thesis2

Neutron Activation Using a Farnsworth Fusor
Carl A. Willis
A thesis submitted to the faculty of Guilford College
in partial fulfillment of the requirements for
the bachelor’s degree in Physics
Physics Department
May 5, 2003
Committee:
________________________________
Rex Adelberger, Chair
________________________________
Thom Espinola
_________________________________
Rob Whitnell
________________________________
Lisa McLeod

Table of Contents
Abstract
1
Introduction: Nuclear Fusion, Neutrons, and the Farnsworth Fusor 1
Experimental Apparatus and Procedures
I. Overview 11
II. Fusor Construction
a. Vacuum Chamber and Cathode Grid 12
b. Gas Handling and Vacuum System 13
c. High Voltage Power 13
d. Automated Measurements 14
III. Neutron Counting 15
IV. Activation Experiments
a. Target Preparation 16
b. Irradiation Methods 18
c. Gamma Spectroscopy 18
Results
I. Activation Gamma Spectra 19
II. Prompt Gamma Spectra 19
III. Other Measurements and Observations 19
Discussion: Activation Analysis, Cheap and Easy 31
Conclusions 33
Acknowledgments 35
References
Appendices
Appendix A: Fusor Machine Shop Drawings
Appendix B: Photographs of Apparatus
Appendix C: Power Supply Schematics and Details; Gas Flow Diagram
Appendix D: Neutron Fluence-to-Dose Chart
Appendix E: LabVIEW Data Handling Programs (CD-ROM)
2

Abstract
The Farnsworth Fusor is a spherically focused, electrostatic, particle
accelerator and collider. It has received little scientific attention since conception
in 1962 as an experimental nuclear fusion reactor, but offers unique potential
benefits as a fusion neutron source. We explored a simple embodiment of the
Farnsworth Fusor for use as a deuterium fusion neutron source, and tested its
performance in neutron activation studies with eight natural isotopes. Our Fusor
emitted a measured neutron flux of up to 3.0×106
neutrons / sec., similar to results
from other comparable Fusors. Activation of 55
Mn, 127
I, 51
V, 27
Al, and 197
Au was
successful, and prompt capture gamma rays were identified from 10
B, 113
Cd, and
1
H targets. The Fusor whose construction is described here is built on a budget of
about $2500. Throughout its 10+ hour operating history, it has proven to be a
stable and reliable device. For these reasons it should appeal to students, small
schools and laboratories, hobbyists, and others who have an interest in neutrons.
Introduction: Nuclear Fusion, Neutrons, and the Farnsworth Fusor
The “Farnsworth Fusor,” a little-known device invented in 19621
, is a fully
functional nuclear fusion reactor approximately the size of a soccer ball.
Conceived as a potential method to satisfy society’s energy demands by
producing power from the same nuclear reactions that fuel the stars, the Fusor has
instead faded into obscurity. (Philo Farnsworth’s other major invention—called
television—has surpassed all initial expectations and amassed a huge following.2
)
Actually, the reality in 2003 is that no fusion reactors (Farnsworth Fusors
included) have been able to release more usable energy than they consume.
Today’s finest experimental fusion systems include the 120-foot-diameter “Z
Machine” at Sandia National Laboratory3
; the 1.8 megajoule laser at the National
Ignition Facility4
, and the multibillion-dollar ITER5
tokamak. Unlike any of these
giant and complex machines, a Farnsworth Fusor can be built on a bench top by a
student for no more than the cost of a “set of golf clubs.6
” In 1997, amateur
scientist Richard Hull built the first homebrew Fusor to verifiably cause fusion of
hydrogen isotopes7
. The simplest Fusor can release more fusion energy in only a
few hours of operation than a firing of the massive Z Machine3
. Even so, the
Fusor does not embody the magic energy solution of the future—it has numerous
and serious shortcomings when compared to today’s cutting-edge attempts at
break-even fusion. But there are other reasons besides energy production to be
interested in a desktop fusion reactor.
Fusion reactions can release energetic subatomic particles such as protons
and neutrons that have immediate utility in research, medicine, and even national
security applications. The Farnsworth Fusor, as a fusion-based particle source,
may be worth a closer look. The central aim of this thesis has been to examine
3

the Fusor as a neutron source. As means of introduction, I shall first summarize
the physics of nuclear fusion, and then explain how the Farnsworth Fusor
accomplishes fusion and how it is built. Finally, I will delve into some detail
about the properties and important applications of neutrons, and how these
applications could benefit from the advantages the Farnsworth Fusor has to offer.
Fusion occurs when two nuclei combine to form a new, heavier nucleus.
Nuclei themselves are composed of protons (which have a positive charge,
symbol p) and neutrons (which are electrically neutral, symbol n). The number of
protons in a nucleus determines to which element the nucleus belongs. The
hydrogen atom has one proton in its nucleus, while helium has two. An element’s
nuclei do not, however, all have the same number of neutrons. Hydrogen nuclei
can have no neutrons (1
H), one neutron (2
H), or two (3
H). These isotopes of the
element differ in atomic mass because they contain different numbers of neutrons.
It so happens that nuclei of certain size and p / n ratio are more energetically
stable than other nuclear configurations, and achieving greater stability is the
driving force behind both fission (the breaking up of heavy nuclei) and fusion (the
combination of light nuclei). An example of fusion that will be revisited often in
this work involves combining two deuterium (2
H) nuclei to yield helium (3
He) and
a free neutron. This reaction is depicted both in equation form and pictorially in
Fig. 1.
Figure 1. A fusion reaction schematically (top), and described in
two notations (middle, bottom). Shown is the fusion of two
deuterium nuclei (also called deuterons) to form 3
He and a
neutron. Not indicated is the large amount of energy released
in this reaction, which causes the products to move away at
high velocity. In the middle notation, the reactants and
products are described by their elemental symbol (e.g. H), the
atomic mass (superscript), and the atomic number—the number
of protons (subscript). The bottom notation is a shorthand in
the form: target(projectile, residual)residual. The d is a
special projectile notation for 2
H.
The barrier that prevents fusion from occurring among all the light nuclei
on earth is the electrostatic repulsive force that nuclei feel from each other due to
their positive charge. This is known as the “Coulomb barrier.” Fig. 2 shows the
potential felt by two deuterons as a function of separation.8
Unless nuclei collide
with high enough energy, they are simply repelled from one another and no
nuclear reaction occurs—they do not come within range of the strong nuclear
4

force that is responsible for binding nuclei. Various technologies have been
developed for accelerating nuclei to high velocity to allow significant probability
of nuclear fusion occurring. The oldest method of doing this is the linear
accelerator, sometimes called an “atom smasher” in popular parlance (see Fig. 4).
Small linear accelerators are currently used for deuterium fusion neutron sources.
Fortunately, to participate in reactions, nuclei are not required to have enough
energy to fully overcome the Coulomb barrier; the quantum-mechanical tunneling
effect8
allows the barrier to be breached by nuclei of lower energy. Thus,
referring to Fig. 2, one deuteron need not have >500 keV kinetic energy to come
sufficiently close to the other for the strong nuclear force to predominate and
“fuse” the deuterons.
Not all nuclei with enough energy to penetrate the Coulomb barrier will
actually do so. Many will simply collide elastically, like rubber balls. Since
probability of tunneling through the Coulomb barrier increases strongly with the
kinetic energy of the colliding nuclei, the probability of fusion likewise gets
higher. The probability Σ of a reaction between a projectile and a target is
expressed by physicists in terms of the cross section σ9
:
(1)
N is the number of target nuclei per unit volume and l is the thickness of the
target. So having a higher density of target nuclei, or having a thicker target,
increases a projectile’s chance of reaction. The cross section has units of area; in
nuclear physics, the barn (10-28
m2
) is often used. It represents not the physical
cross-sectional area of the target nucleus (although that is part of it), but the
apparent cross-sectional area. Fig. 3 shows the cross section for the deuterium
fusion reaction shown in Fig. 1 above, as a function of energy. As could be
expected from the theory of tunneling, cross section increases as energy increases.
5
σlN=Σ

