- Email: [email protected]
In-beam γ-ray Spectroscopy of 30P via the 28Si(3He,pγ)30P Reaction
Available online at www.sciencedirect.com
Nuclear Data Sheets 120 (2014) 88–90 www.elsevier.com/locate/nds
In-beam γ-ray Spectroscopy of
30
P via the
28
Si(3 He,pγ)30 P Reaction
E. Mcneice,1 K. Setoodehnia,1 B. Singh,1, ∗ Y. Abe,2 D.N. Binh,3 A.A. Chen,1 J. Chen,1, † S. Cherubini,4, 3 S. Fukuoka,2 T. Hashimoto,3 T. Hayakawa,5 Y. Ishibashi,2 Y. Ito,2 D. Kahl,3 T. Komatsubara,2 S. Kubono,6 T. Moriguchi,2 D. Nagae,2 R. Nishikiori,2 T. Niwa,2 A. Ozawa,2 T. Shizuma,5 H. Suzuki,2 H. Yamaguchi,3 and T. Yuasa2 1
Department of Physics & Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada Institute of Physics, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8577, Japan 3 Center for Nuclear Study (CNS), the University of Tokyo, Wako Branch at RIKEN, Wako, Saitama 351-0198, Japan 4 INFN-Laboratori Nazionali del Sud and Dipartimento di Fisica ed Astronomia Universit` a di Catania, 95123 Catania, Italy 5 Japan Atomic Energy Agency (JAEA), Tokai–mura, Ibaraki 319-1195, Japan 6 Nishina Center, the Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan 2
The level structure of 30 P up to 8.25 MeV was investigated via in-beam γ-ray spectroscopy using the 28 Si(3 He,pγ)30 P reaction at 9 MeV at the University of Tsukuba Tandem Accelerator Complex in Japan. An energy level scheme was deduced from γ-γ coincidence measurements. 47 new transitions have been observed from the previously known states (mostly resonances), thereby reducing the uncertainties in the excitation energies of 17 states from 3 to 10 keV to values of < 1 keV. Furthermore, spin assignments based on measurements of γ-ray angular distributions and γ-γ directional correlation of oriented nuclei (DCO ratios) were made for several observed levels of 30 P. I.
INTRODUCTION
30 15 P is an odd-odd self-conjugate nuclide, whose level density is reasonably high (34 proton-bound excited states below Sp = 5594.5(3) keV [1]) relative to that in neighboring isobars. Excited states with isospin T = 0 and T = 1 coexist in 30 P from the ground state upwards. Such isospin symmetry breaking can change the statistical distribution of electromagnetic transitions from the standard Porter-Thomas distribution to another form [2]. Furthermore, small isospin symmetry breaking can have a large impact on the level statistics [3, 4]. Thus, spectroscopic studies of 30 P provide an experimental test of the effect of isospin symmetry breaking. The effect of symmetry breaking on nuclear level statistics can be accessed with confidence if all of the states in an energy region in the nucleus of interest have been identified and their quantum numbers determined. The results of all previous measurements on 30 P are summarized in the most recent data compilation [5]. Despite several measurements, including that of Ref. [6], the γ-ray transitions of several states have never been observed before,
∗ †
Corresponding author: [email protected] Present Address: Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA.
http://dx.doi.org/10.1016/j.nds.2014.07.014 0090-3752/© 2014 Elsevier Inc. All rights reserved.
and many of the proton resonances with Ex > 5.7 MeV still have tentative spin-parity assignments. The proton-resonance structure of 30 P has astrophysical interest significance in the determination of the 29 Si(p,γ)30 P reaction rate at the temperature characteristic of explosive hydrogen burning (T > 0.1 GK). We have performed an in-beam γ-ray spectroscopy experiment [7–9] to investigate the level structure of 30 S populated via the 28 Si(3 He,nγ)30 S reaction. The main background channel in this experiment was the 28 Si(3 He,pγ)30 P reaction, which has a higher cross section [10] than the 28 Si(3 He,nγ)30 S reaction at our beam energy discussed later. The only 28 Si(3 He,pγ)30 P experiment carried out in the past was performed in 1968 [11], where the protons and γ-rays were detected in coincidence. Since we used high purity Ge-detectors whose energy resolutions are far superior than those of the NaI(Tl) detectors used in Ref. [11], we analyzed the existing data corresponding to the 28 Si(3 He,pγ)30 P channel to investigate whether or not we could improve our understanding of the level properties of 30 P. We specifically aimed to explore the region just above the proton threshold (Sp = 5594.5(3) keV [1]), where the γ-rays from decays of a number of resonances whose excitation energies are around 6 MeV have never been observed before [5].
