{'fraction_non_alphanumeric': 0.03885342239885004, 'fraction_numerical': 0.05031074282332051, 'mean_word_length': 4.426473961917871, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 2, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Discovery of the Barium Isotopes

Thirty-eight barium isotopes have so far been observed; the discovery of these isotopes is discussed. For each isotope a brief summary of the first refereed publication, including the production and identification method, is presented.


INTRODUCTION
The tenth paper in the series of the discovery of isotopes, the discovery of the barium isotopes is discussed. Previously, the discoveries of cerium [1], arsenic [2], gold [3], tungsten [4], krypton [5], einsteinium [6], iron [7], vanadium [8], and silver [9] isotopes were discussed. The purpose of this series is to document and summarize the discovery of the isotopes. Guidelines for assigning credit for discovery are (1) clear identification, either through decay-curves and relationships to other known isotopes, particle or γ-ray spectra, or unique mass and Z-identification, and (2) publication of the discovery in a refereed journal. The authors and year of the first publication, the laboratory where the isotopes were produced as well as the production and identification methods are discussed. When appropriate, references to conference proceedings, internal reports, and theses are included. When a discovery included a half-life measurement the measured value is compared to the currently adopted value taken from the NUBASE evaluation [10] which is based on the ENSDF database [11]. In cases where the reported halflife differed significantly from the adopted half-life (up to approximately a factor of two), we searched the subsequent literature for indications that the measurement was erroneous. If that was not the case we credited the authors with the discovery in spite of the inaccurate half-life.

DISCOVERY OF 114−151 BA
Thirty-eight barium isotopes from A = 114 − 151 have been discovered so far; these include seven stable, 18 proton-rich and 13 neutron-rich isotopes. According to the HFB-14 model [12], 181 Ba should be the last particle-stable odd-even neutron-rich nucleus, with the even-even barium isotopes reaching up to 188 Ba. Along the proton dripline two more isotopes are predicted to be stable and it is estimated that five additional nuclei beyond the proton dripline could live long enough to be observed [13]. Thus, about 42 isotopes have yet to be discovered and almost 50% of all possible barium isotopes have been produced and identified so far.

Mass Number (A)
Year Discovered Undiscovered, predicted to be bound Undiscovered, unbound with lifetime > 10 s -9 FIG. A. Barium isotopes as a function of time when they were discovered. The different production methods are indicated. The solid black squares on the right hand side of the plot are isotopes predicted to be bound by the HFB-14 model. On the proton-rich side the light blue squares correspond to unbound isotopes predicted to have lifetimes larger than ∼ 10 −9 s. Figure A summarizes the year of first discovery for all barium isotopes identified by the method of discovery. The range of isotopes predicted to exist is indicated on the right side of the figure. The radioactive barium isotopes were produced using heavy-ion fusion evaporation (FE), light-particle reactions (LP), neutron-capture reactions (NC), spontaneous fission (SF), neutron induced fission (NF), charged-particle induced fission (CPF), and projectile fragmentation or fission (PF). The stable isotopes were identified using mass spectroscopy (MS). Heavy ions are all nuclei with an atomic mass larger than A = 4 [14]. Light particles also include neutrons produced by accelerators. In the following paragraphs, the discovery of each barium isotope is discussed in detail. 114

Ba
In 1995 Guglielmetti et al. announced in Identification of the new isotope 114 Ba and search for its α and cluster radioactivity the discovery of 114 Ba [15]. 114 Ba was produced at the Gesellschaft für Schwerionenforschung Unilac via the fusion evaporation reaction 58 Ni( 58 Ni,2n) 114 Ba at 4.2 MeV/u and identified using an on-line mass separator. "With ∆E-E telescopes we measured the total (β -decay) half-life to be T β = 0.43 + 0.30 −0.15 s and the partial α-decay half-life to be T α ≥ 1.2 × 10 2 s (1 MeV le E α ≤ 4 MeV) for 114 Ba." This half-life agrees with the currently accepted value of 0.53(23) s.

