Precision energy measurements of γ-rays occuring in the decay of 110mAg

N U C L E A R I N S T R U M E N T S AND METHODS 151 (1978)

549-553 ; O

N O R T H - H O L L A N D PUBLISHING CO.

PRECISION ENERGY MEASUREMENTS OF y-RAYS OCCURRING IN THE DECAY OF n°mAg JEAN KERN and W. SCHWITZ

Physics Department, University, CH-1700 Fribourg, Switzerland Received 31 October 1977 Energies of several transitions occurring in the decay of l l0mAg, ranging between 400 and 1600 keV have been determined relative to the 411.8044 keV 19SAu 7-ray reference. The measurements have been performed with the Fribourg crystal spectrometer and with a 50 cm 3 Ge(Li) detector. The relative accuracy of the results is - 5 × 10 -6. A new transition in l~°Cd has been disclosed which has an energy of (708.3+_0.2)keV. A value of (661.661_+0.003)keV for the 137Cs decay transition has been determined.

1. Introduction The energies of the most intense transitions in the decay of 192Ir have recently been determined ~-3) with high accuracy, so that reliable calibration standards are available up to an energy of about 600 keV. They supersede the data of Murray et al. 4) which have long been used as secondary standards. The purpose of this work is to extend the energy range up to 1600 keV by the calibration of transitions in the decay of 11°mAg. This isotope is very suitable since several energy combinations can be used to test the results (see fig. 1). The transitions had been calibrated previously in Ge(Li) detector experiments (see e.g. ref. 5) on the Chalk River energy scale4). Some of the y-ray lines have been observed with crystal spectrometers by Gardulski and Wiedenbeck 6) and recently by Borchert et al.7). The results of the present investigation will be given in a scale where E(19SAu 411 keV)= 411.8044 keV, as proposed by Deslattes3). Data from previous investigations have been rescaled to this new value. Only relative errors will be given. Absolute errors can be obtained by compounding the relative error with the 0.4 ppm error in the reference energy itself. Therefore the absolute errors will differ only very slightly from our relative values.

2. Curved crystal spectrometer experiments Eight of the most intense transitions from been observed with the Fribourg curved crystal spectrometer s) after it had been equiped with a laser interferometer9,~°) and moved to SIN/Villigen. The sources were metallic foils with the following dimensions: thickness 11°mAg have

0.05 mm; height 38 mm and depth 3 or 4 mm. The y-rays were diffracted on the (110) planes of a 6 mm thick quartz crystal whose radius of curvature was 5 m. The window in the clamping block had an aperture of 50×50 mm 2. In order to minimize any geometrical aberration11), the gold and silver foils were tightly pressed against each other. The width (fwhm) of the reflexes was approximately 3.7". The measurements were divided into periods of one or two days during which unknown and calibration lines were observed several times. Because of the relatively short half-life of 19SAu, the strong 657 keV line of 11°mAg w a s used as a secondary reference after the 411 keV line had become too weak. This method presents the advantage of showing time-dependent effects. It could thus be observed that the source was slowly drifting. Corrections for this effect were subsequently made. At each spectrometer position, the scintillation spectrum of the NaI(TI) detector was recorded on magnetic tape. After completion of the experiments, windows were set 12) on the full energy peaks corresponding to the observed lines to obtain the number of events as a function of the Bragg angle 0. The method has been described in more detail by Raemy et al.13). It has been shown by Schwitz et al. ~) that the geometrical aberrations can be best accounted for if the centers of gravity of the reflexes are taken as reference angles. In our geometrical and angular conditions (0 still smaller than 2.5°), we have computed ~4) that the line asymmetries due to the aberrations are small, so that negligible errors result when they are ignored. The angular distance be-

550

J. KERN AND W. S C H W I T Z

5° 11°mAg~)1~i

i(i"6~) 2*

1542. 1475.74

2*

6577

O°

O

.oc d Fig. 1. Partial decay scheme of l l°mAg to l l0Cd showing the transitions, labeled by their energies, which have been observed

