TEMPERATURE GRADIENTS
~!
DRY WILSON CLOUD CHAMBER
by
FREDERICK
SUBMITTED
m
SOLCMON EPSTEIN
PARTIAL FULFILlMENT
REQUmEMENTS
OF THE
FOR THE DEGREE OF
BACHELOR OF SOIENCE
at the
MASSACHUSETTS INSTITUTE
OF TEOHNOLOOY
(1957)
Signature of Author •••••
Certified by ••••••••••••
aaaa
u ••••
•••••••••••
.
~~
a.a
•••••••••••••••••••
•••••••••••••••••
Thesis Supervisor
ABSTRACT
A vertical
temperature
gradient
dry Wilson Oloud Chamber operating
was found to exist
on a three-minute
one fast
expansion and two sloW expansions.
believed
tobs
due to natural.
from temperature
gas layers
1) increase
after
differences
adjacent
convection
with increasing
chamber, the temperature
difference
chamber was found to be approximately
between top and bottom when no plate
over-compression
this
gradient.
is considered
originating
was found to
and ~) increase
compression.
was placed horizontally
is
body of gas and
The gradient
expansion and decrease after
.aluminum plate
currents,
expansion ratio
cyc!~~of
This gradient
between tbemain
to the walls.
in a
Whenan
in the middle of the
in each port ion of the
half
of the difference
was used. The method of
as a possible
means of eliminat ing
ACKNOWLEOOEMENTS
I wish to thank Dr. H. S. Bridge and Professors
,Caldwell andR, W. Williams for their
suggestions.
I am also gr-ateful
advice and technical
assistance
D. O~
guidance and 'helpful
to Mr. W. B. Smith, whose
were invaluable.
CONTENTS
I. INTRODUOTION ••••••~••••••••••••••••••••••••••••• Fage 1
II. TBEORY •••••••••••••••••••••••••••••••••••••••••
Pageh4
III. THE APPARATUS ••••••••••••••••••••••••••••••••• Fage 8
IV. ANALrSIS OF RESULTS •••••••••••••••••••••••••••• Fage 11
V. DATA •••••••••••••••••• ~••••••••••••••••••••••••• Page 16
VI. SOME THOUGHTS ON OVER-COMPRESSION •••••••••••••• Page 28
BIBLIOGRAPHY ••••••••••••••••••••••••••••••••••••••• Page ~O
t.
I -- INTRODUOTICtT
'!be object of much recent recearch in cloud chamber techniques
has been to' reduce the recovery time, i.e., the time necessary for
the chamber to reach equilibrium after a fast expansion has occurred
and tracks have been photographed.
This research is necessary be-
cause high energy accelerators often supply bursts of radiation
more frequently than the"chamber .can record ·theirtracks.
Long
recovery times also tend to make experiments involving random events
more
t ed,ious.
In undertaking an improvement· of this kind, one must first
consider what conditions must be fulfilled after a fast expansion
before the chamber is again prepared to record tracks. Generally
speaking, there are three such equilibrium conditions:
(1) After a fast expansion, condensation' ta-kesp1ace on' ions and the
resu1t'ing drops are photographed.
Because the process of drop for-
mation is not reversible, recompression does not completely evaporate these dr ops , Instead, they are reduced to a somewhat smaller
size, and are then fairly stable.
Such so-called "re-evaporation
nuclein are effective as centers of condensation at lower values of
supersaturation than that required for condensation- on lons.
They'
w~ll thus cause a dense background fog on subsequent expansions if
they are not first removed.
The removal of,these small droplets is
our first equilibrium condition.
This condition is fulfilled by a series of'"cleaning expansions"
relatively slow expansions in which the supereaturation is adjusted
so that condensation takes place on these nuclei.
The
drops thus
formed grow a.ndare removed to the bottom of the chamber under gravity.
2) Secondly, a uniform distribution
throughout the chamber before a fast
of vapor must exist
expansion takes place.
is necessary to achieve good track qua1ity,aa
This
a non.,uniform
vapor densi·ty gives heavy tracks' in some places and 11ght tracks
in others.
