Isobaric Analog State Research Articles

Beta-delayed proton decay of 73Sr.

The ${\mathit{T}}_{\mathit{z}}$=-3/2, A=4n+1 nuclide $^{73}\mathrm{Sr}$ produced in the $^{40}\mathrm{Ca}$${(}^{36}$Ar,3n) reaction has been observed via beta-delayed proton emission. A single proton group at a laboratory energy of 3.75\ifmmode\pm\else\textpm\fi{}0.04 MeV has been observed, corresponding to decay of the T=3/2 isobaric analog state in $^{73}\mathrm{Rb}$ to the ground state of $^{72}\mathrm{Kr}$. Combining this measurement with a Coulomb displacement energy calculation yields a mass excess for $^{73}\mathrm{Sr}$ of -31.82\ifmmode\pm\else\textpm\fi{}0.24 MeV based on a predicted mass for $^{72}\mathrm{Kr}$ of -53.94\ifmmode\pm\else\textpm\fi{}0.24 MeV.

Measurement of the resonance at ER=1422 keV in 36Ar(p, gamma )37K.

A new resonance at [ital E][sub [ital R]]=1422 keV has been found in the reaction [sup 36]Ar([ital p],[gamma])[sup 37]K. From its measured resonance strength of [omega][gamma]=(6.0[plus minus]1.5)[times]10[sup [minus]4] eV the proton partial width [Gamma][sub [ital p]] can be calculated. Our data suggest a value of [Gamma][sub [gamma]]/[Gamma][sub [ital p]][approx]40 for the ratio of [gamma] to proton partial width for the corresponding state at [ital E][sub [ital x]]=3241 keV in [sup 37]K. We find that the corrected distribution of Gamow-Teller strength below the isobaric analog state as obtained from the [beta]-delayed proton decay of [sup 37]Ca is in agreement with the results inferred from the [sup 37]Cl([ital p],[ital n])[sup 37]Ar reaction.

Beta-neutrino recoil broadening in?-delayed proton emission of32Ar and33Ar

In high-resolution measurements of delayed protons following theβ-decays of32Ar and33Ar we have observed significant line broadening caused by theβ —gn recoil motion of the daughter nuclei. The measured line shapes agree well with the calculation for both Fermi and Gamow-Teller decays and yield new spectroscopic information on the transitions and excited states involved. For the Gamow-Teller strength in theβ+-decay of33Ar to the isobaric analogue state in33Cl an upper limit of BGT<0.029 was determined.

Double charge exchange and generalized seniority

The double charge exchange transition matrix elements from an even-even target to the double isobaric analog state and the ground state are derived in the generalized seniority model as a function of the number of valence nucleons. Crucial differences with the seniority model are found and their consequences are discussed.

Energy dependence of double giant resonances in pion double charge exchange.

The energy dependence of the giant dipole resonance built on the isobaric analog state was measured in pion-induced double charge exchange on $^{13}\mathrm{C}$ at incident pion energies of 140--295 MeV. The cross section increases with energy. The excitation function has a typical non-spin-flip signature, expected from simple considerations and verified by sequential model calculations. Three narrower previously reported low-lying states in $^{13}\mathrm{O}$ (${\mathit{E}}_{\mathit{x}}$=3.1, 4.5, and 6.1 MeV) exhibit a spin-flip signature and may correspond to the giant Gamow-Teller strength built on the isobaric analog state.

Beta-delayed proton decay of 65Se.

The ${\mathit{T}}_{\mathit{Z}}$=-3/2, A=4n+1 nuclide $^{65}\mathrm{Se}$ produced in the $^{40}\mathrm{Ca}$${(}^{28}$Si,3n) reaction has been observed via beta-delayed proton emission. A single proton group at 3.55\ifmmode\pm\else\textpm\fi{}0.03 MeV has been observed, corresponding to the decay of the T=3/3 isobaric analog state in $^{65}\mathrm{As}$ to the ground state of $^{64}\mathrm{Ge}$. Combining this measurement with a Coulomb displacement energy calculation yields a mass excess for $^{65}\mathrm{Se}$ of -33.41\ifmmode\pm\else\textpm\fi{}0.26 MeV.

Charge independence and charge symmetry breaking interactions and the Coulomb energy anomaly in isobaric analog states.

Effects of CIB (charge independence breaking) and CSB (charge symmetry breaking) interactions on the Coulomb displacement energies of isobaric analog states are investigated for $^{48}\mathrm{Ca}$, $^{90}\mathrm{Zr}$, and $^{208}\mathrm{Pb}$. The mass number dependence of the Coulomb energy anomalies is well explained when we employ CIB and CSB interactions which reproduce the differences of the scattering lengths as well as those of the effective ranges of low energy nucleon-nucleon scattering.

