ABSTRACT
High Voltage Engineering developed, built, and tested a unique 2 MV single-ended accelerator (SingletronTM) for light ions. The system combines a beam current of up to 2 mA for protons and helium in direct-current mode with nanosecond-pulsing capability. Compared to other chopper-buncher applications with Tandem accelerators, the single-ended accelerator increases the charge per bunch by about a factor of 8. The all-solid-state Singletron 2 MV power supplyTM supports high-current operation and features a large dynamic range of the terminal voltage and good transient performance to support the high-current operation. The terminal accommodates an in-house developed 2.45 GHz electron cyclotron resonance ion source and a chopping-bunching system. The latter features phase-locked loop stabilization and temperature compensation of the excitation voltage and its phase. The chopping bunching system further features the selection of hydrogen, deuterium, and helium as well as a pulse repetition rate, ranging from 125 kHz to 4 MHz, that are fully computer controlled. In the testing phase, the system demonstrated smooth operation for 2 mA proton and helium beams at terminal voltages from 0.5 to 2.0 MV, and somewhat reduced current at a voltage down to 250 kV. In pulsing mode, pulses with a full width at half maximum of 2.0 ns reached a peak current of â¼10 and â¼5.0 mA for protons and helium, respectively. This is equivalent to a pulse charge of about 20 and 10 pC. Applications range from various fields requiring direct current at multi-mA levels and MV light ions, including nuclear astrophysics research, boron neutron capture therapy, and deep implantation for semiconductor applications.
Subject(s)
Helium , Protons , IonsABSTRACT
The primary challenge in directly measuring nuclear reaction rates near stellar energies is their small cross sections. The signal-to-background ratio in these complex experiments can be significantly improved by employing high-current (mA-range) beams and novel detection techniques. Therefore, the electron cyclotron resonance ion source at the Laboratory for Experimental Nuclear Astrophysics underwent a complete upgrade of its acceleration column and microwave system to obtain high-intensity, pulsed proton beams. The new column uses a compression design with O-ring seals for vacuum integrity. Its voltage gradient between electrode sections is produced by the parallel resistance of channels of chilled, deionized water. It also incorporates alternating, transverse magnetic fields for electron suppression and an axially adjustable beam extraction system. Following this upgrade, the operational bremsstrahlung radiation levels and high-voltage stability of the source were vastly improved, over 3.5 mA of target beam current was achieved, and an order-of-magnitude increase in normalized brightness was measured. Beam optics calculations, structural design, and further performance results for this source are presented.
ABSTRACT
The 14N(p,gamma)15O reaction regulates the rate of energy generation in the stellar CN cycle. Because discrepancies have been found in the analysis and interpretation of previous capture data, we have measured the 14N(p,gamma)15O excitation function for energies in the range E(lab)(p)=155-524 keV. Fits of these data using R-matrix theory yield a value for the S factor at zero energy of 1.68+/-0.09(stat)+/-0.16(syst) keV b, which is significantly smaller than the previous result. The corresponding reduction in the stellar reaction rate for 14N(p,gamma)15O has a number of interesting consequences, including an impact on estimates for the age of the Galaxy derived from globular clusters.
ABSTRACT
We report on the observation of a new resonance at E(lab)(R)=190 keV in the 17O(p,gamma)18F reaction. The measured resonance strength amounts to omegagamma(pgamma)=(1.2+/-0.2)x10(-6) eV. With this new value, the uncertainties in the 17O(p,gamma)18F and 17O(p,alpha)14N thermonuclear reaction rates are reduced by orders of magnitude at nova temperatures. Our significantly improved reaction rates have major implications for the galactic synthesis of 17O, the stellar production of the radioisotope 18F, and the predicted oxygen isotopic ratios in nova ejecta.
ABSTRACT
Production of the radioisotope 18F in novae is severely constrained by the rate of the 18F(p,alpha)15O reaction. A resonance at E(c.m.)=330 keV may strongly enhance the 18F(p,alpha)15O reaction rate, but its strength has been very uncertain. We have determined the strength of this important resonance by measuring the 18F(p,alpha)15O cross section on and off resonance using a radioactive 18F beam at the ORNL Holifield Radioactive Ion Beam Facility. We find that its resonance strength is 1.48+/-0.46 eV, and that it dominates the 18F(p,alpha)15O reaction rate over a significant range of temperatures characteristic of ONeMg novae.
ABSTRACT
The energy derived from the CN cycle at low stellar temperatures is regulated by the 14N(p,gamma)15O reaction. A previous direct measurement of this reaction has been interpreted as showing evidence for a subthreshold resonance which makes a major contribution to the reaction rate at low temperatures. This resonance, at E(c.m.) = -504 keV would correspond to the known Ex = 6793-keV state in 15O. We have measured a mean lifetime of 1.60(+0.75)(-0.72) fs (90% C.L.) for this state using the Doppler-shift attenuation method. This lifetime is a factor of 15 longer than that inferred from the (p,gamma) data and implies that the contribution of the subthreshold resonance is negligible.