Mass Equilibration and Fluctuations in the Angular Momentum Dependent Dynamics of Heavy Element Synthesis Reactions

Mass and angle distributions for the ^{52}Cr+^{198}Pt and ^{54}Cr+^{196}Pt reactions (both forming ^{250}No) were measured and subtracted, giving new information on fast quasifission mass evolution, and the first direct determination of the dependence of sticking times on angular momentum. TDHF calculations showed good agreement with average experimental values, but experimental mass distributions unexpectedly extended to symmetric splits while the peak yield remained close to the initial masses. This implies a strong role of fluctuations in mass division early in the collision, giving insights into the transition from fast energy dissipative deep-inelastic collisions to quasifission.

Report on G4-Med, a Geant4 benchmarking system for medical physics applications developed by the Geant4 Medical Simulation Benchmarking Group

BACKGROUND

Geant4 is a Monte Carlo code extensively used in medical physics for a wide range of applications, such as dosimetry, micro- and nanodosimetry, imaging, radiation protection, and nuclear medicine. Geant4 is continuously evolving, so it is crucial to have a system that benchmarks this Monte Carlo code for medical physics against reference data and to perform regression testing.

AIMS

To respond to these needs, we developed G4-Med, a benchmarking and regression testing system of Geant4 for medical physics.

MATERIALS AND METHODS

G4-Med currently includes 18 tests. They range from the benchmarking of fundamental physics quantities to the testing of Monte Carlo simulation setups typical of medical physics applications. Both electromagnetic and hadronic physics processes and models within the prebuilt Geant4 physics lists are tested. The tests included in G4-Med are executed on the CERN computing infrastructure via the use of the geant-val web application, developed at CERN for Geant4 testing. The physical observables can be compared to reference data for benchmarking and to results of previous Geant4 versions for regression testing purposes.

RESULTS

This paper describes the tests included in G4-Med and shows the results derived from the benchmarking of Geant4 10.5 against reference data.

DISCUSSION

Our results indicate that the Geant4 electromagnetic physics constructor G4EmStandardPhysics_option4 gives a good agreement with the reference data for all the tests. The QGSP_BIC_HP physics list provided an overall adequate description of the physics involved in hadron therapy, including proton and carbon ion therapy. New tests should be included in the next stage of the project to extend the benchmarking to other physical quantities and application scenarios of interest for medical physics.

CONCLUSION

The results presented and discussed in this paper will aid users in tailoring physics lists to their particular application.

Systematic Study of Quasifission in 48Ca-induced reactions

The production of superheavy elements through the fusion of two heavy nuclei is severely hindered by the quasifission process, which results in the fission of heavy systems before an equilibrated compound nucleus (CN) can be formed. The heaviest elements have been synthesised using 48Ca as the projectile nucleus. However, the use of 48Ca in the formation of new superheavy elements has been exhausted, thus a detailed understanding of the properties that made 48Ca so successful is required. Measurements of mass-angle distributions allow fission fragment mass distribution widths to be determined. The effect of the orientation of prolate deformed target nuclei is presented. Closed shells in the entrance channel are also shown to be more important than the stability of the formed CN in reducing the quasifission component, with reduced mass widths for reactions with the closed shell target nuclei 144Sm and 208Pb. Comparison to mass widths for 48Ti-induced reactions show a significant increase in the mass width compared to 48Ca-induced reactions, highlighting the difficulty faced in forming new superheavy elements using projectiles with higher atomic number than 48Ca.

External beam for the Heavy Ion Accelerator Facility

Radiotherapy using protons and heavier ions is emerging as an alternative to traditional photon radiotherapy for cancer treatment. Ions have a depth-dose profile that results in high energy deposition at the end of the particle’s path, with a relatively low dosage elsewhere. However, the specifics of ion interactions with cellular biology are not yet fully understood. To study the induced biological effects of the ions on cell cultures, an external beam is required as biological specimens cannot be placed in vacuum. The Heavy Ion Accelerator Facility (HIAF) at the Australian National University hosts accelerators for a wide variety of ion-beam research applications. However, HIAF does not currently have an external beam capability. Here, we present an initial design for a radiobiological research capability at HIAF. A systems engineering approach was used to develop the architecture of the apparatus and determine the feasibility of adapting the current facilities to external beam applications. This effort included ion optics calculations, coupled to a Geant4 simulation, to characterise ion beam transitions through a thin window into the air. The beam spread, intensity distributions, and energy of proton and carbon ions were studied as a function of distance travelled from the window, as well as the effects of alternative window materials and thicknesses. It was determined that the proposed line at the HIAF would be suitable for the desired applications. Overall, this feasibility study lays the foundations of an external beam design, a simulation test framework, and the basis for a grant application for an external beam at the HIAF.

