Volume 34B. number 7 PHYSICS LETTERS 12 April 1971

THE 9,‘2- [514] STATE IN 185Re

M. EVANS, A. E. ELLIS, J. R. LEIGH* and J. 0. NEWTON** Department of Physics. University of Manchester. Manchester, England

Received 2 March 19’71

Gamma rays following the 186 W(d, 3n)

185 Re reaction have been observed. Evidence is given for

rotational bands based on the 5/2+[402] and 9/2-[514] states. The half-life of the 9/2- state has been measured and found to be 33 f 3 ns.

The levels of lS%e have been studied by Cou- lomb excitation [l], inelastic deuteron scattering [2] and from the decay of 1850s [3,4]. From this data it can be concluded that 185Re is a deformed nucleus. Observed levels have been assigned to a number of Nilsson states and associated rotational or vibrational bands. Most of the states seen in

l81,183,187Re have also ~~i?i$?!?;~~~,c?~t the 9/2-[514] state has not. It was probably not seen in the previous ex- periments, since both Coulomb excitation and in- elastic deuteron scattering preferentially popu- late collective states based on the ground state and since 1860s has a spin of l/2-.

We have studied 185Re through the reaction 186W(d, 3ny)lf15 Re. Such reactions tend to popu- late states of high spin and, being statistical in nature, to be nonselective regarding the struc- ture of the states. Recently, similar studies of the 181Ta(o!, 2ny)183 Re reaction [5] showed popu- lation of rotational bands based on the 5/2+[402] ground state, 9/2-[514] and l/2-[541] states and also of a 25/2+ isomeric state [6].

Deuteron beams, produced by the Manchester University HILAC, bombarded targets consisting of self-supporting foils of 186W-oxide powder suspended in Formvar. A 25 cm3 Ge(Li) detector, having a resolution of 2.2 keV at 1173 keV, was used to detect the y-rays. In order to make iso- topic assignments for the y-rays, measurements were made at bombarding energies of 12, 16 and 19.6 MeV. The latter is the maximum deuteron energy from the accelerator and the best for production of 185Re. Spectra were also taken

Present address: Lawrence Radiation Laboratory, University of California, Berkeley, California 94720, USA. Present a.ddress: Department of Nuclear Physics, The Australian National University, Canberra, A.C.T. 2600, Australia.

with a target consisting only of Formvar, SO that y-ray originating from this could be identi- fied.

The y-rays which we assigned to 185

Re could all be attributed to the population and decay of two rotational bands (see fig. 1). The assignment of states to the 5/2+[402] ground state band was made on the basis of the excitation functions, accurate energy sums, intensity flows and the systematics of rotational bands. The energies of the states of this band up to that with spin 13 ‘2

'fv*!

t

loo0 ‘%2+ 949.5

32

767-3

13/2+ 697.0 ‘3+- +l- 52

w*- 6 546-8

‘r,+ 475.6

9/2+ 264.1

_1 77

9/2- 366.1

s14

208 5/2+ J

5/2+[402] 165

9/2- [ 5141

RI

Fig. 1. Partial level scheme for 185

Re showing observed y rays and relative transition intensities.

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Volume 34B, number 7 PHYSICS L E T T E R S 12 April 1971

a re consis tent with prev ious data [2] but more accura te . The evidence for this band up to the 15/2 + state s eems ve ry well founded; that for the 17/2 + state is weaker s ince it is based only on sys temat ics .

In the 181Ta(ot, 2ny)183Re reac t ion [5] the 9/2-[514] rota t ional band is s t rongly populated and the 9/2- s tate decays via a 381 keV t r ans i - tion to the 7/2 + m e m b e r of the ground band. This y - ray is the s t ronges t in the spec t rum because it is E l , thus having a low convers ion coefficient . The 11/2- to 9 /2 - t rans i t ion in 183Re has an energy of 163 keV. In 185Re this t rans i t ion will be expected to have a somewhat higher energy, s ince 185Re is l e ss deformed than 183Re. We es t ima te it to be roughly 183 keV, if we assume that the ra t io of its ener - gy in 185Re to that in 183Re has the same value as the cor responding ra t io for the 7/2 + to 5/2 + t rans i t ion of the ground-s ta te band.

The 242.9 keV y - ray is the most intense of those at t r ibuted to 185Re and is t he re fo re most probably analogous to the 381 keV line in 183Re. If this is c o r r e c t it p laces the 9/2-[514] state in 185Re at an energy of 368.1 keV, which f i ts in well with the sy s t ema t i c s of the odd rhenium isotopes [5]. The s trong y - r a y with an energy of 178.7 + 0.1 keV, quite c lose to the es t imated 183 keV, is l ikely to a r i se f rom the decay of the f i r s t ro ta t ional s tate of the K = 9 /2- band. On the bas is of excitat ion functions, in tens i t ies and ro- tat ional band sys t ema t i c s the y - r a y s of energy 210.5 + 0.1 keV and 277.0 ± 0.3 keV are ass igned to the decay of the 13/2- and 17/2- m e m b e r s of the band. The t rans i t ion de-exc i t ing the 15/2- state would be expected to have a lmost the same energy as the s trong 242.9 keV y - r ay and indeed the width of this line in the spec t rum was ob- s e rved to be 0.15 keV g rea t e r than that of the 238.6 keV line f rom a thor ium emanation source taken under s imi l a r conditions with a h igh- re so lu - tion Ge(Li) X - r a y detector . As in 183Re the c r o s s - o v e r t rans i t ions in this band are r e l a t ive ly weak and were , with one poss ible exception, not observed in this work. Thus the re is no good supporting evidence f rom energy sums for the band.

