Isospin Assignments Research Articles

Level structure of 28Si by the 27Al( 3He, d) 28Si reaction

The level structure of 28Si up to an excitation energy of 15.5 MeV has been studied by the 27Al( 3He, d) 28 reaction at 35 MeV bombarding energy. The experimental angular distributions were analyzed with distorted-wave Born approximation calculations. Values of the transferred orbital angular momenta l and spectroscopic factors were obtained for about sixty levels. Strong transitions to about twenty odd-parity states up to 15.5 MeV excitation were observed and identified with the (1 d 5 2 −11 f 7 2 ) and (1 d 5 2 −12 p 3 2 , 1 2 ) configurations. Tentative isospin assignments were made by comparison with 28Al levels.

Excitation of Unnatural-Parity States inC12by 800-MeV Polarized Protons

Cross sections and analyzing powers have been measured for 800-MeV proton inelastic transitions to unnatural-parity states in $^{12}\mathrm{C}$. Data for the 15.11-MeV ${1}^{+}$, $T=1$ state are well explained by a distorted-wave impulse-approximation calculation based on proton-neutron charge-exchange cross sections. Negative analyzing powers were observed for the first time at 800 MeV, for the 12.71-MeV ${1}^{+}$, $T=0$ state. Values of ${A}_{y}$ appear to be characteristic of the isospin transfer, and support isospin assignments for states at 18.3 and 19.4 MeV.

Photonuclear cross sections forO18

All the major photonuclear cross sections for $^{18}\mathrm{O}$, including $\ensuremath{\sigma}(\ensuremath{\gamma},p)$, $\ensuremath{\sigma}[(\ensuremath{\gamma},n)+(\ensuremath{\gamma},np)]$, and $\ensuremath{\sigma}(\ensuremath{\gamma},2n)$, were measured as a function of photon energy from threshold to 42 MeV. The photon energy resolution was between 200 and 300 keV. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for both single- and double-photoneutron events were determined simultaneously with the cross-section data by the ring-ratio technique. The photoproton cross section was determined by counting the delayed neutrons from the $\ensuremath{\beta}$-decay of the residual $^{17}\mathrm{N}$ nuclei. Integrated cross sections were extracted from the data and compared with sum-rule predictions. Resonances in the pygmy-resonance region exhibit substantial decay branching to negative-parity states in $^{17}\mathrm{O}$. Significant differences in the branching of resonances in the giant-resonance region to the various partial cross sections, together with the photoneutron-energy data, allow isospin assignments to be made for all the resonances, with the consequent delineation of the isospin strength distribution for $^{18}\mathrm{O}$. Evidence was found for two weak but broad resonances above the giant-resonance region. Finally, comparison with the corresponding cross sections for $^{16}\mathrm{O}$ shows considerable differences between the two nuclei.

Energy levels of A = 21–44 nuclei (VI)

The experimentally determined properties of the A = 21–44 nuclei are compiled and evaluated, special emphasis being given to nuclear structure. Separate tables for each of the nuclides reviewed give all available information about the following properties in this order: - excitation energies ( E x ), • - γ-ray branching ratios, • - γ-ray mixing ratios (δ), • - lifetimes (τ m) or widths (Λ), • - spin and parity ( J π ) and isospin ( T) assignments, and • - any other data, on single-particle transfer, resonance levels, etc. The “best” (or adopted) values for E x , τ m or Λ, J π and T are summarized in a master table. The review of each A-chain concludes with - a discussion of isospin multiplets, • - a few remarks about the experimental situation, and • - a compilation of references to relevant theoretical papers.

P31(He3,d)S32reaction at 25 MeV

The $^{31}\mathrm{P}(^{3}\mathrm{He},d)^{32}\mathrm{S}$ reaction was investigated at 25 MeV incident energy. One-hundred and eleven levels up to an excitation energy of 12.5 MeV were observed using a split-pole magnetic spectrograph. The experimental angular distributions were analyzed with the distorted-wave Born approximation. The optical model parameters used in the distorted-wave Born-approximation calculations were obtained from a fit to elastic $^{3}\mathrm{He}$ scattering data taken on $^{31}\mathrm{P}$ at 25 MeV. Gamow functions were used as form factors for the transferred proton in the case of unbound states. Values of the transferred orbital angular momenta $l$ and spectroscopic strengths were obtained for sixty levels, with many odd-parity levels being observed above 9 MeV excitation. Spin and parity assignments were made upon the basis of the $l$ values obtained from the shapes of the angular distributions and upon comparison with the results of other reactions. Isospin assignments were made by comparison with $^{32}\mathrm{P}$ levels. Except for the ${l}_{p}=1$, $T=0$ transfers, most of the observed spectroscopic strength is concentrated into a few levels. The existence of a $T$-mixed doublet of levels, ${J}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$, is suggested in the 11 MeV region of excitation. The excitation energies and spectroscopic strengths are compared with results of a recent shell-model calculation.NUCLEAR REACTIONS $^{31}\mathrm{P}(^{3}\mathrm{He},d)$, $^{31}\mathrm{P}$($^{3}\mathrm{He}$, $^{3}\mathrm{He}$), $E=25$ MeV; measured $\ensuremath{\sigma}({E}_{d},\ensuremath{\theta})$, $^{32}\mathrm{S}$ deduced levels, $l$, $J$, $\ensuremath{\pi}$, $T$, spectroscopic strengths; measured $\ensuremath{\sigma}(\ensuremath{\theta})$, deduced optical model parameters. DWBA analysis using Gamow functions as form factors.

