Gamma-gamma Coincidence Measurements Research Articles

Decay of the 25-min Activity ofTe131

The decay of the 25-min activity of ${\mathrm{Te}}^{131}$ has been investigated with scintillation spectrometers and also with an intermediate image beta-ray spectrometer. The energies of the gamma rays which have been observed are 145, 350, 445, 490, 590, 650, 930, 985, 1130, and 1280 keV. On the basis of gamma-gamma coincidence measurements, it is proposed that the 350-, 930-, and 985-keV gamma rays are doublets of gamma rays with energies close to each other. The observed coincidences are explained by proposing levels at 145, 490, 590, 1065, 1130, 1425, and 1480 keV. The total absorption gamma-ray spectrum and sum-coincidence spectra have also been studied to confirm these levels and their decay modes. Spin and parity assignments have been made to these levels on the basis of the $log\mathrm{ft}$ values of the beta transitions feeding them and also their decay modes.

The De-excitation of the 3 MeV Doublet in27Al

One of the levels of the 3 Mev doublet of aluminum-27 decays strongly by the emission of a (790 plus or minus 30) kev gamma from the 2.209 Mev level. This mode of decay suggests that the 2.209 Mev level and one of the doublet states are the (7/2)/sup +/ and (9/2/)/sup +/ members of a rotational band. A gamma-gamma coincidence measurement was made to establish the energy of the low energy gamma ray with greater precision and to verify that the cascade does not proceed by the 842 kev level. The 790 kev transition is found to occur between the higher energy member of the doublet at 3.000 Mev and the 2.209 Mev level. The 3.000 yields 2.209 transition can be interpreted as an M1 transition between these high spin states. The 840 kev gamma ray can be attributed to the de- excitation of the 3.68 Mev level which is weakly excited at the bombarding energy used. (N.W.R.)

The 5.3 sec isomer of W183

A 5.3 sec isomer of W183 has been chemically separated from its parent 5.2 d Ta183. The radiations from the isomer have been observed with a xenon proportional counter and a NaI scintillation crystal and have energies of 46, 52, ≈ 105and≈ 160 keV. Gamma-gamma coincidence measurements involving the photons have been made. On the basis of these results and previously reported high resolution measurements of the decays of Ta183 and Re183, the energy of the isometric level is assigned as 309.49 keV, and its spin and parity as 9/2+.

