Heavy Negative Ions Research Articles

Recent development of negative heavy ion sources

Various types of plasma sputter negative heavy ion source have been developed at many institutes for various purposes, after the first negative heavy ion source using plasma sputtering was successfully operated at KEK in 1988. This type of negative heavy ion source has the capability of generating various negative heavy ion beams with a relatively large beam current in the mA range.

Radio frequency plasma sputter type heavy negative ion source

A rf plasma sputter type heavy negative ion source with mA-class ion currents in dc operation has been developed. Although maximum negative ion production efficiencies by sputtering on a cesiated sputtering target surface were high (about 10% or more for Cu, C, Si, Ge and W), electron-detachment cross-sections were also considerably high (about 10 −15 cm 2). Therefore, in order to extract an intense negative ion beam through a generated plasma region from a sputtering target it is essential that the plasma should be discharged at low gas pressures in the order of 10 −4 torr. By using a rf (13.56 MHz) discharge with a three-turn rf coil 50 mm in diameter, a dense plasma of 10 11 cm −3 order was obtained in the xenon gas pressure range of 10 −5-10 −4 torr. In this source a sputtering target 42 mm in diameter whose surface was part of a sphere was used, and negatively biased by 600 V against the plasma. Extracted total negative ion currents were 6.5 mA for a copper target and 4.2 mA for a graphite target, in which electrons were eliminated by a magnetic field near the extraction hole. The concentration of Cu - current in the total copper negative ion current was 96.6%. The percentages of C - and C 2 - currents in the carbon negative ion current was 38.1 and 54.5%, respectively.

A pulsed-mode, high intensity, heavy negative ion source

A high intensity, plasma-sputter, negative ion source, which utilizes multicusp, magnetic-field, plasma confinement techniques is described. The source is an axial-geometry version of a radial-geometry source which has demonstrated a pulsed-mode peak intensity level of several mA for a wide spectrum of heavy negative ion species. The mechanical design features include provisions for fast interchange of sputter samples, ease of maintenance, direct cooling of the discharge chamber, and the use of easily replaced coaxial LaB6 cathodes.

Detection of artificially created negative ion clouds with incoherent scatter radar

Theoretical incoherent scatter (IS) spectra from collisionless F region plasma are calculated taking into account the effect of negatively charged ions. Such ions can significantly change the composition of the local ionosphere when they are created artificially during active chemical release experiments. If IS radar observations are conducted upon such a modified plasma, the interpretation of the scattered radar signal will be complicated by the negative ions, which are not found in the natural (e.g., O+/e−) ionosphere. Specifically, the presence of heavy negative ions will broaden the IS spectrum, making the shoulder peaks sharper, and creating an additional small central hump. A radar spectral analysis program that assumes only the naturally occurring plasma constituents will erroneously attribute such spectral changes to changes in other plasma parameters, such as the temperature ratio Te/Ti. In order to detect heavy negative ions we fix the temperature structure of the ionosphere to a preevent average measurement and interpret any changes in spectral shape during the experiment as being caused by changes in composition, and not by changes in Te/Ti. The Millstone Hill 440‐MHz IS radar was used to observe the spatial and temporal development of heavy negative ion plasma clouds created during four active chemical release experiments: the Ionospheric Modification Study (IMS) in 1983, the Space‐Plasma Negative Ion Experiments (SPINEX 1 and 2) in 1984 and 1986 (all SF6 releases), and NICARE 1 in 1989 (CF3Br release). Each experiment led to the creation of a region of heavy, negatively charged ions. Concentrations of 10–40% SF6− (146 amu) are reliably detected in the SPINEX 1, SPINEX 2 and IMS data sets. An average uncertainty of ±10% SF6− is present in all three experiments. Concentrations of 30% Br− (80 amu) are further detected in the NICARE 1 release, with uncertainties of ±4%.

