Neutron separation energy

Figure 2.2 Neutron separation energy for the lead isotopes.

We need to find the neutron separation energy and the fission barrier for this nucleus in order to evaluate the ratio ... 

KRU81]). The B strength function for nuclei along the decay back paths [coupled with neutron separation energies (Sn), fission barrier heights (Bf) and B"decay Q-values (Qg)] determines the amount of B delayed fission and neutron emission that occurs during the cascade back to the B stability line. 

A recent analysis by Thielemann et al. [THI83] of the effects of B" delayed processes on the progenitors of the Th-U-Pu chronometers showed that these processes (delayed fission in particular) did indeed significantly influence the final abundances of the chronometer progenitors. This leads to a long age for the Galaxy. In view of the importance of this result, it is useful to re-examine the calculation with a nuclear model that includes the effects of nuclear deformation on the B decay rates, fission barriers, and neutron separation energies self-consistently. 

Level Densities near the Neutron Separation Energy in Sr-93—97... 

CLARK ET AL. Level Densities nearthe Neutron Separation Energy... 

Be is not a stable isotope despite its large neutron separation energy, because it can beta decay to the more stable isobar 7Li (see under Mass excess in Glossary). As a resultno 7Be exists on Earth or in the meteorites, except by transient production by cosmic rays, and none has been seen (yet) in stars. But it is stable against breaking up into nuclear particles (as opposed to beta decay) and is an observable isotope in nature in two ways. 

Stellar nucleosynthesis No 9Be is produced in stellar interiors, so its source must be found elsewhere. Its low neutron separation energy and its lownuclear charge mean that it will be easily destroyed in stars. 

The number 20 conveys such stability to nucleons that it was called a magic number. This isotope is special in having magic numbers of both neutrons and protons. It has strong nucleon bonding, as evidenced by its neutron separation energy, Sn. It is the most massive of the stable alpha nuclei, those that could be assembled jrom integer numbers of 4He nuclei (ten in this case). 

The aforementioned requirements on neutron concentration and temperature suffice to fix qualitatively several of the main features of the nuclear flow associated with the r-process and to identify the involved nuclear physics. Figure 22 depicts the situation very schematically. In the course of the transformation of a given seed into more neutron-rich isotopes by a series of (n,y) reactions, (y,n) photodisintegrations have a rate increasing with the neutron excess or, equivalently, with the associated decrease of the neutron separation energy Sn. For low enough Sn, the (y,n) reactions counteract efficiently the radiative neutron captures. At this point, the nuclear flow may proceed to... 

Fig. 33. Location in the (N, Z)-plane of the stable isotopes of the elements between Fe and Bi. The p-isotopes are represented by black squares, while both the s-, r-, sr- or sp-isotopes are identified with open squares (see Figs. 13 - 15 for details). The p-nuclides are the progeny of unstable neutron-deficient isobars located on the down-streaming p-process flow (thick black line for more details on the p-process flow, see [32]). Some possible r-process flows derived from a high-temperature parametric model (Sect. 7) are also shown, as well as the up-streaming s-process flow (thin black line) confined at the bottom of the valley of nuclear stability. The proton and neutron drip lines correspond to the locations of zero proton and neutron separation energies...

Figure 3.12 A complex decay scheme. For complete explanation of all the symbols and numbers see Ref. 4. Half-life is given for each element s ground state, and energy of each level is given at intermediate states. Q is the neutron separation energy. Transition probabilities are indicated as percentages (from Ref. 4).

The proton separation energies (Sp) generally increase with increasing neutron number (N) and neutron separation energies (Sn) increase with increasing proton number (Z). Consequently, a strong attractive interaction must exist between protons and neutrons. 

Based on the S2n and S411 neutron separation energies and systematics of P-decay energies one can expect also P-delayed 2n and even 4n emission in strongly neutron rich nuclei (Gelbke et al. 2006). 

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