Level structure of 100Tc

Nuclear Physics A321 (1979) 2 5 - 4 4 ; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

LEVEL

STRUCTURE

OF

1°°To

J. A. PINSTON t, W. MAMPE, R. ROUSSILLE and K. SCHRECKENBACH

Institut Laue-Langevin, 156 X Centre de Tri, 38042 Grenoble Cddex, France D. HECK

Institut ffir Anoewandte Kernphysik, Kernforschunoszentrum Karlsruhe, Karlsruhe, Germany H. G. BORNER and H. R. KOCH

Institut fffr Kernphysik, Kernforschunosanlaoe Jfflich, Jfflieh, Germany and S. ANDRE and D. BARNEOUD

lnstitut des Sciences Nucldaires (IN2P3), Grenoble, France Received 14 December 1978 Abstract: Gamma and electron spectra following thermal neutron capture on 99Tc have been studied

with a bent-crystal spectrometer, Ge(Li) and Si(Li) detectors and a magnetic spectrometer. Prompt and delayed ~,-~coincidences with Ge(Li) detectors have been performed. A level scheme is proposed for l°°Tc comprising 21 excited states up to 640 keV. The binding energy of the last neutron in l°°Tc was deduced. For most levels, spin and parity values were assigned. Two isomeric transitions of respective half-lives 10.2 and 4.6 /~s have been identified using the l°°Mo(d, 2n)l°°Tc reaction with a pulsed beam of deuterons. From ,the comparison of the present (n, y) study and the collaborative study of the 99Tc(d, p) reaction, several members of the multiplets ng9/2vgT/2, rtg9/2vds/2, ltg9/zvsl/2 and 7tPl/2vds/2 have been identified. NUCLEAR REACTIONS 99Tc(n, ~), E = thermal ; measured E~, l~, Ece,lcc, yy-coin, delayed y~-coin, l°°Tc deduced levels, J, ~, ~,-multipolarities, neutron binding energy Bn. Ge(Li) and Si(Li) detectors, bentcrystal and magnetic spectrometers, l°°Mo(d, 2n), E = 12 MeV; measured E~, Iv(t). t°°mTc levels deduced TI/2. Pulsed beam, enriched target.

1. I n t r o d u c t i o n

T h e l o w - l y i n g s p e c t r a o f o d d - o d d T c i s o t o p e s w i t h 94 < A < 98 h a v e b e e n s u b j e c t to a n u m b e r o f i n v e s t i g a t i o n s 1 - 3). F o r t h e s e n u c l e i t h e l o w e s t states a r e a s s i g n e d to t h e n e u t r o n - p r o t o n m u l t i p l e t g g ] v d ] a n d t h e n - p r e s i d u a l i n t e r a c t i o n c a n b e s t u d i e d in t h e c a s e o f a v a r i a b l e n u m b e r o f n e u t r o n s . T h e s y s t e m a t i c b e h a v i o u r o f * Present address: Centre d'l~tudes Nucl6aires de Grenoble, D~partement de Recherche Fondamentale/ CPN, Grenoble, France. 25

26

J . A . PINSTON et al.

this multiplet can be extended to the case of ~°°Tc. This nucleus is also a good candidate for the study of other multiplets, less well known, corresponding to the neutron in the s½, d~ and g~ orbitals. In order to gain the maximum information on the completely unknown a°°Tc nucleus, it was decided to examine the 99Te(d, p) reaction at the University of Bradford and also the complementary reaction 99To(n, )'). The results of the (d, p) reaction have been previously published 4). Preliminary results of the 99To(n, y) reaction have been presented at different conferences s-7). The main purpose of the present investigation was to construct the low-lying level scheme of a°°Te and to assign unique spin and parity values. For this purpose experimental information on )' and conversion electron spectra was obtained with a bent-crystal spectrometer (GAMS 1), a fl-spectrometer (BILL) and various Ge(Li) and Si(Li) detectors. Possible isomeric states, with half-lives in the ~s region, have been investigated from l°°Mo(d, 2n) reaction with a pulsed beam of deuterons.