,) Gross temperature gradients
third
eqUilibrium condition
for vapor density
vapor density
turbulence,
must also'be
is closely
related
eliminated.
to the second,
is a .function·· of' temperature.
requires
Thus uniform
a uniform temperature throughout.
causing track distortion,
This
also results
Gaa
from temper-
ature variations.
One possible
evident.
source of' temperature
gradients
Whenthe drops formed during the fast
subsequent slow expansions fallout
is immediately
expansions and
under gravity,
they cool
the bottom of the chamber, leav'1ng it at a lower temperature
than
the top.
However, the thermal behavior of the gas inside the chamber
is another source of temperature gradients~
While this
gas is
cooled by an expansion, the chamber walls remain at a higher
ambient temperature.
Heat conduction therefore
begins to take
place immed.iately, warming the gas layers next to the walls.
The main body of gas, however, remains cool with respect
these' warmed layers.
The existence
in the gas causes convection
of this
currents
temperature
which car~
to
difference
the warmer
gas to the top and cooler gas to the bottom, thus producing a
vertical
gradient.
The experiments ch»scribed herein arepriaarily
this
convect ~ve effect.
discussed.
In pert iculer,
the following
concerned with
results
are
3.
1) The temperature
increasing
expansion
gradient wae found to increase with
ratio.
2) While the gradient
normally
persists througbout
increased after an expansion
the cycle, it
and decreased
upon
compression.
~) When an aluminum
middle
plate was placed horizontally
of the chamber, effectively
temperature
gradient
dividing
in the
it in two, the
in each half of the chamber was found to
be lees than that exist ing in the chamber as a wbole wben no
plate was, used.
For the purposes
of this study, the chamber was operated
with dry argon as a gas and no vapor.
II -
THEORY
Two modes of heat transfer -- namely, conduction
convection
-- are responsible
Wilson Cloud Chamber.
gradients
Although the two processes
as is shown below, convection
For example,
for temperature
and natural
in a "dry"
are interdependent,
seems to be the dominant
consider the thermal behavior
effect.
of the gas immedi-
ately after a fast expansion has taken place. While this (nearly
adiabatic)
expansion has cooled the gas considerably,
walls remain at constant temperature.
the chamber
This temperature
between wall and gas causes heat to flow by conduction,
difference
thus
warming a layer of gas near the walls.
As E.:'<·J.
Williams
bas shown~8 this initial heat conduct ion
limits the cloud chamber sensitive time. For the heated boUndary
layer expands, and the main body of gas therefore
an adiabatic
corresponding
compression.
The compression
rise in temperature
value of supersaturation.
when the supersaturation
for condensation
However,
Furthu~ore,
is accompanied
which,
by a
in turn, diminishes
The chamber then becomes
the
"insensitive"
has dropped below that value necessary
on ions to take place.
all of this happens
Williams
in a matter
rise of the boundary
of milliseconds.
shows that the temperature
bulk of the gas is quite small compared
sitive t~e,
experiences
layer. Therefore,
rise of the
to the temperature
at the end of the sen-
the boundary .layer is at a higher temperature
than
the main body of gas.
Such a temperature
distribution
on the gas near the walls.
results
in a net upward force
The warmer gas thus begins to move
towards the top of the chamber,
process is known 'as convection
displacing
the cooler gas. This
-- heat transfer by mass motion.
412/.
dz
I
z
WALL
F1gure 1 -- Forces acting on a volume element
dV adjacent to a vertical wall.
In particular, we are dealing with natural convection, for the mass
motion is caused by internal temperature differences instead of
some external device, such as a fan or blower.
In order to gain a better physical understanding of natural
convection, consider a volume element dV.
Adz
of the gas next
to a vertical wall (see Figure 1). When the entire body of gas
inside the chamber is at a uniform temperature (i.e., before
expansion), the systemlsin
equilibrium and the forces acting
on dV must add up to zero. More specifically, there must be a
balance of forces in the z-direction. There are three such
forcesa The pressure of the gas at z·,the weight of the volume
element
f
dV (both of which aot in the downward direction),
and the pressure at z + dz. Hence the condition for static
equilibr.ium is
where ~
= density
and g
=
aoceleration due to gravity.
