I
SLAC -PUB-Z’75
HEPL-500
February 1967
ELECTRON
OBSERVED
TRIPLETS
AND PAIRS
IN A STREAMER
D. Benaksas?
CHAMBER*
tt
and R. Morrison
High Energy Physics Laboratory
Stanford University,
Stanford, California
ABSTRACT
We have studied electron
and in the field
of the atomic
pair photoproduction
electrons
of neon.
many ways behaves as a bubble chamber
The recoiling
electron
ture in a magnetic
is observed
field.
plet cross section,
We also report
determine
and its momentum
Most results
chamber
deduced from
agree with theory,
distribution
of several
of streamer
(To be submitted
supported
of the recoiling
tt Now at Universidad
the trielectron
tests that have been made in order to
chambers
to The Physical
by the U. S. Atomic
“fNow at Laboratoire
its curva-
in particular
with incident
electron
photon beams.
*Work
which in
than expected.
the results
the usefulness
A streamer
of the nucleus
has been used for this experiment.
although the momentum
seems to fall more rapidly
in the field
de l*AccQl&ateur
National
Energy
Review)
Commission
Lineaire,
de Ingenieria,
Orsay,
Lima,
Peru
France
and
I.
Electron
processes
pair and triplet
entirely
proximations
Bethe-Heitler’
production
described
are necessary
formula,
1.3%.
from
in the field of a charged particle
by quantum electrodynamics;
for a satisfactory
cross section;
the Born approximations
In the triplet
case the recoiling
rise to a retardation
solution
using the Fermi-Thomas
well the total pair production
correction2
INTRODUCTION
effect;
gives rise to exchange effects.
between the photon and the electron
of these problems.
one has to subtract
which,
describes
only the coulomb
changes the kinematics
of two identical
one has to consider
and gives
electrons
the Y-e interaction
in whose field the pair has been created.
has been shown that the y-e
interaction
energy and have an opposite
effect on the total cross section,
ing; neglecting
The
in the case of neon, is of order
the presence
Finally
many ap-
model for screening,
electron
in addition,
however,
are
and exchange effects
are small
at high
thus partially
them therefore
leads to a small error
in the total cross section.
these effects can be important
in the high momentum
transfer
periment.
The retardation
screening,
while Wheeler and Lamb5 calculated
properly
the best approximationfor
The momentum
Bethe
from
Triplet
the triplet
distribution
of the recoil
was first
by Gates et al.
obtained in these experiments
plet experiment
It has been suggeste cf that
givenby thefollowingexpression:
electron
has been extracted
by Suh and
calculation.
photoproduction
later
who neglected
cross section with screeningtaken
effect neglected.
cross sectionis
However,
-
Borsel line’s
cloud chamber,
the triplet
cancel-
as is found in this ex-
effect has been taken into account by Borsellin$
into account but with the retardation
Ot =%heeler-Lamb
region,
It
directly
9
measured
in a bubble chamber.
agree well with the theory.
has been performed
by Hart et al.
on nuclei heavier
-2-
8
in a
Most of the results
So far no direct
than hydrogen,
tri-
primarily
because of the difficulty
triplet
in detecting
electrons
cross section to pair cross section
for neon and 1 for hydrogen.
Therefore,
with neon than with hydrogen
to observe
the production
screening
process
study,
the target
Momenta are measured
this ratio
the same number
chamber
of triplets.
0.1
is a suitable
instrument
the chamber.
using the curvature
the
for such a
The gas is a mixture
from helium.
and dip angle in a weak magnetic
and 0.4 MeV/c
Photons from
Although
may change the behav-
(10%); about 0.5% of events originate
the range energy method.
is roughly
on the Z of the material,
and the binding forces
(2 Kg), but between 0.2 MeV/c
of
one has to take many more pictures
being the neon which fills
of neon (900/0)and helium
field
electrons
A streamer
ior of the process.
goes as l/Z;
does not depend strongly
of the atomic
Also the ratio
of low energy.
we have used in many cases
the bremsstrahlung
spectrum
were
tagged in order to reduce the data taking to the photon energy band 600 - 800
MeV; the tagging was also helpful
in reducing
substantially
the number
of pic-
tures due to background.
