Directional correlation of gamma rays in Rh105

Nuclear Physics 68 (1965) 401--412; ~ ) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

DIRECTIONAL

CORRELATION

OF GAMMA

R A Y S I N R h l°s

J. F. NEESON and R. G. ARNS t Department of Physics, State University of New York at Buffralo, Bu6ralo, New York tt

Received 18 January 1965 Abstract: Gamma-gamma coincidence measurements on the decay of 4.4 h Ru xe5 to Rh xe6 form the basis for the decay scheme proposed. Directional correlation measurements have been made on the 315-470keV, 485-470keV and the 317-148 keV cascades. The respective correlation functions are: IV = 1-- (0.0124-0.010)Pa(cos 0), IV = 1+ (0.079-4-0.019)P2(cos 0)+ (0.031 -q-0.022)P,(cos 0), W = 1-I-(0.161-4-O.020)Pl(cos O)-- (0.036 ±O.023)P,(cos 0). Spin assignments consistent with these measurements have been made to the levels involved. RADIOACTIVITY. RuX°S[from RuXO4(n,7)l, measured I:,, yT,-coin, 77(0). Rh 1°6 deduced levels, J. Enriched target.

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1. Introduction T h e d e c a y o f 4.4 R u 1os has been s t u d i e d b y several investigators x-4). B r a n d h o r s t a n d C o b b l e 1) have m a d e g a m m a - g a m m a coincidence a n d b e t a decay m e a s u r e m e n t s . C o n v e r s i o n electron a n d g a m m a - g a m m a coincidence m e a s u r e m e n t s b y J o h n s et aL 3) have s h o w n t h a t several new g a m m a t r a n s i t i o n s exist. T w o d i r e c t i o n a l correlations in R h 1°5 have been m e a s u r e d b y A r y a 4) involving the 318-475 k e V a n d 485--475 k e V cascades t t t . T h e results discussed in the present p a p e r are b a s e d on g a m m a - g a m m a coincidence m e a s u r e m e n t s a n d d i r e c t i o n a l c o r r e l a t i o n m e a s u r e m e n t s . Several w e a k cascades have been f o u n d a n d new w e a k l y - f e d levels have been i n t r o d u c e d . Three d i r e c t i o n a l corr e l a t i o n m e a s u r e m e n t s were m a d e involving the 485-470 keV, the 315-470 k e V a n d the 317-148 k e V g a m m a - g a m m a cascades. Consistent with the d e c a y scheme presented a n d o t h e r p e r t i n e n t e x p e r i m e n t a l data, spin assignments have been m a d e to the levels o f R h l ° s . A s h o r t discussion is given o f the spins o f these levels in terms o f a p p l i c a b l e nuclear models. t Present address: Department of Physics, Ohio State University, Columbus, Ohio. tt Work supported by the National Science Foundation. ttt These are referred to in this work as the 315-470 keV and 485-470 keV cascades. 401

402

J. F. NEESON AND R. G. ARNS

2. Experimental Procedure G a m m a - g a m m a scintillation measurements were made on sources produced by irradiating Ru metal samples, electromagnetically enriched to 89.3 ~o in Ru 1°4 in the thermal neutron flux of the Western New York Nuclear Research Center reactor. Samples were irradiated for approximately 2.5 h and the sources produced were used for approximately 2 to 2.5 half lives of Ru 1o5 in order to minimize any contribution f r o m the 36 h decay of Rh 1°5 to Pd 1°5. The coincidence measurements employed a fast-slow coincidence circuit with a resolving time of 50 ns. The detectors consisted of two 5.1 cm x 5.1 cm NaI(TI) crystals mounted on R C A 6655A phototubes. G a m m a rays in coincidence with an energy range selected by a differential analyser were recorded on a 256-channel analyser. Standard precautions were taken to minimize background and Compton backscatter. G a m m a - g a m m a directional correlation measurements employed the previously mentioned fast-slow coincidence unit with the appropriate g a m m a rays being selected by two differential analysers. The sources used were Ru 1o5 metal dissolved in NaOC1 solution. The source to detector distance for all the g a m m a - g a m m a directional correlation measurements was 7 cm. The correlation data for all the g a m m a - g a m m a cascades were taken at intervals of 15 ° f r o m 90 ° to 180°.'The measurements were corrected for source decay and chance coincidences. The least-squares fit was made to the function s):

W(O) =

1 +A2P2(cos

O)+A4P4(cos0);

A2 and A 4 were then corrected for geometry 6). Analysis was made graphically of the possible spin sequences and mixing ratios of the quadrupole and dipole radiation involved 7).

