A Negative Ion Cookbook A Negative-Ion Cookbook Roy Middleton Department Of Physics, University of Pennsylvania Philadelphia, PA 19104 October 1989 (Revised February 1990) BNL TVDG Page 1 of 194 A Negative Ion Cookbook 1H 2He Hydrogen Helium 3Li 4Be 5B 6C 7N 8O 9F 10Ne Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon 19K 20Ca 31Ga 32Ge 33As 34Se 35Br 36Kr Potassium Calcium Gallium Germanium Arsenic Selenium Bromine Krypton 37Rb 38Sr 49In 50Sn 51Sb 52Te 53I 54Xe Rubidium Strontium Indium Tin Antimony Tellurium Iodine Xenon 55Cs 56Ba 81Tl 82Pb 83Bi 84Po 85At 86Rn Cesium Barium Thallium Lead Bismuth Polonium Astatine Radon Transition Elements 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium 57La 72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt 79Au 80Hg Lanthanum Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury Lanthanides 58Ce 59Pr 60Nd 61Pm 62Sm 63Eu 64Gd Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium 65Tb 66Dy 67Ho 68Er 69Tm 70Yb 71Lu Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Actinides 90Th 91Pa 92U Thorium Protactinium Uranium Michael Wiplich at the Tandem Van de Graaff Accelerator located at the Brookhaven National Laboratory prepared the electronic version of the Negative-Ion Cookbook. The original paper version was converted to electronic form using an H.P. Scanjet 6200C scanner and H.P. Precision-Scan Pro OCR software followed by manual fix up in MS Word 2000 Pro which was also used to convert the document to HTML. Please address any comments, suggestions or corrections to [email protected]. BNL TVDG Page 2 of 194 A Negative Ion Cookbook Acknowledgements The author is particularly grateful to the National Science Foundation who, for many years, has supported the Tandem Accelerator Laboratory at the University of Pennsylvania and made possible close to two decades of negative-ion source development. Many people have participated and contributed and it is a pleasure to acknowledge Mr. Paul Harduk for superbly crafting and building all the many ion sources. My thanks are also due to Mr. Charles T. Adams who participated extensively in the early development work and, more recently, to Mr. Harry White for his many contributions and his assistance with the measurements reported here. I would also like to thank Jeff Klein for many stimulating discussions and Mrs. Karen Walter for typing and editing this manuscript. BNL’s Contribution This document was produced at the Brookhaven National Laboratory Tandem Van de Graaff Accelerator in response to our own need to have a readily available copy of the document that would not deteriorate with age and use. We were also responding to requests by members of the SNEAP mailing list for Internet access to the Cookbook. Our original cookbook was a spiral-bound document and our copy was beginning to show excessive wear and tear. It was decided that the entire document would be converted to electronic form. MS Word was chosen as the word processor because of its ability to produce properly formatted HTML 4 documents for use on the Internet and to retain the original in the popular and easily accessed .doc format. Michael Wiplich prepared the electronic version. The original paper version was converted to electronic form using an H.P. Scanjet 6200C scanner and H.P. Precision-Scan Pro OCR software followed by manual fix up in MS Word 2000 Pro which was also used to convert the document to HTML for use on the internet. If, in your use of the information contained in this document, you find any errors or omissions please bring these to the attention of: Michael Wiplich Brookhaven National Lab 59 Cornell Avenue Building 901A Upton, NY 11973 Voice Phone: 631 344 5468 Fax Phone: 631 344 4583 e-mail: [email protected] After review, they will be included in the document at our earliest convenience. Thank you. BNL TVDG Page 3 of 194 A Negative Ion Cookbook Introduction The present work was undertaken to provide a means of producing negative ion beams, of as wide a range of elements as possible, with particular emphasis on their suitability for injection into a tandem DFFHOHUDWRU6LQFHFXUUHQWVJUHDWHUWKDQDERXW $DUHDSWWRORDGPRVWWDQGHPVDQGH[SHULPHQWHUV UDUHO\UHTXLUHFXUUHQWVODUJHUWKDQ $RXUHIIRUWVKDYHFRQFHQWUDWHGRQSURGXFLQJFXUUHQWVLQWKLV range. We have also placed higher priority on convenience of production and reliability than on current. Not all elements form stable negative ions, and in these cases it is necessary to use molecular ions. Molecular beams have two disadvantages in a tandem: 1) they reduce the final energy of the elemental ion, and 2) the coulomb explosion in the stripper foil can degrade the quality of the final beam. Both effects are minimized by choosing the lightest possible