115,116 Ba
In the paper Decay studies of the neutron-deficient isotopes 114−118 Ba Janas et al. reported the first observation of 115 Ba and 116 Ba in 1997 [16]. A 4.9 MeV/u 58 Ni beam was accelerated by the linear accelerator UNILAC at GSI and bombarded enriched 58 Ni and 60 Ni targets. 115 Ba and 116 Ba were identified by measuring the energy and time of β -delayed protons following on-line mass separation. "The least-square fit yielded T 1/2 = 0.45 ± 0.05 s for the decay half-life of 115 Ba and a lower limit of 15% for b β p ." In addition, β -delayed X-rays were detected for 116 Ba. "The time characteristics of the Cs KX-rays intensity, analyzed under the assumption of a single decay component, yielded T 1/2 = 1.3±0.2 s for the half-life of 116 Ba." These observed half-lives are currently the only measured values for 115 Ba and 116 Ba. The same group had previously mentioned the observation of these isotopes in a conference proceeding [17]. 117

Ba
Bogdanov et al. reported the discovery of 117 Ba in 1977 in their article New Neutron-Deficient Isotopes of Barium and Rare-Earth Elements [18]. An enriched 92 Mo target was bombarded with an 180-190 MeV sulfur beam produced by the JINR Laboratory of Nuclear Reactions U-300 Heavy Ion Cyclotron and 117 Ba was produced in the fusion evaporation reaction 92 Mo( 32 S,2p5n). The isotope was separated with the BEMS-2 on-line ion source and identified by its delayed proton emission. "As the proton emission of the A = 117 isobar, it is unambiguously related to 117 Ba, since the reaction leading to the formation of 117 Cs was energetically impossible in our experiments." The measured half-life of 1.9(2) s is consistent with the accepted value of 1.75(7) s. 118

Ba
In the 1997 paper Decay studies of the neutron-deficient isotopes 114−118 Ba Janas et al. reported the first observation of 118 Ba [16]. A 4.9 MeV/u 58 Ni beam was accelerated by the linear accelerator UNILAC at GSI and bombarded enriched 58 Ni and 60 Ni targets on a 63 Cu backing. 118 Ba was produced in the fusion-evaporation reaction 63 Cu( 58 Ni,1p2n) and identified by measuring β -delayed X-rays and γ-rays following on-line mass separation. "From the time characteristics of the cesium KX-rays intensity the 118 Ba half-life of T 1/2 = 5.2 ± 0.2 s was determined under the assumption of a single decay component." This observed half-life is currently the only measured value for 118 Ba. The same group had previously mentioned the observation of this isotope in a conference proceeding [17]. 119

Ba
In 1974 Bogdanov et al. observed 119 Ba, which they reported in the article Delayed-proton emitter 119 Ba [19]. An enriched 106 Cd target was bombarded with an 85 MeV oxygen beam produced by JINR Laboratory of Nuclear Reactions U-300 Heavy Ion Cyclotron. 119 Ba was produced in the fusionevaporation reaction 106 Cd( 16 O,3n). The delayed proton spectrum was measured with a telescope of a thin proportional-counter and a surface-barrier detector. "The observed activity was due mainly to two radiators with half-lives T 1/2 = 5.0 ±0.6 sec, with an excitation function peaking at E16 O = 85 MeV, and T 1/2 = 15.0±1.0 sec with its maximum yield at a higher energy... Thus, the most probable identification of the 5-second emitter is 119 Ba." The extracted half-life agrees with the currently accepted value of 5.4(3) s. 120

Ba
In 1974 Conrad et al. reported the observation of 120 Ba in their article Quasi-Rotational Bands in Neutron Deficient Doubly Even Ba Isotopes [20]. 120 Ba was produced in the fusion evaporation reaction 106 Cd( 16 O,2n) by bombarding cadmium with a 66 MeV oxygen beam provided by the MP Tandem of the Max-Planck-Institut für Kernphysik in Heidelberg, Germany. The isotope was identified by chargedparticle-, neutron-, and gamma-gamma coincidence measurements: "To identify 120 Ba, which has 18 neutrons less than the most abundant barium isotope, neutron-gamma coincidences had to be applied in addition to charged particle and gamma-gamma coincidence measurements. The upper limit for the lifetime of the ground state of 120 Ba is 90 sec." The first three γ-transitions in 120 Ba were measured. The upper limit of 90 seconds is consistent with the accepted half-life value of 24(2) s. 121

Ba
In the 1975 paper New Delayed-Proton Emitters 119 Ba, 121 Ba and 116 Cs Bogdanov et al. reported the discovery of 121 Ba [21]. The U-300 cyclotron of the Nuclear Reactions Laboratory at Dubna accelerated a 32 S beam to a maximum energy of 190 MeV. "The isotope 121 Ba was observed in bombardment of niobium with sulfur ions in the reaction 93 Nb( 32 S,p3n) 121 Ba and with substantially greater yield in the reaction 92 Mo( 32 S,2pn). The half-life is 29.7±1.5 sec." 121 Ba was identified with the BEMS-2 mass separator. The observed half-life is currently the only available measurement. A month later the authors submitted the results to a different journal where they were published first [22]. 122