Radioactive Decay ofLu171and Level Schemes forYb171andYb169

Ytterbium oxide enriched to 93.8% in the mass number 171 was irradiated with 6-Mev protons. An activity decaying by electron capture with a half-life of (8.28\ifmmode\pm\else\textpm\fi{}0.04) days was produced and its assignment to ${\mathrm{Lu}}^{171}$ confirmed by the identification of the ytterbium $K$ x ray and by comparison with the activities produced by similar proton irradiations of each of the other enriched isotopes of ytterbium. The radiations observed in this activity were the $L$ and $K$ x rays of ytterbium and gamma rays with energies of 72-76, 668\ifmmode\pm\else\textpm\fi{}7, 740\ifmmode\pm\else\textpm\fi{}3, and 841\ifmmode\pm\else\textpm\fi{}8 kev. Because neither particle nor annihilation radiation exists in this activity, the mode of decay is solely by electron capture to ${\mathrm{Yb}}^{171}$. Gamma-gamma coincidence measurements were performed for the observed radiations. An energy level scheme has been constructed for the decay of ${\mathrm{Lu}}^{171}$ using the results of the gamma-gamma coincidence measurements, an energy analysis of the conversion electron data, and calculations of transitions intensities for various trial multipole orders. Levels in ${\mathrm{Yb}}^{171}$ at 0 (\textonehalf{}- [521]), 66.7 ($\frac{3}{2}\ensuremath{-}\frac{1}{2}$), 75.9 ($\frac{5}{2}\ensuremath{-}\frac{1}{2}$), 95.1 ($\frac{7}{2}+[633] \ensuremath{\tau}g{10}^{\ensuremath{-}5}$ sec), 122.4 ($\frac{5}{2}\ensuremath{-}[512]$), 167.3 ($\frac{9}{2}+\frac{7}{2}$), 208.1 ($\frac{7}{2}\ensuremath{-}\frac{5}{2}$), 230.5 ($\frac{7}{2}\ensuremath{-}\frac{1}{2}$), 246.5 ($\frac{9}{2}\ensuremath{-}\frac{1}{2}$), 317.3 ($\frac{9}{2}\ensuremath{-}\frac{5}{2}$), (350), 835 ($\frac{7}{2}\ensuremath{-}[514]$), 862 (possibly $\frac{5}{2}+[642]$), 935 ($\frac{9}{2}+[624]$), and 949 ($\frac{7}{2}$) kev account for thirty of thirty-one transitions reported for this activity. Transition probabilities and branching ratios for the electron capture decay have been calculated. Eighty-four percent of the disintegrations of the $\frac{7}{2}+[404]$ ground state of ${\mathrm{Lu}}^{171}$ occur to the $\frac{7}{2}\ensuremath{-}[514]$ state at 835 kev in ${\mathrm{Yb}}^{171}$. The previously reported 600-day activity of ${\mathrm{Lu}}^{171}$ was not found. An energy level scheme for the decay of ${\mathrm{Lu}}^{169}$ (1.5 days) as reported by other workers is proposed with levels in ${\mathrm{Yb}}^{169}$ at 0 ($\frac{7}{2}+[633]$), 24.2 (\textonehalf{}- [521]), 70.9 ($\frac{9}{2}+\frac{7}{2}$), 87.0 ($\frac{3}{2}\ensuremath{-}\frac{1}{2}$), 99.3 ($\frac{5}{2}\ensuremath{-}\frac{1}{2}$), 161.7 ($\frac{11}{2}+\frac{7}{2}$), 191.4 ($\frac{5}{2}\ensuremath{-}[512]$), 244.0 ($\frac{7}{2}\ensuremath{-}\frac{1}{2}$), 264.5 ($\frac{9}{2}\ensuremath{-}\frac{1}{2}$), 278.7 ($\frac{7}{2}\ensuremath{-}\frac{5}{2}$), 389.7 ($\frac{9}{2}\ensuremath{-}\frac{5}{2}$), 523.1 ($\frac{11}{2}\ensuremath{-}\frac{5}{2}$), 570.5 ($\frac{5}{2}+[642]$), 648.2 ($\frac{7}{2}\ensuremath{-}[514]$), 962.2, 1451.6 ($\frac{9}{2}+[624]$), 1456 ($\frac{9}{2}$), and 1465 ($\frac{9}{2}$) kev and with less certainity, at 488.9 ($\frac{11}{2}\ensuremath{-}\frac{1}{2}$), 798, 870, 953, and 973 kev. This scheme accounts for 53 observed transitions following the decay of ${\mathrm{Lu}}^{169}$. The ground state of ${\mathrm{Lu}}^{169}$ is probably the $\frac{9}{2}$ [514] orbital also assigned to the ground state of ${\mathrm{Lu}}^{173}$. This scheme is based upon an energy analysis of the reported transitions, intensity calculations for various trial multipole orders, and analogy with the scheme proposed for ${\mathrm{Yb}}^{171}$.

Decays of Photoreaction Products of Selenium

The decay properties of Se 73m , Se 81 and As 79 have been studied by scintillation methods. Gamma-ray spectra showed lines of energy 170, 275, 335, 410, 480, 550, 650 and 840 KeV belonging to the decay of Se 81 . The decay scheme of Se 81 was proposed with the aid of single and coincidence measurements of radiations. The mass difference between the ground states of Se 81 and Br 81 is represented by the 1.56–MeV beta ray. In As 79 , new gamma rays of 360, 440, 725 and 890 KeV were found. Beta-gamma and gamma-gamma coincidence measurements confirmed that a 44–min activity in selenium is attributed to Se 73m . It is found that the beta-decay energy is 1.86–MeV and that there is no appreciable gamma ray.

Radioactive Decay ofYb167

Erbium oxide enriched to 35.1% in the mass number 164 was irradiated with 17- and 24-Mev alpha particles. An activity decaying by electron capture with a half-life of (17.7\ifmmode\pm\else\textpm\fi{}0.2) minutes was produced and its assignment to ${\mathrm{Yb}}^{167}$ confirmed by the identification of the thulium $K$ x-ray and the well-known daughter activity of ${\mathrm{Tm}}^{167}$. Gamma rays with energies of 106, 113, and 176 kev were observed in the spectrum of ${\mathrm{Yb}}^{167}$. Gamma-gamma coincidence measurements were performed on the observed radiations. Transition probabilities, multipole order admixtures, and electron capture branching ratios have been calculated. Approximately 89% of the electron capture transitions occur to the 292.7-kev ($\frac{7}{2}\ensuremath{-}$) level of ${\mathrm{Tm}}^{167}$.

