Electron Beam Production by Pyroelectric Crystals
James D. Brownridgea
Department of Physics, Applied Physics, and Astronomy
State University of New York at Binghamton, P.O. Box 6016 Binghamton, New York
13902-6016
Stephen M. Shafrothb
Physics and Astronomy Department, University of North Carolina at Chapel Hill, Chapel
Hill North Carolina 27599-3255
Abstract
Pyroelectric crystals are used to produce self-focused electron beams with energies greater than 170 keV.
No high voltage power supply or electron gun is needed. The system works by simply changing the
temperature of a crystal of LiNbO3 or LiTaO3 by about 100oC in dilute gas. Electron beam energy spectra
as well as positive-ion-beam energy spectra and profiles are shown. A change in the crystal temperature of
100oC will cause a spontaneous change in polarization. The change in polarization will be manifested by a
change in charge on the surface of the crystal. It is this uncompensated charge that produces the electric
field, which accelerates the electrons, or the positive ions and gives rise to the plasma, which in turn
focuses them. The source of the accelerated electrons or positive ions is gas molecules ionized near the
crystal surface. When the crystal surface is negative electrons are accelerated away from it and positive
ions are attracted to the surface. These positive ions reduce the net negative charge on the surface thereby
reducing the electric field, which causes the electron energy to decrease over time even though the focal
properties remain unchanged. When the surface is positive the reverse obtains and the positive ion beam
energy decreases over time as well. We will present video clips, photographic and electronic data that
demonstrate many of the characteristics and applications of these electron beams.
Introduction
Even though pyroelectric crystals
have been known since ancient Greek
times1, surprising new effects and
practical applications are constantly
being discovered 1-9.
There is a
voluminous literature on electron
emission by pyroelectric crystals.
Reference 10, a comprehensive review
article and (11-15) are representative
but none of them suggests that energetic
electron beams are produced by such
crystals. Much of the current industrial
use of these crystals is in infrared
detection, sensitive temperature change
detectors and photonics, e.g., the
detection of THz radiation16 in LiNbO3.
For these reasons few studies of the
behavior of these crystals in dilute gases
have previously been done and so no one
has
previously
discovered
the
phenomena being reported on here.
The
first
known
naturally
occurring pyroelectric crystals were
kidney stones of lynx1. All pyroelectric
crystals including artificially produced
LiNbO3 and LaTaO3 are asymmetric
crystals.
They become electrically
charged on heating and cooling and give
rise to electric fields at their surfaces of
up to 106 V/cm18. The opposite side of
a crystal which is heated from its + z
base becomes positively charged on
heating and negatively charged on
cooling. This is due to the movement of
Li+ and Nb- ions relative to the oxygen
lattice12. If the crystal is immersed in a
dilute gas the net charge on the crystal
surface at any time will depend on the
gas type and pressure as well because
when the surface is negative, positive
ions from the gas will bombard it and
when the surface is positive, electrons
from the ionized gas will also bombard
it. Crystal x-ray emission occurs on
heating and target x-ray emission on
cooling when the -z base is exposed and
light is produced both on heating and on
cooling in the plasma adjacent to the
crystal.
Chronologically, JDB2 reported on
the production of characteristic x-rays by
cesium nitrate crystals in 1992. Then in
1999 he and S. Raboy wrote a
comprehensive paper3 describing the xray emission and light production in the
plasma for several different crystals
and gases. Also in 1999 Shafroth et. al.
studied the time dependence of x-ray
emission4. Further they studied the
effect on total x-ray yield for five
different gases and found a marked
decrease in yield for O2 which was due
to the production of ozone in the plasma.
Also in 1999 Shafroth and Brownridge
reported on the use of pyroelectric
crystals in the teaching of x-ray physics5.