Figure 2. A model of electrostatic and strong nuclear potentials
(superposed), as a function of separation between two deuterons.
As the separation becomes smaller, the potential rises and peaks
at about 550 keV. Upon further decreases in separation, the
strong nuclear force enters the picture and binds the nuclei
together (fusion).8
6

Figure 3. Cross section (in millibarns) as a function of energy
for the 2
H(d,n)3
He reaction.22
Figure 4. A linear accelerator (linac), in which an electric field
is established between two electrodes (+) and (-) by a
high-voltage power source (HV). A deuteron is shown
being accelerated towards the negative electrode, on
a trajectory that will cause it to collide with other
stationary nuclei in a target zone. Linear accelerators like
these are used as fusion neutron sources.28
The Farnsworth Fusor is a variant on the linear accelerator, having a
spherical arrangement of the electrodes. A cathode (-) frequently made from a
spherical cage of fine wire sits concentrically within a surrounding anode shell
(+). When charged particles, such as deuterons, accelerate radially inward toward
the cathode, most of them pass through it and collide centrally with other
particles. Those that avoid collision pass out the opposite side of the cathode,
only to be recirculated back through it by the electric field until a collision occurs.
Fig. 5 shows a conceptual cutaway view of a Farnsworth fusor.
7

Figure 5. Conceptual cutaway of the Farnsworth Fusor. In this
spherical electrostatic accelerator, positively-charged nuclei
collide at the center after being pulled through the cathode
by its high negative potential. Thermonuclear fusion is a result
of some of these collisions when nuclei such as 2
H, 3
H, or 3
He
are present. The anode shell on most operating Fusors is
about the size of a soccer ball; HV power supply requirements
are usually <100 kV.
The first apparent advantage of the Fusor over the linear accelerator for
doing fusion is that, for a given acceleration potential, it can manage collisions
that are up to four times as energetic. This is possible when two nuclei collide
head-on in the center. In the head-on collision, both particles are moving with
speed v after “falling” through the potential difference ∆V between anode and
cathode. The following equations express relationships between particle velocity
and maximum collision energy E in both the simple linear accelerator and the
Fusor in a center-of-mass reference frame. Here q is charge and m is particle
mass:
(2)
(3)
(4)
The Farnsworth Fusor can cause collisions four times as energetic as collisions in
a linear accelerator using the same accelerating potential ∆V, and therefore it has
the capability of operating with higher effective energy on the cross-section curve.
8
linacfusor
linac
EVqmv
mv
E
Vq
vmE
m
q
Vv
42
2
2
2
)2/(
2
2
2
2
=∆===
∆
==
∆=

The recirculating mechanism is another way in which the Fusor is advantageous
with respect to the linear accelerator discussed earlier. In the linear accelerator,
particles that fail to collide with the target are lost and their energy is dissipated
unproductively, whereas in the Fusor, deuterons ideally circulate through the
cathode grid until they collide and react. Finally, although the figures presented
cannot convey it adequately, the Fusor is a much simpler device in reality than the
linear accelerator. Its spherical geometry effects all the focusing of ions to a
central collision point. The linear accelerator often needs physically complicated
ion optics to focus its otherwise strongly divergent beam into the target region.
The Fusor built and studied at Guilford College as part of this thesis
performs the deuterium fusion reaction
2
H + 2
H → 3
He + n, Q = +3.26 MeV (5)
The energy yield Q is comes from conversion of rest mass of the deuterons into
kinetic energy as 3
He forms. Conservation of momentum governs the distribution
of this energy between the 3
He and the neutron, and assuming comparatively
small net momentum in the incoming deuteron system, we may take the
assumption that net momentum of the products is zero:
(6)
(7)
where mn and mHe are masses of the neutron and 3
He, and vn, vHe are the respective
velocities. Solving Eqs. 6 and 7 simultaneously, we find that the neutron carries
away mnv2
n/2 = 2.44 MeV. I shall now briefly explain some properties of neutrons
before concluding with a discussion of how the Fusor’s fast neutrons may be
useful.
On earth, most neutrons are tightly bound together with protons in nuclei;
free neutrons are extremely rare. Artificial neutron sources in common use
include nuclear reactors; radioisotope sources (in which alpha particles from
radioactive decay dislodge neutrons from light nuclei); and small linear
accelerators performing fusion reactions.
Occasional elastic scattering or capture by another nucleus comprises the
chief interactions of free neutrons in matter. Neutrons flowing out of a neutron
source collectively behave very much like an invisible gas, but because of the
neutron’s charge neutrality and lack of orbital electrons, the material barriers that
confine gases like air do not stop neutrons effectively. Thick lead and steel are
nearly transparent to neutrons. Light nuclei are somewhat more effective at
stopping neutrons because more kinetic energy can be lost by a neutron in an
elastic collision. These differences between the neutron-stopping ability of heavy
and light nuclei can be visually appreciated in the neutron radiograph shown in
Fig. 6.
9
MeV
vmvm
vmvm
HeHenn
HeHenn
26.3
2
0
22
=
+
=+

Figure 6. Neutron radiograph of a
35 mm camera (top) compared
with x-ray radiograph of the same
camera (bottom). The neutron
image shows dark areas
principally where light isotopes
are concentrated: plastic, for
example. Light nuclei are more
effective at stopping neutrons than
heavy ones. The camera’s film
spool in the left of the photos is
likely made from a hydrogenous
plastic since it is black in the
neutron image but transparent in
the x-ray image. Also, it can be
seen that many details of the
plastic camera case are more
evident in the neutron image. On
the other hand, the metal shutter
mechanisms and other parts in the
middle of the camera are more
transparent to neutrons but are
very opaque to x-rays. (Atomic
Institute of the Austrian
Universitites10
)
Neutrons that have been slowed are easily captured because they spend
more time around potential captor nuclei. When a neutron is captured by a
nucleus, the resultant new nucleus typically forms in an excited state and
promptly de-excites with the emission of gamma rays. In addition, the new
isotope formed by neutron capture is often radioactive and emits gamma radiation
during beta decay. Finally, the neutron itself is radioactive with a half-life of
some ten minutes, splitting into an energetic proton, beta particle and antineutrino.
For the reasons just mentioned, neutrons are considered to be very biologically
hazardous—even more so than alpha, beta or gamma radiation on a per-particle
basis. But these are also the properties that make neutrons useful.
Boron Neutron Capture Therapy (BNCT) is an example of a medical
application of neutrons11
. In this experimental cancer treatment, surgically
inaccessible tumors are loaded with boron-containing drugs, and are then
bombarded with neutrons. The isotope 10
B has a very high cross-section for
neutron capture, meaning that the boron is much more likely to absorb neutrons
than any isotopes present in normal tissue. Charged alpha particles are released
when the neutron is absorbed. These kill tumor cells in the immediate vicinity.
BNCT is a very promising concept, but is still experimental and has not yet been
put through FDA trials in the United States. Part of the problem in developing
BNCT as a treatment option has been the dependence on nuclear reactors for
producing neutron beams; few hospitals have the facilities (or the interest) for
10

