Study of proton resonances in 26Si and 27P by using the 25Al and 26Siradioactive ion beams

J. Y. Moona, C. S. Leea∗, J. H. Leea, C. C. Yuna, J. C. Kimb, M. Younb, S. Kubonoc, T.Teranishic, J. J. Hec, M. Notanic, S. Nishimurad, M. Nishimurad, V. Guimaraese, R. F.Lichitenthalere and S. Katof

aDepartment of Physics, Chung-Ang University, Seoul 156-756, Korea

bDepartment of Physics, Seoul National University, Seoul 151-741, Korea

cCenter for Nuclear Study, University of Tokyo, Wako, Saitama 351-0198, Japan

dRIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

eInstituto de Fısica, Universidade de Sao Paulo, Brazil

fDepartment of Physics, Yamagata University, Yamagata 990-8560, Japan

An explosive hydrogen burning process occurring in novae or X-ray bursts has beensuggested to be important for observation of 1.809-MeV gamma rays of 26Al. In order tostudy relevant proton resonances in 26Si and 27P above the proton threshold, we performedelastic scattering experiments at low energies by using 25Al and 26Si radioactive ion beams.We investigated proton resonances in 26Si up to Ec.m. = 3.016 MeV as well as in 27P up toEc.m. = 3.290 MeV. We compared observed proton resonances in 27P with those obtainedby the OXBASH shell model calculation.

1. Introduction

The nine-year survey of COMPTEL has updated the all-sky map of the 1.809-MeVgamma ray produced by β decay of 26Al. Knowledge of its source is not yet clear thoughmassive stars, novae, and X-ray busters have been suggested to be possible sites. Spec-troscopic information on resonance states relevant to production of 26Al(g.s.) is crucial tocalculation of reaction rates for contributing proton-capture reactions at stellar tempera-tures. In a thermonuclear runaway under the explosive hydrogen burning, 25Al(p,γ)26Sihinders production of 26Al(g.s.). In spite of previous measurements on resonance states in26Si, the astrophysically important 3+ state above the proton threshold could be neitherseen [1] nor definitely identified [2]. On the other hand, 26Si(p,γ)27P prevents 26mAl frombeing produced. It has been suggested that novae at higher temperatures T ≥ 0.4 GKmay be hot enough to establish an equilibrium between 26Al(g.s.) and 26mAl [3]. Thus,study on resonance states in 27P should also be needed to determine the reaction rate

∗This work was supported by Korea Research Foundation Grant (KRF-2002-070-C00031).

Nuclear Physics A 758 (2005) 158c–161c

0375-9474/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.nuclphysa.2005.05.033

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-20 -15 -10 -5 0 5 10 15 20Histogram ID = 101

(A)

(B)

(C)

RF1 (ns)

TO

F (

ns)

Figure 1. Identification of (A) 24Mg, (B) 25Al, and (C) 26Si secondary beams.

of proton capture by 26Si. Until now, only one state at 1.199 MeV above the protonthreshold has been reported [4]. We investigated the resonance states in 26Si and 27P byusing inverse elastic scattering experiments of 1H(25Al,p)25Al and 1H(26Si,p)26Si with athick target.

2. Experimental procedure

The radioactive ion beams were produced by the CNS radioactive ion beam separator(CRIB). A 24Mg8+ primary beam, accelerated by the RIKEN AVF cyclotron (K = 70), wasbombarded on a 3He gas target at 7.434 A MeV. The thickness of the 3He gas target was0.32 mg/cm2. In two primary reactions of 3He(24Mg,n)26Si∗(p)25Al and 3He(24Mg,n)26Si,two kinds of secondary beam, 25Al and 26Si, were simultaneously produced and used forelastic scattering of 1H(25Al,p)25Al and 1H(26Si,p)26Si, respectively. The secondary beamswere identified by using TOF (time of flight) between the two PPAC’s (parallel plateavalanche counters) on an achromatic focal plane (F2), beam energy, and TOF betweenthe production target and a PPACb on the F2 plane. With the slit on a momentum-dispersive focal plane (F1), beam purity was enhanced. Additionally, an energy degradermade of a 2.6 µm mylar was used for another particle separation.

Identification of the secondary beams is shown in Fig. 1. Beam specification is listedin Table 1. The secondary beams were bombarded on a polyethylene (CH2) target of

Table 1Beam specification for the present work.

