Measurement of QEC values of 126La isomers

Appl. Radiat. Isot. Vol. 49, No. 7, pp. 829-834, 1998 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain P I I : S0969-8043(97)00303-5 0969-8043/98 $19.00 + 0.00

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Measurement of QEC Values of 126La Isomers Y. K O J I M A *l, M. A S A I 2, A. O S A 3, M. K O I Z U M I 3, T. S E K I N E 3, M. S H I B A T A 1, H. Y A M A M O T O l a n d K. K A W A D E l ~Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan, 2Department of Nuclear Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan and 3Department of Chemistry and Fuel Research, Japan Atomic Energy Research Institute, 1233 Watanuki-cho, Takasaki, Gunma 370-12, Japan (Received 8 May 1997; in revised form I0 June 1997)

Beta-ray endpoint energies of the neutron-deficient nuclide of ~26Lahave been measured with an HPGe detector. The ~26Laactivities were produced by the 94Mo(36Ar,3pn) reaction and mass-separated on-line. From the 13-7 coincidence measurements, QEc values of the high- and low-spin isomers of ~26Lawere determined for the first time to be 7700 ___100 keV and 7910 + 400 keV, respectively. The experimental atomic masses were compared with predicted values of mass formulas. © 1998 Elsevier Science Ltd. All rights reserved

Introduction The atomic masses are one of the most fundamental quantities because they manifest all interactions that contribute to nuclear binding. Throughout the history of nuclear physics, studies of the atomic masses have greatly contributed to our knowledge of nuclei. Today the region far from stability is open for study by the use of accelerators or reactors. Precise atomic masses of unstable nuclei provide an essential input to theoretical efforts to refine and develop models to predict nuclear masses (Haustein, 1988). The measurement of Q~ values is convenient to determine atomic masses, especially for shortlived nuclei. For high-resolution 13-ray measurements, usefulness of a small HPGe detector has been demonstrated by the present authors (Osa et al., 1993, 1996; Ikuta et al., 1995; Kojima et al., 1997) and others (Rehfield and Moore, 1978; Decker et al., 1982; Born, 1983; Greenwood and Putnam, 1993). The advantage of the use of these detectors is that an accurate energy calibration can be done with standard 7-ray sources. On the other hand, measured [3-ray spectra are distorted due to incomplete absorption of the incident particle energy. In our work for neutrondeficient Cs and Ba isotopes, the experimental response functions of an HPGe detector for monoenergetic positrons, including all the distortions, were used in unfolding 13÷-ray spectra. The accuracy of the 13+-ray maximum energy determination in this method was estimated to *To whom all correspondence should be addressed. Fax: + 81 52 789 3844.

be better than 20 keV in an energy range below 5 MeV for good statistics. For neutron-deficient lanthanides in a region of A ~ 130, systematic QEC measurements have not been performed. This is partly because of large QEc values expected for the nuclides and difficulties in obtaining reliable response functions covering these QEc values. However, we have recently obtained experimentally response functions in an energy range of 6-9 MeV (Kojima et al., 1997). We estimated the systematic error to be less than 50 keV in the 13-ray endpoint determination above 5 MeV. In the present work, the above-mentioned [3÷ray measurement system, which was combined with an isotope separator on-line (ISOL) installed at the TIARA (Takasaki Ion Accelerators for Advanced Radiation Application), was applied for determining QEC values of ~6La. In an earlier work, the isomerism in !26La was reported by Genevey et al. (1987). They observed 13-feedings to several excited states in 126Ba and constructed a decay scheme. To check their decay scheme, 7-7 coincidence measurements were also performed. The atomic mass of ~2~La derived from the QEc value was compared with the predicted values of theoretical mass formulas.

Experiment Source preparation

The t26La activities were produced by the fusion-evaporation reaction of 94Mo(36Ar, 3pn)t26La, followed by the on-line mass separation at the TIARA-ISOL facility (Sekine et al., 1994). A

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Energy (MeV) Fig. 1. Experimental response functions for monoenergetic positrons of 2.54 and 8.90 MeV. Solid lines represent calculated response functions for positrons of 2.54 + 0.03 and 8.90 ___0.18 MeV. 195 MeV 36Ar8+ beam from the A V F cyclotron was directed upon a 3.08 mg cm -2 thick molybdenum foil, enriched in 94Mo to 93.9%. The target was obtained from Euriso-top group C E A as a metallic foil. The averaged intensity of the beam on target was 0.12 particle gA (7.5 x 10 H atoms s-~). Reaction products were ionized in a thermal ion source and mass-separated as a chemical form o f LaO ÷. The mass-separated activities were implanted into an aluminum-coated Mylar tape in a tape-transport system and periodically transported to a detector station for T-ray measurements at time intervals of 100 s. Measurements

