A direct measurement of the 18F(p,α) 15O reaction

29June1995

PHYSICS

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LETTERS

B

Physics Letters B 353 (1995) 184-188

A direct measurement of the 18F(p,a) 150 reaction Labar a,P. Leleux a, M. Loiselet a, C. Michotte a, R. Neal b, G. Ryckewaert a, A.S. Shotter b, J. Vanhorenbeeck ‘, J. Vervier a, M. Wiescher e, Ph. Woods b a Institut de Physique NucMaire, Centre de Recherches du Cyclotron et Tomographie ci Positrons, Universite Catholique de Louvain. B-1348 Louvain-la-Neuve, Belgium b Department of Physics and Astronomy. University of Edinburgh, Edinburgh EH9 3JZ United Kingdom c Universite’ Libre de Bruxelles, B-1050 Bruxelles, Belgium ’ Instituut voor Kern- en Stralingsfysika, Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium e Department of Physics, University of Notre-Dame, IN 46556, USA Received 21 March

1995; revised manuscript received 25 April 1995 Editor: R.H. Siemssen

Abstract

The cross section for the 18F(p,a) I50 reaction has been obtained in reverse kinematics ( “F beam on CH2 target) between 550 and 740 keV above threshold, i.e. in a region of astrophysical interest. The reaction yield is dominated by a wide resonant state whose spin, parity, total width and partial widths were deduced from the analysis of the a-particle and elastic proton data.

is im p ortant both in The reaction ‘8F(p,a)‘“0 the hot CNO cycles and in the rp-process. In novae burning, at “low” temperature (T 5 3 x lo8 K), 160 is converted into 150 by the reaction sequence: ‘60(p,y) ‘7F(~+~)‘70(~,~) ‘8F(p,4’50, while at “high” temperature, ‘*C is converted into I50 by the reaction chain ‘*C(p,y) 13N(p,y) 140(cz,p) 17F(p,y) ‘8Ne(P+v) 18F(p,a) 150. Network calculations [ 1 ] performed for various temperature and density conditions show that the reaction ‘8F(p,a)‘50 determines the flow in both chains at all temperatures T 5 lo9 K. This also indicates that the strength of this reaction will strongly influence the production of I50 which is considered a key isotope for the escape from the hot CNO cycles to the rp-process through the 150( (~,y) “Ne(p,y)*‘Na sequence. The currently used reaction rate for 18F(p,a) is rather uncertain. It 0370-2693/95/$X)9.50 @ 1995 Elsevier Science B.V. All rights reserved SSDr0370-2693(95)00550-l

was first determined [ 21 from the analysis of the level structure of the 19Ne compound nucleus above the 18F+p threshold at 6.411 MeV. Recent measurements of the 19F( ‘He&) 19Ne and 160( 6Li,t) 19Ne reactions performed at Notre-Dame and Princeton [ 31 have revealed the existence of several additional unbound levels which could contribute to the (~,a) channel among which a level at 657 keV, of 40 keV total width and unknown spin. However the present knowledge of the 19Ne levels (and of the spins) is rather poor at high excitation energy, as compared with its stable isobar 19F: between 6 and 7 MeV excitation energy, 11 levels are known in the former and 19 in the latter, whereas below 6 MeV energy, a near one-to-one correspondence exists (22 vs. 23 levels) [ 3,4]. A direct measurement of the ‘8F(p,a) I50 reaction has been performed in reverse kinematics using the

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radioactive beam facility at Louvain-la-Neuve [ 51. The full details of the 18F beam production will be the subject of another paper [6]. In summary a 15MeV, 12PA proton beam bombarded a static water target enriched in 180 (>_ 95%). The 1 Ci ‘*F activity obtained after an irradiation of 2h was then transformed into [ “F] -CHsF molecules by different chemical processes, including the nucleophilic substitution on iodomethane (CHsI) with an activated 18F-fluoride anion. The overall efficiency of these operations is SO%, taking into account the “F decays (synthesis time N 45 min). The labeled CH3F (b.p. -78”C), carried by 4He, is condensed in a small volume stainless steel coil and transported (presently by hand in a shietd) to the inlet of the ECR source of the radioactive beam facility [ 51. 18F2+ ions are thereby obtained and accelerated during about 3 hours, after which a new CHsF supply is brought in. Two loadings were used in the present run. Fine tuning of the cyclotron was needed to separate ‘*F from ‘*O, the relative mass difference being only 9.7 x 10e5. The 14 MeV “F beam, with a typical intensity of lo6 s-‘, bombarded a 200 pg/cm2 thick polyethylene target, in which the beam energy loss [7] was 3.6 MeV, in reverse kinematics, the excitation energy range from 740 keV down to 550 keV above threshold was thereby scanned in one single step. Particles emitted between 12’ and 28’ in the laboratory system were detected by the Louvain-Edinburgh Detector Array (LEDA), a large annular multistrip silicon detector, 300-,um thick and 26-cm external diameter, with a 10 cm diameter hole in the center. LEDA consists of 8 segments, each covering 41” azimuthal angle; each segment has 16 strips of 5-mm width. Data were thus obtained at 16 angles with a lo width. Scattered 18F and 12C recoil heavy ions were prevented from reaching the detector by a 13-pm thick Al foil in front of it. The timing of the events with respect to the RF pulses of the cyclotron was measured; energy-versustime two-dimensional spectra were reconstructed, in which two zones corresponding to a-particles and protons were clearly located (Fig. 1) . A 3-line Ly-source was used to calibrate the a-spectra. Proton spectra were calibrated using two known energy points, i.e. the entrance (exit) energy of the beam into (from) the target. Figs. 2a) and 2b) show typical a-particle and proton spectra taken at 20”. Both spectra, as well as the spectra at other angles, are dominated by a