3. Results 3.1. GAMMA RAY SCINTILLATION SPECTRUM Fig. 1 shows the g a m m a ray spectrum of Ru l°s as recorded on the 256-channel analyser using a 7.6 cm x 7.6 cm NaI(T1) crystal. Certain peaks are seen to be broader than would ordinarily be expected for a single gamma-ray. The peak at 72 keV is interpreted as being the Pb X-ray f r o m the detector shields. The appearance of this peak in the following spectra is similarly interpreted. 3.2. COINCIDENCE MEASUREMENTS Measurements have been made of the g a m m a rays coincident with the photopeaks in the following regions (in keV): 120-138, 145-155, 185-210, 255-280, 310-325, 390-415, 465-490, 715-735, 860-925 and the region above 920. Fig. 2 shows the spectrum of g a m m a rays in coincidence with the energy region f r o m 145-155 keV. This particular experiment was performed with the detectors at

DIRECTIONAL CORRELATION

403

90 ° and with suitable shielding between the detectors to prevent Compton scattering between the detectors. Gamma rays in coincidence with this region are noted at 210, 317, 470, 575, 650 and 870 keV. The peaks appearing at 150 and 263 keV are due to interference from the Compton tails of higher energy gamma rays entering the window of the differential analyser. Other coincidence work summarized in table 1 shows that the 650 keV gamma ray is also in coincidence with the 725 keV gamma ray. The evidence that the 650 keV gamma is in coincidence with both the 725 keV ground state transition and the 150 keV gamma ray leads to the introduction of the low-lying level at 150 keV fed by the 575 keV transition from the 725 keV level. Other evidence discussed in connection with fig. 6 shows that the strong 470 keV ground state tranI 4

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counts/channel • see in arbitrary units. sition is also in coincidence with a gamma ray of energy about 150 keV. Therefore two gamma rays of energies 148 and 150 keV have been introduced into the decay scheme on the basis of these data. No coincidence is noted here with the 188 keV gamma ray. Fig. 3 shows the spectrum of gamma-rays in coincidence with the 263 keV photopeak. The peak at 400 keV is interpreted as the only true coincidence. External conversion data and gamma-gamma coincidence work by Johns et aL 3) have shown that this peak at 400 keV is really complex and composed of two gamma rays o f energies 393 and 414 keV with relative intensities of the order of 5 to 3, respectively. Their work has also identified the crossover between the level at 806 keV and isomer

404

J'. F. NEESON AND R. G. ARNS

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at 130 keV. Other peaks in fig. 3 at 150, 263, 315 and 480 keV arise d u e to the C o m p t o n tails of higher energy gamma rays entering the window of the differential analyser.

DIRECTIONAL CORRELATION

405

Fig. 4 shows the gamma-ray spectrum in coincidence with the region from 310 to 325 keV. Photopeaks at 148 and 470 keV are noted. The other peaks arising in the spectrum at 263 and 315 keV can be attributed to the Compton distributions of the 393, 414 and 470 keV gamma rays. Care was taken to rule out the possibility that the 148 keV photopeak might be arising from counter to counter scattering of the 470 keV gamma ray. The experiment was performed at 90 ° with suitable shielding between the detectors to prevent counter-to-counter scattering. The additional precaution of a direct radiation shield of 0.16 cm lead was placed in front of the detector selecting the energy range to further reduce counter-to-counter scattering. The absence of any coincidence with energy greater than 500 keV in this measurement rules out the posI