In-beam γ-ray Spectroscopy . . .
NUCLEAR DATA SHEETS
E. Mcneice et al.
TABLE I. Weighted average (between both phases of the present experiment) energies of the selected observed transitions in 30 P (for the full table, see Ref. [15]). The γ-rays which are observed for the first time are denoted by italic font. ΔJγ is the difference in spins of the states involved in the transition (only given when RDCO is available from the data). Ref. [5] Ex Jπ (keV) 5597(5) 4+
5701.3(2) 5715(4) 5788(5)
5808(5)
5896(5)
5907.8(8) 5934.0(1) 5997.2(8) 6006.0(1) 6093.5(1) 6178(4) †
Ex (keV) 5595.3(5)†
Jπ
Eγ (keV) 4+ 569.3(5) 2659(4) 2756.1(13) 2872.1(14) 1+ 5701.40(19) 1+ 1518.5(3) 2763.7(2) (5,7)+ 5716.0(5)† (5+ ) 3742.1(5) (3 – 5)+ 5788.7(5)† (3+ – 4+ ) 1860.7(7) 2851(2) 3064.3(11) 4334(2) (3,5)+ 5808.7(5)† 1463.8(6) 3270.9(7) 3835.5(11) (2− ) 5895.5(5)† (2− ) 869.0(5) 1713.0(6) 2957.9(12) (2− ,1− ) 5908.7(7) 4451(2) 5197.3(13) (3+ ) 5934.7(8) (3+ ) 1590.8(7) (1+ ) 6000.0(14) 5290.8(14) (3+ ) 6006.2(6) 3165(2) 3467.3(5) 3− 6093.4(3) 3− 1467.5(3) 1861.2(5) (6)+ 1946.5(1) (5,6,7)+ 6178.50(17)† 2872.1(12)
Present work ΔJγ Bγave A2 /A0 A4 /A0 Coin. Gate (%) RDCO Eγ (keV) ΔJγ 0 20(2) 1.5(3) 1385.1 1 8.0(11) 12(4) 2 60(5) 1.4(1) 2723.5 1 9.0(8) 1 91(14) 1.3(3) 677.1 1 (2) 100 1.00(4) 1264.4 2 (0,1) 33(5) 0.8(1) 2260.4 2 4(1) 32(2) (1,2) 31(5) 1.3(4) 1454.24 1 7(3) 28(4) 65(15) 5(2) (0) 43(8) 0.84(4) 708.7 0 (0) 52(11) 1.3(3) 1454.24 1 21(4) 79(24) 2 100 1.2(2) 2370.3 2 100 52(4) 48(4) 0 59(7) 1.5(3) 1454.24 1 1 41(2) 0.7(3) 1264.4 2 (0,2) 75(9) +0.59(21) −0.10(18) 0.9(1) 1973.23 2 (2) 25(5) 0.9(2) 708.7 0
No γ-ray transition was observed previously from this level.
II.
γ-ray transitions observed in the singles spectra were normalized to the intense 677.1-keV 0+ → 1+ transition in 30 P. Normalized relative yields for each peak of interest were plotted against θ, where θ is the detection angle, and these data were fitted to the function: W (θ)exp = A0 + A2 P2 (cos θ) + A4 P4 (cos θ), where W (θ)exp represents the experimental γ-ray angular distribution function, the coefficients Ai are extracted from the fit, and P2 (cos θ) and P4 (cos θ) are Legendre polynomials. The γ-γ angular correlations of 30 P γ-rays were determined by measuring the Directional Correlations of Oriented states (DCO ratios) for the strong 30 P γ-ray transitions.
