A Ge(Li)--Nal(T1) angular correlation system with a fast-slow electronic coincidence system was used to remeasure the directional angular correlation of the 71.66 - 249.69 keV cascade in 177Hf. After corrections for finite solid angles the anisotropy ratio of A = -0.162 ± 0.026 was obtained. The upper limit | δ | M2/E1 mixing ratio for the 71.66 keV transition was deduced. The results for angular correlation coefficients are A22 = -0.114 ± 0.018 and A44 = 0.Sistem za mjerenje kutnih korelacija s Ge(Li) I NaI(Tl) detektorima i brzo-sporom koindidencijom upotrebljan je za mjesernje direkcione kutne korelacije kaskadnog prijelaza 71.66 keV – 249.69 keV u 177Hf. Nakon korekcija zog konačnih prostornih kutova dobivena je anizotropija A = -0,016 ± 0,026. Određena je gornja granica | δ | < 0,05 za mjer miješanja M2/E1 u 71.66 keV prijelazu. Dobiveni korelacioni koeficijenti iznose A22 = -0,114 ± 0,018 i A44 = O

Basar, I.

Dikšić, M.

Hrastnik, B.

Ilakovac, K.

Jurčević, M.

Knapp, V.

Ljubičić, A.

Vojnović, B.

English

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,1 ,1 i. \1 ,, FIZlKA, 1 (1969) 117-126 DIRECTIONAL GAMMA-GAMMA ANGULAR CORRELATION OF THE 71.66 - 249.69 keV CASCADE IN 177Hf B. HRASTNIK, A. LJUBICIC, I. BASAR, M. DIKsIC, K. ILAKOV AC, M. JURCEVI(;,V. KNAPP and B. VOJNOVIC:Institute »Ruder Boskovil:«, Zagreb Received 5 October 1968 Abstract: A Ge(Li)"-Nal(Tl) angular correlation system with a fast-slow electronic coinci­dence system was used to remeasure the directional angular correlation of the 71.66 - 249.69 ke V cascade in 177Hf. After corrections for finite solid angles the aniso ..tropy ratio of A = -0.162 ± 0.026 was obtained. The upper limit I o I < 0.05 for the M2/El mixing ratio for the 71.66 keV transition was deduced. The results for angular correlation coefficients are A22 = -0.114 ± 0.018 and A44 = 0. 1. IntroductionNuclear levels of 177Hf obtained in the decay of 177Lu (6.8 d) and 177mLu (150 d) have been recently a subject of intensive investigations. Relative inten­sities and branching ratios for the gamma ray transitions in the decay of K 7/2· 7-"h=-·---, 177 LU stable 177Hf Fig. 1. Decay scheme of 177Lu (6.8 d). 0,67 ns 0,1 ns i18 HRASTNIK et ai. 171Lu (6.8 d) have been determined by several groups1• 2• 3l, and together with internal conversion measurements4l and angular correlation measurements for the 208.36 -112.955 keV5l and 71.66-249.69 keV6• 7l cascade transitions have established the decay scheme (see Fig. 1). The underlying intrinsic states of 177Hf nucleus which is situated close to the middle of the deformed region, are successfully interpreted in the Nilsson scheme. The spins and parities of the excited states have been confirmed by different experimental approaches. Counts/ channel 9000 208.36 keV 8000 7000 6000 112.!iS keV 5000 4000 3000 321,34 keV X 20 2000 1000 so 100 150 200 Channel number Fig. 2. 1771.u (6.8 d) direct spectrum measured with the Nal(Tl) scintillation counter. The anisotropy of the weak 71.66 - 249.69 ke V cascade transition was pre­viously measured with Nal(Tl) -Nal(Tl) scintillation counter coincidence systems6 • 7l. In these measurem.ents it was difficult to estimate the contribution of the 208.36 keV peak in the region of the 249.69 keV peak in coincidence with the Compton scattered 112.95 keV quanta yielding counts in the region of the 71.66 keV peak. Also the contribution of the K X-ray in the region of the 71.66 keV peak in coincidence with the 208.37 keV and 249.69 keV gamma rays yielding counts in the region of the 249.69 keV peak was somewhat un­certain. The main reason is that the 71.66 ke V gamma ray measured in co­incidence with the 249.69 keV gamma ray was masked due to a strong back­ground and a relatively poor resolution of the Nal(Tl) detector. An impro­vement was expected if the 71.66 keV gamma ray was detected in a Ge(Li) detector. t I brnEcfIONAt GAMMA ... 