Heck, D.

Michaelis, W.

Ottmar, H.

Schmidt, H.

English

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August 1969Institut für Angewandte Kernphysik96 qSGamma Decay of . Mo arid . 1'110 aft e r Therma.l Neut r on. 95 97Captur-e rn 1'1:0 and MoD. Heck, W. Mlc ha efi.s , H. Ottrnar , H. Sch:midtKFK 1092Reprint from"NEUTRON CAPTUREGAMMA-RAY SPECTROSCOPY"INTERNATIONAL ATOMIC ENERGY AGENCYyrE:NNA. 1969GAMMA DECA Y OF 96Mo AND 98Mo AFTERTHERMAL NEUTRON CAPTURE IN95Mo AND 97MoD. HECK, W. MICHAELIS, H. OTTMAR,H. SCHMIDTInstitut für angewandte Kernphysik,Kernforschungszentrum Karlsruhe,Karlsruhe, Federal Republic of GermanyAbstractGAMMA DECAYOF 96Mo AND "Mo AFTERTHERMAL NEUTRONCAPTUREIN 95Mo AND 97Mo. Thegamma-ray spectra following neutron capture in the odd-rnass molybdenum isotopes have been studied withGe(Li) and NaI(TI) detectors. The analysis of measured angular distributions yields detailed information onthe structure of some levels. The results are compared with theoretical predictions.1. EXPERIMENTAL PROCEDUREThe measurements were performed at theFR-2 research reactor inKarlsruhe. The targets consisted of MoOj, powder (enriched to 96. 2% in95Mo, o-c = 14.5 b) and metallic molybdenumpowder (enriched to 92.8%in 97Mo, o-c =2.2 b), ericlosed in thin-walled polyethylene capsules. Forthe energy r-ange up to 2 .. 7 MeV a Ge{r...i) diode with anti-Compton shield[ 1]has been used. Coincidences and angular correlations of some gamma-gamma cascades have been measured with two 4-in. diam. X 5-in. NaI(Tl)crystals in a fast-slow coincidence set-up [2] coupled with the on-linecomputer system MIDAS [3].2. RESULTS FROM 95Mo(n, '}')96MoThe decay scheme known from the radioactive beta decay of 96gTc,96mTc and 96Nb [4] couldbe extended considerably by the present investi-gation (Fig.1). Noteworthy is a new level at 1625.93 keV. Because of thegood resolution of the anti-Compton spectrometer [1), it could be estab-lished that theknown 850-keV gamma-ra.y [6] de-exciting the 4+ level at1628 keV is really a triplet with two intense lines at 847.67 and 849.95 keVand a weak one at 853.03 keV (Fig. 2). Comparing the intensities of thegamma rays obs er-ved inthe Ge(Lü spectrum with those observed i ntheNaI(Tl) spectrum coi.ncident withthe 778~keV transition, one must concludethat both intense gamma rays at 850 keV are in coincidence with the778-keV transition. Theintensity balance does not allow the strong847. 67-keV line to befittedbetween higher energy levels. Another factconfirming this,neWle"el is the energy difference of 128 keV betweenthe7656 and 7528-ke\lprimary transitions, which is too small to fit theenergy differencepetween the known 1498 and 1628-keV levels. The1625. 7-keV line is probably a ground-state transition, in which case thespin of the new level is limited to 1 or 2. From the angular correlationmeasurement of the unresolved 848 to 778 and 850 to 778-keV cascades,30995Mo+n '"I-'o~.3·t tftti ttfr 9154~ ! &;. & 51;,"':8 !i}!:~12:J±H±B±E=----'---------.e---,-- ~.=MO~778.26oE (keV)..~3441.4I I32811... ..:.1 gN cD2 t! i CJt2790.3:~.'J r 2481.1,,-~m~2234.63~-]- I ,-- 2219.311978.27 ~1869.51 ():"'i628.21 ~625.93~1497.81CD ~ '". ~ ~~±--i--i--......--'----'-----I.-~--I I~.,.§l iIll;t~ l;i.., ~w............--..-........1N~ m Mg 0 ~~~.--l } !------..~ :ri'" Ni r~o·(FIG.l.Levelscheme of 96Mo observed in the reaction 95Mo(n,y)96Mol. Levels above 2.6 MeV found only by Ritz ' s combination principleare not shown,GAMMA DECAY OE '"Mo AND ''Mo 311COUNTS719.53O·24XE OS 1j\ 736.860.= OS I\721.57 I(755.66705.94 i' ,\740.73-------,OO'.N="'-erOOn:&i ~__~tl g,778.26/• 1075810.82 812.48i i 800. .36\ (e19.54• I 790.57 'I.,~: ~. 1 I 11 1~ .ut =:1;> L41080 1085 1090891.5illis d,CHANNELFrG.2. Portion of the '"Mo spectrum, measured with a Ge(Li) detector in the anti-Compton spectrometer [1].The inset shows the triplet at 850 keV with computer fit analysis (solid Iine).the spin 1 for the 1626-keV level is ruled out (Fig. 3). Assuming that the850-keV transition de-exciting the 4+ level is a pure quadrupole radiation,the mixing parameter of the 848-keV line is near 1. This value impliesa positive parity for the 1626-keV level and should be compared with thequadrupole admixture of only 15% (0"'" 0.45) found for the 720~keV transi-tion from the 1498-keV level. This shows that the new level has muchmore collective character than the second 2+ state. The spinjparityvalues of 4+ and 3+ for the levels at 1869.51 and 1978.27 keV, respective-Iy, found in beta decay [4, 5J, coulcl be confirmed by angular correlationmeasurements of the 1091 to 778 and 1200 to 778-keV gamma cascades.An additionaltransition.of 352.27keVfromthe 3+ level to the1626-keVlevel could be assigned. Probably this transition is identical with the351. 