Calibration of a positive ion space charge detector

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CALIBRATION OF A POSITIVE ION SPACE CHARGE DETECTOR

A Thesis Presented to the Faculty of the Department of Physics The University of Southern California

In Partial Fulfillment of the Requirements for the Degree Master of Science

by Richard Ray Eggleston January 1950

UMI Number: EP63350

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This thesis, w ritten by

R i c h a r d Ra_y E g g l e s t o n ........ under the guidance of

A.i.?....Faculty Committee,

and approved by a ll its members, has been presented to and accepted by the Council on Graduate Study and Research in partia l fu lfill­ ment of the requirements fo r the degree of

Master of Science

De an D ate ......

Faculty Committee

ft. k . / j b l u l w

(7

Chairman

TABLE OF CONTENTS CHAPTER

PAGE

INTRODUCTION ........ I.

GENERALIZED DISCUSSION

1 OF THESPACE CHARGE

DETECTOR ...................................... II. III.

EXPERIMENTAL PROCEDURE

............

4 11

DATA AND R E S U L T S ................................

22

RECOMMENDATIONS FOR FURTHERSTUDY

..............

31

BIBLIOGRAPHY ..........................................

3^

IV.

LIST OF FIGURES FIGURE

PAGE

1.

Compton and Van Voorhis Results . . . . . . . .

2.

Electron Gun and Associated C i r c u i t ...........

3.

Electron Gun and Detector Diode Mounting D e t a i l ......................................

8 13

13

4.

Bridge Circuit Schematic

......................

16

5.

Electrometer Tube Circuit Schematic ...........

18

6 . Characteristic Curves of the Electrometer Tube C i r c u i t ....................... 7.

20

Final Calibration Curves of Space Charge D e t e c t o r ....................................

23

INTRODUCTION In the varied types of electrical discharges through gases,

such as glow discharges,

Geiger counters,

spark3, arcs, lightning,

ionization chambers, etc., numerous p r o ­

cesses are active, each contributing to a greater or lesser extent to the propagation of the discharge.

The most Impor­

tant, and best known process, is that of electron multipli­ cation by electron collisions.

In addition to this, there

are others which may for reasons of simplicity be divided into two categories:

(l) Contributing processes occuring

at the electrodes (either cathode or anode), and (2) pro­ cesses occuring in the body of the gas itself.

Of the second

type photoionization in the gas is of profound significance in the explanation of sparks, lightning, Geiger counter dis­ charges, the ionization of the upper atmosphere, and other phenomena.

Yet little quantitative work has been done on

the ionization produced by photons in the energy range of from 10 to 25 e.v.

This corresponds to a wavelength range

in the vacuum ultraviolet of from 12p0 to 500°A. It is planned by the research group in gaseous electronics at the University of Southern California to determine quantitatively the photoionization in gases pro ­ duced by radiation ranging from 10 to 25 e.v. in energy. For this purpose a normal incidence vacuum spectrograph has

2 been constructed to act as a monochromater for the corre­ sponding wavelength range.

In order to carry out this study

it is necessary to have a means of detecting either of the products (electrons or positive ions) of the photoionization occuring within a certain volume of gas through which radi­ ation of known wavelength is passing.

A literature search

has revealed several types of detectors for electrons and positive ions of approximately thermal energies. the two most widely used are,

Of these

(l) the electrometer method,

where the positive ions or electrons are drawn to a plate of appropriate potential and subsequently measured by sensitive I O Q Ji , x current measuring devices, and (2 ) the space charge detector utilizing the effect of positive ions on the space charge limited electron flow in a diode.

It was thought

that the latter detector, if sensitive enough, might prove to be the most appropriate for the proposed measurements of the photoionization in gases.

Its amplifying character­

istics, together with the fact that spurious electrons will

1 H. Wayland, Phys. R e v . 52, 31 (1937)2 R.M. Sutton, Phys. R e v . 33, 364 (1929). 3 R.M. Sutton and J.C. Mouzon, Phys. Rev. 35, 694 (1930). ^ E.O. Lawrence,

Phil. M a g . 50, 345 (1925).

