The 205TI level scheme as investigated by nuclear resonant scattering of 7646 keV γ-rays

Nuclear Physics A141 (1970) 561--576; (~) North-Holland Publishing Co., Amsterdam

Not to be reproducedby photoprint or microfilmwithout written permissionfrom the publisher

T H E 2°STlLEVEL S C H E M E AS I N V E S T I G A T E D BY N U C L E A R R E S O N A N T S C A T I T J ~ I N G OF 7646 keY 7-RAYS

R. CESAREO, M. GIANNINI, P. R. OLIVA, D. PROSPERI and M. C. RAMORINO Laboratorio di Fisica Nucleare Applicata C.S.N. Casaccia C.N.E.N., Roma, Italy

Received 23 July 1969 Abstract: Monochromatic photons, with energy 7646 keV, produced by thermal neutron capture in iron, resonantly excite a a°STl level, with spirt ½. The scattering isotope was identified by coincidence measurements. From the study of the primary inelastic photons de-exciting the resonant level, 16 new levels were observed, between 1300 and 3300 keV, in addition to 7 previously known. The 2°ST1 level scheme is discussed and compared with theoretical predictions. E

NUCLEAR REACTIONS =°STl(~',7"), E=7646keV, measured resonance fluorescence o(E~,,, ~) 2°ST1 deduced levels, J. Natural targets, Ge(Li), NaI(TI) detectors.

1. Introduction

In the last few years, the use of Ge(Li) detectors has allowed the application of the technique o f resonant scattering of monochromatic photons to spectroscopic studies by relating the primary y-transitions to the level structure of the resonant isotope t - s). The commonly employed y-ray source consists of a thick target submitted to a thermal neutron flux. This type of measurements gives information similar to that obtainable employing (n, y) reactions. In this paper, the 2°STl energy levels were studied using the (y, y') reaction on natural thallium. The y-beam was provided by the Fe(n, y) reaction. Its spectrum is characterized by the existence of a strong doublet at 7631.6 and 7645.6 keV in addition to several weaker y-lines 6). The scattered radiation was analysed using a 30 em 3 coaxial Ge(Li) detector and consists of 24 y-lines, of which the highest in energy is 7645.6 keV, belonging also to the direct spectrum. The spectrum of the low-energy secondary photons in coincidence with the primary y-transitions was observed. The analysis of this spectrum supports the attribution of the resonant level to the 2°ST1 isotope. The directional correlation of the elastically scattered y-rays was measured and a v a l u e j = ½ for the resonant level was obtained. 561

562

a. cm~n~o et al.

The decay scheme of the 7646 keV resonant level in 2 o STl was deduced and new levels were found, by assuming that all strong high-energy 7-rays are primary transitions. The justification of this latter assumption is given in subsect. 3.1. The values obtained for the partial radiative widths connected to the de-excitation of the resonant level are compared with the expected ones in subsect. 4.1. Finally, the 2°5TI low-lying states are briefly analysed in subsect. 4.2. Resonant scattering of T1 of ?-rays produced by neutron capture in Fe, first observed by Ben-David et al. 7) was then studied by Ramchandran and McIntyre s) and Moreh et aL 9), using NaI(TI) detectors. During recent measurements of the 2°6pb(t, g)2°ST1 reaction lo) 14 levels below 3 MeV were observed, with j~ = (½, ], ~r, 5) + and ~ - . By using the 2°STl(p, p') reaction, a doublet at 2.61 and 2.69 MeV with j~ = ~- and 5- respectively, was recently identified 11). Finally, from the 2°SHg decay, only the 205 keV first excited level is populated lZ). After the present work was completed, a similar study of the (?, ?') reaction in Z°STl using Ge(Li) detectors came to our knowledge t. The results are in agreement with the present one, except for a few minor points.

