PB 161592

^ecknical vtote 91

DETERMINATION OF THE

K FLUORESCENCE YIELD OF ARGON BY

PROPORTIONAL-COUNTER SPECTROMETRY

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards; the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devices,

and structures; advisory services to government agencies on scientific and technical problems; in- vention and development of devices to serve special needs of the Government; and the development

of standard practices, codes, and specifications. The work includes basic and applied research,

development, engineering, instrumentation, testing, evaluation, calibration services, and various

consultation and information services. Research projects are also performed for other government

agencies when the work relates to and supplements the basic program of the Bureau or when the

Bureau's unique competence is required. The scope of activities is suggested by the listing of

divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's work take the form of either actual equipment and devices or published papers. These papers appear either in the Bureau's own series of publications or in the journals

of professional and scientific societies. The Bureau itself publishes three periodicals available from

the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers; the Technical News Bulletin presents summary

and preliminary reports on work in progress; and Basic Radio Propagation Predictions provides

data for determining the best frequencies to use for radio communications throughout the world.

There are also five series of nonperiodical publications: Monographs, Applied Mathematics Series. Handbooks, Miscellaneous Publications, and Technical Notes. Information on the Bureau's publications can be found in NBS Circular 460, Publications of the

National Bureau of Standards ($1.25) and its Supplement ($1.50), available from the Superintendent

of Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS

technical Ntote

91

MAY 1961

DETERMINATION OF THE

K FLUORESCENCE YIELD OF ARGON BY

PROPORTIONAL-COUNTER SPECTROMETRY

Claire Godeau

The work described in this paper was carried out at the National Bureau of Standards, Washington, D. C, U.S.A.

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $0.50

Determination of the K Fluorescence Yield of Argon

by Proportional-Counter Spectrometry

Claire Godeau

A proportional counter spectrometer designed for investigation of low energy particles has been used for the determination of

the K fluorescence yield of argon.

X-rays following K capture in iron-55 were used as a source for excitation of the

argon atom and the spectrum was measured

„

The K fluorescence yield o^. was deduced from

the comparison of the two peaks shown in the

spectrum and corrections were made, taking

into account photoelectric absorption in outer

shells and reabsorption of secondary X-rays in

the counter.

The fluorescence yield co^ of argon was found to be 0.087 ± 0.007.

1 o Introduction

A proportional-counter spectrometer has been designed which can

be used for the investigation of low-energy particles. It has been

used for the determination of the K fluorescence yield of argon.

Different authors [1-6] have shown that a proportional-counter

device is suitable for that purpose.

2. The Proportional-Counter Description

The counter, a sketch of which is shown in figure 1, is a brass

cylinder with an inside aluminum lining. It is 12" long and 3" in

diameter. The central electrode made of tungsten wire (2 mils in diameter) is held by Kovar seals, providing the electrical insulation

Present name and address: Mme. Claire Stievenart, 53 Avenue George

Bergmann, Bruxelles 5, Belgium. The work described in this paper was carried out at the National Bureau of Standards, Washington, D. C.

,

UquoAo
from the external electrode. The counter has an o 5-mil mylar window

which can be easily removed. The vacuum seal at the window is provided

by an O-ring held in position by means of a brass ring (figure 1-a).

The filling gas is a mixture of argon and methane (90$ argon, 10%

methane) o The counter must be operated at pressures such that the

energy of the electron is completely absorbed in the sensitive volume

of the counter. For the measurement of the K fluorescence yield of

argon, work was carried out at pressures lower than atmospheric and

the cap shown in figure 1-b was used in order to have the same pressure

on both sides of the mylar window.