5 K.H. Kingdon, Phys. Rev. 21, 408 (1923).

not be measured, gave considerable weight to this choice. The experimental data presented here concern themselves with the direct calibration of the detector in terms of numbers of positive ions produced per second, or numbers active in the detector at any instant.

Estermann and Stern®*

investigated the sensitivity of the “Kingdon Cage” as a function of gas pressure in order to apply it to molecular beam studies.

Varney and others®* 9 *10* H *12 usec3 this

device to study ionization in gases and vapors by acceler­ ated alkali or other atoms. i

-3 i 4 ir

Lawrence J

Mohler, Foote, and

used the same technique to measure photo­

ionization in alkali metal vapors.

6

I. Estermann and 0. Stern, Zeit. f. Physik 8p, 135 (1933). 7 I. Estermann, R e v . M o d . Phys. 18, 31^ (1946). 8 R.N. Varney, Phys. Rev. 50, 159 (1936); Phys. Re v. 47, 483 (1935); Phys. R e v . 50, 1095B (1936). 9 H.W. Berry, Phys. R e v . 62, 378 (1942). Berry, Varney, and Newberry, Phys. Rev. 6l . 63 (1942). •*■1 Varney, Gardner, and Cole, Phys. Rev. 52, 526 (1937)• 12 Varney and Cole, Phys_. R e v . 50, 26l (1936). 18 Mohler, Poote, and Chenauit, Phys. Rev. 27, 37 (1927). Foote and Mohler, Phys. R e v ■ 2 b , 195 (1925). 18 Lawrence and Edlefsen, Phys. R e v . 34, 233 (1929).

CHAPTER I GENERALIZED DISCUSSION OF THE SPACE CHARGE DETECTOR An Investigation of the general case of the motion of a positive ion in an electric and magnetic field between coaxial cylinders was made by Hull

16

in 1921.

In 1923*

Kingdon1^ applied this to the case of zero magnetic field, and performed experiments showing the effects of positive ions on space charge limited currents in cylindrical diodes. Because of the lower mobility of the positive Ion in com­ parison to an electron in an electric field, it effectively neutralizes many electrons as it moves in the electric field of the diode.

Since this action causes a rather large

increase in plate current of the diode this mechanism is utilized as a positive ion detector. When a positive ion finds itself in the field of a cylindrical diode, it will generally make many trips around the filament in a spiraling motion before it will be finally captured by the cathode filament.

This has been explained

by the fact that the positive ion initially has a thermal velocity component transverse to the radius of the cylindri­ cal anode so that the positive ion when accelerated toward

16 A.W. Hull, Phys. R e v . 18, 31 (1921). 7 Kingdon, op. clt., p. 409.

the cathode will miss the filament and spiral around it.-^ Eventually it will be lost from the diode volume by any one of four p r o c e s s e s N e u t r a l i z a t i o n charge at the

of its positive

cathode filament, (2 ) escape through the ends

of the cylinder,

(3) recombination with electrons, (4) by

collisions with neutral molecules which reduces the positive ion energy and leads to neutralization at the cathode. Processes (l) and (4) cause the most serious losses of positive ions.

It is because of this spiraling motion of

the positive ion and the subsequently longer period of time it remains within the space charge region, that large ampli­ fication factors have been achieved.

In helium at a

pressure of 10"^ m.m. a positive helium ion will spiral 200 times around the filament and thereby neutralize as 21 many as 50,000 electrons. The effectiveness of the positive ions

in neutralizing space charges in theplane

parallel case

isin inverse ratio of their velocity to

of an electron.

22

Kingdon,

that

If it is assumed that both have equal

ojd.

cit., p. 409.

19 Ibid., p. 412. L.B. Loeb, Fundamental Processes of Electrical Discharge in Gases (New York: John Wiley and Sons, Inc.), p. 32o. 21 Loeb, op. cit., p. 327. 22 Ibid., p. 325.

