Decay Data Evaluation Project (DDEP): Updated evaluation of the 133 Ba, 140 Ba, 140 La and 141 Ce decay characteristics

. Within the Decay Data Evaluation Project (DDEP) an updated comprehensive assessment has been made of the decay characteristics of 133 Ba, 140 Ba, 140 La, and 141 Ce. Experimental data published up to 2016 along with other information (new compilations, analyses and corrections) were taken into account. Newly evaluated values of the half-lives and a number of other key decay characteristics are presented in this paper for all four radionuclides.


Introduction
The Decay Data Evaluation Project (DDEP) is an international collaborative effort aimed at producing highquality comprehensive nuclear decay data evaluations for important applied radionuclides [1,2], for example, gamma ray standards for detector efficiency calibrations, medical radioisotopes, and essential dosimetry calculations. Such evaluations are based on the comprehensive assembly of all relevant published experimental and theoretical data accumulated up to the present.
The mass region 135 to 141 contains some important applications-oriented calibration standards. Both 133 Ba and 141 Ce are regularly used as detector efficiency calibrants [3], while 140 Ba, 140 La are burn-up monitors and constitute evidence of nuclear explosions. The latter are also neutron reaction residuals included in the International Reactor Dosimetry and Fusion File (IRDFF) [4].
Previous DDEP evaluations for 133 Ba, 140 Ba and 140 La decay characteristics were published in Monographie BIPM-5, 2004 [5,6]. Newly recommended half-lives and a limited number of other important decay characteristics for these radionuclides are presented in this paper. While the 2000 DDEP evaluation of 141 Ce decay was fairly recently updated by one of the authors in 2012 [8], a newly recommended half-life has arisen from the current exercise and is reported below. The comprehensive decay data evaluations and detailed comments can be found on the DDEP web site maintained by the CEA/LNE-LNHB [7].

Background and evaluation methodology
The updated evaluations were obtained using the approaches and methodology adopted by the DDEP working group [2,9]. All experimental data published up to 2016 along with new compilations, analyses and corrections were taken into account in the current evaluations. In particular, as a consequence of a major a e-mail: chechev@khlopin.ru problem discovered in the NIST ionization chamber used in their radionuclide calibration studies over many years, the NIST corrections to these half-life data in 2014 were taken into account [10].
Statistical processing of published experimental data sets has involved the development and applying of the LWEIGHT computer program, which uses the Limitation of Relative Statistical Weight method (LWM) [11] to obtain well-defined averages throughout the evaluation. The uncertainty assigned to these average values is either greater than or equal to the smallest uncertainty of the values used to calculate the average. This uncertainty of the best result ("smallest experimental uncertainty") reflects the type B standard uncertainty (systematic error of the method) and the recommended uncertainty cannot be less than this value [2,9,12].
Two important compilations that are regularly reviewed and updated lead to the need to correct some of the parameters in decay data evaluations: atomic mass evaluations (AME) [13], and calculations of theoretical internal conversion coefficients (ICCs) by means of the BrIcc computer program [14]. AME leads to updated Qvalues and relevant corrections in nuclear transition energy values, while BrIcc calculations have improved the ICCs used in the decay data evaluations.
The evaluated decay characteristics includes: half-life, decay energy, energies and probabilities of beta or electron capture transitions, energies and probabilities of gammaray transitions, internal conversion coefficients, energies and absolute emission probabilities of gamma-rays, energies and absolute emission probabilities of X-rays, and energies and absolute emission probabilities of electrons.
Newly evaluated values for all these characteristics can be found on the DDEP web site [7], as mentioned above. Our brief review of the updated evaluations focuses on half-lives, decay energies and gamma-ray energies and emission probabilities (intensities).

133 Ba
An earlier DDEP evaluation of 133 Ba decay data was undertaken by the authors of this paper in 2000 with a minor correction in 2004 [5]. The current evaluation was completed with the same month/year cut-off in the literature, and has now been placed on the DDEP web site [7]. 133 Ba decays primarily by allowed electron capture branches to the 1/2+ and 3/2+ 133 Cs levels at 437.0 and 383.8 keV. Evaluated intensities of the electron capture transitions (I (EC), %) at these levels have been obtained from balances of the gamma-ray transition intensities. Upper limits for other possible electron capture branches to the 133 Cs ground state and levels at 81.0 and 160.6 keV have been estimated from log ft systematics (Table 1).

Half-life
We updated the 2004 DDEP half-life evaluation of 133 Ba on the basis of enforced adjustment to earlier NIST and PTB measurements in previous years [10,15]

Gamma-ray transitions and emissions
ICCs adopted for gamma-ray transitions in 133 Ba decay were obtained by means of the BrIcc computer program [14] using the multipolarities and mixing ratios δ from an ENSDF evaluation [16].
The absolute gamma-ray intensities (Iγ , %) listed in Table 2 have been calculated from the relative emission intensities of each gamma-ray evaluated from experimental data sets for (number of values from 21 to 37) and a normalisation factor of 0.6205 (19) adopted from the absolute emission probability of the 356-keV γ -ray which resulted from an international intercomparison ICRM -S -6 [17].

Gamma-ray transitions and emissions
ICCs for the gamma-ray transitions in the β − decays of 140 Ba and 140 La were obtained via the BrIcc program [14] in an identical manner to the 133 Ba decay data evaluation.
Absolute gamma-ray intensities (Iγ , %) in 140 Ba and 140 La β − decays (Tables 4 and 5) have been determined from the evaluated relative gamma-ray intensities and scaling factors calculated from gamma-ray transition probability balances to the ground states of 140 La and 140 Ce, respectively (Tables 4 and 5). This particular approach proved to be highly appropriate and valid because of the negligible amount of direct β − decay to the two ground states [7]. The small differences between the 2004 and 2016 evaluations of the gamma-ray emission probabilities and  (7) 7.04 (4) 7.04 (7) 1596.203 (13) 95.428 (25) 95.40 (5) 2521.390 (14) 3.412 (24) 3.41 (5) their uncertainties are primarily caused by the adoption of more realistic ICCs in the current work as derived by means of BrIcc.

141 Ce
The initial DDEP evaluation of 141 Ce decay data was made by Schönfeld in 1999 [18] and updated in Ref. [8].

Energy conservation
The reliability of the evaluated data was checked for each nuclide by comparison of the total average emission energies E per disintegration as calculated from the  (11) current evaluated data with Q values from mass tables [13] ( Table 6). The good agreement of the E value with the Q value for each nuclide confirms the consistency of the adopted decay scheme and reliability of the evaluated emission energies and intensities [26].