IMPLOVEiENTS IN METHODS OF EXTRACTION, PURIFICATION, A1D 1ASUREIENT OF EADIO

GENIC ARGON IN MINEAiLS

by

LAWRENCE STRICKLLND

S.B., Massachusetts Institute of Technology

(1952)

SUBMITTED IN PARTIAL FULFILLIENT OF THE REQUIREIENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

June, 1956

Signature of Author...... *0.-.. *a *.. , ...... Department 4f Geology and Geophysics / X, Of .I Septe;nber p3, 1955

Certified Thepc SupgrvAsor

Accepted by.\...... 4...... Chairman, Departmental Committee on Gradua Students -Ming==

A CINOWLEDGEMvENTS

The author is indebted to the many people who helped in the completion of this research. He wishes to thank Dr. Leonard Herzog, who was always available for consultation when problems arose involving mass spectrometry and who was an invaluable help in the early stages of this re- search. Professor Patrick Hurley, who suggested the author undertake this research and who was always willing to take time from his busy schedule to help. He was a source of inspiration whenever forward progress was slow. The author will remember his association with Professor Hurley for many years. Mr. Milo Backus, whose companion- ship made the many hours spent on this research seem short. The typist, Joan Whitehouse, for her untiring efforts to complete this manuscriot in a tight schedule. His wife, Shirley, without her unlimited confidence in the author, and unselfish devotion, the successful completion of this research would have been impossible. The entire staff of the Geology Department. The research presented in this thesis was a part of a program supported by the Atomic Energy Commission urAer Contract AT(30-1)-1381. ABSTRACT

Title: Improvements in methods of extraction, purification, and measurement of radio genic argon in minerals. Author: Lawrence Strickland Submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Massachu- setts Institute of Technology Age measurements by the A 40 /K4 0 method have shown promising results in recent tests. It is important that the possibility of small errors in analysis be investigated and that the techniques of analysis be simplified and shortened. In this investigation new instruments and facilities were constructed and tested to these ends. It was planned that isotope dilution analysis would be used to monitor experiments leading toward a possible re- liable volumetric method of analysis. For this purpose a mass spectrometer was constructed, after Nier's design, with a 60 magnet sector and 6 inch radius, and with changes made in the method of collection and measurement. The method of measuring the isotopes of argon was a dynamic one, in order to allow most of the sample to be used during an analysis. Molecular flow conditions exist throughout the entire gas-flow sheet. The isotope ratios measured at time intervals were then extrapolated to the time the sample started to flow into the ionization chamber. Experimentation showed that argon could be lost if the sample was absorbed on charcoal at too low temoeratures for too long a time. It was also found that quantities of gas containing argon c8uld be purified by selective adsorption on charcoal at -78 c. The mass spectrometric procedures were checked for discriminatio]0and reproducibility by measuring the atmos- pheric argon ratio and the radiogenic argon content of a sample of llpidolite of known age. Results of these tests were as follows: A T1OSPHERIC 36 ARGON 40 / Fractionation and (measured) Nier (1950) Discrimination 310 2 96+l 1.047 311 1.050 308 1.042 or approximately 2 percent discrimination per mass unit. This value is different for each spectrometer. The value obtained is reasonable. LEPIDOLITE SAIIPLE A4 0 / gm sample (x103 cm3 ) age (m. y. ) .79 + .08 This work Aldrich (1954) .74 + .03 1710 + 90 1610 .73 + .03

The volumetric analysis apparatus was checked by analyzing air for its argon content with .993 percent, .990 percent, and .992 percent the values obtained. This is to be comoared with a value of .993 percent obtained by Paneth. 'BLE OF CONTENTS

.page

Acknowledgements...... *00 ...... * ...... 00

Abstract...... * ** o.*eoo .0 000 ...... 0 i11

Section I. Introduction...... 0.0...... 00.0. 1 Methods of Determining Geologic Age Lead Strontium-Rubidium Argon-Potassium Comparison of age Methods Research Problems Section II. Mass Spectrometer...... 7 Introduction Isotope Analysis of Argon Theory of Mass Spectrometer Refocusing of Divergent Beams Causes of Ion Beam Spread Resolution Physical Arrangement of the Equipment Section III. Vacuum Techniques and Gas Flow Conditions in the Mass Spectrometer...... 45 Introduction Gas Flow Through the Mass Spectrometer Cold Traps Background Mass Spectre Gas Flow into the Mass Soectrometer Section IV. Production of Positive Ions...... 59 Introduction Methods and Workmanship The Orthodox Source Mass Discrimination of Ion Source Emission Regulator Sensitivity Stability Section V. Collection and Measurement of Ion Beams..... 70 Collector Design Preamxolifier D-C Current Amplifier Measurement of Ion Beams Treatment of Data Section VI. Isotope Dilution Techniques...... 77 Tracer Introduction System Calibration of the Tracer Possible Errors in Tracer Calibration Isotope Dilution Measurements Possible Errors in the Determination of Radiogenic Argon Section VII. Volumetric Analysis of Argon...... 85 Introduction Separation Procedure (Introductory Remarks) Description of Equipment Calibration of Volumes Problems to be Solved Loss of Argon Extraction of Small Quantities of Argon from Min- erals Atmospheric Argon Contamination Hydrogen Removal Operation of the Barium Furnace Gas Circulation System Results of Volumetric Analyses Section VIII. Standardized Procedures...... 104 Volumetric-Analysis Isotope Dilution Analysis Section IX.Measurement of Age by the Potassium-Argon Method...... *. 110 Section X. Recommendations for Future Research...... 115 Appendix I. Use of Radio Frequency Induction Heater Appendix II. Condensed Procedure Sheet Biographical Sketch I __:i

LIST OF ILLUSTRATIONS Figure 1. Refocusing properties of magnet sector. 11. 2. Effect on refocusing by shifting magnet .1 inch upward from correct position. 13. 3. Effect on refocusing by shifting magnet .2 inch down and .1 inch towards source from correct oosition. 14. 4. Bean spread due to various aberrations. 17. 5. Gas inlet system. 19. 6. Magnet poles. 21. 7. Right side view of mass spectrometer. 23. 8. Left side view of mass spectrometer. 24. 9. Schematic diagram of high voltage supply . 25.

10. Front panel view of high voltage supply. 2. 11. Bottom view of high voltage supply. 27. 12. Rear view of high voltage supply. 28. 13. Schematic diagram of ion current amplifier. 29. 14. Front panel view of ion current amolifier. 30. 15. Bottom view of ion current amplifier. 31. 16. Rear view of ion current amplifier. 32. 17. Schematic diagram of magnet current supply. 33. 18. Schematic diagram of balancing panel. 34. 19. Front panel view of balancing panel. 35. 20. Rear view of balancing panel. 36. 21. Schematic diagram of regulated D.C. power supply. 37. 22. Front panel view of regulated D.C. power supply. 38. Figure 23. Bottom view of regulated D.C. power supply. 319. 24. Rear view of regulated D.C. power supply. 40, 25. Schematic diagram of emission regulator. 41. 26. Front panel view of emission regulator. 42. 27. Rear view of emission regulator. 43. 28. Diagram of mass spectrometer tube. 44. 29. Schematic diagram of mass spectrometer with possible appropriate pressures. 47, 30. Residual spectra using solid carbon dioxide as coolant. 50. 31. Residual spectra using liquid nitrogen as coolant. 51. 32. Increase of background spectra with time. 52. 33. Variation in 4 ratio with time. 56. 40 34. Variation in 40 ratio with time. 57. 35. Schematic diagram of ion source. 61. 36. Schematic diagram of electron gun. 66. 37. Schematic diagram of ion gun. 67. 38. Peak height vs. electron accelerating voltage. 69. 39. Design of collector. 72. 40. Characteristics of CK5886 tube. 75. 41. Typical recorded ion beams. 76. 42. Percent error in Qh for 1 percent error in Rmn. 83. 43. Furnace for extraction of gases. 87. Figure 44. Gas separation system. 89. 45. Adsorption of argon on charcoal at liquid nitrogen temperature. 93. 46. Clean up of sma 1 quantities o gas in presence of 1.20 x 10-3 and cm argon. 100. 40 47. Decay scheme of K . 112. LIST OF TABLES

Table Ak Comparison of Argon ratios (mass discrimination) 64. Table B Calibration of spike 79. Table C Percent error in volume of pure tracer determined per E percent error in ratio or quantity 79. Table D Percent error in the determination of radio- genic argon for a given percent error in ratio and quantity 81. Table E Results of volumetric analysis 103. Table F Branching ratio 111. Table G Comparison of ages 113. Section I

INTRODUCTION

It was the purpose of this research to construct and calibrate

equipment and techniques for the extraction and quantitative separation

and isotopic measurement of the argon from potassium bearing minerals,

the ultimate objective being to contribute data toward the establishment

of the potassium-argon method of age determination.

It was also the purpose of this research to determine if it is

possible to make contamination-free volumetric analysis of argon in min-

erals. The section that follows will acquaint the reader with the recent

developments which prompted the present research, and will present an

introductory statement of the research problems.

Methods of Determining the Geological Age of Rocks and Minerals

Natural radioactivities have provided a means of studying absolute

time in earth history. The more important of these are the breakdowns of 28 25 232 87 40 U2 3 8 , U , Th23, Rb and K

Several excellent reviews of the methods of age determination have appeared in recent years. A detailed account of the historical develop- ment of the potassium-argon decay has been published in a paper by Birch

(1951), while Faul (1954) has an excellent review of all methods of age determination. Kohman (1954) presents the most recent developments in all fields. In order to present this research in its proper prospective a brief review of the development of the methods of age determination is presented.

Lead

The fact that the three heavy radioactive elements U2 38 , U 2 3 5 and Th 2 3 2 produce the three lead isotopes Pb2 0 6 , Pb207 and Pb2 0 8 has led to the

possibility of determining the age of uranium and thorium bearing minerals

be measuring their Pb/U+Th ratios.

Early lead age measurements were made by determining, chemically, the

total lead and uranium plus thorium content of the mineral. Although

several hundred age determinations were made by the chemical lead method,

progress was slow until Nier's (Nier, 1939) work appeared in 1939. It was not until the development of a simple mass spectrometer (Nier, 1940) for isotope abundance measurements that analysis of the lead isotope con- tent of minerals became the practice. It was then possible to make correc- tions for primary lead contamination and to study the effect of losses of parent and daughter elements.

The decay schemes involved allow computation of three ages for each mineral. Since it is evident that the three computed ages seldom agree, loss of parent or daughter elements may be quite common. Enough discrep- ancies in lead age determinations, when compared with other methods of age determination, have been indicated in recent years to warrant a concen- trated program of investigation (Kulp 1954),(Kohman 1954). However, when the computed ages agree the "age" may be considered as reasonably accurate and is being used as a common base point.

Strontium - Rubidium

The use of Rb8 7 decay as a method of age determination was first suggested by Goldschmidt (1937) but, after the initial work by Hahn,

Strassman and Walling (1937) little was done until the late 1940's and early 1950's. Then work by Mattauch (1947), Ahrens (1949), Ahrens and

Gorfinkle (19501 Herzog (1952), Aldrich, Doak and Davis (1952), and others increased the available information on the use of the Rb87 decay. The requirement that there should be a high Rb8 7 /Sr8 7 ratio in any

mineral used made the method applicable, initially, only to such minerals

as lepidolites. The increased use of isotope dilution techniques has

made the determination of small quantities of Rb 87 and Sr8 7 very accurate.

Other minerals such as biotites and potassium feldspars can now be utilized.

Rb/Sr ages at the present time seem to be as reproducible as lead ages,

although, some doubt exists as to the correct half-life to be used for the

Rb87 decay. The method seems to be consistent within itself and the ages

derived agree with lead ages for the same region if a half-life of about +10 4.9x10 is correct for Rb8 7

Argon-Potassium

Evans (1940), and independently Thompson and Rowlands (1943) first

suggested the use of the potassium-argon decay as a possible method of age

determination. Birch (1951) in a review of previous literature stated:- "Since 1930 over one hundred papers or letters have been published concerning the radioactivity of potassium-40."

In the same review he says,

"Few determinations of a es have as yet been made by use of the radioactivity of K 0, but the existence of reason- ably reliable constants should encourage efforts to obtain ages of potassium minerals."

The large terrestrial abundance and ubiquitous nature of potassium,

and intermediate half-life value of potassium-40, has made age determina-

tion by the argon-potassium method appear very promising. Research must

be undertaken in two separate areas if the potassium-argon decay is to

become reliable as a method of age determination. One, it must be estab-

lished whether the contamination reported by other workers is primary argon

or contamination argon introduced during an analysis; and two, the decay constant must be more firmly established. 4.

The problems associated with these areas of investigation have made

it difficult to obtain reliable analytical results. The first problem is

the difficulty of quantitatively extracting small volumes of argon from

the mineral. The second is associated with the necessity of determining

the isotopic composition of the argon. A third problem not connected with

analytical procedures but of prime importance in determining the age of a

mineral, is the doubt that exists regarding the correct value of the branch-

ing ratio.

The first problem is discussed more completely in section VII. However,

the author now believes that by direct fusion or with the use of fluxes

quantitative extraction of argon is possible.

The second problem is not so easily handled. The literature gives

no information regarding the quantity and isotopic composition of

"contamination" argon. It is usually reported as "varying from less than

1% to 10%". If it is not possible to obtain contamination free argon

and if it is true that all minerals contain some "common" argon (Kohman,

1954), then corrections must be made and mass spectrometric measurements

are necessary. Since few laboratories are equipped with mass spectrometers which can be devoted exclusively to the isotope analysis of argon, the

application of methods of age determination to the solution of geological

problems will be considerably curtailed.