in the present experiments. The line widths are approximately proportional to the transition intensities. tween the positive (to the right) and negative (to the left) reflexes was therefore determined by the superposition method 15'16). The quoted errors take into account the statistical uncertainties of the angular differences between pairs of reflexes and the rated errors of the angle measuring systemt°). The results are presented in table 1, col. 3. Best energies were obtained by a least-squares procedure which constrains the energies to satisfy the combination principle. The nuclear recoil effects were corrected for in the adjustmentlT). The results are given in table 1, col. 4. For comparison, the direct and adjusted energies obtained by Borchert et al. 7) are listed in the same table, col. 1 and 2. The agreement is generally good. We note, however, some discrepancies for the 620 and 763 keV transitions. In order to resolve them, complementary experiments with a Ge(Li) diode were performed. 3. Measurements with a 50 cm s Ge(Li) detector The measurements and data analyses were done along the lines described in ref. 5 but with improved conditions: better resolution, higher statis-

tics and improved linearity of the detection system. The spectra have been observed with a 50 cm 3 Ge(Li) detector having a resolution fwhm of 1.6 keV at 661 keV. Several combinations of amplifiers (TC 205 and 205A) and ADCs (various Nuclear Data models) have been used. The data have been fitted with the Fortran program ISABELLES,18). Radioactive 192Ir sources were run simultaneously with ll0mAg. In some cases a 137Cs source was added to the set. They were always aligned at a distance of at least 50 cm from the detector. We have determined for each measurement non-linearity deviations, which are defined as the differences between the transition energies and the energies calculated from the line positions (channels) and an arbitrary calibration straight line (a parabola in some cases). The calibration procedure was divided into three steps: 1) Non-linearity curves in the energy range 290-660 keV were obtained by using the energies of the 295, 308, 316, 468, 604 and 612 keV transitions in the decay of ~92Ir as they have been determined by Deslattes3). These values are the

551

DECAY OF l l 0 m A g "]'ABLE 1

Energies of several transitions in the decay of ll°mAg to ll0Cd determined in a scale where E(198Au 4.11 k e V ) = 411.8044_0 keV. The errors are relative and include the measurement error on the standard. The adjusted energies have been computed with the program LEVELFIT 17) which takes into account the nuclear recoil energy. This work Borchert et al. 7) direct adjusted

Crystal spectrometer direct adjusted

direct

Ge(Li)-diode adjusted

E~, a

AE~,

Era

AE r

Ez,

AE r

E~,

AE 7

E~,

AE~,

E~,

AE r

(keV)

(eV)

(keV) 2

(eV)

(keV)

(eV)

(keV)

(eV)

(keV)

(eV)

(keV)

(eV)

1

3

4

5

446.810 620.366

5 17

446.810 620.352

5 9

446.818 620.363

5 6

446.817 620.365

5 5

657.760 677.611 687.007 706.681

4 14 14 5

657.760 677.622 687.001 706.683

4 9 10 5

657.762 677.621

2 7

677.621

6

706.681 b

5

706.681

4

744.264 763.955 818.046 884.687 937.489

20 4 15 17 19

744.272 763.954 818.043 884.692 937.494

11 4 12 13 10

763.934

7

763.937

6

937.490

13

937.484

7

1384.335

80

1384.301

9

1384.310

30

1384.297

6

1505.089

90

1505.039

6

446.815 620.359 626.25 657.761 677.628 687.012 706.675 708.3 744.281 763.944 818.029 884.682 937.488 1334.42

4 3 30 2 4 5 7 200 7 5 6 7 8 60

1475.789

10

14

446.815 620.359

4 3

657.761 677.628 687.012 706.675

2 4 4 5

744.281 763.944 818.029 884.682 937.488

5 4 5 5 5

1384.299 1475.785 1505.036 1562.301

4 5 5 6

a Data rescaled to the new 198Au scale. t, Only data at diffraction orders n>~3 have been used to avoid a shift due the presence of the 708.3 keV line.

most precise and are very close to the weighted average of the best previous results compared in ref. 3. To this energy set the transition in ~l°Cd with an energy of (657.761-+-0.002)keV has been added. This value was obtained by averaging the crystal spectrometer results reported in table 1 which are precise and in good agreement. Using the non-linearity deviations implied by the curves tbr their energies, the ~°Cd transitions at 446 and 620 keV were calibrated. 2) For the transitions above 670 keV a special procedure was used. When using the adjusted energies obtained from the crystal spectrometer experiments, it was observed that the non-linearity deviations scattered relatively widely around an average smooth curve for each set of data. Because of the good accuracy of the line positions, many error bars did not intercept the curve. The pattern was similar in each measurement and indicated that the transition energies had to be altered. This was done independently for each set by an iterative procedure such that (a) the energies

still satisfied the constraints imposed on their combinations by the level scheme, and (b) the resuiting deviation between the computed non-line-

[eV

Run No. 24 Tc 205 A. ADC ND 50/50

300

400

500

600 700 ENERGY

800

900 [keV]

Fig. 2. Sample non-linearity curve. The deviations z/E are computed for the average energies given in table 1, col. 5. The points denoted by dots correspond to the calibration secondary standards. The crosses apply to the lines calibrated by the usual method, denoted as step (1) in the text. The open circles correspond to the transitions calibrated by the special procedure denoted as step (2) in the text. The error bars are statistical only.