reactor ownership. Developing alternate neutron sources could turn BNCT into a
successful reality.
Neutron activation analysis12
(NAA) is an important analytical use of
neutrons. In standard NAA, a sample under investigation is irradiated with
neutrons. Some of the nuclei in the sample absorb a neutron and become
radioactive—frequently emitting gamma rays as they decay. These gamma rays
have well-known energies depending on which isotopes produce them, and in
examining these gammas, the elemental composition of the sample are
determined. Mining companies sometimes use this type of activation analysis to
determine the composition of buried rock layers. In another type of activation
method, neutrons are used to detect fissile material—isotopes that can act as
chain-reacting nuclear fuel in reactors or bombs. In this technique, known as
differential dieaway detection, samples suspected of containing fissile material are
bombarded with neutrons. If a neutron chain reaction occurs, neutrons will
continue to be produced in the sample even after the neutron source is removed.
The time it takes for neutron emission from the sample to return to zero can be
correlated with the quantity and type of fissile material present. Neutron
activation analysis techniques clearly have relevance in national security and in
controlling the proliferation of weapons material. Making such analysis
techniques widely available, however, requires cheap and safe neutron sources
that can be installed at harbors and airports.
In this thesis, we present the construction of a deuterium-fueled
Farnsworth Fusor; results showing neutron activation of five common natural
isotopes with neutrons from this Fusor; and results showing that the Fusor’s
neutrons are fast, characteristic of the energetic 2
H(d,n)3
He reaction.
Experimental Apparatus and Procedures
I. Overview
The central purpose of this project was to determine whether a simple two-
electrode Farnsworth Fusor emits enough neutrons to be suitable for activation
analyses. Constructing a Fusor and the associated electrical and gas-handling
systems was the starting point of the work presented. Following the completion
of the apparatus, the next step was to measure neutron flux using standard neutron
dose measuring equipment. Finally, actual neutron activation of selected common
isotopes was attempted with the Fusor. Subelements of this stage of the research
included preparing targets, irradiating them, and collecting gamma spectra.
Besides assessing feasibility, another goal of certain activation experiments was
to verify that Fusor neutrons are fast, resulting from deuterium fusion reactions.
This study of the Fusor was originally intended to measure relationships
between neutron flux and voltage, current, and pressure. While the primary focus
of the project has changed to neutron activation, we will discuss our automated
data gathering system that logs values of neutron count rate, voltage, current, and
pressure. It has been a valuable asset in understanding Fusor operating
conditions.
11

II. Fusor Construction
a. Vacuum Chamber and Cathode Grid
The vacuum chamber of our fusor was made from two eight-inch stainless
steel (alloy 304) hemispheres sold as architectural ornaments. One hemisphere
was fitted with two QF-25 vacuum ports and a 2 ¾-inch Conflat port, while the
other was fitted with a nonstandard port to accept a large nonstandard high-
voltage feedthrough. The hemispheres were sealed to each other equatorially by
ten-inch Conflat flanges. With the exception of the power feedthrough, the ports
and flanges were TIG-welded onto the hemispheres at machine shops (see
Acknowledgements) specializing in high-vacuum system fabrication. The shop
drawings for assembling the chamber are shown in Appendix A. An external
soft-silver-solder seal (contrary to high-vacuum practice) was used to attach the
feedthrough, the thought being that this would facilitate replacing the cathode grid
in the event of damage.
The cathode is patterned after the structure illustrated in Fig. 7, in which a
spherical cage is composed of six wire hoops. We chose 0.025-inch (0.64 mm)
diameter stainless-steel (alloy 316) wire. Pieces of 22 cm length were cut and
spot-welded into circles with about one centimeter overlap. The circles were then
arranged into the spherical configuration and spot-welded together. Spot-welding
was one of the more challenging phases of construction; a commercial resistance
welder was not available and apparatus had to be built ad hoc from available
parts. The homemade spot-welder consisted of a microwave-oven transformer in
which the high-voltage winding had been removed and replaced with a three-turn
secondary of extremely heavy wire. A bank of 400 V electrolytic capacitors with
a combined stored energy of about 200 J was discharged through the original
primary winding to create a high-current impulse. Spot-welding was done with
the current from the heavy secondary. Many joints were not strong enough and
resulted in breakage of unfinished grids. Ultimately, the few successfully finished
cathodes contained many imperfections and asymmetries. Being somewhat
misshapen does not preclude a grid from being viable in the fusor, however.
Figure 7. Cathode grid structures made from six closed loops
of steel wire. At left is the “polar” view from the support
stalk, while at right is an “equatorial” view.
12

A length of OFHC copper rod supports the cathode grid as close to the center of
the chamber as possible. This stalk was soft-silver-soldered into the insulator
lead-in. Both this and the previously mentioned feedthrough joint were made
with a 4% Ag / 96% Sn alloy. (Solders containing Cd, Zn, or Pb should be
avoided in vacuum systems because of their high vapor pressure.)
b. Gas Handling and Vacuum System
Deuterium of 99.7% purity was purchased in a lecture bottle (containing
about 20 standard liters of gas). The vacuum system schematic in Appendix B
traces the path of deuterium from the bottle, through a single-stage regulator set at
10 PSIG, through a shutoff valve, to a set of twin metering needle valves to adjust
flow into the fusor. Gas enters through one of the QF-25 ports. Continuous gas
throughput was used for fusor operation, as opposed to backfilling.
The fusor is exhausted by a Varian V-70 pumping station, comprised of a
small turbomolecular pump with a quoted speed (air) of about 50 l / s, backed by
a rotary-vane mechanical pump. Other parts of the vacuum apparatus are shown
in Appendix B.
Some order-of-magnitude measurements of gas load, at base pressure as
well as with deuterium flowing, were made. The 48-inch (1.22 m) QF-25 steel
hose linking the pumps to the fusor is highly constrictive in the molecular flow
regime, having a conductance C approximated by the empirical Knudsen
formula13
(8)
where L is the length of the hose, d the diameter, T the temperature, and M the
average molar mass of the gas being pumped (the formula shown has been
converted to accept SI units). With air, the approximate conductance comes out
1.5 l / s at room temperature, while with deuterium the approximate conductance
is 4.5 l / s. In air, the fusor’s base pressure was 3 • 10-6
torr. Throughput is the
product of pressure and hose-determined speed, or about 4.5 • 10-6
Torr • l / s.
This value is explained by normal rates of permeation of the system’s Viton and
Buna-N o-rings, indicating an otherwise leak-free system. During operation, the
deuterium throughput is in the range of 40 mTorr • l / s, or 3 sccm, with the
normal chamber pressures of about 8 mTorr. However, deuterium usage is
probably an order of magnitude lower when the recommended procedures of
nearly closing the chamber exhaust valve and slowing down the turbopump speed
are followed. A 20-l lecture bottle of gas is therefore estimated to provide days of
continuous operation.
c. High Voltage Power
The Fusor’s power requirements are 0-70 kV, 0-30 mA. Small
commercial x-ray machines are an ideal power supply, provided they can be
current-limited or ballasted to protect the Fusor from massive current in the event
of pressure fluctuations. We used a Fischer Imaging Systems mammograph
13
M
T
L
d
C
3
2.1
=

