Adekola, A

Brune, C R

Grimes, S M

Hadizadeh, M H

Heinen, Z

Hornish, M J

Massey, T N

Matei, C

O'Donnell, J E

Voinov, A V

CERN Document Server

PoS(NIC-IX)119Level Structure of 19Ne from studies of the17O(3He,n)19Ne ReactionM.J. Hornish, C.R. Brune, S.M. Grimes, M.H. Hadizadeh, T.N. Massey, A.V. Voinov,J.E. O’Donnell, A. Adekola, C. Matei, and Z. HeinenDepartment of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USAThe astrophysically-relevant region of the level structure of 19Ne around the proton thresholdremains incomplete when considering that analogs for several states in the mirror nucleus 19Fhave not yet been identified in 19Ne. This structure is particularly important to understandingthe eventual fate of the long-lived radioisotope 18F in explosive environments like novae and x-ray bursts, where rates of proton-induced reactions on 19F depend critically on the properties of19Ne excited states above threshold. To study this system, a measurement of the 17O(3He,n)19Nereaction has been performed at Ohio University’s Edwards Accelerator Laboratory, which featuresa 4.5-MV tandem Van de Graaff accelerator. Utilizing pulsed beams and neutron time-of-flighttechniques, this experiment has been conducted at forward angles with a 3He beam energy of4.2 MeV. This energy has been chosen such that the compound reaction model is the dominantmechanism, which should ensure that this reaction will in principle populate all excited statesincluding any newly observed ones. In addition to precisely determining the excited energiesfor individual levels, this measurement will provide experimental differential cross sections forindividual states that can be compared to calculations employing the Hauser-Feshbach statisticalmodel in an attempt to extract information on the spins of the excited states. The observed 19Nelevel structure from this reaction will be presented.International Symposium on Nuclear Astrophysics — Nuclei in the Cosmos — IXJune 25-30 2006CERN, Geneva, Switzerlandc© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)119Level Structure of 19Ne1. IntroductionDetermining the rates of proton-induced reactions on the long-lived radioisotope 18F will im-prove our understanding of the role that it plays in annihilation gamma-ray emission from novaeand in possible heavier-element nucleosynthesis in x-ray bursts. For temperatures in the range of(1-4)×108 K, the rates of the 18F(p,α)15O and 18F(p,γ)19Ne reactions depend critically on indi-vidual 19Ne resonances above the proton threshold(Qpγ = 6.411 MeV) [1].Unfortunately, there remains considerable uncertainty in the level structure of 19Ne, where asmany as seven levels in the excitation energy range of interest (6.4-7.4 MeV) appear to be missingwith respect to analog states of the corresponding mirror nucleus 19F (see Figure 1). This dilemmaleads to large uncertainties in the reaction rate calculations [2]. As a result, a considerable efforthas been undertaken over the past decade to study this system. Indeed, a variety of laboratoryexperiments utilizing an array of different stable- and radioactive-beam studies (for a partial list,see references cited in Ref. [2]) have been conducted to search for excited states of 19Ne and toidentify their properties.The Edwards Accelerator Laboratory is ideally suited for performing neutron time-of-flightmeasurements with its 30-meter tunnel in conjunction with the swinger magnet assembly andpulsed beams supplied by the 4.5-MV Tandem. With these capabilities, a measurement of the(3He,n) reaction on 17O was determined to be a useful tool for conducting studies of the levelstructure of 19Ne. This reaction has a Q value of 4.299 MeV with respect to the 19Ne ground state,which enables the use of low-energy 3He beams to readily access the excitation energy range ofinterest, while producing optimal neutron energies with respect to energy resolution and detectionefficiency. Moreover, the study of this reaction at low beam energies will ensure that it is dominatedby the compound nuclear mechanism. As such, the particularly nice feature of this study, unlikemany others which preferentially access only certain spin states, will in principle be the populationof all 19Ne excited states.2. Experimental ApproachTargets of tantalum oxide, Ta2O5, on tantalum plates were fabricated via an monodic oxida-tion technique [5] using a sample of water enriched to 76.7% in 17O. A group of three large (5-indiameter, 2-in thick) NE213 and BC501A liquid scintillator detectors were used for the measure-ments and were located at a distance of 8.7 meters from the target, at an angle of θ = 20◦. The useof these detectors enables the implementation of pulse-shape discrimination (PSD) techniques toseparate neutrons from gamma-rays, which constitute a significant albeit almost entirely randombackground in the experiment. The absolute efficiencies of the neutron detectors have been deter-mined via a measurement of the 27Al(d,xn) reaction (Ed = 7.44 MeV, θlab = 120◦) with a stoppingtarget, the neutron yield of which has been normalized to a fission chamber [6].The 17O(3He,n)19Ne measurement was accomplished with a 4.2-MeV beam of 3He++ ionsincident on a 64.25 µg/cm2 (25-keV) thick target. The beam was chopped and bunched, therebyproducing beam pulses