2. Experimental methods 2.1. THE (n, ~) R E A C T I O N

Direct )'-spectra. The targets for the different measurements consisted of metallic technetium powder (obtained from Union Carbide Corp., Oak Ridge, USA) of different weights up to 1 g. Gamma rays, in the energy range 30 < E~ < 800 keV, have been measured with the 5.76 m curved crystal spectrometer GAMS 1 at the High Flux Reactor of the Institute Laue-Langevin in Grenoble s, 9). An angular resolution of 6" was achieved, corresponding to AE(keV) = 1.2 x 10-5x E2(keV)/n (n = reflection order). The absolute energy calibration was performed with two )'-lines previously observed in the decay of a°°Tc [ref. 10)] [539.59 and 590.83 keV] and reported with an accuracy of 50 eV. This error is larger than the relative precision, 6E(keV) = 5 x 10-Sx E~(keV)/n, which could be achieved and it should be noted that the relative precision only is used in the construction of the level scheme. The )'-rays not reflected by the curved crystal have been analysed simultaneously with the anti-Compton spectrometer 1~). This spectrometer is positioned behind GAMS 1 and consists of a 32 cm a Ge(Li) detector (2.1 keV FWHM at 1332 keV) surrounded by a 50 cm diameter x 40 cm plastic scintillator and a 10.2 cm diameter x 15.2 cm NaI(T1) detector. Measurements with this spectrometer covered the energy ranges 0.1 < E < 3.9 MeV in the Compton suppression mode and 3.7 < E~ < 7.5 MeV in the pair mode. At the Karlsruhe research reactor FR2 a Si(Li) X-ray detector (0.5 keV FWHM at 14 keV) was used to measure ),-transitions within the energy range 10 < E < 100 keV Prompt )'-)' coincidences. At the Karlsruhe research reactor FR2 a multidetector arrangement surrounding an external beam tube with a neutron flux of 5 x 10~

t O0Tc

27

n" c m - 2 . sec-1 (filtered by a 40 cm long cooled Bi crystal) was used to determine ?-7 coincidence relationships. This apparatus consisted of two coaxial Ge(Li) detectors (40 cm 3 with 2.5 keV F W H M at 1332 keV) and a planar Ge(Li) detector (4 cm 3 with 2.0 keV F W H M at 1332 keV). The detectors were coupled to a computer-based data acquisition system 12) which reduces the 4096 x 4096 channel coincidence matrix by digital window selection, and accumulates on-line 32 spectra of 4096 channels. Background substraction was done off-line by the double window technique. Delayed ?-~ coincidences were performed at the (n, 7) facility of the Jtilich reactor FRJ2, in order to find delayed coincidences with the very strong 172 keV transition (Iy ~ 50 ~o per neutron capture) in the/as time scale. In this experiment the coincidence condition was fulfilled when a signal of detector 1 was followed within 20/as by a signal corresponding to the delayed 172 keV transition, registered with detector 2. The energy was selected by means of a single channel analyzer. The period of 20/as preceding the 172 transition was devided into four groups each of a duration of 5/as. Four different energy spectra of detector 1 were recorded corresponding to each of these intervals. Conversion electron spectra, in the energy range 20 < E e < 300 keV, were measured with the/3-spectrometer BILL 13, 14-) at the Grenoble High Flux Reactor. A target of 60/ag/cm 2 thickness and 30 x 100 mm 2 area was used. The electrons were detected by a five wire thin window electron detector. The electron energies were calibrated with ?-ray energies measured with the curved crystal spectrometer GAMS 1. 2.2. THE (d, 2n) REACTION The investigation of the isomeric transitions in the ms region was performed on line at the Grenoble variable energy cyclotron. The l°°Tc nucleus was excited by the X°°Mo(d, 2n) reaction with 12 MeV deuterons on an isotopically enriched l°°Mo target. The 8 keV deuteron beam from the external ion source was pulsed in the injection channel (axial injection) by means of 300 V square waves applied between a pair of parallel beam-deflecting plates. The deflection signals were driven by a quartz pulser. The conditions of beam pulsed on for 3/as and off for 30/as were used. The 7-rays emitted by the target after the beam was switched off were detected with a Ge(Li) detector. The delay time of an event with respect to the beam burst was analysed by a time-to-amplitude converter (TAC). The "stop" pulse for the TAC was generated only by one of two beam pulses in order to improve the time calibration. The energy of the ?-events and delay time were registered simultaneously.

3. Results

3.1. GAMMA-RAY SPECTRA AND COINCIDENCE RELATIONSHIPS Part of the ?-spectrum of G A M S 1 spectrometer is shown in fig. 1.

28

J . A . P I N S T O N et al.

