3Present Address: Department of Physics, Duke University, Durham, NC 27708-0305, USA.

This experiment is being done so that we can determine whether anti-hydrogen acts the same as hydrogen; we want to know if anti-hydrogen falls up or down. It consists of making gravity measurements with hydrogen to find out whether we can repeat the process with hydrogen. The experiment at hand requires a number of steps to be done before it is assembled and tested, they are as follows:

• Build the source vacuum assembly
• Build the water cooling system
• Design, build and test LN₂ cooling system for the nozzle
• Design and build the skimmer
• Measure atomic hydrogen flow from the source using analyzing magnets and a residual gas analyzer
• Work on interferometer mechanical design
• Design the plasma coil and build a prototype

From 1970 until 1989 a Lamb-shift source of polarized H^- and D^- ions was used for approximately 50% of the research program at the Triangle Universities Nuclear Laboratory (TUNL). By early 1980’s it had been realized that further significant improvement in its output beam intensity was not possible because of fundamental physics limitations.

In 1986 construction began on a new atomic-beam –type polarized source for the laboratory. The choice of this type of source was made in part to satisfy the predominant experimental need for tensor- and vector-polarized deuterium beams, often efficiently pulsed at ~ 4 MHz for time-of flight experiments. Such sources were traditionally the choice for producing positive polarized beams of hydrogen and deuterium. Although they had been used less often for negative polarized beams, the promise of improved performance with a new ionizer utilizing plasma heated by electron resonance (ECR) enhanced the attractiveness of this well-established technique.

This new source was completed and installed by 1989. Used now for five years and recently for ~ 75% of the scheduled experimental days, the variety and magnitude of the output beam polarizations available have improved steadily and are now high, up to 80% of the theoretical maxima expected for H operation. Typical polarized H^-beam intensities obtained for the laboratory’s traditional tandem accelerator program at energies between 1.5 and 20 MeV are ~ 20 times larger than those previously available. In addition, using gas admitted directly to its electron-cyclotron-resonance (ECR) ionizer, the source can also produce unpolarized beams of H± , D± , and He± . All these beams are available directly from the source with energies from 10 to 85 keV. Using a mini-tandem accelerator facility, negative beams can be further accelerated (for singly charged ions up to ~ 320 keV) to facilitate a new program of low-energy measurements of interest to the astrophysics and fusion communities.

The increasing interest in nuclear and particle physics for polarized ion beams has greatly stimulated the investigations of a variety of production schemes for high intensity polarized hydrogen atomic beams. Besides many new production schemes and proposals, the classical atomic beam method using the Stern-Garlach separation has maintained the role of a very powerful technique to produce high intensities of highly polarized ion beams. In addition, the possibilities to use polarized atomic beams as jet targets in storage rings, or increase the particle density by filling storage cells with polarized atoms for use as dense gas targets have been investigated. However, for a fixed room temperature of the atomic beam, strong limitations occur due to the magnetic saturation of the iron poles in the multipole separation magnet and the stray fields between the poles.

Optical interference phenomena were first described in Newton’s day. Optical interferometers began to be used for measurement (that is other than as objects of study in themselves) at the end of the last century. A notable example is the Michelson-Morley experiment.

The first interferometers for materials particles were realized in 1954 with the near simultaneous demonstration of two kinds of electron interferometer. Marton used a three crystal geometry, and required 1230 6 min exposures to find interference. Marton argues in the paper (1954) that a two slit geometry would be "almost impossible". A single wire (or double slit) biprisim was demonstrated by Mollenstedt in the same year.

In 1962 Maier-Leibnitz demonstrated the first neutron interferometer, a simple biprisim. It was 10 m long but was only able to separate the beams by 60 m m. Due to the low flux through this design, little was done with it. The first perfect crystal neutron interferometer was demonstrated by Rauch in 1974. Interferometers of this type use three crystalline diffraction gratings that are aligned by virtue of being cut from a single perfect crystal, and typically enclose an area of a few 10’s of cm². Many such neutron interferometers have now been built, and a large body of work has been done, using them to study basic quantum phenomena. Some highlights are: coherent spinor rotation (Rauch, Zeilinger et al. 1975), effect of the earth’s rotation (Sagnac effect) (Werner, Staudenmann et al. 1979), and the measurement of the phase shift due to the gravitational field (Colella, Overhauser et al. 1975, known as the COW experiment).

The first observation of diffraction for atoms was by Estermann and Stern in 1930. They observed diffraction in the scattering of H₂ and He of a cleaved LiF surface. In the 1980’s the advent of light force slowers for atom beams made atom interferometers were not demonstrated until 1991, and that none have yet used laser cooled atoms.