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detector (crystal size 25 m m * × 13 mm') for 13+-rays and an n-type H P G e detector for T-rays at the beam collection position. The efficiency of the T-ray detector was 28.4% relative to a 3 inch × 3 inch standard N a I detector. Beta-ray spectra were measured through a 50 gm ~ Mylar window on the tape chamber. In the 13+-ray measurements, the main amplifier was operated in a short shaping time constant of 0.5 ~ts and the counting rate was kept below 3000 counts s-~ to reduce a pulse-pileup effect. Coincidence data were recorded on a magnet-optical disk in event by event mode. A b o u t 7.2 × 10 7 events were collected during a measuring period of 57 h.

Beta-ray singles and 13-y coincidence measurements were performed with a planar type H P G e "~C

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Fig. 4. Fermi-Kurie plots of the unfolded 13+-ray spectra coincident with (a) 1227.8 keV and (b) 455.0 keV T-rays in the decay of the high-spin isomer of )26La. Fitting regions are represented by closed marks. A partial decay scheme is shown in the inset.

QEc values of ~26Laisomers

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Energy calibration of the 13÷ -ray detector extended to 8.6 MeV using a standard 7-ray source of 56Co (energy range 0.85-3.45 MeV) and prompt 7-rays in the thermal neutron capture of 35C1 (energy range 0.79-8.58 MeV). The thermal neutrons were generated by a 0.18 Ixg 252Cfneutron source (Amersham) of 950 kBq sealed within a stainless steel cylinder of 7.8 mm ~ x 10 mm' (Ikuta et al., 1992). Gamma-ray singles and 7-7 coincidence measurements were performed with n-type coaxial HPGe detectors and 8192 channel pulse height analyzers. The relative efficiency and energy resolution of the detectors were about 30%o and 1.8 keV at 1333 keV, respectively. About 5.8 x 106 coincidence events were accumulated.

Response function and data analysis Response functions of the HPGe detector for monoenergetic positrons in an energy range of 1.2-3.7 MeV were previously determined (Osa et al., 1993) with a sector type double focusing 13-ray spectrometer at the Research Reactor Institute, Kyoto University (KURRI). Monoenergetic positrons were obtained with a standard 13+-ray source of 6SGe-rSGa and the 34mC1source, which was produced by the 35C1(7, n) reaction on polyvinylidene chloride at the K U R R I electron linear accelerator (KURRI-LINAC). In another energy range above 6 MeV, the response functions were measured at the K U R R I - L I N A C (Kojima et al., 1997). A pulsed 30 MeV electron beam produced electron-positron pairs in a 2 mm thick, air-cooled tungsten target as an electron-positron converter. A subsequent positron transport line made a part of the positrons pass through with a resolution of about 1.5%. Examples of the response functions are shown in Fig. 1.

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In interpolating the response function of an energy from those experimentally obtained, it was assumed that the response function consisted of four components as shown in Fig. 2. The components of A and B correspond to the full energy peak of positrons and to the escape of bremsstrahlung photons generated in the germanium crystal, respectively. The component C corresponds to the back and side scattering of positrons which escaped from the surface of the crystal. The component D corresponds to summing of annihilation photons and a full energy absorption event of positrons. For each of the response functions obtained, the areas of the components were carefully determined. The energy dependence of the relative intensities of A, B, C and D to the total counts is shown in Fig. 3. We tried to reproduce the response function on the basis of the function form described above. The calculated response function, taking account of the energy resolution, was shown in Fig. 1 together with the experimental spectrum. While a deviation was observed in an energy range below 2 MeV due to scattering positrons, the calculated and the experimental spectrum are in general agreement. We concluded that the function form of the response function adopted in the present work was reasonable. The analysis procedure of a 13÷-ray spectrum is as follows (Osa et al., 1993). A distorted 13+-ray spectrum S(E') is calculated from the response function R(E, E') and a theoretical 13÷-ray spectrum T(E, E,,):

S(E') = c~R(E,E')T(E,Em) E

where c is a normalization factor. In the calculation, we must assume 13+-ray maximum energy E,,. Reasonable values of E,, and c are decided by comparing the calculated spectrum S(E') and the experimental one Mo(E'); We subtract the distorted components from Mo(E') and obtain the corrected 13÷-ray spectrum Mc(E'): M,.(E') =

[Mo(E')--~{R(E,E')--RA(E,E')}T(E,Em)]E R~ (E',E') - i. An endpoint value of the corrected 13÷-ray spectrum is derived from the fit of a straight line to the Fermi-Kurie plot using the maximum likelihood method for Poisson distribution (Awaya, 1979). The same procedure is repeated until the deduced endpoint energy agrees well with the assumed value.