Radioactive Decay ofLu170

Ytterbium oxide enriched to 81.4% in the 170 mass number was irradiated with 6-Mev protons. An activity decaying by electron capture with a half-life of (2.05\ifmmode\pm\else\textpm\fi{}0.05) days was produced and assigned to ${\mathrm{Lu}}^{170}$ by the identification of the ytterbium $K$ x ray and by comparison with the activities produced by similar proton irradiations of each of the other enriched isotopes of ytterbium. The observed activity consists of the ytterbium $K$ x ray, gamma rays with energies of 84, 193, 245, 1010, 1030, 1165, 1275, 1415, 2035, 2365, 2665, 2890, and 3085 kev, and a small amount of annihilation radiation. Because no particle radiation exists, the mode of decay of ${\mathrm{Lu}}^{170}$ is solely by electron capture to ${\mathrm{Yb}}^{170}$. Gamma-gamma coincidence measurements have led to the postulation of levels in ${\mathrm{Yb}}^{170}$ at 2120 (0), 2365 (1 or 2), 3170 (3), and 3395 (1 or 2) kev in addition to the previously assigned 84 (2+), and 278 (4+) levels. A partial energy level scheme with approximate electron capture branching ratios is proposed.

The 85 minute activity of Ba 139

The decay scheme of Ba 139 has been studied using scintillation detectors. The half-life is found to be 85 ± 3 min. Beta-gamma and gamma-gamma coincidence measurements show this decay to consist of three beta groups of 2.38, 2.23 and 0.95 MeV, leading to the ground state and two excited states of La 139, the energies of the excited states being 0.167 ± 0.003 and 1.43 ± 0.05 MeV. Transitions from these two states are to the ground state. Branching ratios of 0.72, 0.26 and 0.02 were found for the 2.38, 2.23 and 0.95 MeV beta groups, respectively. Beta-gamma delayed coincidence measurements made on the 0.167 MeV state show the half-life of this state to be 1.7 ± 0.2 ns.

Radioactive Decay ofTm166

Erbium oxide enriched to 72.9% in the;66 mass number was irradiated with 6-Mev protons. An activity decaying by electron capture and positron emission with a half-life of 7.69 combination rat 0.05 hours was prcduced by a (p,n) reaction and its assignment to Tm/sup 166/ confirmed. The observed activity consists of the K x-ray of erbium, gamma rays with energies of 81, 184, 289, 406, 460, 598, 674, 694, 707, 759, kev, annihilation radiation, and partic;e radiation with an end-point energy of 2090 combination rat 40 kev. Gamma-gamma coincidence measurements and consideration of the energies and relative numbers of the observed radiations led to the assignment or confirmation of energy levels at 81 (2+), 265 2168 (0) kev in Er/sup 166/. The 2139-kev level is highly populated by electron capture and the positron transitions occur to the 265 (4+)- kev level. The pcsitions of the observed radiations and the branching ratios of electron capture are shown in a proposed energy level scheme. (auth)

On the First and the Second Excited States in Ge74

The first and the second excited states in Ge 74 were studied in detail from the decay of As 74 . The possibility of the presence of 0+ excited level was checked by searching for an internal conversion line corresponding to E0 transition by means of a beta-ray spectrometer, but no detectable peak was observed. From the gamma-gamma coincidence measurements the relative intensity T (2'→2)/ T (2'→0) was obtained to be 1.7. From the gamma-gamma angular-correlation measurements the spin of the second excited state was assigned to be 2 and the E2/M1 mixing ratio δ in the 2'→2 gamma transition was found to be \begin{aligned} -\left(1 \begin{array}{l} +9\\ -0.5 \end{array} \right). \end{aligned} These results were discussed in view of systematics on the medium-weight even-even nuclei.