2. Experimental method The experimental arrangement is shown in fig. 1 and is extensively described in ref. 4). The neutron source was provided by the 1MW RC-1 reactor of the C.N.E.N. Casaccia Center. The 7-ray source, consisting of about 2 kg of natural iron, was placed near the reactor core along a tangential-piercing beam tube. The source was maintained at room temperature through a dosed circuit of demineralized water. The spectrum of the scattered y-rays was detected by means of a 30 cm 3 Ge(Li) detector and recorded with a 4096-channel pulse-height analyser. A Pb screening about 50 cm thick, was placed between the detector and the reactor core in order to shield the detector from the direct beam. To measure the directional correlations and the spectrum of the incident beam, the detector was mounted on a platform pivoting on an axis passing through the center of the scatterer. The beam was monitored with a second 30 cm 3 Ge(Li) detector arranged in such a way as to select a convenient line of the incident beam. Coincidence measurements were performed by selecting primary inelastic photons by means of a 12.7 cm x 12.7 cm NaI(T1) detector and by observing the low-energy secondary transitions with either a second 12.7 cm x 12.7 em NaI(Tl) or a 30 cm a Ge(Li) detector. In this measurement, the face of the target was oriented perpendicular to the incident beam, and the two detectors were placed symmetrically about the beam direction, both at ~ = 135 °, with a target-detector distance of 15 era. The resolving time of the coincidence circuit was 2z = 35 nsec. t The authors would like to thank Dr. R. Moreh for sending a preprint of his work x3).

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3. Experimental results 3.1. SPECTRUM OF THE SCATTERED PHOTONS

The scattered spectrum, measured at an angle of ~ = 145 ° with the NaI(TI) detector, is shown in fig. 2. The background due to non-resonant processes is also shown and was determined by replacing the T1 target with a Bi one. In addition to the 7.64 MeV resonant line, other T-rays at lower energy are clearly observed. The same spectrum, measured with the Ge(Li) detector, is shown in fig. 3. Energies and intensities of all T-lines are reported in table I. The energy resolution is 15 keV 104

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at about 5 MeV. Such a value, higher than expected, is partially due to pile-up effects of small energy pulses. To reduce these effects a Pb absorber, 5 mm thick, was placed in front of the detector. In the energy range 4 to 8 MeV, 21 T-lines were dearly observed. For 10 of these it was possible to carry out a careful analysis of the intensity ratio between doubleand single-escape peaks, which has given the result d.e.p. r = - = 4.0+0.2, s.e.p. in agreement with previously reported values 14). However, the 4923 and 5556 keV

2°STI LEWLscI-m~

565

TABLE 1 Energies and intensities of elastic and inelastic y-rays scattered from a TI target Ey (keV)

1~, (rel) %

pres. work (keV)

6505 4-3

3.604-0.55

0 211±5 623±6 1141±3

63094-3 6213 4- 3

0.804-0.15 1.75 4-0.30

13374-3 1433 -4-3

6089 4- 3 60714-5 a) 57724-4 d) 56814-2 5645±3

1.25 4-0.20 0.754-0.15 0.50+0.15 7.7 4-1.2 3.354-0.50

15574-3 15754-5 18744-4 1965±2 2001 ±3

55564-3

2.90±0.45

2090±3

5483 - I - 3 54364-5 a) 54244-3 5346±3 5331 4-3

1.00-4-0.20 0.454-0.15 2.004-0.30 0.654-0.15 0.754-0.15

2163 4-3 22104-5 22224-3 23004-3 23154-3

7646

61.7 ±9.2

Energy levels ") (MeV)

b) (MeV)

c) (keV) 0

2054-2 615-I-5

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2.43 2.49 5088±3

3.4 -4-0.5

25584-3

49774-4

0.304-0.10

2669±4

4941 ±5 4923 ±3 4895 ±3 4750±3 4625 -4-3 44704-5 4357:t:3

0.25-4-0.10 2.05 ±0.30 1.05 4-0.20 1.75 ±0.30 0.85 ±0.15 0.50+0.15 0.654-0.15

2705-I-5 2723 ±3 2751 ±3 2896±3 3021 ±3 31764-5 3289±3

2.61 2.69

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~°ePb(t, u)2°STl reaction. The errors are -t-20 keV. Ref. 1o). Ref. xx). No error is given. Ref. 12). The evidence for thisF-transitionis regarded as uncertain.