The third problem, that of establishing a reliable branching ratio, received most of the early research effort. The branching ratio (see

Table F) may be determined in three ways. The first is by observation of the x-rays produced in the K-capture process; the second is by analysis of the argon 40 produced during a known period of geological time; and the third is by comparison of the quantities of A 4 0 and Ca4 0 produced in the same mineral. Comparison of Age Methods

It now appears possible to make accurate analysis of the parent and daughter elements associated with the different methods of age determina- tion.

Kulp (1954) lists a set of lead 207/206 ages that were determined by three different laboratories on specimens from the same locality. The agreement is very good and regardless of the accuracy of the calculated

207/206 ages the mass spectrometric analyses must not be contributing any variation in lead age determination. Further a few samples from the same mineral and same locality have been measured, for strontium and rubidium, by both the Carnegie Institution in Washington and Nuclear Geophysics

Section of the Department of Geology and Geophysics at the Massachusetts

Institute of Technology. In two samples the quantities of Rb and Sr deter- mined agreed to withint 5% (Herzog, 1954 and 1955). A third sample had to be discarded because of large rubidium contamination corrections. Thus, the few existing interlaboratory checks indicate that quantities of lead, rubidium, and strontium in a mineral can be accurately measured.

Therefore, it is now necessary to assure that radiogenic argon can be measured with the same accuracy. Although few interlaboratory checks have been made, accurate measurements appear possible at the present time.

With assurance that quantities of rubidium, strontium, argon and lead can be accurately measured a concentrated program of age determination is possible. Few laboratories in the world are equipped to make accurate analyses for all these elements. An interlaboratory program is necessary.

With such a program the discrepancies now evident in the ages determined by the various methods may be resolved. 6.

Research Problems

A concentrated program of research devoted to the potassium-argon method of age determination requires the use of a mass spectrometer and a system to extract, purify, and measure the gases in a mineral.

Initial research effort was devoted to the construction of a mass spectrometer that could be used for isotope analysis of argon. The many problems connected with the construction and calibration of a mass spectro- meter are fully discussed in later sections.

A background of information concerning the extraction and purifica- tion of helium was available in the Department of Geology and Geophysics.

The problems associated with atmospheric contamination and the separation of small quantities of gas in the case of argon analysis, however, are more severe. It was necessary to design and construct a furnace to extract argon from minerals since the furnace currently in use for helium analyses could not be sufficiently outgassed.

Argon, as well as other gases, is adsorbed on charcoal at liquid nitrogen temperature. Therefore, this method for the separation of gases, commonly used in helium analysis, could not be employed. The literature contained little specific information regarding the separation and measure- ment of small quantities of argon. Two papers, one by Soddy (1907) and one by Arrol, Chackett, and Epstein (1949) provided the basic information upon which the present separation system is built. The many stages of development through which this system went are discussed in Section VII. 7.

Section II

MASS SPECTROMETER

Introduction

The measurement of radiogenic argon in potassium bearing minerals

requires a knowledge of the isotopic composition as well as the quantity

of the gas. A mass spectrometer was constructed similar to that described

by Nier (1947) with a six inch radius and 600 sector. In selecting this

type of instrument thought had to be given not only to the problem of

argon analysis but to the availability of material, ease of construction, and adaptability to other research problems that may arise after the present research in completed.

Isotope Analysis of Argon

After the initial work of Aston and Dempster around 1918-1919, many persons contributed to the development of mass spectrometry. Several excellent books have been written describing this development. Among the best are Ewald and Hinterberger (1954), Barnard (1952), and Aston (1942).

Aston (1920) first made use of the mass spectrometer to investigate the isotopes of argon. He gave 40.00 ± 0.02 as the mass of the most abundant argon isotope and reported the presence of "a faint line at mass

36, which may be about 3% of the total". It was not until 1934 that

Zeeman and deGier (1934) announced the presence of an isotope of argon of mass 38. This was later confirmed by Nier (1936) when he also demonstrated the lack of other isotopes with a high degree of precision. The ratio of the isotopes of 40 and 36 was estimated by Vaughn as 304112. Nier (1936) later gave 325 for this ratio and 5.1 for the ratio of the isotopes 38 and

36. The latest determination of these ratios give 40/36 = 296t 1 and 38/36 = .188 t .001 (Nier 1950). These later ratios are used throughout

this research.

Theory of the Mass Spectrometer

An analysis of the path of an ion beam in electrostatic and magnetic fields has been carried out by Herzog (1934) and Stephens (1934). Ewald and Hintenberger (195 2 ) in their book "Methoden and Anvendungen Der Massen-

spectroscopic" have an excellent discussion of ion optics. The discussion here will be limited to applications of the general theory to the 600 sector spectrometer.

It is useful to write down, first, the equation for the passage of a charged particle of mass m and charge e through a magnetic field of intensity H. The particle is projected into the magnetic field with a velocity v. The path of the particle will be a circle with the radius dependent upon the velocity, mass, and charge of the particle and intensity of the magnetic field. The particle will experience a centrifugal force, 2 mv /r and for equilibrium this must be balanced by the force exerted by magnetic field, Hev. That is

mv 2/r = Hev or r = mv/eH (1)

If (mv) and H are held constant then r is a constant. If another particle has mass m (14-.m) and velocity v (1* A v) it will have a momentum of my (1 + & m +- v). If it is similarly projected into the magnetic field it will have a radius of curvature of r = my (1+ A (mv))/eH

The magnetic field generates a momentum dispersion. If it is now assumed that the charged particle has acquired its velocity by falling through an electrostatic potential V, the kinetic energy developed will be equal to the potential energy of the particle eV, before acceleration, or

1/2 mv2 = eV/300 (2) There will be a definite velocity associated with all particles having

the same energy (constant mV2 ) which is inversely proportional to the square

root of their mass. Each particle will, as a consequence, describe a path

through the magnetic field with a radius of curvature proportional to the

square root of its mass. The equations (1) and (2) can be combined into

a single equation eliminating v,

or m/e = r2 H2 /2V. (3)

If the radius is expressed in inches, the field intensity in gauss,

the mass in atomic mass units, the charge in terms of a single charge unit,

and V in volts, the equation (3) may be written,

m/e = 3.09 x 10-4 r2 H2 /V (4)

In the case of the mass spectrometer with a six inch radius this

equation becomes

m/e = .0343 x H2/V (5)

Several points should be mentioned. (1.) A more detailed discussion

of the focusing properties of a magnetic sector field follows, however,

it should be noted here that focusing is with respect to direction only.

(2.) It has been assumed that each particle is monoenergetic. The ion

source must be designed so that a small energy spread is achieved. (3.)

The mass spectrometer equation, (3), is followed in the source region before

the ions have passed through the acceleration potential as well as in the

magnetic analyzer. (4.) More intense ion beams are possible if direction

focusing only is undertaken. The mass spectrometer may be considered as

a constant deviation device.

Refocusing of Divergent Beams

The discussion may be extended now to include the refocusing properties of the magnetic sector field. Although the spectrometer was in use in 1918 10.

it was not until 1934 that a general discussion of the refocusing effects

of magnetic and electrostatic fields was published by Herzog (in Ewaldt&

Hintenberger, 1952).

A divergent beam of monoenergenic ions of one mass is directed into

the homogeneous magnetic field. (See figure 1.) The central ion ray is

incident at an angle,6, , with the normal to the field boundaries and

emerges at an angle C. with the normal to the field boundary. The semi-

angle of divergence of the ion beam is q . The condition to be satisfied

in order that a beam divergent from point P1 should be refocused at point

P2 was given by Herzog (1934). (In Barnard, 1952).

r sink -- 1 cos(-E 1) 12 cos(t- 2) - 1, sin( -6 -6 ) 0(6) cos6 1 cos 2 r cose 1 cosE 2 3 If a symmetrically arranged instrument has a radius of 6 inches, a

magnetic field of 600, and an ion beam incident normal to the field

boundaries, the following equations are obtained,

rsin + ll(cos ) +1,cos f - (11) sinj: 0 (7) with 4E =6 2 - 0,

(1 = 12 1 r(cot4p 4- cosec# ) (8) with a symmetrically arranged instrument, and

1 = 6.00( 1 2 ) - 10.39 inches (3)6 (3)2 with a six inch radius and 600 magnetic sector field. The arrangement of the source point, image point, and magnetic field for this case are shown in figure 27. An instrument of these dimensions was constructed for this research.

Causes of Ion Beam Spread

Refocusing is not perfect even with correct alignment. The "focusing" error if 2r(l-cosr). This may be written as rie 2 with the approximation of small e . 11.

K

P A= l1 P2B=12

General case of first-order focusing of ion beam in homogeneous magnetic field with sharply defined boundaries of any arbitrary shape. 12.

The minimum beam spread that can be achieved for first order focusing 2 is rCr . To achieve this P and P2 must be positioned correctly with

respect to Pm. There are additional aberrations introduced if the source

and collector slits are not aligned parallel to each other and perpendicular

to the central plane of the magnet.

If the magnet is displaced from its correct position there are further

aberrations. R. A. Davies (in Barnard, 1953) has derived the following

equations for beam spread due to three possible directions of misalignment.

(1) Magnet displaced distance x1 along x axis. 2 rC 2 ~ 2~*2 Spread = rr + 4 1+ r4C ( 2/2 - gxI/r + 4x /r2

(2) Magnet displaced distance g, along the g axis

Spread r = d2 - y + re ( 42/2 - ffy/2r + 2y 2 - .) (10)

(3) Magnet notated angle i about its nominal apex, Pm

Spread = r 42 .r e +-re ( 2/2 + rR - 9/4 e 2 + .) (11)

Figures 3, 4 show diagrammatically the effect of the refocusing properties of the magnet sector a displacement of the magnet from its correct position. The figures show that a displacement in the x direction, for any given a and r, causes the greatest beam spread.

It has been assumed throughout the above discussion that the magnetic field has well defined boundaries. Barnard (1954) has a discussion of fringing flux corrections and states that additional adjustments are necessary after the magnet has been positioned on a theoretical basis. A good approximation is obtained then by considering that the boundaries of the magnetic fields extend out to a distance of one gap width. It has been further assumed throughout the above discussion that the ions of each mass are monenergetic with a velocity characteristic of that mass. However, A Ii' ill 'II 'II I'

00100I 0l .1-100I

-*100 -*9 -'-0 - -K;-

Figure 2 '00, Effect on refocusing by shifting the magnet .1 inch upward from it- correct position. / '0 I III IIII I'i

I |

~/ / // // //

-- ,9000 0-- 100

009 000 .00,1000

--000 -.- 10000,- -:: ,'000, .0001

Figure 3 -9 -;-- -O, Effect on refocusing by shifting the magnet .2 inch down and .1 inch towards source from its correct position. 15. a small energy spread is unavoidable. There are three causes of energy spread. (1.) When ions are obtained by electron bombardment, a potential gradient across the electron beam is necessary to withdraw the ions from the beam. Even with a beam of small cross-sectional area some ions acquire potential of 6V in excess of that acquired by other ions. The aberration introduced is 2 AR - 2r S/%= r SV/V. (2.) a broadening of the ion beam can arise because of energy changes associated with collisions between similar ions. The pressure must be below 10-6, if collisions are not to cause excessive broadening. (3.) Ions formed from molecular dis- sociation products have associated with them a varying amount of energy of dissociation. Aberrations arising from this source are a cause for concern in hydrocarbon analysis. However, the gases encountered in this research were monatomic and consequently have no energy of dissociation associated with them.

Resolution

Ewaldtand Hintenberger (1952) have derived the following expression for the resolution:

M ------M 2(12) K" v where, theoretically, for the spectrometer used in this research

K" = 2r and G = -1.

The expression for the revolution then becomes

M = 1 . (13) A V Si f.. S2 V + r 16.

For the spectrometer used in this research values of 6.00, .008, .050 are observed for r, S, and S2, respectively. A value of 15/2300 is obtained for dV/V. The resolution is

M/ 4A M = l/.0013 + .0096 92.

That is to say there is a separation of one mass unit at mass 92.

The resolution is sufficient for this research and can be improved, if necessary, by reducing the width of the collector slit.

A picture of possible aberrations introduced is shown in figure .

(Barnard). This figure shows the effects of misalignment of source and collector slits; of spherical aberration for r = 150 mm, r = 1/30 radian; of chromatic aberrations of &V = 0.5V, V = 100OV, and r 150 mm; and of non-uniformity in the z direction of the magnetic field for a pole gap of 20 mm in relation to an ion ribbon width of 10 mm symmetrically disposed abput the central plane.

Physical Arrangement of Equipment

The equipment used in this research to produce, analyze, and collect positive ions consisted of:

(1) Ion Source

(2) Collector

(3) Regulated high voltage supply 0 -5000V with taps for drawing out

and focusing potentials.

(4) Regulated power supply 225V

(5) Regulated magnet current supply 0-300ma.

(6) Magnet

(7) D-C Current Amplifiers

(8) Preamplifier

(9) Emission Regulator 17.

a b C d e f

Figure 4

Beam spread due to various aberrations.

(a) Exact reproduction of source slit (0.2 x 10 mm) with collector slit. Accurately aligned; no aberrations. (b) Spherical aberration added. (c) Chromatic aberration added. (d) Curvature of image due to variation of magnetic flux density across pole gap and to ion rays passing obliquely through central plane. (e) Image broadening due to oscillatory component in acceleration potential. (f) Distortion due to misalignment by 10 of collector slit with source slit. (g) Superposition of aberrations; for clarity each flank considered separately; one flank shown extended by aberrations (b). (c), (e) and (f); the other flank shown extended by aberrations (d) and (f) only. 18.

(10) Vacuum System

(12) Spectrometer tube

(13) Source magnets and aligning mechanism

The ion source (1) is discussed in Section IV. The collector (2),

D-C current amplifier (7), and preamplifier (8) are discussed in section V.

Pictures and schematic diagrams of the electronic equipment are shown on

the following pages. All the electronic equipment with the exception of

the magnet current supply and preamplifier were built by Dunn Engineering

Associates of Cambridge.