6 energy, then this effectiveness is measured by the square root of the ratio of the masses; m. /m , . A s can be iom electron seen from the helium example given above, the spiraling motion results in a much greater neutralizing effect than that given by the above expression. It is possible to increase further the effectiveness of the neutralization by the positive ions, thus Increasing 23 the sensitivity of the diode as a detector. Kingdon has pointed out that this may be done by decreasing the chance of a positive ion being captured, either by making the ratio of anode to cathode radius as great as possible, or by elevating the temperature in the tube and thereby increasing the transverse velocity component of the positive ions. Greater sensitivity also may be obtained by increasing filament temperatures. Loeb

24

This effect has been explained by

as apparently due to the electron space charge cloud

which forms a negative potential through around the filament acting as a trap for the slower positive ions.

Knowledge of

the possible losses may also allow one to increase the sensitivity of the detector by proper design.

In order to

realize the utmost in sensitivity in the detector, it is usually used In one arm of a balanced bridge circuit with a

23

Kingdon, op. cit., p. 412.

oil

Loeb, op. cit., p. 328.

7 similar diode in the other arm.

The purpose of the second

diode is to compensate for small amounts of contaminating foreign gases or vapors.

They may ionize,

causing changes

in positive ion density; or they might attack the filament, causing a change of work function of the cathode changing the emission current.

These contaminants would affect

strongly the balance of the bridge if their effect were not canceled out by the other diode. An electron gun was used to accelerate thermionic electrons through a known voltage into the space charge region of the positive ion detector, where the pressure of the nitrogen was known, and in conformity with K i n g d o n 2 ^ the temperature was assumed to be 300°K. accelerated electrons,

This beam of

on passing parallel to the diode

filament through the detector, undergoes collisions with gas molecules and produces a known number of positive ions. These will cause a deflection in the null indicator of the bridge circuit proportional to their number. The efficiency of ionization by electrons of known energy in nitrogen was measured by Compton and Van Voorhis whose data are reproduced in Figure 1.

2b

To calculate the

number of positive ions present within the detector diode

25 Kingdon,

op. c i t ., p. 417.

Compton and Van Voorhis, P h y s . R e v . 2 7 * 729 (1926).

NO. OF

CHARGES

PER ELECTRON PER M.M. PRESSURE

120 160 200

VOLTS

240 280 320 360 400

at any one time, a number of assumptions has to be made. The first assumption is that under equilibrium conditions in the detector every electron measured produces the same number of positive ions.

This will be true, on the average,

because there is a large number of electrons in the beam. The second is that the electrons formed when ionization occurs are not measured with the ionizing or beam electrons. This may not be strictly true, however.

Since the electrons

formed in the ionization process have little or no energy, they become associated with the space charge density of the diode rather than with the accelerated electrons.

The third

assumption is that equilibrium between the positive ions formed and lost is established in 10“^ seconds. correct in order of magnitude only.

This is

Kingdon2^ calculated

the time necessary to establish equilibrium by dividing the mean free path of the ion by the velocity it derived from the field in the diode.

He showed in the case of mercury

that this calculated time agrees in order of magnitude with experimental data obtained by ascillographic means. experimental conditions used in this work were:

The

(l) An

average nitrogen gas pressure of 8.8X10“^ m.m. of mercury, (2 ) a gas temperature of 25° C ., (3) 8 volts as the plate potential.

Using the above method one could calculate the

2^ Kingdon, op. cit., p. 4l6.

10 time necessary to establish equilibrium of the positive nitrogen ions, and one obtains 8.55X10“^ seconds.

This

time is probably correct only as to order of magnitude. The number of positive ions formed in this time is actually the number of ions active in the neutralization of the space charge within the detector.

It is realized that since this

work was done at several different pressures, the time for the establishment of equilibrium is not truly constant since the mean free path is certainly pressure dependent.

For the

moment, however, we shall assume that it is a constant value and later discuss the pressure dependence of the results.

CHAPTER II EXPERIMENTAL PROCEDURE The vacuum system used was of the dynamic type using a four inch oil diffusion pump.

Nitrogen pressure was

adjusted in the main chamber containing the electron gun, the positive ion detector diode, and balancing diode, with a needle valve.