Although an important literature exists about the formation of molecular beams, one has to be aware that the required conditions for the production of hydrogen atomic beams can strongly differ from the generally studied case. The necessity to keep the recombination rate of the atoms low requires beam forming systems operating at low density (<10¹⁷ atoms/cm³) and large nozzle diameters. Therefore the conditions for the beam formation cannot be chosen a priori, but they are a compromise imposed by other parameters. Depending on the gas density in the beam generating vessel and the nozzle geometry different modes of beam formation are possible.

A dry vacuum pump removes gases by a simple compression stroke of a piston in a cylinder. Four stages of compression are used. The pump is totally oil-free, making it well suited for use on vacuum systems with ultra-clean operating requirements. The use of composite nonmetallic materials on the moving surfaces allows the pump to work without the use of sealing or lubricating fluids. Thus, the oil backstreaming found with rotary vane, oil-sealed mechanical vacuum pumps is eliminated. Backstreaming is that small amount of pump fluid vapor that goes in the wrong direction toward the chamber. This is our primary reason for choosing this vacuum pump.

The dry vacuum pump is used for pre-evacuation of the vacuum system, which is then pumped down by a turbo pump.

Turbo pumps are very clean mechanical compression pumps. They pump using a high-speed rotating surface to give momentum and direction to gas molecules. They operate smoothly and contribute little vibration to the operating system. They are the only purely mechanical vacuum pump that can reach pressures of less that 5 * 10^-10 torr without using traps. (Metal gaskets, and a mild bakeout of the vacuum system are necessary to reach this pressure.)

The turbo pump used runs at a speed of 75,000 rpm. The compressed gases are expelled from the pump via a foreline which is evacuated by the dry vacuum pump. In the appendix, you can find charts that show that using the turbo pump makes a big difference in the vacuum system.

The ionization gauge uses the property that if you can energize an atom or molecule, it may lose an electron and become charged. These charged molecules (ions) are attracted and "counted" as they pick up an electron to become neutral again. The ionization gauge has a hot filament, a grid, and an ion collector. A control unit provides power, amplification, and metering. The hot filament supplies ionizing electrons. The grid attracts these electrons. The ion collector attracts the ions and gives up electrons as ions are neutralized. This process creates a small "ion current" which is then amplified. On their way to the grid, electrons may collide with gas molecules, ionizing them and releasing more electrons. The longer the flight of these electrons, the greater the chance of collision. Therefore, a stronger, more usable signal is produced. The positively ionized gas molecules are attracted to the collector. This produces an ion current proportional to the pressure in the chamber. To get meaningful pressure readings, the sensitivity of the gauge must be known, and the emission current must be well regulated. For this reason the manufacturer went through a great deal of trouble to insure that the emission current is constant by using a Ratiomatic circuit to directly measure the ratio of the ion current to the emission current. This allows small variations in the emission current to occur without affecting the gauge reading.

The convectron gauge is used to measure the pressure produced from the dry vacuum pump.

The thermocouple feedthrough measures the temperature by probes that are placed in the copper rod.

The 1" rod is used to measure the temperature difference from the cold trap to the resistor. The resistor applied heat to the rod. The 1" rod proved to conduct more heat than the ¼" rod. The temperature difference with the ¼ " rod was about 50° , and 20° with the 1" rod.

The cooling fan is used to cool down the turbo pump that becomes very hot after being in use for a long period of time, and after it gets too hot, the ion gauge is automatically turned off. The fan is connected to the top of the turbo pump so the cool air blows down on the pump.

The dissociator is used to produce the directed beam of hydrogen. Atoms are obtained from dissociation of H₂ inside a ~ 30 cm long Pyrex tube placed on the main source axis. The tube is 1 cm diameter at each end and has a 1.4 cm diameter, 10 cm long region near the middle where the intense region of the discharge is excited.

We have not decided yet is how to cool the nozzle, whether a water or nitrogen system should be used. Dissociated atoms leave the discharge region and are precooled by contact with the water-cooled or nitrogen-cooled tube. Then they emerge through a 3 mm diameter aperture at the end of the tapered bore in an oxygen-free, high-conductivity (OFHC) copper nozzle. The nozzle is cooled and thermally isolated from the discharge tube by a Macor accomodator. (MACOR is a machinable ceramic.)

The RGA measures partial pressure of each gas present in the vacuum system as well as the total pressure. In this experiment it is operating in high vacuum, and can also operate in ultrahigh vacuum. The residual gas analyzer separates, identifies and measures the partial pressures of residual, or remaining gases in an evacuated chamber. The gases produce peaks in the display. The position of the peaks identifies the gases producing them. The partial pressures of the gases are measured by the heights of the peaks.

I began by reading a thesis by David Keith on "An Interferometer for Atoms", a study done at Massachusetts Institute of Technology; it gave me background information about an interferometer. I also read two papers dealing with polarized hydrogen atomic beams, they gave me information about what hydrogen beams are, how they are made, etc. Then I started reading a book called "Basic Vacuum Practice"; this book gave me a better understanding about vacuum fundamentals, roughing, high, and ultrahigh vacuum pumps, gauges (for instance, a thermocouple gauge was used to measure temperatures produced by the liquid nitrogen cooling system for the nozzle), and leak detection.