From equation (1) the dependence of pressure on height
z can easily be calculated.
A (/J~d~-
f~) =/pfV
I
da
f;: J di
=1 J ~
cli
(3J
(4)
_here·'po is the pressure at z e 0, 1.e., at the top of the
chamber'. Density variations with height z have been neglected in
the above integration (equation
(4) ).
6.
Immediately after a fast expansion the same general situation
exists. The pressure and density have dropped to new values PI and
('I , but,
except for variations due to gravity, unifo~
conditions
still exist throughout.
Now consider the situation a few milliseconds after a fast
expansion when the gas layer next to the walls has been heated by
conduction. Although this warmed gasexpanas,
the pressure remains
essentially constant at PI as there is little change in temperature
in
the bulk of the gas.
Assuming the gas tobe
perfect, the equation of state
:sJ
(where M is the molecular weight) indicates that a rise in temperature will result in a corresponding drop in density.
This is the case close to the walls, where the gra~itatlonal
force acting on dV
will decrease ae a result of the drop in density. It should be noted,
however, that the average density'at any height z remains essentially
unaltered bya
density change close to the waIls.
The magnitude of the net force on dV can be calculated from
equation (2). The pressures at z and z+
dz remain constant because
the density remains essentially constant. only the deBsity of the
volume element changes, and the net upward force on dV is therefore
Where
f'/
is the density of the heated gas layer. The warm gas is
forced upwards, displacing the cooler gas towards the bottom of the
chamber. A temperature gradient from top to bottom therefore results.
This combination of conduction and convection adds heat to the
gas. Thus, on compression, the gas near the top will be warmed above
the ambient wall temperature. Heat flow now takes place near the
top into the walls from' the adjacent gas layers. This gas, now
cooled, as at a higher density than the warmer gas furthur from
the walls, and will thus experience a net downward force. Gas
nearlY'at wall temperature will therefore move towards the
bottom of the· chamber.
Since the compression takes place more slowly than the expansion,
the temperature rise of the gas on compression is less than the
drop due to the nearly adiabat 1c expansion. Therefore the gas
being carried towards the bottom will tend to raise the temperature
at the bottom of the chamber and force cooler gas upwards. The net
effect is a drop in the temperature gradient on compression.
III .....
THE APPARATUS
A. 1h!. ~
Chamber
The cloud chamber used in these experiments is a semi-volume-.
seml-pressure-controlled chamber. When expanded, the piston position
is determined by a perforated brass plate rigidly attached to the
chamber frame. That is, the piston cannot move back beyond
the
brass plate-,and the chamber volume' 1a thus determined. However,
when the chamber is compressed, the position of the piston is determined only by the air pressure in the back chamber, not by any
volume-eontrolllng device. The piston moves horizontally.
The piston itself is a sheet of 1/16 inch aluminum cemented
to a rubber sheet. This rubber sheet, in turn, is attached to the
chamber frame. It forms the boundar,y between front and back
chambers. Thus the front chamber gas ie compressed by filling the
back chamber to the desired pressure, thereby pushing the piston
forward by stretching the rubber on the periphery of the aluminum.
A constant temperature bath reduces fluctuations in the
temperature of the air coming into the back chamber. It consists
of a galvanized garbage pail (thlrteengallon
vaflety)meulated
with fiberglass. The pail is filled with water, whose temperature
is oontrolled by a DeKhotinsky bimetallic thermoregulator wired
in
series with an electric immersion heater. A helical copper
tube immersed in the bath conducts air into the back chamber.
A pop valve is used to effect the fast expansion. It is actu~
ated by a pulse from the electronic control circuit, and is closed
by air from a reservoir maintained at approximately 60 Ibs , per
square in. pressure. The valve is held shut by air pressure working
against a Bridgman seal.
A differential manometer, is also booked into the circuit as a
safety device. If the pressure in the front chamber falls more than
one cm/Hg. below that of the back chamber (due to a leak 1n the front
chamber or some other failure), the mercury in the differential
manometer closes a switch which opens the outlet valve on the
back, thus red.ucing the back pressure. This prevent s the p1ston
from going sO far in the forward direction that it ruptures.
