The experiments of the present work were completed at Triangle Universities Nuclear Labo- ratory’s (TUNL) Laboratory for Experimental Nuclear Astrophysics (LENA). Funded by the Department of Energy, LENA is devoted to making direct measurements of astrophysically important charged-particle nuclear reactions. The laboratory houses two accelerators. An electron cyclotron resonance (ECR) ion source sits on a 200 kV air-insulated platform and produces high-intensity beams used to measure cross sections at energies below 200 keV. A 1 MV model JN Van de Graaff accelerator is used for measurements above 200 keV. Beams from both accelerators are directed to a bending magnet used to transport the beams to a single target station. A schematic view of LENA is shown in Figure3.1.
3.1.1 1 MV Model JN Van de Graaff Accelerator
An upgraded High Voltage Engineering Corporation (HVEC) 1 MV model JN Van de Graaff can produce H+ beams on target with energies between approximately 0.15 and 1 MeV.
The terminal of this machine has been modified to accommodate a high-output ion source and associated RF power supply. Other modifications were made to improve stability and control, including modification of the generating voltmeter and slit feedback circuitry. The beam current attainable on target is∼250 µA at a terminal voltage of 300 keV. The typical energy resolution is 1 to 2 keV full width at half maximum (FWHM), which can be improved if the output intensity of the ion source is reduced.
ECR Ion Source Acceleration Column Turbo Pump Analyzing Magnet Object Slits Image Slits Cryogenic Pump Cryogenic Pump Liquid Nitrogen Trap Target 200 kV Platform 1 MV JN Van de Graaff Turbo Pump Beam Stop Beam Profile Monitor Magnetic Quadrupole X-Y Steerer
Figure 3.1: Laboratory for Experimental Nuclear Astrophysics. The laboratory is equipped with two accelerators, a 200 kV ECR ion source accelerator and a 1 MV JN Van de Graaff accelerator. Shown above are the essential beam tuning and monitoring instruments. Each beam is directed to target by a dipole magnet.
Table 3.1: List of reactions, energies, and widths used for energy calibration of the LENA accelerators. All energies and widths listed are from Reference [Uhr85] unless noted otherwise.
Reaction Eplab (keV) Γ (eV)
18O(p, γ)19F 150.82±0.09a ≤300b 26Mg(p, γ)27Al 292.06±0.09 <37 338.4±0.1 <40 453.8±0.1 <81 27Al(p, γ)28Si 202.8±0.9c 326.97±0.05 <38 405.44±0.10 <42 a Reference [Bec95]. b Reference [Til95]. c Reference [End90].
Graaff through the following [Ili07]
B = k q
p
2mc2E+E2 (3.1)
wheremc2 and q are the rest mass and charge state of the ion, respectively. The calibration
constant,k, has been determined through the measurement of the resonance energies of well- known reactions. Table3.1lists the measured reactions used for the JN Van de Graaff energy calibration.
3.1.2 200 kV ECR Ion Source and Accelerator
Ion Source Design
The electron cyclotron resonance ion source and its beam acceleration system are relatively new additions to the laboratory [Ces10]. At the start of this project, the previous system was overhauled, and completely redesigned with the hope of developing a reliable, compact ion source capable of producing milliampere hydrogen beams. Our starting point was a design by Wills et al.[Wil99] at Chalk River Laboratory. The plasma discharge chamber is placed on the axis of a surrounding solenoidal permanent magnet array, which produces the B = 87.5 mT magnetic field necessary for electron cyclotron resonance at a frequency off = 2.45
The magnet consists of two independent sub-assemblies: an outer solenoidal array that produces the axial magnetic mirror field for plasma confinement and an inner multipole array for additional radial confinement. Before fabrication, the arrays were thoroughly modeled usingradia, a package developed at the European Synchrotron Radiation Facility (ESRF) [Chu97], which runs inmathematica. As mentioned above, the overall magnetic field resem- bles that used by Willset al.[Wil99], but was modified to produce an axial mirror field, which peaks twice above 90.0 mT within the plasma chamber. This modification was introduced to try to improve both electron confinement and the ionization efficiency of the entering gas, with the goal of producing high intensity beams of H+ and He++.