lines have s h o w n the a n o m a l o u s values r = 2.1-t-0.2 a n d r = 2.3-t-0.2 respectively. F o r this reason, the existence o f a d d i t i o n a l t r a n s i t i o n s a t 5436 a n d 6071 keV was deduced. Also the 5772 keV ~-line was deduced i n order to a c c o u n t for the a n o m a l o u s d.e.p./f.e.p, value o f the 4750 keV ~-line. F r o m a c o m p a r i s o n between scattered a n d i n c i d e n t ~-ray spectrum, n o n e o f the scattered fines were f o u n d to a p p e a r i n the direct b e a m , except the 7646 keV one.

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568

R. CESAREO et aL

Such a remark, together with the fact that the background spectrum shows no evidence of v-lines, led us to the assumption that all the v-rays we observed are due to primary transitions de-exciting the 7646 keV resonant level. However, further different interpretations could be given in order to explain the nature of some v-lines appearing in the spectrum of fig. 3. They are: (i) v-lines produced by radiative capture of epithermal neutrons in the Tl target or in the Pb surrounding the detectors. However, the 7.38 MeV line produced in the Pb(n, ?) reaction, and the most intense lines produced in the Tl(n, V) reaction were not found in our spectra. (ii) v-lines originating in the Tl target and due to nuclear elastic scattering of weak undetected photons present in the incident beam and/or to inelastic components connected to unobserved elastically scattered y-rays. This hypothesis is more sophisticated and cannot be excluded apriori. However a recent study of Moreh and Nof 3) shows that the probability of these processes occurring is very small. (iii) y-lines corresponding to secondary transitions de-exciting the single resonant level. Such a hypothesis does not seem very reliable. First of all, there is a low probability that the low-energy primary transitions occur because of the E 5 (E 3) dependence of electric (magnetic) dipole transitions. Secondly, even if a high-energy level is populated, its decay leads - in most cases - to more than one v-ray. Highenergy strong y-rays are then more probably primary rather than secondary transitions, and such a statistical argument can be questioned only in the case of middle levels. Since the levels observed here are all below 3.3 MeV, we then conclude that all the examined scattered photons can be thought of as primary transitions de-exciting the 7646 keV resonant level.

3.2. ENERGY AND INTENSITY CALIBRATION The energy calibration was performed using two v-lines of the scattered spectrum: the 7646+_+_1 keV v-ray 6) with its single- and double-escape peaks and the doubleescape peak of the 5681 + 2 keV. The first line was identified as the highest of the Fe doublet. The identification was made possible by the comparison with the wellknown elastic component of the Fe-Cd resonance which has energy 7632_+ 1 keV [ref. 4)]. The 5681 keV v-line was identified as the transition populating the 1965 keV level, whose energy was measured in the coincidence spectrum by using the Ge(Li) detector. The non-linearity of the recording system was determined to be less than 1 channel between the channels 1000 and 3500 and the corresponding error ( ~ 2 keV) was added to those due to the statistics and to the energy calibration. The relative efficiency for the detector was measured using the relative intensities of the well known Cl(n, V) V-lines, employing the external neutron beam facility at the RC-1 reactor. A minimum relative error of 15 ~ was assumed for the measured intensities.

=°s'rl LSV]BI.~

569

3.3. COINCIDENCE MEASU1LEMENTS AND IDENTIFICATION OF THE SCAT]'ERING ISOTOPE The low-energy spectrum measured in coincidence with the photons selected in the energy range 3.5-6.5 M e V is s h o w n in fig. 4. The spectrum o f random coincidences is also shown, for comparison purposes. Transitions at 2 1 1 , 4 1 5 , 935, 1135, 1240, 1380, jr-5tl 211

415

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21 a

aoo

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Fig. 4. Spectrum of the secondary low.energy y-rays in coincidence with the primary inelastic photons

scattered from a T1 target. The coincidences were taken with two 12.7 ~ × 12.7 cm NaI(TI) both at # = 135°. The high-energy window selected an energy range of 3.5-6.5 MeV. The intense peak at 0.51 MeV, which appears in the spectrum and the full-line are made up of accidental and background coincidences. The peaks are labelled by numbers which indicate the energy of the ~,-lines in keY.

