Empirical evidence for the existence of pair fluctuations in rapidly rotating nuclei in connection with the pair gap is reviewed. The quantities considered are single-particle energies (routhians) and alignments. While the cranked shell model in the presence of static pair correlations provides an accurate description of data at rotational frequencies below the critical frequency corresponding to the collapse of the static pair gap, conspicuous discrepancies are found in the region of and above the pairing phase transition. In particular, a group of excitations is observed displaying lower excitation energies and smaller alignments than those predicted by the cranked shell model. Such excitations can be characterized as behaving as if the correlations induced by the presence of a pairing condensate were not totally obliterated after the "phase transition." A theoretical model, based on the renormalization of the single-particle motion mixed by the coupling to pairing vibrations, is quite successful in explaining the overall trend of the data at rotational frequencies larger than the critical frequency. Smaller alignments and excitation energies are correlated with configurations displaying particle coupling schemes which profit most from fluctuations of the pair gap about its zero equilibrium value. While this model reproduces many of the experimental features, it still overpredicts the alignments by 2-3 units of $\hslash$ in the crossing region. Thus other degrees of freedom (both static and dynamic), e.g., deformations, also must play a role at large rotational frequencies.

The observed pattern of the population and decay of the superdeformed band of ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$ is related to the large splitting of the giant dipole resonance based on the superdeformed minimum as well as to the low level density associated with it, and to the sudden onset of static pairing correlations taking place at the rotational frequency $\hslash \omega \approx 0.3$ MeV. A new method for spectroscopic studies of superdeformed nuclei is suggested.

To develop a stand-alone code to make any application coded with the GEANT4 (GEometry ANd Tracking, version 4) toolkit capable of reading and writing phase-space (phsp) files in the format created by the IAEA (International Atomic Energy Agency), so that the exchange of phsp files between other validated Monte Carlo (MC) codes and GEANT4 is possible. We present a stand-alone code, written in C++ object-oriented language, developed in a way that ensures the compatibility with future versions of the IAEA phsp format. The aim of the reader part is to get the information from a given IAEA phsp file and create the primary particles in a GEANT4 user application. On the other hand, the writer part of the code is the responsible for writing the IAEA phsp files during a run of the GEANT4 application. A testing simulation was written with GEANT4 to verify the performance of this code, with satisfactory results. An example of use in a GEANT4 application which simulates the treatment head of a radiotherapy linear electron accelerator (linac) is also shown, comparing dose calculations with experimental data. This stand-alone package, which can be used in any GEANT4 application, allows the exchange of validated phsp files between different MC codes and the use of phsp data from many different accelerators and fields in dosimetry studies. Furthermore, it also offers additional utilities of interest in medical applications.