Nuclear level studies of 99Ru

jl.E.1:3.AI

Nuclear Physics

Al80 (1972) 31I-320; @ North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

NUCLEAR LEVEL STUDIES OF “Ru D. K. GUPTA, C. RANGACHARYULU,

R. SINGH

and G. N. RAO

Department of Physics, Indian Institute of Technology, Kanpur, India t

Received 29 June 1971

Abstract: The radioactive decay of 16.1 d g9Rh has beenstudiedusing

a Ge(Li)-Ge(Li) coincidence spectrometer and a NaI(Tl)-NaI(T1) sum-coincidence spectrometer. Evidence for the existence of a new y-ray of energy 910.8 keV is obtained. A level scheme for g9Ru is established. Four new cascade transitions from the energy levels at 850.3, 1000.2, 1295.2 and 1382.9 keV are reported. The relative intensities and energies of all the y-transitions are determined. The half-life of the 89.4 keV level is measured using the delayed coincidence method. The value obtained is 20.5sO.l nsec.

E

RADIOACTIVITY 99Rh [from 99Ru(p, n)]; measured Eu, +,, w-coin, yy-delay. 99Ru deduced levels, T+. Chemical separations; enriched target, Ge(Li) and NaI(T1) detectors.

1. Introduction

The levels of 99Ru are populated by the decay of 16.1 d 99Rh through positron emission and electron capture. The information available so far on the level scheme of 99Ru is the result of the studies made by Moss and McDaniels ‘), Connors ‘) and Anton’eva et al. “) using solid-state detectors. The very limited information that has resulted from studies reported before 1965 using scintillation spectrometers was discussed by Moss and McDaniels ‘). We refer to this paper for all the earlier references. The study of Connors ‘) was, however, restricted to the excited states having energies less than 700 keV. Moss and McDaniels have studied the level structure up to an energy of 2.06 MeV. They have proposed the decay scheme mostly with the help of a computer programme which formed all possible sums and searched for the excited states and the associated y-transitions. Anton’eva et al. “) recorded singles y-ray spectra up to 2.06 MeV using a Ge(Li) spectrometer and measured the energies and the relative intensities of the y-radiation. Our work reported in this paper involves a restudy of the level scheme using Ge(Li) detectors, a NaI(Tl)-NaI(T1) sum-coincidence spectrometer and a Ge(Li)-Ge(Li) slow-fast coincidence spectrometer. Using the delayed coincidence method, we have also measured the lifetime of the 89.4 keV level. An enriched target was used and the chemical separations were also performed. 7 Work partly supported grant NBS(G)-127.

by the National

Bureau of Standards, 311

Washington,

DC through

the

D. K. GUPTA er al.

312

2. Experimental 2.1. SOURCE PREPARATION

The source was obtained in solid form from the Oak Ridge National Laboratory (ORNL). According to the given specifications, the enriched target material (99.95 p/o) was enclosed in an aluminium capsule and was bombarded with the proton beam of the ORNL cyclotron. The 16.1 d 99Rh activity was produced by means of a (p, n) reaction. Chemical separations were carried out to separate the rhodium activity from the impurities, particularly al~inium. 2.2. EXPERIMENTAL

SET-UP

Two Ge(Li) detectors of depletion depths 7 mm and 5 mm with sensitive areas of 6 cm2 each were used for the studies of the singles y-spectra and for the slow-fast y-y coincidence studies. The energy resolution (FWHM) for the 7 mm and 5 mm detectors respectively were 2.8 keV and 3.5 keV for the 662 keV y-rays of 137Cs. The resolving time (22) of the fast coincidence circuit used in the Ge(Li)-Ge(Li) fast-slow coincidence spectrometer was 40 nsec while that of the slow coincidence circuit was 2 psec. The sum coincidence spectrometer consisted of two NaI(T1) scintillators coupled to 56 AVP photomultipliers. The linear signals from the two detectors were summed and operated in a slow-fast coincidence modes. The resolving time (2~) of the fast coincidence circuit was 30 nsec. The details of the sum-coincidence spectrometer were described elsewhere “). The measurement of the lifetime of the 89.4 keV level was performed using a delayed coincidence spectrometer; the details of this can be found in an earlier publication “). A NaI(T1) scintillator and a lead-loaded plastic scintillator coupled with 56 AVP photomultipliers were used respectively in the start and the stop channels. All the spectra were recorded with the help of a Packard 400~channel analyser. 2.3. GAMMA-RAY

