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This is to certify that the

thesis entitled
“An Expansion Cloud Chamber

for
Demons trati on“

presented by

Winston Harold Hencveld

has been accepted towards fulfillment
of the requirements for

Adamo 111%

fez“...—

Major professor

Date W—

“ .1. W_._._ - .
l

 

 

 

 

PLACE IN RETURN BOX to remove this chedcout from your record.
TO AVOID FINE retum on or before date due.
MAY BE RECALLED with earlier due date if requested.

DATE DUE DATE DUE DATE DUE

-

 

AN EXPANSION CLOUD CHAMBER FOR DE‘EONSTRATION

by

‘Winston Harold Heneveld

A THESIS
Submitted to the School of Graduate Studies of Michigan
State College of.Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

MASTER OF SC IENCE

Department of Physics

1952

The author wishes to express his sincere appreciation
to Dr. J C. Lee for suggesting the problem and assisting in its
execution.

He is also indebted to Mr. Albert Smith for the
valuable advice and aid which he rendered on the photographic
problems which were encountered during the course of the

investigation.

Table of Contents

Page

I. Introduction and Historical Review . . . . . . 1
II. Dmnonstration Cloud Chambers . . . . . . . . . 8
III. Design of the Chamber . . . . . . . . . . . . 10
IV. Operation of the Chamber . . . . . . . . . . 17
V. mta....................28
VI. Summary . . . . . . . . . . . . . . . . . . . 36

VII. BibliograPhYOOOOOOOOOOOO0.00.37

I. Introduction and Historical Review

The Wilson cloud chamber has played an important role in

the development of modern physics. It enables one to photograph
and study the canplicated interactions taking place between individ-
ual atans, nuclei, and charged particles. The operation of the
cloud chamber may be stated briefly as follows. A definite volume
of vapor-saturated, dust-free gas is adiabatically expanded to
produce a super-saturated state. The excess vapor then condenses
on ions as nuclei forming a visible trail along the path of the
ionizing particle. While Wilson's original design has been modi-
fied and developed to suit different investigations, all experi-
mental chambers still operate on this basic principle. The majority
of cloud chamber researches has been carried out with the idea of
understanding and improving the technique of the Wilson Chamber.
The ultimate goal is a chamber which is capable of producing sharp
undistorted tracks which can be easily photographed. Sane of the
more prominent adaptations of Wilson's original design are descri-
bed below.

Wilson:l originally utilized a piston as a movable chamber
floor to produce a sudden volume expansion. The rapid motion of
the piston was accomplished by connecting the rear of the chamber

to a highly evacuated vessel. (Fig. 1) Although this gave excellent

tracks the procedure was much too slow to be used in investigating
nuclear phenomena. Therefore, the first step was to develop an
automatic chamber which would be capable of producing repeated

expansions.

 

 

 

 

7L

W 5 IF U

i a- Piston - To Pom
c- Wooaa blocks to reduce air space

0' Rubber shook mounts for piston

E-l Evacuated vessel

F-AIr Intake tor raising pisba to IaItIaI position

 

 

 

 

Fig. 1. Wilson's original cloud chamber.

Shimizug designed a reciprocating piston chamber which
operated in continuous cycles. However, the relatively slow expan-
sions resulted in indistinct tracks. There were many modifications
of this type of chamber. The most important was Blackett's model
in 19275. By replacing the reciprocating piston with a more rapid
spring action, he succeeded in obtaining sharp tracks with a repeat-

ing mechanism. All the cloud chambers used before 1955 were of the

type in which a definite volume change was produced by the motion
of a piston or plunger forming the floor of the chamber.

.An hmportant change in the method of producing expansion
was made by'Wilsonh'when he replaced the rigid piston with an
elastic diaphragm. Compressed air admitted to the back of the
chamber forces the diaphragm inward and raises the pressure in
the working chamber. Expansion is effected when this air is
released to the atmosphere and the elastic diaphragm returns to
its original position. There are several advantages to this type
of chamber. It is simple to construct; it can be operated in any
position; and it is easily modified for the inclusion of various
automatic features. Since the diaphragm is elastic a rise in temp-
erature in one part of the cloud chamber, due to heat conduction
frcm the walls or condensation to form tracks, does not cause
simultaneous compression and consequent rise of temperature in
the rest of the chamber. Thus a longer sensitive time exists in
which supersaturation and condensation can take place.