(10) Vacuum System

The vacuum system consisted of an umbrella-type diffusion pump, a cold

trap and fore pump. The diffusion pump was designed by Homer Priest of the

Research Laboratory for Electronics, and built by Ryan, Velluto and Anderson who also did most of the glass work necessary on the spectrometer and the gas extraction and analysis system. The fore pump was a Welch Duo-seal two-stage rotory pump. It was possible to attain vacua of 3-5x10~ mm of mercury after prolonged heating of metal parts and with liquid nitrogen as a cold trap coolant. It was necessary to use liquid nitrogen as a cool- ant to reduce hydrocarbon background (see section III).

(11) Gas Inlet System

The gas inlet system consisted of an inlet to which the sample con- tainer could be attached, a small calibrated volume, a 50 cm3 sample reservoir, a 5 liter sample reservoir, a cold trap, evacuating system and variable gas leak. A diagram of the inlet system is shown in figure 5.

All glassware was Pyrex and all stopcocks were mercury seal stopcocks with a 4 mm bore. 10,11M I- M- F j IMMOMMOOM

Gas Leak

To spectrometer Sample tube Cold trap ITo fore pump

Cold trap

Diffusion pump

Figure 5

Gas inlet system.

WIN 20.

The evacuating system consisted of a nozzle-type mercury diffusion

pump and a cold trap. Vacuums of less than 2-3x0~7 mm of mercury were

attainable. It was necessary to maintain liquid nitrogen on the cold

trap during sample analysis because of the presence of hydrocarbons in

the background spectra.

The variable leak was variable over a wide range. With a small open-

ing it was possible to analyze air for atmospheric argon, although some

distortion of peak shape was observed due to the presence of large oxygen

and nitrogen beams. It was also possible to accurately control the flow

rate of samples of the size encountered in this research. All parts of the

leak were made of stainless steel and nickel-plated to prevent outgassing.

Some difficulty was experienced with air leaks developing around the press-

fit connectors. It is recommended that in a permanent installation these

be replaced by silver soldered connections.

(6) Magnet

A diagram of the magnet poles is shown in figure 6. The magnet poles

were made of Armco ingot magnet iron. Each core was wound with 20,000

turns of #22 magnet wire covered with double formex coating. With a gap

width of .625 inches, 100 ma magnet current produced a field intensity of

3060 gauss. A plot of field intensity vs magnet current shows that the

field variation is linear in the region of interest.

Ion beams can be located approximately by the use of the mass spectro-

meter equation,

m/e = 3.09 x 10~4 r2H2 /V = k I2 /V (14) where I is in milliamperes and V in volts. The constant k is equal to 8.b6.

If one papameter is fixed the other may be found using equation (14) and the ion beam located. ALLdimensions in inches

Figure 6

Magnet poles.

|||||| Il li AI - ~

22.

(12) Spectrometer tube

The spectrometer tube was made from 2-inch copper tubing with all

joints being silver soldered. All flanges were made of non-magnetic

stainless steel. The diagram on page 44 shows the location of the source,

collector and vacuum outlets. Copper is very gassy and it was necessary

to sandblast the inside of the tube and maintain periodic heating periods

to obtain an adequate vacuum. In order to obtain satisfactory recorded

ion beam shapes the source and image points must be accurately located.

The tube was assembled and swedged by R. Thorness, machinist for A. 0.

Nier. After the tube had arrived it was necessary to position the source

and collector flanges relative to each other and to the central plane of

the tube. With the aid of a competent machinest and a larger optical

flat it was possible to locate these flanges accurately to within .01

inch. Figures 7 and 8 show right and left sideviews of the spectrometer,

showing especially the method of mounting the spectrometer tube.

(13) Source magnets and aligning mechanism

The source magnets were two 2xl inch rods of Alnico V magnetic

material. They could be correctly aligned with the aid of the alignment

mechanism shown in figures 7 and 8. Once correctly aligned it was

possible to lock them securely in position. It was necessary to lock

them in position as any change in their position would make a redetermin-

ation of the discrimination value necessary. Figure 7. Right side view of the Mass Spectrometer

--- ="W- -WO 24.

Figure 8. Left side view of the Mass Spectrometer 25.

Figure 9.

Schematic diagram of high voltage supply. --j

115V 60,,v

5000V REGULATED DC SUPPLY 26.

Figure 10.

Front panel view of high voltage supply. 27.

Figure 11.

Bottom view of high voltage supply. 28.

Figure 12.

Rear view of high voltage supply. 29.

Figure 13.

Schematic diagram of ion current amplifier. CK5886

ION CURRENT AMPLIFIER 30.

Figure 14.

Front panel view of ion current amplifier. 31.

Figure 15.

Bottom view of ion current amplifier. 32.

A a VP

ftftftwmw

Figure 16.

Rear view of ion current amplifier. 33.

Figure 17.

Schematic diagram of magnet current supply. ZP

T, CHICAGO F-65 5A T2 STANCOR P-6134 60oT3 T HORDARSON T-21F04

NEON T4 UTC CG - 301

T2 6.3V I Meg 50K IPWW 0.25 8 600V S TRIAD C-ISA 5V_ 6SF5 I0K 90V 15KW P3K1 T3 8A-iJi MOTR 1W COILSERI 2______5

B 10 SEC TIME TD DELAY 7W lOK 50K 50K D 5U4 V1 50K

-~ 5U I~wCURN C05- 0 2 9 _ _ _j - C C 12P

V2PW IW *00 50K 3 2 5 V5 _MICA 20WR 50W LINE ADJ. 51.14SU2_>30K

20W -- ID COCL C C

15K 30K 47K loW . 20W

R TREEN WHITEUT

NEODN NEON 0 3AE BODEi5A 2N

ALLIED BUU 12A MOMENTARY CONTACT SNHNSO REVERSING SWITCHES

D o____!______

REMOTE CONTROL UNIT REGULATED D.C. MAGNET SUPPLY 34.

Figure 18.

Schematic diagram of balancing panel. INPUT*2 FROM AMPLIFIERS INPUT *1

I - - I OFF R RI6 R 17 13 Rii Rg Ry7 R5 R3 R, + R2 R4 R6 Re Rio R12 R24 R516 Ri OFF

1000 500 200 100 50 20 10 5 2 1 25K 15K 5K Io2.5K 1.5K5001 2500 50f 50i1 0-2MA DC 0-20MADC 50n1 150nl 25010 500n0 1.5K 2.5K 5K 15K 25K ALL RESISTORS 1% PWW- 50fl% 50f1±1% ALL RESISTORS 1% PWW SW5 SW4 A -- B 10 ODIRECT 2 2 DECADE+ 3 3 DECADE R SW 3 4 4 99K O BAT+ R 3 2 9K 25BAT-

2 2.5 V E R24 PUT S TAKE IOOK DECADE 1%PWW RESISTANCES

2

R29 99n R33 04 OFF I nD2 BALANCING PANEL 35.

Figure 19.

Front panel view of balancing panel. 36.

Figure 20.

Rear view of balancing panel. 37.

Figure 21.

Schematic diagram of regulated D.C. power supply. LI STANCOR C-1003

TI UTC R- 102

OUTPUT A5

Al A2 A3 A4 GROUND CONNECTORS

REGULATED D.C.SUPPLY 225V, 180M. A. 60-% 38.

Figure 22.

Schematic diagram of regulated D.C. power supply. 39.

Figure 23.

Bottom view of regulated D.C. power supply. 40.

Figure 24.

Rear view of regulated D.C. power supply. trap sheitd filoment

Figure 25. Schematic diagram of emission regulator.

Rl 150 ohm 50 watt rheostat R2 150 ohm 50 watt adjustable R3 250,000 ohm wire would potentiometer R4 20,000 ohm precision wire wound R5 50,000 ohm wire wound potentiometer B1 , B2 , B, B 45 volt B batteries M mete', 0O10 amps. a.c. meter, 0-1.5 milliamps d.c. M3 meter, 0-500 microamps d.c. T, filament transformer, secondary 2.5 volts 10 amps. 7500 ras volts insulation SWi SPST toggle switch SW2 two position selector switch, 2500 voLts insulation 42.

Figure 26.

Front panel view of emission regulator. 43.

Figure 27.

Rear view of emission regulator. 44.

Figure 28.

Diagram of mass spectrometer tube. * 7 Drill(.201) thru * 25 Drill - 2 holes NOTE: V4-20 N.C. Top 6 holes '/"deep for i x 'W 2'Copper Tubing used throughout equally spaced on long Dowel Pins unless otherwise specified 3.5" B.C.

125

5" .375 2.75 DETA I L OF 0OUTER FACE OF FLI ANGES

.9" 0.D. I.D. =" but central I" to be held parallel 6"R to ±0.005"

li"Copper Tubing

Copper reducing nipple 2"-l1" (shave nipple O.D. and Tee I.D. to snug fit for silver brazing) Cut off"Tee"oand silver -( solder 2"-60* "Ell" l7 g91"

Stainless Steel Flange

Scale tolerance on tube dimensions except where 600 MASS SPECTROMETER TUBE otherwise stated scale j". i" 45.

Section III

VACUUM TECHNIQUES AND GAS FLOW CONDITIONS IN THE MASS SPECTROMETER

Introduction

The gas handling system required for introduction of the sample into

the mass spectrometer depends upon the type of gas to be analyzed. If the

sample is a single gas introduced for isotope assay, fractionation may,

in general, be ignored and a simple handling system is sufficient. If,

however, the gas to be analyzed is a complex mixture consisting of many

isotopes, it is necessary to meet several requirements in so far as possible.

First, there should exist a known relationship between the partial

pressures of each isotope in the sample reservoir and the ionization

chamber. Second, the composition of the sample should not change during

the analysis. Third, the total peak height at any mass should be the

linear sum of all contributing isotopes of the gas mixture. Fourth, the

rate of gas flow should remain constant during the analysis. Fifth, no

gas striking the filament should be allowed to re-enter the ionization

chamber. Sixth, erratic behavior of the diffusion pump should not have

any effect on analysis. Seventh, and last, it is desirable that gas enter-

ing the ionization chamber should have reached temperature equilibrium.

The above requirements are of prime importance in hydrocarbon analysis.

In the present research, however, the problem is one of introducing a

single gas for isotope analysis, complicated by the smallness of the

sample, so that detailed discussion of the above requirements is not

included.

The problems of gas flow can be grouped into two headings. One,

gas flow through the spectrometer and two, introduction of the sample into the spectrometer. These problems are discussed in the present section. 46.

Gas Flow Through the Spectrometer

It is necessary that the pressure in the spectrometer be maintained

such that the mean free path of molecules is greater than the dimensions

of any part of the spectrometer. Figure 29 shows a schematic diagram of

the spectrometer with possible appropriate pressures for the various parts

of the spectrometer.

The rate of molecular flow between any two points is given by

Q = Km dP/MA (15)

where Q is the rate of flow, dP is the pressure difference, and M1 is a

constant depending on the geometry and temperature of the system. Since

the gas flow is proportional to 1/M fractionation must occur in the

source from which the gas is being withdrawn, once steady state conditions

have been established. The peak height of any isotope is dependent directly

upon the partial pressure of that isotope in the ionization chamber. To

determine the steady state partial pressure in the ionization chamber it

is necessary to know the rate at which the sample flows into the ioniza-

tion chamber, v, expressed in litres/sec; the rate at which the sample

is withdrawn from the chamber, S, expressed in litres/sec; and the volume of the ionization chamber, V, expressed in litres.

The ionization chamber gains v dt standard litres of gas in time dt, and loses pSdt litres in time dt, where p denotes the partial pressure of the gas expressed in atmospheres, in the ionization chamber. The net gain in gas then is

d(pV) = (v - pS)dt or V dp/dt = v - pS. (16)

Integration of this equation gives the partial pressure of the gas entering the ionization chamber or also the partial pressure of the gas 10-1 mm of mercury

10-2 J

10~4

10-6

10- 7

10-8

Figure 29.

Schematic diagram of mass spectrometer with possible appropriate pressures. 48.

intersecting the electron beam at a time t after entering the ionization

chamber.

Pt v/S (1 - exp(-St/V)) (17)

The steady state partial pressure then is v/S. The time required

to reach steady state conditions for a given rate of inflow, v, is

dependent upon S and V. The most efficient use of a gas sample is obtained when f/S is large. In order to reach steady state conditions within a

reasonable length of time S/V should be as large as possible. The necessary

information regarding S is not known for the spectrometer used in this research but some idea of the partial pressures attained may be gained from

an examination of the available information. For example, for one particular analysis, the sample size was 2.04 x 10-3 cm3S.T.P. and the rate of inflow was .5 x 10~9 litres sec~ 1. Experience has shown that steady state flow conditions for mass 38 are reached in about 60 seconds. The volume of the ionization chamber is approximately

(2.54 cm x 1.27 cm x 1.27 cm)/1000 or 4.1 x 10-3 litres. The factor exp(-St/V) should have reached a small value, say .01, before steady state conditions are reached. Therefore, e-x is equal to .01 when x is equal to 4.6,or St/V is equal to 4.6. Then,

S = 4.6 x (4) x 10-3 = 3 x 10~4 litres sec~1. 60 60 This is the pumping speed at the ionization chamber slit. The partial pressure of mass 38 then is

.5 x 10~9/s x 10~4 or .16 x 10-5 atmosphere.

At this pressure the mean free path is 7.30/1.6, (Dushman 1949), approxim-

ately 4.6 cm. or about twice the longest dimension of the ionization chamber.

It should be noted that this is a minimum value since t = 60 secs is a

maximum value. Although this is an approximation of the conditions existing 49. in the ionization chamber, it is seen that molecular flow conditions do exist in the spectrometer for samples of the size used in this research.

