Tabletop Nuclear FusionNeutron 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:

• Some fusion reactions are a source for energetic particles such as protons and neutrons.

2H + 2H 3He + n Q = 3.2 MeV2H + 3He 4He + 1H Q = 18.4 MeV2H + 3H 4He + n Q = 17.6 MeV

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,

• It is found that the neutron has 2.44 MeV.

MeVQvmvm

vmvm

HeHenn

HeHenn

26.32

22

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 ProbabilityThe 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 = N l

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

• 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

• For the same power supply potential, V, the Fusor has collisions with four times the incoming energy of the collisions in the linac.

24

2ed

linacVqvm

E

edfusor VqvmE 22

Detecting NeutronsNeutrons react with boron-10 to create alpha particles (4He), and this reaction is frequently used to detect neutrons.

The alpha particles are collected on an electrode in a tube filled with BF3 gas. Each time charge is collected, a count is registered.

10B + n 7Li + 4He

Paul Carter at TUNL checksour 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.

Capture

Decay

Neutron Activation

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 around chamber, a wire grid cathode, andsome high voltage. To do fusion, just addsome deuterium.

In reality, building aFusor requires skilledmachine shop labor, custom welding, andexpensive high-vacuumfittings. Learning “thehard way” occurred onmore than one occasion!

Assembling the Apparatus (Ctd.)

The cathode (negative electrode) was madefrom six loops of stainless-steel wire, spotwelded 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 3106 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 radioactivemanganese 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.

• Targets were made from chemical compounds of the selected test isotopes. The targets had different sizes, shapes, and physical states.

Table 1. Properties of Activation Targets(Sources: CRC4, Rad. Health6)

Isotope Nat.Abundance

c

(barns)RI

(barns)T1/2 of

activationproduct

energiesof activation

product (MeV)55Mn 100% 13.3 14.0 2.58 h 0.847 (99%)

1.811 (29%)2.110 (15%)

127I 100% 6.15 149 25.0 m 0.441 (14%)0.528 (1.4%)

51V 99.8% 4.9 2.7 3.75 m 1.434 (100%)27Al 100% 0.23 0.17 2.31 m 1.780 (100%)

197Au 100% 98.7 1550 2.70 d 0.412 (95%)

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.

• 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.

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.

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 3106

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.

ReferencesKnoll GF. Radiation detection and measurement. New York: John Wiley and

Sons; 1989. 754 p.U.S. Bureau of Radiological Health. Radiological health handbook. Rockville,

MD: U.S. Department of Health, Education and Welfare; 1970. 458 p.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.

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.

Lide DR, editor. CRC handbook of chemistry and physics, 77th edition. Boca Raton: CRC Press; 1996.

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.

Farnsworth PT. U.S. Patent 3258402 (1966). 18 p.Hirsch RL. Inertial-electrostatic confinement of ionized fusion gases. J. App.

Phys. 1967; 38(11): 4522-4534.Eisberg R, Resnick R. Quantum physics of atoms, molecules, solids, nuclei,

and particles. New York: John Wiley and Sons; 1985. 713 p.Glascock MD. An overview of neutron activation analysis.

<http://www.missouri.edu/~glascock/naa_over.html> Accessed 2003 May 1.