Radioactive Decay ofLu172

Ytterbium oxide enriched to 95.9% in mass number 172 was irradiated with 6-Mev protons. An activity decaying by electron capture with a half-life of (6.70\ifmmode\pm\else\textpm\fi{}0.04) days was produced and its assignment to ${\mathrm{Lu}}^{172}$ confirmed by the identification of the ytterbium $K$ x ray and by comparison with the activities produced by similar proton irradiations of the other enriched isotopes of ytterbium. The 4.0-hour positron activity previously assigned to ${\mathrm{Lu}}^{172}$ was not observed and is best attributed to an impurity. The observed activity of ${\mathrm{Lu}}^{172}$ consists of the $L$ and $K$ x rays of ytterbium and gamma rays with energies of 79, 91, 113, 182, 203, 270, 324, 373, 490, 527, 697, 809, 900, 912, 1093, 1402, and 1583 kev. Because no positron radiation exists in the activity of ${\mathrm{Lu}}^{172}$, the mode of decay is solely by electron capture to ${\mathrm{Yb}}^{172}$. Gamma-gamma coincidence measurements, energy considerations, and consideration of the relative numbers of the radiations observed in the activity of ${\mathrm{Lu}}^{172}$ have led to the assignment of energy levels at 530 (6+), 1172 (3+), 1263 (4+), 1375 (5+), 1662 (3-), (1699), and 2072 (4+) kev in ${\mathrm{Yb}}^{172}$ in addition to the previously known 78.7 (2+)- and 260.2 (4+)-kev levels. The positions of all of the observed radiations and some observed only in conversion electron measurements are shown in a proposed energy level scheme for the decay of ${\mathrm{Lu}}^{172}$. Approximate branching ratios for the electron capture disintegrations of ${\mathrm{Lu}}^{172}$ are also shown in the level scheme. Few, if any, electron capture transitions of ${\mathrm{Lu}}^{172}$ occur to the ground and first excited states of ${\mathrm{Yb}}^{172}$. Of the two predicted spins for the ground state of ${\mathrm{Lu}}^{172}$, 4- is more consistent with the proposed energy level scheme.

Radioactive Decay of Lutetium 173

Ytterbium oxide enriched to 92.6% in the 173 mass number was irradiated with 6-Mev protons. An activity with a half-life of 625\ifmmode\pm\else\textpm\fi{}50 days was produced and its assignment to ${\mathrm{Lu}}^{173}$ confirmed by the identification of the ytterbium $K$ x-ray, gamma rays corresponding to transitions among the energy levels established in ${\mathrm{Yb}}^{173}$ by Coulomb excitation of the separated isotope, and the activities produced by the similar proton irradiations of the other enriched isotopes of ytterbium. The observed activity of ${\mathrm{Lu}}^{173}$ consists of the $L$ and $K$ x-rays of ytterbium and gamma rays with energies of 79\ifmmode\pm\else\textpm\fi{}1, 101\ifmmode\pm\else\textpm\fi{}1, 172\ifmmode\pm\else\textpm\fi{}2, 273\ifmmode\pm\else\textpm\fi{}2, 349\ifmmode\pm\else\textpm\fi{}4, \ensuremath{\sim}450, 556\ifmmode\pm\else\textpm\fi{}4, and 630\ifmmode\pm\else\textpm\fi{}4 kev and two other gamma rays not resolved in the gamma-ray spectrum but observed in gamma-gamma coincidence measurements. These two gamma rays have energies of approximately 179 and 282 kev. Because no positron radiation exists in the activity of ${\mathrm{Lu}}^{173}$, the mode of decay is solely by electron capture to ${\mathrm{Yb}}^{173}$. Gamma-gamma coincidence measurements have lead to the assignment of a 633-kev level with a probable spin of $\frac{5}{2}+$ and the confirmation of a 351-kev level in ${\mathrm{Yb}}^{173}$ in addition to the previously known 179.5- and 78.7-kev levels. The disintegrations of ${\mathrm{Lu}}^{173}$ to the 633-kev level are by $L$ capture only. Branching ratios for the electron capture transitions of ${\mathrm{Lu}}^{173}$ to the levels of ${\mathrm{Yb}}^{173}$ and approximate relative probabilities for the transitions in ${\mathrm{Yb}}^{173}$ are given in a proposed energy level scheme for the decay of ${\mathrm{Lu}}^{173}$. The choice of $\frac{9}{2}$- for the ground-state spin of ${\mathrm{Lu}}^{173}$ is the most consistent with the proposed energy level scheme.