One important feature should be noted. If any appreciable volume exists between the leak and the ionization chamber, and if the conductance of this volume is comparable with that of the leak, the time constant in attaining

steady-state pressures will become very large. The leak must be the only controlling factor finally in operation.

Cold Traps: Background Mass Spectra

The diffusion pump used on the spectrometer is a mercury diffusion

pump designed by Homer Priest of Research Laboratory of Electronics at

3 M.I.T. Since mercury has a vapor pressure of .185 x 10- mm mercury at 0QC

it is necessary to prevent mercury from entering the spectrometer. It is

further desirable to keep mercury from diffusing into the interior of the

spectrometer to prevent deterioration of the silver soldered joints.

Diffusion of mercury and hydrocarbons into the spectrometer can be prevented

by cooling a trap with solid carbon dioxide in alcohol or with liquid

nitrogen. A comparison of the residual background spectra using either

coolant is shown by comparing figures 30 and 31. That hydrocarbons

do diffuse into the spectrometer can be seen by comparing figures 30 and

31.. (See also section II ). The hydrocarbons are probably vapors from

the oil used in the forepump and from the stopcock lubricant used on

stopcocks in the gas inlet system. The forepump oils and stopcock lubric-

6 0 ant used have vapor pressures of 10-4 - 10- mm of mercury at 20 C. However,

the temperature of the oil is probably much higher than this due to

continuous operation in a hot room. It is necessary, therefore, to insert

a cold trap immediately adjacent to the gas leak and mercury diffusion pump. 50.

10

w 5 C.

35 36 37 38 39 40 41 42 43 44 MASS NUMBER

Figure 30.

Residual spectra using solid carbon daxide as a coolant. 51.

0

Y-O.5 A Lu 0-

. lI .. I II 35 36 37 38 39 40 41 42 43 44 MASS NUMBER

Figure 31.

Residual spectra using liquid nitrogen as coolant. 52.

36 37 38 39 40 41 42 43 44

10 MIN

36 37 38 39 40 41 42 43 44

30 MIN Figure 32.

Increase of background spec-tra with tim, 53.

Figure 32 shows another source of background mass spectra. The increase in background mass spectra is probably due to outgassing of the filament and electron bombardment of ionization chamber walls.. Note particularly the increase in the carbon dioxide (44) peak.

Recommendations for Further Work

If further work is planned that requires a more sensitive instrument it will be necessary to reduce the background spectra. This can be achieved by plating the spectrometer tube with chromium and by vigorous torching or prolonged baking of glass parts.

It would also be desirable to degas all metal parts in the source and collector by heating them in a vacuum furnace with an industion heater.

Control of Gas Flow Into the Ionization Chamber

In an earlier section reference was made to the existence of molecular flow conditions in the ionization chamber. It is necessary now to consider how these flow conditions are established. The physical arrangement of the leak was discussed in section II. Only the effect of the leak upon the gas flow conditions is discussed here.

Suppose that the volume of the sample reservoir is V litres, and the gas is withdrawn from the reservoir at molecular flow rates. Since the sample reservoir is a closed system there is a steady loss of gas. Let

S be the rate of withdrawal of the gas at the pressure in the reservoir.

Then for any particular gas the loss per time dt is PS dt where P is the partial pressure of the gas. Therefore:

d(PV)= - PS dt or dp/p = - S/V dt. 54.

Hence, if P is the initial partial pressure in the system and Pt the partial pressure at time t,

Pt Po exp(-St/V) (18) Thus, the pressure time characteristic is different for each gas simply because S is proportional to 1/M.

If the reservoir contains a binary mixture, say argon 40 and argon

38, the relationships become

In Pil .948 Pt 4 P (It -P (19) In- Pt 384 Po0)40 n( Pt) \/38

With molecular flow conditions, the gases in the mixture are mutually independent and each ion current decays by a factor exp(-Smt/V) in consequence of the pressure decay. If measurements of the unknown mixture and calibration mixture are taken at exactly the same time t, then the percentage decay will be the same for each isotope in the unknown and calibration mixture. In this research the time of comparison was taken to be t = 0. This is the most easily reproducable time. For most all samples of the size encountered in this research the initial decay is very approximately linear. It is, therefore, easier and more accurate to extrapolate to zero time than interpolate between measured points on an exponential curve.

Fractionation Patterns

Molecular flow exists where the mean free path of molecules is long with respect to the diameters of the tubes through which flow takes place.

The pressure in the ionization chamber of a mass spectrometer is always low enough so that this condition prevails. If the pressure in the sample reservoir is also low enough to allow molecular flow, then molecular flow 55.

prevails throughout the spectrometer and regardless of the nature of the

leak the composition of the gas in the ionization chamber is the same

as that of the sample (Inghram 1954). However, with this low pressure

in the sample reservoir, the leak must be fairly large to keep the pressure

in the analyzing region at an acceptably high value. Thus there is a

fairly rapid depletion of the sample in the reservoir and because the

flow rate of a gas component in molecular flow varies inversely as the

square root of the mass of this component the sample reservoir, and hence

the ionizing region becomes in time depleted in the lighter components.

From equation 18 the following relations can be derived. 40 40 40 (1) Pt = Po exp(-Sm t/V) (19)

(2) P38 P 3 8 e 38 t/V) (20) 40 38) Dividing (1) by (2) and setting Sm ' m equal to 1/40 and 1/382

respectively, we have:

(3) P4 0/P 3 8 P exp (40)2 t/v (21) t) 40/pf(8)' p38 -_ 3 V (40)2 - (38)-

Therefore, the ratio argon 40/argon 38 increases with time at a

definite rate. The variation of the ratio 40/38 in most analyses resembled

that shown in figure 33.

Since the peak height is proportional to the partial pressure in the

ionization chamber, the peak height measurement may be considered represent-

ative of

40 38 40 38 40 38 40 38 t and P 0 ,P 0 , or ht/ht h /h0 ext( t) From the graph on page 56, we have 1.0525 1.0400 exp( g t) and

= 2x 10- 5 sec-1.

The value of d , theoretically is ( 1/(38) - 1/(40)1) 1/V or 4.2x10-3/V.

A computation of the volume of the gas inlet system gives as the volume approximately 200 cm 3 or theoretically 0- 2.lxlO-5sec~1 . 3

1.061

0 5 10 15 MINS.

Figure 33.

Variation in 40/38 ratio with time. E CD

SNIH SL OL S , ------I , , , , , , , , ,O LE 58.

A similar calculation can be made for the ratio 40/38 in atmospheric argon.

40 36 - 40 38 ht /ht ho0 /h8 exp( U t)

From the graph in figure 34 we have 321.5 = 311 exp(d t). The value of f is 5.5x10-5sec~ . The value of f , theoretically, is

(1/(36)1 - 1/(40)1) 1/V = 8.6x10-3/200 = 4.3x10-5sec -1

The agreement here is not as good but definitely indicates that molecular flow conditions are established. 59.

Section IV

PRODUCTION OF POSITIVE IONS

Introduction

In section II it was shown that the mass spectrometer is a constant

deviation spectrometer in which focusing is in respect to direction only.

A spectrometer with adequate resolution is possible only if the positive

ions have a small energy spread. This limits the type of source that may

be used. For example, the gaseous discharge type of source has an ion

energy spread of 1000 ev.

There are two main types of sources; (1) the hot anode or solids

source and (2) the electron bombardment or gaseous source. Use of hot

anode source requires that the sample can be applied to a filament in a

solid form, while the electron bombardment source requires that the sample

be introduced in a gaseous or vapor form.

The electron bombardment source was first used by Dempster (1922) and

subsequently developed by Bleakney (1932), Tate and Smith (1934), Nier

(1940, 1947), and others. The Nier-type source has been called the orthodox

electron-bombardment source because of its almost universal use in routine

mass spectrometric application.

Materials and Workmanship

Careful selection of materials for construction of the ion source is

necessary. Metals should be used which do not corrode or oxidize easily, which have a permeability less than 1.005 and which are not gassy. The

non-magnetic nichromes and tantanlum are very satisfactory. Tantalum, how- ever, should not be used in the presence of hydrogen since it becomes brittle and weak. Adequate insulation as well as mechanical stability have 60.

to be considered in selecting insulators. Fused silica, glass or hydrogen

fired lavite have the best insulation and stability characteristics.

Three features should have careful consideration; (1) maintenance of

design geometry, (2) elimination, in so far as possible, of edges, and

(3) a surface finish. In (1) where requires, alignment, parallelisms,

and squarenesses, should be held to .001 inch. In construction, elimin-

ation of sharp edges (2) is necessary to prevent the intense electrostatic

field disturbances that sharp edges exhibit. Uncontrolled cold field

emission due to these high fields may give rise to background peaks in

the mass region of interest. A source will function if these points are

not fully considered. A more carefully constructed source, however, will

give more satisfactory over-all performance.

The Orthodox Source

An ion source of this type may be said to consist of four parts;

(1) a device for introducing the gas into the source; (2) an ionization

chamber; (3) an electron gun and (4) an ion gun. In this section it is

assumed that the gas has been properly introduced into the ionization chamber and is representative of the original sample.

Figure 35 shows the physical arrangement of the source used in the present research.

(3) The Electron Gun

In the source, the electron gun (see figure 36 ) consists of a heated tungsten filament and an anode. The potential applied to the ionization chamber, the thermal energy, and the potential disturbances in the chamber determine the energy of the electrons. The electon beam is collimated by the use of source magnets. The poles of the magnet are aligned so that the major component of the electron velocity is parallel to the lines of 61.

GA S IONIZATION ELECTRON CHAMBER BEAM 17 JJFILAMENT TRAP CONTROL PLATE

FOCUSING COLLiMNATING PLATESI PLA TES

Figure 35.

Schematic diagram of ion source. 62.

force. Those electrons with a velocity component transverse to the magnetic

lines of force experience a force causing them to rotate in circles whose

plane is perpendicular to the magnetic lines of force. The motion of each

electron, therefore, is in a circular helix. Two important features should

be noted. (1) The electron beam should be aligned so as to pass through

the ionization chamber and be collected without bombarding any slit edges.

A wider slit at the collecting end of the chamber does not help, since

excessive penetration of the collecting voltage into the ionization chamber

may cause serious deflections of the ion beam. A larger source magnet is

the only solution.

(2) Correct alignment may be made empirically from scale drawings,

but final small adjustments are necessary. The best position is indicated

by a compromise between maximum trap current and maximum ion current.

Even this is no guarantee that secondary electrons do not contribute an

important percentage of the ionization.

(4) The Ion Gun

The ion gun (see figure 37 ) consists of a drawing-out potential,

accelerating potential, and collimating system.

The drawing-out potential is variable up to 14% of the accelerating

potential. In adjusting the drawing-out field care must be taken to avoid

extreme penetration of the field into the ion chamber. This will cause

deviation of the electron beam with a resultant spreading of the ion beam.

The accelerating potential is variable from 0-5000 volts, with 2500 volts the voltage most commonly used.

The collimating system consists of two plates with eight-thousandths

inch slits. The half angle of divergence of this system is approximately

2.25 0 63.

Mass Discrimination

Incorrect isotope abundances can arise from two main causes (1) fraction-

ation in the gas handling system and (2) mass discrimination in the ion

source. The former is discussed fully in the section III on gas flow in

the mass spectrometer. The latter is caused by the presence of a magnetic

field in the source region. Mass discrimination has also been observed when electrostatic scanning is used. Since magnetic scanning and not

electrostatic scanning is used, the latter is not a factor in this research.

The source magnets used in aligning the electron beam are a source of mass discrimination. Ions of lighter masses are made to move in circular

paths more easily than the heavier masses, hence the lighter masses will

appear in less than their true abundance. The energy of the ion before

it has passed through the accelerating potential is low and as a consequence

the ion is easily made to move in a circular path.

Since the energies of all ions of the same mass may not be equal it is

impossible to predict the mass discrimination. The mass discrimination must be determined empirically. It was determined by measuring the atmospheric A4 0 /A36 ratio.

A comparison of the ratios of 40/38, 40/36, 48/36 by Nier (1950) and the ratios obtained using the mass spectrometer employed throughout this research are shown in Table A . The difference between the two is due to the mass discrimination of the ion sources. The discrimination values for the ratios 40/38, 40/36, 38/36 may be computed by knowing only the 40/36 ratio. This is standard procedure used by mass spectrometrists. The 40/36 ratio is related to the 40/38 ratio as follows:

(40/36)A/(40/38)i : 310/296 40/38 1575 64.

The value 40/38 is computed as 1604. In the same manner the value for the ratio 38/36 is computed as .192.

Table A

RATIO 40/36 40/38 38/36

296 t 1 1575 .188 Nier (1950)

310 ±-3 1604 .192 This work

Emission Regulator

The physical arrangement of the emission regulator used in this research has been discussed in Section II. It was patterned after a design by Winn and Nier (1949). A schematic diagram of the emission regulator appears in figure 25. Regulation is achieved by control of the electric field at the filament by a control plate placed in front of the filament. This is known as a space-charge-controlled regulator. This is electrically analogous to running a common triode vacuum tube with a positive grid.

Voltage for the control plate is obtained from battery B1 . The electron current to the control plate flows through the battery B1 and the resistors

R3 and R4. Any variation in the electron emission current causes the control plate voltage to vary which tends to oppose the change in electron current.

The filament is a seven mil tungsten wire bent into the shape of a hairpin. It was necessary to use a filament of this shape to obtain an intense electron beam. With a flat or straight filament most of the electron emission would go to the control plate. The emission density from a hairpin- shaped filament is considerably greater at the point than elsewhere. When the point is placed close to the hole in the control plate a considerable 65.

portion of the emission goes through the hole in the control plate while

still being subjected to the controlling field.

It was found by experimentation that the filament should be placed about

one-half millimeter from the control plate hole. With too great a filament-

control plate spacing too much of the electron emission goes to the control

plate and not enough goes through the hole as ionizing electrons. With too

close spacing more electrons go through the control plate hole, but not

enough current goes to the control plate to maintain good stability.

