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Abstract Experimental Qß-Values for Fission Products Are Presented. the Sources Were Produced As Mass Separated Fission Product

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Abstract Experimental Qß-Values for Fission Products Are Presented. the Sources Were Produced As Mass Separated Fission Product NUCLEAR Qß-VALUES FOR FISSION PRODUCTS. A COMPARISON WITH MASS FORMULA PREDICTIONS K. Aleklett, P. Hoff, E. Lund and G. Rudstam The Studsvik Science Research Laboratory S-611 82, Nyköping, Sweden Abstract 2. Experimental technique Experimental Qß-values for fission The OSIRIS on-line-isotope-separator, products are presented. The sources were described in detail in ref, has been produced as mass separated fission products used for the 3y-determinations. With a at the OSIRIS on-line isotope separator normal ion source the elements zinc, Recently determined Qo-values for 79,81ca, gallium, germanium, bromine, krypton, ru• P 79,81,8¿Ge/ 89,90Br, 116-121Ag, 119-121Cd bidium, strontium, silver, cadmium, in• and 139j are,together with 40 earlier mea• dium, tin, antimony, tellurium, iodine, sured values, compared with mass formula xenon, cesium, and barium can be processed predictions. When exposing the target to carbon tetra- fluorid molecular ions are obtained. The 1. Introduction efficiency of the separation of some of the above-mentioned elements is improved, The shape of the nuclear mass surface and a number of other elements appear as far out on the neutron-rich side of 3-sta- fluorides, e.g. yttrium, zirconium and bility is of considerable interest both the lanthanides. The addition of alumi• from the nuclear structure point of view nium vapour to the target produces qood and because of important applications e.g. yields of the ions AlBr+ and All" 2>". theories about the r-process in nucleo• The latter method has been used for the synthesis. Indirect mass determinations Qß-determinations of 89,90Br3) and 139jm have formed part of the experimental It is advantageous since these nuclides programme at the OSIRIS facility for many then appear at mass numbers 116, 117, and years. The mass is deduced from the total 167, respectively. Around mass 116 3-decay energy which is experimentally ob• the situation for Qß-determinations of the tained by measuring 3y-coincidences and bromine isotopes is more favourable than summing the energy of the 3-branches and around A = 89 because of the absence of the energy of the states depopulated by rubidium. The large difference in the the y-rays. The mass of the nuclide is fission yield between the bromine isotopes then determined by adding Qß-values to and the isotopes of silver and cadmium known daughter masses. This method re• makes the presence of the latter activi• quires good knowledge of the level struc• ties little disturbing. At mass 167 ture. In cases where such information is the measurement is undisturbed by con• lacking it is only possible to give a taminating activities. lower limit for the Qß-value. The 3y-coincidence spectrometer used The accuracy of the Qß-determinations in the experiments consists of a system is typically 2-3 % and in some cases of Si (Li)-detectors for 3-detection and, better than 1 %. This accuracy is suffi• for the y-registration, either two Nal- cient for tests of mass formulas. detectors or two 80 cm3 Ge(Li)-detectors coupled in a multiplexing mode. With Very far out on the neutron-rich side the tape system used for collecting of stability the nuclides are delayed samples it is possible to perform accurate neutron precursors. This provides another Qß-determinations for nuclides with half- possibility to get a lower limit for the lives down to 0.5 s. Details about the Qß-value of the precursor by adding the experimental procedure and the calibra• neutron separation energy of the emitter tion procedure are given in refs.