0.10 0.20 0.14 0.25 0.18 0.25 0.60

0.15

_

0.05

0.30

0.015

0.15

VIII. PIXE

692

D. K. Wilson et al. / Trace element analysis

Fe Ka

Fe L

10

I

4

I 0

Energy

(keV)

Fig. 2. PIXE spectra for sample 12N.

ofsteelsamples

total beam current, filter thicknesses, and a list of elements to search for in the spectra, and then fits the spectra using a modified Marquardt least-squares curve fit algorithm with piecewise linear background removal in order to find the number of counts under each X-ray peak. In the thick target mode a calibration parameter calculated from the spectra of a similar standard of known composition is input and GUPIX assigns an absolute concentration to all the elements present in the spectra. This was the procedure followed for the thick targets, the standard used being NBS SRM 1270 (Low Alloy Steel). When analyzing element concentrations for the thin targets, GUPIX was used only for curve fitting purposes to provide the total number of counts under each peak. The amount of each trace element present on the target in units of pg/cm* was then computed from the expression A(DT)F rJx =N,N,E’

tegrated. The X-ray spectra were collected using an IBM PC-based Nucleus PCA-II multichannel analyzer. A typical X-ray spectrum with major peaks labeled is shown in fig. 2. For the thin targets, RBS measurements were collected from either the forward or back-angle detectors.

4. Analysis and results The thick target data was analyzed using the Guelph PIXE Analysis software program (GUPIX) developed at the University of Guelph (Canada) and described in detail elsewhere [7]. The PCA-II program was used to strip the noise peak from the spectra, and conversion software developed in-house was used to format the spectra for input into GUPIX. This program, which runs on IBM PCs and compatibles, takes as its input the X-ray spectra, energy calibration parameters, experimental geometry, detector parameters, proton energy,

Table 2 Thin target pre-evaporation target thickness computed Sample

sample weights, pre- and post-etch, from backscattering measurements

Sample weight [mg] Pre-etch

Post-etch

12L

33.3

24.0

12N

45.0

40.0

12P

65.3

54.6

Target

(1)

where % = X-Ray production cross section for the element in units of cm*, A = counts under the peak, N, = target thickness in pg/cm*, N, = total number of incident projectiles, E = total efficiency of detector at peak energy centroid including geometric factor, DT= dead time correction factor, F = units correction factor. This expression was used to calculate the thickness, No, of the constituent elements on the target in pg/cm*, as well as the total target thickness, assuming the total target thickness to be thin enough to neglect any attenuation and/or matrix effects. The X-ray production cross sections were calculated using GUPIX, the results being based on literature values [8]. Charge integration was used to compute N,, and the detector efficiency was determined by a method detailed in a paper elsewhere

Fe and total target thicknesses

Target

thickness

computed

[ pg/cm’]

X-ray

from X-ray yields, and total

Diff. [W] RBS

Fe

Total

7.84 6.09 16.13 9.13 13.65 12.15

8.25 6.60 17.00 9.57 14.40 12.78

8.34 6.79 18.38 10.53 13.87 12.71

1.05 2.70 7.52 9.13 3.81 0.58

D. K. Wilson el al. / Trace element ana[vsis of steel samples in these

conference proceedings [9]. Total target thickness was calculated by summation of contributions from the major trace elements and the result was compared with the value obtained independently by RBS measurements [lo]. The results of the thin target analysis were not conclusive. In general, it appears that hot filament evaporation did not yield stoichiometric deposition on the carbon foils for many of the elements in this study. We did, however, use the thin target PIXE results to compare directly the iron concentrations in the samples to those measured simultaneously by Rutherford backscattering analysis. These results agreed to within 5%, thus giving support to the accuracy of the GUPIX program and our thick target PIXE results. These results are shown in table 2. We feel that the target analysis can be quite valuable if the evaporation is stoichiometric. We are in the process of preparing thin samples by laser ablation with a rare gas eximer laser. Evaporation by the technique is reported to be stoichiometric, and hence should be quite useful in preparing thin samples for the studies. The trace element concentrations found by the thick sample PIXE are shown in table 1. The ASTM specification for the material is also given, at the bottom of the table, for comparison. It can be seen that for most elements our measurements showed that the samples were within specification. The exceptions are as follows. The concentration found for Si in most cases was below specification. The concentration of Mn was always found to be at the extreme low end of the specified range; however, the uncertainty in the Mn concentration is large due to the Cr K, peak overlap. The MO concentration was also low, but it was computed from the L-shell peak, which is overlapped by the S K-shell peak, and hence the uncertainty in this case was also large.

693

5. Conclusion PIXE has been used to determine trace element concentrations in steel samples from power generating equipment. In most cases the concentrations found showed that the samples were within specification; however, the values obtained from Mn and MO, which did not agree with the specification, could not be relied upon due to peak overlaps. Results from simultaneous thin target PIXE and RBS give support to the GUPIX program and the PIXE results.

References PI S.A.E. Johansson, Nucl. Instr. and Meth. B22 (1987) 1. PI M.A. Respaldiza, G. Madurga and J.C. Soares, Nucl. Instr. and Meth. B22 (1987) 446. 131 J. Rlislnen, Nucl. Instr. and Meth. B22 (1987) 442. [41 J.W. Palmer, M.G. Hollander, P.S.Z. Rogers, C.J. Duffy and T.M. Benjamin, Nucl. Instr. and Meth. B22 (1987) 419. [51 D.L. Weathers et al., Proc. 15th World Conf. of the INTDS, Santa Fe, NM, 1990, Nucl. Instr. and Meth., to be published. WI J.L. Duggan, F.D. McDaniel, S. Matteson, D.E. Golden, J.M. Anthony, B. Gnade and J.A. Keenan, Nucl. Instr. and Meth. B40/41 (1989) 709. 171 J.A. Maxwell, J.L. Campbell and W.J. Teesdale, Nucl. Instr. and Meth. B43 (1989) 218. I81 M.H. Chen and B. Crasemann, Atom. Data Nucl. Data Tables 33 (1985) 217. [91 D.L. Weathers et al., these Proceedings (11th Int. Conf. on the Application of Accelerators in Research and Industry, Denton, TX, 1990) Nucl. Instr. and Meth. B56/57 (1991) 964. 1101 W.K. Chu, J.W. Mayer and M.A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978). 1111 J.M. Anthony, private communication.

VIII. PIXE