25-Minute Isomer ofSe83

BS>Se/sup 83/ was produced by irradiation of a selenium sample, enriched to 75% in Se/sup 82/, in the Pennsylvania State University research reactor. The gamma-ray spectrum of the 25-minute isomer was investigated by means of a 3-in. thick x 3-in. diameter NaI(Tl) crystal scintillation spectrometer. Gamma rays of 2.294 sintering time (0.833 sintering time 0.005, 0.712 sintering time 0.010, 0.524 sintering time 0.012, 0.358 sintering time 0.005, and 0.225 sintering time 0.005 Mev were found. The beta-ray spectrum was investigated by means of a plastic scintillator. In order to eliminate a competing activity from Se/sup 81/ it was found essential to restrict the observation of beta rays to those in coincidence with the Se/sup 83/ gamma spectrum. A coincidence circuit of 0.25microsecond resolving time was developed for this purpose. The beta spectrum was found to be complex with end-point energies of approximately 0.45, 1.0, and 1.7 Mev. From these data, together with gammagamma coincidence measurements, a decay scheme is proposed. (auth)

Scintillation Studies of Some Neutron Deficient Isotopes of Lutecium

${\mathrm{Lu}}^{170}$, ${\mathrm{Lu}}^{172}$, ${\mathrm{Lu}}^{173}$, and ${\mathrm{Lu}}^{174}$ activities were produced by bombarding ${\mathrm{Lu}}_{2}$${\mathrm{O}}_{3}$ with bremsstrahlung from the University of Illinois betatrons. Gamma rays of energy 0.083, 0.190, 0.245, and 2.04 Mev were associated with the decay of ${\mathrm{Lu}}^{170}$. A gamma-gamma coincidence experiment showed that the 2.04-Mev gamma ray was coincident with the three low-energy transitions. Gamma rays of energy 0.079, 0.113, 0.181, 0.203, 0.325, 0.370, 0.525, 0.820, 0.900, and 1.09 Mev were associated with the electron capture decay of 6.7-day ${\mathrm{Lu}}^{172}$, the isotope studied in the most detail. Levels of energy 0.0787, 0.2602, 0.3731, 0.5769, 0.9015, 1.082, and 1.99 Mev above the ground state have been assigned to ${\mathrm{Yb}}^{172}$ by gamma-gamma coincidence measurements and energy considerations. Gamma rays of energy 0.022, 0.079, 0.113, 0.145, 0.176, 0.274, 0.335, 0.440, 0.550, and 0.640 Mev were assigned to transitions between levels of ${\mathrm{Yb}}^{173}$ while gamma rays of energy 0.077, 0.084, 0.113, 0.176, 0.230, 0.275, 0.990, and 1.245 Mev were associated with the decay of ${\mathrm{Lu}}^{174}$. A summary of all gamma-gamma coincidence experiments involving ${\mathrm{Lu}}^{173}$ and ${\mathrm{Lu}}^{174}$ is included in this paper. The 0.084-Mev transition associated with the decay of ${\mathrm{Lu}}^{174}$ was interpreted to be the first excited level of ${\mathrm{Hf}}^{174}$. A rough calculation of the $K$-conversion coefficient of this transition yielded ${\ensuremath{\alpha}}_{K}<~2.5$.

The level scheme of 112Cd

The level scheme of 112Cd has been investigated by studying the gamma ray spectrum from the decay of 112Ag which excites levels in this nucleide. A source of 112Pd 112Ag in equilibrium was used. Gamma rays of 615, 690, 785, 855, 980, 1090, 1210, 1310, 1400, 1490, 1620, 1810, 1930, 2110, 2240, 2300, 2520, 2540, 2680, 2710, 2830, 2950, 3100 and 3280 keV were found to follow the decay of 112Ag. The summing spectrum was examined using a well type NaI(Tl) crystal with the source in different geometries; summing peaks of 3560, 3300, 3130, 2830, 2710, 2450, 2230, and 2050 keV were observed. Gamma-gamma coincidences were performed using a conventional fast-slow coincidence circuit. The results of these coincidence measurements are listed in table I. A level scheme for 112Cd, based on the intensity values of the various gamma rays following the decay of 112Ag and on the summing and gamma-gamma coincidence measurements is presented in fig. 6.

