Coulomb excitation of neutron-deficient polonium isotopes studied at ISOLDE

The polonium isotopes represent an interesting region of the nuclear chart having only two protons outside the Z = 82 closed shell. These isotopes have already been extensively studied theoretically and experimentally. The heavier isotopes (A > 200) seem to follow a "regular seniority-type regime" while for the lighter isotopes (A < 200) a more collective behavior is observed. Many questions remain regarding the transition between these two regimes and the configuration mixing between quantum states. Experiments in the lighter polonium isotopes point to the presence of shape coexistence, however the phenomenon is not fully understood. A Coulomb excitation study of the polonium isotopes whereby the dynamic properties are investigated can provide helpful insights in understanding the shape coexistence phenomena. In this thesis $^{202}$Po was studied via Coulomb excitation. The $^{202}$Po isotope was part of an experimental campaign in which the $^{196,198,200,206}$Po isotopes were studied as well via Coulomb excitation. In a Coulomb excitation event, a beam of nuclei (projectile) is inelastically scattered on target nuclei. After scattering, one or both of the nuclei are left in an excited state due to the exchange of virtual photons of the electromagnetic interaction. The cross section for Coulomb excitation depends in first order on the reduced transition probability B(E2, $I_i$ $\rightarrow$ $I_f$) and in second order on the quadrupole moment of the populated state and multistep excitations. The B(E2, $I_i$ $\rightarrow$ $I_f$) value is a measure of the collectivity of the nucleus and the quadrupole moment indicates how much the nucleus deviates from spherical symmetry. These two observables can be determined by Coulomb excitation and thus this technique lends itself perfectly for studying the polonium isotopes. The experiment studying the $^{202,206}$Po isotopes was part of an experimental campaign studying the Coulomb excitation of the neutron-deficient polonium isotopes. At REX-ISOLDE a beam of polonium isotopes was produced and accelerated to 2.85 MeV/u. This beam impinged on the target ($^{104}$Pd or $^{94}$Mo) in the collision chamber. The scattered particles were detected by a position-sensitive double-sided silicon-strip detector (DSSSD). Emitted $\gamma$ rays originating from the deexcitation of the nuclei were detected by the Miniball gamma spectrometer. The detector is also position sensitive. The $\gamma$ rays are emitted in flight and thus the detected $\gamma$-ray energy is Doppler broadened and shifted. In order to perform a proper correction for this effect, position sensitivity of the detectors is needed. Both these detectors need to be calibrated and an absolute efficiency curve for detecting the $\gamma$ rays was determined using decay data from a $^{133}$Ba and $^{152}$Eu source. The first part of the analysis consists of selecting the interesting events. Normally coincidence between two detected scattered particles and a detected gamma ray is employed. This procedure was rendered impossible to use due to problems with the downscaling of the detected events. Therefore a milder constraint had to be used: coincidence between a detected target nuclei and a gamma ray. Once the interesting events were selected the gamma spectrum can be visualized. Two clear photo peaks coming from the deexcitation of target and projectile are present as well as a large amount of X rays. These X rays can originate from two different processes: heavy-ion induced K vacancies due to collisions and conversion electrons. The estimated and observed amount of X rays were then compared. A large discrepancy between estimation and experiment was observed and the origin of these excess X rays was further investigated. A first possible origin was the E0 transition between the second 0$^+$ state and the ground state. A $\beta^+$/electroncapture-decay study by Bijnens et al. indicated that the E0 transition was highly favored since no transition from the 0$^+_2$ state to the 2$^+_1$ state was observed [Bij98]. Strong population of this 0$^+_2$ state during the Coulomb excitation experiment will give rise to a large amount of X rays. In light of this question the data on $^{206}$Po was analyzed. For this isotope the 0$^+_2$ state is expected to lie very high in energy and thus excess X rays in this isotope must solely come from the atomic effect. A large amount of unpredicted X rays was also present in this isotope so the question still remains as to were these X rays originate from. The information obtained from the Coulomb excitation experiment was then analyzed using the CLX code. The CLX code uses first order perturbation theory to calculate the cross sections for Coulomb excitation of the populated states. As crucial input the CLX code requires the matrix elements that connect the populated states. Using the known matrix elements of the target the polonium beam flux can be determined. The predicted amount of polonium deexcitation counts can then be compared to the observed amount. The matrix element connecting the 2$^+$ to the 0$^+$ state, and thus the cross section, was then varied such that the prediction and observation agreed with one another. A B(E2, $2^+ \rightarrow 0^+$) value of 28 (6) W.u. was extracted. An attempt was also made for extracting the diagonal matrix element, related to the quadrupole moment of the populated state, but since only a limited angular range of the DSSSD could be used no conclusion could be drawn. The B(E2, $2^+ \rightarrow 0^+$) value was then compared to values predicted from theory and experimentally known values for other polonium isotopes. In conclusion, a more detailed analysis of the X rays should be performed in order to understand where the large amount of excess X rays originate from. Once the origin is known a more complete analysis can be performed using a more suitable programme (e.g. GOSIA) and the transitional and (possibly) the diagonal matrix element can be determined more accurately. These results can then be added to the result from the other polonium isotopes investigated during the same campaign to gain a more complete understanding of the transition from "regular seniority-type regime" to collective behavior in the polonium isotopes.