Coulomb excitation experiments in inverse kinematics using heavy postaccelerated radioactive ion beams often result in multiple step Coulomb excitation whereby several low-lying excited states are populated. At REX-ISOLDE, the Miniball gamma spectrometer is used for the detection of gamma rays originating from electromagnetic transitions in the investigated nuclei [1]. The nuclear levels populated by Coulomb excitation do not always necessarily decay to a lower-lying energy level by emitting a gamma ray, but also by conversion electrons. Conversion and 0+20+1 E0 transitions are important in the neutron-deficient lead region due to the high proton number and nuclear structure arguments [2]. Hence, observed gamma ray intensities do not suffice to analyze Coulomb excitation data in the neutron-deficient lead region in a correct way: the decay from populated levels involving electrons should be included as well.
The vacancy created in an atomic shell by the electron is filled by another atomic electron, accompanied by the emission of a characteristic X ray. The Miniball gamma spectrometer can be used to detect the more energetic K X rays. Conversion and E0 transitions are not the only sources of X rays at Miniball. The beta decay of a fraction of the radioactive ion beam scattered in the neighborhood of Miniball can give rise to the detection of X rays and the REX linear post accelerator yields a broad spectrum of ‘room background’ X rays. These processes are random as they are not related to a beam particle hitting the target. This hints to the fact that it is crucial to do a proper prompt to random scaling when considering the particle gamma coincidences. As the amount of random gammas is large in the X-ray region and the time behavior of these ‘random’ events in the X-ray region is different than for the atomic X rays from the beam and conversion, the amount of X rays is very sensitive to the prompt to random scaling factor.
A last source for X rays is the creation of a K vacancy induced by the collision of the heavy ion beam with the target. This process gives rise to prompt and Doppler broadened X rays as they are created in the interaction of the projectile with the target. These prompt X rays cannot be distinguished from X rays originating from nuclear effects. In order to determine the number of E0 transitions the cross section for the collision-related X rays has to be estimated. The total cross section for K-shell ionization can be inferred from a theoretical prediction [3]. In the Hg coulex analysis 188Hg is used as a reference point since no E0 transitions are expected there. All the detected X rays, corrected for converted E2 transitions, can then be attributed to the collision between projectile and target [4].
In both the 2009 and 2012 experimental campaigns of the Coulomb excitation of the neutron-deficient polonium isotopes X rays were detected at Miniball. A similar role as 188Hg can be played by 206Po where no E0 transitions are expected.
In this presentation the X-ray evaluation will be discussed in the Coulomb excitation data on 196,198,200,202,206Po.
[1] N. Warr et al, Eur. Phys. J. A 49 (2013) 40.
[2] J.L. Wood, K. Heyde, Rev. Modern Phys. 83 (2011), 1467.
[3] C.M. Romo-Kröger, Phys. Scripta T118 (2005) 9.
[4] N. Bree, to be published.