Ba
In 1974 Conrad et al. reported the observation of 122 Ba in their article Quasi-Rotational Bands in Neutron Deficient Doubly Even Ba Isotopes [20]. 122 Ba was produced in the fusion evaporation reaction 108 Cd( 16 O,2n) by bombarding cadmium with a 66 MeV oxygen beam provided by the MP Tandem of the Max-Planck-Institut für Kernphysik in Heidelberg, Germany. The isotope was identified by gammagamma coincidence measurements: "Up to now a level scheme of 122 Ba has not been published. For the lifetime of the ground state a value between 2.5 and 5 sec has been suggested [23]. From our data the partial level scheme shown in fig. 4 was obtained." The first six γ-transitions of the ground state band in 122 Ba were measured. 123

Ba
Preiss and Strudler reported discovery of 123 Ba in 1962 in their article New Neutron Deficient Barium Isotopes [24]. 123 123 Ba and natural Sn( 12 C,xn) 123 Ba; the beams were produced by the Yale University Heavy Ion Accelerator and had a maximum energy of 10.5 MeV/nucleon. 123 Ba was identified measuring characteristic X-ray spectra following chemical separation. "Mass assignments for the new Ba activities were based on the parent daughter genetics using Cs half-lives and γ-ray energies previously reported and/or found in the present study. The proposed half-lives and mass assignments are: 123 Ba, 2±0.5 min; 125 Ba, 6.5±0.5 min; and 127 Ba, 10.0±0.5 min." The observed half-life for 123 Ba is consistent with the currently accepted value of 2.7(4) m. 124 Ba 124 Ba was first observed in 1967 by Clarkson et al. as reported in Collective Excitations in Neutron-Deficient Barium, Xenon, and Cerium Isotopes [25]. 124 Ba was produced by the reactions 115 In( 14 N,5n) at 84 MeV and 116 Sn( 12 C,4n) at 80 MeV where the ions were accelerated with the Berkeley heavyion linear accelerator (HILAC). Gamma-ray spectra were measured with a lithium-drifted germanium counter. "Since 124 Ba was produced by two reactions with different targets and projectiles, which both give the same transitions, this mass assignment is likewise considered to be quite certain." The first two transitions of the ground-state band were correctly identified.  [26]. 125 Ba was produced in the fusionevaporation reaction 115 In( 14 N,4n) at Yale University. The isotope was chemically separated and βγand X-ray spectra were measured. "Previously unobserved and unassigned γ rays resulting from the decay of Ba 125 were detected at 56±3, 76±2, 84±2, and 141±2 keV, decaying with a composite halflife of 3.0±0.5 min." This half-life agrees with the currently accepted value of 3.5(4) m. D'Auria et al. interpreted this state as the high-spin ground state in addition to the presence of an 8(1) m isomeric excited state. The existence of the isomeric state has not been confirmed. Preiss and Strudler had previously reported a half-life of 6.5(5) m for 125 Ba [24]. However, because this value is almost a factor of two larger than the accepted value and closer to the claimed isomeric state, it is likely that the state had been misidentified. 126

Ba
In the 1954 article New Chain Barium-126-Cesium-126 Kalkstein et al. announced the discovery of 126 Ba [27]. Indium oxide was bombarded with a nitrogen beam produced by the Berkeley Crocker 60 inch cyclotron with a maximum energy of 140 MeV. 126 Ba was produced in the fusion-evaporation reaction 115 In( 14 N,3n) and identified using a scintillation spectrometer, a scintillation coincidence spectrometer, and a time-of-flight mass spectrograph. Element assignments and genetic relations have been verified chemically, and the mass number assigned with the isotope separator. " 126 Ba decays principally by orbital electron capture with a half-life of 96.5±2.0 minutes." This half-life is consistent with the currently accepted value of 100(2) m. 127

Ba
In 1952 Linder and Osborne reported the discovery of 127 Ba in The Nuclides Ba 127 , Ba 128 and Cs 128 [28]. A cesium nitrate target was bombarded with 190 MeV deuterons at Livermore. 127 Ba was chemically separated and its activity measured with an end-window argon-alcohol-filled counter. "A barium isotope of 12-minute half-life was found whose radiations were not directly characterized. However, positron emission is probable since electromagnetic radiation seemed to comprise no more than five percent of the total activity detectable on an end-window counter. By four rapid chemical separations made at ten-minute intervals a cesium activity was obtained from the barium whose half-life and radiation characteristics agree with those reported for Cs 127 . Furthermore, the yield of this nuclide diminished roughly by a factor of two in each of the four successive separations. The 12-minute barium activity is thus Ba 127 ." The observed half-life agrees with the currently adopted value of 12.7(4) m.