^'5)^ to the sum of the "beta branch" energy in The time-resolution for the system is of coincidence with a y-transition in the the order of 15-20 ns yielding a negli• final nucleus. gible accidental coincidence rate. The y- gates contained contribution from the Until now, the technique for direct Compton background, which was corrected mass measurements is only elaborated for for by subtracting background spectra ob• few elements. Our systematic study spans tained by setting gates in the neighbour• over many elements covering around 55 hood of the chosen y-lines. fission products in the mass regions A - 75 - 90 and A = 116 - 139, including 3. Experimental results and compa• isotopes of zinc, gallium, germanium, ar• rison with mass formula predic• senic, bromine, silver, indium, tin, an• tions timony, tellurium, and iodine. Total 3-decay energies have recently Although total 3-decay energies are been determined at OSIRIS for the nuclides 6 sometimes difficult to determine with high 79-81Ga/ 79,81,82Ge ), 89,90Br3), 116-121Agf accuracy for nuclides far from stability, 119,121cd7) and 139i. some of these a systematic mapping of the mass surface nuclides have been investigated before will always give important information at this laboratory, but the accuracy is about its structure and will, in addition, now improved. The resulting Qß-values and serve as a useful tool for testing mass those of about 40 cases studied earlier8-ll) formula predictions. have been compared with a number of - 124 - predictions from current mass formulas. mass relations. The comparison is given We have chosen for this comparison the in Table 1. The differences between pre• formulas of Myers 12a)f Seeger- dicted and experimentally determined Qß- Howardl2t)) , and Möller-Nix^3) represen• values for two types of theoretical in• ting liquid drop model formulas, Liran- vestigations are shown in Fig. 1 for the Zeldes1^0) representing a semi-empirical liquid drop model formula of Myers and in shell model formula and Jäneckel2d) and Fig. 2 for the shell model calculation Comay-Kelson12e) representing empirical by Liran and Zeldes. Table 1 Experimental Q -values and differences between predicted and experimental Q -values pred. - Qf exp. 12a) 12b) 13) 12c) T12d) 12e) Nuclide Exp.Q rvalue M S-H M-N L-Z C-K 75 Zn 5.62±0.20 £ 0.85 <; -0.29 * 0.05 £-0.02 ¿0.29 ¿0.15 76 Zn 3.98±0.12 -1.00 -0.93 -0.72 -0.06 -0.17 -0.34 77 Zn 6.91±0.22 -0.76 -0.27 0.23 0.32 0.30 0.18 78 Zn 6.01±0.18 -1.64 -1.43 -0.59 -0.20 -0.65 -0.89 76 Ga 6.77±0.15 -0.74 0.17 0.46 0.15 0.09 -0.04 77 Ga 5.34±0.06 -1.07 -0.63 -0.38 -0.32 -0.58 -0.72 78 Ga 8.14+0.15 -0.72 0.12 0.44 0.13 0.03 -0.14 79 Ga 6.77±0.08 -1.12 •0.51 -0.15 -0.11 -0.45 -0.75 80 Ga 10.0 ±0.3 -1.3 -0.5 0.2 -0.4 -0.3 -0.8 81 Ga 8.32±0.15 -1.35 -0.45 -0.30 0.22 -0.38 -0.96 79 Ge 4.1H0.10 -0.33 -0.06 -0.32 0.35 -0.05 -0.04 80 Ge 2.6410.07 -0.61 -0.56 -0.08 0.32 -0.37 -0.61 81 Ge 6.2310.12 -1.09 -1.00 -0.21 -0.13 -0.61 -0.94 82 Ge 4.7010.14 -1.30 -1.08 -0.88 0.03 -0.81 -1.34 80 As 5.3710.12 -0.30 -0.34 0.62 0.27 0.26 0.32 81 As 3.7610.08 -0.44 0.08 0.50 0.16 0.03 0.01 83 As 5.4610.22 -0.80 0.05 0.45 0.16 -0.05 -0.40 85 Br 2.8710.02 -0.45 -0.14 0.31 -0.02 0.14 -0.07 86 Br 7.6210.06 0.10 -0.08 0.91 0.09 0.18 -0.04 87 Br 6.8310.12 -0.86 -0.40 0.54 0.08 0.06 -0.23 88 Br 8.9710.12 -0.04 0.21 1.47 0.68 0.23 -0.10 89 Br 8.1410.14 -0.94 -0.30 0.05 0.44 0.03 -0.24 90 Br 9.8 10.4 0.3 0.9 1.59 1.5 0.8 0.80 116 *) Ag 6.2 10.2 -0.7 -0.2 -0.4 0.0 -0.2 117 Ag 4.1710.05 -0.21 -0.15 -0.31 0.10 -0 .11 118 Ag 7.1310.10 -0.64 -0.06 -0.49 0.15 -0.17 119 Ag 5.3710.04 -0.45 -0.28 -0.61 -0.05 -0.37 120 Ag 8.2110.10 -0.79 -0.7 0.35 -0.77 0.08 -0.18 121 Ag 6.3910.15 -0.54 0.26 -0.74 -0.10 -0.27 119 Cd 3.7910.06 -0.23 0.82 -0.10 -0.15 -0.25 121 Cd 4.9610.09 -0.45 0.2 0.72 -0.29 -0.26 -0.49 *) Preliminary result - 125 - Table 1 cont. Q3, pred. - Q3, exp. ,12a) 12b) -Tcl2e) Nuclide Exp.Q -value S-H M-N13) L-Zl2c) Jl2d) C-K 120 In 5.30d 0.17 0.61 -0.47 0.52 0.04 0.07 0.07 121 In 3.41H 0.05 0.24 -0.58 0.27 0.21 -0.02 -0.02 122 In 6.51d 0.23 0.84 -0.60 0.40 -0.31 -0.16 -0.16 123 In 4.44H 0.06 0.33 -0.41 0.37 0.16 -0.07 -0.07 124 In 7.18d 0.05 0.44 -0.29 0.68 0.04 0.01 0.01 125 In 5.48d 0.08 0.48 -0.35 0.22 0.01 -0.20 -0.20 126 In 8.21d 0.08 0.75 -0.24 0.53 -0.30 -0.25 -0.25 127 In 6.49H 0.07 0.56 -0.24 0.19 -0.20 -0.
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