552

J. KERN AND W. S C H W l T Z

arity distribution and a smooth curve became minimal. The results obtained by this analysis for each of the various runs differ very little. 3) Finally the 1475 keV ~°Cd line was calibrated with procedure (1) by taking as additional secondary standards the 1384 and 1505 keV transitions whose energy can be determined by combinations of transitions with lower energy (taking into account the recoil energy). The average results are reported in table 1, col. 5. A sample non-linearity curve, for which these y-ray energies have been used, is shown in fig. 2. The bars indicate the statistical errors only. Concerning our experiments, the following remarks must be added: 1) The ll°mAg source was obtained by thermal neutron irradiation of natural silver, so that l°8mAg was also produced. In the decay of the latter isotope, a 614.2 keV transition is present which has to be taken into account in the analysis of the 612 keV 192Ir decay transition. 2) Special care had been taken in the adjustment of the electronics to obtain peaks which could be fitted by the analytical function proposed in ref. 5 over their entire profile. It was thus possible to discover the presence of a small, hitherto unknown transition at (708.3 _+0.2) keV. The line is evident as a small tail on the high-energy side of the 706 keV peak and has an intensity of only (2.0+__0.5)% of the close-lying partner. The weak satellite peak cannot be neglected, since this would result in a 10 eV shift in the calibration of the 706 keV transition. Two unassigned 7-rays with energies 626.2 and 1334.4 keV, which have been reported in the decay of ¿l°mAg and l~°In to ~°Cd (see ref. 19), have also been observed in the present experiments (see table 1, col. 5). Their energy difference corresponds within the estimated error to that of the new line. On this basis it appears likely that the 708 keV transition belongs to ~°Cd. It was not possible to find another reasonable isotopic assignment. 3) A small 884 keV 192Ir line near the 884 keV ~°mAg peak has been taken into account, although under our experimental conditions the presence of this component has a very small effect. 4) It can be seen in fig. 2 that the energy of the 744 keV transition determined by the use of the non-linearity curve is about 3 eV larger than the value listed in table 1. This behaviour is common to all measurements. The origin of this deviation is not known.

The measured energies from the Ge(Li) diode experiments and their estimated errors are reported in table 1, col. 5. The adjusted 17) values are given in col. 6. In view of the calibration procedure, it is clear that the adjusted energies do not differ from the input data, except for the 1475 keV transition. The value of g 2 has been set equal to one for the calculation of the errors. 4. Discussion of the results 1) Comparison of the results in table 1, col. 6 with the crystal spectrometer data shows general agreement, with the few following exceptions: 818 keV (col. 1 and 2) and 763 keV (col. 1-4) transitions. In the latter case, the Ge(Li) results are just intermediate between the crystal spectrometer values. 2) In their work on ),-ray energies, Heimer et al. 2°) have determined in a series of measurements with Ge(Li) detectors the difference in energy of a number of doublets involving the 884 keV line in either ll°mAg or 192Ir and some other line, from another isotope, close in energy. When the latter transition is common in two such measurements, it is possible to obtain the energy difference between the two 884 keV ~-rays by simple subtraction. The following results are obtained:

Intermediate line, 871 keV 94Nb, 874keV ~85Os, 879 keV 16°Tb, 880 keV 185Os, average:

energy difference; (136_5) eV; (145+9)eV; (145___4) eV; (147___8) eV; (142___ 3) eV.