Figure3.2shows the complete magnetic geometry that was modeled inradia. The main, outer solenoidal array has twelve magnetic bars aligned axially and spaced evenly in a cylinder of inner diameter 120 mm. Each bar consists of two 25 mm×25 mm ×50 mm neodymium iron boron (NdFeB) magnets (remanent magnetic field of 1.42 to 1.47 T) and one 25 mm × 25 mm × 50 mm piece of low-carbon steel sandwiched between them. The NdFeB magnets are magnetized through the 25 mm×25 mm face. Low-carbon steel rings (ID: 120 mm, OD: 190 mm, thickness: 5 mm) are placed at each end of the cylindrical array of magnetic bars to shape the rise and fall of the axial field. The permanent magnets fit snugly into slots in a cylindrical sleeve, which was fabricated using a 3-dimensional acrylonitrile butadiene styrene (ABS) plastic printer. Additional plastic parts fully enclose the magnet array to isolate it from the interior plasma chamber, which sits at high voltage.
The plasma chamber is made of two copper parts for ease of construction and maintenance. The first surrounds a 60 mm diameter×60 mm long cylindrical volume, which contains the plasma. Its interior is completely lined with 2 mm of insulating boron nitride to reduce electron loss and increase the ionization efficiency within the plasma. The second copper part contains a cooling channel for chilled, deionized water and is machined to accept an aperture electrode for ion extraction. The aperture is made of molybdenum, press fitted into a copper disc. This disc is removable so that the aperture diameter can be changed. A tapered waveguide provides the 2.45 GHz microwaves into the chamber through a 63.5 mm diameter, 3 mm thick aluminum oxide end window, which is vacuum sealed to the copper with
Figure 3.2: The magnetic geometry as modeled in radia. The outer array provides a solenoidal field, where the NdFeB is indicated by green and steel is indicated by red. The gray discs represent low carbon steel to shape the rise and fall of the solenoidal field. The inner array provides a multipole (cusp) field without affecting the field on axis. Here, NdFeB is represented by khaki and the steel ring in gray. Each piece of the geometry is segmented (black lines) for calculation inradia. The axes are in units of mm.
a VitonR o-ring. The microwave system consists of a 500 W magnetron, three-stub tuner,
circulator, and dummy load where reflected power is absorbed and monitored. A waveguide break electrically isolates the microwave system from the plasma chamber. During first tests we observed nonuniform heating of the Al2O3window, causing it to crack and vent the plasma
chamber to atmosphere. The interior side of this window was subsequently covered with a 2 mm thick disc of boron nitride to reduce thermal stress on the window.
The plasma aperture described above is the first of a three-electrode, ‘accel-decel’ beam extraction system [Coo72,Wil00]. The second and third electrodes are each made of molyb- denum and press fitted into a stainless steel disk, which mounts on the end of a stainless steel tube. Each tube is mounted to a corresponding stainless ring on a precisely aligned, brazed stainless steel-to-aluminum oxide extraction column ensuring collinearity of the electrodes. The electrode spacing and aperture diameter are variable for diameter sizes between 3.5 and 5 mm. Typical voltages applied are +10 to 15 kV to the plasma chamber (plasma aper- ture) and -0.3 to -2.5 kV to the second (accel) electrode. The third (decel) electrode is held at ground potential. Immediately following the extraction column is a water-cooled copper plate containing a variable (13 to 25 mm) diameter collimator (labeled as “collimator” in Figure3.3). Its function is to reduce any beam halo, and to transport only the least divergent beam particles. The ion source and its essential components are shown in Figure3.3.
The entire ECR ion source is installed on a high-voltage platform biased using a Glassman +200 kV, 8 kW power supply. Combining this with an ECR plasma chamber voltage of +10 to 15 kV, the source can deliver H+ beams on target with energies between 50 keV and 215 keV. The beam is transported to ground through a 24-gap air-insulated acceleration column designed to provide optimal transport for beams of 200 keV (manufactured by High Voltage Engineering Corporation (HVEC)). The 200 kV platform potential is stepped down with inter-electrode resistances of 10 MΩ. When energies lower than the maximum are required, downstream gaps of the column are shorted to provide optimal beam focusing. For example, for a 100 keV H+ beam, the voltage is stepped down over the first 12 gaps, which provides
Figure 3.3: The LENA ECR ion source. Microwave power of 2.45 GHz at 100 to 350 W is input into a plasma chamber with low gas pressure to match the 0.0875 T necessary for electron cyclotron resonance. The axial field is plotted with reference to the location of the plasma chamber. The field peaks twice at∼0.09 T and defines an ECR zone, which lies within the plasma chamber. Positive-ion beam is extracted using an ‘accel - decel’ lens system, with typical extraction potentials of 10 to 15 kV. Beam is collimated before acceleration.