40

20

0

I

I 1000

I 1500

2000 Energy (channel)

Fig. 1. Two-dimensional spectrum (time versus energy) from the ‘*F + p interaction showing half the data at 20° lab. The proton and cY-particlesregions are indicated. The vertical stripe is due to positrons from ‘sF decays.

resonant state which interferes with Coulomb scattering in the proton data [ 8,9]. From the a-spectra using Northcliffe-&hilling Tables [ 71, the energy of the state is determined to be 638 keV above threshold with a conservative uncertainty of 5 15 keV taking into account uncertainties in the tables, its total width amounts to 37 & 5 keV (energy loss effects were taken into account) and the angular distribution of the LYparticles from the decay of this state is isotropic over the angular range 23” to 54” in the c.m. system, as will be shown below. A Breit-Wigner one-level formula [lo] was used to calculate the proton elastic scattering cross section, taken as the squared sum of a Coulomb and a resonant amplitude, resulting in an unambiguous assignment for the orbital angular momentum of the resonance as e = 0, the cases e = 1 and e = 2 being clearly excluded (Fig. 3a). The J” values of the corresponding 19Ne state are thus restricted to l/2+ and 3/2+. The latter value is strongly favored (Fig. 3b). Although Fig. 3b presents a calculation containing f, = 0.5 Itot as an input, the data restrict the allowed Fp /Itot ratio to the 0.4-0.6 domain for a 3/2+ spin and 0.7-1.0 for a l/2+ spin; for example, a ratio of 0.25 is definitely excluded for both spins (Fig. 3~). A resonance energy of 630 keV and a total width of 40 keV, in agreement with

R. Cosmch et al. /Physics

(4

0 4000

Leners B 353 (1995) 184-188

n

6000

8000

12000 10000 ElabNW

1000

1500

Fig. 2. a) Typical a-specuum from the **F(p ,a)150 reaction at a lab angle of 20.3&; a non-resonant contribution wouid be flat over the 7-9 MeV energy range b) Spical proton spectrumfrom the ‘*F(p,p) “F elastic scattering at a lab angle of 20.3’. the line is an eye guide.

the a-particle data, provided a fair agreement with the proton data. In addition, the calculation sets the absolute scale of the spectra in mb/sr using the energy EC where the cross section is purely Coulomb [ 81; former works in this laboratory [ 8,9], dealing with C = 0 resonant states of comparable widths, allow us to determine EC with a fair level of confidence. It appears that, at all angles, the mean elastic cross section over the target thickness is about ( 1. IL 0.1) times the Rutherford cross section. In Fig. 3, typical data obtained at f3,, = 36” have been presented and used to get informations on level properties; excitation functions at other angles show the same trend, in particular the pronounced minimum at 600 keV energy. Normalizing the a-particle spectra to the elastic proton cross section, the cross section for the “F( p,~) I50 reaction over the 550-740 keV energy range was thus obtained (Fig. 4). The isotropy of the angular distribution is expected from the C = 0 value of the orbital angular momentum transfer of the resonant state that dominates by far the spectra. Extrapolating over the entire angular range, a yield Y = (3.0 f 0.3) x lop6 is obtained, where Y is the number of resonant reactions per incoming ‘*F particle. The streng th oy of the resonance is related to the yield Y by:

where: ee is the ‘*F stopping power [7], Mt ( MP) is the target (projectile) mass, A is the c.m. wave length. A correction due to the finite target thickness was introduced, and a resonant strength oy = 5.6 -f 0.6 keV was then deduced, with: wy = wTp . ra/rtot where: w is the statistical factor (2hj$$+,), JR, Ji and 52 being the spins of the resonant state, the projectile and the target, respectively; rp and r, are the partial proton and alpha widths, respectively; rtot is the total width ( rtot = rr, +r,, since Tr is negligible). It should be stressed that a J” = l/2+ assignment for the 19Ne level would imply w = 2/6, and hence a rp . r, product incompatible with the experimental total width ( rp + r, ). Moreover, the rp/rtot ratio of the 657 keV level deduced from 19F(3He,tp) and 19F(3He,ta) measurements [ 31, i.e. 0.40 f 0.05, is in agreement with the domain (0.4 to 0.6) allowed by the present data for a 3/2+ spin, and in contradiction with the domain (0.7 to 1.0) for a l/2+. Table 1 summarizes the range of the rP iTtot ratio allowed by the proton data, for different spin assignments. Finally, it should be mentioned that the ‘*O(P,(Y)15N reaction was also measured with the same experimental techniques. An ‘*O beam of the same energy as the r8F beam can easily be obtained by changing slightly the cyclotron magnetic field. The ‘*O(p,p) elastic scattering and the ‘*O( p,cu)15N reaction were

