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RA3. Experimental Equipment/Technique - A Novel Approach for Measurement of (p,α) Reactions
(J.C. Blackmon, Spokesperson)

We have developed a new technique for measurements of low energy (p,α) reactions. The approach uses a differentially pumped windowless gas target and is optimized for studies of narrow resonances using radioactive ion beams. We have demonstrated this new approach by applying it to measure a recently reported resonance in the 17O(p,α)14N reaction at Ecm=183 keV [CHA05] that is important for understanding nucleosynthesis in giant stars and novae. In this article we briefly summarize the technique, the results from the 17O(p,α)14N measurement, and plans for future measurements using the technique with radioactive ion beams.

In our demonstration experiment, beams of 17O from the HRIBF tandem accelerator bombarded a large scattering chamber filled with hydrogen gas at a pressure of 4 Torr. The scattering chamber was connected to the accelerator beamline by a series of 4 differentially-pumped chambers separated by 5-mm diameter apertures. A schematic illustration of the experimental setup is shown in Fig. RA3-1. No windows or foils obstructed the beam or contained the gas, and the hydrogen gas in the scattering chamber served as a spatially extended target for the (p,α) reaction. Ultra-high purity gas was used in a single-pass system, and the differential pumping stages reduced the pressure to about 10-7 Torr over a distance of less than half a meter.

Figure RA3-1: Schematic of the experimental setup. Only 2 of the 4 differential pumping chambers are shown.

The α and 14N reaction products were detected in coincidence by an array of silicon strip detectors operating within the hydrogen gas environment. Α particles were detected by the SIDAR array, while recoiling 14N (emitted at angles less than 21 degrees) were detected by a CD-style detector (Micron semiconductor Type S1 detector) mounted just downstream of SIDAR. The kinematics and relative timing of the two detected particles allowed the 17O(p,α)14N reaction to be cleanly distinguished. The energies of particles detected by the S1 detector are plotted against the energy of the coincident particle detected by SIDAR in Fig. RA3-2 for events that are coincident within 0.4 microseconds. Events from the 17O(p,α)14N reaction are distinguished as a straight line with a constant sum energy indicative of the reaction Q value and independent of the reaction angle.


Figure RA3-2: The energy of particles detected in the CD (S1) detector is plotted versus the energy of coincident particles in SIDAR for (a) E(17O)=3.29 MeV and (b) E(17O)=3.34 MeV. Any bins with at least 1 count have a uniform black fill. The incident beam on target is comparable for both (a) and (b).

Each segment of the detector array views reaction products from a wide range of angles depending on the point of origin along the beam axis. However, the reaction angle of each 17O(p,α)14N event can be determined from the measured α energy, which varies rapidly with angle. We found all of the 17O(p,α)14N events to originate from a narrow range of positions within the target chamber, indicating that the entire yield is due to a narrow resonance. The centroid of the reaction vertex was found to vary linearly with the incident energy, allowing the stopping power for oxygen ions in hydrogen gas near the peak of the Bragg curve (E(17O)=193 keV/u) to be determined for the first time. Our experimental result, (631)x10-15eV/cm2 is in good agreement with popular semi-empirical models [ZIE03,PAU03].

The integrated beam current at each energy was determined by normalizing to 12C(17O,17O)12C elastic scattering measured simultaneously with the 17O(p,α)14N reaction using a carbon foil and two single-collimated silicon surface barrier detectors. The dominant systematic uncertainty results from the thickness of this carbon foil used for beam current normalization, which was determined in separate measurements to a precision of 6%.

The thick target yield curve is shown in Fig. RA3-3 along with a best fit to the data, which results in a resonance strength of 1.700.15 meV including both statistical and systematic uncertainties added in quadrature. We also measured a yield curve at a target pressure of 1 Torr with consistent (but less precise) results. Our result is in good agreement with the recent first measurement of this resonance strength (1.60.2 meV) using a high-intensity, low-energy proton beam [CHA05]. We also determined the resonance energy to a precision of about 0.3%, with a value also in good agreement with Ref. [CHA05].

Figure RA3-3: Measured yield as a function of the incident 17O energy. Triangles represent upper limits. The curve is a best-fit to the data.

Our new approach to (p,α) reactions was designed for high sensitivity for narrow resonances using radioactive ion beams. The pure nature of the target increases the reaction yield by about a factor of 3 over that from polypropylene targets, and the gas pressure can be adjusted to match the expected resonance width, decreasing the yield from non-resonant and background sources. We believe this technique is also well suited to measurements of (α,p) reactions using helium gas. We next plan to apply this technique to measure the strength of low-energy resonances in the 18F(p,α)15O reaction (Moazen et al., RIB-165) that are important for understanding 511-keV gamma-ray production in novae.

[CHA05] A. Chafa et al., Phys. Rev. Lett. 95 (2005) 931101; 96 (2006) 019902.
[ZIE03] J. F. Ziegler, SRIM-2003.10 (2003); see http://www.srim.org.
[PAU03] H. Paul and A. Schinner, At. Data Nucl. Data Tables 85 (2003) 377.

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