Tabletop Nuclear Fusion Neutron Activation with a Farnsworth

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: + 2 H 3 He + n 2 H + 3 He 4 He + 1 H 2 H + 3 H 4 He + n 2 H Q = 3. 2 Me. V Q = 18. 4 Me. V Q = 17. 6 Me. V • 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 m. He be the masses of the neutron and helion, and vn, v. He 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 Me. V.

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 • 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.

Detecting Neutrons react with boron-10 to create alpha particles (4 He), and this reaction is frequently used to detect neutrons. 10 B + n 7 Li + 4 He The alpha particles are collected on an electrode in a tube filled with BF 3 gas. Each time charge is collected, a count is registered. Paul Carter at TUNL checks our BF 3 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 239 Pu). • 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 m. Rem / 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 k. V. 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 mm. Hg.

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 Lab. VIEW were used to write data files containing voltage, current, pressure and neutron count information several times per second. • Our Fusor initially only produced 2 m. Rem / hour of neutrons, as measured 9” from the center. But performance was honed until regular yields in excess of 60 m. Rem / 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. • 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 e. V) than for fast neutrons, a moderator to slow the Fusor’s 2. 4 Me. V 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 Mc. Leod. • 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