Sensitivity

In ion production two efficiencies are considered. One with respect

to the gas molecules and one with respect to electrons. The problem of gas

flow in the spectrometer and efficient use of gas molecules are discussed

in section III. It should be mentioned here that the total gas flow through

the spectrometer, Q , is expressed in liters-micron-sec 1. That is, the number of liters of gas at one micron pressure flowing through the source

per sec. The over-all sensitivity, then, is expressed as the number of

liters-micron-sec 1-needed to produce a given number of amps at the collector. The sensitivity of the spectrometer used in this research is

6.3xl0~4lit-micron-sec~ for 10-12 amps at mass 40. This sensitivity is

limited only by the background at mass 40 which is generally below 10-12 amps. (see section III for a more complete discussion of background).

The number of ions produced may be expressed as

i - noQisie (21) where n0 is the density of the gas molecules, Q is the collision cross- section of the molecules for a given electron energy, s is the path length of the electron in the gas, and i is the electron current. n0sQi is a small quantity and is taken to be the ionization probability. (Barnard 1952). 66.

IONIZATION CHAMBER

TRAP CONTROL PLATE

FILAME NT

454 40"45V

Figure 36.

Schematic diagram of electron gun. 67.

CHAMBER GAS

ELECTRON BEAM

DRAWING-OUT VOLTAGE

ION BEAM I ACCELERATING VOLTAGE

- .9- I 1 I-10

if,"

Figure 37.

Schematic diagram of ion gun. 68.

A plot of the observed peak height vs. electron energy is shown in figure 38.

Stability

In all ion sources, adequate electronic equipment must be provided to stabilize the voltages applied to the electrodes in the ion and electron guns. Of consideralbe importance is the use of proper insulation. (see section II). A leaky insulator can result in an unstable ion beam. This type of instability is difficult to locate and can best be prevented by adequate attention to cleanliness in the source.

If in equation (21) Qisn0 is a constant for any given set of conditions

(as it usually is) the stability of the ion current depends upon the stab- ility of the electron current, i e . Since secondary electron emission is, to some extent, always present, it is desirable to control the total electron current immediately adjacent to the filament. As discussed in an above subsection this is the method used in the present research.

The density of gas molecules no, is directly proportional to the rate of gas inflow Qgffl and inversely proportional to the pumping speed

S, or no Cc Q9f 1/S. It is necessary, therefore, to control not only the rate of gas inflow but also the rate at which the gas is pumped from the ionization chamber. The pumping speed can be controlled by proper control of the heating element in the diffusion pump and by proper design of the ionization chamber. 1.0 +

++

.8-

. Os 5- + n m

J0 4

.2-

0 10 20 30 40 50 60 70 80 90 100 ELECTRONACCELERATING VOLTAGE (VOLTS) 70.

Section V

COLLECTION AND MEASUREMENT OF ION BEAMS

Collector Design

Several different collector designs have been considered in the present

research. The main decision to be made being that of selecting a single

or multiple collecting system. The wide mass separation of the isotopes

of argon made null method measuring impractical without multiple collection.

This, however, would introduce unwanted mass discriminations. (Barnard 1952).

Further because of the smallness of the sample usually encountered in this

research, the decay of ion beams was rapid and null method measurement

would be impossible. The design of the first single collector in use was

similar to one used on a solid source instrument in the same laboratory.

This collector, however, had two major faults. One, it was mechanically

unstable and consequently gave rise to large background noises when the

instrument was subjected to any small vibration; and two, several negative

peaks were noted, the one occuring at mass 36 being the more important.

The exact reason for the presence of these peaks in unknown. The most

probable reason being that the ion beams of other mass spectra would be

glancing off the sides of the tube, picking up electrons and being

collected as negative ions at the time mass 36 was being collected. The

design of the collector now being used is shown in figure 38 . This col-

lector has given excellent results, It is mechanically stable, and negative

peaks have not been observed.

Preamplifier

The ion currents measured in mass spectrometers range from a maximum of the order 5 x -10~9 A to a minimum determined by the limits of present day techniques. Measurement of these low currents requires the use of a 71. stage of preamplification before they can be effectively used and measured.

Several considerations are of importance in preamplifier design and tube selection. One, grid insulation of tube should be satisfactory; two, adequate shock mounting is necessary; three, maintainance of a dry atmosphere surrounding the preamplifier is desirable; four, adequate electronic shield- ing must be provided; and, five, proper adjustment of condensor and resistor values are necessary such that the time constant of the recorder and not the preamplifier is the determining factor in recorded peak shape.

A schematic diagram of the preamplifier used in the present research is shown in figure 13 . The tube used is a Raytheon CK5886. The preamplif- ier as it is now designed gives very satisfactory service. The grid leak current of the tube used in this preamplifier is listed by the manufacturer as being less than 2x10-13 amps. This is extremely satisfactory since the current is at leact a factor of 500 below the ion currents that it would be desirable to measure. Complete characteristics are shown in figure 40 . The tube is mounted on its own leads, as are the leads from the collector box itself and the grid resister. The aluminum housing is gas tight, so it is possible to maintain a dry helium atmosphere around the preamplifier. This housing also serves to electronically shield the preamplifier. The current from the preamplifier and voltages necessary to run the preamplifier are carried in a shielded cable from the power supply and D-C amplifiers housed in the main electronic contact panel.

D-C Current Amplifier

The physical arrangement of the D-C amplifier has been discussed in

Section II. A schematic diagram of the amplifier appears in figure 13

No attempt will be made here to discuss the theory of D-C amplifiers.

Instead, the interested reader is referred to Aiken (1947) 72.

springs

TANTALUM OR NICHROME NO. 303 STAINLESS STEEL

FUSED SILICA

SCALE

I1 I 1/500 - 0 I

0.100 a 5/" SLIT

39 K C040 K41 B

/7 \ 0.042%1/2" SLIT

0.060x 5/9" SLIT

II

0.100x5/9~ CAGE

Figure 39.

Design of collector 73.

and the many other excellent works on the subject.

The amplifier is simple in construction and maintainance time is

negligible. Two features warrant mention. One, it is necessary to

adjust the zero step-adjust resistor whenever a new tube is inserted;

and, two, the amplifier is extremely sensitive to microphonics. Tubes

which are microphonic can be eliminated by tapping them lightly. The

resultant noise increase in immediately apparent.

Measurement of Ion Beams

After the ion current has been amplified the resulting current is

passed through a meter and series of scaling resistors (See figure 18 ).

The voltage thus set up is recorded by a Brown recorder. A typical set

of ion peaks for isotopes argon 38 and argon 40 are shown in figure 41

The peaks are measured, after drawing in zero and peak top lines, to the

nearest .01 inch. The peak measurements are then plotted on semi-log

graph paper and the ratios calculated. Section V contains a more complete

discussion of the ratios thus obtained and their interpretation.

Treatment of Data

When a sufficient number of peaks have been recorded, the record is removed from the recorder and the peak heights measured and timing-lines added. Peaks are measured to the nearest .01 inch and times recorded to the nearest .1 minute. The 40, 38 and 36 peak heights are plotted on semi-log graph paper and the decay extrapolated to t = 0. In all analyses the plotted peak heights have plotted as straight lines on semi-log graph paper. The values of the peak height extrapolated to t = 0 are read and any background correction subtracted. Ratios are obtained for 40/38 and

38/36. The raw data is then corrected for discrimination. The factors used to correct each ratio are given in section IV. The quantity of radio- 74. genic argon and atmospheric contamination observed can then be computed. 75.

DESCRIPTION

The CK5886 is an electrometer pentode of subminiature construction having extremely low filament current, high emission stability and low microphonics. Operated as a triode, the tube has an unusually high ratio of transconduct- once to control grid current for single stage circuits. As a pentode, the amplification factor is high enough to afford considerable voltage gain in the electrometer stage of a multi-stage circuit. The flexible terminal leads may be sol- dered or welded directly to the terminals of circuit components without the use of sockets. Standard subminiature sockets may be used by cutting the leads to 0.20" length.

MECHANICAL DATA

ENVELOPE: T-2X3Glass D BASE: None (0.016" tinned flexible leads. Length: 1.5" min. Spacing: Leads 4- 7 0.150" center - to - center; Other Leads 0.050" center -to - center.)

TERMINAL CONNECTIONS: (Red Dot is adjacent to Lead 1) Lead 1 Plate Lead 4 FFilament Negative; One Deflector Lead 2 Screen Grid Lead 7 ontrol drid Lead 3 Filament, Positive; One Deflector MOUNTING POSITION: Any Press Width ELECTRICAL DATA 0.4 10" max. DIRECT INTERELECTRODE CAPACITANCES: (wufd.) Control Grid to Filament Control Grid to Screen Grid and Plate DESIGN CENTER MAXIMUM RATINGS: F iloment Voltage (dc)* 1.25 volts Plate Voltage 45 volts 7 4 3 2 1 Screen Grid Voltage 45 volts CHARACTERISTICS AND TYPLCAL"OPERAT ION: Pentade Filament Voltage (dc) 1.25 1.25 volts Filament Current 10 10 ma. Plate Voltage 10.5 12 volts Screen Grid Voltage 4.5 volts Control Grid Voltage .3 -2 volts Plate Current 200 6 uo. Screen Grid Current 3.6 ua. Amplification Factor 2.0 Transconductance 160 14 umhos Plate Resistance II meg. Max. Control Grid Current 2 X 10~ 3 X i1 amp.

* For use nigh- batteries having an initial voltage of 1.55 voltes max. * Screen Grid connected to plate. AVERAGE TRANSFER CHARACTERISTICS

-4 I C425v Tff~~ fltM- T TW-

-5

.6 -4 4 . 44( ; Y 4 7FT7~t--- -7 * : ; ; . 1 L- 4t

t -8

Ec - VOLTS Q 0

-43- /f]

/8 II

Figure 41.

Typical recorded ion beams. 77.

Section VI

ISOTOPE DILUTION TECHNIQUES

The Atomic Energy Commission has made available quantities of material

artificially enriched in rare isotopes. Materials enriched in the isotopes 87 84 48 42 38 of Rb , Sr , Ca , Ca , and A are available. The use of these

enriched samples, commonly called "spikes" or "tracers" have made it possible

to greatly improve the absolute accuracy in the measurement of small quantities

of these elements. The isotope dilution technique consists of the addition

of an accurately known quantity of a tracer, T, artificially enriched in

isotope X, in which the isotope abundance ratios Xl/X2 are accurately

known, to a known total quantity of sample, S, in which the quantity of the

isotope, X2 to be determined is unknown.

By determining the isotope ratio (Xl)T/(X2)S V (X2)T in the mixture

of sample and tracer, and by knowing the ratio (Xi/X2)T and quantity of

the tracer added, it is possible to determine the amount of isotope X2 in the original sample. The section that follows explains the use of

isotope dilution techniques in determining small quantities of radiogenic

argon in minerals.

Tracer Introduction System

The tracer used in this research was enriched in argon 38. It was

introduced into a 3 liter bulb from which it could be withdrawn when

needed. The bulb was previously prepared by flushing with hydrogen and

evacuating for 24 hours. Any small quantity of gas left in the bulb was

evacuated by adsorbtion on charcoal cooled with liquid nitrogen. The tracer was then added from the break seal tube in which it had been stored. The

tracer could be withdrawn into a small volume when needed and expanded

into the McLeod gauge for measurement. The tracer was added to the sample 78.

before the purification procedure.

Calibration of the Tracer

Since the tracer was not pure argon but contained unknown quantities

of hydrogen, nitrogen and carbon dioxide, it was necessary to determine

the quantity of tracer per unit volume of total gas in the bulb. This

quantity can be determined if the isotope abundance ratios of the tracer

and of spectroscopic argon, and a mixture of the two, are known.

If the following notation is used,

R = (4 0/ 38 )x

Qx = quantity of material x

where x may stand for S for spectroscopic argon, m for mixture of the

two, and T for tracer, the following ratios are determined.

4 0 3 8 QT T - T QS = 40S 4- 38 4- 3 6S

40T QT/l t- (3 8 /4 0 )T = RT QT/ 1 + RT (22) 40S = QS/l+ (38/40) -4- (36/40)S

38T =QT/1 RT

Using the 40/38 ratios measured in the tracer, spectroscopic argon, and mixture of the two, we obtain

(40/38)m = 40T + 4 0s/ 38 T

Rm RTQTA1 + RT) * QSl + (36/40) + (38/40)S) QT/1 + RT

QT (Rm - RT/l + RT) Q/1+ (36/40)s + (38/40)s

QT = S/l + (36/40)S + (38/40)Si f(1 - RT/Rm - RT The quantity of tracer in the mixture is then

QT = (40/36)S QS/1 t (40/36)S I (1 -+RT/Rm- RT (23) 79.

Table B shows the ratios measured and the final results. The gas in the bulb was found to contain 93.6% tracer per unit volume of gas.

Possible Errors in the Calibration of Tracer

The determination of the quantity of tracer per unit volume of gas in the bulb is subject to error dependent upon the error in measurement of the ratio Rxard quantities Q . Table C shows a tabulation of the errors introduced in the determination of the volume of tracer for a given errcr in the determination of Rs, RT, Rm' QS, and QT

Table B

CALIBRATION OF SPIKE

Trace r Spectroscopic Mixture Argon 1

40/38 .0825 1575 1.039

40/36 161 296 281

38/36 1965 .188 272 5 6 Q 2.04 x10-cm3 1.80 x10 3 cm3 3.99 x 10-3 cm3

Contamination in tracer 6.4% per unit volume

Table C

Error In Volume Of Pure Tracer Determined Per E% In Ratio Or Quantity

RATIO OR QUANTITY

S Rm T QS T -. 1% per -10% -t.7%per -1% -.1% per -4% -. 8% per -1% -h.8% per -1% 80.