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sibility of the 148 keV peak arising from a transition of 320 keV from the 470 keV level to the 150 keV level. The absence of higher energy gamma ray in coincidence with the 315 keV photopeak as contrasted with the higher energy gamma rays in coincidence with the 470 keV gamma ray indicates that a 320 keV gamma ray does not de-excite the 470 keV level through the 150 keV level. This evidence along with that discussed in connection with figs. 2 and 5 leads to the introduction of two transitions of energies 148 and 150 keV and two transitions of energies 315 and 317 keV. Fig. 5 shows the gamma ray spectrum in coincidence with the complex 400 keV photopeak. The 263 keV gamma ray is the only true coincidence. The weak photopeaks at 150 and 480 keV are due to the Compton tails of the 470, 485 and 575 keV gamma rays. Comparison of fig. 5 with fig. 4, in which the relative source strengths and length of measurement are the same, indicates a negligible contribution to the

40~

J. F. NEESON AND R. G. ARNS

148 keV in coincidence with the 315 keV gamma ray due to the Compton interference of higher energy gamma rays. Fig. 6 shows the gamma ray spectrum in coincidence with the region from 465 to 490 keV. This measurement shows coincidences with gamma rays of energy 148, 188, 315, 478 (470 and 485 keV), 875 and 920 keV. Evidence discussed in connection with figs. 2 and 4 has shown that the 148 keV gamma in coincidence with the 317 keV gamma does not de-excite the low-lying 150 keV level. It is believed that the strong 315 keV'gamma ray from the 785 keV level is in coincidence with the 470 keV ground state transition. The 317-148 keV transition is interpreted as a separate cascade feeding the 470 keV level. Coincidence work with higher energy gamma rays seems to indicate that the 188 keV gamma ray is fed by the 920 keV gamma ray. I

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F i g . 7. S p e c t r u m o f g a m m a r a y s c o i n c i d e n t with the energy range 860-925 keV.

Fig. 7 shows the gamma ray spectrum in coincidence with the region from 860 to 925 keV. This spectrum shows that there are at least three gamma rays in coincidence with this region of energies 150, 188 and 470 keV. The peak at 315 keV is due to a weak coincidence with a high-energy gamma ray in this region which is not well enough defined to be entered on the decay scheme. There is some indication of a peak above 600 keV but this is not clear enough to be specified. Table 1 contains a summary of all the coincidence measurements made on the decay of Ru 1o5. Fig. 8 is the decay scheme proposed as a result of the measurements contained in this paper and measurements reported previously by other investigators 1-3). The relative intensities of the gamma transitions are presented in table 2 and have been

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3. F. NF,E$ON AND R. G. ARNS TABLE 1 Summary of coincidence measurements Energy range selected (keV) 120-138 145-155 185-210 255-280 310-325 390--415 465--490 715-735 860-925 above 920

Gamma rays in coincidence with selected energy range (keV) none 210, 317, 470, 575, 650, 870 150, 470, 575, 725, 920 400 I) 148, 470 263 148, 188, 315, 478, 875, 920 210, 650 150, 188, (315), 470 none

s) Complex.

TABL~ 2 Relative intensities and percentage decay o f gamma rays from Ru 105 Energy (keY)

Relative intensity (263 keV-----100)

Decays per 100 decays o f Ru x°6

Estimated uncertainty

(70) 130 148 150 188 210 263 315 317 393 413 470 485 575 650 677 725 870 875 920 955 1345 1375 1578 1730

288 a) 26 20 22 18 1O0 104 26 37 22 204 30 13 25 59 469 7 22 22 9 1 <1 <1 <1

27.4 2.5 1.9 2.1 1.7 9.5 9.9 2.5 3.5 2.1 19.4 2.8 1.2 2.4 5.6 44.5 1.0 2.1 2.1 1.0 0.1 <0.1 <0.1 <0.1

• ) The intensity o f the 130 keV transition has been corrected for internal conversion.

