Il Nuovo Cimento (1955-1965)

, Volume 32, Issue 4, pp 1023–1036 | Cite as

Isotopic-spin conservation in allowed β-transitions and coulomb matrix elements

  • S. D. Bloom


A survey has been made of all available experimental data for atomic weight >20 yielding a value for |MF|, the Fermi matrix element, in allowed transitions. Useful measurements have been made for 15 different radio-nuclides, three being ostensibly pure Fermi transitions (0+→0+), the rest being mixed. From |MF| are deduced the values of |α T (T−1)|, the absolute value of the impurity coefficient for isotopic spin (T) in state (T−1), and |〈HC〉|, the effective Coulomb matrix element. In deducing |〈HC〉| recent results on (p, n) reactions joining isobaric states in medium and heavy-weight nuclei were used in order to locate the excitation of theT-state. The findings are that there is a marked tendency for |α T (T−1)| to be ≲5·10−32≲3·10−5) and for |〈HC〉| to be ≲30 keV. Four measurements yield values higher than these limits, but in three of these alternative experimental results give values which are lower. The fourth measurement is the64Ga (0+→0+) transition which gives one of the biggest and most precise values for α T (T−1), (2.00±0.05)·10−2. However, this deduction depends on the spin assignment for64Ga, expected to be 1 from shell-model theory. A direct confirmation of the64Ga spin would clarify the situation here. The general findings described above are in good qualitative accord with the results of the survey made byWilkinson of α T (T−1) in light nuclei (A⩽20). One is led therefore to suggest that |〈HC〉| tends to be ≲30 keV in the vicinity of the ground state (≲5 MeV excitation) as a general rule for all atomic weights up to at least 134. This is in keeping with the finding from the above-mentioned (p, n) reactions that isospin is (qualitatively) well conserved forall atomic weights at low excitations (≲10 MeV).


Si è fatta una rassegna di tutti i dati sperimentali disponibili, per pesi atomici >20, che dànno il valore di |MF|, l’elemento di matrice di Fermi, nelle transizioni permesse. Si sono fatte utili misure in 15 diversi radionuclidi, di cui tre erano ostensibilmente pure transizioni di Fermi (0+→0+), ed il resto transizioni miste. Da |MF| si deducono i valori di |α T (T−1)|, il valore assoluto del coefficiente di impurezza per uno spin isotopico (T) nello stato (T−1), ed |〈Hc〉|, l’elemento effettivo della matrice di Coulomb. Per dedurre |〈Hc〉| si sono usati, per localizzare l’eccitazione dello statoT, i recenti risultati sulle reazioni (p, n) che uniscono stati isobarici nei nuclei medi e pesanti. Si trova che |α T (T−1)| ha una forte tendenza ad assumere un valore ≲5·10−32≲3·10−5) ed |〈HC〉| ad assumere un valore ≲30 keV. Quattro misure dànno valori superiori a questi limiti, ma in tre di queste altri risultati sperimentali dànno valori più bassi. La quarta misura è quella della transizione (0+→0+) del64Ga, che dà uno dei maggiori e più precisi valori di α T (T−1), (200±0.05)·10−2. Tuttavia questa deduzione dipende dall’assegnazione dello spin al64Ga, che si prevede essere 1 in base alla teoria del modello a gusci. Una conferma diretta dello spin del64Ga chiarirebbe qui la situazione. I risultati generali descritti sopra sono in buon accordo qualitativo con i risultati della rassegna fatta da Wilkinson di α T (T−1) nei nuclei leggeri (A⩽20). Si è quindi indotti a suggerire che |〈HC〉| tende ad essere ≲30 keV in prossimità dello stato fondamentale (eccitazione ≲5 MeV) come regola generale per tutti i pesi atomici sino almeno a 134. Questo concorda con il risultato delle suddette reazioni (p, n), che cioè l’isospin è (qualitativamente) ben conservato per tutti i pesi atomici alle basse eccitazioni (≲10 MeV).


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© Proprietà Letteraria Riservata 1964

Authors and Affiliations

  • S. D. Bloom
    • 1
  1. 1.CERNGeneva

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