The level scheme of 112Cd

The level scheme of 112Cd has been investigated by studying the gamma ray spectrum from the decay of 112Ag which excites levels in this nucleide. A source of 112Pd112Ag in equilibrium was used. Gamma rays of 615, 690, 785, 855, 980, 1090, 1210, 1310, 1400, 1490, 1620, 1810, 1930, 2110, 2240, 2300, 2520, 2540, 2680, 2710, 2830, 2950, 3100 and 3280 keV were found to follow the decay of 112Ag. The summing spectrum was examined using a well type NaI(Tl) crystal with the source in different geometries; summing peaks of 3560, 3300, 3130, 2830, 2710, 2450, 2230, and 2050 keV were observed. Gamma-gamma coincidences were performed using a conventional fast-slow coincidence circuit. The results of these coincidence measurements are listed in table I. A level scheme for 112Cd, based on the intensity values of the various gamma rays following the decay of 112Ag and on the summing and gamma-gamma coincidence measurements is presented in fig. 6.

Radioactive Decay ofSe83

An experimental investigation, using scintillation techniques, has been made of the gamma radiation emitted by the 69-second isomer of ${\mathrm{Se}}^{83}$. The ${\mathrm{Se}}^{83}$ was produced by irradiating selenium, enriched in the isotope ${\mathrm{Se}}^{82}$, in the reactor neutron flux. Gamma rays of energy 2.02\ifmmode\pm\else\textpm\fi{}0.05 Mev, 1.01\ifmmode\pm\else\textpm\fi{}0.02 Mev, 0.650\ifmmode\pm\else\textpm\fi{}0.005 Mev, and 0.350\ifmmode\pm\else\textpm\fi{}0.006 Mev were found, each with a (70\ifmmode\pm\else\textpm\fi{}1)-second half-life. Of these, only the 0.350-Mev gamma ray was found in the 25-minute isomer. From these measurements, together with beta-gamma and gamma-gamma coincidence measurements and a rough beta-ray end-point determination, a decay scheme is proposed for the 69-second isomer of ${\mathrm{Se}}^{83}$.

Decay ofCe144andPr144

The beta and gamma spectra from the decay of ${\mathrm{Ce}}^{144}$ and ${\mathrm{Pr}}^{144}$ have been investigated by means of a magnetic-lens beta spectrometer and single-channel scintillation spectrometers. Three partial beta spectra with end-point energies and relative intensities of 184 kev (30%), 245 kev (5%), and 320 kev (65%) were observed for ${\mathrm{Ce}}^{144}$. Gamma rays and internal conversion lines corresponding to 54, 80, and 134-kev transitions were also detected. Internal conversion coefficients indicate an $M1$ multipolarity for the 80- and 134-kev transitions. ${\mathrm{Pr}}^{144}$ yields three partial beta spectra with end-point energies and relative intensities of 0.90 Mev (2%), 2.45 Mev (3%), and 3.15 Mev (95%). Corresponding gamma rays were also detected. Beta-gamma and gamma-gamma coincidence measurements substantiate the proposed decay scheme.

Decay Scheme ofPt199

The radiations of the 30-min activity of ${\mathrm{Pt}}^{199}$ have been investigated with 180\ifmmode^\circ\else\textdegree\fi{} magnetic photographic spectrometers and a 10-channel coincidence scintillation spectrometer. This activity of platinum has been found to decay by the emission of four beta rays with maximum energies of 0.8, 1.1, 1.3, and 1.7 Mev. Gamma rays with energies of 0.074, 0.197, 0.246, 0.318, 0.475, 0.54, 0.71, 0.79, and 0.96 Mev were detected. Extensive beta-gamma and gamma-gamma coincidence measurements were made; and an energy level scheme is proposed which is consistent with the results of all of the experiments.

Beta Decay ofTb161

The radiations emitted in the beta decay of 7-day ${\mathrm{Tb}}^{161}$ have been studied with a 180\ifmmode^\circ\else\textdegree\fi{}, 40-cm radius of curvature, shaped magnetic field spectrometer and a scintillation counter. Gamma-gamma coincidence measurements have also been made. The following transitions with multipolarities indicated were observed (energies in kev): $25.5(E1)$, $48.9(M1)$, $56.9(E1, \mathrm{tentative})$, and $74.6(E1)$. The following beta groups were also observed (energies in kev): 571, 522, 496, and 439. A level scheme incorporating these data is presented.