Nuclear structure of 18F: (IV). Negative-parity states

Studies via the ¹⁶O(³He, pγ)¹⁸F, ¹⁴N(α,γ)¹⁸F and ¹⁷O(p, γ)¹⁸F reactions have resulted in new J^π assignments for 11 states or negative parity: E_x(keV) (J^π) = 3791(3^?), 4226(2^(?)), 4398(4^?), 4860(1^(?)), 5502(3^(?)), 5785(2^?), 6097(4^?), 6108(1^(?), 2^(?), 3^(?)), 6241(3^?, T = 1), 6643(2^?, T = 1) and 6878(3^(t-), 4^?). The 6241 keV state is probably isospin mixed. New information for 5 states of positive parity has also been obtained: E_x(keV) (J^π) = 3838(2⁺), 4115(3⁺), 4652(4⁺, T = 1), 4753((0⁺), T = 1) and 4964(2⁺, T = 1). Mean lives, branching and mixing ratios are reported for all states. The results for the negative-parity states are discussed in the framework of the various models available. The states at E_x = 1080(0^?), 2100(2^?) and 4398(4^?) keV are interpreted as the first three members of a K^π = 0^? rotational band.     

A new level of 18F

A new $\text{T = 0 level of}{}^{18}\text{F, E}{}_{\mathrm{x}}\text{= 4848.3 ± 0.5}\text{keV}$, has been studied using the ⁶Li(¹⁶O, αγ)¹⁸F reaction. The γ-decay by a 65 ± 4 % branch to the 1121 keV (5⁺) level and 35 ± 4 % to the 3791 keV (3^?) level has been observed and a lifetime limit τ > 2 ps has been given. A tentative assignment J^π = 5^? is proposed which is supported by the α-γ angular correlation measurements. The possible structure of the new level and reasons why it had not been observed in past experiments are discussed.     

Electroexcitation of isovector magnetic dipole and quadrupole transitions in 18O and their relation to radiative pion capture

In a high-resolution ¹⁸O(e, e′) experiment sharp states at E_X=16.38±0.01 MeV (J^π=2^?, ground state analogue of and at E_X=18.86±0.01 MeV (J^π=1⁺) and clustering of strength (possibly with J^π=1^?, 2^?) at E_X≈23.7 MeV have been observed and compared to theoretical predictions. The transition strengths for the narrow states are B(M2) ↑ = 58± 8 μ_K² fm² and B(M1) ↑ = 0.28± 0.04 μ_K², respectively. These results agree qualitatively with the resu lts from the analogous ¹⁸O(π^?, γ)¹⁸N reaction.     

The Jπ = 3− doublet at Ex = 6241 keV in 18F: Isospin mixing

The resonance previously observed at E_x = 6241 keV in the reactions ¹⁷O(p, γ)¹⁸F and ¹⁴N(α, γ)¹⁸F has been found to be a closely separated doublet (ΔE_x = 2.09±0.04 keV). Resonance strengths in ¹⁷O(p, γ)¹⁸F, ¹⁷O(p, α)¹⁴N and ¹⁴N(α, γ)¹⁸ have been measured and partial widths for the resonance states have been obtained. The doublet was further investigated by means of the ⁴He(¹⁴N, α)¹⁴N reaction using the differentially pumped gas target at CRNL. Analysis of these data using R-matrix theory yielded J^π = 3^? for both states and more precise values of the α-particle widths (Γ_α(lower) = 133 ±4 eV; Γ_α(upper) = 137 ±4 eV). The reduced α-particle widths and the E1 and M1 transition strengths indicate that the doublet arises from the nearly complete mixing of T = 0 and T = 1 eigenstates which in the absence of an isospin breaking interaction would have been separated by less than 40 eV. The α-particle scattering experiments included the J^π = 1⁺ resonance at E_x = 6257 keV and yielded the new values of Γ_α = 580±12 eV and Γ_p = 25⁺³⁵_?25 eV.     

Lifetimes of low-lying levels in 18F

Mean lifetimes of levels in ¹⁸F have been measured using the Doppler-shift attenuation method and the inverse reaction ³He(¹⁶O, p)¹⁸F. Targets of ³He implanted into Al, Nb, and Au foils were employed in the measurements. The Doppler-broadened lineshapes observed at 0° to the beam were analyzed to obtain the following lifetime values: 0.971 ± 0.030, 0.605 ± 0.029 and 0.435 ± 0.041 ps for the 1.70(1⁺), 2.52(2⁺) and 3.36(3⁺)MeV members of the K^π = 1⁺ rotational band, 5.12 ± 0.56, 0.403 ± 0.018 and 1.91 ± 0.17 ps for the 2.10(2^?), 3.13(1^?) and 3.79(3^?) MeV members of the K^π = 0^? bands, and 〈1.2, 2.7^+4.1_?2.7 and 20 ± 2 fs for the 3.06(2⁺, T = 1), 3.72(1⁺) and 3.84(2⁺) MeV states, respectively.     