EXPERIMENTAL SETUP AND DATA ANALYSIS
A 3 He2+ beam (0.2 – 0.5 enA) was accelerated to 9 MeV via the tandem accelerator at the University of Tsukuba Tandem Accelerator Complex (UTTAC) in Japan. The beam impinged on a self-standing 25 μmthick foil of natural silicon. The experiment consisted of two phases, which are fully explained in Refs. [7–9]. During each phase, our γ-ray detector setup consisted of two high-purity Ge-detectors located at 90◦ (phase I), and 90◦ and 135◦ (phase II) with respect to the beam axis and on the opposite sides of the beam line. Details on energy resolutions, energy and efficiency calibrations, and γ-ray angular distribution and γ-γ angular correlation measurements, and their analysis are given in Refs. [7–9]. γ-γ coincidence and γ-ray singles data were accumulated over a total of 7 days. A γ-γ coincidence matrix was produced and analyzed to obtain information on the 30 P decay scheme. In our γ-ray angular distribution measurement, for every (θ1 , θ2 ) angular pair corresponding to the positions of the two detectors, the intensities of the strong 30 P
III.
RESULTS AND CONCLUSIONS
After placing software coincidence gates on the strong P peaks observed in the singles spectra, decay cascades from higher-lying states were observed in the γ-γ coincidence spectra (see Fig. 1). For the sake of brevity, only part of our final results are given in Table I; full results will be available online at the Experimental Unevaluated Nuclear Data List (XUNDL) database [15]. 30
89
In-beam γ-ray Spectroscopy . . .
NUCLEAR DATA SHEETS
E. Mcneice et al.
With the exception of the 4502.66(10)-, 5574.25(20)-, 7051.6(7)-, 7287.0(4)-, and 7601.9(8)-keV states, all the other measured energies of the observed levels are in agreement within 1 to 2σ with those reported in the most recent compilation [5]. We have observed for the first time 47 new transitions from 30 P excited states, most of which are proton resonances. Only selected transitions are given in Table I. Observation of these new transitions has enabled us to reduce the uncertainties (over the range 3 to 10 keV down to < 1 keV) on the excitation energies of 17 states, denoted by † in Table I of Ref. [15], for which no γ-ray transitions have been observed prior to the present work. Figure 1 presents a sample of the coincidence spectrum obtained during phase I from a coincidence gate on the newly observed 1946.5-keV peak, revealing several previously known transitions from 30 P excited states. The A2 /A0 and A4 /A0 coefficients, as well as the DCO ratios, were adopted from the typical values given in Ref. [14]. With the results of our γ-ray angular distribution and γ-γ directional correlation (DCO ratios) measurements, we have been able to confirm the previously determined spins of several states in 30 P. Consider the 6178.5-keV level, assigned previously as (5, 6, 7)+ from L-transfer in (3 He, t) reaction data [16]: we can rule out 7+ and consider 5 less likely, supporting an assignment of (6)+ . However, our data are generally not adequate to assign firm spins to the observed levels whose spin-parity assignments were previously determined to be tentative.
FIG. 1. An example of γ-γ coincidence spectrum measured at 90◦ obtained from gating on the 1946.5-keV transition of 30 P (with J π = (6)+ → J π = 4− ), which is observed here for the first time. This γ-ray de-excites the 6178.50(17)-keV excited state of 30 P. Peaks are labeled with their energies (in keV) that are weighted averages between both phases of the experiment.
Final uncertainties in the energies are due to the statistical uncertainties in the corresponding centroids only. The relative intensities were calculated separately at 90◦ and 135◦ with respect to the strongest 30 P γ-ray measured at the same angle and branching ratios were then determined for each angle. Unweighted averages between branching ratios measured at 90◦ and 135◦ are given in Table I. It should be noted that these still carry angular distribution effects. The γ-ray energies given in Table I are corrected for the recoil energies, corresponding to the de-exciting states, which were taken into account when constructing the final excitation energies of 30 P from the γ-ray decay scheme. By performing a least-squares fit to the recoil corrected γ-ray energies, the 30 P excitation energies were reconstructed to obtain the level scheme of 30 P. This fit (χ2red. = 0.99) was obtained using the GTOL code [13].