TABLE 1 G a m m a - g a m m a coincidence relationships observed in ~°°Tc

Lines observed in coincidence spectra (keV) 76 63 76 87 91 98 106 108 113 128 134 139/141 144/146 153/154 163 166 169 172 180 186 196 206/210 211/213 223 225/226 229/23 I 239 249 252/253 257 264 270 277 287/289 291/293 299 301 309 318/322 323/324 329 345/346 349 357/359 365/366 376 380/382 385/387

Digital gate on transition c) (keV) 87

91

106 128 141 144/146166 172 196 213/217223 225/226252 264 277 299 301

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TABLE I (continued) Lines observed in coincidence spectra (keV) 76 87 91 390 396/398 399/402 413 422 430 457 464 471 485/486 489/491 497 523 544/547 551 565/570 574 605 615 622 629 634 640 653 666/669 683 704 721 785 806 814 830 969/972

+

Digital gate on transition 0 (keV) 106 128 141 144/146166 172 196 213/217223 225/226252 264 277 299 301

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•) Uncertain. b) Probably contributed by line in neighbouring gate. 0 Only the strongest transitions within the gate are listed.

T h e m e a s u r e m e n t w i t h the G A M S 1 s p e c t r o m e t e r a n d w i t h t h e different G e ( L i ) s p e c t r o m e t e r s r e v e a l e d ~ 1150 y-lines a t t r i b u t e d to l ° ° T c . T h e y-energies a n d yi n t e n s i t i e s a r e g i v e n e l s e w h e r e 15). T h e a b s o l u t e y-ray yields a r e d e d u c e d f r o m t h e i n t e n s i t y v a l u e s xo) o f t h e 540 a n d 591 k e V t r a n s i t i o n s b e l o n g i n g to t h e X°°Tc ~ l ° ° R u decay. T h e results of t h e c o i n c i d e n c e m e a s u r e m e n t s a r e s u m m a r i z e d in t h e c o i n c i d e n c e m a t r i x p r e s e n t e d in t a b l e 1. T h e s p e c t r a p r e s e n t e d i n fig. 2 a r e c o r r e c t e d for c o i n c i d e n t

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background. The gates on the peaks and those used for background subtraction are shown on the upper diagram of fig. 2. 3.2. ISOMERIC TRANSITIONS IN t°°Tc

The y-spectrum of the l°°Mo target irradiated with deuterons is shown in fig. 3. It corresponds to a 2.6 ps counting interval, beginning 3 /~s after the beam burst. Counts

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J,

32

A. PINSTON et al.

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Three lines decrease effectively during the 30/~s, which separate two bursts of deuterons and the analysis of the intensity versus time distributions is shown in fig. 4. The ~,-line with 98 keV energy and a half-life of 18.3___1.5 #s has been identified with a known transition in 9 9 M o [ref. 16)]. The two other y-lines of 43.3 keV (T½ = 4.6 ps) and 172.1 keV (T½ = 10.2 #s) have been assigned to l°°Tc produced by the l°°Mo(d, 2n) reaction and already known from the (n, ~) reaction work. These results have been previously published in a short note 7). Recently a half-life of 8.2/~s associated with a y-line of 172.3 keV was reported by Bartsch 17) who produced l°°Tc by the l°lRu(~, p) reaction. The analysis of the intensity versus time distribution of the 172 keV y-line reported in fig. 4 shows clearly that the counting rates of the first two points are too low. This effect cannot be associated with a possible dead-time problem which has been verified from the counting rates of the 306.9 keV line, belonging to the l°lTc ~ l°lRu decay and plotted on the same figure. It is thus concluded that the 172 keV isomeric transition is itself fed by another isomeric transition, the half-life of which is impossible to deduce with precision from the decay curve of fig. 4, The explanation of the behaviour of the intensity versus time distribution of the 172 keV y-ray will be given in the next section, in the discussion of the decay scheme of l°°Tc. The y-rays feeding the 172 keV isomeric transition in a direct or indirect way and measured by the ?-~ delayed coincidence measurements are reported in table 2.

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34

J. A. PINSTON et al. TABLE 2

Delayed coincidences with the 172 keV transition E~ (keV) 62.9 86.8 140.3 144.2 299.8

339.0 385.0 399.1 413.9" 429.7*

* Questionable assignment.

3.3. CONVERSION ELECTRONS

The results of the conversion electron measurements are summarized in table 3. Multipolarities of strong transitions were determined by the L (and M) subshell ratios and were used for the relative intensity calibration between the (n, 7) and (n, e-) measurements. Fig. 5 demonstrates the resolution of the subshell lines of the 28.5 keV

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36

J.A. PINSTON et al.

