REVIEW OF SCIENTIFIC INSTRUMENTS 85, 02B912 (2014)
Ion source developments for the production of radioactive isotope beams at TRIUMFa) F. Ames,b) P. Bricault, H. Heggen, P. Kunz, J. Lassen, A. Mjøs, S. Raeder, and A. Teigelhöfer TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T2A3, Canada
(Presented 13 September 2013; received 15 September 2013; accepted 20 October 2013; published online 10 December 2013) At the ISAC facility at TRIUMF radioactive ions are produced by bombarding solid targets with up to 100 μA of 500 MeV protons. The reaction products have to diffuse out of the hot target into an ion source. Normally, singly charged ions are extracted. They can be transported either directly to experiments or via an ECR charge state breeder to a post accelerator. Several different types of ion sources have to be used in order to deliver a large variety of rare isotope beams. At ISAC those are surface ion sources, forced electron beam arc discharge (FEBIAD) ion sources and resonant laser ionization sources. Recent development activities concentrated on increasing the selectivity for the ionization to suppress isobaric contamination in the beam. Therefore, a surface ion rejecting resonant laser ionization source (SIRLIS) has been developed to suppress ions from surface ionization. For the FEBIAD ion source a cold transfer line has been introduced to prevent less volatile components from reaching the ion source. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4833926] I. INTRODUCTION
The operation of ion sources at an ISOL (isotope separation online) facility imposes several special challenges. Requirements for the ion sources are: (1) a high efficiency, many radioactive ions are produced only in small quantities; (2) robustness and high reliability, the ion source is located close to the hot target (typically at 2000 ◦ C) in a high radiation area and not accessible during operation; (3) fast ionization process, the half life of many isotopes of interest is short (down to some ms); and (4) low cost and easy manufacturing, after a typical target life time of 4 weeks the ion source has to be disposed together with the target material due to high levels of contamination and activation. In order to cover a broad range of elements, several ion sources have been in operation during the last decade at the ISAC (Isotope Separation and Acceleration) facility at TRIUMF. The sources are closely coupled to a target container which consists of a 20 cm long tantalum tube with a diameter of 2 cm. The tube can be filled completely or partially with the target material. A proton beam of 500 MeV up to a current of 100 μA intersects it in the longitudinal direction. The target material is chosen to optimize the production and release of the desired isotope. It consists of a stack of D shaped foils. Those foils are made of refractory metals or carbide or oxide materials, which are deposited on graphite or metal backing foils to enhance the overall thermal conductivity. The material is chosen for its capability to operate at high temperature in order to allow the radioactive products to diffuse out of the target material and to effuse into the ion source within a sufficiently short time frame. The heat conductance of the foils has to a) Contributed paper, published as part of the Proceedings of the 15th
International Conference on Ion Sources, Chiba, Japan, September 2013. b) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2014/85(2)/02B912/3/$30.00
be sufficiently high to allow the energy deposited by the proton beam to be transported to the outer radius of the target container, where it can be radiated away. In order to increase the radiative cooling, target containers can be equipped with cooling fins. This allows a power deposition from the proton beam up to about 10 kW in the target at a center temperature of about 2000 ◦ C.1 In case of a lower proton beam intensity, the target container can be resistively heated by applying a current of up to 600 A to it. Thus, the required temperature for an optimum release of the products can be maintained. Atoms are transported from the target container through a 3 mm inner diameter tantalum transfer tube, which is heated separately. II. ION SOURCES USED AT ISAC
As there is no single ion source with optimum properties for all elements, a variety of sources have been developed. Principle operation of those sources has been described in earlier publications.2 A. Surface ion source
The surface ion source is the simplest design in use at ISAC. A rhenium foil is inserted into the transfer tube to increase the work function and the ionization probability. The tube is operated at up to 2200 ◦ C. That means mainly alkaline elements and some other elements with an ionization energy below about 6 eV can be ionized. B. Resonant laser ionization source