Acknowledgements: We would like to thank the staff of UTTAC for their contributions. This work was supported by the Natural Sciences and Engineering Research Council of Canada; the U.S. Department of Energy; the Grant-in-Aid for Science Research KAKENHI of Japan; the JSPS KAKENHI and JSPS Bilateral Joint Project of Japan; and Japan Society for the Promotion of Science Core-to-Core Program on International Research Network for Exotic Femto Systems.
[9] K. Setoodehnia, A.A. Chen, D. Kahl et al., Phys. Rev. C 87, 065801 (2013). [10] R. Bass, U. Friedland, B. Hubert et al., Nucl. Phys. A 198, 449 (1972). [11] C.W. Vermette, W.C. Olsen, D.A. Hutcheon, D.H. Sykes, Nucl. Phys. A 111, 39 (1968). [12] A. Kr¨ amer-Flecken, T. Morek, R.M. Lieder et al., Nucl. Instrum. Methods Phys. Res. A 275, 333 (1989). [13] http://www.nndc.bnl.gov/nndcscr/ensdf pgm/analysis/gtol/. [14] Summary of basis for spin and parity assignments, Nucl. Data sheets 113, iv (2012). [15] http://www.nndc.bnl.gov/xundl. [16] B. Ramstein, L.H. Rosier, R.J. De Meijer, Nucl. Phys. A 363, 110 (1981).
[1] M. Wang, G. Audi, A.H. Wapstra et al., Chin. Phys. C 36, 1603 (2012). [2] J.F. Shriner Jr., C.A. Grossmann, G.E. Mitchell, Phys. Rev. C 62, 054305 (2000). [3] G.E. Mitchell, E.G. Bilpuch, P.M. Endt, Phys. Rev. Lett. 61, 1473 (1988). [4] J.F. Shriner Jr., E.G. Bilpuch, P.M. Endt, Z. Phys. A 335, 393 (1990). [5] M.S. Basunia, Nucl. Data Sheets 111, 2331 (2010). [6] C.A. Grossmann, M.A. LaBonte, G.E. Mitchell et al., Phys. Rev. C 62, 024323 (2000). [7] K. Setoodehnia, A.A. Chen, T. Komatsubara et al., Phys. Rev. C 83, 018803 (2011). [8] K. Setoodehnia, PhD thesis, McMaster University (2011), http://digitalcommons.mcmaster.ca/ opendissertaions/6313/
90
Nuclear Data Sheets 120 (2014) 88–90 www.elsevier.com/locate/nds
In-beam γ-ray Spectroscopy of
30
P via the
28
Si(3 He,pγ)30 P Reaction
E. Mcneice,1 K. Setoodehnia,1 B. Singh,1, ∗ Y. Abe,2 D.N. Binh,3 A.A. Chen,1 J. Chen,1, † S. Cherubini,4, 3 S. Fukuoka,2 T. Hashimoto,3 T. Hayakawa,5 Y. Ishibashi,2 Y. Ito,2 D. Kahl,3 T. Komatsubara,2 S. Kubono,6 T. Moriguchi,2 D. Nagae,2 R. Nishikiori,2 T. Niwa,2 A. Ozawa,2 T. Shizuma,5 H. Suzuki,2 H. Yamaguchi,3 and T. Yuasa2 1
Department of Physics & Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada Institute of Physics, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8577, Japan 3 Center for Nuclear Study (CNS), the University of Tokyo, Wako Branch at RIKEN, Wako, Saitama 351-0198, Japan 4 INFN-Laboratori Nazionali del Sud and Dipartimento di Fisica ed Astronomia Universit` a di Catania, 95123 Catania, Italy 5 Japan Atomic Energy Agency (JAEA), Tokai–mura, Ibaraki 319-1195, Japan 6 Nishina Center, the Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan 2
The level structure of 30 P up to 8.25 MeV was investigated via in-beam γ-ray spectroscopy using the 28 Si(3 He,pγ)30 P reaction at 9 MeV at the University of Tsukuba Tandem Accelerator Complex in Japan. An energy level scheme was deduced from γ-γ coincidence measurements. 47 new transitions have been observed from the previously known states (mostly resonances), thereby reducing the uncertainties in the excitation energies of 17 states from 3 to 10 keV to values of < 1 keV. Furthermore, spin assignments based on measurements of γ-ray angular distributions and γ-γ directional correlation of oriented nuclei (DCO ratios) were made for several observed levels of 30 P. I.