Ba
Fink and Templeton identified 128 Ba in 1950 as described in Radioactive Isotopes of Barium [29]. Cesium chloride was bombarded with 85 MeV protons in the 184 inch Berkeley cyclotron. The induced barium activities were chemically separated and the half-life was measured with a Geiger counter: "The 2.4-day period is not 127 Ba, otherwise it would produce 5.5-hour 127 Cs as a daughter. The most probable assignment is 128 Ba from the p,6n reaction, but this assignment lacks direct proof." The measured halflife of 2.4 d agrees with the presently accepted value of 2.43(5) d. Thomas and Wiig had measured a 2.4(1) d half-life in barium but they were only able to determine that the mass number was smaller than 129 [30]. 129

Ba
In 1950 the discovery of 129 Ba was reported first by Thomas and Wiig in On Neutron Deficient Isotopes of Barium [30]. 250-MeV protons accelerated by the Rochester 130-inch cyclotron were used to bombard spectroscopically pure cesium chloride. The half-life of the chemically separated barium fraction was measured by deflecting positrons into a counter tube with a permanent magnet. "Parentdaughter separations performed more than 24 hours after the bombardment failed to show any 129 Cs activity whereas earlier milkings did show the activity. This led to the conclusion that the 1.8-hour barium is 129 Ba." The measured half-life of 1.8 (2) h is close to the currently accepted value of 2.23 (11) h. Less than a month later Fink and Templeton submitted their half-life measurement of 2.0(1) h for 129 Ba [29] which was published in the same issue of Physical Review immediately following the article by Thomas and Wiig. 130 Ba In the 1936 paper The Isotopic Constitution of Barium and Cerium Dempster reported the first observation of 130 Ba [31]. The mass spectra were measured at the Ryerson Physical Laboratory at the University of Chicago: "I have photographed several mass spectra of the barium ions formed in a high frequency spark between two barium electrodes, which show two still fainter isotopes at 130 and 132." 131 Ba 131 Ba was first observed by Katcoff in 1945 and reported in Plutonium Project Records [32]. The results were subsequently published in a refereed journal in the article New Barium and Cesium Isotopes: 12.0d Ba 131 , 10.2 Cs 131 , and Long-Lived Ba 133 [33]. 131 [36]. 133 Ba was produced by bombarding cesium with 9.5 MeV deuterons at the University of Michigan. Following chemical separation the isotope was identified with a magnetic beta-spectrometer and by absorption measurements. The observed 40.0(5) h half-life was identified as an excited state and could be ascribed to either 133 Ba or 134 Ba. "However, it is known that the Rochester group have found this same activity by bombarding cesium with protons. In their bombardment it is possible to produce barium by the (P,N) and (P,γ) reactions. Although the latter process it known to occur, the former is much more probable and favors the assignment to Ba 133 ." The observed half-life of this isomeric state agrees with the present value of 38.9(1) h. 134

Ba
In 1936 Blewett and Sampson reported the discovery of 134 Ba in Isotopic Constitution of Strontium, Barium, and Indium [37]. The mass spectrographic study of barium was performed at Princeton University by heating barium oxide from a tungsten filament. "The curves for barium showed a peak at mass 134 making up 1.8 percent of the total emission...[which] lead us to believe that this is due to a new isotope of barium." 135−137 Ba 135 Ba, 136 Ba, and 137 Ba were discovered by Aston as reported in his 1932 article The Isotopic Constitution and Atomic Weights of Caesium, Strontium, Lithium, Rubidium, Barium, Scandium and Thallium [38]. The mass spectra were measured at the Cavendish Laboratory in Cambridge, UK: "The production of sufficiently intense rays from barium salts is a matter of great difficulty, but after many  [40]. Neutrons with energies up to 20 MeV, produced by bombarding lithium with 6.3 MeV deuterons at the University of Michigan, were used to irradiate many stable elements. In the summary table the observed half-life of 85 m was assigned to 139 Ba. This assignment is supported by a previously published contribution to a conference: "Barium becomes strongly radioactive with a half-life period of 85.6 min. The β -particles have the negative sign. Chemical analysis shows that the activity is most probably due to Ba 139 ." [41]. This half-life agrees with the currently accepted value of 83.1(3) m. Amaldi et al. had reported a 80 m period in barium in 1935; however, no mass assignment was made [42]. 140