A ,/16 inch thick aluminum plate was placed inside the
chamb&r in the second part of theeocperiment. The plate was supported by a wooden frame which rested on the bottom of the chamber.
The plate was· positioned horizontally midway between the top and
bottom walls, effectively dividing the chamber in two. However, the
plate area was somewhat less than the horizontal cross-sectional
area of the chamber, leaving small gaps between the edges of the
plate and the vertical walls of the chamber.
B. Temperature Measurement
All temperatures were measured with iron-constantan thermocouples. The best time response to a change in gas temperature
was recorded when the bare tip of the thermocouple was exposed to
the gas. A thin plastic rod ran vertically through the chamber,
being cemented to the top and bottom walls. The thermocouple
wires were then attached to the rod so that the bare tip was
exposed.
The temperature data was' recorded with a single-pen Brown
Recording Potentiometer, which gives voltage as a function of
time. The voltages-recorded were then converted to temperature
readings for-the type thermocouDle used.
If) .
Temperature differences were measured directly by "bucking" two
thermocouples and recording the net voltage output. For example,
to measure the temperature difference between the gas near the top
and that near the bottom of the chamber, the following procedure was
employedl Two thermocouples were placed in the regions of interest,
the bare tips exposed. These couples were brought out through
the chamber frame to'the Bl»ownRecorder. '!betwo negative leads
(constantan) were oonnected together. The positive lead(iron)
from the top couple (measuring the higher temperature) was
hooked to the posit ive terminal of the recorder, and the
positive leads from the bottom couple (measuring the lower temper.
ature) was hooked to the negative terminal of the recorder.
Absolute temperatures (as distinguished from temperature
differences) weremeaeured
in a similar manner. In this oase
all temperatures were meaauredwith
reference to a lead brick,
which has a high thermal inertia. A thermocouple plaoed at the
point of interest inside the chamber was "buckedn against a
couple attached to the brick in the manner described above. Thus
the potentiometer recorded the desired chamber temperature with
reference to the (constant) brick temperature. The temperature
of the brick itself was measured with a thermometer.
1 t.
IV A. Operation
Without
AN1LYSIS OF RESULTS
Plate
Figures 2, , and
4
show the measured
temperature
differ~nce
between top and bottom of the chamber vs. time. '!be 'salient feature
of these graphs
gradient
is, of course, the existence
in a dry cloud chamber. The details
tend to show that the gradient
of a temperature'
of these graphs
is in fact due to natural con-
vection.
First, it is seen that the average gradient
in.creasing expansion
ratio. (Expansion
the front chamber pressure
ded pressure).
Assuming
batie, a higher
temperature
ratio is defined here as
at compression
divided by the expan-
that the fast expansion
expansion
increases with
is nearly adia~
ratio will result in a larger gas
drop_ That is,
(8j
where the subscript 0 refers to conditions
and the sUbscript
r
expansion.
1 refers to conditions
immediately
at To, a higher expansion
that a larger initial temperature
the gas and the walls.
expansion
expansion,
after
is the ratio of specific heats • Since the walls
remain essentially
proportional
before
difference
ratio also means
exists between
The rate of heat conduction
to this temperature
ratio the temperature
difference.
is nearly
Hence for a higher
of gas layers next to the walls
will rise at a faster rate, and this rise will be accompanied
a correspondingly
larger decrease
on a gas element dV is proportional
in density.
by
The upward force
to this density' drop ~equation
7).
/2.
This increased upward force will, of course, result in a higher
rate of heat transfer by convection.
difference
Thus a larger temperature
between top and bottom of the chamber is to be ex-
pected for a higher expansion
Secondly,
ratio.
the general time variation
a cycle seems to indioate that natural
place. The gradient
after a compression.
surroundings,
convection
An expansion
over
is taking
is seen to rise after an expansion
and drop
leaves the gas cooler than its
leading to heat conduction
and convective heat transfer
compression
of the gradient
into the boundary
to the top of the chamber.
layer
Conversely,
heats some of the gas above the ambient wall temper-
ature. Heat will then be conducted
and convection
from the gas to the wall,
will take place in the opposite direction,
reducing the size of the temperature
difference
thereby
between top and
bottom.