INTRODUCTION
30 15 P is an odd-odd self-conjugate nuclide, whose level density is reasonably high (34 proton-bound excited states below Sp = 5594.5(3) keV [1]) relative to that in neighboring isobars. Excited states with isospin T = 0 and T = 1 coexist in 30 P from the ground state upwards. Such isospin symmetry breaking can change the statistical distribution of electromagnetic transitions from the standard Porter-Thomas distribution to another form [2]. Furthermore, small isospin symmetry breaking can have a large impact on the level statistics [3, 4]. Thus, spectroscopic studies of 30 P provide an experimental test of the effect of isospin symmetry breaking. The effect of symmetry breaking on nuclear level statistics can be accessed with confidence if all of the states in an energy region in the nucleus of interest have been identified and their quantum numbers determined. The results of all previous measurements on 30 P are summarized in the most recent data compilation [5]. Despite several measurements, including that of Ref. [6], the γ-ray transitions of several states have never been observed before,
∗ †
Corresponding author: [email protected] Present Address: Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA.
http://dx.doi.org/10.1016/j.nds.2014.07.014 0090-3752/© 2014 Elsevier Inc. All rights reserved.
and many of the proton resonances with Ex > 5.7 MeV still have tentative spin-parity assignments. The proton-resonance structure of 30 P has astrophysical interest significance in the determination of the 29 Si(p,γ)30 P reaction rate at the temperature characteristic of explosive hydrogen burning (T > 0.1 GK). We have performed an in-beam γ-ray spectroscopy experiment [7–9] to investigate the level structure of 30 S populated via the 28 Si(3 He,nγ)30 S reaction. The main background channel in this experiment was the 28 Si(3 He,pγ)30 P reaction, which has a higher cross section [10] than the 28 Si(3 He,nγ)30 S reaction at our beam energy discussed later. The only 28 Si(3 He,pγ)30 P experiment carried out in the past was performed in 1968 [11], where the protons and γ-rays were detected in coincidence. Since we used high purity Ge-detectors whose energy resolutions are far superior than those of the NaI(Tl) detectors used in Ref. [11], we analyzed the existing data corresponding to the 28 Si(3 He,pγ)30 P channel to investigate whether or not we could improve our understanding of the level properties of 30 P. We specifically aimed to explore the region just above the proton threshold (Sp = 5594.5(3) keV [1]), where the γ-rays from decays of a number of resonances whose excitation energies are around 6 MeV have never been observed before [5].
In-beam γ-ray Spectroscopy . . .
NUCLEAR DATA SHEETS
E. Mcneice et al.
TABLE I. Weighted average (between both phases of the present experiment) energies of the selected observed transitions in 30 P (for the full table, see Ref. [15]). The γ-rays which are observed for the first time are denoted by italic font. ΔJγ is the difference in spins of the states involved in the transition (only given when RDCO is available from the data). Ref. [5] Ex Jπ (keV) 5597(5) 4+
5701.3(2) 5715(4) 5788(5)
5808(5)
5896(5)
5907.8(8) 5934.0(1) 5997.2(8) 6006.0(1) 6093.5(1) 6178(4) †
Ex (keV) 5595.3(5)†
Jπ
Eγ (keV) 4+ 569.3(5) 2659(4) 2756.1(13) 2872.1(14) 1+ 5701.40(19) 1+ 1518.5(3) 2763.7(2) (5,7)+ 5716.0(5)† (5+ ) 3742.1(5) (3 – 5)+ 5788.7(5)† (3+ – 4+ ) 1860.7(7) 2851(2) 3064.3(11) 4334(2) (3,5)+ 5808.7(5)† 1463.8(6) 3270.9(7) 3835.5(11) (2− ) 5895.5(5)† (2− ) 869.0(5) 1713.0(6) 2957.9(12) (2− ,1− ) 5908.7(7) 4451(2) 5197.3(13) (3+ ) 5934.7(8) (3+ ) 1590.8(7) (1+ ) 6000.0(14) 5290.8(14) (3+ ) 6006.2(6) 3165(2) 3467.3(5) 3− 6093.4(3) 3− 1467.5(3) 1861.2(5) (6)+ 1946.5(1) (5,6,7)+ 6178.50(17)† 2872.1(12)
Present work ΔJγ Bγave A2 /A0 A4 /A0 Coin. Gate (%) RDCO Eγ (keV) ΔJγ 0 20(2) 1.5(3) 1385.1 1 8.0(11) 12(4) 2 60(5) 1.4(1) 2723.5 1 9.0(8) 1 91(14) 1.3(3) 677.1 1 (2) 100 1.00(4) 1264.4 2 (0,1) 33(5) 0.8(1) 2260.4 2 4(1) 32(2) (1,2) 31(5) 1.3(4) 1454.24 1 7(3) 28(4) 65(15) 5(2) (0) 43(8) 0.84(4) 708.7 0 (0) 52(11) 1.3(3) 1454.24 1 21(4) 79(24) 2 100 1.2(2) 2370.3 2 100 52(4) 48(4) 0 59(7) 1.5(3) 1454.24 1 1 41(2) 0.7(3) 1264.4 2 (0,2) 75(9) +0.59(21) −0.10(18) 0.9(1) 1973.23 2 (2) 25(5) 0.9(2) 708.7 0
No γ-ray transition was observed previously from this level.