power supply ballasted with a saturable-core inductor (or “magamp”). A variac
enables the operator to raise or lower the voltage of the x-ray unit, while a 0-65 V
DC power supply magnetizes the saturable-core inductor for power control.
Appendix B provides a schematic and a more in-depth discussion of the power
supply.
Safety is an issue with an x-ray power supply. We satisfied concerns for
electrical safety by 1) having a light that turns on when the high voltage is
powered, and 2) placing the exposed end of the Fusor’s cathode feedthrough in a
rather inaccessible location.
d. Automated Measurements
A program was written in LabVIEW (Fusor3.VI, Appendix E) to
automatically collect pressure, voltage, and current data using HP34401
multimeters linked by GPIB bus to a computer. The multimeter responsible for
pressure measurement reads the raw output voltage from the capacitive
manometer on the vacuum system. Another multimeter reads cathode potential
via a standard high-voltage probe connected to the cathode lead-in. Finally, a
third multimeter reads Fusor current by measuring the voltage drop over a
resistance in the x-ray transformer’s ground lead.
Converting raw voltage from the manometer to pressure is
straightforward. The manometer’s output is 0-10 V, corresponding to pressures
from 0-10 Torr. It was found that the instrument’s uncertainty was about 0.1
mTorr. The manometer requires calibration so that 0 Torr (high vacuum) reads 0
volts; this adjustment is usually made by pulling a high vacuum and turning a
trimpot on the back of the manometer until a 0 V reading is obtained. However,
physically zeroing the manometer is an annoying task. Our labVIEW code
simply subtracts the manometer’s reading at high vacuum (measured each time
before the Fusor is operated) from the readings during operation.
The high-voltage probe has a nominal 1:1000 step-down ratio so that, for
example, 30 kV reads 30 V. However, because of the uncertainty in the resistors
used in the probe, the manufacturer claims a 10% uncertainty in this ratio. Better
precision was desired for our studies. Thus, the probe required calibration using a
known high-voltage source. In this case, known was taken to be the reading on an
HP34401 multimeter when used to measure the voltage of a nominally 400 VDC
supply. Another reading from this supply was made simultaneously with the HV
probe and a second HP34401 multimeter to obtain the correct probe ratio. In the
LabVIEW program, raw voltages from the HP34401 are multiplied by this probe
ratio to obtain cathode potential to several significant digits. Also, the probe was
modified for voltages above its 40 kV rating by filling it with mineral oil. This
prevents deleterious corona discharges and also affords a measure of convection
cooling for the divider resistors.
The current-measuring resistor is shown in the power supply schematic,
Appendix B. The actual resistance of this resistor was measured with an
ohmmeter to several digits; Ohm’s law (I = E/R) calculations were done in
LabVIEW using the measured value of resistance to obtain cathode current.
14

Fusor3.VI is set to make and record nearly simultaneous measurements of
voltage, current, and pressure three times per second. It writes these numbers in a
tab-separated-variable text file.
III. Neutron Counting
Neutrons were detected using a Ludlum Model 12-4 neutron dose rate
meter. Our unit was obtained from surplus working, but improperly adjusted, and
was unofficially calibrated at Duke University prior to use at Guilford (see
Acknowledgements). The Model 12-4, like virtually all commercial neutron dose
rate meters, contains battery-powered electronics and a separate detector head
holding a BF3 proportional tube within a high-density polyethylene sphere
(known as a “Bonner ball”). Neutrons thermalized in the polyethylene are
detected in the proportional tube via the 10
B(n, α)7
Li reaction. The “Bonner ball”
detector has a characteristic inverse-RPG (Radiation Protection Guide) curve
response, meaning that the count rate is closely proportional to biological efficacy
of the incident neutrons rather than just neutron fluence rate14
. An important
consequence is that in order to use this type of dosimeter to measure neutron
fluence rate, it is necessary to know the neutrons’ energy so that dose or count
rate can be converted to fluence rate on the manufacturer’s calibration curve.
We modified the neutron counter to facilitate automatic data logging by
computer. The signal from a non-inverting (positive) output of the Model 12-4’s
second stage of amplification was coupled to the Receive Data (pin 2) input of a
computer serial port through a 0.1 µF silver-mica capacitor. The port input is
triggered high by voltages in excess of about 2 V; the 10 Vpk pulse from the
counter is more than adequate to cause a string of binary data to accumulate at the
port. To detect counts on the computer, we wrote a program in LabVIEW (called
Neutcount3.VI, included in Appendix D) that repetitively reads the serial port at
high speed. When a non-zero number of bits are detected, a counter is augmented
by one to signify the detection of a pulse. In addition, Neutcount3.VI averages
the number of collected counts over a user-selected time unit using a moving-
average algorithm, yielding the count rate. Finally, the program writes a file
recording count rate (in CPM) and the corresponding time of the measurement.
We have found the serial port method to be satisfactory for computerized
data collection from radiation counters in general, provided the program reading
the serial port can keep up with the rate of incoming counts. Initially there was
worry that significant errors might result from pileup of more than one count at
the serial port during a single iteration period of the program. This would cause
the count rate calculated by computer to be erroneously low, since only one count
can be detected during each program iteration. However, the pileup problem is
not manifested either in observation or in theory. If we assume that the fusor
neutrons are created by a stochastic process, then the probability I that a count
will arrive in the time t elapsed since the previous count is given by9
(9)
15
∫
−
=
t
rt
dtretI
0
)(

The time t required for a single iteration in Neutcount3.VI depends on the
computer platform, with about 1 / 10,000 seconds required on the Macintosh G4
used in our data gathering. To find the theoretical count rate r at which there is a
1% chance of pileup, Eq. 3 is integrated, solved for r, and t is set at 1 / 10,000
seconds. It is found that, for our Mac G4, a 1% loss due to pileup is expected
when r = 6000 CPM, representing a dose rate of about 200 mRem / hr. In
experimental observations, dose rates were at or below 60 mRem / hr.
Furthermore, dose rate readings from the instrument itself and count rate data
gathered by computer appeared consistent.
To measure the neutron flux emitted by the Farnsworth fusor, the round
detector head of the Ludlum Model 12-4 dose rate meter was placed next to the
fusor such that a 1.0 cm space remained between the fusor wall and the neutron
counter head. In this position, the detector’s BF3 tube was centered on a point
22.6 cm radially distant from the center of the fusor. Knowing the detected
neutron fluence rate φ at this radius r, and making the reasonable assumption that
fusor neutron emission is isotropic, the flux dN / dt can be calculated by
multiplying the fluence rate by the area A of the spherical surface around the
Fusor on which the BF3
tube is centered:
(10)
CPM, as recorded by the data logging software, is first converted to dose rate via
the manufacturer’s conversion of 30 CPM / mRem / hr.15
, and dose rate for the
neutrons with assumed energy of about 2.5 MeV is converted to a fluence rate via
the inverse-RPG relationship provided in Appendix D. At this energy, a dose rate
of 1 mRem / hr is caused by a neutron fluence rate of 8.06 n / cm2
/ sec. As an
example, the highest observed count rates from the Fusor of about 1800 CPM =
60 mRem / hr are caused by the isotropic emission of 3.0×106
neutrons / sec.
IV. Activation Experiments
a. Target Preparation
Potential target materials for activation were identified by searching a data
table of naturally-occurring isotopes16
for large thermal neutron capture cross
sections (σc) and resonance integrals (RI). Out of the possibilities thus identified,
availability and cost were considered. Finally, only those isotopes yielding short-
lived, gamma-emitting activation products were selected so that gamma
spectroscopy could be used to confirm activation. Table 1 lists the five isotopes
chosen for this study. Two targets were made to attempt prompt neutron
activation analysis as well.
16
πφφ 2
4rAd
dt
dN
=•= ∫