The main concern for sources of errors in the determination of this

value is in the measurement of Rm' QS' and the discrimination value

of the ion source.

The discrimination of the ion source, that is the percentage difference

between the actual and recorded ratio introduced in the ratio measurement

of two masses, is caused by the presence of the electron beam aligning

magnets. A complete discussion of the discrimination of the ion source

is given in section IV. This factor should not introduce any large error

since the reproducibility of the discrimination value was within .3%.

Measurement of Q and QT, if in error, would be most probably in error

by the same percentage in the same direction. An error in the determin-

ation of these two quantities would tend to cancel each other. An error

in Rm is likely to be small since QS and QT were so chosen that the value of Rm would be nearly 1.000. The error in the determination of peak

heights on the same scale and of approximately equal heights is likely to be small. Any discrimination or non-linearity that may exist in the amplification system would then have no effect on this ratio. As a con- sequence the determination of the value of 93.6% tracer per unit volume of gas in the bulb is believed to be accurate to within t 1%.

Isotope Dilution Measurements

The quantity of radiogenic argon in any sample of gas may be determined by isotope dilution techniques if the quantities QT and ratios 40/38, 40/36, and 38/36 in the tracer gas mixture of gases are known. If use is made of the notation

40/38X = Rx; (4 0 / 36 )x = Px, (38 / 3 6 )x = Tx and Qx is the quantity of x, where x may be T for tracer; C for contamination; m for mixture, or R for 81.

radiogenic, we have the following ratios: 40 40 402 38 4 0/ 38 m T +-4 QR * C (24)

38/36m 36 (24a)

38 QT/l - R = Q'/1.082 40 O/ +4/8 4o G/l 4 3 8 /4 0T = RQT/l + R .082 1.082 From(249)we have the volume of contamination QC or

3 8 / 3 6 m (r6 3 =)= 8 QC/l t ( 4 0/ 3 6 )C - (38/36)c 38 1 _ 36 1-T 3 38/36) or QC= 1 - 3 6 ) 3 36 3 8 /3 6 m (25) 38/36m r 4M Combining 24 and 25 and simplifying, we have as the quantity of radio- genic argon

QR= QT (.924 (4 0 /3 8 )m ~ (38/36) (274) t .0642). (26)

Errors in the Determination of Radiogenic Argon

Aside from the error introduced by improper technique in handling the sample, the quantity of radiogenic argon determined may be in error due to errors in the determination of the ratios 4 0/3 8 m, 4 0 /38 T, and 38/36m and the quantities of tracer QT. Table D shows the errors introduced in the determination of the quantity of radiogenic argon for a given error in the

4 0 3 8 ratios / m, 4 0/ 38T' and Q,

Table D

Error In QR Per E% Error In Ratio Or Quantity

Ratios 4 0 38 / T 40/38m 4r

t .5%/ ±- 2% (1) 4 1%/ ± 1% (2) t 1%/ 1%

(1) Rm > 1.25 and Qc/h ^/ 0.

(2) Qc/QT -.~ 0, Rm > 1.25, see figure 42 and discussion below when Qc/Q :P 0 - 82.

The errors were determined by use of the following equations:

4 0 3 8 .0642 QR O 924 / m - 275 + 3/6 ,)OQR/ RmR QT ( .924)

) QR / ) Rm -- .924 4 0 / 3 8 m QR Rm .924 40 - 275 + .0642 38 m 38 / 3 6m

3 8 3 6 Further; / m Tm QT (.924) = .924 QT 4 QV 1& Qc . 1 2127 297 2127 0T 297

Figure 42,showing the per cent error in the determination of the

quantity of radiogenic argon for a given percent error in 4 0/ 38 m,was

derived using the above equation and the following relationship between

QR, Qc, and QT'

OR/Qc .924 Rm QT - .07589Q - 1 Qc Qc

For any value of 4 0/ 38 T there is an optimum value of the ratio,

tracer argon to radiogenic argon.

If the quantity of atmospheric argon contamination is small, the assigned percent error in QR has the same value as the error in Rm'

However, if the ratio QR/Qc is less than 5, more accurate results are obtained if the value of QR/QT is greater than 1. This is shown particul- arly clearly in figure 42 where it will be noted that to keep the percent- age error in QR near one, for any value of Qc/QT the value of the ratio

QR/QT must be greater than one. 3. m 0T/0 C1 0 2.0 1.0

0 m

02.

1.- --

10 agaC 1.0 ligure42.

7kError in Q for 1% error in Ra' 84.

Each analysis can be in error due to errors in 4 0/ 3 8T' QT' Qc, and

3 8/ 3 6m. Further, a 1% error in the (4 0/ 38 )T ratio would introduce a very small error in QR, The error in the determination of QT should be small since the value of Q~T was quite reproducible. The main source of error then is due to Qc and 38/36m . When the value of the ratio 38/36m is large, approximately 1900, the error in QR, due to a 1% error in

3 8/ 3 6m is very small. 85.

Section VII

VOLUMETRIC ANALYSIS OF ARGON

Introduction

As stated earlier in this work, it is also the purpose of this research to determine if it is possible to make volumetric analysis of radiogenic argon without atmospheric argon contamination. If this is possible, and the mineral contaiisno primary argon, then it would be possible to make volumetric analyses of radiogenic argon without continuing mass spectro- metric analysis. Isotope dilution and mass spectrometric analysis require techniques and equipment not available in all laboratories.

The volumetric analysis equipment described below, with exception of the R. F. Induction heater, although a far less expensive model would surfice, are easy to obtain, less expensive than mass spectrometer equip- ment, and more easily manipulated.

Separation Procedure

A detailed description of the standardized argon separation procedures in included in section VIII. A brief description is included here to facilitate understanding of the detailed discussions that follow.

The mineral sample, after being weighed and wrapped is placed in the vacuum furnace and the furnace flushed and evacuated. The crucible is degassed at a high temperature. The crucible is allowed to cool and the pumps and the crucible is heated until the mineral has melted and the gas sample extracted. The gas sample is transferred to the separation system.

The hydrogen present is removed by converting it to water. If a large quantity of water is present it is adsorbed on a charcoal trap cooled to

0 -78 C. The charcoal is heated to room temperature. The remaining small 86.

sample is transferred to the gas circulating system for final purification.

The volume of gas in the system is measured periodically until a constant

reading is obtained. The sample is then transferred to a break-seal tube

and transported to the mass-spectrometer for isotopic analysis.

Description of Equipment

In order to facilitate any discussion concerning techniques of volumetric-

analysis a section is included here describing the equipment used.

Radio-Frequency Induction Heater and Furnace

The induction heater used in this research is a 10 KW induction heater

manufactured by the Lepel High Frequency Laboratories. (For a full discussion

of operating procedures see Appendix I ).

The furnace as is shown in figure 4 3 consisted of a graphite crucible,

2" x 2V" with j" walls and base. This was supported on two tungsten rods

on the inside of an alundum cylinder 2" x 8" with {" walls. The crucible

and alundum cylinder were surrounded by a bell jar which was surrounded

by a water jacket. The bell jar and assembly were supported on an aluminum

plate.

The bell jar seal was made with high melting point vacuum grease and

apeizon Q sealing wax. Ideally this assembly would be connected directly

by glass to the rest of the system. For experimental purposes it was

connected to the system through a ground ball and socket joint. Such an

arrangement would ordinarily hold a vacuum for 24 to 36 hours. It was

not, however, entirely dependable and might leak air at any time. Since

an average heating lasted no more than one or two hours this assembly was

satisfactory for early experimental procedures. With such an arrangement 0 temperatures of 2000 C or more could be maintained without raising the temperature of the cooling water more than a few degrees. 8mm tubing sample / 3/4 in. tubi ng N11 LLway Steel ball

-/in. Glass pipe Graphite crucible 2 X 1 1/2 -100mm Glass pipe

Water inlet Alundum 2XBX1/4in.

Apiezon Q

Ground 0 Ring Aluminum__ 6.0 in.

Figure 43.

Furnace for extraction of gases. - U w-

88.

The sample was admitted without breaking vacuum. The sample was

wrapped in aluminum foil and held in a side tube. It was admitted to

the crucible at the proper time by pushing it ahead with a steel ball

activated by a magnet. The steel ball was then returned to the sample tube.

Charcoal Traps

The charcoal traps used in this research contained Cenco activated nut

charcoal.

The traps contained varying amounts of charcoal depending on the

purpose for which they were used. Traps for removal of contaminating

gases contained approximately 40 gms. of charcoal, while traps for transfer

of small quantities of gas contained approximately 6 gms. of charcoal.

It was necessary to bake all charcoal traps for 48 hours before initial

use. This time was necessary to completely activate the charcoal.

Calcium and Barium Furnaces

Several different furnace designs were tried. Originally furnaces

were vertically mounted with the metal in the bottom of the furnace. In

order to provide more surface for adsorption of gas horizontally oriented

furnaces were used. The final design was a horizontal furnace through

which the gas could be circulated. (see figure 44.).

Careful control of the temperature of the barium furnace was necessary.

If the furnace was allowed to become too hot and a thick barium mirror form-

ed, the active barium would react with the quartz. When the furnace was

cooled severe strains would develop in the quartz. In most instances

the quartz would crack and a new furnace would have to be installed.

Calcium turnings were used initially since they were easy to obtain

and not easily oxidized. However, barium was used in later research because

calcium was found to be very gassy at elevated temperatures. 210cc Pipette

Transfer U L tube CuO 1 2 3 4 5 CharcoaL traps

Cold tCopraps Diffusionpump 533cc STP Manometer

McLteod

Figure 44.

Gas separation system. 90.

A complete discussion of the operation of the barium furnace for

cleanup of gases is given below.

McLeod Gauge

The McLeod gauge used in this research was built by Ryan, Velluto,

and Anderson, glass blowers. The capillary and volume was calibrated

by the author (see figure ). The capillary was calibrated by weigh-

ing and measuring the length of a mercury column at various positions in

the capillary. All length measurements were made with a vernier caliper readable to .0005 inches using a jewelers glass. Three determinations of the volume per centimeter length of the capillary were made. They appear in the Table H. The volume of the McLeod gauge bulb was determined by weighing and measuring the quantity of water in the bulb. Temperature equilibrium was attained and bottles used which were calibrated to deliver a known volume.

All volumes except those of the McLeod and capillary were measured two times each by two methods, the high pressure helium method and low pressure helium method. In the high pressure helium method, pressure of cm. of mercury were used. In the low pressure method pressures of 10-2 to 10-3 mm mercury were used.

Problems to be Faced

Early experimentation showed that four major problems would have to be solved if volumetric analysis was to be quantitative. One, loss of argon; two, incomplete extraction of argon from the mineral; three, the possibility of atmospheric argon contamination; four, incomplete removal of contaminating gases other than atmospheric argon. These problems will be discussed in the following sections. 91.

Table H

Volume of Gas Separation System

Volume of capillary .002101 cm 3/cm

.002108

.002101

Volume of McLeod gauge 533 + 1 cm3

Volume of gas circulating system 168 cm 3 92.

Loss of Argon

There are three ways in which argon may be "lost". One, semi-permanent

adsorption on charcoal; two, semi-permanent adsorption on glass; three,

solution of argon in the molten mineral.

Three experiments showed that semi-permanent adsorption on charcoal is possible (see figure 45). The argon was adsorbed on the charcoals at -189.5 0C for varying lengths of time. In experiment A (Curve A) and experiment B

(Curve B) the adsorption time was 30 minutes. In experiment C the adsorption time was 60 minutes. In experiments A and B the charcoal was allowed to reach room temperature slowly. In experiment C the charcoal was heated to room temperature by immersion in water. In all cases the quantity of argon lost was proportional to the quantity of charcoal in the trap. In another experiment the argon was adsorbed on several charcoals in sequence at -189.50C.

In this experiment 20% of the argon sample present in the system was lost.

Other adsorption temperatures were tried and it was found that at -780 C

(dry ice and alcohol mixture) 82.5% of the argon sample present in the system was adsorbed on the charcoal. However, when the charcoal was heated to room temperature the argon was completely desorbed.

No direct confirmation of these results has appeared in the literature.

Paneth (1953) states that,

"For separation of argon from krypton and xenon the charcoal should be kept at -780 C. In one complete fractionation 95% of the argon is removed (recovered)1 *".

Wetherill (1954) states, in a mass spectrometric investigation of argon and neon that "most" of the argon comes off the 0 charcoal at -100 C. (Liquid N2 and acetone mixture).

(1) Inserted by the author. U- -- I -z--

/ AuA B.*

I.. I I I I I I 0 5 10 20 30 MINS.

Figure 45.

Adsorption of argon on charcoal at liquid nitrogen temperature. 94.

It may be that in the first instance the xenon preferentially occupies

the adsorption sites, for Wetherill further states that he makes use of

xenon in his argon 38 spike containers to assure that the argon 38 is not

adsorbed on the glass. No experimental evidence was offered, however,

that argon 38 is adsorbed on glass to any appreciable extent.

Loss of Argon by Adsorption On Glass

As stated above no experimental evidence was offered by Wetherill

that argon was adsorbed on glass. Solution of helium in glass has been

noted by Paneth (1953). The quantity, however, was not important until

analysis of quantities of He of approximately 10~9 cm were undertaken.

It is probable that no appreciable quantity of argon is soluble in glass at room temperature.

Loss of Argon by Solution In the Molten Mineral

No evidence can be offered that argon is soluble in the molten mineral sample under the conditions of a temperature 15000c and pressures less than 1 or 2 mm. (Naughton (1953) has experimented with molten pyrex glass and found that argon is not soluble at pressures of 10 cm or more.

It is improbable therefore, that solution of argon in the molten mineral can cause any error in quantitative determinations.