ENERGIES AND INTENSlTIES

A typical singles y-ray spectrum recorded with the help of a Ge(Li) detector with a chemically separated source is given in figs. l-4. Fig. 1 gives the y-rays observed in the energy range of O-442 keV. We have found no evidence for the existence of the 119.4 keV y-ray which was reported by Moss and McDaniels ‘). The spectra recorded with the separated impurity source showed the existence of a welldefined photopeak at 119.4 keV. This shows that the 119.4 keV y-ray does not belong to the decay of 99Rh. The rest of the y-rays in this region, however, agree well with the earlier work of Moss and McDaniels ‘). The y-ray spectrum in the energy range 440-1200 keV is given in fig. 2. In addition to the y-rays reported earlier “), we have observed an additional y-ray at an energy of 910.8 keV. This was found to decay with the 16.1 d half-life. The peak at 778.1 keV does not belong to the decay of 16.1 d g9Rh. In fig. 3, we have plotted the y-ray spectrum in the energy range 1200-1750 keV. Fig. 4 gives the y-spectrum in the high-energy region (> 1960 keV). The y-rays

9 9Ru LEVELS

313

observed by us in the singles spectrum essentially confirm the results reported by Moss and McDaniels except for minor variations. The energy calibration was done using 241Am, 57Co, ‘03Hg, 22Na, 13’Cs, “Mn, 6oCo and **Y standard sources, providing y-rays with energies in the region 604850 keV. The energies of the y-rays observed by us in the 16.1 d decay of 99Rh are listed in table 1. The energies of the y-rays obtained from our energy calibration are very close to the values reported by Moss and McDaniels. Therefore, we have adopted their energy values except in the case of the 1295

I 50

lo2 0

I 100

I 150

0 200 Channel

I 250

1 300

I 350

400

Number

Fig. 1. Gamma-ray singles spectrum of 99Rh in the energy region O-443 keV obtained with a Ge(Li) detector (livetime 60 min).

lo2

’

20

I

I

I

I

I

I

I

I

50

100

150

200

250

300

350

400

Channel

Fig. 2. Gamma-ray

Number

singles spectrum of 9gRh in the energy region 443-1208 keV obtained Ge(Li) detector (livetime 600 min).

with a

D. K. GUPTA et a/.

314

1

10

!

100

I

I

1

t

I

150

200

250

300

350

Channel Fig. 3. Gamma-ray

Fig. 4. Gamma-ray

4

Number

singles spectrum of 99Rh in the energy region 1208-1749 keV obtained with a Ge(Li) detector (livetime 1200 min).

150 200 Channel Number singles spectrum of 99Rh in the energy region 1749-2059 keV obtained with a Ge(Li) detector (livetime 1800 min).

keV level in 99Ru. The starred peaks in figs. l-4 do not belong to the 16.1 d decay of 99Rh* the origin of these lines has not been explicitly determined. The full-energy-peak efficiency of the detector used for recording the singles spectra was experimentally determined using standard sources giving energies in the range of interest, at a fixed geometry. The same geometry was maintained throughout the ysingles studies. The areas under the photopeaks of the y-rays of 99Rh were determined after subtracting the background and were used for the evaluation of the relative intensities. The relative intensities obtained by us are listed in table 1 along with the values reported by Moss and McDaniels ‘) and Anton’eva et al. “). The intensities reported by us are the relative y-ray intensities and no attempts were made to correct the data for internal conversion. 2.4. Ge(Li)-Ge(Li)

SLOW-FAST

COlNClDENCES

The coincidence spectra were recorded with gates accepting the peaks at energies 322.4 and 353.0 keV. The widths of the gate settings used were mainly to include the photopeaks of the y-rays of interest. The results are summarized in table 2. The coin-

31.5

9gRu LEVELS TABLE 1

-

~

Gamma rays observed in the decay of 16.1 d g9Rh and their relative intensities

-~ ~~-Energy U$) (keV)

Relative y-ray intensities (f,,) Moss

89.36 119.4 175.2 232.4 295.7 322.4 353.0 442.8 486.5

et al. ‘)

Anton’eva

76 0.13 5.8 1.3 2.1 16.5 81.3 4.3 1.5

et al. 3)

