The beta strength function and the astrophysical site of ther-process

Abstract

The beta strength function has been calculated for ~ 6 000 nuclei between the line of beta stability and the neutron drip line. The calculations -performed by using a schematical Brown-Bolsterli model -yield more reliable beta decay half lives, P, values and/?-delayed fission rates than the strongly oversimplified assumptions on S~ used up to now in astrophysical calculations. The calculated beta rates are shown in this paper to be decisive in the discussion of the problem of the astrophysical site of the r-process, which is responsible for the production of the heavy elements in the universe. In particular we concentrate on the neutron capture processes during the explosive burning of He in massive stars, initiated by the outgoing shock wave from a supernova explosion. It is shown that as consequence of the revised fi-decay rates explosive He-burning represents a very convincing alternative scenario to the classical r-process, which avoids the presently existing problems of the latter: 1. The computed relative abundances of r-nuclei resemble on the average their solar system counterparts. 2. The calculated abundance peaks essentially coincide in position with the observed ones. 3. The new site of the r-process avoids the problems of overproduction of heavy elements and of the mass-cut. Our results are based on realistic stellar models and hydrodynamical explosion calculations which for the first time are applied here to the problem of heavy element nucleosynthesis. The results turn out to be rather insensitive to the details of those models. The shorter/3-decay half lives obtained are of importance also in the investigation of further astrophysical sites producing heavy elements such as the n-processes in explosive C or Ne burning.

Calculation of beta-decay properties of neutron-rich nickel isotopes

“…While with the use of the -on the average -distinctly shorter T1/2 of [11] in an explosive He-burning scenario [29] considerable improvement over earlier attempts could be obtained as far as the reproduction of positions and relative heights of the r-abundance peaks at A ~ 130 and 195 are concerned [32], difficulties were met in the region around A=80 [12,33]. These calculations showed that, when starting from the Fe group seed nuclei, not even the N = 50 shell (corresponding to the A ~ 80 abundance peak) could be reached during the short duration of the neutron flux of a few tenths of a second in explosive He-burning, and solar-system r-abundances could not be re-produced anymore (see also [34]).…”

“…These calculations showed that, when starting from the Fe group seed nuclei, not even the N = 50 shell (corresponding to the A ~ 80 abundance peak) could be reached during the short duration of the neutron flux of a few tenths of a second in explosive He-burning, and solar-system r-abundances could not be re-produced anymore (see also [34]). Therefore, to overcome this problem, the r-process calculations presented in [32] started from 7aNis0. As is discussed in [12], also the assumption of an (n, 7)-(% n) equilibrium in the attempt of [32] seems to be inconsistent with the adopted astrophysical model.…”

“…Therefore, to overcome this problem, the r-process calculations presented in [32] started from 7aNis0. As is discussed in [12], also the assumption of an (n, 7)-(% n) equilibrium in the attempt of [32] seems to be inconsistent with the adopted astrophysical model. For scenarios like explosive He-burning, characterized by neutron densities of G=IO is cm -3 and stellar temperatures of T<109 K, the neutron-capture times for the Fe group nuclei are already comparable to the T1/2 predicted by the TDA model [11], thus requiring detailed network computations rather than a simple flflow approach.…”

Absence of delayed fission in the? ?-decay of 2.3 min238Pa

“…In order to achieve a homogeneous mixture of 233pa tracer and the uranium salt, 233pa solution in 6M HC1 was added to UO2(NO3)2.6H20 powder, stirred to give a wet mass, dried for one day at room temperature in vacuum, and finally milled to give the finely powdered material used as target for the neutron bombardments. Furthermore, in some cases we determined chemical yields via 7-ray activities of 6.7 h 234gPa present in the decay chain of 23sU with a branch of 0.13% [1][2][3][4][5][6][7][8][9][10][11][12]. Within the statistical accuracy the chemical yields determined via the artificially added 233pa tracer, and, alternatively determined via 234gPa, a natural decay product of 23SU, were the same.…”

“…Therefore, in the energy interval 2.56-3.46 MeV one has to rely on theoretical calculations of S~. Microscopic calculations of S~ are available [1,2], however the predictive power of such calculations, in particular for deformed nuclei, is still unsatisfying [3]. For the purpose of a simple estimate of the order of magnitude of P~I we use the extremely gross approximation S~=const.…”