Energy levels of Pm149

Nuclear Physics 63 (1965) 233--240; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced

by photoprint or

microfilm without written permission from the publisher

E N E R G Y L E V E L S O F P m 149 C. H. C H E N t and R. G. A R N S t t

Department of Physics, State University of New York at Buffalo, Buffalo, N.Y. 14214 tit Received 18 September 1964 Abstract: The gamma rays o f Pm 149 following the beta decay of Nd 1'9 have been studied using fast-

slow coincidence techniques. A total of 18 gamma rays have been identified. Levels are established in Pm 149 at 114, 188, 210, 265, 386, 535, 650 and 750 keV. The gamma-gamma directional correlation of the 270 keV-265 keV cascade has been measured and the expansion coefficients have been found to be As = --0.22-4-0.01 and A4 = 0. The energy levels found in this investigation are discussed in terms of recent nuclear models. E

RADIOACTIVITY Nd 14~ [from NdX48(n,~)]; measured I~, ??,-coin, 7?(0). Pm x49 deduced levels, J, zr. Enriched target.

1. Introduction T h e g a m m a transitions o f P m 149 following the b e t a d e c a y o f 1.8 h N d 149 were first studied in detail b y R u t l e d g e et al. 1). A level scheme consisting o f 14 g a m m a r a y s f o l l o w i n g three b e t a g r o u p s was p r o p o s e d b a s e d on c o n v e r s i o n electron m e a s u r e ments. G o p i n a t h a n a n d Joshi 2) have recently p r o p o s e d a n e w decay scheme for N d 149 b a s e d on b e t a decay, g a m m a - g a m m a a n d b e t a - g a m m a coincidence m e a s u r e ments. T h e present g a m m a - r a y coincidence 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 were u n d e r t a k e n in o r d e r to o b t a i n i n f o r m a t i o n which w o u l d p e r m i t a c o m p a r i s o n with recent nuclear theories. A t the same t i m e it is h o p e d t h a t the present w o r k will help to resolve the discrepancies b e t w e e n the results o f the previous investigators.

2. Experimental Procedure and Results 2.1. SOURCE P R E P A R A T I O N A N D G A M M A - R A Y SPECTRUM

T h e samples were p r e p a r e d b y i r r a d i a t i n g N d 2 0 3 with the N d electromagnetically enriched to 93 ~ N d 148 in the r e a c t o r o f the W e s t e r n N e w Y o r k N u c l e a r Research C e n t e r for 30 m i n periods. T h e samples were used for a p e r i o d o f no m o r e t h a n 5 h, b e g i n n i n g 1 h after i r r a d i a t i o n , in o r d e r to a v o i d the activities o f 53 h P m 149 a n d 12 rain N d 151 followed b y 28 h P m l s l . T h e latter were f o r m e d b y n e u t r o n c a p t u r e in the 1.06 ~o N d 15° in the N d o f the samples. r Present address: Physics Department, Long Beach State College, Long Beach, California. tt Present address: Physics Department, Ohio State University, Columbus, Ohio. ttt Work supported by the National Science Foundation. 233

234

C. H.

CHEN

AND

R.

G. ARNS

The gamma-ray scintillation spectrum (fig. 1) was obtained using a 7.6 cm x 7.6 cm NaI(TI) crystal and recorded on a 256-channel analyser. Certain g a m m a rays recorded by the previous investigators are not seen in this spectrum. For example, the 62 keV g a m m a ray recorded by Gopinathan and Joshi 2) is not seen here. This line may be attributed to the strong 65 keV g a m m a rays of the P m 151 decay 3, 4) resulting from the much larger Nd xS° fraction in their samples. 9 X-RAY 8

If4 420

7

202

~"

N(E) T

536

Z Ioo

200

300 400 500 ENERGY(keY)

600

700

Fig. 1. Gamma-ray spectrum of Pmt~9.

