Transcript Tabletop Nuclear Fusion Neutron Activation with a Farnsworth Fusor Carl A. Willis 2 May 2003

Tabletop Nuclear Fusion
Neutron Activation with a Farnsworth Fusor
Carl A. Willis
2 May 2003
Overview
1. Background Information on fusion and neutrons
2. Constructing a Farnsworth Fusor
3. Experiments: Methods and Results
4. Conclusions
5. Acknowledgements…and your questions!
Background: Fusion, Neutrons,
and the Farnsworth Fusor
The Farnsworth Fusor, a soccer-ball-sized fusion
reactor, was invented in 1962 by Philo Farnsworth.
(<www.richmond.infi.net/~rhull/>)
Nuclear Fusion
• In nuclear fusion, light nuclei bind together to form a heavier
nucleus, with the release of kinetic energy.
• Fusion reactions power the Sun and hydrogen bombs, but
controlled power production from fusion is a distant dream for
humans. Fusion reactions can be done in labs with particle
accelerators.
• Examples of fusion reactions:
+ 2H  3He + n
2H + 3He  4He + 1H
2H + 3H  4He + n
2H
Q = 3.2 MeV
Q = 18.4 MeV
Q = 17.6 MeV
• Some fusion reactions are a source for energetic particles such
as protons and neutrons.
Fusion of Deuterium
• By conservation of energy and momentum, we can calculate how
much energy the neutron has. Let mn and mHe be the masses of
the neutron and helion, and vn, vHe be their velocities. Assuming
that the collision energy is very small compared to the Q-value,
mnvn  mHevHe
2
mnvn2  mHevHe
 Q  3.26 MeV
2
• It is found that the neutron has 2.44 MeV.
The Strong Force and the
Coulomb Barrier
• For fusion to occur, nuclei must get close enough for the strong
nuclear force to bind them.
• Nuclei are positively charged, and because like charges are
electrostatically repelled, energy is required to get the nuclei
close together. The electrostatic potential barrier is called the
Coulomb barrier.
• Nuclei do not need enough kinetic energy to overcome the full
height of the Coulomb barrier and get close enough for fusion.
A quantum-mechanical effect called tunneling gives rise to
probability for barrier penetration at low energies.
Potential
Electrostatic and strong nuclear potentials as a function
of separation between two deuterons.
Reaction Probability
The probability P that a given nuclear reaction will occur
between a projectile and a target depends on the
concentration N of target nuclei, the thickness of the target l,
and the cross-section  :
P=Nl
Particle Accelerators
In the linear accelerator pictured, an electric field
accelerates a positively-charged deuteron towards a target
of stationary deuterons. This accelerator design is used
frequently as a fusion neutron source.
The Farnsworth Fusor
• A spherical, electrostatic, particle accelerator and collider
• Allows head-on collisions between accelerated nuclei
• Nuclei circulate radially through a negatively-charged grid until
they collide at the center of the Fusor.
Deuterium Fusion in the Fusor
• Let md be the mass of a deuteron and qe be its charge. In the
ideal Fusor, two deuterons, each traveling at speed v, collide
head-on in the center. Thus, the collision energy Efusor is
E fusor  md v 2  2Vqe
• In the linear accelerator with fixed target, pictured before, two
deuterons traveling at 0.5 v (in the center-of-mass frame of
reference) collide with energy
md v
Vqe


4
2
2
Elinac
• For the same power supply potential, V, the Fusor has
collisions with four times the incoming energy of the collisions in
the linac.
Detecting Neutrons
Neutrons react with boron-10 to create alpha particles (4He),
and this reaction is frequently used to detect neutrons.
10B
+ n  7Li + 4He
The alpha particles are collected on an electrode in a tube
filled with BF3 gas. Each time charge is collected, a count is
registered.
Paul Carter at TUNL checks
our BF3 counter. The
detector tube sits in the
middle of a 9” plastic ball to
slow down fast neutrons. By
itself, the boron reaction will
only detect low-energy
neutrons.
How are Neutrons Useful?
• Neutron activation analysis (NAA) is used to determine the
elemental composition of samples. When neutrons are
captured by natural nuclei, the resulting new nuclei are often
radioactive. The elements in a sample can be determined by
looking at the radiation emitted after neutron bombardment.
• In differential dieaway detection (DDD), neutrons are used to
detect the presence of fissile materials (such as weapons-grade
239Pu).
• Cancer therapy with neutrons is expected to be very effective
against brain tumors. In boron-neutron capture therapy
(BNCT), boron that has been pharmaceutically localized in a
tumor absorbs neutrons and emits alpha particles that kill the
tumor.
Neutron Activation
Capture
Decay
Neutron Activation
Gamma radiation emitted after a pottery sample was
exposed to neutrons in the Missouri University Reactor.
(<www.missouri.edu/~glassock/>)
Building a Farnsworth Fusor
“Holy shit, it works!” After two years under
construction, our Farnsworth Fusor gives
birth to her first neutrons (2 mRem / hr.)
in January, 2003.