TABLE Conversion electrons from

E~(AEv)

I7

E~(AE~)

(keV)

100 n

(keV)

0.29 ~)

25.477 (2) 25.726 (2) 25.842 (2) 27.962 (6) 28.084 (2) 28.479 (3) 28.321 (3) 36.447 (5) 22.242 (2) 40.240 (3) 40.608 (2) 42:856 (3) 22.517 (2) 25.004 (3) 43.010 (10) 41.845 (2) 59.839 (5) 42.815 (3) 71.536 (12) 65.489 (4) 72.514 (10) 65.801 (5) 69.648 (6) 70.151 (14) 70.370 (5) 88.358 (8) 70.997 (14) 84.666 (6) 145.613 (20) 147.776 (9) 151.1043(11) 169.1043(19) 169.439 (26) 171.591 (10) 175.395 (12) 185.121 (21) 192.145 (12) 196.084 (20) 202.406 (14) 242.511 (6) 255.533 (19) 278.421 (26)

31.370 (1) 39.489 (1) 43.286 (1)

0.78 0.63 0.39 0.55 ")

43.562 (1) 46.049 (1)

0.79 0.19

62.8887(5)

2.22

63.8531(5) 71.600 (3) 75.5330(2)

1.00 0.22 2.40

86.8498(3) 90.6820(4) 91.177 (5) 91.4074(2)

1.62 1.07 0.08 3.69

92.0183(4) 105.7083(2) 166.6825(3) 168.8302(2) 172.1484(4)

0.20 2.34 1.23 1.80 48.79

196.4402(5) 206.1568(5) 213.2006(4) 217.1398(6) 223.4682(4) 263.5555(9) 276.5525(7) 299.4806(5)

1.50 0.90 1.76 1.49 4.91 5.20 3.10 I 1.49

") y-intensity deduced from electron measurement intensities.

Shell

Etrans (Co+ e,~o,,)

Lt L2 L3 M~ MzM 3 N L, L, K L1 L3 M2M 3 K K L: K L1 K K K L1 K K K K L~ K K K K K Lt L2L 3 M K K K K K K K K

28.520 28.519 28.519 28.506 28.519 28.518 31.364 39.490 43.286 43.283 43.295 43.291 43.561 46.048 46.053 62.889 62.882 63.859 71.580 75.533 86.845 90.692 91.195 91.414 91.401 92.041 105.710 166.657 168.820 172.1483 172.1473 172.17 172.14 196.439 206.165 213.189 217.128 223.450 263.555 276.577 299.465

I°°Tc

37

3 the 99Tc(n, e - ) reaction

i

Mo

100 n

(%)

El

E2

MI

1.36 6.51 10.90 0.23 3.27 0.62 0.79 0.23 7.59 0.82 2.84 0.75 2.35 0.55 0.10 1.62 0.19 0.85 0.13 0.96

14 13 13 28 13 14 13 16 15 12 10 9 15 16 20 8 12 9 20 10

0.33 0.08 0.13 0.02 0.01

1.07 7.02 10.90 0.20 3.81

1.02 0.07 0.02 0.06 0.006

E2

0.26 0.09 0.50 0.04 0.01 0.002 0.98 0.20 0.02 0.98 0.09 0.42 0.07 0.43

2.81 1.00 5.97 0.49 1.70 0.76 11.72 2.39 0.20 10.69 0.91 4.58 0.69 6.32

0.77 0.25 1.07 0.12 0.003 0.004 2.09 0.43 0.05 2.02 0.23 0.88 0.14 1.30

M1 MI E2

0.59 0.31 0.05 1.28 0.15 0.12 0.54 0.059 0.106 3.50 0.42 <0.044 0.086 0.074 0.042 0.061 0.044 0.059 0.177 0.047 0.060

9 30 39 8 14 30 9 25 12 1 5

0.28 0.16 0.01 0.56 0.05 0.03 0.23 0.033 0.046 1.18 0.12 0.016 0.024 0.029 0.013 0.023 0.019 0.057 0.038 0.020 0.060

2.66 1.52 0.11 5.09 -0.46 0.27 1.95 0.207 0.290 7.32 0.71 0.40 0.20 0.17 0.072 0.125 0.100 0.295 0.177 0.090 0.26

0.59 0.34 0.02 1.16 0.13 0.06 0.49 0.075 0.104 2.98 0.30 0.020 0.058 0.069 0.031 0.055 0.045 0.136 0.094 0.050 0.151

