INTRODUCTION
This
research is focused on understanding the pyroelectric effect
in order to use it to effectively accelerate
electrons and ions. The accelerated electrons
and ions can be used to produce beams of electrons,
ions, x-rays, and possibly monoenergetic neutrons and gamma rays. The first application
of the pyroelectric effect
for x-ray production was reported by Brownridge.
The first commercial compact x-ray source was
manufactured by Amptek.
The goal of our research is to further develop
this technology in order to achieve higher acceleration
potential and current. Such improvements will
allow us to use this technology to create useful
highly compact, battery operated, high energy
x-rays, and neutron sources.
Theory of Operation
Pyroelectric crystals
are spontaneously polarized along an axis. In
other words, they have a bulk dipole moment that
exists in equilibrium conditions and does not
sum to zero over the volume of the
crystal. This polarization is usually masked
by the gradual accumulation of surface charges. The
distinguishing trait of pyroelectric crystals
is that the polarization of the crystal is altered
dramatically with changes in the temperature
of the crystal, such that the polarization is
no longer masked and a large voltage can be observed
across the crystal.
The pyroelectric effect
allows to easily achieve very
high electric potentials by heating a small (~4mm
x 4mm x 4mm) crystal in a low-pressure (0.5-10 mTorr)
environment. We successfully used this effect
to demonstrate production of x-rays with energies
up to 200 keV.
EXPERIMENTS
Most
of the work is done with LiTaO3 but
some was also done with LiNbO3. The
crystals range in thickness from 1 to 10 mm with
typical area of 14-25 mm2. During
operation, the crystals are maintained in
a vacuum chamber with a pressure of 0.5-10 mTorr.
The crystals are heated to about 160 ºC and the
measurements are done during the crystal heating
or cooling phases. Most of the emissions occur
at temperatures above 30 ºC.
Electrons
and ions
Several
measurements were done using a surface barrier
detector to directly detect electrons and ions. A
typical electron spectrum and ion spectra are
shown in figures 1 and figure 2. The electron
spectrum is obtained during a cooling cycle with
Z- side of the crystal is facing the surface
barrier detector. The ion spectrum is obtained
by revering the crystal polarity such that during
the cooling cycle the Z+ is facing the detector.
The spectrum in figure 1(a) shows unique pileup
peaks, calculations indicates that this pileup
is much higher than what is expected from random
coincidence pile-up. This gives an indicates
on the "bunched"
nature
of the electron emission from these crystals.
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(a)
|
(b)
|
Figure 1 – Typical electron
spectrum from a single LiTaO3 crystal,
measured using a surface barrier detector (a)
pileup spectrum (b) high energy spectrum
Figure 2 – Typical ion spectrum
obtained during cooling of a LiTaO3 crystal with
the Z+ facing the detector.
X-ray Production
When accelerate electrons hit
the crystals or other metallic targets bremsstrahlung and
characteristic X-rays are produced. These photons
are detected using an Amptek x-ray
detector XR-100CDT.
We examined the possibility of multiplying the
acceleration potential by using two pyroelectric crystals.
Figure 3 shows the geometry of such an experiment
with paired crystals and Figure 4 shows the typical
spectrum, A 6 mm thick SST absorber was used
in order to reduce
the
low
energy x-ray
rate.
|
Figure 3 –paired crystal
system.
|
Figure 4 – The x-ray spectrum
sum from 5 cooling cycles.
|
Such spectrum has high enough
energy to fluoresce lead and thorium. The geometry
use to demonstrate thorium fluorescent is shown
in Figure 5; such geometry utilized the pyroelectrically generated
x-rays to fluoresce the thorium sample in transmission
geometry and the spectrum obtained is shown in
figure 6. In this case the sample is radioactive
and will fluoresce by excitations from the radioactive
decay. This emission is treated as background, figure
6 shows the background and the pyro-x-rays
induced fluorescent.
|
Figure
5 – The geometry
used for lead florescent measurement.
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Figure
6 – x-ray florescent
spectrum from thorium.
|
Lead fluorescet
was done in a “reflection” geometry and the results
are show in Figure 7.
Figure 7 – x-ray florescent
spectrum from lead.
NEUTRON PRODUCTION
Neutron production from such
device is now under investigation. The idea is
to use a paired crystal accelerator with a low
pressure fille (5-20 mTorr)
of D2 gas. In addition to the gas
we would use a deuterium enriched target
(such as deuterated plastic).
The deuterium will be accelerated towards the deuterated target
and if the right conditions exist, a D-D reaction
will take place. This reaction is accompanies
by a 2.5 MeV neutrons
that can be detected outside the vacuum chamber.
This type of accelerator can also be used on
a target containing tritium to obtain a higher
yield of D-T neutrons with energy of 14 MeV.
Check our latest publication below which provides
all the details of our neutron generator.
This figure shows the neutron
output from our "Doubel Crystal Fusion" device.
The neutron spectrum is measured using a liquid
scintilator. The spectrum was calcibrated using
gamma sources and thus
the
x-axis energy
is given
in
units
of recoiled electron energy.
PUBLICATIONS
Jeffrey
Geuther, Yaron Danon, Frank Saglime, Bryndol
Sones, "Electron Acceleration for X-ray Production Using Paired Pyroelectric Crystals", abstracts of the sixth
International Meeting on Nuclear Applications
of Accelerator Technology, AccApp’03, p.
124, San Diego, June 1-5, 2003
Jeffrey Geuther and Yaron
Danon, "Increasing X-Ray Energy By Paring
Pyroelectric Crystals", (Invited), 18th
International Conference on the Application
of Accelerators in Research and Industry (CAARI
2004), Ft Worth TX, October 10-15, 2004.
Jeffrey
A. Geuther, Yaron Danon," Pyroelectric
Electron Acceleration: Improvements and Future
Applications",
ANS Winter Meeting Washington, D.C, November
14 – 18, 2004.
Jeffrey
A. Geuther, Yaron Danon, "Electron and
Positive Ion Acceleration with Pyroelectric
Crystals", J. Appl. Phys.
97,
074109 (2005).
Jeffrey
A. Geuther, Yaron Danon, "High-Energy
X Ray Production with Pyroelectric Crystals",
J. Appl. Phys. 97, 104916 (2005).
Jeffrey
Geuther, Yaron Danon and Frank Saglime, “Nuclear
reactions induced by a pyroelectric accelerator”, Physical
Review Letters, 96 054803 (2006).
Media Coverage of "Double
Crystal Fusion"
"RPI claims battery-fueled, room-temp fusion", EETimes,
02/20/2006.
"Double Crystal Fusion' Could Pave The
Way For Portable Device", Science
Daily, February 14, 2006.
"Tabletop nuclear fusion device developed", Physorg.com,
February 13, 2006.
This
research is sponsored in part for DOE NEER
grant no. 04ID14596