This is, of course, a gl'ossly simplified
picture.
A de-
tailed analysis would show that many side effects must also be
considered
for a complete understanding
sider, for. example, figures
time variation
of the chamber.
constant
of temperature
5 and
of the gradient.
6 showing,
respectively,
the
at the top and at the bottom
In both cases, the temperature
for tbefirst
Con-
few seconds
remains nearly
of the compression
stroke.
Iltring this time the back chamber pressure -is being raised to
the expanded argon pressure
cm/Hg. above atmospheric
in the front chamber
pressure),
(some twenty
and the piston is not moving.
Figures 5 and 6 also show that the temperature
expansion
is much sharper than the temperature
sion. The compression
drop after an
rise on compres-
takes place slowly (approximately
twenty
/3.
seconds), for the valves and temperature
bath offer a high re-
sistance to air flow into the back chamber. Thus heat conduction
into the argon during compression
expansions
are all relatively
the expansion
cannot be neglected.
fast. It therefore
But the
follows that
strokes are more nearly adiabatic
than the comM
press ions.
The fact that there is a net temperature
from this thermally
unbalanced
batic expansion were followed
pression,
cycle. Presumably,
immediately
there would be no gradient.
each expansion
is followed
grad,ient follows
by an adiabatic
In reality,
Since the amount of heat transferred
proportional
to the temperature
an expansion
however
~
variation
I
slow
by convection
between the gas and
to the top after
than to the bottom after a compression.
a characteristio
B. Operation
difference
more heat will be transferred
of course, a steadystate
com-
a few seconds later by a relatively
compression.
its surroundings,
if each adia-
eJ
Eventually,
.
is reacbAin which the grad1ent
shows
Over each cycle.
Plate
When an aluminum
plate wae placed horizontally
in the middle
of the chamber, ther thermal behavior was as shown in figures
7 through 12. The chamber was again operated with dry argon
gas and no vapor. An expansion
measurements
with the plate.
Comparison
of figures
pro6f that a gravitational
gradients.
ratio of 1.11 was used for all
7 and 8 with
figure
effect is responsible
Figure 7 shows the temperature
4
offers furthur
for the measured
difference
between the
gas near the top of the chamber and that near the upper side of
the aluminum
plate, while figure 8 shows the differenoe
the lower side of the plate and the bottom
between
of the chamber.
is
/4.
Figure 4 shows the temperature
gradient
from the top to the bott om
with no plate and using the same expansion
ratio (1.11). As ex-
pected, the average temperature
in each section
approximately
difference
half as great as that measured
chamber without
cloud chamber
dimension
of the whole chamber.
and the density at any height Z
pressure variation
a temperature
divides the
into two equal parts, each part having
due to gravity varies linearly with vertical
4),
for the entire
a plate. For the plate effectively
half the vertical
3).
(equation
gradient
is
due to convection,
would also vary linearly with vertical
only one"
The pressure
distance
proportional
Therefore,
is
(equation
to the
one would expect that
a gravitational
distance.
effect,
This was found
to be the case.
However,
temperature
there is one curious feature
differences
of the variation
over a cycle. The lower half of the cham-
ber (figure 8) exhibits the eame time variation
chamber (~igure
4).
of
That is, the gradient
as the plate-free
rises after an expansion
in both cases. But the 8radient
and drops after a compression
in
the upper part of the chamber (figure 7) shows an opposite time
variation;
1.e., it rises on compression
This can be understood
ing temperature
and falls on eXpQ;h::'i017;.
by considering
figures 9 and 10 show-
vs. time near the top of the chamber and near the
upper side of the plate, respectively.
Both of these temperatures
are at all times higher than the lead brick temperature,
which
approximately
in part
the ambient wall temperature.
As mentioned
is
III, there were gaps between the edges of the plate and the
vertical walls
of the chamber.
flow upwards along the vertical
Since there is a net convective
heat
walls, these gaps allow the upper
portion of the chamber to fill with warm gas.