Table 1. Properties of Activation Targets
(Sources: CRC16
, Rad. Health17
)
Isotope Nat.
Abundance
σc
(barns)
RI
(barns)
T1/2 of
activation
product
γ energies
of activation
product (MeV)
55
Mn 100% 13.3 14.0 2.58 h 0.847 (99%)
1.811 (29%)
2.110 (15%)
127
I 100% 6.15 149 25.0 m 0.441 (14%)
0.528 (1.4%)
51
V 99.8% 4.9 2.7 3.75 m 1.434 (100%)
27
Al 100% 0.23 0.17 2.31 m 1.780 (100%)
197
Au 100% 98.7 1550 2.70 d 0.412 (95%)
Activation targets were prepared in various ways as described below:
55
Mn
1, 2. Anhydrous MnSO4(s) was dissolved in boiling
deionized water. After cooling, the saturated solution was
used to fill two Ocean Spray two-quart PETE juice bottles
(selected for their flat, rectangular shape).
3, 4. MnO2(s) was dispensed into two identical rectangular
plastic cases, approx. 2.5 x 5 x 0.5 cm., such that each
contained 17.0 g.
127
I
5. ~100 mL boiling deionized water was saturated with
HIO4(s) and transferred to a flat plastic rubbing alcohol
bottle.
51
V
6. 70.0 g NH4VO3(s) was dispensed into a 1.5-inch
polyethylene bottle.
27
Al
7. ~500 g Al powder in original 3-inch plastic bottle.
197
Au
8. 25 sheets of Monarch 23 kt gilding leaf (one pack or
about 0.4 g) were rolled together to form a flat foil, ~15
cm2
. This foil was glued between two sheets of paper for
protection.
17

Targets for prompt neutron activation analysis were prepared as follows:
113
Cd
9. A stick of Wood’s metal (50% Bi, 25% Pb, 12.5% Sn,
12.5% Cd) was melted in hot water and poured into a
uniform patty approximately 2 mm thick.
10
B
10. Borax (Na2B4O7 •10H2O(s)) was dissolved in boiling
deionized water to form a saturated solution, which was
poured into a 2-inch polyethylene bottle.
b. Irradiation Methods
Slow (< 0.5 eV) neutrons are required for activation experiments, but fast
(2.44 MeV) neutrons are expected from deuterium fusion. The activation targets
were thus surrounded by suitable moderators made of water and paraffin. Based
on available data18
and on previous experimental work19
, a water moderator
thickness of three inches interposed between the fusor and the target was chosen
as reasonable. Target #1, the Mn++
solution, was already in a bottle exceeding
three inches in width, so no additional moderator was used. All other targets were
irradiated in the neutron “hearth” pictured in Appendix C, constructed from
paraffin and “water bricks” (plastic VHS cases filled with deionized water).
Very thin targets (#8, #9) were placed behind three “water bricks;” moderately
thick targets (#5, #6, #7) were placed behind two; and target #10 was placed
behind a single brick. An experiment was done with identical manganese targets
#3 and #4, in which each target was irradiated at a set distance for a fixed time,
but in which only one of the targets was behind a water moderator.
Exposure durations were roughly decided on the basis of activation
product half-life and expected rate of formation, while attempting to keep
comfortable fusor operating times. An internet-based JavaScript activation
calculator20
greatly simplified this figuring process. The calculator has inputs of
target nuclide, target quantity, neutron source flux, and distance from source to
target. The calculator uses the geometry and source flux, as well as cross section
data from Brookhaven National Laboratory, to determine the rate of formation of
expected activation products.
c. Gamma Spectroscopy
Gamma emissions from the irradiated targets were detected with a
1” x 1.5” NaI(Tl) scintillator and a pulse-height analyzer (Spectrum Techniques
UCS-20 for Macintosh). Immediately prior to any experiment, or upon moving
the scintillator, we calibrated the energy scale of the gamma spectrum. 0.511
MeV and 1.2745 MeV peaks from a 22
Na source, and the 0.835 MeV peak from a
54
Mn source, were employed for this purpose.
Counting very weak emissions requires a carefully optimized method to
help improve signal-to-noise. Two guidelines were followed for determining the
duration of counting for a given target. First, we wanted to maximize counts in
the desired photopeak channels while minimizing background, and thus, three
18

half-lives of the activation product of interest was taken to be an upper limit for
the counting time in any experiment. Second, we wanted to insure that the
expected photopeak would have a statistically significant number of counts. This
was accomplished simply by watching the accumulation of counts, and not
stopping data collection until it was visually apparent that at least 100 counts had
accumulated in the peak relative to background. (It was found that this condition
could not be strictly met for the 28
Al photopeak, which only contained some 70
counts relative to background.) A second aspect of optimization is to control the
resolution of the pulse-height analyzer. Resolution was controlled by setting
conversion gain and amplifier gain such that the FWHM (full width at half
maximum) of the anticipated photopeak would encompass no more than about ten
channels. Knoll9
recommends five channels.
Results
I. Activation Gamma Spectra
Activation was observed in manganese, gold, iodine, vanadium, and
aluminum, with the exception of the manganese target irradiated with no
moderator (Fig. 8). The gamma spectra of the activated targets can be seen in
Figs. 8-13, accompanied by information on the irradiation and counting times. In
each graph, dashed red lines are inserted where photopeaks are expected.
II. Prompt Gamma Spectra
Prompt capture gamma spectra from the boron (#10) and cadmium (#9)
targets are shown in Figs. 15 and 16. These spectra are more difficult to interpret.
Peaks which could be identified are indicated with arrows. There are also peaks
that remain unidentified but are labeled. In both spectra the 1
H capture peak is
very prominent.
III. Other Measurements and Observations
Finally, three graphs are inserted as part of additional observations. These
include a graph showing the relationship between neutron output and power
supply voltage / current (Fig. 17); a plot of pressure and neutron measurements as
a function of time (Fig 18).
19

Figure 8. Gamma energy spectra of Mn targets #3 and #4, the former behind three “water bricks” and the latter

behind an equivalent distance of air. The target behind the water moderator showed considerable activation
while the target behind air showed no significant activation. The peak observed in both spectra at 1.45 MeV is
attributable to natural 40
K.
22

Figure 9. Neutron activation of Mn target #1. Red dashed lines indicate where peaks are expected in the spectrum.
Excellent agreement exists with the 0.847 MeV peak; the 1.81 MeV peak is visible, and the 2.1 MeV peak is not significant.
The peaks occur in 99%, 29%, and 15% of decays, respectively. Background was subtracted by counting non-irradiated
target #2 immediately after counting target #1.
23

Figure 10. Gamma spectrum from iodine target #5. Expected peaks at both 441 keV and 528 keV are both
prominent. The 441 keV gamma is emitted in 14% of decays and the 528 keV in 1.4%.
24

Figure 11. Gamma spectrum from vanadium target #6. The spectrum here shows plenty of counts above background
from scattered radiation at energies below the Compton edge.
Figure 12. Overlaid target and background spectra showing evidence of activation of the gold target, #8.
This was the first activation attempt on the gold target, and therefore background could be collected from
the target itself prior to irradiation. It can be seen above that the only region in which the irradiated
25

spectrum departs from the background is around 418 keV, in good agreement with the expected 198
Au peak
at 412 keV. Large peaks show up in both spectra at 515 keV and at 1.45 MeV, due to background
annihilation radiation and 40
K gammas, respectively. The principal difficulty in detecting gold activation is
the long 2.7-day half-life of 198
Au.
26