1965, 2580 and 3010 keV were observed. From a spectrum decomposition, additional ?-lines at about 630, 1470, 1565, 1700, 2020, 2720 and 2900 keV could be tentatively identified. All these ?-lines could be inserted in the level scheme and explained as secondary ?-transitions.

570

it.

C~SAr.EO e t al.

The same coincidence spectrum was also measured using the Ge(Li) detector, and y-rays at 412, 936, 1135, 1965 keV were dearly observed. In order to analyse the 211 and 415 keV y-transitions better, the coincidence spectrum was more carefully measured with the NaI(T1) detector and analysed in the energy interval (0-600 keV). The two lines are both dearly present, and this gives an unambiguous support to the condusion that the resonant isotope is the 2°5T1. This result is in disagreement with the assumption of previous authors s, 9), who assigned the resonant level to the 2°3T1, because of the (y, n) thresholds which are at 7.66 and 7.53 MeV for 2°3T1 and 2°5T1 respectively is). 3.4. DIRECTIONAL CORRELATION MEASUREMENT OF THE 7646 keV y-LINE

In order to measure the directional correlation of the 7646 keV scattered photons, the scattered spectrum was measured at six different angles (120 °, 125°, 130°, 135°, 140°, 145°) with respect to the beam direction. A dipole angular distribution W(#) = Ao+A2 cos 2 was assumed. Corrections due to the finite dimensions of the scatterer and of the detector, to the self-absorption in the scatterer and to the efl~ciencies of the NaI(T1) crystals employed, were calculated by means of Monte Carlo programmes le). From the experimental data the value A_._~2= _(1.4+3.3)10_ 2 Ao was deduced, in agreement with the theoretical value A2 = 0 for the accepted spin sequence ½(1)~(1)~. The measured spin of the resonant level is then ½, in agreement with the previously reported values s,9). Let us note that the ground state of both the T1 stable isotopes has spin ½. 4. Discussion 4.1. RADIATIVE WIDTHS

The branching ratios F ~ / F 7 for the decay of the 7646 keV level can be directly obtained from table 1. In this ratio FT~ is the partial radiation width to the ith level, and F 7 = ~ Fr, is the total radiation width. Our experimental results give for the decay to the ground state FT----~*-- 0.617+0.092. F7 Ramehandran and MeIntyre s) and Moreh et aL 9) have obtained from scattering and absorption cross section measurements F~./Fy = 0.85 and Fro/F ~ = 0.69 respectively. These values were established on the assumption that the resonant isotope was 2°3T1, but the branching ratio is, to a first approximation, independent of the resonant isotope 17).

20$T1 LEVEL

SCHEME

571

In order to analyse the multipolarity of the v-transitions involved in the decay of the 7646 keV resonant level, the measured rT, values can be compared with the average values expected for dipole transitions in heavy nuclei. To this purpose we define the reduced width K~I for E1 and M1 radiations by means of the relationship

K¢I = F,,E~"D -1, where D is the average spacing at the initial state of levels with the same spin and parity as the radiating state and E~ is the energy of the transition. The average reduced width for E1 transitions to the ground state in the 6-8 MeV range was estimated by an extrapolation of the giant resonance is). This calculation gives an E# dependence for r~o and the approximate relationship

g~.~ = (F,oE~'SD -1) ~ 6.10-1SA I

(1)

was deduced. Here Fro and D are in the same units, Er is in MeV and A is the mass number. An extensive study of the partial radiation widths in resonance capture was made by Carpenter x9). He found for r~,, the same dependence on Ey and A as in eq. (1), and the best fit to the data is given by

i~B, = (F~,EfSD -l> m 3.10-1SA 1

(2)

in a satisfactory agreement with eq. (1). The available data on M1 transitions are still quite fragmentary. However all data seem to be consistent with an E~ dependence of the radiation widths 2 o). The relationship

KMt = (Fv,E~'3D -1> ~. 2 . 1 0 -s

(3)

was obtained 21), which is intended to be meaningful only over the limited energy range 6-8 MeV and for A > 90 t. Eqs. (1), (2) and (3) can be summarized for 2°5T1 by the relationships = 9 . 1 0 -9 /~nt = 5" 10 -9 K'Mt = 2" 10 - s

from giant resonance extrapolation, from resonance capture data, from resonance and thermal neutron capture data.