The resonance laser ion source uses the same mechanical setup as the surface ion source. In addition up to 3 laser beams are injected from the front into the transfer tube. The laser wavelengths are selected in such a way that a stepwise resonant excitation and final ionization of the atoms
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of interest is achieved. At TRIUMF a full solid state laser system has been chosen. It consists of several tunable titanium sapphire lasers which are pumped by a frequency doubled Nd:YAG laser at a pulse repetition frequency of 10 kHz. The output of the titanium sapphire lasers can be frequency doubled, tripled, or quadrupled to cover most of the wavelength range between 200 and 1000 nm. Usually one or two excitation steps to excited atomic states are used. For the final ionization step from an excited state to the continuum either a non-resonant excitation with another frequency doubled Nd:YAG laser, an excitation of an auto ionizing state or of a high lying Rydberg state with subsequent ionization in the extraction electrical field, via blackbody radiation or via collisions can be chosen. The choice of a specific excitation scheme depends on the atomic level structure of the element. A maximum ionization energy of more than 9 eV can be reached. In some cases development work has to be performed to determine the optimum excitation scheme. As a byproduct of such a development the source can be used for atomic spectroscopy and determination of ionization potentials. In the case of At, this led to the discovery of more than 40 formerly unknown high lying atomic states and more than 50 transitions.3, 4 For most elements, where this source has been used so far, the available laser power is sufficient to saturate the ionization process. Ionization efficiencies of some 10% can be achieved. In the periodic table of Fig. 1, 24 elements, which have been ionized with the laser resonance ionization process at TRIUMF, are marked. C. Surface ion rejecting resonant laser ionization source
Although, the ionization process in the laser ion source is element selective, surface ionization still takes place at the same time in the hot tube. For many isotopes, especially those which have alkaline elements as isobars, the intensity of such ions can be orders of magnitude higher than the desired rare isotope and imposes severe problems for the planned experi-
Rev. Sci. Instrum. 85, 02B912 (2014)
ments. In the ion guide laser ion source the laser resonant ionization takes place in a small quadrupole RF ion guide in front of the transfer tube.5, 6 An electrical potential barrier between the transfer tube and this ion guide suppresses all ions, coming from the hot surfaces of the target and transfer tube. Only neutral atoms or molecules can reach the ion guide, where they can be ionized by the laser beams. The ion guide confines the ions and guides them to an extraction electrode. This source has been used on-line for the first time in April 2013. Resonantly ionized Mg and Al isotopes have been produced while suppressing the isobaric Na ions by up to 6 orders of magnitude. The intensity of the Mg and Al ions was about a factor of 50 lower as compared to a normal resonant laser ionization source. This can be explained mainly by the smaller spatial overlap of the atom cloud with the laser beams due to the expansion of the atom cloud, when leaving the transfer tube. D. FEBIAD ion source
For elements or molecules with an ionization energy higher than about 9 eV, which cannot be ionized efficiently either by surface or by the laser ionization, a FEBIAD (forced electron beam induced arc discharge)7 ion source is used. In the TRIUMF design the tip of the transfer tube acts as an electron emitter. An anode grid in front of it accelerates the electrons up to about 200 eV into a small plasma chamber made of tantalum. The plasma chamber is surrounded by a coil to form a magnetic field for confinement of the electrons. Neutral atoms or molecules emerging from the transfer tube can be ionized via collisions with the electrons and are extracted through a small hole in the plasma chamber. The source has been used so far mainly for noble gases, halogens, and molecules like for example CO with one radioactive component. E. FEBIAD ion source with cold transfer line
As the plasma chamber of the FEBIAD source is not heated directly, the temperature at the wall is low compared
FIG. 1. Periodic table with elements marked, which have been either on-line ionized with the TRIUMF laser ion source, or where excitation schemes have been developed at TRIUMF or other places or are theoretically possible.