To solve the problem of small leaks tahl, Halstead, and
Tuve5 added sylphon bellows to produce a permanently sealed chamber.
The bellows can be expanded or contracted.mechanically to adjust for
changes in the expansion ratio. This type of chamber had less tur-
bulence than any of the earlier types.

C. T. R.‘Wilson and J. G.‘Wilson6 deve10ped the radially
expanding chamber. Expansion is produced by a sudden reduction of

pressure in the annular space by means of a valve which opens to the

-h-

atmosphere. Then with the chamber inserted between the poles of
the magnet the tracks may be illuminated from the rear. This
simplifies the problem of photography.

One of the chief factors in track distortion is the
effect of gravity on the drops. This is particularly noticeable
in the case of the thin, beaded tracks of electrons which dis-
integrate almost immediately after they are formed. Wilson and
Wilson6 have also tried to develop a falling chamber in which
both the chamber and the camera fall freely under gravity. Since
distortion due to the effect of gravity on the drops relative to
the gas in the chamber is eliminated the time of photographic
exposure and the interval between expansion and photographing
can be greatly increased. The falling chamber was never used
extensively because of the mechanical difficulties involved.

There have been several attempts to construct a contin-
uously sensitive cloud chamber. Vollrath7 produced continuous
supersaturation by allowing HCl and H20 vapors to diffuse.
Brinkman8 constructed a continuously running chamber which makes
several expansions per second. The most successful solution to

the problem was one due to Langsdorfg, He allowed a warm, satura-

ted vapor to diffuse downwards through a non-condensible gas into
a refrigerated space. 'When the vapor has cooled sufficiently,
supersaturation and condensation on ions takes place. The depth
of the sensitive layer depends on the height of the chamber. Best

tracks are Obtained when the temperature gradient between the roof

-5-

and the floor is fron 3° to 7°C per centimeter. This is usually
done by placing warm water at the top and dry ice at the bottom

as shown in Fig. 2.

 

LW'WL

 

 

 

 

 

L o

A! Heat Reservoir

BI Vapour Source

c- Sensitive Layer

0* Low Temperature Sill:

 

 

 

 

Fig. 2. A typical continuous diffusion cloud chamber.

A highly significant step in the development of the cloud
chamber has been the introduction of the counter-controlled chamber
by Blackett and Occhialinilo. The chamber is normally under compres-
sion until a cosmic ray or ionising particle passes through a series
of counters placed above and below the chamber. The resulting co-
incident discharge of the counters excites the grid of a thyratrom
which short circuits and trips the expansion and light circuits.

The chamber is recanpressed and camera reset by means of cams so

that the entire operation is completely automatic. More than
80 percent of the photographs taken with counter control contain
tracks.

RandanLy operated slow chambers are still used occasion-
ally to obtain statistical evidence regarding cosmic ray activity.
A chamber is made slow, that is, having a large sensitive time, by
increasing the depth. Chambers 50 to 60 centimeters deep have been
constructed having sensitive times ranging from one-half second to
over one second. 'With a slow chamber it is possible to use a longer
exposure time with less light and to photograph more tracks per
expansionll.

Two other modifications are the high and low pressure
chambers. Low pressure chambers are particularly useful in record-
ing emissions such as fission fragments whose normal range is only

a few'millimeters at atmospheric pressures,

High pressure chambers are used.when a long equivalent
path length or high absorption is desired. Thus it is possible to
have an equivalent path length of 200 feet of atmospheric argon in
a chamber only thirty centimeters in diameter. This is important
in observing such phenanena as the disintegration of the meson at
the end of its range, the Heisenberg explosion type of shower forma-
tion, or neutron energies. High pressure chambers have become
increasingly important in observing nuclear effects excited by
artificial radiations from cyclotrons and betatrons. The first

high pressure chamber, designed to measure neutron energies, was

-7-

operated at pressures up to 50 atmosphereslz. one of the later
additions has been that of Johnson, Benedetti, and Sohutt13 we
designed a hydrostatically supported chamber 30 centimeters in
diameter and 9 centimeters deep, that was capable of operating at
pressures up to 300 atmospheres.