Oxygen and water vapor are the most active

contaminants likely to be present.

It is obvious that both

of these would cause fluctuations of emission, reduction of filament life, and possible electrical shorts between leads due to layers of tungsten on the inside of the vacuum chamber.

These undesirable contaminations were removed by

using a purification train between the main chamber and needle valve.

The nitrogen was first made to flow through

a liquid nitrogen cold trap to remove water vapor, then over copper pellets at a temperature between 450 and 500° C . for oxygen removal.

The gas was then passed through another

liquid nitrogen cold trap to remove any water vapor that might have been formed If there had been a small amount of hydrogen present.

Nitrogen pressure was read directly at

the main chamber by means of a thermocouple gauge. In the construction of the detector and balance diode the ratio of anode to cathode radius was made as large as practicable, since it was known that the detector sensitivity

increased with this ratio.

The positive ion loss from the

ends of the diode cylinder was minimized by using end plates.

The detector and balance diode anodes were made

of 5-mil nickel sheet rolled into cylindrical shape 2.5 cm. in diameter and 3*5 cm. long.

The balance diode had end

plates constructed out of 5-mil nickel sheet in the form of caps which were held in place friction tight on the anode cylinder.

The detector diode had end plates constructed in

the same manner; one cap was fastened friction tight to the cylindrical anode and drilled .625 cm. from its center with an .052 inch diameter hole to allow entrance of the ionizing electron beam.

The other cap for the detector was mounted

separately, and since it was slightly larger, covers the end of the cylinder without making electrical contact with it. All end caps were drilled with a .116 inch diameter center hole to allow entrance of the nickel support wires which the 5-mil tungsten filament was spot welded.

It was anticipated

that leakage resistances of the order of 10^2 to 1 0 ^ ohms might be present.

It was therefore necessary to keep the

emitting portion of the cathode within the cylinder end plates to prevent stray currents from flowing to other portions of the electrode system. Figure 2 has a scale drawing of the electron gun which shows the position of the elements in relation to each other and to the detector diode.

(A) is a hair pin filament

13

® i 03335

W V W V W N A

12 VOLTS

ELECTROMETER TUBE CIRCUIT

2 VOLTS

I

FIGURE 2 ELECTRON GUN AND ASSOCIATED CIRCUIT

14 made from 15 -mil tungsten wire which has been thinned to 7-mil at its tip by electrolysis so that the emission from the filament would be confined to the tip.

(B) is a nickel

shielding plate which is connected electrically to the positive end of the gun filament and protects other chamber parts from receiving electrons from the gun filament.

(C) is the first and (D) is the second accelerating anode both made from 5~mil nickel sheet.

The second accelerating

anode was made large enough to cover the detector and plate to protect it from electrons from the gun filament. Tungsten-pyrex glass seals to which 50-mil nickel wire had been hard soldered were used for mounting the chamber parts.

The electron gun and detector diode were

arranged on the inside part of a 103/75 spherical ground glass joint.

The glass area aro na the seals was kept free

of a conducting layer of evaporated tungsten by using glass aprons over the lead stems.

The mounting of the electron

gun and the detector diode on the spherical joint together with the glass aprons can be seen in the photograph of Figure 3 .

The balance diode was fastened to a 55/50 tapered

ground glass joint. The schematic diagrams of the bridge and electron gun circuits are given in Figures 4 and 2 respectively.

It

should be noted that the gun circuit is not electrically linked with the bridge circuit so that trouble from leakage

FIGURE 3 ELECTRON GUN AND DETECTOR DIODE MOUNTING DETAIL

16

DETECTOR DIODE

NULL DETECTOR

B A LA N C E DIODE 12 VOLTS

RiGURK

BRIDGE CIRCUIT SCHEMATIC

17

currents between the two could be kept to a minimum. The null detector in the bridge circuit was a Rubicon galvanometer with a sensitivity of .OOO69 microamperes per m.m. scale division.