9-keV line observed in the decay of 96Nb and 96m Tc, but placed in adifferent position [4]. The energy of the 2234. 63-keV level correspondswith the energy value of the 3- octupole vibrational state at 2.25 MeVfound in (d, d") measurements [7]. The negative parity of this level is con-firmed by the assumption of multipolarity EI for the strong primary6 919-keV transition feeding this level from thepositive parity capturestate. Between 2.6 and 3.6 MeV a large numberof levelscan easily beobtained by applying Ritz i s combination principle. Only three levels,312 HECK et al....l.ttUh assumption of Q purequadrupo18 maroder of the850keV transition0·5A40·42(1,2)2(2)0~, -10 Gi)0·3-0·2-0·3-0·4Angular Correlation Coefficients of'Y-'Y-Cascades from95Mo(n;y)96Mo1(1,2)2(2)0Pffi.3. Angular eOllclation mcasurcmcnls on '"Mo easeades with the 2 0 transition 77S keV. The erossesshow positions of the measured A2 , A4 coefficients with errors in the Az ' A4 plane; solid lines are theoreticalvalues as functions of the mixing parameters (,.namely those at 2790.3, 3287.1 and 3441. 4 keV, have been taken over inthe level s cherne, who s e existence was confirmed e ither- by coincidencemeasurements or by the feeding with intense primary gamma rays. Thespin determinations of the levels at 2790.3 and 3287.1 keV are not unique.For the latter, with the 2509 to 778-keV cascade, spin 2 seems veryprobable, but spin 4 mayaIso be possible, as shown in Fig. 3. The2668--,keV transition is supposed to be a pure EI transition because of itshigh intensity. This will give us spin and parity 1-, 2- or 3- for the3441. 4-keV level. Because M2 admixtures to an EI radiation are usuallyvery weak, one would expect a ö-value near 0 for the 2668-keV radiation.With this assumption one can extract from the angular correlation m eas-:urement, despite the great errors, a spin sequence of 2-2-0 for the2668- to 778-keVcascade. FurthercoincidencemeasurementswithtwoGe (Li) detectors are in progress and should be usefulin adding to andconfirming this level scheme.3. RESULTS FROM 97Mo(n, ')') 9sMoIn the 9sMo level scheme (Fig.4) the first excited level at 734.77 keVhas the spinjparity 0+ [8]. The existence of this remarkable level isassured by the weak primary 7909. 6-keV transition (the binding energy ofGAMMA DECAY OF 96MoAND 98Mo 31397Mo + n787.35734noE(keV)2419.52333342223142206142104.762017.48~":1758.4410'" 1509971432232+.3+rrnITlf----------- 8642.60) <DOO)Ln~~i~ii~~~II,FIG.4. 98Mo·level seherne observed in the reaetion 9'Mo(n,y)98Mo. Levels above 2.5 MeV found only byRIZ's combluatlon principle are not shown.the last neutron is 8642.5 keV). The following levels (spin and parity inparentheses) 787.35 keV(2+), 1432.21 keV (2+) and 1509.97 keV (4+) areweIl known [9] and look like the usual one and twc--phonon vibrationalstates, respectively. The beta decay studies of 98Nb (2.8 sec) give twoalternative spin and parity assignments (1+ and 2+) for the 1758. 44-keVlevel [9]. This discrepancy has been removed by angular correlationmeasurements on the 971 to 787-keV cascade. From this measurementthe 1758. 44-keV level is unambiguously assigned a spin of 2. Interestingis the strong E2 admixture of 82±20/0 to the971. 08-keV transition. Thevery strong primary 6624. 8-keV transition must be an EI transition fromthe positive-parity capture state. With a negative parity for the2017. 48-keV state the analysis of the angular cor-r-e'lation measurement ofthe 1230 to 787-keV gamma cascade permits onlya spin of 3 for this level.Indeed Kim and Cohen [7]have foundthe collective 3-octupole state near2.04 MeV via the (d , d!) reaction. Noteworthy is the atr-ong 2017. 