±20 ± 30 -4- 50 ±50 ± 50 ± 20 ±20 =[=30 ±30 ±50 ±20 ± 30 ±50 ± 30 ±20 ±20 ± 50 ± 50 ± 50 =[=50

DIRECTIONAL CORRELATION

409

corrected for efficiency and photofraction. The beta group percentages and log f l values have been calculated using the gamma ray transition intensities. The percentage beta feeding of the ground state of Rh 105 is due to Brandhorst and Cobble 1). The spin assignments to the various levels has been based where possible on the results of the directional correlation measurements, log ft values and other experimental evidence. The ground state spin of Ru I o5 is assumed to be ~+. The ground state spin of Rh 1°5, discussed below, is assumed to be -~+. The isomeric transition in Rh 105 from the 130 keV level to the ground state has been identified as E3 by Ricci et al. 8). Other investigators 2, 9) studying the beta decay of Rh 1°5 to Pd 1°5 have reported that the 560 keV beta group terminates in the ~+ ground state of Pd 1°s and is an allowed transition. Beta-gamma coincidence measurements 2, 9) have indicated that the 319 keV gamma ray in Pd 1°5 is in coincidence with a maximum beta energy of about 250 keV. With this evidence, the value of ~+ for the ground state of Rh 1°5 has been adopted in this paper. Rosen lo) has made measurements indicating that the 319 keV gamma ray t of Pd 1°5 is in coincidence with a maximum beta of energy about 560 keV and also that the 319 keV gamma ray shows coincidence with other gamma rays of energy 220, 310 and 415 keV. Gammagamma coincidence measurements made at this laboratory with the 319 keV line in pd 1o2 have shown no coincidences, thus supporting the ~+ assignment for the Rh x05 ground state. 3.3. DIRECTIONAL CORRELATION MEASUREMENTS The 485-470 keV directional correlation was measured and the coefficients A 2 and A 4 computed and corrected. The coefficients for the directional correlation are A 2 = 0.079___0.019 and A 4 = 0.031+_0.022. The log fl value of the beta group feeding the 955 keV level indicates that it is either an allowed or first-forbidden transition allowing spins of ½, { or 7 for this level. The existence of the 955 keV crossover to the ground state and the absence of a transition to the ½ isomer rules out a spin lower than ~r to the 955 keV level. The analysis o f the directional correlation data shows that a spin of ~ is not consistent with the expansion coefficients. The remaining value ½+ has been assigned to the 955 keV level. Analysis of the directional correlation data indicates that spin assignments of ~, -~, -~ or ~ - to the 470 keV level are consistent with the directional correlation data. The absence of beta feeding to the 470 keV level indicates that the spin should be no less than ~-. An assignment of ~ - cannot be ruled out but seems less likely due to the relative intensities of the 955 and 485 keV transitions. The 315-470 keV directional correlation was measured in the standard manner and the corrected coefficients were found to be h 2 = -0.012+_0.010 and A4 = 0. The l o g f t value of the beta group feeding the 785 keV level indicates that it is either an allowed or first-forbidden transition. Spins of ½, ~ or -~ are possible but the indication from the previously discussed 485--470 keV directional correlation that the 470 keV * The 319 keV level as reported in N.D.C. Rosen 10) refers to this as 315 keV.

410

J. IF. NEESON AND R. G. ARNS

level is § or greater favours an assignment of-~ to the 785 keV level. These correlation data contain interference from the 317-470 keV cascade. The relative intensities of the transitions involved show that the 315-470 keV cascade is at least four times stronger than the 317-470 keV cascade and the directional correlation has been interpreted for the 315-470 keV cascade. keY 7/2-*

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Fig. 9. Partial decay scheme o f R u 1°6 s h o w i n g the levels involved in the directional correlation analysis. The figures above the individual g a m m a transitions represent transitions per h u n d r e d decays o f Ru 1°5.