Then in 2001 JDB and collaborators
reported on the production of energetic
electrons by pyroelectric crystals in
dilute gases6. They found that multiple,
energetic
(~100
keV)
nearly
monoenergetic electrons were produced
on cooling these crystals when the - z
base faced the detector. Later JDB and
others7 found that the maximum energy
of the emitted electrons depended
somewhat on gas type but strongly on
pressure. Then JDB and SMS found8
that if the crystals were ground to a
cylindrical shape that focused beams of
electrons were produced as the crystal
cooled and focused beams of positive
gas ions were produced as the crystal
was heated. Most recently, S. Bedair et.
al., using a LabVIEW program have
studied the effect of pressure on the time
dependent yield of crystal x-rays and
target current9.
Fig. 1. (a) Photograph of the experimental
arrangement with ZnS screen, crystal, and heater
resistor, (b) Beam spot at ~0.5 mtorr, (c) ~3
mtorr, (d) ~8 mtorr, where the beam blows up.
Note how the beam is defocused as pressure
increases.
Focused Electron Beams
This report describes the production
of self-focusing electron beams arising
from near pyroelectric crystal surfaces in
dilute gases after the crystal has been
heated
and
returned
to
room
temperature. In one case a 5 mm diam x
5 mm cylindrical LiNbO3 crystal in an
atmosphere of <10 mtorr of dry N2 was
contained in a glass tube.
30000
~170 keV
25000
Counts
8000
o
Temp. 65 C
~127 keV
6000
4000
2000
~254 keV
~381 keV
0
16000
0
50
100
150
200
250
300
350
400
450
Counts
500
550
600
o
~149 keV
Temp. 44 C
12000
8000
4000
~298 keV
~447 keV
0
0
50
100
150
200
250
300
350
400
450
500
550
600
6000
Counts
o
Temp. 25 C
~124 keV
4000
2000
~248 keV
~372 keV
0
0
50
100
150
200
250
300
350
400
450
500
550
600
keV
Fig. 2. Electron Energy Spectra taken with a 10
mm x 4mm dia LiNbO3 crystal in 2.7 mTorr of
N2 .
20000
Counts
The crystal was heated to about 115 0C
and then allowed to cool to room
temperature. Heat was supplied by
passing ~100 mA through a wire-wound
60 ohm resistor which was epoxied to
the crystal's + z base. After cooling to
room
temperature,
spatially-stable
electron beams were produced. They
were made visible by placing a ZnS
screen at the focal length (17 mm) of the
crystal as illustrated in Fig. 1(a), which
shows a photograph of the ZnS screen;
the cylindrical crystal and the 60 ohm
wire-wound resistor used to heat the
crystal.
We studied the effect of
pressure on the beam. Typical results at
~0.5 mtorr 1(b), ~3 mtorr 1(c) and ~8
mtorr 1(d) are shown in Fig 1(b), 1(c),
1(d) respectively.
As the pressure
increased, the beam spot became more
diffuse and brighter. At ~8 mtorr the
beam blew up and its intensity dropped
to zero. The dynamical behavior of the
electron beam with pressure, which is
illustrated in the video clip17 suggests a
gas-multiplication effect. The focusing
effect may be due to plasma focusing.
All results reported on in this paper were
repeated many times.
15000
10000
5000
~340 keV
~511 keV
~680 keV
0
0
100
200
300
400
500
600
700
800
900
1000
Electron energy (keV)
Fig. 3. Electron energy spectrum for highest
energy electron beam yet obtained. Same
conditions as Fig. 2.
Fig. 4. Schematic diagram of the experimental
arrangement for scanning the electron beam.
The slit and detector move. The dotted lines
show an idealized cartoon of the crystal focused
electron beam.
Next a 100 μm surface-barrier
electron detector was installed in place
of the ZnS screen so that electron energy
spectra could be recorded. The timedependent
electron-beam
energy
spectrum, which was taken in "snapshot
mode", i.e. over an interval of <60 s,
typically, exhibited discrete peaks at
integral multiples of the lowest energy
peak. This indicated that nearly
monoenergetic
multiple
electron
production6 was occurring.
Typical
electron energy spectra are shown in Fig.
2. where the multiple peak effect due to
pile up is shown at three different crystal
temperatures. Fig. 3. shows the highest
energy electron spectra so far obtained.