Ba
In the 1939 paper Nachweis der Entstehung aktiver Bariumisotope aus Uran und Thorium durch Neutronenbestrahlung; Nachweis weiterer aktiver Bruchstücke bei der Uranspaltung Hahn and Strassmann identified 140 Ba for the first time at Berlin-Dahlem in Germany [43]. 140 Ba was produced by irradiating Uranium with neutrons from a Ra-Be-neutron source. Decay curves were measured following chemical separation. A previously reported 300 h activity originally labeled as "Ra IV" [44,45] now identified as the fission product "Ba IV" was again observed. Based on the measured half-life of the daughter activity it was tentatively assigned to 140 Ba: "Was die anderen Barium isotope aus dem Uran anbelangt, so läßt sich für das Ba IV vielleicht die Hypothese machen, daß es die Muttersubstanz des in der Literatur beschriebenen Radiolanthans von 31-46 Stunden Halbwertszeit mit dem vermutlichen Atomgewicht 140 ist." (Concerning the other from uranium produced barium isotopes, it is hypothesized that Ba IV may be the parent of the radioactive lanthanum which was reported with a half-life of 31-46 hours with the probable atomic weight of 140.) Hahn and Strassmann did not officially assign the 300 h activity to 140 Ba in subsequent papers [46,47,48,49], although they confirmed the relationship of the activity to 140 La [49]. Subsequently this assignment was specifically made by other authors [50,51] and it was generally adopted in the 1944 Table of Isotopes [52]. The final proof was given by mass-spectroscopic measurements in 1947 [53]. 141

Ba
Katcoff established the identification of 141 Ba in 1945 in the Plutonium Project Record Radiations from 3.7h La 141 [54]. Uranyl nitrate was irradiated with slow neutrons produced with the Chicago cyclotron and the A=141 mass chain of the 141 Ba-141 La-141 Ce relationship was established: "About 75 min was then allowed for 3.7h La 141 to grow into the solution from its 18m Ba 141 parent...The βdecay curve shows a long-lived component (probably the 28d Ce 141 daughter of 3.7h La 141 ) and small amounts of 30h and 1.5h components; but the 3.7h component greatly predominates." The currently accepted value for the half-life of 141 Ba is 18.27(7) m. Hahn and Strassmann had originally reported this half-life in barium for the first time [55] modifying a previous observation of a single 14 m component [43] into two components of 18 m and 6 m. Hahn and Strassmann also observed the relationship of the 18 m half-life with a 3.5 h component in lanthanum. However, no specific mass assignment was made. In another paper in the Plutonium Project Record Goldstein mentioned the established relationship of the mass chain [56] referring to the work by Ballou and Burgus which was not included in the published record [57].   [70]. 149 Ba was produced and identified at the OSIRIS isotope-separator online facility at the Studsvik Neutron Research Laboratory in Nyköping, Sweden. In the large table of delayed-neutron branching ratios and half-lives the half-life of 149 Ba is quoted as 0.324 (18) s. This value agrees with the currently adopted value of 344(7) s. Warner and Reeder had reported a half-life measurement for 149 Ba seven years earlier in a conference proceeding [71].

150−151 Ba
Bernas et al. discovered 150 Ba and 151 Ba in 1994 at GSI, Germany, as reported in Projectile Fission at Relativistic Velocities: A Novel and Powerful Source of Neutron-Rich Isotopes Well Suited for In-Flight Isotopic Separation [72]. The isotopes were produced using projectile fission of 238 U at 750 MeV/nucleon on a lead target. "Forward emitted fragments from 80 Zn up to 155 Ce were analyzed with the Fragment Separator (FRS) and unambiguously identified by their energy-loss and time-of-flight." The experiment yielded 13 individual counts of 151 Ba. As shown in Figure 3 of the article many more counts of 150 Ba were recorded though not explicitly mentioned in the text since Mach et al. had reported the discovery of 150 Ba in a conference abstract [73]. However, since this observation was never published in a refereed journal we credit Bernas et al. with the discovery of 150 Ba.

SUMMARY
The discovery of the barium isotopes has been cataloged and the methods of their discovery discussed. Many of the barium isotopes have a long and interesting history. The discoveries of 140 Ba and 141 Ba were directly linked to the discovery of fission. The half-lives of four isotopes ( 128 Ba, 139 Ba, 142 Ba, and 143 Ba) were first measured without accurate mass assignments and two measurements were initially wrong ( 125 Ba and 142 Ba). 129 Ba and 147 Ba were identified essentially simultaneously by two groups independently. It is also interesting to note that 120 Ba, 122 Ba, 124 Ba, and 146 Ba were first observed in γ-ray spectroscopy studies. Finally, the discovery of 149 Ba and 150 Ba had been reported in conference proceedings seven years prior to a publication in refereed journals. Year of discovery