11 13 21 12 20 13 5 15 19

Conclusion

MI MI MI MI MI MI MI MI E2, M 1 MI E2, MI M1 M1 M1 MI +(18__+ 10)~oE2

M1 M1 M1 MI E1 E2 MI E1

38

J.A. PINSTON et aL

transition. The multipolarities of nuclear transitions have been deduced from comparison of the electron intensities with the product Ira, where the • are the theoretical conversion coefficients of ref. 18) and the y-intensities I r come from ref. 15). 3.4. LEVEL SCHEME

The level scheme as proposed in this paper is presented in fig. 6. It is based on the application of the Ritz combination principle, which is made possible by the very precise 7-ray energies. Table 4 gives an impressive example of a cascade energy sum in comparison with the crossover energy. There is agreement with the measured coincidence relationships, which are also indicated in fig. 6. TABLE 4 Gamma-ray deexcitation of the 264 keV level Measured transition energy (keV)

Recoil correction energy (keV)

91.4074 (2) 172.1484 (4)

0.00005 0.00016

263.5555 (9)

0.00038

Recoil corrected transition energy (keV) 91.4075 172.1486 sum

263.5561 263.5559

The greater part of the levels are populated by primary transitions and we have deduced the value for the binding energy of the last neutron to: B, = 6764.4 _ 1.0 keV This value agrees well with the value of 6780 +__20keV deduced from the measurement of 4554___20 keV for the ground state Q-value of the 99Tc(d, p) reaction 4). These two results fall clearly outside the error bars of the value 6600 + 60 keV listed in the compilation of Wapstra (1971) [ref. 19)]. The spin and parity assignments are based on the multipolarities obtained from the conversion electron spectrum. Further information comes from primary transitions which can feed levels with spin and parity values I s = 3 ±, 4 ±, 5 ±, 6 ± and 2 ÷, 7 + by dipole transitions or very weak E2 transition, from the I" = 4, 5 ÷ compound state. An excellent agreement has been found between the (n, 7) work and the accompanying study of the (d, p) reaction 4). Nevertheless the better energy resolution obtained in the (n, ~,) reaction has allowed some groups of levels, not resolved in the (d, p) reaction, to be separated. We shall not discuss the level scheme in detail but only some special points will be considered below. The ground state. A s p i n and parity assignment I s = 1 + has previously been

100Tc

39

associated to the ground state of l°°Tc [ref. lO)] on the basis of the small log ft value (4.7) for the ground state//-decay l°°Tc -~ t°°Ru. The 172 k e V level. A unique spin and parity assignment I ~ = 2 + was assigned to this state which is fed directly by a very weak (I~ = 10 -4 per neutron capture) primary transition and which decays by an M1 transition to the I ~ = 1 + ground state. The 201 and 244 k e V levels. The very strong (I v ~ 32 % per neutron capture) E2 transition of 29 keV, observed in the conversion electron spectrum only was placed above the 172 keV level, in order to fullfill the intensity balance requirement. The existence of this level was confirmed by the presence of a primary transition and the existence of a 199 keV level fed in the (d, p) reaction. A spin and parity I ~ = 4 + was assigned to this state. As it is very hard to assign the half-life of 10.1 ps to the 172 keV M1 transition, it is assumed that the isomeric state is in fact the 201 keV level which decays by the 29 keV E2 transition, as also discussed by Bartsch 17). The existence of a 244 keV level, I ~ = 6 +, which decays by the 43.3 keV E2 transition, to the 200 keV (I ~ = 4 +) was based on the presence of a primary transition and the existence of a 244 keV state fed in the (d, p) reaction 4). This placement of the 43.3 keV isomeric transition (T~ = 4.6 ps) could explain the behaviour of the intensityversus-time distribution of the 172 keV )'-ray reported in fig. 4 and discussed in subsect. 3.2. The half-lives and hindrance factors of the E2 isomeric transitions are reported in table 5. The 500.15 k e V level. A 299.5 keV ),-line was measured in delayed coincidence with the 172 keV transition (table 2). Because of its large intensity (I v = 11.5 % per neutron capture), this line can only feed the isomeric state at 201 keV. TABLE 5

Half-lives and hindrance factors in E2 isomeric transitions of l°°Tc Transition energy (keY)

Tt/2 (#s)

28.5. 43.3

10.2 4.6

Fhindr

TI/2(exp) TI/2(s.P.) 1.1 0.95

4. The neutron-proton multiplets One can predict in l°°Tc from simple shell-model considerations multiplets based on the g~ proton particle and the p~ proton hole coupled to the neutron hole d~ and the neutron particles g~, s~ and d~. The comparison of the (d, p) and (n,)') reactions is a powerful tool in identifying the different members of these multiplets and making spin and parity assignments. Only states originating in the ~p~ proton-hole cannot be fed in the (d, p) reaction.