II.
Figure
beam,
1 shows the general
arrangement
after passing through the radiator,
some shielding
The electrons
blocks.
600 - 800 MeV were deflected
mon backing
counter
60 cps).
These tagging counters
amount of hardener
necessary
counter
was deflected
The electron
by a magnet towards
radiation
in the energy range
a group of 3 tagging counters
Beam intensity
and radiator
counted about one electron
were used as a monitor
(0.2 radiation
to cut down substantially
of this experiment.
that emitted
towards
in coincidence.
chosen such that each front
small
APPARATUS
-3
-
thickness
were
per pulse (1 /-Ls,
for the photon beam.
length of Be) in the X-ray
the number
with a com-
beam was
of low energy Compton electrons
A
which otherwise
might be mistakenly
near an electron
pair vertex.
regarded
A collimator
vented the large angle photons from
A.
Streamer
Chamber
Streamer
chambers
have rarely
tion of very large streamer
We constructed
a small
perform
a series
erator.
This chamber
sets of wires
are isolated
chamber
stretched
allowing
Driving
through
on an aluminum
photography
to decrease
following
paper.
14
Mark III accel-
frame
constitute
a
above the chamber.
lucite plate,
assuring
at
0.6 cm thick alumi-
corona effects
and extend
field uniform
A slow flow of Ne-He mixture
and
gas is main-
small holes in the walls.
in the streamer
mode, the chamber
8 to 10 ns wide and of 15 to 20 kV/cm.
When directly
we use only a shorting
the Marx.
Although
the chamber
-4
-
Marx
generator
(10
ZSO-kV pulses with a tail of about
such a pulse. produces
of an ionizing
gap operating
needs a very short
A classical
applied on the chamber,
charge between the plates along the track
pulse,
from
in order to make the electric
stages) with a power supply (f 30 kV) delivers
100 ns.
in a detailed
System
To be operating
pulse,
and the opera-
beam at the Stanford
The ground plate is made from
in the chamber.
tained continuously
ago, but as yet
to be used in a magnet in order to
from the neon gas by a 0.6-cm
2.5 cm outside the chamber
avoid breakdowns
walls.
of construction
have been discussed
Both plates have rounded corners
num.
some years
pre-
is 38 X 30 X 13 cm3 with walls made of 1.2 cm lucite.
the same time a gas seal.
B.
The techniques
streamer
high voltage plate,
The wires
10-13
when produced
after the hardener
the spark chamber
of tests using the electron
Two perpendicular
transparent
reaching
chambers
electrons
immediately
have been developed
been used in physics.
as recoil
particle.
a dis-
To shape the
under 40 pounds pressure
of SF6,
is not long enough to behave as a
transmission
line,
terminated
it was foundthat
by 250 ohms.
sophisticated;
the best pulse is obtained when the chamber
This arrangement,
nevertheless
such a system
such as the one used in this experiment.
to the chamber.
For chambers
between the chamber
a Blumlein
seems the best solution.
Operation
About three streamers
may vary from
one has a series
directly
photography.
We usually
visible
particles
the top.
longer.
gap may
for very large chambers,
so that,
directed
of the ava-
instead of a spark
alongthe electric
field.
such that the streamer’s
Below 4 mm the streamers
a mirror
was used with an f/1.4
of the chamber
of their
look very faint when seen by
The chamber
crossing
although the sensitive
This is not a disadvantage
chamber.
been removed
Vertices
on specific
Fig-
are per-
in the bright-
than those parallel
events just as any conven-
time of the streamer
chamber
at Mark III where the beam pulse length
-5-
to allow
of all ionizing
there is an anisotropy
the plates being brighter
may be triggered
bright when seen
lens in this experiment.
as are the trajectories
However,
angle.
length is ad-
but are sufficiently
events obtained in the small
the tracks
spark chamber
and change the width by varying
A part of the top pole of the magnethas
in the volume
ness of the tracks,
tional
and a series
on the average in the gas and their length
gap in a manner
Kodak 2475 film
independent
to the plates.
particle
keep the voltage constant
from the side view through
ures 4 and 5 show typical
fectly
pulse applied
a few mm to a few cm depending upon the voltage and the width of the
in the shorting
from
a capacity
of streamers
per cm are produced
justed between 4 to 10 mm.
the camera
size chambers
3 we show a typical
and the Marx generator;
after the passage of an ionizing
the plates,
the pressure
size,
small
the pulse to about 10 ns, one can stop the development
lanches initiated
pulse applied.