Subtracting (142__+3) eV from the l l0mAg (884.682___0.005) keV energy value (table 1, col. 6), one obtains (884.540_+ 0.006) keV for the 192Ir transition. This is in good agreement with the value (884.542_0.001) proposed by Deslattes3). 3) The 588 keV 192Ir transition has been considered as unknown. We have obtained (588.582+__003)keV, close to the latest result, (588.584+__0.001) keV, by Deslattes3). 4) As noted in sect. 3, in some of our Ge(Li) experiments the 661 keV 137Cs line was observed simultaneously with the other spectra. We have determined a difference of (3.902 _+0.002) keV between the 137Cs and the 657keV ll°mAg transitions. Helmer et al. 20) had obtained (3.898 ___0.002) keV. Using an average value of (3.900 +_0.002) keV, we obtain the energy (661.661___0.003) keV for the 137Cs transition. This

DECAY OF l l0mAg

is in agreement with a recent result, (661.666 +_0.005) keV, obtained by Borchert21). 5. Conclusion Several transitions in the decay of l l 0 m A g have been measured with the Fribourg curved crystal spectrometer. Some discrepancies with respect to the data of Borchert et al. 7) w e r e noted which have been eliminated by complementary measurements (sect. 3) with a Ge(Li) detector by which it is possible to make precise energy difference determinations for not too distant lines. The results presented in table 1, col. 6, can be regarded as a "physical average" of the crystal spectrometer data. They are therefore the more reliable. The few checks of these results which could be made (see sect. 4) indicate good consistency with other data. The authors thank Prof. O. Huber for his permanent support and Mr. CI. Nordmann for his help in the adjustment of the electronics. They acknowledge the help of the collaborators of Prof. H. J. Leisi (SIN) in the crystal spectrometer experiments. They are very grateful to H. Tchopp and his collaborators of the mechanical workshop for their careful work on the crystal spectrometer. Note added in proof R. A. Meyer (private communication to F. E. Bertrand, quoted in Nucl. Data Sheets 22 (1977) 135) has independently disclosed the 708.3 keV transition. His results confirm our isotopic assignment and the energy relationship noted in sect. 3. The existence of a new line at 676.6 keV implies an energy ,;hift of the 677.6, 706.7 and 744.3 keV transitions. The small deviation for the latter line, noted in sect. 3, is removed by this adjustment. The results in table 1 have been accordingly corrected.

553

References t) W. Beer and J. Kern, Nucl. Instr. and Meth. 117 (1974) 183. 2) G. L. Borchert, W. Scheek and K. P. Wieder, Z. Naturforsch. 30a (1975) 274. 3) R. D. Deslattes, Proc. Course 68 Metrology and fundamental constants, Summer School of Physics - Enrico Fermi, Varenna (1976, preprint revised Sept. 1977; E. G. Kessler, Jr., R. D. Deslattes, A. Henins and W. C. Sander, Phys. Rev. Lett. 40 (1978) 171. 4) G. Murray, R. L. Graham and J. S. Geiger, Nucl. Phys. 63 (1965) 353. 5) j. Kern, Nucl. Instr. and Meth. 79 (1970) 233. 6) p. L. Gardulski and M. L. Wiedenbeck, Nucl. Instr. and Meth. 105 (1972) 169. 7) G. L. Borchert, W. Scheck and O. W. B. Schult, in Atomic masses and fundamental constants, vol. 5 (eds. J. H. Sauders and A. H. Wapstra; Plenum, New York, 1975) p. 42. s) O. Piller, W. Beer and J. Kern, Nucl. Instr. and Meth. 107 (1973) 61. 9) W. Schwitz and J. Kern, Proc. 2nd Int. Symp. on Neutron capture y-ray spectroscopy (Petten, 1974) p. 697. 10) W. Schwitz, Nucl. Instr. and Meth. (in print). tl) W. Schwitz, J. Kern and R. Lanners, Nucl. Instr. and Meth. (in print). t2) Fortran program ANALYSE, written by W. Schwitz. 13) A. Raemy, W. Beer, J.-CI. Dousse, R. Eichler, J. Kern, Th. von Ledebur and W. Schwitz, Helv. Phys. Acta 49 (1976) 645. 14) Fortran program BRAGG, written by W. Schwitz and R. Lanners, 15) j. Kern and Th. Dubler, Proc. 2rid Int. Symp. on Neutron capture y-ray spectroscopy (Petten, 1974) p. 702. 16) Fortran program CRUCHE, written by J. Kern. 17) j. Kern, Fortran program LEVELFIT, University of Fribourg report IPF-SP-003 (1974). 18) j. Kern, Fortran program ISABELLE, University of Fribourg report IPF-SP-001 (1970). 19) F. E. Bertrand and S. Raman, Nucl. Data Sheets B5 (1971) 487. 20) R. G. Helmer, R. C. Greenwood and R. J. Gehrke, Nucl. Instr. and Meth. 96 (1971) 173. 21) G. L. Borchert, Z. Naturforsch. 31a (1976) 387.