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In a search for a chiral symmetry in 102Rh
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XX International School on Nuclear Physics, Neutron Physics and Applications (Varna2013) IOP Publishing Journal of Physics: Conference Series 533 (2014) 012041 doi:10.1088/1742-6596/533/1/012041
In a search for a chiral symmetry in
102
Rh
M.S. Yavahchova1 , D. Tonev1 , N. Goutev1 , P. Petkov1 , G. de Angelis2 , R. K. Bhowmik3 , R.P. Singh3 , S. Muralithar3 , N. Madhavan3 , R. Kumar3 , M. Kumar Raju4 , J. Kaur5 , G. Mohanto3 , A. Singh5 , N. Kaur5 , R. Garg6 , A. Sukla7 , Ts. K. Marinov1 , S. Brant8 1
Bulgarian Academy of Sciences, Institute for Nuclear Research and Nuclear Energy, 1784 Sofia, Bulgaria 2 INFN, Laboratori Nazionali di Legnaro, I–35020, Legnaro, Italy 3 Inter–University Accelerator Center, New Delhi, India 4 Nuclear Physics Department, Andhra University, Visakhapatnam, India 5 Department of Physics, Panjab University, Chandigarh, India 6 Department of Physics and Astrophysics, Delhi University, New Delhi, India 7 Department of Physics, Banaras Hindu University, Varanasi, India 8 Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia E-mail:
[email protected] Abstract. Excited states in 102 Rh were populated in the fusion-evaporation reaction 94 Zr(11 B, 3n)102 Rh at a beam energy of 36 MeV, using the INGA spectrometer at IUAC, New Delhi. The angular correlations and the electromagnetic character of some of the γ-ray transitions observed in 102 Rh were investigated in detail. A new candidate for a chiral twin band was identified in 102 Rh for the first time.
1. Introduction Chirality is a phenomenon which is often found in nature. Examples of systems demonstrating chirality are present in chemistry, biology, high energy physics etc. A spontaneous breaking of the chiral symmetry can take place for configurations where the angular momenta of the valence protons, valence neutrons, and the core are mutually perpendicular [1]. The projections of the angular momentum vector on the axes of the intrinsic system can form a left- or a right-handed system. Since the chiral symmetry is dichotomic, its spontaneous breaking by the angular momentum vector leads to a pair of degenerate ΔI = 1 rotational bands, called chiral twin bands. Pairs of bands, presumably due to the breaking of the chiral symmetry in triaxial nuclei, have been recently found in the mass regions A ∼ 130 [2, 3], A ∼ 105 [4, 5, 6, 7], A ∼ 195 [8] and A ∼ 80 [9]. However, only in few cases (126 Cs [10], 128 Cs [11]), the systematic properties [12] of the chiral bands, which originate from the underlying symmetry, were confirmed including the transition from chiral vibrations to static chirality in 135 Nd [3]. Thus, the yrast and side bands should be nearly degenerate. In many cases, the energy degeneracy of the chiral candidate bands was almost observed but the transition probabilities are different, like in the case of 134 Pr [13, 14]. The main goal of the present work was to check for the existence of chirality in the mass region A ∼ 100. According to the works [15] and [16], the nucleus of 102 Rh is one of the candidates to express multiple chirality. The goal of this work is to check for the existence of chirality in the mass region A ∼ 100. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1
XX International School on Nuclear Physics, Neutron Physics and Applications (Varna2013) IOP Publishing Journal of Physics: Conference Series 533 (2014) 012041 doi:10.1088/1742-6596/533/1/012041
2. Experiment Excited states in 102 Rh were populated through the reaction 94 Zr(11 B,3n)102 Rh. The beam of 11 B with an energy of 36 MeV was provided by the 15-UD Pelletron accelerator at the Inter University Accelerator Centre (IUAC) in New Delhi, India. The target consisted of 0.9 mg/cm2 94 Zr, enriched to 96.5%, evaporated onto a 8 mg/cm2 gold backing. The recoils were leaving the target with a mean velocity, v of about 0.9% of the velocity of light, c. The de-exciting gamma rays were registered with the Indian National Gamma Array (INGA), whose 15 Clover detectors were accommodated in 4π geometry [17]. Gain matching and efficiency calibration of the Ge detectors were performed using 152 Eu and 133 Ba radioactive sources before sorting the data in matrices and cubes. 3. Data analysis and results To investigate the level scheme and electromagnetic properties of the transitions of interest in 102 Rh, we have performed four different types of data analyses as follows. The ordering of transitions in the level scheme was determined according to the γ-ray relative intensities, γγ coincidence relationships, and γ-ray energy sums. The character and multipolarity of the transitions was deduced from the investigation of linear polarization and angular correlations measurements, respectively. For this purpose, the clover detectors positioned close to 90◦ were used as a composite Compton polarimeter [18]. The efficiency with which the Compton scattering events were registered was determined with the 152 Eu and 133 Ba sources that emit γ rays of natural polarization isotropically. We have determined the spins, the levels and the multipolarity of the corresponding transitions using angular correlation analysis of data taken with the INGA spectrometer with the computer code CORLEONE [19]. In figure 1 the negative lines correspond to transitions of predominantly magnetic character while the positive lines correspond to transitions of predominantly electric character. The negative character of the lines 304, 306 and 363 in the spectrum, proves that the corresponding transitions are of predominantly magnetic character.