The values of the reduced widths obtained in the present work, calculated both for electric and magnetic dipole transitions are shown in table 2. The value r 7 = 1 eV was taken from the paper of Moreh et aL 9), and the value D = 1 keV has been adopted as obtained from the well known Lang-Le Couteur level density formula. In the assumption that all transitions are M1 or El, we can calculate the average of t In ref. 2x) the experimental characteristics of radiative transitions from highly excited states are extensivelydiscussed.

572

g. ~

et al.

our d a t a o v e r the final states. W e thus o b t a i n /~m =2'10

-7 ,

/~E1 = 6 " 1 0 - 9 I t is clear t h a t the best a g r e e m e n t with the systematics is o b t a i n e d b y s u p p o s i n g the o b s e r v e d t r a n s i t i o n s to be p r e d o m i n a n t l y E l . T h e s a m e result is o b t a i n e d i n t r o d u c i n g in t h e average o n l y the t r a n s i t i o n s t o the lowest-lying levels which have a positive p a r i t y . Then, the p r e v i o u s a r g u m e n t s suggest a negative p a r i t y f o r the r e s o n a n t level. TABLE2 Reduced widths for E1 and M1 radiation of the transitions from the 7646 keV resonant state of 2°STl Transition energy (keV)

Level energy (keY)

KMz • 10a

K,~I • 109

7646 6505 6309 6213 6089 6071 5772 5681 5645 5556 5483 5436 5424 5346 5331

0 1141 1337 1433 1557 1575 1874 1965 2001 2090 2163 2210 2222 2300 2315

138 13 3 7 6 3 3 42 19 17 6 3 12 4 5

24 3 1 2 2 1 1 13 6 6 2 1 4 2 2

5088

2558

26

I0

4977 4941 4923 4895 4750 4625 4470 4357

2669 2705 2723 2751 2896 3021 3176 3289

2 2 17 9 16 9 6 8

1 1 7 4 7 4 3 4

A f u r t h e r analysis o f t a b l e 2 allows the o b s e r v a t i o n o f a large s p r e a d i n g o f the r e d u c e d w i d t h s a r o u n d the average value. G r o u p s o f transitions, c h a r a c t e r i z e d b y large r e d u c e d widths, a r e o b s e r v e d at a b o u t 5600 a n d 4600 k e V c o r r e s p o n d i n g t o e x c i t a t i o n energies a t a b o u t 2 a n d 3 MeV. I n o r d e r to e x p l a i n this p o i n t we m u s t clearly classify these states as h a v i n g large o n e - p a r t i c l e - o n e - h o l e core excitations. This p h e n o m e n o n is p r o b a b l y c o n n e c t e d to the b u m p o b s e r v e d a t a b o u t 5 M e V in the ~-ray s p e c t r a f o l l o w i n g the n e u t r o n c a p t u r e in nuclei n e a r the closed shell w i t h

573

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.574

x. ~

et al.

l l 0 =< ,4 ~_ 140 and 180 < .4 _~ 205 [ref. 22)]. This bump is known to be prevalently E1 radiation. 4.2. D E C A Y SCHEME

In column 3 of table 1 the energies of the low-lying levels populated in our experiment are given and compared with the results of other authors lo-12) (columns 4-6). Even if the different accuracy of the measurements does not allow a reliable comparison, nevertheless 9 levels seem to coincide. The decay scheme of the ½- resonant level in 2o ST1 as deduced by our measurements is shown in fig. 5. On the basis of the considerations of subsect. 4.1 the most intense 25G0

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1+

T THEOR, dJ

Fig. 6. A comparison of the 2°STl level scheme as determined by the experimental data and by the theoretical predictions. On the left the full lines refer to results of the (7, ~') reaction (present work) and dashed lines refer to results of the 2°ZPb(t, =)2°5T1 reaction. The spin and parity assignment shown in brackets are discussed is subsect. 4.2. The subscripts on the levels havethe following meaning: a) the evidence for this level in regarded as uncertain (see subsect. 3.1). b) the spin attributions of these levels were proposed in ref. ,o). c) Ref. 23). d) Ref. 24).