Further refinements of cloud chambers in recent years
have been largely confined to the develognent of portable models
for use in cosmic ray research in the upper atmosphere. However,
the most recent innovation (1951) has been the integration of an
ionization chamber and a cloud chamber for selective studies of the
cosmic-ray nuclear interactions in which heavy ionising particles
are produced. An ionization chamber placed within the cloud chamber
and utilizing a canmon gas is used to trigger the expansion of the
cloud chamber (Fig. 3). Then by removing the high potential after
the electrons have been collected and before the positive ions have
moved appreciably, it is possible to observe the path of the pulse-
generating particle within the collector. Geiger counters and
proportional counters have also been used successfully in a similar

mmerlhe

 

 

 

 

 

 

A' Compression Chamber

8' Viealng Chamber

0 ' Ionization Cylinder Well (‘ I800 volts)
0' Collecting Wire (Grounded)

 

 

Fig. 3. The cloud - ion chamber.

II. Ebmonstration Cloud Chambers

To function successfully as a demonstration device a
cloud Chamber should be relatively simple to construct and Operate
and should be portable and self-contained as much as possible. In
addition, it should provide the observer with an idea as to the
principles of cloud chamber Operation and be representative of the
chambers currently in experimental use.

An expansion type chamber meets the above requirements.

Its one disadvantage, the rather lhnited viewing angle, can be
eliminated by a projection system if large groups are to be encounter-
ed. Once this type of chamber is set up it will continue to function
‘without adjustments for long periods of time. ‘Another advantage is
the elimination of cumbersome heating and cooling agents. The effect
of various factors on the degree of supersaturation can also be
clearly demonstrated. The expansion chamber to be described later

has worked quite successfully as a demonstration device at the
University of Minnesota)"5 .

The continuous diffusion cloud chamber has recently become
quite popular as'a demonstration device because of its simplicity and
low cost of construction. No timing problems are involved and the
viewing angle is much larger than that of the expansion chamber with-
out the need for a projection system. Cosmic rays can also be obser-
ved without using any of the complicated counting arrangements.

However, clearing with electric fields and insertion ofznetal plates

do disrupt the operation sanewhat. One other disadvantage is the
limited depth of the sensitive layer. The continuous diffusion
chamber is primarily a demonstration device and has not been used

f-

as yet in any serious experimental work.

- 10 -

III. Design of the Apparatus.

Designed for use in lecture demonstrations, this equip-
ment is portable and self-contained except for the power and air
supply. Since all operations are automatic, the unit will function
without supervision once the initial adjustments are completed.
‘With its twelve inch diameter the chamber has an ample viewing
angle for small classroom groups without exceeding the space

requirements for a compact unit. An exploded view and a scale

drawing appear in Figures h and 5, respectively.

 

Fig. h. Exploded view of the cloud chamber.

 

-11..

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

v
s

L """“"“ ..

 

 

 

 

A ’ B C D E F
A8 Back Plate and Valves E=Front Plate
8 3 Compression Chamber F 8 Retaining Ring
Cs Transition Plate
0: Viewing Chamber Scde 2": 1'- o"

 

Fig. 5. A side view scale drawing of the chamber canponents

Conpression Chamber.

 

A 12 x 1% inch brass ring and a diaphragm of two pieces
of l/6h inch gum rubber dam comprise the cmpressicn chamber which
is capable of producing a maximun expansion ratio of 1.31. It was
also necessary to place a piece of amber 18 mesh screen on both
sides of the canpression chamber to prevent the diaphram from
being forced into the holes and valves during the compression and

equilibriun periods .

_. w—v—

All gaskets throughout the chamber are of one—quarter

inch pure gum arabic rubber.

 

Fig. 6. Escape valves and back plate

Thelthree electranagnetic escape valves (Fig. 6) each
have a winding of 2,000 turns of #30 copper wire and require 12
to 16 volts d.c. for successful operation. Although the valve
is returned immediately after expansion to its normally closed

position by a concentric spring, the compressed air continues to

escape until the valve current is applied.

- 13 -

Viewing Chamber.

The aluminun transition plate at the back of the viewing
chamber is covered with black velvet to reduce turbulence and to
provide a suitable photographic background. .Approximately 200
half-inch holes were drilled in this plate leaving a three-quarter
inch strip around the edge intact. A hole is also drilled through
the edge and a nozzle added to accomodate a manometer tube. Then,
with a sponge fixed on the glass cylinder directly in front of this
hole, the solution can be added without Opening the chamber.