40

J . A . P I N S T O N et al.

The n9~vgi multiplet. The ground state ( F = 1 +) and first excited state (172 keV, I ~ = 2 ÷) fed in the (d, p) reaction via In = 4 transitions belong to this multiplet. The small log J/ value (4.7) [ref. lo)] for the ground state//-decay l°°Tc ~ l°°Ru is well explained by the nuclear structure proposed for l°°Tc. Indeed the B-transition could be interpreted microscopically, in a first approximation,

{rcg~vgl}, + ~ {~g~}o+ + e which corresponds to the single particle transition: vg i ~ ngl + e-.

The third level (424 keV, I ~ = 3 +), well established in the (n, ~) reaction, is also fed in the (d, p) reaction but the I. value is not reported. The assignment to this multiplet is tentatively based on the fact that this level feeds preferentially, by a ?-ransition, the 172 keV level. The last two levels, 709 and 776 keV, assigned to this multiplet are fed in the (d, p) reaction only. The identification is based on the I. = 4 values found and, in addition, the small I, = 0 admixture limits the spin to the values 4 and 5. In fig. 7 the five lowest spin members of the multiplet under consideration, in l°°Tc, are compared with the levels of 98Tc fed in (p, d) reaction by In = 4 transitions a). Although spins are unknown in 98Tc the energies are very near in the two nuclei and analogous states are certainly observed. The 3 ÷ member seems not to have been observed in 98Tc but two new members, probably 6 ÷ and 7 ÷, are established in the energy region between 700 and 800 keV. The two Tc nuclei are also compared in fig. 7 with the rrg~- lvg~- 1 multiplet observed in 116in [ref. 20)]. The main difference between the three spectra is a compression of

4"- .....

813 4~,5.

776

8*-..

,,,,666 ,.=--"~'--709

3" /

~658 3"

2"

273

1"

0

"I

1161n

-

-

_

_

895 794

- 752 - - 7 0 9

424

2"

172

I"

0

lOOTc

142 - - 0 . 3 0 5

98Tc

E(keV)

Fig. 7. Comparison of the lrgg/2VgT/2 multiplet in 9STc,Z°°Tcand 116In. This multiplet is 305 kcV above the ground state in 98Tc.

l°°Tc

41

the multiplet in the technetium isotopes which can be theoretically explained by the presence of three protons in Tc [ref. 21)], provided only the seniority v = 1 is taken into account. It is interesting to note that, in the three nuclei, the 1 + ground state is considerably below the center of gravity of the multiplet in question. This situation is a consequence of the strong attractive interaction between the neutron and the proton coupled to the spin I = 1 when the two particles have the same orbital angular momentum 1 (1rg~ and vg~ orbitals). The ~rg~cs½ doublet (461-522 keV). Two levels are strongly fed in the (d, p) reaction via I, = 0 transitions. The higher energy member (552 keV) is well established from the (n, ~) and (d, p) reactions and the spin and parity assignment, I n = 4 +, is based on this (n, ~) work. In contrast, the second member, I ~ = 5 +, is not precisely known from the (d, p) reaction where two levels 444 and 460 keV are not completely resolved. Two arguments have been used for the definitive identification of this member with the 461 keV, I ~ = 5 +, state found in (n, y) reaction: (i) it is the unique level with af good spin and parity assignment in this energy window and (ii) the two members of the multiplet are connected by a y-transition. There is another case, 116in ' where the doublet ng~vs½ is also known and the energy splittings, 96 keV (116in ) [ref. 20)] and 91 keV (l°°Tc), are very close together. This is consistent with the theoretical consideration that the energy splitting is a constant independent of the number of protons in the ng~_ orbital, if the neutron is in an I = ½ orbital 22). The r~g~vd~ 1 and zty~vd~ multiplets. The two multiplets are fed in the (d, p) reaction by I, = 2 transitions. In fact, only the ground state, the 172 keV and the 552 keV states do not contain a I, = 2 component, up to i MeV. Slater and Booth 4) suggest, from the summed spectroscopic strengths of the I, = 2 transitions, that the 2d{ neutron state is incompletely filled in the ground state of 99Tc and consequently the nglvd~ multiplet can be fed in the (d, p) reaction. Another interesting point is connected with the very small spectroscopic strengths experimentally measured in the (d, p) reaction. This situation is normal for the d~ neutron state which is rather full but more difficult to understand for the d~ neutron state, certainly practically empty. Then it is clear from these preliminary comments that (i) it will be very difficult to separate the two multiplets and (ii) the multiplet rcg~vd~ will not be very pure if we consider the fragmentation of the I, = 2 strength between a considerable number of states. Nevertheless we have made the reasonable hypothesis that the lower energy states fed in (d, p) reaction via I. = 2 values belong to the ng~vd~-1 multiplet. This assertion is based on the fact that the states of this multiplet are the ground state and first excited states in odd-odd Tc isotopes lighter than l°°Tc [refs. i-3)]. Among the low-energy levels, a group of four states at 244 keV (6+), 287 keV (5+), 295 keV (4 +) and 341 keV (3+), fed in the (d, p) reaction via I. = 2 transitions with a relatively important strength, are assigned to the multiplet (Ttg~ v = 1)vd~-1 where the three protons have the seniority v = 1 and are coupled