In Fig.
2, is far from being
of the Chamber
By shorting
connecting
is able to drive
of medium
be inserted
C.
shown in Fig.
is
is much
is 1 psec.
We can use either the neon of the chamber
this experiment,
example,
or an internal
target.
as target,
A thin sulphur
plate has been used, for
to study the high energy end of the bremsstrahlung
uniformity
of electric
field does not appreciably
ers nor the apparent position
iment the incident
measured
and the final electrons
while the emitted
test we inserted
the mid-plane
streamers
a cylinder
of mylar
the non-
the formation
were observed
and their
of stream-
and passed an electron
and therefore
containing
In another
hydrogen
beam through
it.
one could not see the vertex
from
visible
Up to 200 quanta per pulse can be injected
a chamber
similar
studies.
in design but physically
15
We have attempted
do electroproduction
be injected
chamber
is limited
of final
pair visible
in such
A system,
much larger, will be used for photoproduction
to use electrons
cylinder
without
to about 3000 per pulse.
which spiral
charged particles
in the chamber.
in a similar
We found that the number
studies.
in the hydrogen
energy electrons
field
the trajectories
with less than one electron
in
The
which could be reconstructed
outside the cylinder.
exper-
momentum
in a shower counter.
(2.5 cm in diameter)
in hydrogen
spectrum;
In this bremsstrahlung
photon was detected
of the chamber
did not form
disturb
of the trajectories.
as done in
producing
manner
in order to
of electrons
which can
a large background
This background
above and below the cylinder
is mainly
in the
low
when a magnetic
is applied.
III.
PROCEDURE
In this experiment,
to the emission
converts
AND MEASUREMENTS
a signal in any one of the tagging counters
of a photon in the energy range 600 - 800 MeV.
in the chamber,
its energy greater
one member
than 300 MeV.
at least of the electron
The magnetic
-6-
field
corresponds
If such a photon
pair produced
in the chamber
and the
has
position
of the trigger
electrons
counters
(and positrons)
The chamber
a fitting
field.
with a signal from
Pictures
All tracks
were calculated.
written
was not clearly
The typical
resolution
is calculated
visible
on the picture,
Momentum
The typical
mass.
recoil
mainly
momentum,
When the recoil
angles is 5 milliradians
because the streamers
pulse length.
in electron
particle
happened
In these cases,
on projected
The differential
pair production,
is an electron,
it in the streamer
all events are measured
For momenta
and its dip angle.
Experimental
and angles
values only.
RESULTS
use the same method for some events,
was used.
with
Distribution
than 0.4 MeV/c,
coil electron
by an
only which viewed the top
and the momenta
and gave information
large enough to enable us to observe
greater
in the picture
5 to 10%. In some cases the side view
in the electric
IV.
the
for the case of an inhomogenous
for small
within
count-
Events were measured
to a common vertex
only the top view was measured
electron
was recorded
for us by D. Fries
to be too long due to variations
Recoil
the left or right trigger
were taken by one camera
were fitted
while the momentum
A.
between 300 MeV and 800 MeV.
and the side view through a mirror.
program
detected
any of the 3 tagging counters;
which gives the coincidence
light.
view directly,
ranging
such that these counters
when a signal from either
ers was in coincidence
appropriate
of energies
was fired
tagging counter
were adjusted
recoil
is of order
the kinetic
chamber.
energy is
For momenta
using the curvature
smaller
of the
than 0.4 MeV/c,
of the rewe could
while for others the range energy relation
momentum
distribution
points are shown with the statistical
band resolution;
the solid points represent
using curvature
and dip angle,
6.
and with the energy
events where momenta
the square points
-7-
error
is shown in Fig.
correspond
were measured
to events measured
mostly
by the range-energy
to be 0.2 MeV/c
streamer
The minimum
method.
or 36 k?V in our streamer
length; the first
chamber,
point below 0.2 MeV/c
rect.