Clearing the chamber of ions is accomplished by means
of 300 volts d. c. applied to an aluminum foil ring and a #36
tungsten wire cemented to the front glass plate. The wire and
a tab of foil project outside the chamber to serve as the positive
connection while the aluninum plate and chamber chassis serve as
ground. The voltage doubling circuit shown in Fig. 7 is connected
directly to the 110 volt main switch to provide a convenient voltage

source.

 

 

 

 

 

A D
. = \1
us volts 4 —L c +
no c T ‘ 300 volts
1‘ 70_r\_ .1. 0 0° _ '
. T l

 

_ A-IZOv Selenium Rectitier
i auto [to

 

Fig. 7. Voltage doubling circuit used to obtain
the clearing field voltage.

Mounting Arrangement .

 

 

Fig. 8. Front view of the canpleted unit

 

‘ ‘O- ‘-

BURCE‘ES

 

Fig. 9. Mounting of the chamber with accessories

Figures 8 and 9 show the mounting of the canplete unit.

- 16 -

 

Fig. 10. Rotary timing switch.

The timing control for operation of the light and valve-
relays utilizes a one r.p.mc synchronous motor as a rotary switch.
The upper bronze strip, mounted on the plexiglass base as shown in
Fig. 10, serves as a contact in the illuminating relay circuit and
is adjusted to close the circuit two seconds before expansion and
to open the circuit five seconds after expansion. The lower strip
is a contact in the valve relay circuit and provides for a 21
second compression time and a 39 second recovery period to allow
the chamber to attain temperature equilibrium before the next

expansion.

- 17 -

IV. Operation of the Chamber.

Procedure. .

 

The following procedure should be followed if the
chamber has been opened. First the glass surfaces are washed
thoroughly with ethyl alcOhol to remove any grease and other
impurities that may be present. The chamber is then assembled
and evacuated. Helium is added to a pressure of one atmosphere.
After adding about ten cubic centimeters of solution and allow-
ing a few minutes to elapse for the helium to become vapor sat-
urated the chamber will be ready for operation. A few preliminary
expansions are usually sufficient to wash down any suspended dust
particles in the chamber. Compressed air is now admitted to the
rear of the chamber to provide a pressure of 9 cm Hg upon ex-papcomprmfi...
(4M helm". ashram».
s-i-en'.A Then for each succeeding expansion the pressure is increased
until best tracks are obtained. ‘With a solution of 65 percent.
ethyl alcohol and 35 percent water in helium at 22°C this should
occur at about 10 (27%) cm.Hg. The critical pressure will then

remain at this point until the operating temperature of the chamber

changes.

Parameters 2: Operation.

 

Since the principle of the cloud chamber is based upon
a condition of supersaturation and condensation of vapor on ions,
it is important to recognize the factors involved in producing this

condition and their effect on the overall efficiency of the chamber

-18..

in producing sharp tracks with a minimum background. The brief
development below will serve as the basis for a discussion on the
variation of supersaturation with temperature, expansion ratio,
vapor, and pressure.

It is assumed that just prior to expansion there exists
a non-condensible gas at a pressure, PE, and a vapor at a pressure

P1, contained in a volume, V1, at a temperature, 01, Then

<1) 40‘,” ("%~)Re,

where M1 : total mass of vapor present in V1 and M 3 the gram
molecular weight of the vapor.

The gas undergoes a rapid expansion to a new volume
V2 with a resultant drop in temperature from 01 to 02' which

can be expressed in the adiabatic relation
4-/

<2) Me; = ( ‘4/14)

where ’7’ is the ratio of the specific heats of the gaseous
mixture in the chamber. Before condensation takes place, the
original mass of vapor, M, is distributed over the new volume

I
V2. Therefore, the pressure P2 is given by

m f’vz = (M) Re;

- 19 -

At this lower temperature, 92', less vapor can be held in suspen-

sion so condensation takes place reducing the mass to M2. When

equilibrium is again attained, the vapor pressure falls to P2,
the saturation pressure at temperature 02. The slight difference

between the temperature immediately after expansion, 92" and the

equilibrium temperature, 92, can be neglected so

(MP r-(M/m) Re

after condensation. The vapor density, 6;: , after eXpansion

and before condensation when M1 is contained in V2 is

Q; = ”W.

but after condensation the vapor density is

(6) a =- Mat/v,

By definition supersaturation, S, is the ratio of the density of

the vapor to the saturation density at the same temperature. Then

(7) S .=: eel/(If ”flag: 8K9‘/ v

I.