100

&lo cc2 4.5Al.O 1.5*0.5 3.5kO.5 18 &2 100 +10 4.5kO.5 1.5kO.5

present 56.3 3.8 0.72 2.6 9.1 78.3 5.4 0.89

annihilation radiation 527.7 575.2 618.0 734.2 763.9 806.6 850.3 897.2 910.8 941.5 1000.2 1061.2 1089.4 1208.3 1295.2 1323.9 1382.9 1442.1 1484.3 1505.1 1532.9 1572.4 1618.0 1662.1 1749.1 1970.3 2058.6

100 0.6 10.0 0.8 1.0 3.5 0.6 1.7 3.8 1.7 0.4 0.8 0.4 0.9 0.4 0.4 0.2 0.4 0.2 1.4 0.58 0.6 0.15 0.15 0.4 0.08

The values reported earlier by Moss and McDaniels

100 0.8 kO.2 11 &i 0.9&0.2 3x&0.5 51 1.7AO.3 3.3+0.5 2.0*0.2 0.6kO.2 51 (= 0.8 l.OkO.3 I_ 0.5 5 0.4 < 0.4 < 0.4 < 0.4 1.910.4 0.710.2 _I 0.6 < 0.4 < 0.4 5 0.4

100 0.47 11.9 0.61 0.88 3.5 1.1 2.1 0.13 3.4 2.0 0.55 1.0 0.66 0.72 0.66 0.29 0.34 0.14 0.1 1.3 I.3 0.9 0.23 0.6 0.21 0.43

I) and Anton’eva ei ai. 3, are also given.

cidence spectrum obtained with the gate at 322.4 keV is shown in fig. 5. The peaks observed in fig. 5 at 175.2, 322.4 and 353.0 keV have probably resulted from coincidences with the Compton part of the annihilation radiation present under the gate setting of 322.4 keV. The 295.7 keV y-ray observed in coincidence with 322.4 keV y-ray is due to a transition leading from the level at 618.0 keV going to the level at 322.4 keV. The peak at 511 keV is due to the coincidences between the 322.4 keV y-ray

316

D. K. GUPTA et al.

and the 511 keV annihilation radiation feeding the level at 322.4 keV. No evidence for the existence of the 119.4 keV y-ray reported by Moss and McDaniels is seen in fig. 5.

Summary of the Ge(Li)-Ge(Li) y-ray energy E (keV)

TABLE2 fast-slow coincidence measurements y-rays in coincidence with E

322.4 353.0

295.7 89.4, 175.2, 941.5

CHANNEL

NUMBER

Fig. 5. The spectrum

of y-rays in the region O-511 keV coincident with the 322 keV y-ray recorded with a Ge(Li)-Ge(Li) fast-slow assembly.

2.5. NaI(Tl)-NaI(T1)

SUM-COINCIDENCE

MEASUREMENTS

Coincidence spectra were recorded by fixing the gates at energies of 575, 618, 734, 850, 897,941, 1000 and 1295 keV. The sum-coincidence experiments were carried out for establishing the existence of the cascade transitions. The 16.1 d 99Rh decays to the excited levels in 99Ru through EC and j?’ decay. The intense sum-coincidence peaks at 511 keV that were obtained in the spectra were mostly due to the coincidences between the intense 511 keV annihilation radiation feeding the levels and the Compton part of the following y-rays. The results are summarized in table 3. The sum-coincidence spectrum obtained with the gate at 1295 keV is given in fig. 6. 2.6. LIFETIME

MEASUREMENTS

OF THE 89.4 keV LEVEL

The lifetime of 89.4 keV level in 99Ru was measured by the delayed coincidence method. One lead-loaded plastic scintillator (5.1 cm x 5.1 cm) was used in the stop channel to accept the 89 keV y-ray and a NaI(T1) crystal of (5.1 cm x 5.1 cm) in the

g9Ru LEVELS

317

TABLE3 Summary of the NaI(Tl)-NaI(T1) sum-coincidence Sum coincidence gate energy (keV)

y-y cascades observed

(a)

486 575 --, 528 618 3

(b)

618 z

322 z

0

(c)

618 2

443 “2

0

134 2

0

(a)

850 %

618 2

(b)

850 z

0

897

897 !$

89+

941

941 z$

1000

911 lOOO+

575 618

734 850

1295

0 0

20

40

CHANNEL

Fig. 6. Nal(TI)-NaI(T1)

measurements

sum-coincidence

89%

0

89 -%

0

89

0

0

175 89%

0

(a) 1295 %

lOOft 2

0

fb) 1383 2

443 2

89

(c) 1383 z

618 2

89

60

80

NUMBER

spectrum

recorded

with the gate setting at 1295 kcV.