II.
γ-ray transitions observed in the singles spectra were normalized to the intense 677.1-keV 0+ → 1+ transition in 30 P. Normalized relative yields for each peak of interest were plotted against θ, where θ is the detection angle, and these data were fitted to the function: W (θ)exp = A0 + A2 P2 (cos θ) + A4 P4 (cos θ), where W (θ)exp represents the experimental γ-ray angular distribution function, the coefficients Ai are extracted from the fit, and P2 (cos θ) and P4 (cos θ) are Legendre polynomials. The γ-γ angular correlations of 30 P γ-rays were determined by measuring the Directional Correlations of Oriented states (DCO ratios) for the strong 30 P γ-ray transitions.
EXPERIMENTAL SETUP AND DATA ANALYSIS
A 3 He2+ beam (0.2 – 0.5 enA) was accelerated to 9 MeV via the tandem accelerator at the University of Tsukuba Tandem Accelerator Complex (UTTAC) in Japan. The beam impinged on a self-standing 25 μmthick foil of natural silicon. The experiment consisted of two phases, which are fully explained in Refs. [7–9]. During each phase, our γ-ray detector setup consisted of two high-purity Ge-detectors located at 90◦ (phase I), and 90◦ and 135◦ (phase II) with respect to the beam axis and on the opposite sides of the beam line. Details on energy resolutions, energy and efficiency calibrations, and γ-ray angular distribution and γ-γ angular correlation measurements, and their analysis are given in Refs. [7–9]. γ-γ coincidence and γ-ray singles data were accumulated over a total of 7 days. A γ-γ coincidence matrix was produced and analyzed to obtain information on the 30 P decay scheme. In our γ-ray angular distribution measurement, for every (θ1 , θ2 ) angular pair corresponding to the positions of the two detectors, the intensities of the strong 30 P
III.
RESULTS AND CONCLUSIONS
After placing software coincidence gates on the strong P peaks observed in the singles spectra, decay cascades from higher-lying states were observed in the γ-γ coincidence spectra (see Fig. 1). For the sake of brevity, only part of our final results are given in Table I; full results will be available online at the Experimental Unevaluated Nuclear Data List (XUNDL) database [15]. 30
89
In-beam γ-ray Spectroscopy . . .
NUCLEAR DATA SHEETS
E. Mcneice et al.
With the exception of the 4502.66(10)-, 5574.25(20)-, 7051.6(7)-, 7287.0(4)-, and 7601.9(8)-keV states, all the other measured energies of the observed levels are in agreement within 1 to 2σ with those reported in the most recent compilation [5]. We have observed for the first time 47 new transitions from 30 P excited states, most of which are proton resonances. Only selected transitions are given in Table I. Observation of these new transitions has enabled us to reduce the uncertainties (over the range 3 to 10 keV down to < 1 keV) on the excitation energies of 17 states, denoted by † in Table I of Ref. [15], for which no γ-ray transitions have been observed prior to the present work. Figure 1 presents a sample of the coincidence spectrum obtained during phase I from a coincidence gate on the newly observed 1946.5-keV peak, revealing several previously known transitions from 30 P excited states. The A2 /A0 and A4 /A0 coefficients, as well as the DCO ratios, were adopted from the typical values given in Ref. [14]. With the results of our γ-ray angular distribution and γ-γ directional correlation (DCO ratios) measurements, we have been able to confirm the previously determined spins of several states in 30 P. Consider the 6178.5-keV level, assigned previously as (5, 6, 7)+ from L-transfer in (3 He, t) reaction data [16]: we can rule out 7+ and consider 5 less likely, supporting an assignment of (6)+ . However, our data are generally not adequate to assign firm spins to the observed levels whose spin-parity assignments were previously determined to be tentative.