2.2. COINCIDENCE MEASUREMENTS A conventional fast-slow coincidence spectrometer with a resolving time of 45 ns was used in the coincidence measurements. The detectors were 5.1 cm x 5.1 cm NaI(T1) crystals mounted on R C A 6655-A phototubes. A shield was placed between the detectors to prevent counter-to-counter scattering. Measurements were made o f g a m m a rays in coincidence with the following energy ranges (in keV): 70-80, 105-120, 140-160, 185-215, 260-275, 315-335, 415-440, 530-560, 635-665. G a m m a rays in coincidence with the energy region 105-120 keV are shown in fig. 2. The 75, 150, 420 a n d 536 keV lines are interpreted as direct coincidences with the 114 keV g a m m a ray. As will be seen below, the 270 keV line is an indirect coincidence through the 150 keV transition: The strength of the 114 keV line, the coincidence measurements, and the previous beta-decay measurements 2) argue strongly for a level at 114 keV. The presence of the 114 keV line in this spectrum is the result of interference due to the Compton portion of the spectrum of higher energy coincident transitions.

P m 149 E N E R G Y

235

LEVELS

Fig. 3 shows the g a m m a - r a y s p e c t r u m in coincidence with the w e a k 150 k e V p h o t o p e a k . D i r e c t coincidences were n o t e d as the 114 k e V a n d 270 keV transitions. 9

X-RAY

- -

8

N_~7 T 6

5 150

420

4

556

3 198 2

I

I

I

I ~'~l

0

I00

200

300 400 ENERGY (keY)

~ 500

600

Fig. 2. Spectrum of gamma rays in coincidence with 114 keV gamma ray. 7 --

X-RAY

114

6

5

Ng! T 4

3

2

I

0

I

0

t

I I I00 200 ENERGY (keV)

270

÷

300

Fig. 3. Spectrum of gamma rays in coincidence with 150 keV gamma ray.

The 76 a n d 200 k e V p h o t o p e a k s a r e i n t e r p r e t e d as interference f r o m coincidences with the n e a r b y 114 a n d 202 k e V p e a k s .

236

c.H.

CHEN

AND

R.

G.

ARNS

The gamma-ray spectrum in coincidence with the composite photopeak of the 188, 198 and 210 keV transitions 1) is shown in fig. 4. The photopeaks at 325 and 440 keV are interpreted as direct coincidences with the 210 keV g a m m a ray. This is confirmed by the coincidence measurements with the 325 keV g a m m a ray and the 440 keV g a m m a ray (not illustrated). The line at 550 keV is interpreted as a composite o f two lines of 540 and 560 keV, corresponding to transitions from a level at 750 keV 114 9 197

X-RAY 8

8-325

T

7

6

--

4--

114

3--

2 --,

440

I--

I 50

I

I

I00 150

o

I I00

I 200

300

400

500

ENERGY (keY)

Fig. 4. Spectrum of gamma rays in coincidence with 202 keV photopeak. (The lower end has been spread out).

to levels at 210 and 188 keY, respectively. This is confirmed by coincidence measurements with the 530-560 keV range (fig. 6). The strong photopeak at 197 keV is interpreted as due to a 198-188 keV cascade from a level at 386 keV. The 75 keV line is apparently due principally to a transition from the 188 keV level to the 114 keV level, but a 77 keV transition between the 265 and 188 keV levels m a y also contribute to this coincidence. The 114 and 270 keV lines arise partly through the 74 and 77 keV lines and partly from interference. Fig. 5 shows the spectrum in coincidence with the composite photopeak of the 265 and 270 keV transitions. I f a level at 265 keV is postulated then the weak line at 385 keV can be interpreted as a transition f r o m the 650 keV level. Similarly, the 77, 150 and 265 keV lines are interpreted as direct coincidences with a 270 keV transition

P m 149 E N E R G Y

237

LEVELS

X~,RAY

I

X~cRAY

i

8

i

7

5 4-

II

150

II

'

2 t I#

o Fig.