119 2. Measurements and resultsThe two-detector system8) consisted of a planar 2.5 cm2 and 3 mm thick Ortec Ge(Li) detector with a FET-preamplifier and a P/2'' dia X l" thick Nal(Tl) scintillator mounted on a XP 1020 phototube. The direct spectrum of 177Lu gamma rays taken with the Ge(Li) detector is shown in Fig. 2. The spectrum taken with the Nal(Tl) scintillation counter with a 0.5 mm tin ab­sorber which was applied to reduce the intensity of Hf X-rays, can be seen in Fig. 3 .. The fast-slow electronics with a TAC unit and two-dimensional analysis permitted energy-energy and energy-time analysis. The 177Lu (6.8 d) source was obtained by irradiation of lutetium oxide in the Vinca reactor at a flux of 3 · 1013 neutrons/cm2 • s for 5 days. The source prepared for angularcorrelation measurements was in the form of lutetium nitrate m a dilute aqueous solution. It was shown by Wiedling9) that the anisotropy of such li­quid sources is almost independent of the chemical composition and that the maximum anisotropy can be obtained with chloride and nitrate sources. To establish the absence of internal perturbations in our source, we measured the anisotropy of the well-known intensive cascade 208.37 - 112.95 keV. The result obtained without the finite solid angle correction was A = -0.197 ± 0.006. Counts / channel 50000 ,0000 10000 lOU6 • ..., 10000 10000 71,66 llt\J 249.69 ktV V x,o ]21,HkeV � )\ 100 150 200 259 300 350 Channel number Fig. 3. 177Lu (6.8 d) direct spectrum measured with the Ge(Li) detector. 120 HRASTNIK et al. which is in good agreement with other measurements5• 10l. If the 208.87 -112.95 keV cascade is not perturbed, then the weak 71.66 - 249.69 keV cascade should also be unperturbed. The total time-integrated attenuation cocfficient for liquid sources is 1 dt = 1 + A.k r 'where r is the mean life of the intermediate level in the cascade transition. ,h is the relaxation constant given by11l 3 'tc _k(k+1) [41 (/+1) -h(k+1) -1] A.k = 80 Ii.2 (eQ)2 V2,, z� (21-1)2 ' where re is the correlation time, Q the electric quadrupole moment of the inter­mediate state with angular momentum I, and V;. the average electric field gradient at the nucleus. Calculating A.k for two intermediate levels of 112.95 ke V and 249.69 ke V one obtains the ratios ----�--------------------------, 1000 900 800 700 O EXPERIMENTAL POINT:> 600 500 400 JOO 150 CHANNEL NUMBER Fig. 4. The scintillation spectrum in coincidence with the 71:66 keV gamma ray detected in the Ge(Li) detector at 90°. Also shown are the Gaussian curves G1 (k) and G2 (k) and the exponential base E (k) used to unfold the composed 208.36 and 249.69 keV peak, and the sum curve G1 (11) + G2 (k) + E (k).DIRECTIONAL GAMMA ... Using the lifetimes of the intermediate states5l we further obtain J.4 '250 0.067 - - =--=02 J.4 '113 0.335 . , 121 which confirms the above statement that the weak cascade ought to be per­turbed less than the intense cascade. The angular correlation of the weak 71.66 -249.69 keV cascade was measur­ed by the integral method at three angles, at 90°, 135° and 180°. The coinci­dence rate was about 2.5 counts per minute. Lead absorbers were suitably disposed to prevent coincidences arising from the Compton scattering of higher energy gamma rays from one counter into the other. The angle was changed every half an hour to minimize systematic errors. Energy spectra were ob­tained in the photomultiplier branch at 90°, 135° and 180° in coincidence with the 71.66 keV transition in the Ge(Li) branch. The interesting part of the spectrum obtained at 90° can be seen in Fig. 4. The total area of the 249.69 ke V photopeak was obtained by the unfolding of the composed peak which consists of the accidental coincidences of the 208.36 keV photopeak and the true coincidences of the 249.69 keV photopeak. The unfolding was performed assuming- that the part of the spectrum of interest can be written in the form N,h(k)=G1(k)+G2(k)+E(k), where k is the channel number and G1 (k) and G2 (k) are the Gaussian distri­butions for the 208.36 and 249.69 ke V peaks, respectively, r (K-b1)2 ] G,(k)=a, exp --2cf- ,E (k) is the exponential function E (k) = A exp ( -). k) i = 1,2The eight parameters a1, b1, c1, a2, b2, c2, A and ). were computed from therequirement L [ G1 (k) + G2 (k) + E (k) - N (k)] 2 = ruinFrom the parameters of the Gaussian distribution representing the 249.69 keV photopeak, the integrals at 90° , 135° and 180° were computed. We obtained 122 HRASTNIK et al. N (90) = 10134 ± 262 , N (135) = 9597 ± 292 , N (180) = 8623 ± 273 . These values were used to determine the anisotropy N (180) A = N(90) -l 'and the ccefficients A22, A44 of the unperturbed