The 6− analog-antianalog states of 28Si

A resonance is observed in the ²⁷Al(p, γ)²⁸Si reaction at E_p=2876±2 keV, which corresponds to an excitation energy of 14356±2 keV. The 14.36 MeV level decays to a new level at 11577±2 keV, which is turn decays to the known level at 9701.8±0.5 keV. With previous information on the 9.70 MeV level and the present γ-ray angular distributions, obtained from singles spectra as recorded by a 40 cm³ Ge(Li) detector, the spins of the three levels can be limited to J=5, 6; J=6; and J=5, respectively. Transition strength arguments based on measurements of the strength of the 2876 keV resonance and the lifetime of the 11.58 MeV level indicate that the 14.36 MeV level has J^π=6^?, T=1 and that the 11.58 MeV level has J^π=6^?, T=0.     

High-spin states in 38K

High-spin states in ³⁸K are investigated with the ²⁴Mg(¹⁶O, pnγ)³⁸K reaction at $\text{E(}{}^{16}\text{O}\text{) = 36–44}\text{MeV}$. A recently developed Compton-suppression spectrometer with 120 msr solid angle and a pulsed beam are employed to study their γ-decay. For the E4 transition from the isomeric level at E_x = 3458 keV to the ground state a branching ratio of (0.15 ± 0.02)% is found. On the basis of angular distribution and polarization measurements, in which the delayed feeding component is eliminated, spin-parity assignments are obtained of J^π(2646 keV) = (2, 4)^?, J^π(3420 keV) = (4, 6)^? and J^π(3458 keV) = (5, 7)⁺. Prompt-delayed and prompt γγ coincidence experiments are performed to locate high-spin levels above the isomer. Hitherto unobserved levels of high spin are found at E_x = 5254, 7397, 8693, 8747 and 10980 keV and assignments of J^π = (9⁺), (10^?), (12^?), (11^?) and (13^?) respectively, are suggested by weak-coupling considerations. The experimental results are compared with a large-scale shell-model calculation performed in a configuration space with a ²⁸Si core and ten active particles distributed over the ($\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{, 1}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{, 1}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{, 2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ shells. The high-spin states appear to have a rather simple shell-model structure.     

Decay of63Zn

Investigation of the 38.8 min decay of⁶³Zn has resulted in 31 gamma-ray transitions in⁶³Cu. New levels not earlier reported inΒ ⁺-decay of⁶³Zn, but found in nuclear reactions, are suggested at 2,010.9 and 3,101 keV. It is assumed that the excited core model generates a multiplet of levels at 669.75 keV (1/2^?), 962.1 keV (5/2^?), 1,327.1 keV (7/2^?) and 1,546.6 keV (3/2^?). Intensities for intramultiplet transitions were found to be: 219.4 keV, <0.004 (in units of 200 for annihilation radiation), 292.4 keV, <0.004, 365.0 keV, 0.03±0.03, 584.4 keV, <0.005, and 876.8 keV, <0.005.     

The β-decay of98Y and the level scheme of98Zr

Study of theβ-decay of⁹⁸Y and of the level scheme of⁹⁸Zr was undertaken by the use of the two recoil fission product separators JOSEF and LOHENGRIN. Twoβ-decay modes have been detected for⁹⁸Y, the half-lives of which are 0.65±0.05 s and 2.0±0.2 s. The spins and parities of the two states which undergoβ-decay are proposed to be 1⁺ and 4^?, respectively, with the configurations (π1g _9/2)(ν1g _7/2) and (π2p _1/2 (ν1g _7/2). The measuredQ _β -value is 7300±400 keV for the 0.65 s decay. The level scheme of⁹⁸Zr was deduced fromγ,γ-coincidence data. Absoluteγ-intensities were obtained from two independent measurements. The half-life of the first excited state of⁹⁸Zr at 853.0 keV with spin and parity 0⁺ was determined through delayedγ, e^?-measurements to be 63 ±7ns. No indication of the coexistence of two different nuclear shapes of⁹⁸Zr was found.     