The corrected coefficients for the 317-148 keV cascade are A2 = 0.161-t-0.020 and A4 = -0.036___ 0.023. These coefficients have been corrected for the contribution made to the correlation function due to the Compton interference from the 315-470 keV cascade. The logft value of the beta group feeding the 935 keV level indicates that the beta group involved is either allowed or first-forbidden. The absence of beta feeding to the 618 keV level would indicate that a spin assignment of not lower than 9 be made to this level. The values shown in fig. 9 as -~, { for the 935 keV level and 9, ~ - for the 618 keV level are consistent with the expansion coefficients. Coincidences arising from counter-to-counter scattering of the 470 keV gamma ray at high angles was found to be negligible. The most probable choices for the spin assignments discussed above have been entered on the partial decay scheme (fig. 9), which shows the levels involved in the directional correlation measurements. With the available data definite parity

DIRECTIONAL CORRELATION

411

assignments could not be made to most of the levels involved. The results of the 315470 keV and 485-470 keV measurements presented in this section are not in agreement with those of Arya 4). 4. Discussion

The Rh l°s nucleus with 45 protons and 60 neutrons lies in a region in which the coUective nuclear excitations are usually thought of as vibrations about a spherical equilibrium shape. Kisslinger and Sorensen 11) have recently calculated the energy levels of spherical nuclei with residual forces between the nucleons outside of closed shells. Their results have been particularly successful in the case of nuclei with a closed shell for either protons or neutrons. For Rh 1o5, which has several particles (or holes) outside of both proton and neutron shells, the theoretical calculations have not been particularly fruitful. Kisslinger and Sorenson point out that possibly the coupling scheme employed is breaking down in this instance. In addition they have noted that for even nuclei with Z = 44, 46 the low-lying 2 + levels are the lowest-lying 2 + levels in this Z range although these nuclei do not exhibit rotational structure. Kisslinger and Sorenson point out that the spins of low-lying states of the odd-mass nuclei of this region appear to indicate some degree of deformation. It would be desirable to have a calculation by the Kisslinger and Sorenson method which accurately reflects the complexity of these nuclei. Some insight may be gained by applying the suggestion of de-Shalit 12) which attempts to consider the same problem more qualitatively. In the approach of deShalit one attempts to identify excited states of odd-mass nuclei which result from the coupling of the odd particle to the collective excitation of the adjacent even nucleus. Ideally one would expect to find that the lowest of these collective states in the same energy region as the 2 + first excited state of the adjacent even nuclei. In general it is observed that these collective states fall below the energy which one would expect from considering the 2 + first excited states of the adjacent even nuclei. If the 2 + state of Ru 1o, is coupled to a {+ proton a set of states with spins from ½+ to 1~+ could be produced in Rh 1°5. The 2 + state of Ru 1°4 occurs at 358 keV and the 2 + state of Pd 1°6 occurs at 513 keV. Information from the directional correlation has led to a probable spin assignment of-92 to the 470 keV level and might identify the 470 keV level as a member of the core multiplet. Other levels in this region with positive parity are possible states of this core multiplet. Negative parity levels may arise out of the coupling of the single phonon with the ½-, 130 keV isomer. Levels at higher energies may be either two phonon states or intrinsic states.

References 1) H. W. Brandhorst and J. W. Cobble, Phys. Rev. 125 (1962) 1323 2) Nuclear Data Sheets, 61-4-15 (National Research Council, Washington, D.C. 1961)

412 3) 4) 5) 6) 7) 8) 9) 10) 11) 12)

J. F. NEESON AND R. G. ARNS M. W. Johns and S. Schriber, private communication A. T. Arya, Nuclear Physics 40 (1963) 387 M. E. Rose, Phys. Rev. 91 (1953) 610 R. G. Arns, R. E. Sund and M. L. Wiedenbeck, University of Michigan Research Institute Report 2375-4-T (Feb. 1959) R. G. Arns and M. L. Wiedenbeck, University of Michigan Research Institute Report 2375-3-"1" Oan. 1958); Phys. Rev. 111 (1958) 1631 S. A. Ricci, S. Monaro and R. van Lieshout, Nuclear Physics 16 (1960) 339 S. E. Karlson, O. Bergman and W. Scheurer, Ark. Fys. 27 (1964) 61 A. Rosen, Thesis, University of California (1958) L. S. Kisslinger and R. A. Sorensen, Revs. Mod. Phys. 35 (1963) 853 A. de-Shalit, Phys. Rev. 122 (1961) 1530