The experimental arrangement for
scanning the electron beam profile is
10 0
22mm
21mm
75
75
50
50
50
25
25
25
0
10 0
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
Counts
0
10 0
10 00
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
10 00
10 0
19mm
0
10 0
75
50
50
25
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
10 0
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
10 00
10 0
16mm
0
10 0
75
50
50
25
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
10 0
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
10 00
10 0
13mm
50
50
25
25
50 0
60 0
70 0
80 0
90 0
10 00
30 0
40 0
50 0
10 00
60 0
70 0
80 0
90 0
10 00
30 0
40 0
50 0
60 0
70 0
80 0
90 0
0
10 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
10 00
0
10 0
Counts
18
16
14
12
12
12
10
10
10
8
8
8
6
6
6
4
4
4
2
2
0
0
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
10 00
Counts
15 mm
18
0
100 200 300 400 500 600 700 800 900 1000
20
16 mm
18
16
16
16
14
14
14
12
12
12
10
10
10
8
8
8
6
6
6
4
4
2
2
0
0
20
18
18
4
0
100 200 300 400 500 600 700 800 900 1000
20
19 mm
18
16
16
16
14
14
14
12
12
12
10
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
0
100 200 300 400 500 600 700 800 900 1000
17 mm
2
100 200 300 400 500 600 700 800 900 1000
20
18 mm
14 mm
2
100 200 300 400 500 600 700 800 900 1000
20
100 200 300 400 500 600 700 800 900 1000
Counts
18
14
18
60
40
40
20
1
2
3
4
5
6
7
8
9
10
11
Focal length
20
13 mm
16
14
20
80
45
Slit position (mm)
16
100 200 300 400 500 600 700 800 900 1000
100
50
25
20 0
20
12 mm
120
55
10 00
Fig. 5. Beam scans taken at different crystal-toslit distances. Each scan lasted ~30 s. The focal
length is ~16.5 mm. Profiles were taken in the
order shown.
18
60
0
20 0
Slit/detector position as it is moved across the beam
20
140
35
16.5mm
50
40 0
90 0
10 0
75
30 0
80 0
65
30
12mm
75
20 0
20 0
0
10 0
75
0
10 0
70 0
25
0
10 0
10 00
60 0
14mm
75
50
0
10 0
50 0
160
10 0
15mm
75
25
40 0
25
0
10 0
10 00
30 0
17mm
18mm
75
50
25
20 0
10 0
75
0
10 0
Counts
20mm
75
10 0
Counts
10 0
70
Electron count rate (c/sec)
Counts
10 0
5. shows results of ~ 30 s scans taken at
different crystal-to-slit distances.
Electron beam energy (keV)
shown schematically in Fig. 4. The
crystal and heater assembly could be
moved toward or away from the
detector, which was useful for checking
the focusing effect.
100 200 300 400 500 600 700 800 900 1000
20 mm
100 200 300 400 500 600 700 800 900 1000
Slit/detector position as it is moved across the beam
Fig. 6. Electron beam profiles taken 16 hours
later. The focal length is still ~16.5 mm. The
beam intensity is down after 16 hours of
continuous electron emission, however it is still
focused.
The detector was covered by a lead
screen with a 0.1mm slit so that the
beam profile could be obtained by
moving the slit-detector assembly at
constant speed across the beam at
different crystal-to-slit distances. Fig.
Fig. 7. The electron beam count rate profile for
a 10 mm x 4 mm LiNbO3 crystal (open circles).
The electron beam energy (solid circles) is
shown for each slit position. Note the linear
decrease in energy as the scans progress and as
time increases. (See Fig 8 for comparison).
All pulses corresponding to electron
energies > 15 keV were counted for
these scans. From this figure it can be
seen that the best focus is obtained 16.5
mm crystal-to-slit distance. Fig. 6. shows
electron beam scans taken 16 hours later.
The focal length is still 16.5 mm. Fig. 7.
obtained with a 4mm diam x 10 mm
LiNbO3 crystal shows the electron beam
count rate profile as the detector and slit
are scanned across the beam at a
distance of 20 mm. The FWHM is about
4 mm. Also shown is the electron
energy at each slit position. The uniform
decrease in electron energy with slit
position and time is similar to that
described below.