318

D. K. GUPTA et nl.

start channel to accept y-rays from 320 to 540 keV. For this measurement two 56 AVP photomultipliers connected to ORTEC fast electronics and an ORTEC Model 437 TPHC were used. The time spectrum recorded is given in fig. 7. The data was leastsquares fitted with the help of an IBM 7044 computer. The final computed value of

..

Ch. No. 70 0

10

20

90 I 60

I’ 40 Time

in

nsec

100

130 7’ 60

(arbitrary

150 : 100

170 190 ‘I 120

210 140

zero)

Fig. 7. The lifetime spectrum of the 89.4 keV level in 99Ru recorded with the delayed coincidence spectrometer.

many runs for the half-life (7”) is 20.5fO.l nsec. Our value compares favourably with the earlier measurements of Matthias et al. “) (20.7kO.3 nsec); Kistner et al. ‘) (20+ 1.0 nsec); and Bodenstedt et al. “) (19.7kO.4 nsec). 3. Results and discussion The level scheme suggested by Moss and McDaniels with some modifications which are warranted by our experimental results is reproduced in fig. 8. Our investigations essentially support the decay scheme suggested by Moss and McDaniels except for the following new results: (i) Our coincidence data gave no evidence for the presence of a 119.4 keV y-ray. The fact that we have observed an intense 119.4 keV y-ray in the singles spectrum of the chemically separated impurity led us to conclude that the 119.4 keV y-ray does not belong to the decay of 16.1 d 99Rh. (ii) The existence of a 232 keV transition from the 322 to 89 keV level was reported by Moss and McDaniels. Though we did observe a small peak at 232 keV in the Ge-Ge coincidence spectrum with the gate at 89 keV, we cannot explain the sumcoincidence spectra obtained with the sum-coincidence gate at 850 keV unless we keep the 232 keV transition between the 850 and 618 keV levels.

99Ru LEVELS

319

(iii) Moss and McDaniels suggested that the 764 keV transition is between the 1662 and the 897 keV levels. Our sum-coincidence data warrants that it should be between 1383 and 618 keV levels. However our results do not rule out the possibility that there could be one more 764 keV transition between the 1662 and 897 keV levels

”

Ru

Fig. 8. Level scheme of 99Ru. Dashed lines shown are the new transitions.

320

D. K. GUPTA et al.

(iv) The sum-coincidence spectrum with the gate at 1000 keV energy gave evidence for a new transition of 911 keV energy. This transition is probably from the 1000 to 89 keV level. A y-ray with 911 keV energy was also observed in the singles spectra recorded with Ge(Li) spectrometer. (v) The sum-coincidence spectrum recorded with the gate setting at 1295 keV energy gave evidence for a new transition of 295 keV energy from 1295 to 1000 keV level. Our lifetime measurement further supports the conclusion of Moss and McDaniels [ref. ‘)I that the lower excited states in 99Ru are the result of a multiplet formed by a coupling of the 2d, neutron to the first 2+ state of the even core of the neighbouring nucleus 98R~. The authors are grateful to Professor G. K. Mehta and Dr. B. V. N. Rao for going through the manuscript and for many helpful discussions and to Professor S. Mukherjee of the Chemistry Department for suggesting the method and for supervising the radio-chemical separations, We are also thankful to Mr. B. K. Jain for his help during the measurements. References 1) G. A. Moss and D. K. McDaniels, Phys. Rev. 162 (1967) 1087 2) P. I. Connors, thesis, Pennsylvania State University, 1966 3) N. M. Anton’eva, E. P. Grigor’eva and L. F. Protasova, Izv. Akad. Nauk SSSR (ser. fiz.) 34, no. 4 (1970) 865; Bull. Acad. Sci. USSR (whys. ser.) 34, no. 4 (1971) 771 4) C. Rangacharyulu, S. N. Chaturvedi, G. K. Mehta and N. Nath, to be published, 1971 5) D. K. Gupta and G. N. Rao, IIT/K report no. 5/71, March 1971, to be published, 1971 6) E. Matthias, S. S. Rosenlelum and D. A Shirley, Phys. Rev. 139 (1965) B532 7) 0. C. Kistner, S. Monara and A. Schwarzschild, Phys. Rev. 137 (1965) B23 8) E. Bodenstedt, G. Gunther, J. Rodeloff, W. Engles, W. Delang, M. Forker and H. Luig. Phys. Lett. 13 (1964) 330