4-keVground-state transition, which must be a pure E3 radiation. A similartransition to the 0+ level at 734.77 keV was not found. The spin assign-ments of the levels above the 3- state are only tentative and not confirmedby angular correlation measurements. The presence of transitions to the734. 77-keV 0+ level limits the possible spin values for the 2104.76 and2206. 74-keV levels to 1 or 2. For the 2333. 34-keV level the assignmentof 2+ will be the only one permitted because of the presence of transitionsto the 0+ level at 734.77 keV as weIl as to the 4+ level at 1509.97 keV.~I-'ol'>FIG. 5. 96Mo level sehemes found by different reactiofrom Refs [Lü, 11].ompared with shell-rnodel ealeulations~:;.:~~J. VervierNJelPhys.75(19E6)pK.H.Bhatt, ..18.&11Nucl.Phys 6Jf1965!,286She// - MOdel CalculationsThis·Work2-Io~)14+Ist-~b-I... 6"6' 8'1/;+/+ 2'I;' I.J+rs+Jlf":rW ~+r~++i22' r:i2 ~ +2+ 2+r0' 0' h+95tvfo(n,yf6Mo'f6Mo3-2'·,0'95Mo(d,pflMoC.F.Moore ef al.ffl/s,Re. I41(1966)/166,a~ __ .... ____(7'.8')6+ -_ ... .. .. . --:r,4J .:6+ 6'5+ 5~+ ~+r r r~ ~ ~+~+ ~+ ~+r : r 2'r z: 2+0+ 0+ 0'S&n-rc_96Mo >JirJ)'C_96tvfo !XiNb_96tvfo11' 11' p-S.Mr:rr1ro et aJ, Can.J:JDflys. '6(1968)/2375./J}2{Jo1.5.15'.10;:~1-2Sl<JGAMMA DECAY OF 96Mo AND "Mo4. COMPARISON WITH THEORETICAL CALCULATIONS315When looking at the level schemes of 96Mo and 98Mo one finds levelswhich cannot be explained by vibrational models, such as one of the two2+ states around 1.5 MeV in 96Mo and the low-lying 0+ state at 735 keV in98Mo. As these two nuclei are not too far off the closed neutron shellN = 50 and the semi-closed proton shell Z =40, one would expect that thelevel scheme will be reproduced bythe shell-modeL Bhatt and Ball [10]and Vervier [11] have performed shell-model calculations, based on the~gZr50 core. For nucleons outside this core they use residual interactionsempirically determined from the known level schemes of ~§M050' ~rNb51,and ~~Zr52 for the protons and neutrons in the 1Tlg9/2 and v2d 5/ 2 orbits,respectively. For 96Mo the results of these calculations are shown inFig.5, together with the level schemes observed in different reactions[4, 7, 12]. The agreement is not very good. Again there is only one2+ level in the region of 1.5 MeV. The second 4+ and the 3+ levels observedbetween 1.8 and 2.0 MeV are reproduced with too high energies. Thisdisagreement is very likely due to the increasing number of neutrons out-side the closed shell. For 98Mo with the closed subshell v2d 5/ 2 the theoryexpects a simple level scheme with a spin sequence 0+ (ground state),2+, 4+, 6 +, 8+, according to a pure proton excitation to higher orbits.This is in complete contradiction to the experimental results and showsthat the influence of the subshell closure cannot be very significant.5. FINAL REMARKSFurther investigations of the (n,'Y) reaction with the odd-mass molyb-denum isotopes are in progress and will give reliable information of higherlevels. In addition, attempts should be made to extend shell-model cal-culations by involving admixtures of further nucleon configurations to im-prove the reproduction of experimentally found levels.REFERENCES[1] MICHAELIS, W., HORSCH, F., "High-preeision neutron eapturegamma-rays spectroseopy usinggermanium deteetors in Compton-suppression teehnique", these Proeeedings.[2] SCHMIDT, H., IiECK, D., "Angular eorrelation measurements on gamma-ray easeades followingthermal neutron eapture", these Proeeedings.[3] KRÜGER, G., ZIPF, G., Rep. KFK 371 (1965).[4] MONARO, S., BARETTE, 1., BOUTARD, A., Can, 1. Phys, 46 (1968) 2375.[5] SIMONS, L., HOLLARD, R.E., WENDT, G., SPRING, E., KÄLD, L., HA GE B<1 , E., Nuel. Phys, 39(1962) 130. . -[6] EARLE, E.D., MONARO, S., Rep. AECL 2639.[7] KIM, Y.S., COHEN, B.L., Phys. Rev , 142 (1966) 788.[8] HÜBENTHAL, K., BERTHIER, J.. HOCQUENHEIM, 1.C., MOUSSA, A., Compt. r, Acad, Sei. 265B(1967) 162.[9] HÜBENTHAL, K., MONNAND, E., MOUSSA, A., tobe published in Nucl. Phys.[10] BHATT, K.H., BALL, J.B., Nucl. Phys. 63 (1965) 286.[11] VERVIER, 1., Nucl. Phys. 75 (1966) 17. ~[12] MOORE. C.F., RICHARD, P., WATSON, C.E, ROBSON, D., FOX, 1.D., Phys. Rev , 141 (1966)1166.Printed by the IAEA in Austria

Gamma decay of ⁹⁶Mo and ⁹⁸Mo after thermal neutron capture in ⁹⁵Mo and ⁹⁷Mo

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Gamma decay of ⁹⁶Mo and ⁹⁸Mo after thermal neutron capture in ⁹⁵Mo and ⁹⁷Mo

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