3. Electronic Equipment

This is shown in block diagram in figure 2<> The pulse due to the

event in the sensitive volume of the counter is amplified in a non- overloading linear amplifier and then presented to a single channel

pulse-height analyzer. The number of events per voltage interval is

determined by a scaler. In order to avoid the high impedance at the

counter output, a cathode follower is mounted directly on the counter,,

4o Calibration of the Spectrometer

Energy calibration of the counter was performed by measuring the

pulse height induced by monoenergetic incident radiation of known

energy A good source of monoenergetic radiation is provided by the

X-rays following electron capture in certain nuclides (iron-55> zinc-65

and barium-133). The process can be described as follows: After K

absorption the iron-55 nucleus becomes a manganese-55 nucleus and an Xray characteristic of manganese is emitted. This X-ray is absorbed in

the filling gas of the counter by photoelectric process. A K electron

of argon is thus knocked out of the argon atom and emitted with an energy equal to the incident energy minus the binding energy of a K

electron of argon. There is now a vacancy in the K shell of the argon

atom which is filled by an electron from an outer shell giving rise to

the emission of an X-ray characteristic of argon. This X-ray can either escape, or be absorbed in the counter according to the Auger effect. If it escapes, the energy spent in the counter is the energy of

the K photoelectron only, but in case of Auger absorption the total

incident energy is spent in the counter. One can thus expect two peaks

in the spectrum obtained with iron-55 source (figure 3), the low energy

peak being due to K photoelectrons only and the high energy peak being

due to K photoelectrons plus Auger electrons..

In the case of iron-55 the X-ray characteristic of manganese has

an energy of 5.9 kev e The binding energy of the argon K electron is

3<>2 kev so that the low energy peak in the spectrum represents

5.9 - 3.2 = 2.7 kev while the high energy peak corresponds to 5.9 kev.

Both these energies are used as calibration points

„
A similar process for zinc-65 gives two more points for the calibration curve. The situation with barium-133 is somewhat different because of the very low photoelectric cross section for cesium X-rays in

argon. The barium source was accordingly placed inside the counter and

the barium Auger electrons were detected and used, to obtain a further

point on the calibration curve.

These points have been used to establish the calibration curve of

the proportional-counter spectrometer between and 26 kev By taking

into account the amplifier gain and the base-line vernier calibration

of the pulse-height analyzer, the pulse height in millivolts at the output of the counter can be plotted against the energy.

Figure 4 shows the curve obtained. One can draw a straight line

for the low energy end, but the 26-kev point is somewhat off this line.

The calibration curve is however linear up to and slightly beyond the

characteristic X-ray energy of iron-55 which was used in the determination of the fluorescence yield.

5. Resolution of the Spectrometer

The resolution was measured for the different calibration points by

measuring the width of the peaks at half maximum. The values obtained

for 2.7 kev and 5.9 kev are respectively 21% and 18%. They are similar

to those obtained by Bertolini, Bisi and Zappa [5] for the same type of

counter

As previously described, when external radiation of a certain range

of energy falls upon an argon atom, a K-shell photoelectron is emitted

followed by an Auger electron or an X-ray characteristic of argon. In

nearly all cases the X-ray will escape o We thus obtain two peaks

(figure 3)> a high energy peak, corresponding to absorption of the argon

K-shell photoelectron and Auger electron, and a low energy peak known as the escape peak, corresponding to absorption of the argon K-shell electron and escape of the argon X-raye

6 Fluorescence Yield

The fluorescence yield cc^ is defined as the probability that a K-shell vacancy results in X-ray emission

7. Determination of the Fluorescence Yield

Considering the spectra obtained, one can very easily deduce the

fluorescence yield from the intensities of the two peaks, assuming that

the process is the one described above (we will see later that corrections must be applied). If I is the intensity of the low-energy

peak, and I is the intensity of the high-energy peak, the fluorescence
yield av is determined by Ij/(l]_ + ^2) > s i- nce *i i- s the number of K

photoelectrons followed by non-measured X-radiations, and Ip is the

number of K photoelectrons followed by Auger electrons.

8. Corrections

Two major corrections have to be made to the data to obtain the

fluorescence yield due to the fact that:

1) The incident electromagnetic radiation can produce

photoelectrons from the L and M shells of the argon

atom. These photoelectrons cannot be resolved from

the high-energy peak;

2) The secondary X-rays can be reabsorbed in the counter

by a photoelectric process in another atom of argon.

In that case, the total incident energy will again be

absorbed in the counter but not by the Auger effect,

and the intensity of the high-energy peak will be un- duly high.