FIG. 1. An example of γ-γ coincidence spectrum measured at 90◦ obtained from gating on the 1946.5-keV transition of 30 P (with J π = (6)+ → J π = 4− ), which is observed here for the first time. This γ-ray de-excites the 6178.50(17)-keV excited state of 30 P. Peaks are labeled with their energies (in keV) that are weighted averages between both phases of the experiment.
Final uncertainties in the energies are due to the statistical uncertainties in the corresponding centroids only. The relative intensities were calculated separately at 90◦ and 135◦ with respect to the strongest 30 P γ-ray measured at the same angle and branching ratios were then determined for each angle. Unweighted averages between branching ratios measured at 90◦ and 135◦ are given in Table I. It should be noted that these still carry angular distribution effects. The γ-ray energies given in Table I are corrected for the recoil energies, corresponding to the de-exciting states, which were taken into account when constructing the final excitation energies of 30 P from the γ-ray decay scheme. By performing a least-squares fit to the recoil corrected γ-ray energies, the 30 P excitation energies were reconstructed to obtain the level scheme of 30 P. This fit (χ2red. = 0.99) was obtained using the GTOL code [13].
Acknowledgements: We would like to thank the staff of UTTAC for their contributions. This work was supported by the Natural Sciences and Engineering Research Council of Canada; the U.S. Department of Energy; the Grant-in-Aid for Science Research KAKENHI of Japan; the JSPS KAKENHI and JSPS Bilateral Joint Project of Japan; and Japan Society for the Promotion of Science Core-to-Core Program on International Research Network for Exotic Femto Systems.
[9] K. Setoodehnia, A.A. Chen, D. Kahl et al., Phys. Rev. C 87, 065801 (2013). [10] R. Bass, U. Friedland, B. Hubert et al., Nucl. Phys. A 198, 449 (1972). [11] C.W. Vermette, W.C. Olsen, D.A. Hutcheon, D.H. Sykes, Nucl. Phys. A 111, 39 (1968). [12] A. Kr¨ amer-Flecken, T. Morek, R.M. Lieder et al., Nucl. Instrum. Methods Phys. Res. A 275, 333 (1989). [13] http://www.nndc.bnl.gov/nndcscr/ensdf pgm/analysis/gtol/. [14] Summary of basis for spin and parity assignments, Nucl. Data sheets 113, iv (2012). [15] http://www.nndc.bnl.gov/xundl. [16] B. Ramstein, L.H. Rosier, R.J. De Meijer, Nucl. Phys. A 363, 110 (1981).
[1] M. Wang, G. Audi, A.H. Wapstra et al., Chin. Phys. C 36, 1603 (2012). [2] J.F. Shriner Jr., C.A. Grossmann, G.E. Mitchell, Phys. Rev. C 62, 054305 (2000). [3] G.E. Mitchell, E.G. Bilpuch, P.M. Endt, Phys. Rev. Lett. 61, 1473 (1988). [4] J.F. Shriner Jr., E.G. Bilpuch, P.M. Endt, Z. Phys. A 335, 393 (1990). [5] M.S. Basunia, Nucl. Data Sheets 111, 2331 (2010). [6] C.A. Grossmann, M.A. LaBonte, G.E. Mitchell et al., Phys. Rev. C 62, 024323 (2000). [7] K. Setoodehnia, A.A. Chen, T. Komatsubara et al., Phys. Rev. C 83, 018803 (2011). [8] K. Setoodehnia, PhD thesis, McMaster University (2011), http://digitalcommons.mcmaster.ca/ opendissertaions/6313/
90