N⟂+ N∥
237
335
(E)
306 304 300
363 400
529 556
(M) 500 E / KeV
669
*10
N⟂- N∥ 600
700
Figure 1. Coincidence spectra registered by the perpendicular and parallel arms of the compton polarimeter. Transitions with energies of 304, 306 and 363 keV have predominantly magnetic character. The lifetimes were derived using the Doppler-shift attenuation methods. The analysis was carried out according to the procedure outlined in [20], where details about the Monte-Carlo 2
XX International School on Nuclear Physics, Neutron Physics and Applications (Varna2013) IOP Publishing Journal of Physics: Conference Series 533 (2014) 012041 doi:10.1088/1742-6596/533/1/012041
simulation of the slowing down process, determination of stopping powers and fitting of line shapes can be found.
2000 1500 1000 500
. E = 768 keV . = 148 . .. .. . . ... ........ ............. o
Counts
2500
0 760
764
768
772
776
Gamma-ray energy [keV]
1200 1000 800 600 400 200
. . = 32 . E = 768 keV . .. . ...... ....... ............. o
0 760
764
768
772
776
Gamma-ray energy [keV]
Figure 2. Fitted line-shape of transitions observed at 32◦ and 148◦ . The spectra for every gated ring were summed up to increase the statistics. For each level, lifetimes were derived independently at the four rings where appreciable Doppler-shifts are observed. In figure 2 is shown an examples for such analysis and the Doppler-shifts are nicely seen. The gate is set from below of the transition of interest. To describe the reduced transition probabilities B(σλ) and the level scheme in 102 Rh we performed two quasiparticles plus triaxial rotor (TQPTR) calculations within the approach given in [21]. Figure 3 illustrates the experimental level scheme of 102 Rh compared to TQPTR calculations. The reduced transition probabilities B(σλ) deduced from the lifetime data are presented in Ref.[22]. The comparison between the experimental and calculated B(M1) transition strengths leads to the conclusion that the TQPTR calculations reproduce roughly the data in Band 1 and are consistent with the transition strength in Band 2 at spin 11 ¯h. The absence of an appreciable staggering of the data in Band 1 indicates that the expectations for the observation of a static chirality in 102 Rh are not realized [22]. If the chirality in 102 Rh exists it has mainly a dynamical character. 4. Summary and conclusions For the investigation to the level-scheme of 102 Rh we have performed an experiment at the IUAC in New Delhi using the INGA spectrometer. To construct the level scheme of 102 Rh we used γ-γ coincidence data, relative γ-rays intensities and energy sums. For the first time we applied angular correlation analysis and polarization measurements for INGA spectrometer data. The results obtained from angular correlation and linear polarization measurements were essential for the spin and parity assignments. As a result from our analysis 4 new exited states in 102 Rh were determined for the first time. In addition 8 new lifetimes have been determined. 5. Acknowledgments This research has been supported by Bulgarian Science Fund under contract DFNI-E 01/2 and by a NUPNET - NEDENSAA project funded by the Bulgarian Ministry of Education and Science. Thanks to the Pelletron Staff of IUAC, New Delhi for their perfect work for our experiment. 3
XX International School on Nuclear Physics, Neutron Physics and Applications (Varna2013) IOP Publishing Journal of Physics: Conference Series 533 (2014) 012041 doi:10.1088/1742-6596/533/1/012041
102
"2"
5000
102
Rh - Experiment "1"
18
4500
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15
1057 528
2500
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-
-
1500
-
359
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452
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811
10
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824
500
462
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510 78
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-
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9
176
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-
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13
-
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-
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-
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-
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Figure 3. Comparison of the experimental and calculated within the TQPTR level schemes of 102 Rh. References [1] S Frauendorf, Jie Meng 1997 Nucl. Phys. A 617 131 [2] K Starosta et al. 2001 Phys. Rev. Lett. 86 971 [3] S Mukhopadhyay et al., 2007Phys. Rev. Lett. 99 172501 [4] J Tim´ ar et al. 2006 Phys. Rev. C 73 011301 [5] C Vaman, D B Fossan, T Koike, K Starosta, 2004 Phys. Rev. Lett. 92 032501 [6] T Suzuki et al. 2008 Phys. Rev. C 78 031302 [7] P Joshi et al. 2004 Phys.Lett. B 595 135 [8] E A Lawrie et al. 2008 Phys. Rev. C 78 021305 [9] S Y Wang et al. 2011 Phys. Lett. B 703 40 [10] E Grodner et al. 2011 Phys. Lett. B 703 46 [11] E Grodner et al. 2006 Phys. Rev. Lett. 97 172501 [12] S Brant, D Tonev, G De Angelis, A Ventura 2008 Phys. Rev. C 78 034301 [13] D Tonev et al. 2006 Phys. Rev. Lett. 96 052501 [14] D Tonev et al. 2007 Phys. Rev. C 76 044313 [15] J Meng et al. 2006 Phys. Rev. C 73 037303 [16] J Peng et al. 2008 Phys. Rev. C 77 024309 [17] S Muralithar et al. 2008 Proc. DAE-BRNS Symp. on Nucl. Phys.53 221 [18] N Goutev et al. 2012 J. Phys.:Conf. Ser. 366 012021 [19] I Wieden¨ over et al. 2008 Proc. DAE-BRNS Symp. on Nucl. Phys. 53 221 [20] P Petkov et al. 1998 Nucl. Phys. A 640 293 [21] I Ragnarsson and P Semmes 1988 Hyp. Int. 43 425 [22] D Tonev 2014 Phys. Rev. Lett. 112 052501
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