Results and Discussion In the 13+ decay of 126La, two different half-lives and 13-feedings to the levels with spin values ranging

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from 0 to 6 were observed together with more than 100 new v-rays. The part of the decay scheme to be used in the present analysis was consistent with the results by Genevey et aL (1987). The detailed decay scheme will be given in a forthcoming paper. Relatively strong [}-feedings to the levels of 710.9 k e V (4 +) and 1938.7 keV ( 5 - ) were observed in the decay of the high-spin isomer of !26La. F e r m i - K u r i e plots of the unfolded 13÷-ray spectra to these levels are shown in Fig. 4. Endpoint energies of the [3÷-rays were found to be 5900 _ 150 keV and 4 7 8 0 _ 120 keV, respectively. After correction on energy losses o f positrons in the collection tape, the Mylar window, the air and the beryllium window of the detector using the energy loss table given by Pages

et al. (1972), the QEC value of 7700(100) keY was

obtained for the first time. In order to derive the QEc value of the low-spin isomer of '26La, the I]+-ray spectrum feeding to the 2029.7 keV 0 + level, which was identified by a 7-7 angular correlation measurement (Asai et al., 1997), was analyzed. The F e r m i - K u r i e plot of the 13÷-ray spectrum gated by 1773.8 keV v-ray is shown in Fig. 5. The 13+ -ray maximum energy and the resulting QEc value for the low-spin isomer of '26La were determined to be 4860 + 400 keV and 7910 _ 400 keV, respectively. The above result is the first experimental data on the energy difference between the two isomers of 126La. Although the experimental error of the QEc

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Fig. 6. Deviations of the experimental atomic masses and the calculated ones, inc,,c, from the evaluated values of La isotopes, move],by Audi and Wapstra (1995). The present value is represented by a closed circle. Open circles are experimental masses, compiled by Audi and Wapstra. The mass formulas are classified in four major classes of (a)-(d) (see text).

QEC values of ~26Laisomers value of the low-spin isomer is large, the high-spin isomeric state may tentatively be assigned to be the ground state. With the present QEc value and the well-evaluated mass excess of ~26Ba ( -- 82676 4- 14 keV) (Audi and Wapstra, 1995), the mass excess of - 74980 _ 100 keV was obtained for ~26La. The present mass excess is 130 keV larger than the evaluated value by Audi and Wapstra (1995). In order to check the predictive capabilities of mass formulas, deviations of the experimental masses and the theoretical ones from the evaluated values are presented in Fig. 6. In Fig. 6, mass formulas are classified into four major classes as follows: (a) a semiempirical formula by Dussel et al. (1988); (b) liquid drop type formulas by Satpathy and N a y a k (1988), Spanier and Johansson (1988) and Tachibana et al. (1988); (c) mass formulas based on the microscopic model by M r l l e r and Nix (1988), M r l l e r et al. (1995) and Aboussir et al. (1995); (d) mass formulas based on mass relations by C o m a y et al. (1988), Jhnecke and Masson (1988) and Masson and J/inecke (1988). Excellent agreement with the experimental masses is obtained for the formulas of Masson and J/inecke (1988) and especially J/inecke and Masson (1988), which is consistent with the results for neutron-deficient Cs and Ba isotopes by Osa et al. (1996).

Conclusions F r o m the 13-y coincidence measurements with the H P G e detectors, QEc values of the high- and low-spin isomers of ~26La were determined for the first time to be 7700 __+ 100 keV and 7910 + 400 keV, respectively. Experimental masses of La isotopes were compared with the predicted values of various mass formulas. The best agreement was observed for the formula of J/inecke and Masson (1988). We note that 13÷-ray endpoint measurements with an H P G e detector are an excellent method to determine atomic masses. Systematic QEc measurements in a mass region of A ~ 1 3 0 are now in progress. Acknowledgements--The authors would like to thank the TIARA cyclotron crew for providing intense and stable beams. This work was partly supported by the UniversitiesJAERI Collaborative Research Project.

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