Neutron Counting: Neutrons were detected using a Ludlum ...



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)

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 55Mn, 127I, 51V, 27Al, and 197Au was successful, and prompt capture gamma rays were identified from 10B, 113Cd, and 1H 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 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 (1H), one neutron (2H), or two (3H). 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 (2H) nuclei to yield helium (3He) 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 3He 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 2H.

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 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 ( of a reaction between a projectile and a target is expressed by physicists in terms of the cross section (9:

(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.

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

Figure 3. Cross section (in millibarns) as a function of energy

for the 2H(d,n)3He 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.

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 2H, 3H, or 3He

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 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 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 2H(d,n)3He 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 (28Al, 198Au, 51V, 56Mn, 128I) was readily detectable using a basic scintillation spectroscopy setup. Additionally, prompt neutron-capture gamma emissions from 10B, 1H, and 113Cd 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.

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.

References

1Farnsworth PT. U.S. Patent 3258402 (1966). 18 p.

2Schatzkin P. The boy who invented television. Accessed 2003 Feb. 19.

3Sandia National Laboratory. Z produces fusion neutrons, Sandia scientists confirm. Accessed 2003 May 1.

4Lawrence Livermore National Laboratory. National Ignition Facility programs. Accessed 2003 May 1.

5ITER. Cost, schedule, and siting. Accessed 2003 May 1.

6Schatzkin P. The open source fusor research forum. Accessed 2003 May 1.

7Hull R. 2001 June 21. Introduction. Accessed 2003 March 17.

8Eisberg R, Resnick R. Quantum physics of atoms, molecules, solids, nuclei, and particles. New York: John Wiley and Sons; 1985. 713 p.

9Knoll GF. Radiation detection and measurement. New York: John Wiley and Sons; 1989. 754 p.

10Atomic Institute of the Austrian Universitites. Neutron radiography. Accessed 2003 April 22.

11Barth RF et al. Boron neutron capture therapy of brain tumors: an emerging therapeutic modality. Neurosurgery 1999; 44(3): 433-449.

12Glascock MD. An overview of neutron activation analysis. Accessed 2003 May 1.

13Strong J. Procedures in experimental physics. New York: Prentice Hall; 1938. 642 p.

14Hankins 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.

15Ludlum Measurements, Inc. Ludlum Model 12-4 manual. 1989. 26 p.

16Lide DR, editor. CRC handbook of chemistry and physics, 77th edition. Boca Raton: CRC Press; 1996.

17U.S. Bureau of Radiological Health. Radiological health handbook. Rockville, MD: U.S. Department of Health, Education and Welfare; 1970. 458 p.

18Libbrecht KG. Neutron experiments. Accessed 2003

May 1.

19Rosenstiel J. 2002 December 17. Indium Activation. Accessed 2003 March 17.

20WISE Uranium Project. Neutron activation calculator. Accessed 2003 May 1.

21Hirsch RL. Inertial-electrostatic confinement of ionized fusion gases. J. App. Phys. 1967; 38(11): 4522-4534.

22Ashley 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.

23White RM, Resler DA, Lawrence Livermore National Laboratory, U.S.A., ENDF/B-VI evaluation, MAT # 128, May 1991; data retrieved from the ENDF database Accessed 2003 April 05.

24White RM, Resler DA, Lawrence Livermore National Laboratory, U.S.A., ENDF/B-VI evaluation, MAT # 225, May 1991; data retrieved from the ENDF database Accessed 2003 April 05.

25Hale GM, Drosg M, Los Alamos National Laboratory, U.S.A., ENDF/B-VI evaluation, MAT # 131, Revision 1, January 1995; data retrieved from the ENDF database Accessed 2003 April 05.

26Cross 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.

27Hull R. The Farnsworth / Hirsch fusor. The Bell Jar 1997; 6(3-4)

28Hansen S. Neutrons and neutron generators. The Bell Jar 1997; 6(3-4)

29U.S. Nuclear Regulatory Commission. Units of radiation dose. Accessed 1 May 2003.

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.

Figure A2. Second half of the fusor. This part holds the high

voltage feedthrough, not shown, with a solder joint.

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 (. 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.

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).

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.

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

Appendix E

Three LabVIEW Data Handling Programs

On accompanying compact disk.

-----------------------

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download