Electron Beam Energy Spectra
Once the crystal reached room
temperature the beam energy decreased
nearly exponentially with time (Fig. 8);
faster with increasing pressure. In order
to produce higher energy electrons the
crystal was heated from the + z base to
about 160 0C and allowed to cool to
room temperature at <10 mtorr.
Electron energy (keV)
100
10
1
0
25
50
75
100 125 150 175 200 225 250 275 300 325
Time (minutes)
Fig. 8. Electron beam energy as a function of
time after the crystal has reached room
temperature. Note the nearly exponential decay
of the beam energy.
The maximum energy electrons (170
keV) were produced after the crystal
temperature had dropped to ~30 0C.
Independent proof that the electron beam
energy was at least 80 keV (Au K edge)
for a significant time was confirmed by
irradiating gold and observing the K Xrays2,3. Gold K electrons were being
ionized thereby producing Au K X-rays
as long as the incident electron energy
was greater than 80 keV i.e. starting as
the crystal cools from 160 0C when the
increasing electron energy reached 80
keV and continuing until the maximum
energy (170 keV) is reached and then
until the electron energy falls to less than
80 keV, sometime after the crystal has
reached room temperature.
Phenomenological
These Processes
Description
of
If the crystal is subjected to a rapid
temperature change the polarization will
be manifested briefly by the presence of
an electric field whose strength is
proportional to the surface charge
density, which in turn is a function of the
change in temperature and depends on
the ambient pressure. When the crystal
is in a reduced pressure environment and
is subjected to the same temperature
change, the polarization change is
manifested by an electric field that lasts
much longer; at a pressure of about 10-6
torr, it takes more than 30 hours for the
resulting surface charge to be
neutralized, whereas at 1 atm
neutralization is virtually instantaneous3.
The neutralization of the electric field
produced by a change in polarization is
due to bombardment of and attachment
to the surface by positive ions. To
summarize, as long as the net surface
charge is sufficiently negative, electrons
are accelerated away from the crystal in
a focused beam whose energy can be up
to 170 keV after which the energy of the
electrons in the beam decreases nearly
exponentially with time until the
electrons become undetectable. It is
remarkable that the beam becomes
spatially stable once the crystal has
reached room temperature, even before
in some cases.
Positive Ion Beams
We believe that singly charged
positive ion beams arising from the
residual gas have also been observed.
For practical reasons they can best be
studied by exposing the + z base of the
crystal and taking measurements as the
crystal cools, although the same
phenomena can be observed on heating a
crystal with its -z base exposed. Figure 9
shows an energy spectrum obtained over
a period of 35 s with a crystal of
LiNbO3, 10 mm x 4mm dia heated to
1700C after the crystal had
mm (the focal length). On the other
hand the N2+ ion beam energy decreases
by about 5% as the slit passes through
the focal point of the beam as indicated
by the count rate increase.
3000
~100 keV
2500
~34 keV
1500
125
1 mm
Crystal
1000
5 mm
d
~62 keV
0
0
25
50
75
100
125
150
175
200
225
250
N2 Ion energy (keV)
Fig. 9. Energy spectrum (30 s snapshot) of N2+
ion beam obtained with a. LiNbO3 crystal on
cooling from 170 0C. The + z base is exposed.
The spectrum was obtained after cooling to 35
0
C. The gas was N2.
Ion beam energy (keV)
120
500
9 mm
115
110
105
d = 23 mm
d = 21 mm
d = 80 mm
100
0
1
2
3
4
5
6
7
8
9
10
11
12
Slit position (mm)
130
45000
40000
125
35000
120
30000
25000
115
20000
15000
110
Ion count rate (c/sec)
Ion beam energy (keV)
Counts
2000
10000
105
5000
100
0
0
1
2
3
4
5
6
7
8
9
10
11
Slit position (mm)
Fig. 10. The N2+ ion beam count rate profile
(open circles) and the beam energy (solid circles)
at each slit position. This should be compared
with the electron beam results in Fig. 7, where
the beam energy is unaffected by the slit being at
the focal position.
cooled to ~350C . Here the gas was N2
so the beam was presumably N2+. The
main component of the beam has an
energy of ~100 keV but there are
secondary components with energies of
~34 and ~62 keV, which we don't
understand.