Plasma sputter negative ion source with ECR discharge (invited)

A plasma sputter type of negative ion source with an electron cyclotron resonance (ECR) discharge has been developed at KEK. The ECR discharge was produced by a 2.45-GHz microwave. In this ion source, negative heavy ions are produced at the surface of the metal which is placed in a Xe plasma confined by a cusp magnetic field. In preliminary experiments, the beam current of 7 mA for Cu− was obtained in pulsed mode operation.

Measurement of negative-ion-production efficiencies and development of a dc operation sputter-type negative ion source

For the development of a high current heavy negative ion source, the production efficiency of the negative ions by Xe+ sputtering with cesium and electron detachment cross sections in xenon gas were measured. The maximum efficiency for Cu− was 12.1% and the single-electron detachment cross section was found to be of the order of 0.6–1.4 × 10−15 cm2 in the energy range of 5–50 keV. Based on the results of these fundamental experiments, a plasma-sputter-type heavy negative ion source has been constructed, in which the plasma was generated at a rf-frequency of 13.56 MHz. A total negative ion current of 2.5 mA was obtained without electrons from the source in dc operation, and the Cu− current was detected to be 91% of the total beam. The operation characteristics are described.

Development of negative heavy ion sources for plasma potential measurement

A plasma sputter negative ion source was evaluated for its applicability for measuring the plasma potentials in fusion devices. Both the beam current density and the beam energy spread are key issues for this application. The energy spectra of self-extracted Au− beams from the source were measured under the condition of a constant work function of the production surface. The FWHM increases from 3 to 9 eV monotonically as the target voltage increases from 50 to 300 V, independently from the target surface work function of 2.2–3 eV.

Recent advancements in high intensity heavy negative ion sources based on the sputter principle

Significant advancements have been recently made in the technology of forming high intensity heavy negative ion beams. Due to their ease of operation, simplicity, long lifetime, and wide range of species capabilities, negative ion sources predicated on the sputter principle are being used for an increasing number of diverse applications, including standard tandem accelerator-based research, high-energy ion implantation, tandem accelerator mass spectrometry, and low-energy atomic physics research. Sources have been developed which utilize either (1) direct surface ionization, or (2) a plasma to form the positive ion beam used to effect sputtering of samples containing the material of interest. This article will include a brief review of the fundamental processes underlying negative ion formation in these source. Emphasis will be placed on the descriptions and performance characteristics of specific sources based on each of the positive ion formation techniques.

A high brightness plasma sputter heavy negative ion source

A high-intensity, pulsed-mode, plasma-sputter heavy negative ion source has been developed; this source holds promise for a number of uses, including tandem electrostatic accelerator/synchrotron injection applications. To date, the source has been used to generate mA intensities from more than 18 sputter probes. A brief description of the source and typical performance data for a number of ion species are given. In addition, basic ion source operational data, such as intensity versus cesium oven temperature, sputter probe voltage, and discharge pressure, along with emittance data, are presented.

A heavy negative ion sputter source: Production mechanism of negative ions and their applications

Heavy negative ions can be produced by sputtering a metal surface at high efficiency by means of a secondary ion emission process. The most important factor for obtaining high negative ion production efficiencies is to maintain approximately one-half monolayer of Cs on the sputter target which produces a minimum in the surface work function. In a dynamic situation in which the surface is being bombarded by heavy ions, the cesium coverage is determined by a balance between the incoming and outgoing cesium fluxes. The supply of additional cesium to the surface and control of the target temperature are then essential for high negative ion production efficiency. The Neutral and Ionized Alkaline metal Bombardment type heavy Negative Ion Source (NIABNIS) in which a continuous negative ion current of a few hundred μA for several elements can be extracted, can be directly used for applications in materials science, such as negative ion beam deposition and negative ion implantation.