Beam Characterization
The quality of beam produced by the ECR accelerator was evaluated by measuring two low-energy resonances. One of these resonances was the well-known Erlab = 150.82 ± 0.09 keV (Γ ≤0.3 keV) [Bec95] resonance in the 18O(p, γ)19F reaction (Qpγ = 7994.8 ± 0.6 keV
[Aud03]). The target was prepared by anodization of a 0.5 mm thick tantalum backing in 18O-enriched water (enrichment to 99%). Before the tantalum was anodized with 18O
water, it was carefully cleaned by etching in a chemical bath and subsequently resistively heated in vacuum (see Chapter4). Such 18O targets have been found to have a well-defined
stoichiometry, Ta2O5 [Phi76]. The second resonance used was at Erlab = 202.8 ± 0.9 keV
[End90] in the27Al(p, γ)28Si reaction (Q
pγ = 11585.11 ±0.12 keV [Aud03]). The target was
prepared by evaporation of Al2O3 onto a 0.5 mm thick tantalum sheet. Promptγ-rays from
the reactions were detected using a large-volume (582 cm3) HPGe detector [Lon06,Car10], placed at an angle of 0◦ with respect to the beam axis and a distance of 1.6 cm from the target (see Section3.2.1).
As a check of the voltages provided by the acceleration supplies and subsequently the energy of the beam, excitation functions (i.e.,γ-ray yield versus bombarding energy) for both resonances were measured and fit to determine resonance energy and the corresponding energy resolution. The fitting routine assumes a Gaussian profile for the beam energy and a uniform target. The beam energy and energy resolution were determined from the maximum and FWHM of the Gaussian distribution, respectively. The excitation functions from each reaction are shown in Figure 3.4. The resonance energy measured for the 18O(p, γ)19F resonance
was Elab
r = 150.6 ± 0.1 keV, with a corresponding energy resolution of 0.84 ± 0.07 keV.
The resonance energy in 27Al(p, γ)28Si was Erlab = 200.9 ± 0.1 keV with a corresponding energy resolution (beam plus target) of 1.44 ± 0.17 keV. Thus, small corrections have been made to normalize the power supply energies to the literature values. The measured energy widths are a factor of 2.8 to 3.6 greater than what is expected from the ripple and stability specifications of the power supplies (better than 0.1% and 0.01%, respectively, [Gla07] for the 200 kV supply). However, we cannot rule out some contribution from contaminants or
150 153 156 159 162 Energy (keV) 0 1 2 3 4 5 6 7
Yield (arbitrary units)
198 200 202 204 206 Energy (keV) 0 0.5 1 1.5 2 2.5 18 O(p,γ)19F 27Al(p,γ)28Si
Figure 3.4: Excitation functions: (left) of the primaryR→ 3908 keV transition for the Erlab = 151 keV resonance in 18O(p, γ)19F and (right) of the secondary 1779 → 0 keV transition for theElab
r = 203 keV resonance in27Al(p, γ)28Si. The solid line represents a fit to the data
when beam energy resolution is considered.
non-uniformities in the targets.
3.1.3 Target Station
The beam enters the target chamber through a 63 cm long, 3.2 cm diameter liquid-nitrogen cooled copper tube, which extends to within 2 cm of the target (as shown in Figure3.5). The primary purpose of the copper tube is to trap impurities that could be deposited onto the target. The beam diameter may be further reduced as needed with copper collimator inserts (6 mm to 25 mm diameter) within the copper tube. Beam current is minimized on this tube by focusing with the second quadrupole magnet. At the exit of the tube is a ceramic break
Figure 3.5: Schematic layout of the target station at LENA. It consists of collimator sets to define the size and shape of the beam. Charge integration is maintained by suppressing secondary electrons with a knife-edge Cu-ring biased at -300 V immediately before the target. The Cu-tube is cooled to liquid nitrogen temperature to minimize contaminants migrating onto the surface of the target. The target backing is directly water cooled by chilled deionized water. Note: Figure is not to scale.
that holds a knife-edge copper ring. The copper ring is biased to -300 V to suppress the emission of secondary electrons from the target and copper collimator. The rear side of the target backing is directly cooled with chilled, deionized water.