Nucleon-antinucleon resonances near threshold: A possible explanation of the S(1930) meson

We suggest a dynamical mechanism (dominance of isoscalar meson exchange) which leads to an interpretation of the S(1930) meson in terms of a doublet of N N resonances with J πC ( I G )=2 −+(0 +, 1 −). The presence of a doublet implies constructive interference in the p p → p p elastic channel and destructive interference in the p p → n n channel, in agreement with recent experimental evidence. Possible spin and isospin assignments are suggested for other structures seen in N N cross sections near threshold.

Structure ofNe20: TheF19(He3, d)Ne20reaction

The $^{19}\mathrm{F}(^{3}\mathrm{He}, d)^{20}\mathrm{Ne}$ reaction has been studied at a bombarding energy of 18 MeV. Levels were identified up to an excitation energy of 13.3 MeV in $^{20}\mathrm{Ne}$. Several new spin-parity and isospin assignments are made. A comparison of the experimental spectroscopic strengths with those predicted by the Nilsson, SU(3), and shell models shows reasonably good agreement.NUCLEAR REACTIONS $^{19}\mathrm{F}(^{3}\mathrm{He}, d)$, $E=18$ MeV; measured $\ensuremath{\sigma}({E}_{d}, \ensuremath{\theta})$; $^{20}\mathrm{Ne}$ deduced levels, $l$, $J$, $\ensuremath{\pi}$, $T$.

Comment on the Existence of Rotational Structure inAl26

New spin-parity and isospin assignments in $^{26}\mathrm{Al}$ allow additional states to be placed in previously proposed rotational bands. Two new rotational bands are suggested with ${K}^{\ensuremath{\pi}}={0}^{+}$ and ${K}^{\ensuremath{\pi}}={2}^{+}$.

Isospin and decay modes of 6Li states from 7Li(τ, α) 6Li and 7Li(τ, α) 6Li∗ (d) 4He reactions at Eτ = 1.5 MeV

The 7Li (τ, α) 6Li and 7Li(τ, α) 6Li∗(d) 4He reactions have been used to study the energy levels of 6Li. Singles spectra provide evidence for five states below 6 MeV of excitation energy. These include the ground state and four excited states at 2.18, 3.56, 4.57 and 5.36 MeV. One- and two-parameter coincidence data confirm the identification of these levels and aid in the determination of decay modes and isospin assignments. The deduced isospin assignments for the five levels are T = 0, 0, 1, 0 and 1 with a small T = 0 impurity, respectively.

Study of (d, α) Reactions on 40Ca and 40Ar

The nuclear structures of 38 K and 38 Cl are investigated by 40 Ca( d , α) 38 K and 40 Ar( d , α) 38 Cl reactions with 11.4 MeV deuterons. The excitation functions for 40 Ca( d , α) 38 K reaction have been obtained for the deuteron energy between 10.65 and 12.17 MeV. The angular distributions have been measured on the low excited states of 38 K and 38 Cl. The zero range DWBA analysis of the angular distributions on these excited states gives the spin, parity and isospin assignments as follows, the 2.63 MeV state (4, 3 or 2) - 0, the 2.85 MeV state (4, 3 or 2) - 0 and the 3.97 MeV state (3, 2 or 1) + 0 for 38 K. Four strongly excited states are found in 38 Cl at 1.82, 1.99, 2.51 and 2.82 MeV and are considered to have positive parity from DWBA calculations. The spin and parity are considered tentatively to be (3) + for the 1.99 MeV state in 38 Cl.

Study ofN14by Several Transfer Reactions

Nitrogen-14 has been studied by magnetic analysis of particles from the reactions $^{13}\mathrm{C}(^{3}\mathrm{He},d)^{14}\mathrm{N}$, $^{12}\mathrm{C}(^{3}\mathrm{He},p)^{14}\mathrm{N}$, $^{16}\mathrm{O}(d,\ensuremath{\alpha})^{14}\mathrm{N}$, and $^{15}\mathrm{N}(^{3}\mathrm{He},\ensuremath{\alpha})^{14}\mathrm{N}$. Measured angular distributions and intensities are consistent with present spin, parity, and isospin assignments. Doublets were observed at 8.97 and 8.99 MeV, and at 10.06 and 10.10 MeV. Evidence is presented that the 7.97-MeV state is not negative-parity. Strong evidence suggests the 6.21- and 6.44-MeV states may be considered to arise by excitation of two particles to higher shell-model orbitals. Other qualitative configurational assignments are also made.