Double giant resonances in pion double charge exchange on 51V, 115In, and 197Au.

Resonances were observed in the continuum in pion-induced double charge exchange (at [ital T][sub [pi]]=295 MeV) on [sup 51]V at [ital Q]=[minus]14.0, [minus]32.4, and [minus]48.0 MeV, on [sup 115]In at [ital Q]=[minus]25.0, [minus]37.3, and [minus]49.6 MeV, and on [sup 197]Au at [ital Q]=[minus]35.0, [minus]44.3, and [minus]54.9 MeV. The three resonances have different characteristic angular distributions. Partial angular distributions were measured on [sup 51]V and [sup 115]In and observed to have [ital J][sup [pi]]=0[sup +], 1[sup [minus]], and 2[sup +] shapes, respectively. Based on their centroid energies, angular distributions, and observed cross sections, we identify the resonances as the double isobaric analog state, the giant dipole built on the isobaric analog, and the double giant dipole resonance. The new observations are in close agreement with the gross features reported recently for the double giant resonances, and provide some of the best signals obtained to date for the double resonance states in nuclei.

Pion double charge exchange on 56Fe at T pi =400 MeV.

Data were measured for pion double charge exchange on [sup 56]Fe at [ital T][sub [pi]]=400 MeV and three laboratory angles: 0[degree], 10[degree], 20[degree]. Angular distributions were measured for the double isobaric analog state, the analog of the giant dipole resonance, and the giant dipole built upon itself. The excitation function of each of these resonances is discussed. Evidence is found for separate [ital J][sup [pi]] components of the double dipole resonance, and cross sections of these components are extracted.

Double charge exchange in 93Nb( pi +, pi -)93Tc at T pi =164, 230, and 294 MeV.

In pion-induced double charge exchange on $^{93}\mathrm{Nb}$, a number of new discrete states, in addition to the ground state and the double isobaric analog state (DIAS), have been observed in the residual $^{93}\mathrm{Tc}$ nucleus. Cross sections for the different states exhibit widely different energy dependence characteristics. In particular, the state just below the DIAS has a very different excitation function from the DIAS. In addition, the cross section of the giant resonance GDR\ensuremath{\bigotimes}IAS has been measured at ${\mathit{T}}_{\mathrm{\ensuremath{\pi}}}$=230 and 294 MeV.

Escape and spreading widths of the isobaric analog states of Sn and Te isotopes

The escape widths for direct proton decay from isobaric analog states in Sb and I nuclei have been obtained by combining calculated single-particle widths with experimental spectroscopic information for neutron pickup reactions on the respective Sn and Te target nuclei. They display a pronounced odd- A/even- A staggering effect with very small widths for the even- A isotopes due to the lower proton decay energies. Only data for odd- A nuclei are available, and there is good agreement. Combining the calculated escape widths with the known experimental total widths yields values for the spreading widths. As expected, and in agreement with theoretical predictions, the spreading widths are essentially constant and dependent only weakly on neutron excess. Interestingly, the spreading widths are independent of whether statistical isospin-violating neutron decay is energetically possible or not, as is the case for the isobaric analog states of the light Sn isotopes with A < 116. The branching ratios for the isospin-forbidden neutron decays from the isobaric analog states average about 95% for the heavier even- A Sb isotopes, and about 60% for the odd- A isotopes. Gamma-ray deexcitation and direct and statistical proton decay compete in the decays of the IAS of the light Sn isotopes, and a branching ratio of about 90% for gamma-ray deexcitation is expected for the IAS in 112Sb.

Systematics of the double isobaric analog state cross section at 50 MeV.

Double charge exchange cross sections for 50 MeV pions to double isobaric analog states in $^{44}\mathrm{Ca}$, $^{56}\mathrm{Fe}$, and $^{93}\mathrm{Nb}$ were measured. The results indicate that the effective double charge exchange operator at this energy has a strong short-range component. In addition, evidence is presented tending to implicate isospin mixing as the mechanism for excitation of the analog of the antianalog in $^{93}\mathrm{Nb}$.

Pion double charge exchange on nickel isotopes and generalized seniority.