Electron detachment cross-sections for heavy negative-ion beam

Cross sections of single and double electron detachment for negative heavy ion beams were investigated from the standpoint of negative-ion beam transport and application. The ion energy dependence of these cross sections was measured in the energy range from 5 to 50 keV. The measurement system, i.e., the negative ion implanter developed by the authors, consists of three chambers for beam collimation, collision and detection. In the detection chamber, an electrostatic deflector was provided to separate the interacted ion beam into negative and positive ion beams, which were independently measured. Argon and nitrogen gases were examined as target gases. Single and double electron detechment cross sections were obtained. Single electron detachment cross sections were 1.15 × 10 1̄5 cm 2 for the C − ion beam and 1.4 × 10 −15 cm 2 for the Si − in argon gas. In nitrogen gas, the cross sectio twice that size. In the energy range corresponding to a velocity of 10 5m/s, the cross section has almost a constant value. Double electron detachment has a cross section of about 10 −17−10 16 cm 2, and increases with the beam energy.

New results from the KEK ion source development projects

Two types of ion sources are currently being developed at KEK. One is the optically pumped polarized ion source and the other the cusp-magnetic-field negative heavy ion source. In this paper, recent results of both are described.

Electron detachment cross-sections in low energy heavy negative ion beam appratus

We have measured the single- and double-electron detachment cross-sections of B, C, O, and Si negative ions on He, Ar, Xe, H 2 and N 2 gas targets in an ion beam energy range of 5–50 keV. The single-electron detachment cross-section in this energy range is nearly constant and around 10 − 15 cm 2 , while the double-electron detachment cross-section increases in proportion to the ion velocity and is much smaller in area than the single one. The order of magnitude of both cross-sections for negative ion species is Si - > B - > C - > O -. The order for target gases is Xe > Ar > He for single atomic gases and N 2 > H 2 for molecular gases. These properties can be quantitatively explained by considering the orbital radius of the electrons in the outermost shell of the negative ion, the number of electrons, the electron affinity (for single-electron detachment), the sum of the electron affinity and first ionization potential (for double-electron detachment) and the atomic radius of the target gas.

Polarization asymmetry in two-electron photodetachment: A cogent test of the ionization threshold law.

A very sensitive test of the electron-atom ionization threshold law is suggested: for spin-aligned heavy negative ions it consists of measuring the polarization asymmetry A(PA) coming from double detachment by left- versus right-circularly polarized light. The respective yields are worked out for the Te(-) (5p)5 2P(3/2) ion. The Coulomb-dipole theory predicts A(PA) to be the ratio of two oscillating functions in sharp contrast to any power law (specifically that of Wannier, 1953) for which the ratio is expected to be a smooth function of energy.

Characterization of a cesium surface ionization source with a porous tungsten ionizer. I

A cesium surface ionization source of the type and geometry customarily used in conjunction with sputter-type negative heavy ion sources has been characterized. Measurements have been made of positive-ion production and probabilities of ionization as functions of extraction voltage and cesium oven temperature for ionizer porosities of 0.7 and 0.8. The perveance P of the source, when operated in the space-charge-limited regime with the ρ=0.7ρ0 ionizer, is found from experiment to be 7.61×10−4 μP, while that for the ρ=0.8ρ0 ionizer is 1.92×10−4 μP. These values are lower by factors of 3.88 and 15.53, respectively, than those predicted by numerical solution to Poisson’s equation for full area emission from the source. Positive-ion current versus ionizer temperature data are also presented along with mechanical design features of the source.

Further evaluation of the high intensity plasma sputter heavy negative ion source

In this report, we provide more detailed information on the high intensity heavy negative ion source described previously. Intensity vs time spectra, mass distribution data and source operational data are presented for Au, Cu, Ni and CuO sputter probes. Sputter probe voltage limited beam intensities of 10.2, 8.2, 5.1 and 4.5 mA, respectively have been realized from these sputter probes. The results of emittance measurements for Au and Ni probes indicate a rather strong dependence on beam intensity, as expected from space charge considerations. The source, when operated in pulsed mode, holds considerable promise for use in conjunction with tandem electrostatic accelerator/synchrotron injection applications. The high intensity capabilities of the source make it a viable candidate for generating mA intensity level, cw ion beams for a variety of other applications, including ion implantation.