The fact that the upper portion of the chamber is above the
ambient wall temperature
throughout,
whereas
the bottom portion
varies from above ambient at the top to below ambient at the
bottom, accounts
for the reversed time variation
in the upper half.
During a compression
of the g.adient
stroke, the gas in the
upper portion near the walls will be cooled by conduction
move down along the walls
gas from belovia
into the lower portion. However,
pr-evented by the plate from moving
since the rate of heat transfer by conduction
to the temperature
difference
cooler
up. Moreover,
is proportional
between wall and gas, more cool
gas (i.e., that at wall temperature)
is leaving from the
the chamber than from the upper side of the plate. Hence
average temperature
and
top of
the
at the top is rising faster than that at
the upper side of the plate, and this gradient
therefore
in-
creases on compre8sion~
A reverse process takes place on expansion.
temperature
Gas near wall
rises to the top of the chamber from the lower halt,
cooling the top. The gradient
thus decreases
on expansion.
/6.
V. DATA
Title
Temperature ,Difference Between Top and
Bottom V8. Time. (Expansion Ratiol 1.05)
17
Temperature 'Difference Between Top and
Bottom vs. Time. (ExpaneionRatiol 1.085)
18
Temperature Difference Between Top and
Bottom ve. Tbne. (Expansion.Ratiol 1.11)
19
5.
Temperature at Top -,of Chamber vs. Time
20
6.
Temperature at Bottom of'Chamber va.- Time
21
7.
Temperature Difference Between Top of
Chamber and Upper' Side of Plate va. Time.
22
2.
4.
8.
./
Temperature Difference-Between Lower Side
of Plate and· Bottom 'of Chamber va. Time
Temperature at Top of Chamber (Plate
Installed) ve. Time
10.
Temperature at Upper Side of Plate
V8.
11.
12.
24
Time
25
Temperature at-Lower Side of Plate
vs. Time
26
Temperature at Bottom of Chamber' (Plate
Installed) vs , Time
27
N.B. The numbers on the graphs, such as
10, 20, etc., have noeignificance here,
and should therefore be disregarded.
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TEM P. 01 FFERENCE,
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DEG. C.
Figure 2 -- Temperature Ditterence Between
'fop and Bott am YI. T1me.
Ixpenlion Ratiol
1.05
18.
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DIFFERENCE,DEG.C.
rigure , -- Temperature Ditterence Between
Top and Bott OIl .e. Time
Ixpanlicn Ratiol 1.085
FA S T
EX P.
EX~
I!J.
I ,
IIIt ~
-j
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Jj
4
TEMP. DIFFERENCE,
_
FA S T EX ~
5
DEG. C.
II'
Pigure
-
4 --
Temperature Ditterence Between
Top and Bottom ve. Time
Ixpaneion Ratios 1.11
20.
eT
!
ILl
2
...
FAST
EXP.
CO M P •
H--:---+o:-"---~----+--~-
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~....o....+~~-~~-
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3 MIN.
~~~~~~~~~-
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5 LOW EX F!
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i ;
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5
6
TEMP.
'igure
5
I
7
8
...
ABOVE
9
10
REF•• DEG. C•
5 -- Temperature
VI.
at Top ot Chamber
Time
Bxpansion Ratiol 1.087
. Reference Temp.1 24.0 Deg. o.
EX R
2/.
I
i
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o
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7
TEMF! BELOW REF., DEG. C.---....-.-- TEM~
COMP.
INCREASING
Figure 6 -- Temperature at Bottom of
Chamber VI. Time
Bxpanlion Ratios 1.081
Reference Temp.I 2~.O Deg. O.
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4
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REF: DEG. C.
7 -- Temperature Difference Between Top of
Chamber and Upper Side of Plate
Expansion Ratioa 1.11
EX~
r.
i
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2
TEMP. ABOVE
FAST
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Figure
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~-t-;.++-H-~~-;---~-t-
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S LOW EX P.
COM F!