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to the transfer tube and thus, the ionization efficiency for condensable elements is poor. In most cases this effect is wanted, as it reduces the background. In order to further decrease the number of those unwanted elements a cold spot can be introduced in the transfer tube. This is realized by bringing the tube in thermal contact with the water cooled heat shield, which surrounds the hot target-ion source assembly. A first prototype using this concept has been tested already a few years ago. In the new version the thermal contact has been done by an AlN piece to allow additional electrical insulation. The operation of this source has been tested off-line by releasing Ne from a calibrated leak into the target volume. The resulting ionization efficiency has been measured to about 10%. This is similar to the efficiency achieved with the normal FEBIAD configuration, whereas the ion current from metallic components, which can be usually seen from impurities out of the hot target container was substantially reduced. On line measurements with this source will be performed in the next year. F. Charge state breeding
The on-line ion sources at TRIUMF described in the preceding paragraphs are mainly producing singly charged ions. The accelerator system at ISAC can accept ions directly if their mass to charge state ratio is below 30 amu/e. After the first acceleration stage electron stripping to mass to charge ratios
Ion source developments for the production of radioactive isotope beams at TRIUMFa) F. Ames,b) P. Bricault, H. Heggen, P. Kunz, J. Lassen, A. Mjøs, S. Raeder, and A. Teigelhöfer TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T2A3, Canada
(Presented 13 September 2013; received 15 September 2013; accepted 20 October 2013; published online 10 December 2013) At the ISAC facility at TRIUMF radioactive ions are produced by bombarding solid targets with up to 100 μA of 500 MeV protons. The reaction products have to diffuse out of the hot target into an ion source. Normally, singly charged ions are extracted. They can be transported either directly to experiments or via an ECR charge state breeder to a post accelerator. Several different types of ion sources have to be used in order to deliver a large variety of rare isotope beams. At ISAC those are surface ion sources, forced electron beam arc discharge (FEBIAD) ion sources and resonant laser ionization sources. Recent development activities concentrated on increasing the selectivity for the ionization to suppress isobaric contamination in the beam. Therefore, a surface ion rejecting resonant laser ionization source (SIRLIS) has been developed to suppress ions from surface ionization. For the FEBIAD ion source a cold transfer line has been introduced to prevent less volatile components from reaching the ion source. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4833926] I. INTRODUCTION
The operation of ion sources at an ISOL (isotope separation online) facility imposes several special challenges. Requirements for the ion sources are: (1) a high efficiency, many radioactive ions are produced only in small quantities; (2) robustness and high reliability, the ion source is located close to the hot target (typically at 2000 ◦ C) in a high radiation area and not accessible during operation; (3) fast ionization process, the half life of many isotopes of interest is short (down to some ms); and (4) low cost and easy manufacturing, after a typical target life time of 4 weeks the ion source has to be disposed together with the target material due to high levels of contamination and activation. In order to cover a broad range of elements, several ion sources have been in operation during the last decade at the ISAC (Isotope Separation and Acceleration) facility at TRIUMF. The sources are closely coupled to a target container which consists of a 20 cm long tantalum tube with a diameter of 2 cm. The tube can be filled completely or partially with the target material. A proton beam of 500 MeV up to a current of 100 μA intersects it in the longitudinal direction. The target material is chosen to optimize the production and release of the desired isotope. It consists of a stack of D shaped foils. Those foils are made of refractory metals or carbide or oxide materials, which are deposited on graphite or metal backing foils to enhance the overall thermal conductivity. The material is chosen for its capability to operate at high temperature in order to allow the radioactive products to diffuse out of the target material and to effuse into the ion source within a sufficiently short time frame. The heat conductance of the foils has to a) Contributed paper, published as part of the Proceedings of the 15th
International Conference on Ion Sources, Chiba, Japan, September 2013. b) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2014/85(2)/02B912/3/$30.00
be sufficiently high to allow the energy deposited by the proton beam to be transported to the outer radius of the target container, where it can be radiated away. In order to increase the radiative cooling, target containers can be equipped with cooling fins. This allows a power deposition from the proton beam up to about 10 kW in the target at a center temperature of about 2000 ◦ C.1 In case of a lower proton beam intensity, the target container can be resistively heated by applying a current of up to 600 A to it. Thus, the required temperature for an optimum release of the products can be maintained. Atoms are transported from the target container through a 3 mm inner diameter tantalum transfer tube, which is heated separately. II. ION SOURCES USED AT ISAC
As there is no single ion source with optimum properties for all elements, a variety of sources have been developed. Principle operation of those sources has been described in earlier publications.2 A. Surface ion source
The surface ion source is the simplest design in use at ISAC. A rhenium foil is inserted into the transfer tube to increase the work function and the ionization probability. The tube is operated at up to 2200 ◦ C. That means mainly alkaline elements and some other elements with an ionization energy below about 6 eV can be ionized. B. Resonant laser ionization source
