Discovery of isotopes of the transuranium elements with 93≤Z≤98

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Abstract

One hundred and five isotopes of the transuranium elements neptunium, plutonium, americium, curium, berkelium, and californium have been observed so far; the discovery of these isotopes is described. For each isotope a brief summary of the first refereed publication, including the production and identification method, is presented.

Introduction

The discovery of neptunium, plutonium, americium, curium, berkelium, and californium isotopes is described as part of the series summarizing the discovery of isotopes, beginning with the cerium isotopes in 2009 [1]. 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 described. When appropriate, references to conference proceedings, internal reports, and theses are included. When a discovery includes a half-life measurement, the measured value is compared to the currently adopted value taken from the NUBASE evaluation [2] which is based on the ENSDF database [3]. In cases where the reported half-life 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. All reported half-lives inconsistent with the presently adopted half-life for the ground state were compared to isomer half-lives and accepted as discoveries if appropriate following the criterium described above.

The first criterium is not clear cut and in many instances debatable. Within the scope of the present project it is not possible to scrutinize each paper for the accuracy of the experimental data as is done for the discovery of elements [4]. In some cases an initial tentative assignment is not specifically confirmed in later papers and the first assignment is tacitly accepted by the community. The readers are encouraged to contact the authors if they disagree with an assignment because they are aware of an earlier paper or if they found evidence that the data of the chosen paper were incorrect.

The discovery of several isotopes has only been reported in conference proceedings which are not accepted according to the second criterium. Examples from fragmentation experiments highlighting why publications in conference proceedings should not be considered are 118Tc and 120Ru, which had been reported as being discovered in a conference proceedings [5] but not in the subsequent refereed publication [6].

In contrast to the criteria for the discovery of an element [7], [8], [4], the criteria for the discovery or even the existence of an isotope are not well defined (see for example the discussion in Ref. [9]). Therefore it is possible, for example in the cases of plutonium and californium, that the discovery of an element does not necessarily coincide with the first discovery of a specific isotope.

The initial literature search was performed using the databases ENSDF [3] and NSR [10] of the National Nuclear Data Center at Brookhaven National Laboratory. These databases are complete and reliable back to the early 1960’s. For earlier references, several editions of the Table of Isotopes were used [11], [12], [13], [14]. Additional excellent resources were the preface of the National Nuclear Energy Series volume containing the Transuranium research papers [15] and the book “The elements beyond Uranium” by Seaborg and Loveland [8].

Section snippets

Discovery of 225–244Np

The element neptunium was discovered by McMillan and Abelson in 1940 [16]. A previous claim of natural occurring element 93 in 1934, named bohemium [17], was very quickly discredited [18]. Also in 1934 Fermi et al. reported the “Possible production of elements of atomic number higher than 92” in neutron bombardment of uranium [19]. In his Nobel lecture he named the element with Z=93 ausenium but in the write-up of the lecture he added a footnote: “The discovery by Hahn and Strassmann of barium

Discovery of 228–247Pu

The element plutonium was discovered by Seaborg et al. in December 1940 and a report was submitted for publication in Physical Review on January 28, 1941. However, it was only published in 1946 as explained in a footnote of the paper: “This letter was received for publication on the date indicated but was voluntarily withheld from publication until the end of the war” [37]. This is an example where the element discovery differs from the discovery of a specific isotopes because no mass

Discovery of 232–247Am

The element americium was discovered in 1944, announced in 1945 [62], and first reported in a publication in 1949 by Seaborg et al. as part of the Plutonium Project Record identifying the isotopes 239Am, 240Am, and 241Am [54]. The name americium was officially accepted at the 15th IUPAC conference in Amsterdam in 1949 [22], [23].

Sixteen americium isotopes from A=232 to 247 have been discovered so far. According to the HFB-14 model [24] about 80 additional americium isotopes could exist. Fig. 3

Discovery of 237–251Cm

The element curium was discovered in 1944, announced in 1945 [62], and first described in a publication in 1949 by Seaborg et al. as part of the Plutonium Project Record identifying the isotopes 240Cm and 242Cm [78]. The name curium was officially accepted at the 15th IUPAC conference in Amsterdam in 1949 [22], [23].

Fourteen curium isotopes from A=237 to 251 have been discovered so far. According to the HFB-14 model [24] about 85 additional curium isotopes could exist. Fig. 4 summarizes the

Discovery of 238–251Bk

The discovery of the element berkelium was published in 1950 by Thompson et al. identifying 243Bk [99]. The authors of this first paper suggested the name berkelium with the symbol Bk: “It is suggested that element 97 be given the name berkelium (symbol Bk), after the city of Berkeley, in a manner similar to that used in naming its chemical homologue terbium (atomic number 65) whose name was derived from the town of Ytterby, Sweden, where the rare earth minerals were first found”.

Thirteen

Discovery of 237–256Cf

The discovery of the element californium was published in 1950 by Thompson et al. [116]. The authors assigned a 45 min half-life tentatively to 244Cf, however, this assignment was later changed to 245Cf [117]. Thus, the original paper by Thompson et al. constitutes the discovery of the element californium but not a californium isotope. The authors of this first paper suggested the name californium with the symbol Cf: “It is suggested that element 98 be given the name californium (symbol Cf)

Summary

The discoveries of the known transuranium isotopes neptunium, plutonium, americium, curium, berkelium, and californium have been compiled and the methods of their production described. The identification of these isotopes was relatively straightforward. The half-lives of 228, 234Np, 232, 234Am, and 241Cm were initially reported with no or only uncertain mass assignments and the first half-life reported for 243Cm was incorrect. In addition, an α-decay energy originally assigned to 241Cf was

Acknowledgments

This work was supported by the National Science Foundation under grants No. PHY06-06007 (NSCL) and PHY10-62410 (REU).

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