of 3-4 ns width (FWHM) and spaced 800 ns apart. The beam currenton target was typically 10-25 pnA and data were accumulated over the course of approximately10 days including time spent studying some additional targets to better understand background2PoS(NIC-IX)119Level Structure of 19Ne19F 19Ne6.26.46.66.87.07.219F and 19Ne level energies (MeV)-0.200.20.40.60.818F+p resonance energies (MeV)6.288     (7/2+)(6.419)     (3/2+)6.437     (1/2-)(6.449)     (3/2+)6.698     (5/2+)6.741     (3/2-)6.861     (7/2-)7.076     3/2+7.210    7.253    6.255     1/2+6.282     5/2+6.330     7/2+6.536     1/2-6.4967   3/2+6.500     11/2+6.5275   3/2+6.554     7/2(+)6.592     9/2+6.787     3/2-6.8384   5/2+6.891     3/2-6.9265   7/2-6.989     1/2-7.114     7/2+7.1662   11/2-7.262     3/2+Proton Threshold:   Ex = 6.411 MeVFigure 1: Level scheme for 19F and 19Ne, taking into account some recent results (e.g., Refs. [3, 4]). Dashedlines represent putative mirror states.contributions. For example, time was spent studying a blank tantalum plate, as well as 12C and13C targets used to study backgrounds associated with buildup over the course of the experimentof carbon on the tantalum oxide target. This effect was minimized as much as possible by frequentrepositioning of the tantalum oxide targets with respect to the beam spot.It should be pointed out that special care was paid to understanding the time-of-flight spectrum.For example, periodic measurements of the time calibration (time per channel) of the TAC/ADCsystem were made and compared to a random spectrum taken after completion of the experiment.This information is essential to acquiring an accurate calculation of the energy of a given neutron3PoS(NIC-IX)119Level Structure of 19Ne0 200 400 600 800Time of Flight (ns)01000200030004000Counts / (1.4 ns)0 2 4 6 719Ne Excitation Energy (MeV)450 500 5502004006008006 6.2 6.4 6.6 6.8Figure 2: Neutron time-of-flight spectrum for the 17O(3He,n)19Ne reaction measurement at E(3He) = 4.2MeV at θ = 20◦. The inset shows a preliminary MINUIT fit to a portion of the spectrum in the region ofinterest, where known levels are indicated by the vertical dashed lines.group. In addition, the timing characteristics of the beam pulsing and bunching were frequentlymonitored and retuned to maintain consistency and correct for drift. Finally, a peak associatedwith prompt gamma-ray emission from the target is used as a reference point to establish the trueneutron time of flight.3. Results and ConclusionsA preliminary analysis of the data has been performed, and the combined data set from allthree detectors is plotted in Figure 2. A few general observations can be made from a brief analysisof the spectrum. First of all, the neutron spectrum sits atop a flat, uniform background resultingfrom random gamma rays and cosmic-induced or otherwise scattered neutrons. Secondly, asidefrom one major background peak – that associated with the ground state of 14O due to reactionson 12C – the rest of the visible peaks in the spectrum are associated with states in 19Ne. Generallyspeaking, the widths of the peaks are consistent with the expected experimental widths, taking intoaccount time, distance and angle uncertainties, beam energy uncertainties and target energy loss.One interesting note is that there is no overwhelming evidence of any newly-observed states.However, there are some hints of potentially interesting features that will be explored in more4PoS(NIC-IX)119Level Structure of 19Nedetail, once the data have been fitted with MINUIT. The final results from this analysis are expectedto be completed in the near future.AcknowledgmentsIt is a pleasure to thank Don Carter and Devon Jacobs for their technical support. In addition,Chris Fox from UNC – Chapel Hill and TUNL supplied helpful information regarding the produc-tion of tantalum oxide targets. This work was supported in part by the U.S. Department of Energyunder grant No. DE-FG02-88ER40387.References[1] A. Coc, M. Hernanz, J. José, and J.-P. Thibaud, Astron. Astrophys. 357, 561 (2000).[2] N. Shu, D. W. Bardayan, J.C. Blackmon, Y. S. Chen, R. L. Kozub, P. D. Parker, and M. S. Smith,Chin. Phys. Lett. 20, 1470 (2003).[3] D.W. Bardayan, R.L. Kozub, and M.S. Smith, Phys. Rev. C 71, 018801 (2005).[4] R. L. Kozub, D. W. Bardayan, J. C. Batchelder, J. C. Blackmon, C. R. Brune, A. E. Champagne, J. A.Cizewski, T. Davinson, U. Greife, C. J. Gross, C. C. Jewett, R. J. Livesay, Z. Ma, B. H. Moazen, C. D.Nesaraja, L. Sahin, J. P. Scott, D. Shapira, M. S. Smith, J. S. Thomas, and P. J. Woods, Phys. Rev. C71, 032801(R) (2005).[5] D. Phillips and J.P.S. Pringle, Nucl. Inst. Meth. 135, 389 (1976).[6] T.N. Massey, S. Al-Quraishi, C.E. Brient, J.F. Guillemette, S.M. Grimes, D. Jacobs, J.E. O’Donnell,J. Oldendick, and R. Wheeler, Nucl. Sci. Eng. 129, 175 (1998).5

Level structure of $^{19}Ne$ from studies of the $^{17}O(^{3}He,n)^{19}Ne$ Reaction

Level structure of $^{19}Ne$ from studies of the $^{17}O(^{3}He,n)^{19}Ne$ Reaction

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Level structure of 19Ne^{19}Ne19Ne from studies of the 17O(3He,n)19Ne^{17}O(^{3}He,n)^{19}Ne17O(3He,n)19Ne Reaction

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Level structure of $^{19}Ne$ from studies of the $^{17}O(^{3}He,n)^{19}Ne$ Reaction