Assembling the Apparatus
In theory, the Fusor is simple. You need a
round chamber, a wire grid cathode, and
some high voltage. To do fusion, just add
some deuterium.
In reality, building a
Fusor requires skilled
machine shop labor,
custom welding, and
expensive high-vacuum
fittings. Learning “the
hard way” occurred on
more than one occasion!
Assembling the Apparatus (Ctd.)
The cathode (negative electrode) was made
from six loops of stainless-steel wire, spot
welded together to form a spherical cage.
Left: A “polar” view looking toward the support stalk.
Right: “Equatorial” view.
Power Supply
The Fusor gets high voltage from an x-ray power
supply that is capable of about 70 kV. It uses about
700 watts of power.
Power Supply (Ctd.)
The x-ray machine will
produce sparks about 8”
long, so adequate
insulation is needed to
provide safety for both
the operator and the
equipment
Vacuum and Gas System
Deuterium flows through the Fusor and is exhausted by
high-vacuum pumps. The Fusor uses less than one
standard milliliter of gas per minute, and normal gas
pressure in the Fusor is 0.01 mmHg.
The Finished Fusor
Measurements
• We modified the neutron counter to allow computerized
collection of data.
• Computer-interfaced voltmeters were configured to measure
pressure, voltage, and current.
• Programs in LabVIEW were used to write data files containing
voltage, current, pressure and neutron count information
several times per second.
• Our Fusor initially only produced 2 mRem / hour of neutrons, as
measured 9” from the center. But performance was honed until
regular yields in excess of 60 mRem / hour were obtained.
Given the geometry of the counter setup, we calculated a
corresponding flux of 3106 neutrons / second.
Measurements (Ctd.)
Dependence of neutron output
on potential and current.
Measurements (Ctd.)
Plot showing neutron countrates and gas pressure
as a function of time. Note the pressure “burp”
after the Fusor has been turned off.
Activation Experiments
This Ocean Spray bottle does not contain
fruit juice. Rather, it is full of radioactive
manganese sulfate and is being counted
with a scintillation spectrometer.
Activation Targets
• Test isotopes were chosen on the basis of thermal neutron
capture cross section (c), resonance integral (RI), availability,
and favorable decay characteristics of the capture products.
Table 1. Properties of Activation Targets
(Sources: CRC4, Rad. Health6)
Isotope
55
Mn
127
51
I
V
Al
197
Au
27
Nat.
Abundance
c
(barns)
RI
(barns)
100%
13.3
14.0
T1/2 of
activation
product
2.58 h
100%
6.15
149
25.0 m
99.8%
100%
100%
4.9
0.23
98.7
2.7
0.17
1550
3.75 m
2.31 m
2.70 d
 energies
of activation
product (MeV)
0.847 (99%)
1.811 (29%)
2.110 (15%)
0.441 (14%)
0.528 (1.4%)
1.434 (100%)
1.780 (100%)
0.412 (95%)
• Targets were made from chemical compounds of the selected
test isotopes. The targets had different sizes, shapes, and
physical states.
Activation Targets (Ctd.)
• Since capture is more probable for slow (< 0.5 eV) than for fast
neutrons, a moderator to slow the Fusor’s 2.4 MeV neutrons
was placed around the activation targets.
The moderator assembly is made
from VHS tape cases filled with water.
Hydrogen nuclei in the water slow
down neutrons in successive collisions.
A manganese dioxide target can be
seen here also.
• Targets were irradiated with Fusor neutrons for a timed period
and were then counted with a scintillation spectrometer. This
equipment measures the energy of gamma radiation.
Does the Fusor Make Fast or Slow Neutrons?
Activation of Manganese
Activation of Iodine
Activation of Vanadium
Activation of Gold
Activation of Aluminum
Conclusions
1. Our Farnsworth Fusor produces about 3106
neutrons / second, very similar to the values
reported by others.
2. The Fusor is suitable for neutron activation
analyses. It is simpler than the linac neutron
sources currently in use. It is technically and
financially accessible to a hobbyist.
3. Building the Fusor even smaller than our
current unit would be beneficial.
Acknowledgments
• I am grateful for the efforts and support of my advisor, Rex
Adelberger, and the rest of my committee: Thom Espinola, Rob
Whitnell, and Lisa McLeod.
• This project was made possible financially by Winslow Womack,
whose generosity is deeply appreciated.
• Thanks to the Chemistry Department for “lending” chemicals and
for tolerating the mess I made while building parts of the Fusor!
• My uncle, Ralph Chapman, deserves credit for his help with safety
and “public relations.” Also my father, Robert Willis, and many of
my fellow students have made valuable contributions to my efforts.
• Paul Carter and Chris Westerfeldt at TUNL helped calibrate our
neutron counter at no charge. I thank them for their time.
• The many contributing members of the Open Source Fusor
Research Forum <www.fusor.net> have provided immense help
and encouragement.
A Portrait of Fusion
Beautiful glowing beams of deuterium nuclei converge in the center of the
Fusor. This photo through the viewport was made while running full power,
and the intense radiation caused damage to the camera’s CCD.
References
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April 05.
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May 1.