8.1. First Correction

According to Bertolini, Bisi and Zappa [5], the percentage of

atoms ionized in the outer shells can be calculated by taking into ac- count the photoelectric absorption coefficient of the various shells,

^k* Tl' "Si* ^ne rel a "tion between the real fluorescence yield ov and

the fluorescence yield 0^>\t modified by absorption in outer shells, is

°^ = (1 + ^°

^~r (1) Tk 1 +-L coV

Tk

k

the contribution of the M shell being negligible. According to [7] the

ratio — for argon is 0.071.
8.2. Second Correction

Calculation' of the reabsorption of a secondary X-ray in the

volume of the counter is very difficult, involving a double volume

integration, but this effect can be evaluated in the following manner:

At pressure zero, it is normal to assume that there is no reabsorption.

If one can plot the ratio I^Ali + ^

against the pressure, the value

extrapolated to zero pressure gives the pressure correction for the

fluorescence yield <^>\»

9. Results

The proportional counter described above was used at different

pressures. The pressure was made the same on both sides of the mylar

window in order to avoid breaking the window (figure 1-b). The

iron-55 source was mounted on the window. The gas (90$ argon - 10$

methane) circulated from the tank through the cap placed on top of the

window before passing through the counter itself. A vacuum pump con- nected on the other side of the counter allowed the pressure, which was

read on a mercury manometer, to be lowered. The flow rate was regulated by a sylphon valve and checked with a bubbler. The iron-55

spectrum was measured at pressures ranging from 0.53 atmosphere to 1.03

atmosphere.

The ratio I,/(l

1 + O was calculated for each spectrum, I, being

the numerical integration of the low-energy peak and Ip the one for

the high-energy peak, the two peaks being resolved after subtraction

of the background (figure 3). I 1/( I 1

+ 1 2)

was Plotted against the

pressure (figure 5).

Extrapolation by least squares to zero pressure was used to draw

the most probable straight line passing through these experimental

points. The extrapolated value of the fluorescence yield so obtained,

taking into account the correction for reabsorption of secondary X-rays,

is

co' k = 0.082 ± 0.006
The error assigned is the standard deviation of the intercept The

correction for absorption of the primary radiation in the outer shells

of the argon atom has to be applied by using formula (l), and the final

result obtained for the K fluorescence yield of argon is

^k = 0.087 ± 0.007

10. Comparison with Existing Values

<°k Method of Measurement Reference

0.07 Cloud chamber 8

0oH9 Cloud chamber 9

0.077 Cloud chamber 10

0.06 Proportional counter spectrometer 1

0.123 Width of X-ray absorption line 11

o 083 Theoretical value 12

0.085 Proportional counter spectrometer 5

0.087 Proportional counter spectrometer Present measurement

11. Background Measurement

A complete background spectrum was obtained by making a run of

measurements without a source and it was used to help interpolate the

background in the actual spectra below the peaks. Further checks were made by counting backgrounds beyond the peaks at the end of eaGh spectrum. It was found that errors in background measurements cannot affect the final result by more than ±5%»
12. Influence of the Presence of Methane in the Argon

Pure argon cannot be used as a filling gas in the proportional

counter because of its having metas table states. One has to add

methane and one may wonder how its presence will affect the peaks intensities,,

The- absorption of primary radiation in methane is much lower than

the absorption in argon. The amount of methane in the filling gas being only 10$, one can calculate that the absorption of primary radiation in methane represents only 0.4$ of the total absorption.

When the primary radiation is absorbed in methane, the energy dissipated in the counter is either the total incident energy, or that

energy minus the binding energy of a K electron, in a similar way with

argon. The binding energy of a K electron in methane is so small, however, that the two processes cannot be resolved and both appear in the

high energy peak of the spectrum. Taking into account the low probability of the absorption in methane and the fact that only the high

energy peak, which has the higher intensity, is affected, it is clear

that the error on the fluorescence yield can be neglected in comparison

with the final error and it can be calculated to be of the order of

0.4-$.

Acknowl edgment

s

The author is grateful to Dr. H. H. Seliger and Dr„ W„ B„ Mann for

helpful discussions and to Mr. G. W. Randel for help in the construction

of the counters.
13. References

[l] S. C. Curran, J. Angus, and A. L. Cockroft, Phil. Mag. 4-0, 36

(1949).