The resonance laser ion source uses the same mechanical setup as the surface ion source. In addition up to 3 laser beams are injected from the front into the transfer tube. The laser wavelengths are selected in such a way that a stepwise resonant excitation and final ionization of the atoms
85, 02B912-1
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of interest is achieved. At TRIUMF a full solid state laser system has been chosen. It consists of several tunable titanium sapphire lasers which are pumped by a frequency doubled Nd:YAG laser at a pulse repetition frequency of 10 kHz. The output of the titanium sapphire lasers can be frequency doubled, tripled, or quadrupled to cover most of the wavelength range between 200 and 1000 nm. Usually one or two excitation steps to excited atomic states are used. For the final ionization step from an excited state to the continuum either a non-resonant excitation with another frequency doubled Nd:YAG laser, an excitation of an auto ionizing state or of a high lying Rydberg state with subsequent ionization in the extraction electrical field, via blackbody radiation or via collisions can be chosen. The choice of a specific excitation scheme depends on the atomic level structure of the element. A maximum ionization energy of more than 9 eV can be reached. In some cases development work has to be performed to determine the optimum excitation scheme. As a byproduct of such a development the source can be used for atomic spectroscopy and determination of ionization potentials. In the case of At, this led to the discovery of more than 40 formerly unknown high lying atomic states and more than 50 transitions.3, 4 For most elements, where this source has been used so far, the available laser power is sufficient to saturate the ionization process. Ionization efficiencies of some 10% can be achieved. In the periodic table of Fig. 1, 24 elements, which have been ionized with the laser resonance ionization process at TRIUMF, are marked. C. Surface ion rejecting resonant laser ionization source
Although, the ionization process in the laser ion source is element selective, surface ionization still takes place at the same time in the hot tube. For many isotopes, especially those which have alkaline elements as isobars, the intensity of such ions can be orders of magnitude higher than the desired rare isotope and imposes severe problems for the planned experi-
Rev. Sci. Instrum. 85, 02B912 (2014)
ments. In the ion guide laser ion source the laser resonant ionization takes place in a small quadrupole RF ion guide in front of the transfer tube.5, 6 An electrical potential barrier between the transfer tube and this ion guide suppresses all ions, coming from the hot surfaces of the target and transfer tube. Only neutral atoms or molecules can reach the ion guide, where they can be ionized by the laser beams. The ion guide confines the ions and guides them to an extraction electrode. This source has been used on-line for the first time in April 2013. Resonantly ionized Mg and Al isotopes have been produced while suppressing the isobaric Na ions by up to 6 orders of magnitude. The intensity of the Mg and Al ions was about a factor of 50 lower as compared to a normal resonant laser ionization source. This can be explained mainly by the smaller spatial overlap of the atom cloud with the laser beams due to the expansion of the atom cloud, when leaving the transfer tube. D. FEBIAD ion source
For elements or molecules with an ionization energy higher than about 9 eV, which cannot be ionized efficiently either by surface or by the laser ionization, a FEBIAD (forced electron beam induced arc discharge)7 ion source is used. In the TRIUMF design the tip of the transfer tube acts as an electron emitter. An anode grid in front of it accelerates the electrons up to about 200 eV into a small plasma chamber made of tantalum. The plasma chamber is surrounded by a coil to form a magnetic field for confinement of the electrons. Neutral atoms or molecules emerging from the transfer tube can be ionized via collisions with the electrons and are extracted through a small hole in the plasma chamber. The source has been used so far mainly for noble gases, halogens, and molecules like for example CO with one radioactive component. E. FEBIAD ion source with cold transfer line
As the plasma chamber of the FEBIAD source is not heated directly, the temperature at the wall is low compared
FIG. 1. Periodic table with elements marked, which have been either on-line ionized with the TRIUMF laser ion source, or where excitation schemes have been developed at TRIUMF or other places or are theoretically possible.
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to the transfer tube and thus, the ionization efficiency for condensable elements is poor. In most cases this effect is wanted, as it reduces the background. In order to further decrease the number of those unwanted elements a cold spot can be introduced in the transfer tube. This is realized by bringing the tube in thermal contact with the water cooled heat shield, which surrounds the hot target-ion source assembly. A first prototype using this concept has been tested already a few years ago. In the new version the thermal contact has been done by an AlN piece to allow additional electrical insulation. The operation of this source has been tested off-line by releasing Ne from a calibrated leak into the target volume. The resulting ionization efficiency has been measured to about 10%. This is similar to the efficiency achieved with the normal FEBIAD configuration, whereas the ion current from metallic components, which can be usually seen from impurities out of the hot target container was substantially reduced. On line measurements with this source will be performed in the next year. F. Charge state breeding
The on-line ion sources at TRIUMF described in the preceding paragraphs are mainly producing singly charged ions. The accelerator system at ISAC can accept ions directly if their mass to charge state ratio is below 30 amu/e. After the first acceleration stage electron stripping to mass to charge ratios