Nine events were found with momentum
recoil
momentum
small,
which has been neglected,
concerned,
The experimental
Evaluation
A direct
the minimum
of the Triplet
measurement
recoil
however,
N of pairs
produced
serve a recoil
electron.
cross
for pair and triplet
The same
transfer.
section
is not possible
of momentum
of the triplet
in the chamber,
production.
which have a recoil
momentum
because
meas-
cross
sec-
One has to measure
whether
or not we ob-
to op + at, the sum of the total
Then we measure
greater
the number
than q,; N(qo) is propor-
This gives:
all systematic
compares
is
the experimental
than some value go.
N is then proportional
at&,)
Almost
cross
a good estimation
first
to a,(y,).
of the distribution
is much lower than our limit
greater
tional
for small
Cross Section
momentum
N(qo) of triplets
not too
to fit the theoretical
curve for high momentum
tion at( qo) for recoil
sections
by Suh and
et al. , in hydrogen.
momentum
the total number
is important
slope than expected,
of the total triplet
urement . One can expect,
the highest
for momentum
points are normalized
it seems to have a steeper
lying below the theoretical
being due to
than 48 MeV/c;
As far as the general behavior
effect has been seen by Gates,
B.
greater
since the screening,
only.
seems
is thus not expected to be cor-
it is expected to be correct
curve at q = 1.05 MeV/c.
points
the limitation
above,
momenta
momentum
The solid curve is that calculated
was 130 MeV/c.
Bethe; as discussed
detectable
errors
at(qo) obtained from
=
N(qo)
N
cat + op)
cancel in the experimental
Suh and Bethe calculations
-8-
ratio
N(qo)/N.
Table 1
and deduced from
this
We have used E,, - 700 MeV in the calculations,
experiment.
taken as the mean value for the photon energy-band
ot(qO) depends only slightly
which has been
600 - 800 MeV.
Note that
on the photon energy when this energy is greater
than 100 MeV.
TABLE
at (4,)
g0
I
a,(s,)
theory
experimental
(millibarns)
ot(4,)
=p.
q(s,)
th-
o&qo)
(n/rev/c)
(millibarns)
0.5
20.2
21.0 f 1.10
1.04 -I 0.05
0.235
1
12.1
12.2 f 0.76
1.01 f 0.06
0.147
2
6.94
6.2 f 0.52
0.91 f 0.08
0.0845
5
3.12
2.74 f 0.37
0.88 f 0.12
0.038
The errors
assigned
the determination
of the triplet
are partly
cross section
cause momentum
preceding
when q, = 0.5 MeV/c,
measurements
due to possible
while we observe
the results
are inaccurate
errors
in this region as explained
however,
drops faster than expected.
between q, = 0.5 and go = 5 MeV/c
where all three electrons
even a large discrepancy
the total cross
section,
in the
between experiment
The possible
discrepancy
with
is about 16% and could be attributed
in the cross section.
since the main contribution
to
col-
in this case, there is a reduction
observed in the high momentum
-9-
be-
there is some slight evidence that
have high energy;
in phase space which produces a reduction
23.5%
only 3.8%
exchange effects since these effects arise mainly in large momentum transfer
lisions
in
for q, < 0.5 MeV/c
Column 4 shows a good agreement
for low momentumtransfer;
Ot(qo) experimental
and partly
We have not calculated
paragraph.
and theory
statistical
The last column of Table I shows that we observe
of q,.
when q, = 5 MeV/c.
theory,
3
One can notice that
region does not greatly
affect
comes from the low momentum
region.
Exchange terms
as well as
y-e interaction
in the case where the two electrons
energy;
the contribution
$%1n
m N 3. lo3
relative
0.02.
Thus,
urement
have high energy while the positron
has low
calculated
of correction
contribute
is consistent
approximately
in Ref. 3.