Figure 10 shows the N2+ ion
beam profile, which is also about 4mm
when the crystal-to-slit distance is 23
Fig. 11. Ion beam energies at different slit
positions and at different crystal-to-slit distances.
Note that the maximum ion beam energy
decrease occurs at the crystal focal length, which
is 23mm.
Finally Fig. 11 shows results of a study
of the N2+ ion beam energy vs slit
position for three different crystal-to-slit
distances. In this case the pressuresensitive energy-drop at 23 mm (the
focal length) is nearly 10%.
In
conclusion
self-focused
energetic (<170 keV) electron beams are
produced by heating to 1600C cylindrical
LiNbO3 crystals with the - z base
exposed in dilute gases at 2-8 mtorr on
cooling. Conversely, positive gas-ion
beams are produced by heating LiNbO3
crystals when the + z base is exposed
again on cooling.
Acknowledgments
We are grateful to our
colleagues, Tom Clegg, Bill Hooke, Sol
Raboy, Brian Stoner and Eugen
Merzbucher for insightful discussions.
a)
e-mail jdbjdb@binghamton.edu
e-mail shafroth@physics.unc.edu
J. Appl. Phys. 88, 6109 (2000).
b)
1. S. B. Lang, Sourcebook of
Pyroelectricity, Gordon and Breach,
New York 1974
2. J. D. Brownridge, Nature (London)
358, 278 (1992)
3. James Brownridge, and Sol Raboy,
J. Appl. Phys. 86, 640 (1999)
4. S. M. Shafroth , W. Kruger and J.D.
Brownridge, Nucl. Instr. and Meth., A
422, 1 (1999)
5. S. M. Shafroth and J. D.
Brownridge, CP 475 American Institute
of Physics , Applications of Accelerators
in Research and Industry, Edited by J. L.
Duggan and I. L. Morgan, p 1100 (1999)
6. J. D. Brownridge, S. M. Shafroth,
D. Trott, B. Stoner, W. Hooke, Appl.
Phys.
Lett, 78, 1158 (2001)
7. J. D. Brownridge and S. M.
Shafroth, Bull Am. Phys. Soc. 46, 106 &
164 (2001)
8. J. D. Brownridge and S. M.
Shafroth, Appl. Phys. Lett. 79, 3364
(2001)
9. S. Bedair ,S. M. Shafroth and J. D.
Brownridge, Bull. Am. Phys. Soc. 47, 65
(2002)
10 G. Rosenman, D. Shur, Ya. E.
Krasik and A. Dunaevsky,
11. B. Rosenblum, P. Brunlich, and J.
P. Carrico, Appl. Phys. Lett. 25,
17(1974).
12. R. S. Weis and T. K. Gaylord,
Appl. Phys. A 37, 191 (1985).
13. G. I. Rozenman, V. A. Bodyagin,
Yu. L. Chepelev, and L. Ye. Isakova,
Scripta Technica, ISSN in
Radiotekhnika i elektronika, No9,
(1987) pp. 1997- 1999.
14. D. Shur, G Rosenman, and Ya. E.
Krasik, Appl. Phys. Lett. 70, 574 (1997).
15. Zdenek Sroubek, J. Appl. Phys. 88,
4452 (2000).
16. C. Winnewisser, P. Uhd Jepsen, M.
Schall,V. V. Schyja and H. Helm, Appl.
Phys. Lett. 70 3069 (1997).
17. J. D. Brownridge, Video clip,
Dynamical Behavior of Beam as
Pressure Increases,
http://www.binghamton.edu/physics/bro
wnridge.html
18. V. S. Kortov, A. F. Zatsepin, A. I.
Gaprindashvili, M. M. Pinaeva, V. S.
Vasil"ev and I. A. Morozov, Sov. Phys.
Tech. Phys. 25(9) 1126 (1981)