Astrophysical analysis of the measurement of (α,γ) and (α,n) cross sections of

T. Rauscher¹,

G. G. Kiss²,

T. Szücs³

et al.

Abstract

Reaction cross sections of 169 Tm(α,γ) 173 Lu and 169 Tm(α,n) 172 Lu have been measured in the energy range 12.6 ≤ Eα ≤ 17.5 MeV and 11.5 ≤ Eα ≤ 17.5 MeV, respectively, using the recently introduced method of combining activation with X-ray counting. Improved shielding allowed to measure the (α,γ) to lower energy than previously possible. The combination of (α,γ) and (α,n) data made it possible to study the energy dependence of the α width. While absolute value and energy dependence are perfectly reproduced by theory at the energies above 14 MeV, the observed change in energy dependence at energies below 14 MeV requires a modification of the predicted α width. Using an effective, energy-dependent, local optical α+nucleus potential it is possible to reproduce the data but the astrophysical rate is still not well constrained at γ-process temperatures. The additional uncertainty stemming from a possible modification of the compound formation cross section is discussed. Including the remaining uncertainties, the recommended range of astrophysical reaction rate values at 2 GK is higher than the previously used values by factors of 2 − 37.

Further explorations of theα-particle optical model potential at low energies for the mass rangeA≈45–209

“…Thus, the new measurements for the (α, n) reaction on the lighter target nuclei 120 Te [5], 127 I [6], 130,132 Ba [7], and 151 Eu [9] as well as for the (α, γ) reaction on 127 I and 151 Eu are rather well described by this potential. However, we The A-dependence of energies E1 (dashed curve) below which the depth WD is constant, E2 (solid curve) corresponding to 0.9B, E3 (dash-dotted curve) and E4 (dotted curve) given in Table II, and the energy ranges of the (α, x) reaction data involved in this work for A≥120 [5][6][7][8][9][10][11][12][13][14] (thick bars) as well as formerly for A<120 [15] (thin bars), and A>113 [16] (medium bars).…”

“…The more recent (α, x) reaction data concern mainly heavier target nuclei and incident energies well below B [4][5][6][7][8][9][10][11][12][13][14] (Fig. 1).…”

“…have met difficulties in describing even the (α, n) reaction data for the heavier nucleus 141 Pr [8] and especially the well-deformed nuclei 165 Ho and 166 Er [10], 162 Er [11], 169 Tm [12,13], and 168 Yb [14], as well as the (α, γ) reaction cross sections for 130 Ba [7] and well-deformed nuclei [11][12][13][14]. Consequently, further efforts had to be devoted to the OMP parameters for heavier nuclei and the distinct case of the deformed nuclei, as well as to the account of the γ-ray strength functions, to clarify the role of the α-particle OMP within the later cases.…”

“…Thus, while the use of this potential led to description of some new data almost as well as local potentials [3-7, 9, 17] or showing the best agreement of global potentials [1,2,[18][19][20], underestimation of newer data [8,[10][11][12][13][14] was also found. Hence a revision of this OMP [16] became desirable and forms the object of the present work.…”

“…The recent accurate α-particle elastic-scattering andinduced reaction data around the Coulomb barrier (B) on heavier mass nuclei [1][2][3][4][5][6][7][8][9][10][11][12][13][14] make possible the improvement of a previous α-particle global optical model potential (OMP) [15,16]. Thus, while the use of this potential led to description of some new data almost as well as local potentials [3-7, 9, 17] or showing the best agreement of global potentials [1,2,[18][19][20], underestimation of newer data [8,[10][11][12][13][14] was also found.…”