correlation function W (8) = 1 + A22 P2 (cos 8) + A44 P4 (cos 8) . The second transition of the cascade (249.69 ke V) is a pure E25• 6l transitionand the first transition (71.66 keV) can contain a small admixture of M2 inthe predominantly El 5• 6l transition. From the known spin sequence 9/2, 11/2,7 /2 one obtains and F2 (11/2 9/2 11) + 2o F2 (11/2 9/2 12) + o2F2 (11/2 9/2 22) A22 = 1 + 02 X X F2 (11/2 7/2 22) o2 AH = F4(11/2 9/2 22) 1 + 02 and the anisotropy A = A ( b) 1 + A22(0) + AH(o) A(o) = 1-o.sA22(0)+0.J75AH(o) -t'where the mixing ratio o is given by = ( Int M2 ) 1/20 Int El From the number of coincidences collected at 90° and 180° we obtained a pre­liminary value of anisotropy A = -0.149 ± 0.035. After correction for the finite solid angles it amounted to A = -0.181 ± 0.042. From the drawing of the anisotropy A = A ( o) for the 9/2 (D, Q) 11/2 (Q) 7 /2 transition (Fig. 5)one obtains for the mixing ratio of the 71.66 keV transition ii � 11 I! ll Ii 1:. ., • 0.800 0 • 0.70 ,0.600 ,0.500 ,G.40 0 , o.Joo ,0.200 ,o.,oo 0.000 -0.100 -0.200 -0.300 -0.40 0 -0.500 -0.600 -0.700 -0.800 '/ '/ .;////////,, 001 DIRECTIONAL GAMMA ... 123 ---o>o/"_/ W//, �/ffffe, � � � �ffe�----...... I/ o<O� _/ E1 M, 01 10 100 lol Fig. 5. Anisotropy versus the mixing ratio 1\12/El for the first transition in the 71.66 - 249.69 ke V cascade in 177Hf. -0.05 < o < + 0.03Taking the maximum absolute value I o I = 0.05 one obtains (Int M2 ) lnt EI max = o2max = 0.0025 , or at most 0.250/o of M2 in a predominantly EI transition.The above value of o2max givesA,14 < 0.00137 "' o . Hence we can neglect the c�fficient A44 and the angular correlation function becomes W (@) = 1 + A22 P2 (cos@). Using the least square method to fit the correlation function to the expen­mental results we obtained 124 HRASTNIK et al. A22 = -0. 1 03 ± 0.016 A22 = -0.114 ± 0.01 8 ,for the experime�tal and for the finite solid angle corrected value of the an­gular correlation c<l!ffic:ient,respectively. From this result one can derive the anisotropy , A =  -0.162 ± 0.026 . · Our results are compared with the previous measurements in the Table. -- ·-- - -· --- ·-A22 -0.143 ± 0.014-0.122 ± 0.017-0.114 ± 0.018Table A44 Author (ref.) � -- -0 S. Ofer6l+ 0.01 L. Simons et al. 7l0 present paperTaking- into account the statistical errors the three results for A22 are in agreement. Our value is absolutely somewhat lower, possibly due to an im­proved separation of the 71.66 ke V gamma ray peak from its continuous background. The value of A44 is expected to be very small. R e f e r e n c e s1 ) P.  Marmier and F. Boehm, Phys. Rev. 97 (1955) 103 ;2) P. Alexander, F. Boehm and E. Kaukeleit, Phys. Rev. 1 33 (1964) B284;3) A.J. Haverfield, F. M. Bernthal and J. M. Hollander, Nucl. Phys. A94 (1967) 337;4) K. Way, Nucl. Data A 1, No 6, August 1966, 521 ;5) Nuclear Data Sheets;6. S. Ofer, Nucl. Phys. 3 (1957) 479 ;7) L. Simons, G. Wendt, and E. Spring, Acta Polytech. Scand. 17 (1962);8) B. Hrastnik, A. Ljubicic, B. Vojnovic, K. Ilakovac, M. Jurcevic, Fizika, 1 (1969) 127;9) T. Wiedling, Thesis, University of Stockholm, 1956;10) E. Matthias, E. Karlsson and C.- A. Lerjefors, Ark. Fys. 22 ( 1962) 139 ;11 ) R.  M. Steffen and H. Frauenfelder in E. Karlsson, E. Matthias, and K.  Siegbahn,»Perturbed Angular Correlations«, North Holland Pub!. Co., Amsterdam 1964. DIRECTIONAL GAMMA . . . 125 DIREKCIONA KUTNA GAMA-GAMA KORELACIJA KASKADNOGPRIJELAZA 71.66 - 249.69 keV U 177Hf B. HRASTNIK, A. LJUBICIC, I. BASAR, M. DIKsic, K. ILAKOV AC, M. JURCEVIC.V. KNAPP i B. VOJNOVIClnstit11t »Ruder Boskovif:«, Zagreb S a d r z a jSistem za mjerenje kutnih korelacija s Ge(Li) i Nal(Tl) detektorima i brzo­sporom koindidencijom upotrebljen je za mjerenje direkcione kutne korelacije kaskadnog prijelaza 7 1 .66 keV - 249.69 keV u 177Hf. Nakon korekcija zbogkonacnih prostornih kutova dobivena je anizotropija A = -0,162 ± 0,026.Odred:ena je gornja granica I c) I <  0,05 za omjer mijesanja M2/El u 71.66 keV prijelazu. Dobiveni korelacioni koeficijenti iznose A22 = -0,114 ± 0,018  i A44 = O .  

Direkciona kutna gama-gama korelacija kaskadnog prijelaza 71.66 - 249.69 keV u 177Hf

Direkciona kutna gama-gama korelacija kaskadnog prijelaza 71.66 - 249.69 keV u 177Hf

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