Therefore,
transfer;
by a factor
of
region is concerned,
3. 10S3/0. 02 or 15%
with the discrepancy
than the exchange effects
performed
leads to a contribution
transfer
of the cross section for high momentum
action is smaller
is of order
The same calculation
have high momentum
will
16
by Votruba
as far as the high momentum
exchange and y-e interaction
This order
involved
to the leading term.
for the case where all particles
order
are clearly
of these terms
terms
observed
in fact,
the
Bn(k) or 6.7,
one can assume that exchange effects
in the meas7-e inter-
as discussed
are responsible
for
the main discrepancy.
C.
Angular
Distribution
The angular
distribution
tron and the direction
presents
In fact,
of the Recoil
d@/d 8, where 8 is the angle between the recoil
of the incident
photon,
a large peak which extends from
the angular
distribution
tion; large angles correspond
momentum
transfer.
momentum
momentum
is strongly
is greater
to a momentum
electrons,
angles greater
common part,
around 55’.
distribu-
angles to high
of the events
centered
around
One can notice that with low
scattering
is very large, therefore
than 90°. The extraction
or Votruba calculations
- 10 -
data
and also for the case where the
Their
of multiple
elec-
The overall
and small
we give the distribution
of about 1 MeV/c.
either the Borsellino
7a.
with the momentum
transfer,
than 1 MeV/c
the probability
for some events, to recoil
lar distributionfrom
correlated
to low momentum
lies between 0.4 and 1 MeV/c.
55’, corresponds
is given in Fig.
0 to 90’ , with a maximum
In the same figure,
for which the momentum
leading,
Electrons
of the angu-
is very complicated;
therefore,
simple
there is no theoretical
kinematic
momentum
considerations
electron:
are the positron
opening angle; the second term
typically
of order 4 m.
cos O/sin2
0.
versus
mentum
cos O/ sin20
distribution
be correct,
and the electron
0 - cr/2kcos
momenta,
of the momentum
by the points
The first
6.
because the recoil
3 points in Fig.
a! being
q versus
group effectively
7c, we show the angular
0;
and w is
is very small,
around the
distribution
instead of 0 ; it has the same behavior
of Fig.
However,
between the
q N 2m cos B/sin2
7b shows the diagram
8 (2m = 1); in Fig.
with.
relation
of the above relation
The events represented
line q = cos O/sin2
plotted
Fig.
our results
lead to the approximate
and the angle of the recoil
CY= p+p- w2 where p+ and p
their
curve to compare
as the n-m-
7c are not expected to
angle could not be measured
for all low momentum
events.
D.
Energy
Sharing Distribution
The energy of both electrons
pair energy,
carried
The f distribution
of the pair was measured.
away by the positron,
is presented
in Fig.
8.
is expressed
for these events,
about 32 cm, which gives an accuracy
Only about 100 triplets
their
clear field.
therefore,
the electron’s
in the momentum
predicted
by the theory,
the dip and the maxima
seem more accentuated.
seems to be preferred.
- 11 -
+ E-).
in the first
length is
of order
which was not enough to study
distribution
with a symmetry
trajectory
measurement
we used only the pairs
The experimental
of the
field was low,
which occurred
The solid curve is that of Bethe and Heitler,
E ,, = 700 MeV and Z = 10.
behavior
the pairs
were found in this region,
energy distribution;
by f = E+/(E+
Because the magnetic
we used only a small part of the data, i.e.,
two inches of the chamber;
The fraction
produced
calculated
presents
around f = 0.5.
in the nufor
the general
However,
An uneven energy sharing
5%.
E . Conclusions
The first
track
conclusion
chamber.
Because it can be triggered
to a bubble chamber,
cessfully
very satisfactory
triplet
tions for the electron
slight divergence
heavier
cross section
recoil
useful for high
q, near 1 MeV/c
observed
mostly
tigations
of the high momentum
standing
of the recoil
the field
of the electron.
momentum
photoproduction
in a
seem necessary
and angular
with calcula-
but might indicate
visible
is of the proper
at high momentum
region
and suc-
than hydrogen.
as is most clearly
when q, increases,
which arise
the streamer
and because it shows isotropy
was found to be consistent
momentum
The discrepancy
exchange effects
many qualities
It enabled us to study triplet
way in a material
The measured
concerns
one hopes that this new tool will be widely
used in the future.
distribution.
this experiment
It has been shown that it possesses
energy experiments.
similar
to be drawn from
a
in the momentum
sign to be attributed
transfer.