The E1 t rans i t ions between the 9/2 9/2-[514] + 181,183,187 e s ta te and the 7/2 5/2 [402] state in R

a re all obse rved to be s lower than the s ingle- pa r t i c l e e s t ima te by a fac tor of a lmos t 10 °. We the re fo re measu red the hal f - l i fe assoc ia ted with the 242.9 keV t rans i t ion to see whether it was s i m i l a r l y hindered in 185Re.

The m e a s u r e m e n t was fac i l i ta ted because the beam f rom the a c c e l e r a t o r cons is ted of pulses

400"

2 0 0

_j 0 w Z Z <[

u 20C u) I - Z

0 u 0

2 0 0

I 0 0

0

COINCIDENCE WITH ENERGY WINDOW AT 2 4 2 . 9 k e V

•,•°

°° ° ~ % °°°~

~° %...'- • •~°~,°•

COINCIDENCE WITH BACKGROUND

• •*% ••~.

• • ,•

,* • .•" • , ••

* . S ' * * • ° ", • %.

COINCIDENCE WITH 2 4 2 . 9 k e V PEAK

I I I I I I I 0 20 4 0 6 0

T IME (Its)

Fig. 2. Time spectra for the 242.9 keV y-ray. The full line is a calculated fit to the data for a half-life of 33 ns; finite time resolution has been taken into account•

Representative errors are shown.

approximate ly 3 ns long occur r ing every 40 ns. The y - r a y s were detected in a 4 cm 3 p lanar Ge(Li) de tec tor and the digital informat ion for each event, re la t ing to the pulse height and to the t ime of a r r i v a l af ter the beam pulse, was s tored on magnet ic tape. T i m e spec t r a for individual y - r a y s were obtained by scanning the tape with appropr ia te digital windows set over the peaks and over the backgrounds adjacent to them. Since a y - r ay continuum is always p resen t in this type of exper iment , subtract ion of the background is essential•

In fig. 2 the t ime spec t r a appropr ia te to the 242.9 keV y - r ay a re shown. F r o m these a value of 33 ± 3 ns is obtained for the ha l f - l i fe of the 9 /2- state, if one a s sumes our ass ignments and no delays of this o rde r in the major y - r ay cas - cades feeding it. Indeed the 178 keV t ransi t ion, which according to our scheme provides most of the population for the 9 /2- state, shows no meas - urable delay.

A sum m ary of the data on this E1 t rans i t ion in the odd rhenium isotopes is given in table 1. All a re highly hindered by about the same fac tor

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Volume 34B. number 7 PHYSICS LETTERS 12 April 1971

Table 1 Half-lives 7s of the g/Z-[5141 states in odd rhenium

have only seen states having spin up to 1’7/2, as

nuclei and partial half-lives ryand energies Eyof the compared with 23/2 for example in 183Re, and

radiative transitions to the 7/2 5/2+[402] states. it is not possible to say whether the alternating

The %.

were obtained from the 7s by correcting for term is required or not. If it had the same branc mg ratios and internal conversion. The hindrance magnitude as that for 183Re, it would not have factors are taken relative to the single-particle estimates. been seen. For the K = 9/2 band we find

Nucleus, [ref.] ‘*ke [7] 183Re [8] 185Re 18’7

Re (91 A = 16.20 * 0.10 keV and B = +0.2 f 1.0 eV. In

this case we did not include the cubic term in

Ev OteV) 144.8 381.8

rs (ns) 156ilO 7.7*0.5

7~ (ns) 180*12 8.350.5

Hindrance x10-6 3.8hO.3 3.2+0.2

242.9 72.0 the calculation as it was felt not to be significant.

33*3 570*50

34+3 1070*100 References

3.4*0.4 2.8kO.3 [l] F. K. McGowan and P. H. Stelson, Phys. Rev. 109 (1958) 901.

of 3 x 10-6. This result can be regarded as further confirmation of our assignment.

Values for the coefficients A, B, C, occuring in the usual power series expansion in Z(Z+ 1) for rotational band energies, were deduced from the measured energies. For the K = 5/2 band we

[2] K.H. Bisgaard and E.Veje, Nucl. Phys. A103 (1967) 545.

[3] M. W. Johns, S.V. Nab10 and W. J.King, Can. J. Phys. 35 (1957) 1159.

[4] B. Harmatz and T. H. Handley, Nucl. Phys. A121 (1968) 481.

[5] J. 0. Newton, Nucl. Phys. A108 (1968) 353. [6] M. J. Emmott, J. R. Leigh, J. 0. Newton and

D.Ward, Phys. Lett. 20 (1966) 56. obtain A = 18.38 h 0.03 keV B = -16.6 f ~I_$E 39 f 15 meV. In 18’Re [5] and in

0.7 eV

Re [lo] a term of alternating sign, in addition to those mentioned above, is required to fit the band energies. In the present case we

[7] P. F. A. Goudsmit, Physica 35 (1967) 479. [a] J. 0. Newton, Phys. Rev. 117 (1960) 1520. [9] H.K. Walter, A.Weitsch and P.Kierle, 2. Physik

175 (1963) 520. [lo] J.R. Leigh, A. E. Ellis, M. Evans and J. 0. Newton,

to be published.

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