Extraction of Small Quantities of Argon from Minerals

The difficulty of extracting small quantities of argon has been reported by many workers. Most of the earlier papers published give no mention of whether the yield was quantitative or not. Several methods have been employed. Aldrich and Nier (1948) heated the mineral to 100000 with no length of time specified in the publication. Thode and Fleming

(1953) in their work on argon 38 in pitchblende minerals 95. heated the sample in an inconel tube to 250-3000C for one hour to drive off adsorbed gases, after having evacuated the tube for 24 hours. Follow- ing this, the temperature was slowly raised to 12500C (maximum attainable).

While there were no tests to determine if the yield was quantitative, tests did prove that no structural argon was lost in heating to 3000C.

The Use of Fluxes

Most recent work has made use of Na fluxes in one form or another.

An example of incomplete removal of argon was found in the work of Russell, et al (1953) who obtained a branching ratio K/ 2/ of 0.06. The sample had been fluxed with metallic Na which subsequently was found to yield

35% less than a NaOH flux. Most workers (Wasserburg, 1955; Wetherill,

1954; Shillibeer, 1954) now make use of NaOH as a flux. Thomson and Mayne

(1955) have experimented both with and without flux, and report that the use of sodium peroxide is superior to either sodium hydroxide or sodium carbonate. This is mainly because of the large quantities of water and carbon dioxide, respectively, released by the two fluxes. In a fusion without flux the atmospheric argon contamination was found to be lower by 40%.

In the present work, tests have been made on the extraction of argon from biotite, lepidolite.

A high frequency induction heater available in the Department of

Geology and Geophysics was used to heat the samples. With proper furnace design it was possible to maintain temperatures of 1500-20000C.

The use of fluxes is unnecessary when such temperatures are used. The mineral structure in all cases was completely destroyed and in most instances the iron oxide in the sample was partially reduced to iron. 96.

Atmospheric Argon Contamination

Atmospheric argon may be introduced into the gaseous sample during two stages in the analysis procedure. (1.) If the graphite crucible is not completely degassed, some quantity of atmospheric argon which may be adsorbed during the sample loading procedure, is collected along with the gaseous sample upon reheating. Dushman (1949) states,

"At 21500C (according to Norton and Marshall) it is possible to degas graphite so that subsequent heating at a higherl* temperature given no further gas. It is very interesting to note that the gas evolved in the range 1700-2200 0C is predominantly nitrogen."

Atmospheric argon may be introduced into the extraction or separation system through a leak in the glassware or around grease-sealed ground joints. (2.) Early in the research use was made of ground glass joints to attach the transfer tube to the extraction and separation systems.

It was not possible to depend on these seals and several analyses were discarded because of large leaks. The use of grease seals was discontinued and the transfer tubes were sealed onto each system by glassblowing it in position. It was then possible to make analyses with less than 5% atmospheric contamination. In the new system being constructed it is not necessary to use transfer tubes and all grease seals have been eliminated.

Mercury seal stopcocks are used throughout the system. These stopcocks will not leak, although some attention is necessary to ensure that the bulb is periodically evacuated. -3 3 In one analysis there was less than .10xlO cm of argon contamination.

If it were always possible to keep the contamination at this level, or lower, other problems not considered, volumetric analyses would be possible.

1. The emphasis is by the author. 97.

Contaminating Gases Other Than Atmospheric Argon

The gases extracted from a mineral consist mainly of large amounts of water, hydrogen if a graphite crucible is used, and smaller quantities carbon dioxide and carbon monoxide. The radiogenic argon is only a very minor percentage of the total quantity. The volume of gas collected depends upon the type and weight of the mineral sample.

Hydrogen Removal

Hydrogen can be removed by oxidation to water. The hydrogen is readily oxidized in the presence of copper oxide heated to 450 0C. The water formed is then adsorbed on charcoal at -780C. Only a small percentage

of water will be desorbed on heating to room temperature. However, it

is not possible to remove the last small quantity (less than .2cm3STP) of water by adsorption.

It is important to note the water should not be removed by freezing.

When the water is frozen argon is trapped. (Smits 1953). In one experi- ment 30% of the radiogenic argon was lost when the water was removed by freezing. When the water is adsorbed on charcoal it may condense and freeze thereby trapping argon. It is thought, however, that the water is

in the gaseous form at room temperature and remains adsorbed while the argon is' liberated.

Cleanup of Final Traces of Gas

After treatment with copper oxide it is necessary to admit the gas to a barium furnace where the remaining water and most of the remaining contaminating gases are removed. 98.

Operation of the Barium Furnace

The use of calcium for the production of high vacua was first proposed by Soddy (1907). Soddy observed that Co, C02, H2 0, C 2 N2 , SO2 , NH3 and oxide of nitrogen were readily cleaned up. Hydrogen was not adsorbed in any appreciable quantity. He states,

"There is no doubt that a low initial pressure not exceed- ing a few millimeters of mercury is as essential in causing calcium to combine with gases as a high temperature. For rapid and continuous adsorption, volatilization is necessary. Argon, helium, and the other rare gases were not adsorbed by calcium."

Since the original paper by Soddy intensive investigation has shown that other metals such as barium and magnesium are effective as adsorption agents. The order in which metals may be rated as cleanup reagents for most gases corresponds roughly to the chemical activity of the alkaline- earths. Barium is the most efficient but is difficult to use because it is readily oxidizable. Calcium is not so readily oxidizable and is almost as efficient. Calcium was used initially as a cleanup reagent. However, because calcium was found to be very gassy at high temperatures attempts were made to make use of barium metal. The difficulty encountered due to the rapid oxidation of barium was overcome by storing the clean barium metal in an atmosphere of helium. When a fresh supply of barium was needed, a small hole was blown in the quartz furnace, the barium inserted, and the hole closed. The system was toen evacuated. The total time necessary was not more than 10 minutes. In this way, it was possible to have a fresh unoxidized supply of barium metal in the furnace.

The gas separation system contains two barium furnaces. The first is used as a rough pump to remove large quantities of gas. The second is part of the gas circulation system (see discussion on next page) and is used for the removal of small traces of gases. 99.

In order to clean up large quantities of gas it was necessary to

volitalize an appreciable quantity of barium metal. 450 0C was the most

efficient temperature for rapid volatilization, (variac setting 36). After

one-half hour a mirror of barium formed and the temperature was reduced

to 250 0C,(variac setting 20). At the end of one hour from the start

of volatilization the pressure had usually been reduced from an initial

value of a few (3-5) mm of mercury to 10~1-10-2mm of mercury. The

gaseous sample was then transferred to the gas circulating system. The

quantity of contaminating gas remaining is small. The barium furnace in

the circulating system was operated at a lower temperature (2500C). A

mirror was not formed when the furnace was operated at this temperature.

The amount of barium vapor formed, however, was adequate for efficient

sample cleanup. Figure 46 illustrated an experiment which involved -3 3 cleanup of a small quantity of gas (8x10 cm ) in the presence of

2x10-3cm 3 of argon. The contaminating gas consisted mainly of carbon

dioxide and oxygen. It will be noted that the system is quite similar

to a differential-thermal-analyzer. The reaction between the vaporous

barium metal and gas is thought, because of the shape of temperature

curves, to be endothermic.

Gas Circulating System

It is necessary, for efficient cleanup, that the gases come in contact

with the barium vapor or metal. If the pressure of argon is approximately

equal to or greater than the pressure of contaminating gases cleanup is

considerably slowed if the gas in the barium furnace is not changed

periodically. That is, the ratio of argon to contaminating gas in the furnace becomes very high. When the pressure in the system is below 10 4mm the gases can only move about by diffusion processes. The thermal agitation 400.- f -

231, 5,

19

1 7

1 X10-2

CM 3

Figure 46.

3 Cleanup of small juantities of pe in presence of I.9xIO cm argon.

X10 180 21.0 300 MINS 0120 120 10 240 300 MINS 101.

of the gas in the furnace and the small opening in the stopcock through which the gas must pass makes the diffusion of gases into the barium furnace an extremely slow process. In order to remove this difficulty a system was constructed consisting of a barium furnace, a copper oxide furnace, a mercury diffusion pump, and isolating cold traps. A diagram of the system is shown in figure 44 .

The gas sample was brought into the gas circulating system by adsorption on charcoal using liquid nitrogen (T -189.5 0C). The circul- ating system was isolated from the rest of the system and the sample desorbed by heating the charcoal to 150 0C. All the argon is desorbed at this temperature. The diffusion pump and barium furnace was heated.

The gas circulated in this system was efficiently cleaned up since the gas samples could be repeatedly exposed to the barium vapor. Further, no gas was allowed to remain in the barium furnace for an appreciable length of time so that the ratio of contaminating gas to argon was the same throughout the system.

Calibration of the System

The volumes of the various parts of the separation system are shown in a diagram of the entire system (figure 44). The entire system was calibrated by analyzing air for the volume of argon present. The values obtained in three analyses were .993%, .990%, .992%. The gas circulating system was not included in these analyses.

Results of the Volumetric Analysis

As stated above, it is possible to make analyses free of atmospheric argon to within 5%. The new system being constructed should further reduce the level of contamination. The graphite crucible will remain the main 102.

source of atmospheric argon. However, if sufficient degassing procedures are observed this difficulty may be eliminated. In any analysis other than for atmospheric argon in air it has not been possible to completely remove all of the contaminating gases. With correct cleanup procedures

(see section VIII describing standardized procedures) it is thought that -3 the quantity of contaminating gases can be kept below .05x10 . The results of several analyses and an explanation of the reasons for failure

(if necessary) is given in table E rlq 7-

Table E

Sample Total Gas Determined A40/gm. Sample x 10-3 Volumetric Isotope Dilution Volumetric Isotope Dilution

Bob Ingersoll

ID #1 4.64 4.78 .83 .79

ID #2 12.00 13.80 (3) .74

ID #4 3.67 .50 (4-)

ID #5 (2) 4.43 (2) .73

(1) It is thought that the total volume of gas as determined by isotope dilution technique is higher than that determined by a volumetric determination because of (1) an extremely small leak in the break seal tube, or (2) outgassing of the glass when the break seal tube is removed from the separation system. (2) Not determined.

(3) Not computed since it would have no meaning.

(4) In this experiment water was frozen in a cold trap. 104.

Section VIII

STANDARDIZED PROCEDURES

Volumetric Analysis

The procedure for determining the volume of the argon in a mineral has been changed many times during the experimental development, and changes may be made if future work indicates such changes are necessary.

Two separate procedures are included in the following subsections.

Procedures are included for making volumetric analyses and isotopic dilution analyses of the argon content of a mineral. Included in the sections are not only the immediate steps necessary but also such cautionary and advisory notes as are deemed expedient to a successful analysis. A condensed procedure sheet has been added in Appendix II.

The procedure for volumetric analysis of argon occupies the present sub- section.

Preparation of the Gas Analysis System

Preparatory to making an analysis any gases in the system should be evacuated. All charcoal traps should be baked and evacuated for periods of not less than 2 or 3 hours. The transfer charcoal trap and break seal tubes should be baked and evacuated overnight if time permits. The barium furnace, copper oxide furnace, and gas circulating system diffusion pump should be heated and evacuated. The gas circulating system should be prepared separate from the remainder of the system. The copper oxide furnace should be heated to 450 0C, filled with oxygen, and heating continued for 2 more hours. This procedure convertsto copper oxide any copper that may have formed during the previous run. The barium furnace should be heated to 350 0C (variac setting 30) and evacuated along with the McLeod gauge and gas circulating diffusion pump (variac setting 60) for a period 105.

of 12 hours if time permits but not less than 2-3 hours. Although very

little gas is admitted to the gas circulating system, the barium in the

furnace becomes poisoned after 2 or 3 runs and cleanup time is considerably

lengthened if adequate pre-analysis treatment is neglected.

Weighing and Wrapping the Sample

All samples should be weighed to the nearest .0005 gms. and wrapped

in aluminum foil. The sample container is made by wrapping the aluminum

foil around a 1/2 oz. sample bottle leaving one end open. After weigh-

ing, the sample is put into the aluminum foil container and the open end

twisted closed. At this stage a series of very small pin holes may be

made in the aluminum foil container to allow any air to escape more

readily when the sample is admitted to the vacuum furnace.

Preparation of Gas Extraction Furnace

The crucible should be cleaned of slag from the previous run and

the sample and steel ball inserted into sample holder. The bell jar

ground glass joint should be cleaned of any old vacuum lubricant (cylvacene-

heavy is recommended) and a thin coating of fresh lubricant applied. The

alundum radiation shield and crucible are placed in position and the bell

jar assembly placed in position over them. The assembly should be rotated

to thoroughly lubricate and seal the ground glass joint. A wedge of

Apeizon Q is applied to the outside of the ground glass joint. The glass

seal between the bell jar and the rest of the system is made at this time.

The water circulation is started and the water seal at the ground glass joint is made. The transfer tube is now sealed on to the system and the crucible is ready for degassing. In the new system under construction transfer tubes are not necessary so this step can be omitted. The crucible 106.

is degassed at approximately 2000oC-22000 C (150 amperes current through

the induction coil). The system is closed from time to time for periods of 2 or 3 minutes. The change in the pitch of the fore pump indicates

the quantity of gas coming off the crucible. When no change in the pitch

is noted, upon opening the system, the crucible is considered as degassed.

The time to degas the crucible is usually 15 minutes to one-half hour.

This is a weak point in the analysis that will be removed in the permanent

system where the evacuating apparatus consists of diffusion pumps and

cold traps and an ion gauge or simple McLeod gauge to measure the pressure

in the system. The evacuation and degassing used, however, was found to

be sufficient for experimental purposes. The crucible is allowed to

cool and the lead to the fore pump closed. The sample is admitted to the

crucible and heating of the sample started. It is extremely important

to heat the sample slowly in the initial stages. A large quantity of gas,

probably water vapor comes off the sample at approximately 300-5000C

(temperature of crucible). The aluminum foil has melted by this time

and the sample will as a consequence spill over the system if care is

not taken to ensure that the gas does not leave the furnace too rapidly.