2°STl LEVELSCHEME

575

y-transitions should populate levels with j~ = ½+ and ½+. On the other hand, since y-transitions having E2 or M2 multipolarity are weaker than E1 transitions, levels with j = ~r should be weakly populated. In fact, we are not able to observe primary transitions to the ~+ second excited level, and to the 2.61 MeV (j" = ~r-) state found in the 205Tl(p ' p,) reaction 11). No direct transition is expected to occur between the 7646 keV resonant level and any level whose spin is greater than ~. For this reason the level at 2669 keV deduced from the present work should not be identified with the 2.69 MeV level observed in the 2°STl(p, p') reaction 11), which has spin ~. As a final remark, let us point out that no primary inelastic y-rays to the 205 keV (j~ = ½+) and 1.21 MeV (j~ = ½+) levels were found in the present experiment, while they were both weakly observed by Moreh and Wolf 13). In fig. 6 the energy levels obtained from experimental data are compared with the results of some recent theoretical calculations 23,24). In a preliminary analysis reported at the Studsvik Symposium 25) a tentative spin assignment based on a comparison between experimental and theoretical data was suggested. However, a further more careful analysis led us to the conclusion that at the present time this comparison cannot be made and that there is a general disagreement between experimental and theoretical results above 1 MeV. It is very likely that the description of levels around 1.5 MeV may require additional theoretical calculations involving a larger configuration space and more realistic effective interactions. The auuaors wish to express their appreciation to Dr. A. Aquili who made the hardware and software for acquisition and handling of data with a PDP-9 computer. The cooperation of Dr. F. Rimondi in evaluating the data of the directional correlation is gratefully acknowledged.

References I) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)

N. Shikazono and Y. Kawarasaki, Nucl. Phys..4,118 (1968) 114 Y. Schlesinger, B. Arad and G. Ben-David, Phys. Rev. 178 (1969) 2013 R. Moreh and A. Nof, Phys. Rev. 178 (1969) 1961 R. Cesareo, M. Giannini, P. Oliva, D. Prosperi and M. C. Ramorino, NucL Phys. A|32 (1969) 512 V. E. Michalk and J. A. Mclntyre, Nucl. Phys. A137 (1969) 115 N. C. Rasmussen, Y. Hukai, T. Inouye and V. Orphan, AFCRL-69-0071, January 1969 G. Ben-David, B. Arad, J. Baldermann and Y. Schlessinger, Phys. Rev. 146 (1966) 852 S. Ramchandran and J. A. McIntyre, Phys. Rev. 179 (1969) 1153 R. Moreh, O. Shahal and S. Shlorao, Bull. Am. Phys. Soc. 13 (1968) 1368 S. Hinds, K. Middleton, J. H. Bjerregard, O. Hansen and O. Nathan, ~ucl. Phys. 83 (1966) 17 J. P. Wurm and E. Grosse, Phys. Lett. 28B (1969) 413 C. Lederer, J. M. Hollander and J. Perlman, Table of isotopes (J. Wiley and Sons Inc., New York, 1967) p. 394 R. Moreh and A. Wolf, private communication and Phys. Rev. 82 (1969) 1236 B. J. Allen, J. R. Bird and S. Engstrom, Nucl. Instr. 53 (1967) 61 J. H. Mattauch, W. Thiele and A. H. Wapstra, Nucl. Phys. 67 (1965) 32 M. Giannini, P. Oiiva and M. C. Ramorino, RT/FI (1969) 15; Nucl. Instr. to be published

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R. C E S A U O

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