Figure 13. Another gold activation attempt using the same target (#8) used in the previous activation
experiment. Because only a few half-lives had passed since the first irradiation, background for this
spectrum was gathered by counting with no target. As in Fig. 12, the major discrepancy between
irradiated target and background occurs only at 413 keV, satisfying our expectation for a peakat 412 keV.
Figure 14. Gamma spectrum from irradiated aluminum, target #7. The most challenging activation material
tried, Al has a small cross-section and 28
Al has a short half-life. This spectrum is plotted against background
27

to highlight the differences. The observed peak centered on 1.782 MeV is in excellent agreement with an
expected value of 1.780 MeV. The 40
K peak is visible in both target and background spectra. The
lower-energy region of the target spectrum is considerably higher than the same region in the background,
probably because of the Compton continuum caused by scattering of 28
Al gammas.
28

Figure 15. Energy spectrum spanning 200 keV – 3.5 MeV, resulting from the neutron bombardment of a borax
solution (target #10). The peak at 478 keV indicated with an arrow is due to neutron capture by 10
B. The
substantial peak at 2.22 MeV is from captureby 1
H. Other peaks are labeled but have not been identified.
Figure 16. Gamma spectrum from target #9, a cadmium-bearing piece of Wood’s alloy. The 557 keV and 655 keV peaks
29

(with arrows) have been linked to expected values for 113
Cd capture at 558.6 keV and 651.3 keV. The 501 keV peak may
also be part of the cadmium spectrum. The 1
H capture peak at 2.2 MeV is visible. An unidentified 841 keV peak is
also prominent.
30

Figure 17. Neutron output versus potential at 8.7 mTorr.
Current increased with potential and was not held constant.
Figure 18. Pressure and neutron count rate recorded simultaneously as a function of time.
31

Note the small burst in gas pressure occurring at the end of the run. This effect was observed
by Hirsch21
, who related it to the release of deuterons trapped in the Fusor’s potential well.
32

Discussion: Activation Analysis, Cheap and Easy
The Farnsworth Fusor is a generator of 2.5 MeV fast neutrons when filled
with deuterium gas. This is experimentally evident from the moderator test with
manganese targets #3 and #4, in which neutron activation only resulted when a
water moderator was interposed between the Fusor and the target (Fig. 8). Other
reactions besides fusion also liberate neutrons from deuterium. The photoneutron
reaction, 2
H(γ,n)1
H, is well-known; an incoming photon causes disintegration of
the deuterium nucleus into a proton and a neutron9
. Energetic massive projectiles
can also cause an identical breakup reaction called “stripping.” Since in both
cases enough energy must be delivered to the deuteron to overcome its binding
energy, these reactions are endothermic with Q-values of –2.225 MeV. Neutrons
characteristic of these reactions would have very low energy as the reaction
threshold is crossed.
The original intent behind investigating the Farnsworth Fusor as a neutron
source was to measure correlations between neutron flux and the experimental
variables of gas pressure, cathode potential, and current. Similar relationships had
been the focus of preceding work with the Fusor; Hirsch21
, for example, found
that neutron flux varied linearly with current at constant pressure and potential,
and varied in a monotonic increasing or exponential relationship with potential at
constant current and pressure. We were challenged by an inability to adequately
separate experimental variables (e.g. simultaneously maintain constant current
and pressure). An ion pumping effect, and also a gradual buildup towards
thermionic runaway at higher power levels, is suspected of causing the trouble
with simultaneously holding variables constant. Interesting data were collected
nevertheless. Fig. 17 in the Results section shows neutron output as a function of
potential at a nearly constant pressure of 8.7 mTorr. It should be noted that
current increases with potential as well and could not be held constant. Thus, the
effects of potential cannot be separated from those of current. But in the sense
that an upward monotonic relationship is evident, the data agree with Hirsch.
Another phenomenon observed by Hirsch is the sharp pressure burst when
operation is ceased (Fig. 18). He attributed this effect to the rapid release of
confined, recirculating particles from the confining potential.
One of the mentioned advantages of the Farnsworth Fusor over linac-
based fusion neutron sources is an increased efficiency due to the ion recirculation
mechanism. Though the fusor described here is structurally akin to Farnsworth’s
invention, it can be shown that its high operating pressure precludes any
significant recirculation. The mean free path λ of a deuterium molecule in low-
pressure deuterium gas is given by13
(11)
where R is the gas constant, T the temperature, d the effective molecular diameter,
A is Avogadro’s number, and P the pressure (SI units). In deuterium gas at 8.5
APd
RT
2
π
λ =

mTorr and 300 K, typical conditions in our fusor, the molecule’s mean free path is
only about 1.5 cm. Mean free path of a deuteron, even at 50 keV, will be even
shorter due to interaction between its positive charge and the electrons on
surrounding molecules. The probability of making even a single round-trip
recirculation through the cathode is indeed small, and pressures several orders of
magnitude lower would be wanted in order to gain an efficiency advantage from
recirculation. This calculation leads to the hypothesis that, in the high mTorr
range, a smaller fusor would be a more efficient neutron source. A smaller
difference between cathode and anode diameter would minimize the energy losses
from deuterons scattering off of neutrals during acceleration. Furthermore, a
smaller cathode would improve the chances of head-on central collision, rather
than collision with stationary deuterium within the cathode space. If we conduct
future Fusor work, a six-inch-diameter, water-cooled chamber would be preferred
to the more expensive and less efficient eight-inch chamber used for these studies.
Our Fusor has proven to be a rugged and reliable device, having a trouble-
free operating lifetime so far of nearly ten hours at full power. The life of a
Farnsworth Fusor such as ours is probably determined by sputtering of metal from
the cathode. Deposition of cathode material occurs visibly on the glass viewport
and also on the exposed ceramic of the feedthrough insulator. Presumably,
deposited metal could eventually lead to a short-circuit of the high-voltage supply.
The cathode will also be structurally degraded by loss of material to the point
where it breaks and must be replaced. Evidently, the sputtering problem is not
severe with the stainless steel wire used in our system.
The 2
H(d, n)3
He reaction has been detected by its penetrating neutrons, but
another competing fusion reaction, 2
H(d, p)3
H, has even higher energy yield (Q =
4.03 MeV) and similar cross-section at low energies, as Figure 19 illustrates26
.
Thus we can expect that measured fusion rates from 2
H(d, n)3
He account for less
than half of the total fusion energy released in the deuterium fusor. In fusors that
might hypothetically be designed for producing high neutron flux or fusion
power, an interesting effect is thus suggested in which the fusor “breeds” more
energetic fuel from deuterium. Also contributing to this effect would be the
buildup of deuterium reaction products 3
He and 3
H; the former undergoes fusion
with deuterium in a very energetic reaction [3
He(d,p)4
He, Q = 18.5 MeV] while
the latter reacts with deuterium according to 3
H(d,n)3
He (Q = 17.6 MeV). Again,
the cross-sections, shown in Figure 19 as well, are significant at low energies.
Ashley et al.22
found that in a 1:1 fuel mixture of D2 and 3
He, DD and D3
He
reactions occurred in a ratio of about 2:1 near –50 kV grid potentials. An energy
production scheme would therefore be optimized by allowing the waste products
of DD fusion to build up, although this might be deleterious in a high-intensity
neutron source.