The β-decay of 91Sr

Both the continuous β-mray spectrum and the internal conversion electron spectrum of ⁹¹Sr have been studied with a π√2 magnetic spectrometer. The measurements yield an intensity of 30.8% for the first-forbidden unique β-decay to the ground state and a Q-value of 2.684 ± 0.004 MeV. The measured internal conversion coefficient of the 555.6 keV isomeric transition in ⁹¹Y is in excellent agreement with the theoretical value for an M4 transition. Seven less intense internally converted transitions were also studied. The results verify the J^π assignments of earlier studies and also provide new information for the transitions from which level structure configurations can be proposed.     

Excitation of anomalous α-14C resonances in the inelastic scattering of 18O on a 12C target

Two broad resonances at 9.33 MeV and 9.65 MeV are observed in the inelastic excitation of ¹⁸O on a ¹²C target at a laboratory energy of 82 MeV. The α-decay of these states has been measured. A coherent sum of J^π = 2⁺ and 3^? is required to fit the correlations. The structure of these states and the excitation mechanism is discussed.     

High-resolution (e,e') study of isovector M1 and M2 transitions in the oxygen isotopes: (III). 18O

The excitation energy region in ¹⁸O from about E_x = 11–27 MeV has been studied with low-momentum transfer, but high-resolution inelastic electron scattering. Two sharp lines are prominent in the spectra, corresponding to the excitation of T = 2 levels at 16.399 ± 0.005 MeV and 18.871 ± 0.005 MeV of J^π = 2^? and 1⁺, respectively. In contradiction to theoretical predictions no more strong M2 transitions could be found. Broad peaks were observed at 18.5, 19.7, 20.2, 22.5 and 23.8 MeV, the latter two are due to the giant dipole resonance as known from photonuclear reactions. The spectra show in addition considerable fine structure and the application of a cross correlation function technique for its analysis resulted in the location of twelve more low multipolarity weak transitions in the excitation energy range between 16 and 19 MeV. Tentative J^π assignments are given for these levels. The spectra of isospin T = 2 states of A = 18 nuclei are discussed in view of the existing experimental and theoretical work. Finally, the pattern of the isovector M1 and M2 strength distributions of all the three oxygen isotopes ^16,17,18O is discussed.     

Electromagnetic transition strengths in 24Na

Mean lifetimes, excitation energies and branching ratios of ²⁴Na states, populated in the ²³Na(d, p)²⁴Na reaction have been measured. Gamma-ray spectra were measured at three angles in coincidence with proton groups at θ_p = 169°. Mean lifetimes obtained from DSA are (excitation energy in keV, lifetime in fs): 563, > 1000; 1341, 95 ± 30; 1344, 38 ± 11; 1346, > 1500; 1512, 38 ± 11; 1846, 260 ± 50; 1885, 36 ± 9; 2513, 15 ± 7; 2563, < 25; 2904, 50 ± 15; 2978, < 25; 3216, 22 ± 8; 3372, 19 ± 5. For higher levels up to 4207 keV upper limits of 30 fs were set. In combination with earlier work the following unique spin(-parity) assignments could be made: J^π(1846) = 2⁺, J(2513) = 3, J^π(3745) = 3^?. A new level at E_x = 3681.7 ± 0.6 keV is reported.     

Radiative width of the first T = 2 state in 8Be

The lowest T = 2 state (J^π = 0⁺) in ⁸Be was observed as a resonance at E_d = 6962.8 ± 3.0 keV (E_X = 27495.8 ± 2.4 keV, λ_c.m. = 5.5 ± 2.0 keV) in the ⁶Li(d, γ)⁸Be reaction. The resonant capture yield to the 17.6 MeV J^π = 1⁺, T = 1 state gives λ_γ = 23 ± 4 eV (1.14 ± 0.20 W.u.), in good agreement with the intermediate coupling model.     

Der Zerfall des Fe53g und des Fe53m

Theγ-ray spectra from the decays of 8.5 min-Fe^53g and a new (2.51±0.02) min-Fe^53m have been investigated by means of Ge(Li)-detectors. As far as the Fe^53g -decay is concerned no other but the known 380 keV-transition (43%) has been observed indicating that the proceedingΒ ⁺-decay leads to the groundstate and first excited state of Mn⁵³ only. This contradicts formerΒ ⁺-spectroscopic results but is consistent with recent spin assignments. In the Fe^53m -decay fourγ-transitions have been found: 701 keV (100%), 1011 keV (76%), 1328 keV (76%), and 2339 keV (24%). The proposed level scheme of Fe⁵³: 0 keV (7/2^?), 1328 keV (9/2^?), 2339 keV (11/2^?), and 3040 keV (19/2^?, 2.5 min) is in agreement with results published recently byEskola. The level schemes of Mn⁵³ and Fe⁵³ are discussed in terms of the individual particle shell model.     

