Neutron-rich isotopes, 6-decay

Because the path of the s process is blocked by isotopes that undergo rapid beta decay, it cannot produce neutron-rich isotopes or elements beyond Bi, the heaviest stable element. These elements can be created by the r process, which is believed to occur in cataclysmic stellar explosions such as supemovae. In the r process the neutron flux is so high that the interaction hme between nuclei and neutrons is shorter that the beta decay lifetime of the isotopes of interest. The s process chain stops at the first unstable isotope of an element because there is time for the isotope to decay, forming a new element. In the r process, the reaction rate with neutrons is shorter than beta decay times and very neutron-rich and highly unstable isotopes are created that ultimately beta decay to form stable elements. The paths of the r process are shown in Fig. 2-3. The r process can produce neutron-rich isotopes such as Xe and Xe that cannot be reached in the s process chain (Fig. 2-3). 

Know that nuclear stability is best related to the neutron-to-proton ratio (n/p), which starts at about 1/1 for light isotopes and ends at about 1.5/1 for heavier isotopes with atomic numbers up to 83- All isotopes of atomic number greater than 84 are unstable and will commonly undergo alpha decay. Below atomic number 84, neutron-poor isotopes will probably undergo positron emission or electron capture, while neutron-rich isotopes will probably undergo beta emission. 

The neutron capture rate in the AGB convective shell is primarily dependent upon the neutron density Nn. Higher neutron densities tend to build up the neutron-rich isotopes. The /T decay rate of an unstable isotope can, in some instances, be sensitive to the temperature at the base of the convective shell, Tcsb. Higher temperatures mean greater excitation of low-lying nuclear levels from which /3 decay may proceed much more rapidly than from the nuclear ground state. 

Secondly, measured beta decay half-lives of neutron-rich isotopes, pro-... 

Beta-decay half-lives were measured for eight neutron rich isotopes produced by fragmentation of E/A-30MeV 180 ions.The first measurements of the half-lives of Be(4.2 0.7 ms.)and 17C(202 17 ms.) have been made along with the half-lives of 9Li, 1 Li, 1 2Be, 1 -B, 15B. The lifetime of -Be is the shortest known beta lifetime.This is the first experiment to use the MSU Reaction Product Mass Separator... 

Beta-decay half-lives for the neutron-rich isotopes measured in this experiment are compared with previous measurements when they exist. Theoretical predictions for Gamow-Teller beta decay are also provided for... 

There is now observational evidence for the existence of isotopic anomalies involving the p-isotopes of Kr, Sr, Mo, Xe, Ba and Sm in various meteoritic materials [32]. These anomalies manifest themselves as excesses or deficits of the abundances of the p-nuclides with respect to the more neutron-rich isotopes, when comparison is made with the bulk SoS mix. In addition, two isotopic anomalies are attributable to the now extinct neutron-deficient radionuclides 92Nbg and 146Sm which have decayed in the meteoritic material where excesses of 92Zr and 142Nd are observed. 

Because the path of the s process is blocked by isotopes that undergo rapid beta decay, it cannot produce neutron-rich isotopes or elements beyond Bi, the heaviest stable element. These elements can be created by the r process that is believed to occur in cataclysmic stellar explosions such as supernovae. 

The combination of collinear fast-beam laser spectroscopy and P-RADOP (radiation-detected optical pumping) has been used to measure nuclear spins and moments of neutron-rich isotopes of the light alkali elements jLi [72-74] and Na [75]. Here, the optically pumped fast atomic beam is implanted into a single crystal placed in a static magnetic field. The NMR signal is destroying the nuclear polarization detected by measuring the p-decay asymmetry. 

Decay by emission of a particles (with ANIAZ — 1) is a proton-rich process, and in the known transactinides results in daughter nuclei that lie closer to the line of /i stability than do their parents. The decays of the superheavy cold-fusion nuclei lead to long chains of sequential a emissions and a progressive increase in neutron richness in the lower members of the chain. In this way, cold fusion can be used to produce isotopes that rival the neutron richness of those produced in hot-fusion reactions. This has been referred to as overshooting [22, 47]. For example, 10-s Hs can be produced directly in the Cm( Mg,5n) reaction with a cross section of 7 pb [179, 180]. The most neutron-rich isotope of hassium that can be produced directly by cold fusion is Hs, in the Pb( Fe,n) reaction [241-243]. However, Hs is also the third member of the Cn decay chain. 

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