‘B

and from (3) and (h)

(a) 5= (azaélflfi/e.)

-20..

'7"/
However, 92; 51’ and from (2) 9/31, = (Vt/f)

<9) 3" V: (“/V. )1 8/3. ( fl’)’,

where E 5 expansion ratio :- the ratio of the volume after
expansion to the volume before expansion. The ’Y for a composite

mixture has been given by a formula due to Richarz as

(10) XI-l = /4’—/ 43/,” 1" /ry;,-/ . E'AT

where’ff 3 the total pressure 3 Pg+ Pv 3 the subscripts g and
v refer to gas and vapor. Now if the expansion ratio is fixed
then the variation of ’7 because important in determining the
value of supersaturation and it is, therefore, necessary to examine
those factors which will influence the of the mixture present
in the chamber.

Reference to Table I shows the variation of S with the
non-condensible gas and vapor. The following conditions apply
for each case: E : 1.25, T1 :- 20°C, P1 : 1.75 cm Hg,

: 1.30, and the total initial pressure : 1.5 atmospheres.
Table I clearly shows the advantage of using a monatcmic gas
as the supersaturation produced means lower expansion ratios

are necessary to produce condensation on ions. The advantage

of an air-water mixture over air-alcohol mixture is also shown.

 

 

Hq.m mn.m m.mmm am.a mo mmmo oaa
m.m no.4 n.mam Hm.H 0mm moo
m.n am.m m.aom 0:.a 0mm tea
o.mH 55.0 m.mmm ce.H 0mm a

we as
m ow Na mo \\ song use

 

 

 

 

 

 

 

Gavanomm ZOHH¢MDa¢mmmmbm $24 Maw mo amaaaz

H mama;

-22..

It has been found experimentally by Beck16 that the best solution

to use if air is the non-condensing gas is 65 percent ethyl

alcohol and 35 percent water by volume. Several other investiga-
tors have obtained good results with solutions ranging from 60
percent alcohol - LO percent water to 70 percent alcohol - 30 percent
water. It is generally agreed that the best solution is an alcohol-
water solution lying in that range rather than solutions of pure
alcohols or other organic liquids.

Equation (1) also indicates the variation of super-
saturation with gas pressure. If the total pressure 1T is
increased by raising Pg then fif/fl decreases while 5/{
remains practically the same for Pg >> Pv’ This increases/y
and, from (2), the drop in temperature is greater and the result-
ing value of supersaturation is increased. The increased pressure
also means an increased number of ions will be formed per centi-
menter of track length which in turn means heavier tracks. Less
illumination is then required for successful photography. This
advantage is somewhat offset, however, by the problems of leakage
and construction that accompany high pressure work.

An increase of initial temperature will result in a

decreased temperature drop and lower supersaturation for a given

expansion ratio since the increase in Pa/h/ 'will be more than

Pg/h' , if V1 and ‘TT are kept constant, resulting in a lower
value for ’7/ . There are other advantages in operating the

chamber at lower temperatures which will be discussed later in

-23-

connection with turbulence.

The most important single factor in the production of
'a supersaturated state is the expansion ratio. An increase in
expansion ratio will raise the value of supersaturation for all
liquids, mixtures, and gases. Therefore, regardless of the initial
temperature, pressure, solution, supersaturation can still be
attained by going to larger values of expansion ratio. ‘Wilson
found that an expansion ratio of 1.25 (S : L) with an air - water
vapor mixture condensation takes place on ions as centers (ion
limit) while at E : 1.37 (S : 8) a cloud-like condensation fills
the chamber (cloud limit) using vapor molecules as centers. To
produce a value of S e h.using an air-alcohol mixture an expan-
sion ratio of 1.29 is required. Beck found best tracks with a
minimum expansion ratio of 1.125 for 65 percent ethyl - 35 percent
water. Later, using a 50 percent ethyl alcohol - 25 percent
acetone - 25 percent water solution a minimum expansion ratio
of 1.112 was found to produce even better results. The majority
of cloud chambers operate at expansion ratios of less than 1.15.