The practical definition for vacuum is what exists in any contained volume where there is less gas than there is in the surrounding atmosphere. These gases exert a force on the surface of the container; this force is called pressure. We can measure the pressure in the chamber by comparing it to the atmospheric pressure on the outside. In this way, we can find out how much gas is left in the vacuum. Pressure is defined as force per unit area. Gases are composed of small particles. These particles are in constant motion. As these particles move around in space, they hit objects. When they hit something, they exert a force, or pressure. We can take a unit of area and measure the number and intensity of particle impacts on that surface. The result is a pressure measurement.

Vacuum pumps are special pumps which remove the air and other gases from the work chamber. There are many, and very different kinds of vacuum pumps. Some of them actually remove the gases. Other pumps trap the gases or change their form. In any case, the pump’s job is to take as many gases out of circulation as necessary. This brings us to the different degrees of vacuum, called rough vacuum, high vacuum, and ultrahigh vacuum. Which one is used depends on the application. The better (or higher) the vacuum is, the less air and gas are present in the work chamber. Atmospheric pressure at sea level (45° N latitude) is 14.7 psia or 760 torr, my experiment was running under a pressure of about 1.5 torr.

Next I began winding and measuring H₂ dissociation coil. This process involved winding a copper rod around a Pyrex Discharge Tube (thin glass tube), and connecting the ends of the coil to the Network Analyzer and recording measurements of frequency, amplitude, and phase, to find out the inductance, which for this experiment is 1E-06. {Inductance chart in appendix}. Next I want a resonant frequency, so I have to find a capacitor that will generate a resonant circuit; we want a resistive load and from the three different capacitors used, I found that the 1500 pf capacitor gives me what I want; when phase is zero, the load and resistance looks alike. {Capacitance chart in appendix}. Afterwards I assembled the liquid nitrogen cooling system with the ¼" copper rod and a 50W resistor was connected to the end of the copper rod to supply heat, then tested for thermal conduction. {Chart showing cooling curve in appendix}. After seeing that there was a temperature difference of approximately 50° , it was then decided to go with the 1" copper rod. All the proper disconnections and reconnections were made, the system was reassembled, and a 125W resistor was used, and more thermal conduction test were run and recorded. { Chart showing cooling curve due to the 1" copper rod and 125W resistor in appendix }.

The turbo pump, cooling fan, and ion gauge are now added to the system, and new conductivity test are run and recorded. The system proved to work better with the turbo pump; the temperature difference has gone down to 20° .

Now the hydrogen source and the residual gas analyzer is added to the system, which has now been mounted on a make-shift unistruts to keep from tilting over from the weight of the residual gas analyzer. Before we begin to leak hydrogen into the system, the Macor accommodator was crushed from being squeezed too tightly, but we ignored it and continued the test. We began to leak hydrogen into the system and then the dry vacuum gauge would not give correct readings, the ion gauge was affected somehow which prevented the emission on the ion gauge to be activated. We stopped the test, took some parts to the machine shop and this is as far as we got.

The purpose of this experiment is to determine whether anti-hydrogen falls up or down. The project was delayed when we tried to leak hydrogen into the system, and most of the equipment went haywire. Theory suggest that anti-hydrogen will fall down, however, we never got far enough to prove or disapprove the theory because of time constraints. Although I was not able to finish the project, I did learn a lot about all aspects of the experiment, and the equipment (mainly vacuum) mentioned in this paper. I feel that with the knowledge of vacuums that I have gained, I can now work on any such system relating to vacuum.

This project is cutting edge technology which can have vast applications in the marketplace, especially in the area of vacuum, pulse power, and radio frequency just to name a few.

The work described here was supported by the Universities Research Association, Inc. who operates Fermilab under contract with the United States Department of Energy. I wish to thank Gerry Jackson without whom this entire project could not have been performed. Lee Benson for helping me get a lot of the equipment for my project and for a better understanding of those concepts which was too much for me to grasp on my own. Tom Phillips for taking time out to help. I also gratefully acknowledge Elliot McCrory and Diane Engram for giving me the opportunity to participate in the Summer Internships in Science and Technology (SIST) for minority students program at one of the most prestigious national laboratories in the world. I would love to return next year.

1. Varian vacuum products, Basic Vacuum Practice, Third Edition, 1992.
2. T.B. Clegg et al. / Nucl. Instr. And Meth. in Phys. Res.A 357 (1995) 200-211.
3. T.B. Clegg et al. / Nucl. Instr. And Meth. in Phys. Res.A 278 (1989) 349-367.
4. David W. Keith, An Interferometer for Atoms, 1991.

 Here are some pictures of my equipment