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Letters B 353 (1995) 184-188

I

I

I

30

40

50 @..,,.(&!

of the ‘*F(p,a)

I50 cross section as

Fig. 4. Angular distribution measured in this work.

Table 1 Spin assignments (Jr), corresponding rp/rtot allowed by the present data

&values

and range

J”

rp 1rtoc

1/2112+ 3/2+ 7/2+

_

of

0.7-l .o 0.4-0.6

.

500

550

600

650 E,,.

700 WV

Rg. 3. a) experimental proton cross section in the cm. system (&Ill = 36’) compared with a Breit-Wigner calculation for an angular momentum transfer e = 1, Jr = l/2(solid curve) and P = 2, Jr = 7/2+ (dashed curve) b) same experimental cross section compared with calculations for e = 0, JP = l/2+ (dashed curve) and JP = 3/2+ (solid curve); in both cases Tr, = 0.5 rioi is assumed c) same cross section compared with the calculation for P = 0, rP = 0.25 riot, JP = 3/2+ (solid curve), JP = l/2+ (dashed curve). In all cases, deviations from the calculation below 575 keV are due to the target thickness, i.e. data with a thicker target would have reproduced the calculations down to lower energy (where the cross section is nearly pure Coulomb).

measured with LEDA over the same angular range on the same target. The alpha spectra are dominated by a broad resonance, which covers the whole excitation energy range spanned by the present work, i.e. 190 keV c.m: a broad state is indeed expected at 680-695 keV, with a controversial width [4]. No conclusion about the latter can however be deduced from our data. The proton spectra are structureless, due to the absence of narrow states in the covered energy region. The 180(p,p) elastic scattering has been measured at 140’ c.m. [ 111, yielding a cross section which represents 93% of the Rutherford cross section over the energy range covered by the present experiment. Normalizing our a-spectra to the elastic (mainly Rutherford) proton cross section of Ref. [ 11] ), a mean cross section of 22 mb/sr was obtained for our ‘80(p cu) cross section at 24” lab (or 135” c.m. in the reterse process), in excellent agreement with a

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previous measurement [ 121. Moreover, our a-particle angular distribution is perfectly flat, as expected from the Jr = l/2+(! = 0) assignment for this state [ 121. The 180(p,a) data were also used to estimate the IgO contamination in the ‘*F beam: cu-particles from the former reaction are expected in the 9-12 MeV energy region i.e. well above the a-particles region in the 18F(p,a) reaction. Integrating the counts between 9 and 12 MeV in the 18F(p,a) data, normalizing to the measured “F( p,cu) events and correcting for the ratio of the (p,a) cross sections, one obtains an upper limit of 4% for the 180 contamination in the 18F beam. The present work has shown that the ‘*F(p,cu) I50 cross section between 550 and 740 keV c.m. above threshold is dominated by a 3/2+ resonant state, at 638 f 15 keV energy above threshold (7.049 MeV excitation energy in 19Ne) and with a 37 f 5 keV total width. Previous estimates [2,3] had predicted that a 40 keV broad state at 657 keV should dominate the reaction rate for temperatures larger than 4 x lo8 K. The intense 18F beam developed in Louvain-la-Neuve together with the large solid-angle LEDA detector have thus allowed to get accurate data and conclusive results on that issue which is of important astrophysical interest. On the other hand, the question of the analog state in 19F is still open: a 3/2+ level at 7.262 MeV is reported in 19F [4], but with an a-width of < 6 keV. It should be mentioned that a broad resonance in thelSN( a, a) elastic scattering had been originally assigned to a 7/2+-3/2+ doublet at 7.120 MeV [ 131, but this was not confirmed later [ 141.

Letters B 353 (1995) 184-188

We wish to thank Mr P. Demaret for the preparation of the targets. Four of us (M.C., F.B., PL. and J. Va.) are senior research associates of the National Fund for Scientific Research, Brussels. M.G. is supported by a grant of the Flemish Institute for the advancement of scientific-technological research in industry. This text presents research results of the Belgian Programme on Interuniversity Poles of Attraction initiated by the Belgian State, Federal Services of Scientific, Technical and Cultural Affairs, and was also supported by the UK SERC and by NATO Grant CRG.940213. Scientific responsibility is assumed by the authors.

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