Beam Energy (A MeV) Intensity (kcps) Purity (%)25Al 3.44 9.325 4.7226Si 3.95 1.573 0.70

J.Y. Moon et al. / Nuclear Physics A 758 (2005) 158c–161c 159c

8.24 mg/cm2, and were fully stopped in the target. For detection of recoiled protons,two sets of the silicon counter telescope, consisting of a PSD (position-sensitive silicondetector) with the thickness of 75 µm and an SSD (surface-barrier silicon detector) withthe thickness of 1,500 µm, were installed at 0 and 17, respectively. They were alsoused as ∆E-E telescopes to identify protons from other heavier charged particles. For thesecond silicon counter telescope located at 17, an additional SSD was added to rejecthigh-energy protons. Geometrically, the first(second) telescope covered the angular regionfrom -4.6 (11.4) to 4.6 (23.2) with the angular resolution of FWHM = 1.36 (1.57)in the laboratory system. Converted into the center-of-mass (c.m.) system, the anglecoverage was ranged from -171.0 (133.7) to 171.0 (157.3) with the angular resolutionof 2.72 (3.14). The energy resolutions of the 0 and 17 telescopes were FWHM ≈36 and 53 keV (in c.m.), respectively. The secondary beams had energies of 3.4407 AMeV for 25Al and 3.9575 A MeV for 26Si on the target, respectively. These beam energiesallowed us to scan up to Ex = 8.8243 MeV for 26Si and Ex = 4.7109 MeV for 27P above theproton threshold, respectively. A carbon target was used for subtraction of a contributionby carbons contained in the CH2 target from the proton spectrum obtained by the CH2

target. Energy calibration was done with protons whose energies were determined by theCRIB magnetic settings.

3. Result and discussion

0.5 1.0 1.5 2.0 2.5 3.0

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.425

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d σ/σ/ σ/σ/d Ω

(Ω

(Ω

(Ω

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r)

EC.M.

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00 (

L =

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Figure 2. Excitation function for 25Al +p. Arrows with excitation energies and spin-parity indicate the known states in 26Si.

0.5 1.0 1.5 2.0 2.5 3.0-0.5

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095

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EC.M.

(MeV)

d σ/σ/ σ/σ/d Ω

(Ω

(Ω

(Ω

( b/s

r)

3.3

39 M

eV

Figure 3. Excitation function for 26Si + p.Arrows indicate the states which are sup-posed to be the elastic resonance peaks.

Figure 2 shows the proton spectrum obtained by the 0 telescope in the 25Al + p elasticscattering run. The energy region above Ec.m. = 2.7 MeV was excluded in the analysis

J.Y. Moon et al. / Nuclear Physics A 758 (2005) 158c–161c160c

Table 2Shell model calculation for states in 27P (in MeV).

2J π; 2T Shell model Emx

a Ref. [4] Present work

1+; 3 0.000 g.s g.s

3+; 3 0.895 0.985 1.199(19)

5+; 3 1.666 1.699 1.631(19)

5+; 3 1.978 1.940 1.951d

7+; 3 3.149 3.109b 3.095

7+; 3 3.300 3.427c 3.453(22) 3.339aExcitation energies for the 27Mg mirror nucleus, taken from Ref. [7].b,cWith spin-parity of (5,7)+; 3.dValue deduced from a 27P spectrum in Ref. [4].

because of a contaminant which was thought to come from PPACs. The observed peakswere compared with the known states in 26Si [1,2]. In this experiment, we could not findthe 3+ unnatural-parity state near the expected resonance energy of ER = 0.427 MeV,corresponding to Ex = 5.945 MeV [2]. Absence of the 3+ state could be attributed to theexperimental condition that our target was too thick for the recoiled protons to escape.In addition, our s-wave calculation yielded a small proton decay width ∼ 34 eV, whichalso hindered observation of the 3+ state. Further analysis with the the R-matrix methodis underway. For the 26Si + p spectrum shown in Fig. 3, we observed several peakseven with low statistics. For states in 27P, we performed the shell model calculation byusing the code OXBASH [5]. The sd-shell configuration and the Wildenthal interactionwere considered in the calculation [6]. Comparison to previous work [4] is summarized inTable 2. In this preliminary analysis, two peaks at Ec.m. = 2.198 MeV and 2.442 MeVcorresponding to Ex = 3.095 MeV and 3.339 MeV, respectively are supposed to be theelastic resonance peaks, as shown in Fig. 3. The data-absent regions in the spectra canbe seen, which are due to the dead-layers in detectors.

REFERENCES

1. D. W. Bardayan et al., Phys. Rev. C 65 (2002) 032801R.2. J. A. Caggiano et al., Phys. Rev. C 65 (2002) 055801.3. A. Coc et al., Phys. Rev. C 61 (1999) 015801.4. J. A. Caggiano et al., Phys. Rev. C 64 (2001) 025802.5. B. A. Brown et al., OXBASH, 1984 (unpublished).6. B. A. Brown and B. H. Wildenthal, Annu. Rev. Nucl. Part. Sci. 38 (1988) 29.7. P. M. Endt, Nucl. Phys. A 633 (1998) 1.

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