The problem of compression time must be considered in
any discussion of cloud chamber operation. It has been found
experimentally that for the chamber under consideration a minimum
compression time of 18 seconds is necessary in order to reach
temperature equilibrium before expansion. If expansion takes
place before 18 seconds the resulting value of supersaturation

is not sufficient to produce tracks. It has also been found

-21;-

advisable to use a pressure regulator valve in the compressed
air line. The pressure in the chamber will then build up
quickly to the critical point and be held there until expansion.
Maintaining the original volume, V1, for about 18 seconds reduces
the background fog and yields sharper, more well defined tracks.
The use of a regulator valve also eliminates the problem of small
fluctuations in the compressed air line pressure. Each expansion
will then take place at exactly the same pressure each time.
Since the valve can be set quite accurately, the problem of
locating the critical pressure is simplified.

The problem.of turbulence and track distortion is very
important in counter-controlled and “slow" chambers and, to a
limited extent, in demonstration chambers. For best viewing the
tracks should be relatively undistorted for approximately one-
half second. The introduction of the copper screen, perforated
plate, and velvet backing to reduce turbulence has been.mentioned
earlier. One should employ a source holder which presents a
minimun amount of surface area in the direction of expansion and
place the exhaust valves symmetrically with respect to the central
axis of the chamber. It is also advisable that the initial con-
ditions (solution, non-condensible gas, temperature, pressure)
be such as to require the lowest possible expansion ratio necessary
to produce good.tracks.

Convection currents are a sizeable factor in track dis-

tortion. Immediately after each expansion the gas near the walls

-25-

warms up first so that a convection current develops causing

a rapid fall of the central mass of colder gas. The forces to
which this motion is due increase with the third power of the

time, thereby introducing appreciable distortion within one-quarter
of a second after expansion. 'Water cooling, insulating, and
limiting the operation time of heat producing devices have proved
to be quite effective in reducing chamber distortion due to this
thermal.motion.

Since heavy, well defined tracks are desired in a
demonstration.chamber is is important that a consideration be
given to the emitting nature of the source. The ions formed by
an emitted particle begin to diffuse out immediately after emis-
sion. If expansion takes place within one second after emission
the resulting tracks are fuzzy and indistinct. If the rate of
emission is too high the clearing field is unable to clear the
chamber of previous ions and the background fog obscures any
tracks that are formed. A chamber 30 centimeters by 10 centi-
meters has a sensitive time of approximately one-tenth of a
second. Therefore, if an average of five good tracks per expan-
sion are desired the source should have an average rate of about
50 disintegrations per second. Also, for the sole purpose of
showing tracks an on anitter should be used.

Since the margin between the condensation on ions and
condensation on other particles is apt to be very small, the effect

of contamination on background fog must be considered. There are

-25-

several factors in cloud chamber practice which reduce the cloud
limit to the extent that it may actually be lower than the ion
limit or, as in most cases, that it may be lowered enough to pro-
duce an objectionable background fog at the ion limit. A survey
of these factors is entirely empirical in nature but the results
are worth noting as they apply to the majority of cloud chambers
C. T. R. Wilson concluded from a study on the effect of
metal surfaces that nuclei in the form of droplets containing
hydrogen peroxide acted as centers for condensation of a general
background fog. These nuclei were effective in the range of
expansion ratios from 1.252 to 1.38 which is the ion limit range
for air-water mixtures. The effect of these nuclei in expansion
ratios less than 1.25 is negligible. Amalgamated zinc is the
largest producer of condensation nuclei followed by polished
zinc and lead. It has been found that the base metals are a
definite source of contamination unless thoroughly oxidized. The
contaminating effect of aluminum.is as yet debatable. Rubber and
neoprene are clean materials when at rest or moving in constant
tension but contribute heavily to background fog if rapid changes
in tension occur, e.g., in receiving the shock of the piston. All
volatile materials such assealing materials and adhesives should
be thoroughly dried before the chamber is put into Operation.
Grease and oils must be carefully removed fran the glass surfaces

as they will Obstruct the view after a short period of operation.

17‘

-27-

Sane chemical action may produce contamination after the chamber
has been closed but this is easily removed by sweeping the chamber
with dry gas. The principal objective of the contamination study
has been to prevent the introduction of large molecules or molecular
aggregates into the gaseous mixture in the chamber. The ultimate
goal is to use the largest available degree of supersaturation in
the ion limit range with a minimum amount of background fog.

The usual precautions in mounting a radioactive source
must be followed to prevent the spread of radioactivity to other
parts of the chamber. Contamination of this type may be extremely
difficult to remove.