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COM ~
FAS T
+~~~~~+-H-"-+~t---
\ !
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1
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Pigure 8 -- 'emperature Ditterence Between Lower
«.
".
Slde of Plate and !ott CI!I 01'. Cha.ber
Te. !1ae
Ixpaneian Ratios 1.11
EX R
2
2
a
24.
iTT----
-rT1 1 -I" .,
-L~;
1-
.'
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J_ ~.-
_1_
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10 I
~-++-t-I-+
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II
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.3 MIN.
~+-+-+-+-~-~-+ --
I
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o
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EXP.
t
_LW~"L
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234
..
TEMR ABOVE REf:,
DEG. C.
Pigure 9 -- f .. perature at fop of Chamber
(Plate In.talled)
••• fiJIe
Ixpan.j.on Ratio. 1.11
Reterence Temp.. 23.9 DeR.
o.
COMPo
FAS T
EXP.
'I
.T 'I'
II? Z? ill
FAST
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3 MIN.
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ABOVE
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3
4
5
REF.tDEG. C.
Figure 10 -- femperature at Upper Side of
Plate va. Time
Ixpanaion Ratiol 1.11
Reference Temp.1 2'.9 Deg. C.
EXR
2
26.
i
20
;
t
I
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t
92
:
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...........
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.~-........-+-O....O.--~-
EXP.
3 MIN.
CO M ~
~~4------+-~----
~ ........~----
SLOW
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~'-"'-i-~~-"""'~-
FAST
!
I
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i
I
U
...
Figure 11 -
4
5
REf:, DEG.C •
!.perature
at Lower Slde ot
.Plate .,.•• !!me
Ixpan.ion Ratios 1.11
Reference !emp.t
2,.6 Deg. O.
EXP.
EXP.
I~
82 Z 222
I
2 2? 2 2 ? 2 Z d
.T
FAST
..... ---4---t~~~++4++++-H-+-H-+-H-++--
EX ~
COM ~
SLOW E XP.
~~+-HI-'--+""""""~+-+-!~-
~~""""''''''''''~+-+-~---''''''''''''''-H-H-3MIN.
CO M ~
S LOW
~--I--+---""'~~~--+---+--"-""'-+---
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\-+-~.....o-+--+------
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----...:=-,
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.'
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-+-,
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t ":.. , ,I . : I . . ; ; I . . .
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TEM~
•
2
3
4
5
BELOW REF:,DEG. C.--....
TEMP.
INCREASI NG
Figure 12 -- Temperature at Bottom ot
Chamber (Plate Inltalled)
VI. Time
Ixpan810n
Reference
Ratiol 1.11
Temp.1 2,.8 Deg.
COM ~
FA ST E X ~
C~:"::::':'...:.-J--H--H-~"'-"-'~"""""'''''''''--
....,..
EX P.
o.
28.
VI. SOME THOUGHTS ON OVER-OOMPRESSION
The thermal gradient found in these experiments is undesirable tor good cloud chamber performance; as turbulence and uneven
track quality result from such temperature variations. Under
normal operating conditions, a "waiting time" is included at the
end of the cycle during which the piston is held in the forward
(compressed) position. This waiting time serves a three-fold
purpose:
1) The drops formed during the fast expansion and subsequent
slow expansions fall to-the bottom of the chamber, where they are
evaporated.
2) This process of drop evaporation takes heat from the -bottom
wall, leaving it somewhat cooler than the rest of the chamber. The
waiting time allows the bottom to again reach thermal equilibrium.
,) The natural convection currents discussed in this thesis.
die out, and nearly uniform thermal conditions are obtained
throughout. However, it has been found experimentally that a
temperature difference of about
0.75
Degrees C. between the top
and bottom of the chamber will produce convection currents -large
enough to redistribute the liquid that has fallen to the bottom,
but not large enough to cause track distortion.
Recently, several researchers9,1,,16
an 'over-compressionl
have experimented with
cloud chamber cycle, as distinguished from
the conventional slow expansion cycle. In this process the-fast
expansion takes place in the normal way. But instead of then going
through a series of slow expansions, the chamber pressure is quickly
raised to a value above the normal compressed value. The piston is
29.
then slowly brought back to its normal position, and a sbort
waiting time is included at the end of the cycle.