Forward angle ([theta][sub lab]=5[degree]) differential cross sections for pion double charge exchange ([pi][sup +],[pi][sup [minus]]) on [sup 60]Ni, [sup 62]Ni, [sup 64]Ni, and [sup nat]Ni have been measured at a [pi][sup +] kinetic energy of 293.4 MeV for reactions resulting in the double isobaric analog states. These cross sections are compared with the general trends seen in other double-charge-exchange reactions and with a generalized seniority model based on a sequential reaction mechanism. Results for nonanalog reactions involving the residual Zn ground states are also presented.

Gamow-Teller strength in A=37 nuclei.

A comparison is made of the recent ${\mathrm{\ensuremath{\beta}}}^{+}$-decay measurements of Garc\'{\i}a et al. and the 1981 $^{37}\mathrm{Cl}$(p,n${)}^{37}$Ar measurements of Rapaport et al. in order to see whether the experiments are consistent. Garc\'{\i}a et al. were not able to determine the amount of Gamow-Teller strength in the isobaric analog state (IAS) or the Coulomb mixing of Fermi strength to a state near the IAS, and were forced to determine the amount of Gamow-Teller strength in the daughter 1371 keV state by making the branching ratios sum to unity. This approach leaves a range of possible values open for these quantities which we investigate. We find that the experiments could be consistent within a theoretically unlikely part of this range. We also examine the effect of this range of allowed values on solar and supernova neutrino cross sections of $^{37}\mathrm{Cl}$ and find weak dependence on the uncertainties described here. We discuss how the consistency of the two experiments can be tested more conclusively.

T=13 double isobaric analog state in 138Ce via pion-induced double charge exchange.

Pion double charge exchange on $^{138}\mathrm{Na}$ (T=13), at a bombarding energy of 292 MeV, and a scattering angle of 5\ifmmode^\circ\else\textdegree\fi{}, has been used to populate the double isobaric analog state in $^{138}\mathrm{Ce}$. The measurements provide determinations of Q value, width, and center-of-mass cross section. Results are compared with systematics and with the isobaric-multiplet-mass equation.

Beta-delayed two-proton decay of39Ti

The beta-delayed two-proton decay of theT z =−5/2 nuclide39Ti has been observed. The39Sc isobaric analog state has been calculated to lie at 8.82 MeV using the measured two-proton sum energy of 4750±40 keV for its decay to the37K ground state. Combining this excitation energy with a Coulomb displacement energy calculation has lowered the energy available for ground state two-proton emission of39Ti from 760 to 530 keV.

Pion double charge exchange on 128,130Te.

Measurements were made of pion double charge exchange on $^{128,130}\mathrm{Te}$ at ${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$=5\ifmmode^\circ\else\textdegree\fi{} and ${\mathit{T}}_{\mathrm{\ensuremath{\pi}}}$=292 MeV. Observations were made of the double isobaric analog, the giant dipole resonance built upon the isobaric analog, and the giant dipole resonance built upon itself. Cross sections for these states were found and compared with all previously known cross sections at ${\mathit{T}}_{\mathrm{\ensuremath{\pi}}}$=292 MeV, and new mass dependences were fitted to these data. A new state below the double isobaric analog state in $^{128}\mathrm{Te}$ was observed and its cross section and width were determined.

Two-to-one relationships between the spectra of selected even-even and even-odd nuclei.

Data are gathered to put to the test of a prediction of the single j shell model, that for selected nuclei the excitation energy of the double isobaric analog state in an even-even nucleus is twice the excitation energy of the single analog state in the adjacent even-odd nucleus. This two-to-one relationship works much better than one would expect. The effects of configuration mixing and the Coulomb interaction are considered.

Sum rules for double Gamow - Teller excitation

Abstract Sum rules are presented for the double Gamow-Teller excitation with ΔT z =±2. The sum rules consist of a model-independent term which is expressed by the numbers of protons and neutrons, and model-dependent terms. It is demonstrated by a numerical calculation that the model-independent term represents a good estimate of the double Gamow-Teller strength for nuclei with N ⩾ Z +4. The calculation indicates that the strength is concentrated in lowest-isospin states, and a giant resonance is constructed near the double isobaric analog state. The double Gamow-Teller excitation favors an angular-momentum transfer of J = 2 rather than J = 0.

0(+) and 2(+) strengths in pion double charge exchange to double giant-dipole resonances.

Coupled-channel optical-model calculations are presented for pion double charge exchange (DCX) to the double giant-dipole resonance (DD). Systematic formulas for the transition matrix element to the giant-dipole resonance are presented. The calculations reproduce the available single-charge-exchange data. No further normalization is required to reproduce the DCX data. A smaller ${0}^{+}$ strength to the DD is calculated than previously expected. The quality of the calculation is reminiscent of that found for similar calculations to the double isobaric analog state.