Figure 19. Cross section of four fusion reactions of potential
interest in the Farnsworth Fusor. Data taken from ENDF
Databases23, 24, 25, 26
at Brookhaven National Laboratory.
Despite its interesting advantages and initial promise, the Farnsworth
Fusor admittedly shows little hope for being a viable power-production solution.
But its configuration as a tabletop source of energetic radiations and particles
shows much immediate promise and untapped potential. Efforts thus far have
demonstrated useful fast neutron production by deuterium and deuterium-tritium
fusion. The 3
He(d,p)4
He reaction looks very promising as a benchtop source of 17
MeV protons, which could be extracted through thin beryllium or aluminum
windows. The 19
F(p,α)16
O reaction might be a good benchtop source of 7 MeV
gamma radiation, provided a higher cathode potential of >100 kV is used.26
These
latter two reactions have a latent advantage in that ions of multiple charge state (q
= 2e, 3e, etc.) can be collided.
Conclusions
Our Farnsworth Fusor is indeed a tabletop fusion neutron source, built
with materials and a budget that should be accessible to many students and
laypersons interested in neutron experiments. Optimal yields of the machine
described here were on the order of 3.0×106
neutrons / sec. This figure is very
similar to measured output from other Fusors. Hirsch21
quotes about 106
neutrons / sec. at -55 kV, 10 mA, and 7.8 mTorr using a somewhat different ion-
gunned Fusor; and Ashley quotes a peak yield of 4.9×107
neutrons / sec. from a
Cross Section vs. Energy
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 20 40 60 80 100 120 140 160 180 200
Energy (keV)
CrossSection(barns)
2H(d,n)3He
2H(d,p)3H
3He(d,p)4He
3H(d,n)3He
35

deuterium Fusor at –55 kV, 117 mA, at approximately 2 mTorr. Rosenstiel19
reported the flux from his hobby fusor—a machine very similar to ours—as 2.35
×106
neutrons / sec. at –47 kV and 16 mA. These similar claims lend credence to
our measurements. Furthermore, we have shown through an activation
experiment that neutrons from the Fusor are fast and must be moderated by
several inches of water to become thermalized. The only indicated source of fast
neutrons is the exothermic 2
H(d,n)3
He fusion reaction; the endothermic stripping
or photoneutron emissions that some have alleged are physically out of the
question.
Fusor neutron activation of five relatively common isotopes (28
Al, 198
Au,
51
V, 56
Mn, 128
I) was readily detectable using a basic scintillation spectroscopy
setup. Additionally, prompt neutron-capture gamma emissions from 10
B, 1
H, and
113
Cd could be easily detected from targets undergoing irradiation. With a high-
resolution GeLi detector, NAA identification of most natural elements should be
possible using Fusor neutrons.
36

Acknowledgments
First and foremost, I would like to express my deep appreciation for
Winslow Womack, whose endowment for funding student research in physics at
Guilford College made this project financially possible.
I am thankful to the Guilford Physics Department (Rex Adelberger,
Steve Shapiro, and Thom Espinola) for their encouragement and support, and
for providing a very generous amount of lab space for this project and others. The
physics faculty and my fellow physics students have been very tolerant of my
“mess.” Throughout the duration of the fusor project, they have also endured the
threats of high voltage, radiation, and flammable gas, and still made me feel a
welcome part of their program. My gratitude is profound. Many students have
provided me with thesis-related transportation, encouragement, and suggestions.
Thanks to all of them, and to Peter Pozzo and Renee Kloefkorn in particular.
The Guilford Chemistry Department played a large role in my neutron
activation experiments by providing chemicals. But also of great significance
were the expressions of support and enthusiasm for my work from Anne Glenn,
Dave MacInnes, and Rob Whitnell. The Chemistry Department also provided
space and resources used to fabricate parts of the fusor, and they too tolerated
various messes of my making.
Paul Carter and Chris Westerfeldt at Triangle Area Nuclear
Laboratory (TUNL) were very helpful with calibrating our neutron counter and
loaning a replacement while work was being done. I am also thankful for the
assistance of Robert Timberlake at the Duke University Instrument Shop, and
Kent McGregor at VTI, Inc., for providing their machine shop services at
reduced cost.
My thesis committee—Rex Adelberger, Thom Espinola, Rob Whitnell,
and Lisa McLeod—all committed their time and suggestions towards making
this thesis a success. Thanks to each one of them for their contributions.
Much experimentation with the Farnsworth Fusor has been carried out at
the hands of “amateur scientists”—people with an appreciation for scientific
quirks, lost inventions, and do-it-yourself resourcefulness, that transcends the
motivation to make money or build reputation. I would like to especially thank
all the contributing members of the Open Source Fusor Research Consortium6
for sharing their efforts and for providing me with insight and encouragement on
my own fusor. Richard Hull7, 27
is really the momentum of this group and, by
divulging information about his three homemade fusors, has been particularly
helpful to me.
My uncle (and a physicist), Ralph Chapman, is owed credit for his
suggestions regarding radiation safety and public relations. I heeded his advice,
which included the comparative health risk explanation on my hazard warning
signs. My father (yet another family physicist), Robert Willis, has provided
numerous insights and support. Finally, the rest of my family deserves
recognition for their support.
37

References
1
Farnsworth PT. U.S. Patent 3258402 (1966). 18 p.
2
Schatzkin P. The boy who invented television. <http://www.farnovision.com>
Accessed 2003 Feb. 19.
3
Sandia National Laboratory. Z produces fusion neutrons, Sandia scientists
confirm. <http://www.sandia.gov/news-releases/2003/nuclear-
power/zneutrons.html> Accessed 2003 May 1.
4
Lawrence Livermore National Laboratory. National Ignition Facility programs.
<http://www.llnl.gov/nif/> Accessed 2003 May 1.
5
ITER. Cost, schedule, and siting. <http://www.iter.org/ITERPublic/ITER/cost-
schedule.html> Accessed 2003 May 1.
6
Schatzkin P. The open source fusor research forum. <http://www.fusor.net>
Accessed 2003 May 1.
7
Hull R. <rhull@richmond.infi.net> 2001 June 21. Introduction.
<http://www.fusor.net/> Accessed 2003 March 17.
8
Eisberg R, Resnick R. Quantum physics of atoms, molecules, solids, nuclei,
and particles. New York: John Wiley and Sons; 1985. 713 p.
9
Knoll GF. Radiation detection and measurement. New York: John Wiley and
Sons; 1989. 754 p.
10
Atomic Institute of the Austrian Universitites. Neutron radiography.
<http://www.ati.ac.at/~neutropt/experiments/radiography/
radiography.html> Accessed 2003 April 22.
11
Barth RF et al. Boron neutron capture therapy of brain tumors: an emerging
therapeutic modality. Neurosurgery 1999; 44(3): 433-449.
12
Glascock MD. An overview of neutron activation analysis.
<http://www.missouri.edu/~glascock/naa_over.html> Accessed 2003
May 1.
13
Strong J. Procedures in experimental physics. New York: Prentice Hall; 1938.
642 p.
14
Hankins DE. Los Alamos Scientific Laboratory Report LA-3595: A modified-
sphere neutron detector. Los Alamos, NM: Los Alamos Scientific
Laboratory of the University of California; 1967. 39 p.
15
Ludlum Measurements, Inc. Ludlum Model 12-4 manual. 1989. 26 p.
16
Lide DR, editor. CRC handbook of chemistry and physics, 77th
edition. Boca
Raton: CRC Press; 1996.
17
U.S. Bureau of Radiological Health. Radiological health handbook. Rockville,
MD: U.S. Department of Health, Education and Welfare; 1970. 458 p.
18
Libbrecht KG. Neutron experiments.
<http://www.pma.caltech.edu/~ph77/labs/exp16.pdf> Accessed 2003
May 1.
19
Rosenstiel J. <jonr@pacbell.net> 2002 December 17. Indium Activation.
<http://www.fusor.net/> Accessed 2003 March 17.
20
WISE Uranium Project. Neutron activation calculator.
<http://www.antenna.nl/wise/uranium/rnac.html> Accessed 2003 May 1.
38