This type of cycle would seem to bave several decided
advantages in eliminating convection currents.
First, the short time lag (about
0.' seconds)
between fast
expansion and over-compression means that heat transfer by
convectlonhas
little chance to take effect. Moreover, the heat
flow by conduction during the time that the cbamber is expanded
is balanced by a flow in the opposite difection while the chamber
18 overcompressed. Thus a thermally balanced cycle can be obtained,
and any convection that has taken place during expansion can be
cancelled by opposite convect1cn currents during over~ompres8ion.
While it was originally thought that over-canpressionmight
evaporate the drops to such a size that they could be swept away
by an applied electric field, several investigators9,17 have shown
thai tracks persist even on· severe over-compression. Thus tbe
slow expansion from overcompression to-normal compression is
necessar,yfor drop formation on re-evaporation nuclei. ,During the
waiting time the drops thus formed fall to the bottom of tbecbamber,
and the vapor is redistributed.
However, the fact that the drops fall to the bottom isn't
necessarily a disadvantage. While this would cool the bottom and
thus produce a temperature gradient, a small gradient is necessary
to redistribute vapor (see above).
The ultimate'advantage
of over-eompression Is that a shorter
cycle is attained. It would seem that this reduction in cycling time
is made possible by the elimination of natural convection currents
in the gas.
3D.
BIBLIOGRAPHY
1. I. Fermi, Thermodynamics ~ New York, 1956
2. A. Findlay,
;.
The Phase Rule and ItsApplications
B. Gr1meehl,
l} •
!
T~'tbook of Physics
London, 1946.
(Volume
4. W. J. Humphreys, Phis ic s E! the Air 5. L. S. Marks, Mechanical Engineers'
6. W. M. Rohsenow., ~
(~Transfer)
Oambridge, Mass.,
7. M. B. at out, Basic Electrical
..~-.New York, 1927
II, ~
and Sound)
New York, 1940
Handbook -- NewYork, 1951
Lecture Notes 1957.
Measurements -
New York, 1950
8. J. G. Wilson, ~
Principles
of Oloud-Chamber Technique Owmbridg~England, 1951
9. N.
o.
Barford,
".Fsst Cycling Olou'd Chambers· 1956, Vol. II, p. ;5.
.
.Q!!!:!! Symposium,
10. H. Brinkman, "A Wilson Oloud Chamber With Several Expansions
Per Second" - Physica, Vol. VI (19;9), p. 519.
11. A. P. Oolburn and A. O. Hougen, ·Studlesin.
Heat Transmission"
Unlv. of Wisconsin, mineering
Experimental Station
Bulletin,
Series 70 19;0)
12. N.N. Ias Gupta and S.X. Ghosh, • A Report on the Wilson Oloud.
in Physics" - ~
Mod. :AiyS .,
Chamber and Its Applications
Vol. 18 (1946), p. 225
1;. E. R. Gaerttner and M. L.Yeater , "A Fast Recycling 010ud
Chamber••• ". - R.S.r., Vol. 20 (1949J, p. 588
14.w.
E. Hazen, "Some Operating Characteristics
of the Wilson
Oloud Chamber" -- R.S.I.,
Vol. 1; (1942), p. 247
of the
15. J. M. W. Mi1atz and 0, van Bearden, "An· Investigation
Wilson Oloud Chamber- - Hlysica, Vol. XIII (1947), p. 21
16. J.Walker,
J.O. Brown, and D.W. Hadley, 'Observations
on the
Behavior of a Small Over-Oompression Chwmber" w- .
.Q!!!!. Symposium, 1956, Vol. II, p. 40
3/.
17.
J. Walker et aI, "A Simple Fast-Recycling Cloud Chamber"
J.o.s.r., Vo1~ ;; (1956), p. 1;;
18. E. "J. Williams, "Note on the Sensitive Time of a Wilson
Expansion Chamber" - hoc. Oamb. Phil. Soc.,
"Vol.;5
(19;9(, p , 512