A critical comparison of ionospheric depletion chemicals

Six chemicals, H2, H2O, CO2, SF6, CF3Br, and Ni(CO)4, are considered as ionospheric modification agents. Each of these species reacts in the F region to produce localized plasma depletions. The first three interact with O+ and yield polyatomic ions which dissociatively recombine with electrons to give neutrals. The last three dissociatively attach electrons to produce heavy negative ions which become mutually neutralized by reactions with O+. The effectiveness of these chemicals depends on the amount which goes into the vapor state upon release. Thermodynamic calculations show that H2O has the lowest vapor yield of about 20% from a heated, pressurized tank. Over 60% of the other substances should be vented in gaseous form. Based on estimates of plasma density reduction and airglow stimulation, nickel carbonyl is the most efficient of the six species for modifying the nighttime ionosphere. During the daytime, CF3Br and SF6 provide the largest depletions.

後段加速を用いた正，負両イオン用高感度検出器

A high sensitive detection system for heavy ions of low energy was described. In order to improve the conversion efficiency from positive or negative heavy ions, to electrons at the conversion dynode of the electron multiplier, the whole assembly of detection sysytem (electron multiplier, d. c. amplifier, high voltage power supply for the electron multiplier and power supply for the d. c. amplifier and A-D converter) were isolated from the ground and were kept at -10kV potential for the positive ion detection and +10kV for the negative ion detection. The d. c. signal from the d. c. amplifier was converted to 12-bits digital signal by the A-D converter and was sent to the ground side by light-emitting-diodes (LED). It was received by photo-transistors and converted back to a d. c. signal by a D-A converter. The conversion cycle of the A-D converter was 8 μsec. With this detection sysytem, the high sensitivity for negative heavy ions was obtained.

Negative ion source (NIABNIS) and preparation of transparent carbon films by negative carbon ion beam deposition

A high current heavy negative ion source of the neutral and ionized alkaline metal bombardment type (NIABNIS) has been newly developed for negative ion beam applications such as deposition and implantation. This source is a sputter type, and the sputtering target is placed on the extractor of a cesium plasma ion source in order to obtain a large current of cesium ions bombarding the target simultaneously with a controlled amount of neutral cesium atoms. The work function of the sputtering target surface in operation is kept as low as possible so that a high efficiency of the negative ion production is realized. Mass-separated ion beams of 0.74 mA for C −, 200 μA for C 2 −, 320 μA for Cu −, and 170 μA for Si − were obtained. A negative ion beam deposition system with the NIABNIS, in which divergent effects of the ion beam were effectively suppressed, has also been developed. A decelerated C − ion current of several tens of μA in the range of a final energy of 25–100 eV, and about 100 μA in the range higher than 100 eV was obtained. By using C − and C 2 − ion beams with an energy of 10–1000 eV, transparent carbon films of several thousands of Å in thickness were deposited.

Radiobiological Background for Negative Pion and Heavy Ion Therapy on Cancers

Recent advances in radiotherapy have been remarkable. However, there are some tumors which are incurable with conventional radiation therapy because of their histological features, localization and the like. In addition, conventional radiations such as photons, gamma rays or electrons show several disadvantages to bring tumors to cure. Hence more and more attention is being paid to new modalities such as fast neutrons, protons, negative pions and heavy ions, which are expected to overcome some of the defects of the conventional radiations. Except for fast neutrons, these particle radiations show excellent physical dose localization in tissue. Moreover, in terms of biological effects, they demonstrate several features compared to conventional radiations, namely low oxygen enhancement ratio ( OER), high value of relative biological effectiveness (RBE), smaller cellular recovery, larger therapeutic gain factor (TGF) and less cell cycle dependency in radiation sensitivity. In the present paper the biological effects of pions on murine tumor cells are shown comparing to the effects of conventional radiation, or heavy ions (Ne or Ar ions).