\ I I00 200 300 ENERGY(keV)

0

400

0

with 265 and 270 keV gamma rays. 149

210

I00 200 ENERGY(keV)

Fig 6. Spectrum of gamma rays coincident with 536 keV photopeak

5 Spectrum of gamma rays coincident

6oNd

I

v

5_~?/-

(,.8h) ~

0~o.~,

920(12%,ZI) \

560 540 650

io2o(2r,3./..6j),

I

I

I

I

I

650 536 4po4403185

ll35(m.8"/o,6.3f~ dz. #2-

,

, ~

~o~>

I

I

I

535

I

I 420 325 270

1284(8 6 % , 6 8 ~

,405(,33%6G~

keV 750

5/E,.~/Z+7/;

1460(26%,667 1482(l.2%,79)/v //2,+5/2+

I

~ ~

5/E #2-

1556(9.6%,Z2#

3e6

I 198

2G5

ZlOle_ _ ~

~E 7/E,9/~-

,/¢. ,/¢

S 74

,,,>

114

114

,/2 + 149 61 P m 8 8

(55h)

Fig. 7. Decay scheme of N d t*9. Information on the beta decay is arranged in the order: the endpoint energy, percentage and log f l value. The end-point energies were calculated from the Q value reported in ref. 2), the percentages and also the l o g f i values were calculated from the relative intensities of the gamma rays as listed above the arrow corresponding to each gamma transition and in terms o f percentage decay o f Nd 14~.

238

¢. H. CHEN AND R. G. ARNS

from the 535 keV level to the 265 keY level. The lines at 114 and 200 keV are due to indirect coincidences and interference. Fig. 6 shows the spectrum in coincidence with the energy range from 530-560 keV. A strong coincidence with the 114 keV transition is noted. This is interpreted as due to a 536 keV gamma ray from the 650 level. In addition, there appear to be weak transitions from the 750 keV level to each of the 210 and 188 keV levels which give rise to the 74, 188 and 210 keV coincidences. The proposed decay scheme, based on the coincidence measuremenls, is given in fig. 7. Although only the most impostant coincidence measurements have been illustrated and discussed in detail, this level structure is consistent with all of the present measurements. Gopinathan and Joshi 2) proposed beta-fed levels at 114, 210, 272, 538, 650 and 740 keY. Corresponding levels appear in the present scheme as well as weakly-fed levels at 188 and 386 keV which are required by the gamma ray measurements. The 62 keV gamma ray found by Gopinathan and Joshi 2) in coincidence with the 270 keV photopeak does not appear in the present work. As was pointed out above, this may arise from the decay of Pm 151 in which there is a strong 65-275 keV coincidence 4). If this explanation is correct, then the 168 keV photopeak in Pm lsl may be responsible for the discrepancy between the energy of 150 keV reported here and the 158 keV gamma ray reported in ref. z). The relative intensities of the gamma rays, as measured in the singles and coincidence spectra, are listed in the level scheme and were essential in its construction. The listed intensities have been corrected for efficiency and photofraction of the crystal and are expected to be correct to within _ 30 70. The beta decay information has been calculated from the gamma ray intensities. The energy difference between the Nd 149 and Pm 149 ground states has been adopted from ref. 2). In order to account for the intensity of the X-ray in the singles and coincidence spectra, it was necessary to assume that the 114 keV transition is nearly pure MI. The 74 and 77 keV transitions are then required to be predominantly El. The listed relative intensities of these low-energy gamma rays have been accordingly corrected for internal conversion. Certain other weak transitions might be inferred from the coincidence spectra. For example, there may be a weak 55 keV transition between the 265 and 210 keV levels or a weak 96 keV transition between the 210 and 114 keV levels. Because the evidence is not strong, and due to the possible interference from Pm 15~, these have not been included in the decay scheme. Rutledge et al. 1) found a 67 keV conversion electron line which they interpreted as a K electron corresponding to a 112 keV gamma ray. This line may, in fact, be also interpreted as an L electron line corresponding to the important 74 keV gamma ray of the present decay scheme. 2.3. D I R E C T I O N A L