After each expansion and condensation it is found that
there exists some condensation nuclei on which further condensa-
tion can take place at very small values of supersaturation. These
nuclei are produced by the evaporation of large droplets and, if
present in quantity, can be removed by a series of two or three
slow expansions. The production of these re-evaporation nuclei
increases rapidly as the critical expansion ratio is exceeded and
a dense cloud is formed. In general, dust particles and re-evapora-

tion nuclei can be effectively removed by slow expansions and longer

recovery periods.

- 28 -
V. mta

Photographs were taken using an Exacta V, 55 mm. camera
with special lens adapter tube for close up work. All track
photographs were taken at f:h, .02 second, using Kodak Super XX
film. It was found that at longer exposures considerable track
distortion and blurring occurred, particularly in the case of
electron tracks of low density. Illumination was provided by
two photoflood lamps placed at an angle of approximately 60° to
the central axis of the chamber. Since the lights were operated
manually it was necessary to determine the exposure times by the
camera shutter. This was accanplished by means of a solenoid
attached to the shutter cable release and activated by the time
delay circuit shown in Fig. 11. There was a delay of .05 seconds

after the valve relay opened and expansion had started before the

shutter was opened.

 

Vein

. I
17—1 a...
! 5.3....

To Romy s-Ien W H
no.0 c .n. I

 

 

 

 

 

34/”
‘ Solenoid Role |
l _ va rec
1 L3.“ .
M-———'
8000'!“

 

 

 

 

Fig. 11. Time delay circuit used in operating
the camera.

-29-

Resu1t8 e

 

Fig. 12. 0\ tracks fran Polonium (Actual size)

The heavy, straight, sharply defined tracks resulting
from the Q emissions of Poloniun are shown in Fig. 12. Also
present are the wide feathery tracks'of emissions which took place
within approximately one second before expansion and have had time
to diffuse out before condensation. The extent to which these dif-

fuse tracks will appear is largely a function of the clearing field

- 50 -

voltage since the stronger voltage will collect the ions more
quickly thereby decreasing the maximum age of the tracks. There
is some evidence of curvature in two of the very diffuse tracks
due to the thermal motion of the gas near the walls of the chamber.
It will be noticed that in cloud chamber photographs in which the
source is contained in the chamber all tracks appear to begin

some distance fran the source. Evaporation of the initial drops
of a track due to the thermal capacitance of the source is the
most probable reason for this phenanena. Therefore, all measure-
ments of track length are made to the surface of the source rather
than the visible beginning of the track. Also apparent in Fig. 12

are the collisions suffered by four of the CX; particles near the

end of their range.

 

Fig. 13. e tracks fran 1131 (Magnification 1% x)

- 31 -

The relatively low ionizing power and high velocity of
the 1.3 Nov 9 particles fran I131 are evident from the track
shown in Fig. 15. Near the source the track consists of faint
groups or clusters of drops gradually increasing in number and
density as the end of the range is approached. The solid white
line in the lower left hand corner is a section of the clearing

field wire.

as

 

Fig. 1b. Canpton and photoelectron tracks produced
by an x-ray beam. (Magnification 1% x)

-52-

A two millimeter collimated beam of continuous x-rays
from a tungsten target, 55 Kv. peak, impinging upon a .01 cm.
thick lead foil produced the Canpton and photoelectrons shown in
Fig. 114. The lower energy and higher ionization of these electrons

is particularly noticeable when comparison is made with the Q

tracks.

 

Fig. 15. Canpton and photoelectron tracks produced
by ’Y rays. (Magnification 1% x)

Figure 15 shows the electron tracks produced by ”)1
60

radiation fran a Co sample encased in several layers of lead

foil. The rather high background fog and lack of contrast could

-55-

be largely eliminated if the radiation were excluded fran the
chamber until the instant of expansion. The resulting photograph
would then contain only the sharp tracks. Also, when photographing
electron tracks high intensity flash lamps rather than photofloods
should be used so as to increase the depth of focus and reduce

the heating effect.

The series of photographs shown in Figs. 16 to 20
illustrate the effect of increasing expansion ratios throughout
the critical range. Such factors as exposure time, lighting,
developing, and printing were kept constant. In general one
observes a gradual increase in the number and density of the
tracks as the expansion ratio is increased from the ion limit.
However, the onset of background fog (Fig. 20) at E : 1.158 is
abrupt. At any higher expansion ratios the tracks are completely
obliterated by a dense fog. The pressure noted for each expansion

ratio refers to the excess pressure above one atmosphere.