Further
inves-
to provide
a better
under-
distributions
in pair production
Acknowledgements
We wish to thank R. Mozley for his constant
A. Odian,
problems.
F. Villa
and D. Yount for their
We are grateful
to D. Drickey
phases of the experimental
preparation
for his help in the analysis
problems.
support
help in solving
of this work,
streamer
and
chamber
for the active part he took in all
and in the data taking,
- 12 -
to
and to D. Fries
in
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J. A. Wheeler
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H. A. Bethe,
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Development:
and G. E. Chikovani,
F. Bulos,
R. Mozley,
A. Odian,
JEPT 18, 561 (1964).
F. Villa,
I. Derado,
D. Drickey,
D. Fries,
A. Odian,
F. Villa,
D. Yount,
Experimental
Proposal
for a Study of High Energy
Acad.
Tcheque Sci. 49, 19 (1948).
at SLAC
16.
and
6, 254 (1963).
13.
15.
11, 113 (1959).
V. Votruba,
Bull.
Intern.
and
and
Photoproduction
List of Figures
1. Experimental
setup.
2.
Streamer
chamber
driving
3.
General
shape of the electric
4.
Typical
triplet
system.
pulse.
field direction,
is represented
large dip angle the streamers
5.
in the right
visible
for the recoil
Positrons
passing through a streamer
Recoil
momentum
a continuous
track.
This
chamber
set up in
a
magnetic
field.
may also be seen.
The solid curve is that of Suh and Bethe.
distribution.
using the curvature
the square points have been measured
Experimental
by range.
to form
notice that for
electron.
The full points have been measured
partly
in the electric
side of the picture;
join together
is clearly
One Compton electron
6.
showing the streamers
event; the side view,
partly
and the dip angle;
by the same technique
points are normalized
and
to fit the curve at
q = 1.05 MeV/c.
7a. Angular
distribution
of the recoil
electrons.
The curves
represent
only a
smooth fit to the data.
‘7b. Cos Q/sin2 19 versus
momentum
7c. Angular
The first
8.
Energy
the momentum
of the recoil
distribution
q where
8 and q are the angle and the
electron.
of the recoil
electron;
the abscissa
3 points are not expected to be correct
sharing
distribution
for pairs
produced
The solid curve is that of Bethe and Heitler.
is cos Q/sin’
8 .
(see text).
in the field of the nucleus.
TRIGGER
TAGGING
MAGNET
FI
FCTRnhlS
I
COLLIMATOR
I
HARDENER
Pnl
“““I.
MAGNET
INTFRS
I 68,”
LEFT
I
STREAMER
CHAMBER
RADIATOR /
TRIGGER
COUNTERS
RIGHT
~
A
I’
TAGGING/
COUNTERS
FIG. I
ZV869
t
4
dW
9N IltlOHS
t
AYOCAlddfIS
t
/\yoc+
U3MOd
1 I03
13N9VW
-
I
10
20
nanoseconds
kV
Fig. 3
FIG. 4
PI
u
P
CT t
d++ /dq (mb/MeV/c)
0
0
\o
>,
t
dd
A
w
/
cn
E
w
-
I
0
p=
.
II
.n
-”
\
i
9
0
Y.’
\
)/‘o
‘\o.,
0
0
/-
l\
70
100
50
.
/*;
.
.
.’
.
.
.
0.2 t
I/
01 VI
‘0.1
/
.
0.2
0.5
.
.
.
. .
.
.
..
.
.
.
. .
.
.
.
.
l
.
.
.
.
.
.
--.rLuL
I
.
.
.
l .:.
.
.
. .
:
C
I
I
2
5
q (MeV/c)
FIG. 7b
IO
I
20
I
I
I Ill11
50
100
69Bbtb
‘0.1
0.2
0.5
2
1
cos8/sin28
FIG. 7c
5
10
696A7C
20
al
ln
-\+
73
++ 0
‘a,
2
-cl 0
in
0
iv
0
-