ATLAS OF NUCLEAR ISOMERS Ashok Kumar Jain, 1, ∗ Bhoomika Maheshwari, 1 Swati Garg, 1 Monika Patial, 1 and Balraj Singh 2 1 Department of Physics, Indian Institute of Technology, Roorkee-247667, India 2 Department of Physics and Astronomy, McMaster University, Hamilton, Ontario-L8S 4M1, Canada We present an atlas of nuclear isomers containing the experimental data for the isomers with a half-life ≥ 10 ns together with their various properties such as excitation-energy, half-life, decay mode(s), spin-parity, energies and multipolarities of emitted gamma transitions, etc. The ENSDF database complemented by the XUNDL database has been extensively used in extracting the relevant data. Recent literature from primary nuclear physics journals, and the NSR bibliographic database have been searched to ensure that the compiled data Table is as complete and current as possible. The data from NUBASE-12 have also been checked for completeness, but as far as possible original references have been cited. Many interesting systematic features of nuclear isomers emerge, some of them new; these are discussed and presented in various graphs and figures. The cutoff date for the extraction of data from the literature is August 15, 2015. I. INTRODUCTION The nuclear isomers are excited metastable states of nuclei, which experience a hindrance in their decays. Nu- clear isomers have been the focus of attention since early days, and were first foreseen by Soddy in 1917 [1], when he remarked, “We can have isotopes with the identity of atomic weight, as well as of chemical character, which are different in their stabilities and mode of breaking up.” Otto Hahn is generally credited with the first experimen- tal observation of isomers in Uranium salts [2]. But, as pointed out by Walker and Carroll [3], the first such ob- servations were in all probability made by Kurtchatov et al of Soviet Union [4] and, Szilard and Chalmers of Britain [5] in the Br and In isotopes. An interpretation of isomers in terms of hindered gamma decay was pro- vided by Weizsacker in 1936 [6]. A correlation of the existence of isomers with the shell structure and levels having considerably different angular momenta was first discussed by Feenberg [7], Feenberg and Hammack [8] and Nordheim [9]. Goldhaber and Hill [10] presented the very first collection of isomers and their detailed discus- sion in terms of the shell structure. Feenberg [11] later on followed this up in his book by presenting the data on nearly 100 isomers and providing their interpretation in terms of the single particle shell model. On the basis of the inhibition mechanism, the isomers are generally classified into four types: the spin isomers, the K-isomers, the shape isomers and the fission iso- mers [12]. A fifth category of isomers known as the senior- ity isomers makes an appearance, mostly in semi-magic ∗ Corresponding author: ajainfph@iitr.ac.in nuclei when the configuration comprises broken pairs of particles [13]. In the past decade, there has been a rapid growth in the experimental and theoretical studies of iso- mers as they are able to provide a unique window into the nuclear structure properties in unusual situations, as well as many known applications. New isomers in different re- gions of nuclear landscape, excitation energies and spins continue to be discovered at a fast pace with the availabil- ity of several radioactive ion-beam facilities and state-of- the-art nuclear experimental techniques, and prompted by possibilities of novel practical applications [14]. It is now possible to measure a broad range of half- lives, extending from picoseconds to years with the ad- vancements in experimental techniques. As a result, the lower limit of the half-life used to define isomers has been decreasing over the years. The very first version of Nubase in 1997 [15] used the definition of an iso- mer as the excited state having a half-life greater than 1 ms; the ENSDF (Evaluated Nuclear Structure Data File) database [16] also defined an isomer likewise. Later on, Nubase 2003 [17] lowered the limit to 100 ns. The Nubase 2012 [18] has maintained the same limit. Many authors in recent works have started to label states with a half-life of several picoseconds also as iso- mers. However, the general trend in the literature has been to consider a half-life of 1 ns or greater as an iso- mer. We note that there are at least 830 cases having half-life between 1 to 10 ns; or about 1150 cases between 0.5 to 10 ns. In the current compilation, we have chosen to keep the limit as 10 ns or greater. Accordingly, the “Atlas of Nuclear Isomers” lists about 2469 isomers with a lower limit of half-life of 10 ns. The number of odd-odd isomers is almost 804 which is nearly double the number of even-even isomers which is almost Available online at www.sciencedirect.com Nuclear Data Sheets 128 (2015) 1–130 0090-3752/© 2015 Elsevier Inc. All rights reserved. www.elsevier.com/locate/nds http://dx.doi.org/10.1016/j.nds.2015.08.001