21
Hirsch RL. Inertial-electrostatic confinement of ionized fusion gases. J. App.
Phys. 1967; 38(11): 4522-4534.
22
Ashley RP et al. Steady-state D-3He proton production in an IEC fusion device.
14th
Topical Meeting on the Technology of Fusion Energy; 2000 Oct 15-
19; Park City, UT. Madison, WI: Fusion Technology Institute. 6 p.
23
White RM, Resler DA, Lawrence Livermore National Laboratory, U.S.A.,
ENDF/B-VI evaluation, MAT # 128, May 1991; data retrieved from the
ENDF database <http://www.nndc.bnl.gov/nndc/endf/> Accessed 2003
April 05.
24
White RM, Resler DA, Lawrence Livermore National Laboratory, U.S.A.,
ENDF/B-VI evaluation, MAT # 225, May 1991; data retrieved from the
April 05.
25
Hale GM, Drosg M, Los Alamos National Laboratory, U.S.A., ENDF/B-VI
evaluation, MAT # 131, Revision 1, January 1995; data retrieved from the
April 05.
26
Cross Section Evaluation Working Group, ENDF/B-VI Summary
Documentation, Report BNL-NCS-17541 (ENDF 201) (1991), edited by
Rose PF, National Nuclear Data Center, Brookhaven National Laboratory,
Upton, NY, U.S.A.
27
Hull R. The Farnsworth / Hirsch fusor. The Bell Jar 1997; 6(3-4)
28
Hansen S. Neutrons and neutron generators. The Bell Jar 1997; 6(3-4)
29
U.S. Nuclear Regulatory Commission. Units of radiation dose.
<http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-
1004.html> Accessed 1 May 2003.
39

Appendices
Appendix A
Machine Shop Drawings
Figure A1. One half of the fusor, showing the attachment of
one 2¾-inch ConFlat port, two QF-25 ports, and one of the
10-inch ConFlat equatorial flanges. In the actual device, the
2¾-inch ConFlat was at the polar position while the QF-25s
were at the 45-degree angles.
40

Figure A2. Second half of the fusor. This part holds the high
voltage feedthrough, not shown, with a solder joint.
41

Appendix B
Farnsworth Fusor Power Supply and Gas Flow
Figure B1. Power supply for fusor.
T1: 0-230 VAC variable transformer, 1.4 kVa / 8 A rating, 120 VAC input
T2: Mammograph x-ray transformer made by Fisher Imaging Systems, Inc. Pri: 0-230 VAC
Sec: 0-135 kV, center tap grounded. Peak current rating of 200 mA.
L: Saturable-core inductor, ~5 kVA. DC winding operated by control voltage Vc = 0-65
VDC.
D1, D2: High-voltage silicon rectifier stacks, part of x-ray unit.
R: Current sensing resistor. Two 1 k 2 W carbon resistors in parallel with 0.01 F high-
frequency bypass capacitor. Measured resistance of 504.03 Ω. Voltage drop of VI
with respect to ground.
HVP: Fluke model 80K-40 high-voltage probe, modified by filling with mineral oil. 1:1000
voltage output at VE.
The x-ray transformer and rectifiers are mounted in an oil-filled tank, and
connected to the fusor via a 5-meter high-voltage coaxial cable. The maximum voltage
available is about –65 kVDC. The variable transformer controls this maximum voltage by
modifying the overall step-up ratio.
The saturable-core inductor essentially controls power by inserting variable
reactance in series with the x-ray transformer primary. As the DC supply current is
increased by the operator, the magnetization of the inductor’s core increases such that the
core is saturated over an increasing phase angle of the current in the AC winding. This
results in a diminished inductive reactance, providing more current to the x-ray power
supply. The saturable-core reactance phase control, or “magamp,” has been largely
replaced by semiconductors in modern equipment.
42

Figure B2. Gas flow diagram for fusor. Deuterium originates from the lecture bottle,
Passes through a single-stage regulator that holds pressure in the 10 psig range, and
Enters a 2-meter coil of 1/8” copper tubing that serves as an intermediate reservoir
(C1). This is followed by two fine metering valves (Hoke valves) in series to enable
easy flow rate control. The fusor is exhausted through a manual right-angle QF-25
vacuum valve that serves as a throttle for very fine pressure adjustments.
Fusor pressure is determined with a capacitance manometer (CM), which is an
Absolute gauge. A cold-cathode gauge (CG) is used to calibrate the bottom of the
Manometer pressure range when air is in the system. The cold-cathode gauge
Reading is nearly meaningless for gases besides air. C2, a 1.22 m QF-25 metal
hose, is the speed-determining element in the vacuum system. The pumps are
T, a Varian V-70 turbomolecular pump, backed by a direct-drive rotary vane
Pump (M).
43

Appendix C
Photographs of Apparatus
Figure C1. Top-down view of complete Farnsworth Fusor and irradiation
apparatus. Numbered components are explained below.
1. Moderator assembly (“neutron hearth”). Distilled water is stored in glued-shut VHS
tape cases approximately one inch wide, which sit on a layer of paraffin blocks. The
entire assembly resides on a jack that is used to raise or lower it. A manganese target is
pictured sandwiched between a group of three VHS-case water bricks and two more
that serve as a neutron reflector.
2. Farnsworth Fusor, covered in 1/8-inch lead x-ray shielding that doubles as an airflow
guide for cooling.
3. Detector head of Ludlum 12-4 neutron dose rate meter.
4. Feedthrough insulator for cathode high voltage
5. Anti-corona ring of 5/8-inch copper tubing
6. Fluke high voltage probe, filled with mineral oil. The probe is supported vertically at its
tip from the cathode lead-in.
7. Air hose to a “Shop-Vac,” the main source of cooling air.
8. Deuterium metering valves
9. Tylan General 10 Torr capacitance manometer on vacuum manifold.
10. Cold-cathode vacuum gauge and vacuum pumpline connection on manifold.
44

Figure C2. The central fusion region
in the Fusor at high power, viewed
through the viewport. Beams of
deuterons may be seen radially
approaching the center. The star-
shaped glow has a reddish-violet
coloration. Part of the cathode wire
structure can also be seen, glowing red
from ion bombardment. During
operation, the discharge must be
monitored indirectly by viewing its
image in a mirror; x-ray dose rates on
the order of 1 R / hour were measured
near the viewport. In fact, several
elements of the camera’s CCD were
destroyed making this photograph. At
least one of the resulting blank pixels
may be seen in this photo.
Appendix D
Neutron Dose-to-Fluence Conversion Chart
Figure D1. Fluence per dose for neutrons as a function of
energy. Source: NRC29
45

Appendix E
Three LabVIEW Data Handling Programs
On accompanying compact disk.
46

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