On parity violation in α-decay

The Fermi liquid model of α-decay and the shell-model wave functions of Zuker, Buck and McGrory are used to calculate the “regular” and “irregular” (parity non-conserving) α-widths to some J^πT = 2±0 low-lying levels of ¹⁶O. The Pauli corrections in the α-channel wave functions are taken into account.     

β-decay studies of the neutron-rich 18,21N isotopes

The β-decay studies of neutron-rich ^18,21N isotopes have been performed using β-n, β-γ, and β-n-γ coincidence methods. The ^18,21N ions were produced by the fragmentation of the ²²Ne and ²⁶Mg beams, respectively, on a thick beryllium target. The time of flight of the emitted neutrons following the β-decay of ^18,21N was measured by a neutron detector system with wide energy detection range and low-energy detection threshold. In addition, several clover germanium detectors were used to detect the β-delayed γ-rays. The half-lives of the β-decays of ¹⁸N and ²¹N were determined to be (619±2) ms and (82.9±7.5) ms, respectively. Several new β-delayed neutron groups were observed with a total branching ratio of (6.98±1.46)% and (90.5±4.2)% for ¹⁸N and ²¹N, respectively. The level schemes of ¹⁸O and ²¹O were deduced. The experimental Gamow-Teller β-decay strengths of ¹⁸N and ²¹N to these levels were compared with the shell model calculations.     

Nuclear orientation of negative parity states of 131Xe

The anisotropies of the 177, 326, and 503 keV γ-transitions between negative parity states of ¹³¹Xe have been remeasured in the decay of oriented ¹³¹I nuclei. In addition the linear polarization of the 503 keV radiation has been determined with a Compton polarimeter consisting of two Ge(Li) detectors. A combined analysis of the reported experiment and earlier internal conversion and angular distribution data yields the unique assignments $\text{9}\text{2}{}^{\mathrm{?}}\text{and}\text{7}\text{2}{}^{\mathrm{?}}$ for the levels at 341 and 667 keV. The $\text{E2}\text{M1}$ mixing ratios of the 177 and 326 keV transitions are δ(177) = ?4.5 ± 1.5 and δ(326) = ?4.4 ± 1.6. The intensity of the L = 2 component in the first forbidden β-decay to the 667 keV level is at most 60 %.     

States of167Ho from the decay of neutron rich nuclide167Dy

Theβ ^?-decay of ₆₆ ¹⁶⁷ Dy produced through the fast neutron reaction¹⁷⁰Er(n, α)¹⁶⁷Dy has been investigated by using several kinds of detectors and a high-capacity two-parameter recording system. The half-life andβ ^?-decay energy of¹⁶⁷Dy were determined to beT _1/2 = 6.20 ± 0.08min andQ _β-=2.35±0.06, respectively. The observed level scheme of ₆₇ ¹⁶⁷ Ho (completely unknown previously) contains 12 states, among them a 6.0±0.1 μsM2 isomer at 259.3 keV. On the basis of theoretical and systematic considerations combined with multipole determinations, the following Nilsson model assignments are proposed for the lowest states of¹⁶⁷Ho: 0 keV (7?/2 [523]), 259.3 keV (3+/2[411]), 319.8 keV (5/2 3+/2[411]), 392.5 keV (1+/2[411]), 410.0 keV (3/2 1+/2[411]), 569.7 keV (3?/2{7?/2[523], 2⁺}). Theβ-decay proceeds mainly to the proposed gamma-vibrational state at 569.7 keV with an anomalously low logft value 5.4, indicating similarity between the microscopic structures of this state and the famous ¦K ₀?2¦ gamma vibration of¹⁶⁵Ho.     

Der Zerfall des 1,5 min-Isomers Co54m

Co^54m (T _1/2=1.43 min) was produced in iron-foils by irradiation with_dE=19 MeV deuterons. The gamma ray spectrum was investigated using a NaJ(Tl) scintillation spectrometer, a coincidence circuit and a Ge(Li)-counter. There were observed three gamma rays having the following energies and intensities per Co_54m decay: 411 keV (0.97±0.07), 1130keV (0.98±0.05), 1407 keV (1.00±0.05). The directional correlation between the pairs of gamma rays were determined. These results correspond to spin and parityJ _π=2⁺ for the 1407 keV,J _π=4⁺ for the 2537 keV, andJ _π=6⁺ for the 2948 keV energy level of Fe⁵⁴. The last-mentioned level was not excited in previous scattering experiments. Our results are compatible withE=210keV andJ _π=6⁺ or 7⁺ of the isomeric state of Co^54m .