[2] S. C. Curran, A. L. Cockroft, and J. Angus, Phil. Mag. 4-0, 929

(1949).

[3] D. H. W. Kirkwood, B. Pontecorvo, and G. C. Hanna, Phys. Rev. 74,

497 (1948).

[4] G. C. Hanna, D. H. W. Kirkwood, and B. Pontecorvo, Phys. Rev. 75,

985 (1949).

[5] G. Bertolini, A. Bisi and L. Zappa, Nuovo Cimento 10, 1424 (1953).

[6] J. P. Mize and D. J. Zaffarano, USAEC Report ISC-347 (1953).

[7] H. A. Bethe, Handbuch der Physik 24/l» 479 (1933).

[8] P. Auger, Ann. Phys. 6, 183 (1926).

[9] G. L. Locher, Phys. Rev. 40, 484 (1932).

[10] L. H. Martin, J. C. Bower, and T. H. Laby, Proc. Roy. Soc.

(London) A148, 40 (1935).

[11] D. L. Dexter and W. W. Beeman, Phys. Rev. 81, 456 (l95l).

[12] E. H. S. Burhop, The Auger Effect and Other Radiationless Transitions, Cambridge University Press, p. 11 (1952).

- USC0MM—DC

4?

§

\^y
Treasure 760m/nhlQ

ss Iron Spectrum

at f950 fo/fis.

Background

too too 300 4oo Sex?

3else //ne rer/?/er

F,$3
Energ y fa keV

Spectrometer Ca/*2>rerh'on Cortfe

F>3

*
/ooo

Pressure in mm Hq

F<3

J. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

1NAL BURfcAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

lie scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and Roulder,

nlorado, is suggested in the following listing of the divisions and sections engaged in /technical work. Tn general, »ch section carries out specialized research, development, and engineering in the field indicated by its title. A brief

!, . ription of the activities, and of the resultant publications, appears on the inside of the front cover. WASHINGTON, D.C.

lectricity. Resistance and Reactance.

Melectrics. letrology.

letrology.

Electrochemistry. Electrical Instruments. Magnetic Measurements.

Photometry and Colorimetry. Refiactometry. Photographic Research. Length. Engineering

Mass and Scale. Volnmetry and Densimetry.

it. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Statistical Physics. tadiat ion Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment.

Jul Iconic Instrumentation. Neutron Physics.

ilytical and Inorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistry. Analytical

Chemistry. Inorganic Chemistry.

Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Rheology. Combustion

Controls. rganic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications,

structure. Plastics. Dental Research. Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy,

lineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals,

lysical Properties. Constitution and Microstructure. luilding Research. Structural Engineering. Fire Research. Mechanical Systems.

«"les and Safety Standards. Heat Transfer. Inorganic Building Materials. Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. Components and Techniques. Digital Circuitry. Digital Systems. Analog Systems*

Lpplications Engineering.

i tomic Physics. Spectroscopy. Radiomelry. Solid State Physics. Electron Physics. Atomic Physics.

Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation. Mechanical Instru- ments. Basic Instrumentation. Thermochemistry. Surface Chemistry. Organic Chemistry.

Polymer

Corrosion. Metal Physics'

Crystal Growth.

Organic Building Materials,

Physical Chemistry

Molecular Kinetics. Mass Spectrometry.

Office of Weights and Measures.

Molecular Structure and Radiation Chemistry.

Molecular Spectroscopy.

BOULDER, COLO.

-ryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Gas Liquefaction.

Ionosphere Research and Propagation. Low Frequency and Very Low Frequency Research. Ionosphere

lesearch. Prediction Services. Sun-Earth Relationships. Field Engineering. Radio Warning Services. Radio Propagation Engineering. Data Reduction Instrumentation. Radio Noise. Tioposphcric Measurements.

Tropospheric Analysis. Propagation-Terrain Effects. Radio-Meteorology. Lower Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. Radio and Microwave

Materials. Atomic Frequency and Time Interval Standards. Electronic Calibration Center. Millimeter-Wave

Research. Miciowave Circuit Standards. Radio Systems. High Frequency and Very High Frequency Research. Modulation Research. Antenna Research. Navigation Systems. Space Telecommunications.

Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. Ionosphere and Exosphere

Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.