An Ayrton shunt was used in con­

junction with the galvanometer to extend the useful range to larger currents. qQ

A type 95^ tube connected as an electrometer tube was used to measure the ionizing beam current.

The

schematic of this circuit is given in Figure 5 .

Small

emission currents could be measured, limited only by the grid leakage current of the electrometer tube, if proper valued grid resistances were available.

Since the charac­

teristic curves are dependent on local conditions in the room and the electro-motive-force of the battery supply, it was necessary to determine them each day.

To keep grid

leakage currents small the tube and its porcelain socket were cleaned before use with carbon tetra-chioride. Determination of the grid current was made in the following w a y :^9

First, the grid voltage vs. plate current character­

istic was obtained; second the grid was given its potential through a series resistance of the order of 1012 ohms and

28 Carl E. Nielson, Rev . Scl . Inst. 18, 23 (19^7). ^ j. Strong, Procedures in Experimental Physics (New York: Prentice-Hall, Inc.), p~ 428.

954

O

PA>

+6V.

130 OHMS

2 0 OHMS GRID 2 OHMS

RESISTOR

Oo

TRS

ELECTROMETER TUBE CIRCUIT S C H E M A T I C

19

the grid voltage vs. plate current characteristic was again measured, the grid voltage being read directly across the source of the bias voltage; third, the grid current was determined by taking the voltage difference between the two plotted curves at that value of the plate current chosen, and divided by the value of the series high resistance used in step two.

Characteristic curves of the 95^ tube used

for calculation of the data and for determination of the grid current

are reproduced in Figure 6.

Such curves as those

make it

possible to determine an operating range of the

plate current so as to keep the grid current below the current to be measured by any factor desired.

The grid

voltage

vs. plate current characteristic is also used to

convert

the plate current readings into emission currents

when a given value grid resistor is used. Extreme precautions had to be used in treating the insulation problem inherent in all low current measurements. All batteries, meters, and resistance boxes were set on sulfur plug insulators.

Panel switches and rheostats were

first attached to polystyrene disks, which were in turn set into the panel.

Polostyrene extension shafts were made for

the panel controls so that the apparatus could not be grounded through the operator.

Since the ordinary cotton

insulation was insufficient, connecting wires that were to be shielded by flexible cable were first strung with

120

110

954

100

90

BIAS VOLTAGE

70

60

CURRENT

80

50 9.4X ICf OHMS 40

30

20

-3 .0

-2 .5

X

x

X

-3 .5

-

2.0

GRID

-

1.5

VOLTAGE

i_ -

1.0

-0 .5

PLATE

954

21

Stupakoff beads to improve the insulation.

For the same

reason, hook-up wiring was made to support itself between tie points and was not allowed to touch any other objects along its length. The control table, main vacuum chamber, batteries, and external connections between the components were completely shielded to prevent alternating current pickup. Since the connections from the control table to the main chamber had to be well insulated and also shielded, a co­ axial line was made from 1 inch brass tubing and l/l6 inch brass rod insulated from the outer tubing and supported axially in it by machined sulfur end plugs.

CHAPTER III DATA AND RESULTS It Is the principal aim of this work to obtain the sensitivity of the detector diode In terms of parameters shown In Figure 7, where the bridge galvanometer deflections are plotted as ordinates and the number of positive Ions present or the number of positive ions gener­ ated per second is plotted as the abscissa.

This latter

number could be varied by changing the number of accelerated electrons entering the detector by adjustment of the gun filament temperatures or by changing the applied gun ac­ celerating voltages.

The procedure in obtaining the data

as plotted was to apply in turn each of the five accelerat­ ing voltages for each gun filament temperature.

For each

voltage then the electrometer tube plate current and de­ flection of the bridge null detector were noted. To complete the data, five or six different gun filament temperatures were used until the usable range of the electrometer tube circuit for emission current measurements was fully utilized.

Electron currents obtained in this

manner were then used to calculate the number of positive ions effective in the space charge detector as has been described above, so that the calibration curve can be plotted as in Figure 7«

The calibration curve as it appears

7.6 x 8.2x 92 x 1.0 X

icf l