J.A. PINSTON et al.

42

to the spin I = 9. Another group of three levels at 200 keY (4+), 264 keV (3+) and 320 keV (5 +) are assigned to the same configuration but with a seniority of the three protons v = 3. Of course the seniority is certainly not a good quantum number and the seniority indicated characterizes the main component of the wave function. A mean ratio of 2.7 has been computed from the strength of the feeding of the two groups in the (d, p) reaction. If we consider that the ground state of 99Tc is a pure v = 1 level, the strengths measured in the (d, p) reaction and reported in fig. 8 are proportional to the component of the wave function where the protons are coupled with the seniority v = 1. In fig. 8 we have compared the members of the appropriate multiplet in 98Tc [ref. 3)] and l°°Tc, each of which has one hole in the d~ neutron orbital. The energies are very similar in the two nuclei for the members of seniority v = 1 and the order of the members is exactly the same as in 9 6 N b [ref. 23)], a pure v = 1 proton state. The main difference between the two Tc isotopes is the presence, in ~°°Tc, of v = 3 states at low energy. In fact a 4 + state becomes the ground state of the rcg~vd~ 1 multiplet. It is an interesting case where the Nordheim rule R3, which predicts a spin I = Ip + I n - 1 = 6 for a particle-hole coupling does not work.

E(keVlz50

:oo

7"

4*

°

7*

_

_

3"

0.37 3

100 032

5"

4"

6" ~

50 a38

6"

6"

0.1~? 3. 0.09

4" ¢

96Nb

"

98Tc

0 -50

10OTc

Fig. 8. Comparison of the gg9/2vds/2 multiplet in 96Nb, 9STc and l°°Tc. In the case of l°°Tc the spectroscopic factors from the (d, p) reaction are reported for each level.

As we have pointed out the strength of the d~ state is fragmented between a large number of levels in the (d, p) reaction and thus, in this case, it is impossible to observe clearly the pure multiplet of this configuration. The rip, vd~ 1 doublet(223-500 ke V).The p½ proton state does not exist in the ground state of 9~Tc and consequently members of the doublet cannot be fed in the (d, p) reaction. In the (n, 7) reaction the two negative parity states at 500 keV and 223 keV connected by a relatively strong v-transition of 276 keV energy are tentatively assigned to the doublet. Spin values cannot be definitively associated with these two levels.

l°°Tc

43

Nevertheless spins and parities 19 = 2- (223 keV) and 1~ = 3- (500 keV) are the only possibilities if our interpretation is correct. The 2- and 3- members of the doublet are known for 8 nuclei 1-3, 24) and the energy splittings are plotted in fig. 9 versus the neutron number. Since one of the particles is in an I = ½orbital, the energy splitting should be a constant, independent of the number of neutrons in the d~ orbital, as pointed out by Talmi 22). The maximum deviations from the mean energy value, E = 208 keV, represent ~ ½ of the energy splittings and could be explained by some admixtures with other configurations. E(keV) i

300 250

• 200

_--~- .

.

.

E = 208 keY .

• 150 - e 1 51 Fig. 9. Plot of the

I 53

I 55

I 57

• Tc •Nb xY N

gPl/2Vds/2 energy-splitting versus the neutron number. The mean value E

= 208 keV

is also plotted.