Single-Isobar Production in Proton-Proton Interactions at 28.5 GeV/c

Two- and four-prong final states in a 28.5-GeV/c $p\ensuremath{-}p$ bubble-chamber experiment were examined for single-isobar production in various decay channels; results are compared with missing-mass-spectrometer data. The 1400 enhancement was found to be an $N\ensuremath{\pi}$ system with the ratio $\frac{n{\ensuremath{\pi}}^{+}}{p{\ensuremath{\pi}}^{0}}\ensuremath{\sim}2$, which establishes the isospin assignment $I=\frac{1}{2}$. The final state ${\ensuremath{\Delta}}^{++}(1238)n$ was measured and compared with low-energy data to show that the cross section decreases according to $P_{\mathrm{lab}}^{}{}_{}{}^{\ensuremath{-}(1.91\ifmmode\pm\else\textpm\fi{}0.08)}$, consistent with single-pion exchange.

Isobaric spin mixing in the 8Be 16.62 and 16.92 MeV states

The 16.62 and 16.92 MeV levels of 8Be are considered, and it is found from theoretical calculations that it is impossible to make a pure isospin assignment to either of these levels. This is in agreement with various experimental data.

Even parity states in O 16

Even parity states in O 16 are studied using the shell model in the jj coupling scheme with a zero-range, spin-dependent residual interaction. This necessitates consideration of both one particle-one hole excitations through two oscillators shells and two particle-two hole excitations. The unperturbed energy spectrum is taken from data of the single particle levels in O 15 and O 17. Energy eigenvalues and eigenfunctions have been obtained for spin and isospin values J π ( T) = 0 +(0, 1, 2), 1 +(0, 1), 2 +(0, 2), 3 +(0), 4 +(0, 1). Transition matrix elements for photo and electro-excitation are computed. An extensive comparison of these results with experimental data on energy levels below 20 MeV has been carried out. Reasonable agreement is obtained for those levels with well-established spin and isospin assignments. In addition, some support is provided for tentative assignments drawn from nuclear reaction data. Comparison above 20 MeV, where there is a high density of predicted even parity levels, is made difficult by the dearth of experimental data. In contrast to the rather satisfactory results for energy values, electron scattering from factors as well as the pair emission rate from the lowest 0 + level are generally much smaller than the observed values, although form factor shapes appear to agree with the limited available data.

States inC12Between 16.4 and 19.6 MeV

The states of ${\mathrm{C}}^{12}$ in the energy region $16.4 \mathrm{MeV}<{E}_{x}({\mathrm{C}}^{12*})<19.6 \mathrm{MeV}$ have been studied by bombarding ${\mathrm{B}}^{11}$ with protons in the range $0.5 \mathrm{MeV}<{E}_{p}<4.0 \mathrm{MeV}$. Many angular distributions and yield curves were measured for the capture gamma rays forming the ground state and first excited state of ${\mathrm{C}}^{12}$ and for the alpha-particle transitions to the ground state and first excited state of ${\mathrm{Be}}^{8}$. The total-neutron-yield curve was determined by measuring the yield of residual ${\mathrm{C}}^{11}$ activity. The absolute inelastic proton-scattering cross section was also determined and used to normalize the relative-yield curve of other investigators. The elastic proton scattering was measured as a function of energy at several angles. Some $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ angular-correlation measurements were made at one resonance. In all, nine distinct resonances were studied and given spin, parity, and isospin assignments with varying degrees of completeness. In most cases in which earlier assignments had been made, these assignments were confirmed. The observed levels are compared with those predicted by particle-hole calculations. It is concluded that the particle-hole model is consistent with most of the observed structure, and that all of the predicted levels in this region have been at least tentatively identified.

On the gauge inlariant theory of elementary particle interactions

The weak interactions of elementary particles are deriled from the principle of gauge inlariance, following the general framework of Sakurai’s inlestigations (3). It is found that only one new boson, an isospinor of the second kind, has to be introduced. The actual weak processes can be obtained by combining the basicV-A interaction, which automatically giles an explanation for the ¦Aδt¦ -1/2 and ¦δS¦ = 1 rules, with larious strong interactions. Next, a systematic study of the electromagnetic interaction is presented, indicating that, besides the photon field, the hypercharge and the isospin boson field also play a role, giling rise to short-range collections for the familiar electromagneticinteraction. It is pointed out that the weak and electromagnetic interactions destroy the strict gauge inlariance of the second kind for bosons which transmit the strong interactions, and therefore make it possible for them to have a non-lanishing rest masss. Finally, the consequences of hypercharge and isospin assignments for leptons are investigated, and it is shown that such an assignment automatically leads to a « muon number » which is conserved in all interactions.