The angular distributions of α-particles emitted by oriented nuclei and their relation to nuclear structure and deformation

Abstract

α-particle angular distribution data from an extensive on-line nuclear orientation study of the favoured decay of nearly spherical, neutron deficient At and Rn nuclei near N=126, as well as of deformed Fr and Pa isotopes are presented. From the comparison of these data with various theoretical models it is found that for nearly spherical nuclei, anisotropic α-emission in favoured decays can be well explained within the shell model but not by a tunnelling model. On the other hand, for the deformed nuclei studied here, a description on the basis of α-particle tunelling through the Coulomb barrier is quite appropriate.The process of α-decay was the first known form of radioactivity. Nevertheless, the mechanisms that determine the α-decay observables are still poorly understood. Indeed, for the description of the total α-decay rate as well as for the angular distribution of α-particles emitted by oriented nuclei [1, 2, 3], various divergent theories exist among which, with the experimental information available up to now, no clear choice could be made.Here, we report on a systematic study of the relationship between the angular distribution of α-particles emitted by oriented nuclei and their shape and structure. For this purpose, we have experimentally determined the angular dependence of the α-radiation pattern of favoured transitions from oriented, nearly spherical nuclei (205)(206)(207)(208)(209)(210)(211)217 At, 205,207,209 Rn) as well as of nuclei with intermediate ( 221 Fr) and strong deformations ( 227,229 Pa). The measurements were performed using the technique of low-temperature nuclear orientation (LTNO) at the NICOLE refrigerator at ISOLDE (CERN) and at the KOOL-facility on-line to LISOL at CYCLONE (Louvain-laNeuve). In this type of experiments, the observed quantity is the angular distribution function W (θ) which is derived from the normalised radiation intensity as a function of the emission angle θ relative to the nuclear orientation axis. The latter is determined by the direction of the magnetic field B. In the case of α-decay in an axially symmetric geometry, W (θ) may be written as [4]: Table 1. The experimental mixing ratios δ 02 for the favoured α-transitions in nearly spherical At and Rn and in deformed Fr and Pa isotopes together with theoretical values calculated in the shell model[5] or in a tunelling model[6, 8]. The nuclear quadrupole deformation is indicated by β 2 nucleus δ 02 (exp) δ 02 (th) β 2 [7] 205 At 0.041(2) 0.035 207 At 0.077(3) -0.035 209 At 0.115(4) -0.035 211 At 0.201(5) 0.207[5] 0.008 205 Rn 0.089(4) 0.081[5] -0.079 207 Rn 0.098(4) 0.089[5] -0.053 209 Rn 0.108(3) 0.096[5] -0.044 221 Fr -0.216(9) -0.204[8] 0.120 (K=1/2) 227 Pa 0.364(27) 0.341[6] 0.168 229 Pa 0.75(16) 0.45[6] 0.190 W (θ) = 1 + fA 2 B 2 Q 2 P 2 (cosθ) + fA 4 B 4 Q 4 P 4 (cosθ) + . . .where f is a two-site model parameter which describes the distribution of the implanted atoms in the host lattice. B k and Q k account for the orientation of the nuclear ensemble and the detection geometry, respectively and P k are ...

14 Particle Decay Study of Oriented Nuclei

“…These data agree well with the former results, although the higher yields give rise to higher precisions [107], Furthermore the Leuven group carried out experiments on the moderately deformed 22 'Fr and the well deformed α-emitters 227 Pa and 229 Pa. The recorded anisotropics are huge.…”

“…More experiments have to be and will be carried out with sources of actinide α-emitters prepared by implantation at isotope separators as well as by recoil implantation. Results of further experiments have already been reported [81,107], Concerning ß-asymmetry experiments, the determination of precise A 1 -values invokes further technical improvement in the way as worked out by the Columbia Unauthenticated Download Date | 6/13/16 9:19 AM group [31]. Among different possibilities the precise measurement of A t -parameters of mirror transitions can answer some very fundamental questions in weak interactions.…”

“…Quin and Gerard pointed out that by measuring the longitudinal polarization of ß-particles emitted by polarized nuclei, the presence of right-handed currents in nuclear ß-decay can in some cases be tested with up to an order of magnitude enhancement in sensitivity compared to measurements for unpolarized nuclei [39]. A recent experiment carried out with 107 In nuclei at the Leuven on-line system, if interpreted in the framework of left-right symmetric models, has provided new constraints on extensions of the Standard Model [6], A lower limit of 225 GeV/c 2 has been deduced for the mass of a possible right-handed charged W gauge boson. More results of these measurements are expected to reduce significantly the hitherto existing range established from other experiments.…”

“…The recorded anisotropics are huge. These recent experiments are analyzed and interpreted more precisely [107].…”

“…Recently, new limits on the mass of a possible right-handed gauge boson in weak interactions have been obtained by studying the polarization-asymmetry correlation of positrons emitted from polarized nuclei [6], and on a precision scale at least comparable to larger-scale experiments in high energy and particle physics. A first experiment has been carried out with 107 In. Further measurements will yield more precise information about the occurrence of right-handed currents or other contributions beyond the Standard Model.…”