molecule, such as a hydride. Consequently, whenever possible, hydrides have been our first preference; but if difficult or inconvenient to make, we have turned to carbide, nitride, or oxide negative ions. Since beams of rare isotopes such as 13C, 17O, 18O, 36S , and 48Ca are often required in nuclear research, we have placed emphasis on developing efficient methods of producing micro-ampere beams of these. In the growing field of accelerator mass spectrometry, samples are often small and contain only ~106 atoms of the isotope of interest. Clearly, ionization efficiency is of paramount importance, and in several cases we have attempted to measure this quantity. BNL TVDG Page 4 of 194 A Negative Ion Cookbook Negative Ion Source Some of the measurements reported here were made with a source of the type described by Middleton1) and shown in Figure 1 (this is commonly known as a high-intensity source). However, most were made with a source in which a spherical one replaced the cylindrical ionizer and a directed spray conveyed the cesium vapor to it. This eliminates the need for cesium vapor containment, and the inner source chamber could be dispensed with -- a significant advantage with gas cathodes. Figure 2 shows details of the ionizer, the electric field shaping shroud, the cesium spray, and the cathode insulator assembly. The ionizer, which is made from molybdenum, has a radius of 17.5 mm and an outside diameter of 25 mm. The negative ion exit hole in the center of the ionizer is 5 mm. in diameter. We do not wish to discuss here the relative merits of the cylindrical and spherical ionizer sources; suffice it to say that negative ion currents rarely differ by more than a factor of two. The most significant feature of the spherical ionizer source is that the Cs+ beam is much better focused and concentrated, and this undoubtedly results in improved emittance. However, there are other effects that we don't presently fully understand. For example, the negative ion currents and ionization efficiencies are very dependent upon the creation of a deep sputter crater and the formation of an intense plasma ball. It is also likeO\WKDWWKLVSODVPDLVUHVSRQVLEOHIRUWKHSURGXFWLRQRIRU $RI 133Cs- ions -- but strangely only with cathodes of aluminum and iron (see Cesium). Figure 1: The high-intensity sputter source developed by Middleton1 in 1982. BNL TVDG Page 5 of 194 A Negative Ion Cookbook Figure 2: The modified high-intensity source that was used to make most of the measurements reported here. Major changes include a spherical ionizer and a spray system for the cesium vapor that eliminates the need for a cesium confinement chamber surrounding the cathode. References 1) R. Middleton, Nucl. Instr. And Methods 214 (1983) 139 BNL TVDG Page 6 of 194 A Negative Ion Cookbook Operating Conditions and Procedures Almost all of the measurements reported here were made on the ion source test facility shown schematically in Figure 3. The major components are a 7.5 cm diameter decelerating einzel lens, adjustable object and image slits, a 30 cm radius double focusing 90º magnet, and an electrically suppressed Faraday cup. The test stand is pumped by a combination of cryo- and ion-pumps, and the vacuum during operation is typically 2 to 3X10-7 Torr. We normally operate the spherical ionizer source with a cathode voltage of 8 kV as compared with 6 kV used with cylindrical ionizers. At this voltage, and with the optimum flow of cesium vapor, the Cs+ current is between 1 and 1.5 mA but the cathode current (the current supplied by the cathode power supply) is usually between 2 and 5 mA. The latter is larger because it includes the electrons emitted by the cathode, most of which are deflected by external permanent magnets and strike the ionizer. The magnitude of the electron current depends on the cathode material and on the development of a sputter crater in the cathode. Usually this forms within 10 or 20 minutes, depending on the cathode, and generally is accompanied by the formation of a small ball of bright (often pale blue) plasma. The ionizer current and voltage are typically 12.5 A and 9 V respectively and at this power the ionizer temperature is ~1100 ºC. However certain cathode materials, such as the rare earth metals, are apt to stick on the surface of the ionizer and because of their low work functions, impair its efficient operation. With these we frequently increase the ionizer power to ~ 200 Watts, raising the temperature to about 1250 ºC -- a dangerously high value. The cesium reservoir temperature is measured by a thermocouple screwed in its base. Since this is not thermally insulated, the temperature reading corresponds to the lowest value of the reservoir and cesium vapor feed tube.