CORRELATION

MEASUREMENT

The gamma-ray directional correlation of the 270-265 keV cascade has been meas-

P m 149 ENERGY LEVELS

239

ured. Data were taken in a double quadrant sequence at every 15°. After least-squares fitting 6) and geometrical correction 7), the expansion coefficients were found to be A2 = -0.22+__0.01 and A 4 = 0. The ground-state spin and parity of Pm 149 are known s) to be -~+. Considering the l o g f t values and using a graphical method of interpretation 9), it was found that only two spin-parity sequences are consistent with the directional data. These are ~- (D)½ + (D, Q)½+ with a quadrupole content Q of %10 % and ~- (D, Q)~+ (D, Q)½+ with the quadrupole content of the first transitions less than 2 % and that of the second transition between 15 7o and 73 %. 3. Summary and Discussion The level structure of Pm 149 resulting from the present work differs from the schemes proposed by earlier investigators 1, 2). The principal gamma transitions found in ref. 1) have been identified here, although the proposed level structure, based on coincidence measurements, is quite different from that constructed from energy considerations only 1). The 270-265 and 325-210 keV cascades, which were reported in ref. 2), have been confirmed. However, the present results differ in the energies, intensities and positions of some of the other gamma transitions. By means of intensity data and the directional correlation measurement, it has been possible to limit the possible spin assignments for most of the levels. More information is needed in order to interpret the low-lying level structure of Pm 149. However, some speculation based on the present results may be useful. The Pin 149 nucleus has 61 protons and 88 neutrons and lies just below the region of stable deformed nuclei. Empirically the transition to deformed shape is observed to take place for neutron number between 88 and 90. The Nilsson model has been successful in predicting the level structure in the deformed region lo). If one attempts to interpret the present level structure in these terms, one is forced to choose the (~+, 404) orbital for the ground state of Pm 149. However, in order to make this choice a very small deformation parameter 6 < 0.075 is required. This is not reasonable compared to other deformation parameters in this region and one concludes that the Nilsson approach is not applicable here. Even if the additional freedom of non-axially symmetric shapes is allowed, in accordance with the calculations of Hecht and Satchler 11), it is not possible to account for the spins and parities found in the present work. Finally, we may compare the level structure with the calculations of Kisslinger and Sorensen 12), in which the spectra of spherical nuclei with residual forces have been predicted. However, these calculations are equally unsuccessful in predicting the Pm 149 structure. References 1) Rutledge, Cork and Burson, Phys. Rev. 86 (1952) 775 2) K. P. Gopinathan and M. C. Joshi, Phys. Rev. 134 (1964) B297

240 3) 4) 5) 6) 7) 8) 9) 10) 11) 12)

c. H. CHEN AND R. (3.

ARNS

Burke, Law and Johns, Can. J. Phys. 41 (1963) 57 C. H. Chen and R. G. Arns, to be published Budick, Doyle, Marrus and Nierenberg, Bull. Am. Phys. Soc. 7 (1962) 476 M. E. Rose, Phys. Rev. 91 (1953) 610 Arns, Sund and Wiedenbeck, Univ. of Michigan Research Institute Report 2375-4-T (Feb. 1959) Cabezas, Lindgren and Marrus, Phys. Rev. 122 (1961) 1796 R. G. Arns and M. L. Wiedenbeck, Phys. Rev. 111 0958) 1631 B. R. Mottelson and S. G. Nilsson, Mat. Fys. Skr. Dan. Vid. Selsk. 1, No. 8 (1959) K. T. Hecht and G. R. Satchler, Nuclear Physics 32 (1962) 286 L. S. Kisslinger and R. A. Sorensen, Revs. Mod. Phys. 35 (1963) 853