5. Isomerism in the mass region 110 < A < 120

Odd-odd nuclei in the mass region 100 < A < 120 are characterized by a large number of long-lived isomers 24). This situation is certainly the consequence of the presence of a 1 + ground state belonging to the multiplet ng~vgi, and with an energy considerably below the center of gravity of the multiplet (fig. 7). An explanation of this situation is given in the former section. Then the isomeric state could be the ground state of one of the two multiplets rigors½ or rtg~vd~. 6. Conclusions

We have presented in this paper a level scheme of l°°Tc, a nucleus whose structure was completely unknown at the beginning of this work. The comparison with the (d, p) study has permitted the identification of a large number of members of 4 multiplets. A qualitative explanation of the isomerism in the mass region 90 < A < 110 is also given and attributed to the nature of the neutron-proton interaction, which considerably splits the rtg~vg~ multiplet.

44

J . A . PINSTON et al.

References 1) S. I. Hayakawa, J. E. Kitching, J. K. P. Lee, S. K. Mark and J. C. Waddington, Nucl. Phys. A277 (1977) 337 2) G. Ch. Madueme and K. Arita, Nucl. Phys. A297 (1978) 347 3) R. A, Emigh and R. E. Anderson, Nucl. Phys. A293 (1977) 379 4) D. N. Slater and W. Booth, Nucl. Phys. A267 (1976) 1 5) D. Heck, J. A. Pinston, H. B6rner, H. R. Koch and R. Roussille, Proc. 2nd Int. Symp. neutron capture gamma-ray spectroscopy and relative topics, Petten, Sept. 2-6, 1974, p. 520 6) J. A. Pinston, H. B6rner, F. Braumandl, P. Jeuch, H. R. Koch, W. Hampe, R. Roussille, K. Scheclenbach and D. Heck, Verhandl. DPG(VI) 10 (1975) 742 7) H. R. Koch, H. B6rner, W. F. Davidson, D. Heck, J. A. Pinston, R. Roussille, P. H. M. van Assche, Proc. Int. Conf. on the interaction of neutrons with nuclei, Lowell (Mass.), July 6-8, 1976, p. 1299 8) H. R. Koch, O. W. B. Schult, J. A. Pinston, R. Roussille and H. B6rner, Contributions to Conf. on nuclear structure study with neutrons, Budapest, (1972) p. 40 9) H. B6rner, P. G6ttel, H. R. Koch, J. A. Pinston and R. Roussille, Proc. 2nd Int. Symp. on neutron capture v-ray spectroscopy and related topics, Petten (1974), p. 691 10) G. Berzins, M. E. Bunker and J. W. Starner, Phys. Rev. 187 (1969) 1618 11) D. Heck and U. Fanger, Kernforschungszentrum Karlsruhe, report KFK 1604 (1972) 12) S. Cierjacks, G. Ehret, H. Hanak, G. Kriiger and H. Schmidt, Kernforschungszentrum Karlsruhe report KFK 982 (1969) 13) W. Mampe, B. Maier, P. Jeuch, J. Larysz and F. Branmandl, Proc. 2nd Int. Symp. on neutron capture v-ray spectroscopy and related topics, Petten (1974), p. 709 14) W. Hampe, K. Schreckenbach, P. Jeuch, B. R. K. Maier, F. Braumandl, J. Larysz and T. von Egidy, Nucl. Instr. 154 (1978) 127 15) D. Heck and J. A. Pinston, Report KFK 2693 (1978) 16) L. R. Medsker, Nucl. Data Sheets 12 (1974) 431 17) H. Bartsch, K. Huber, U. Kneissl and H. Krieger, Z. Phys. A285 (1978) 273 18) R. S. Hager and E. C. Seltzer, Nucl. Data A4 (1968) I 19) A. H. Wapstra and N. B. Gove, Nucl. Data Tables 9 (1971) 283 20) V. L. Alexeev et al., Nucl. Phys. A262 (1976) 19 21) C. Schwartz, Phys. Rev. 94 (1954) 95 22) I. Talmi, Phys. Rev. 126 (1962) 2114 23) J. R. Comfort, J. V. Maher, G. C. Morrison and J. P. Schiffer, Phys. Rev. Lett. 25 (1970) 383 24) Nucl. Data Sheets, Nuclear level schemes A = 45 through A = 257 (Academic Press, New York, 1973)