The heating is continued until the temperature has reached 1500 0C (130

amperes current through the induction coil), a temperature slightly cooler

than was used to degas the crucible. The temperature is then held constant

until all the gas has been collected. In the experimental system difficulty was sometimes experienced in pulling the entire sample of gas,extracted from the mineral, into the charcoal trap. The pressure noticed, however, was thought to be due to continuous reduction of the iron oxide in the

sample. Many magnetic particles are fould to be present in the reduced

slag. In the permanent system the charcoal will be replaced with an automatic 107.

toepler pump and the gas will be evacuated from the furnace region. After

all the gas has been pulled into the transfer tube, the tube is removed

from the system and sealed onto the analysis system. The section in

between the stopcock and the transfer tube and the analysis system is

evacuated by adsorption of any remaining gases on charcoal at liquid

nitrogen temperatures.

Separation of Argon

The sample is admitted to the separation system and copper oxide furnace.

The copper oxide furnace has been preheated to 4500C (variac setting 43).

The transfer tube is heated to 1500C (variac setting 23) to ensure that all

the argon is desorbed. The gaseous sample is left in the copper oxide

furnace until all or most of the hydrogen has been converted to water. If

the pressure of water exceeds a few millimeters of mercury, the sample

should be adsorbed on one of the charcoal traps at -780C for one-half hour.

At this time the coolant is removed and the charcoal trap heated to room

temperature. If the pressure is not below a few millimeters of mercury,

the process is repeated on another charcoal trap. When the pressure in

the system is below a few millimeters, usually after treatment on one

charcoal trap, the roughing barium furnace is opened to the system and the

temperature of the furnace raised to 5000C (variac setting 36). When a mirror has formed the temperature is lowered to 2000c (variac setting 20),

and the sample left to stand for one-half hour. Throughout all the above procedures the copper oxide furnace has remained hot and all charcoal traps are left open to the system. The pressure in the system should now be below .2 mm. The sample is adsorved on the charcoal in the gas circul- ating system and the circulating system isolated from the rest of the system. The adsorption time usually lasts for two hours. Previous to 108.

isolating the gas circulating system, the entire system may be evacuated

along with the charcoal trap on which the sample is adsorbed. It is

thought that in this manner it may be possible to remove any hydrogen

that had not been previously converted to water. This procedure was

tried only once. Thomson and Mayne (1955) state:

"Ventil P was closed and the tap Q opened to the vacuum for two minutes, and thus any helium and neon in the gas sample were removed."

"Trial experiments showed that argon was not lost in this process."

The cleanup barium furnace is heated to 3000C (variac setting 30),

the diffusion pump heater is turned on, and the isolating cold traps covered with solid carbon-dioxide and alcohol mixture (temperature -780C).

The gas is left circulating over night. The volume of the gas is measured periodically until a constant volume is reached. The heaters are turned off and the cold traps heated to room temperature and the volume measured again. The actual volume of gas in the system can be computed using the constants listed in another part of this section.

Isotope Dilution

The procedures followed in an isotope dilution analyses are similar to those observed in a volumetric analysis. The system is prepared in the same manner and the gaseous sample extracted from the mineral sample by similar procedures. The traced is added when the sample is admitted to the separation system. If it is desired to make a volumetric analysis of the sample the procedures to follow are the same as would be observed if the tracer had not been added. When the volume has been measured the gaseous sample is adsorbed on the charcoal in a break seal tube. One hour is usually time enough to adsorb 99% of the sample. The break seal tube is sealed off and blown onto the gas inlet system of the mass spectrometer. 109.

Preparation of the Mass Spectrometer

In order to make satisfactory and accurate mass spectrometric analyses

it is necessary that certain proceduresbe carefully followed. One half to one hour before an analysis is to be made the filament should be turned so

that the source area will have reached temperature equilibrium. Previous to this the spectrometer tube should be baked out for 2-3 hours and allowed to cool. A background spectrum of the mass spectrometer with and without the leak open should be obtained. A comparison of the two will determine if there is an air leak in the gas inlet system. If there is no air leak in the gas inlet system the gas leak is closed and the break seal on the sample tube broken. The sample is allowed to cool for one-half hour, with liquid nitrogen on the cold trap, to condense any hydrocarbons that may be in the sample. The spectrometer is set to record a mass 40 or mass 38 ion beam and the gas leak is opened to the desired setting. The scale changer is set to the appropriate scale and manual recording started.

The recording is continued until a sufficient number of peaks have been recorded. 110.

Section IX

MEASUREMENT OF AGE BY THE POTASSIUM-ARGON METHOD

The age of a mineral may be determined, by the potassium-argon method,

if the quantities of argon and potassium in the mineral are known. It is

necessary that the argon produced by the decay of potassium 40 remain in

the mineral, that is the mineral must be a closed system.

Potassium 40 decays by beta emission to calcium 40 with a decay constant

and by K capture to argon 40 with a decay constant The number of 40* atoms of K40 present at any time t is 40 40 (Kt KO exp(- A t))

where - K # . The ratio of the number of atoms of argon 40 that

decay per unit time to the number of atoms of calcium 40 that decay in the

same length of time, is called the branching ratio,R / . The 40 number of atoms of argon produced from K0 atoms of potassium

4 0 4040 -At A = R -- l/R K 0 (1 -d ) 40 40 or A = R + 1/R K (ea -1)

It is then possible to determine the age of the mineral, or, A 4 0 t ;+ 1R + 1 R K4 0

The Branching Ratio

Table F shows that the branching ratio has varied widely over the

past years. It can be said that the ratio probably lies in the range

.09-.130. This variation introduces a considerable uncertainty in age determination by the potassium-argon method. Several methods have been employed to measure the branching ratio, the physical method and the geo- logical method. The physical method makes use of the beta emission associated with the calcium 40 or If emission associated with the argon 40. 111.

Table F

Year Authority Method k/ b

1943 Thompson and Rowlands X-Rays 3 to 4

1947 Bleuler and Gabriel X-Rays 1.9 ± 0.4

Horteck and Suess Argon 0.1

1948 Ahrens and Evans Calcium 1.4 ± 0.02

Aldrich and Nier Argon 0.02 to 0.09

1950 Ceccarelli, Quarcini,

and Rostagni X-Rays 0.07

Graf and X-Rays 0.127 to 0.67

Inghram, Brown, Patterson

and Hess A40 /Ca40 0.126 t 0.003

Sawyer and Wiedenbeck Auger Electrons

from X-Rays 0.135

1955 Wasserburg and Hayden A40/Pb ages .085

Moljk (1) 0.124 to 0.136

Wasserburg (2) (2) 0.102!: 0.01

(3) (3) 0.128 ± 0.02

Backus and Strickland A40 /Ca40 0.150 t.02 (4)

(1) No mention made of method of determination.

(2) Using Sawyer and Weidenbeck's data and a redetermination of the 42 gamma emission of K . (Kahn and Ryan (1953)).

(3) Computed by the author using the average of the best determinations

of the beta and gamma emission.

(4) Based on the determination of the quantities of argon and calcium

in one lepidolite. 112.

All measurements indicate that the ratio /2p is equivalent to

. (see figure 47).

40

Decay scheme for K 40, after Sawyer and Wiedenbeck; Energies

after Alburger.

The geological method makes use of the fact that known quantities of argon and calcium are produced in a known length of time. If the quantity of argon produced in a mineral is known, and if it is possible to deter- mine the age by some other method, as for example with lead, the branching ratio then is

R = 1 K'U/A 40 ( e- 1) -1

If it is possible to measure the quantities of radiogenic argon and radiogenic calcium in the same mineral, the branching ratio may be deter- mined directly regardless of the age of the mineral.

4 0 4 0 That is A /Ca =

It is necessary to know the abundance of potassium 40 in order to measure the age by the potassium-argon method. Herzog (1955) has a com- plete listing of all the published measurements of the potassium 40 abundance. The value determined by Nier (1950) (.0122% by weight) is probably the most accurate. ~- "-

Table G

Sample Best Geological Age Best Lead Age Rb/Sr A 4 0/K 4 0 Ca (.085) (.125)

Bob Ingersoll 1450 (1) 1600 ± 50 2050 (2) 1610 (8) 1180 + 90 (6) 2060 (3) 1710f 90 (7) 850 t 200 (4) 1500 . 300 (5)

(1) Kulp (1955).

(2) Aldrich, et al (1955).

(3) Herzog (1955), not yet published.

(4) (5) Ahrens (1951).

(6) Backus (1955), not yet published.

(7) This work.

(8) Presented by Wetherill at the American Geophysical Union meeting,

Washington, 1965. 114.

The Bob Ingersoll Lepidolite was used for calibration purposes. Its age is presented so that some comparisons may be made. It can be seen from the table that the age as determined by the Rb/Sr method is 17% higher than the age determined by the argon-potassium method, and 20% higher than the lead method. Several explanations of the differences observed may be offered. (1) There may be rubidium leaching in which case the Rb/Sr age 40 40 would be high. (2) There may be loss of argon in which case the A /K age would be low. (3) There may be loss of calcium, in which case the calcium age would be low. It is necessary that many more measurements be made. Two laboratories in the United States are admirable suited for these measurements, Massachusetts Institute of Technology, Department of

Geology and Geophysics, and the Carnegie Institution in Washington. 115.

Recommendations q Futu Research It is always possible, after the completion of a research problem, to think of better ways to approach the problem, and other problems that should be attacked. This research is no exception.

Mass Spectrometery

For more accurate and sensitive analysis the spectro- meter tube should be plated, the gas inlet system should be converted to an all metal system to eliminate hydro- carbon background, and the ionization chabber should be made gas tight to reduce the residual mass spectra.

Volumetri. Analysis

With a single one piece entire glass system it should be possible to make volumetric analysis of argon without the presence of atmospheric contamination. Such a system should be constructed.

Gezteral

The problem of "common' argon has not been solved. This problem could be solved if the Mass Spectrometer was placed so that gases attracted from the mineral could be measured directly. A system consisting of a radio frequency heater, furnace gas separation system and mass 116.

Spectrometer, all directly connected could form the foundation of a gas analysis system which would aid immeasureably in solving a wide variety of problems. Appendix I

Use of the R.F. Induction Heater

The induction heater is a very useful instrument, however, several precautions should be observed if no one is to be injured. The booklet included with the heater gives the basic information. However, several points should be carefully observed.

1. Become very familiar with the location of all dials and switches,

especially the 100 ampere circuit breaker.

2. Be sure to turn on the main power switch before turning on the

switch for the water circulating pump.

3. Allow at least 20 minutes warmup period after "start" buttom

has been pushed.

4. When the preliminary adjustments have been made step on "step-

switch" for a "split second" and observe readings.

5. If the 100 ampere circuit breaker opens, the water pressure will

go up to 60 p.s.i. To avoid damage to the water pump this switch

must be closed immediately or the water by-pass valve should be

opened.

6. Since the induction heater makes no distinction between metals,

extreme care should be taken not to wear rings too near (6 inches)

the coil or leads.

7. A familiarity with the note-book that came with the induction

heater is an invaluable aid in learning the correct operating

procedures. Appendix II

Condensed Procedure Sheet

1. Bake all charcoal traps and evacuate for period of 2 to 3 hours.

2. Bake and evacuate transfer tube overnight.

3. Prepare the copper oxide furnace and the barium furnace.

4. Weigh and wrap sample.

5. Prepare the gas extraction furnace.

6. Bake out the crucible (150 amperes through induction coil) for

one-half hour or until no gas is being given off.

7. Let the crucible cool.

8. Push the sample into the crucible with the steel ball and magnet.

9. Heat the sample, taking care to heat very slowly in the initial

stages.

10. Put liguid nitrogen on the transfer tube.

11. Continue heating at 140 amperes for at least one-half hour, or

until no more gas is coming off.

12. Close the stopcock to the transfer tube and break it off.

13. Seal on to the gas separation system.

14. Evacuate the section between the transfer tube and the gas

separation system.

15. Heat the copper oxide and transfer tube.

16. Allow the gas to remain in the copper oxide tube for at least

two hours or until the pressure has stopped decreasing.

17. If the pressure is greater than a few mm adsorb the gases on

charcoal in a trap cooled with solid carbon dioxide for one-half hour.

18. Let this trap heat to room temperature for one-half hour.

19. If the pressure is now below a few mm heat up the rough barium

furnace. 20. After a mirror has formed turn the heat down to variac setting 25.

2L After one-half hour the pressure should have decreased below a readable pressure on the manometer.

22. Adsorb the gas on the charcoal in the gas circulating system.

23. Isolate the gas circulating system from the rest of the system.

24. Turn on the barium furnace and the diffusion pump.

25. Let the gas circulate for a period of twelve hours or over night.

26. Measure the volume of gas in the system.

27. Continue circulating the gas until the volume read reached a constant value. BIBLIOGRAPHY

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Winn, E. D., and A. 0. Nier (1949) Simplified emission regulator for mass spectrometer ion sources, A Review of Scientific Instruments, 20, 11, 773-774. Biographical Sketch of the Author The author was born in Providence, R. I. in 1925, the first of a family of two. After graduation from Hope High School he enlisted in the U. S. Navy, spending five years as a pilot. During this time he attended Trinity College in Hartford, Conn. for one year. He was separated from the Navy in 1948 and came to M.I.T. in that year. He received an S. B. from M.I.T. in>1952. His professional experience includes summer work

with Geophysical Services Inc., Atlantic Refining Co., and California Company in the field of geophysics. Part time experience has been varied including employ- ment as a teacher and electronic trouble shooter. He was elected to the Sigma X1 in 1955 and is a member of the A.G.U. and E.A.E.G. Upon graduation he plans to work with Geophysical Services Inc.

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