Q

 

Fig. 16. CK, tracks at an expansion\ratio of
1.112 P;8%anorng

-314-

Q

 

Fig. 17. q tracks at an expansion ratio of

 

Fig. 18. (X tracks at an expansion ratio of
1.125 P : 9% cm of Hg

-55..

 

Fig. 19. Ok tracks at an expansion ratio of
1.151 P : 10 cm of Hg

 

Fig. 20. OL tracks at an expansion ratio of
1.138 P : 10% cm of Hg

- 56 -
VI. Summary

An expansion cloud chamber which complies with the basic
requirements for a demonstration device was constructed and operated
successfully. It is portable, self-contained, autanatic, and of
a size suitable for viewing by small classroan groups. Photographs
of CC and Q tracks and the Conpton and photoelectric
electron tracks produced by x-rays and ’1 rays were obtained.

It was found that while electron tracks can be photographed they
are not of sufficient density to be observed. A series of photo-
graphs were also taken to demonstrate the effect on 0L tracks
of increasing expansion ratios from the ion limit to the cloud
limit. The point of maximum track density and minimun background
fog was found at an expansion ratio of 1.131 using a solution of

70 percent ethyl alcohol - 30 percent water in helium gas at 22° ,

1.

5.

6.

7.

9.

10.

11.

-37-

VII. Bibliography

Wilson, C. T. R., ”Cloud Chamber Technique" , Proc. Roy. _S___oc.
87. 277-292 (1912)

Shimizu, T., “A Reciprocating Expansion Apparatus for Detecting
Ionizing Rays“, Boo. Ra. Soc. _92, 1.25-1431 (1921)

Blackett, P. M. S., "An Automatic Cloud Chamber for the Rapid
Production ofok-ray Photographs" , 1. Sci. Inst. 14,

1433-169 (1927) _- '-

Wilson, C. T. R. “New Type of Expansion Cloud Chamber",
Proc. Roy. Soc. lh’B, 88-91 (1933)

 

mhl, 0., L. R. Hafstead, and M. A. Tuve, “Permanently
Sealed Chamber With Metallic Sylphon Bellows",

Egoig- Inst. 3. 375-378 (1933)

Wilson, C. T. R. and J. G. Wilson, “Falling Cloud Chamber
and Radially Expanding Cloud Chamber”, Proc. Roy. Soc.
1148. 523-535 (1935)

Vollrath, R. E., "Continuously Active Cloud Chamber",
Rev. 8010 IHSte l, h09‘h10 (1936)

 

ms Gupta, N. N. and S. K. Ghosh, "A Report on the Wilson
Cloud Chamber and Its Applications in Physics“,
Rev. Mod. Phys. 12, 2141-290 (1946)

 

Langsdorf, E., "A Continuously Sensitive Diffusion Cloud
Chamber", Rev. Sci. Inst. _1_9_, 91-103 (1939)

Blackett, P. M. S. and G. P. S. Occhialini, "Some Photographs
of the Tracks of Penetrating Radiation”, Proc. Roy.

£93- }22. 699-719 (1953)

Williams, E. J., "Sane Observations on Cosmic Rays Using
a large Randcmly Operated Cloud Chamber“, Proc. Roy.
Soc. 172, 1911—211 (1939)

Mott - Smith, L., “A High Pressure Wilson Chamber“, Rev.
Sci. Inst. 2, 3116-350 (19314)

Johnson, '1‘. E., S. D. Benedetti, and R. P. Shutt., aA
Hydrostatically Supported Cloud Chamber of New Design
at High Pressures" , Rev. Sci. Inst. 115265-273 (19143)

 

1).} o

- 58 1

lewis, H. W., W. W. Brown, D. 0. Seevers, and E. W. Hanes,
"A Cloud-ionization Chamber for the Study of Cosmic
Ray Nuclear Interactions", Rev. Sci. Inst. 22,

259-267 (1951)

 

Christensen, F. E., “Wilson Cloud Chamber“, Amer. J;_Phys.

if}. 1149-150 (1950)

Beck, 0., ‘Optimum Liquid Combination for Cloud Chambers",
Rev. Sci. Inst. _13, 602-606 (19M)

 

Wilson, J. G., The Principles of Cloud Chamber Technique,
pp. 1 - 15, The Cambridg3_University Press,‘Londofi

(1951)

 

 

 

 

HICHIGQN STQTE UNIV. LIBRQRIES

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