Measuring precise fusion cross sections using an 8T superconducting solenoid

A novel fusion-evaporation residue separator based on a gas-filled superconducting solenoid has been developed at the Australian National University. Though the transmission efficiency of the solenoid is very high, precision cross sections measurements require this efficiency to be accurately known and vitally, requires knowledge of the angular distribution of the evaporation residues. We have developed a method to deduce the angular distribution of the evaporation residues from the laboratory-frame velocity distribution of the evaporation residues transmitted by the solenoid. The method will be discussed, focusing on benchmarking examples for 34S+89Y, where the angular distributions have been independently measured using a velocity filter (A. Mukherjee et al., Phys. Rev. C. 66, 034607 (2002)) . The establishment of this method now allows the novel solenoidal separator to be used to obtain reliable, precise fusion cross-sections.

Mechanisms Suppressing Superheavy Element Yields in Cold Fusion Reactions

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of ^{48}Ca, ^{50}Ti, and ^{54}Cr with ^{208}Pb. Moving from ^{48}Ca to ^{54}Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity P_{CN} (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for ^{50}Ti and 15 for ^{54}Cr in comparison to ^{48}Ca. The energy dependence of P_{CN} indicates that cold fusion reactions (involving ^{208}Pb) are not driven by a diffusion process.

Origins of Incomplete Fusion Products and the Suppression of Complete Fusion in Reactions of ^{7}Li

Above-barrier complete fusion involving nuclides with low binding energy is typically suppressed by 30%. The mechanism that causes this suppression, and produces the associated incomplete fusion products, is controversial. We have developed a new experimental approach to investigate the mechanisms that produce incomplete fusion products, combining singles and coincidence measurements of light fragments and heavy residues in ^{7}Li+^{209}Bi reactions. For polonium isotopes, the dominant incomplete fusion product, only a small fraction can be explained by projectile breakup followed by capture: the dominant mechanism is triton cluster transfer. Suppression of complete fusion is therefore primarily a consequence of clustering in weakly bound nuclei rather than their breakup prior to reaching the fusion barrier. This implies that suppression of complete fusion will occur in reactions of nuclides where strong clustering is present.

Evidence for the Role of Proton Shell Closure in Quasifission Reactions from X-Ray Fluorescence of Mass-Identified Fragments

The atomic numbers and the masses of fragments formed in quasifission reactions are simultaneously measured at scission in ^{48}Ti+^{238}U reactions at a laboratory energy of 286 MeV. The atomic numbers are determined from measured characteristic fluorescence x rays, whereas the masses are obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasifission fragments on a broad angular range, the important role of the proton shell closure at Z=82 is evidenced by the associated maximum production yield, a maximum predicted by time-dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the N/Z (neutron over proton ratios of the fragments) evolution in quasifission reactions.

Production of the PET isotope 11C in hadron therapy via neutron knockout

PURPOSE

Positron emitting isotopes such as ¹¹C and ¹⁰C can be used for vital dose verification in hadron therapy. These isotopes are produced when the high energy ¹²C primary beam particles undergo nuclear reactions within the patient.

METHODS

We discuss a model for calculating cross sections for the production ¹¹C in ¹²C+¹²C collisions, applicable at hadron therapy energies.

RESULTS

Good agreement with the available cross section measurements is found for ¹²C(-1n), though more detailed, systematic measurements would be very valuable.

CONCLUSIONS

Nuclear structure plays a crucial role in the reactions of light nuclei, particularly when those reactions are peripheral and involve only a few nucleons. For such reactions, nuclear structure has a strong influence on the energy and angular distribution of the cross section, and is an important consideration for reliable dose verification using ¹¹C in hadron therapy.

Isospin Symmetry at High Spin Studied via Nucleon Knockout from Isomeric States

One-neutron knockout reactions have been performed on a beam of radioactive ^{53}Co in a high-spin isomeric state. The analysis is shown to yield a highly selective population of high-spin states in an exotic nucleus with a significant cross section, and hence represents a technique that is applicable to the planned new generation of fragmentation-based radioactive beam facilities. Additionally, the relative cross sections among the excited states can be predicted to a high level of accuracy when reliable shell-model input is available. The work has resulted in a new level scheme, up to the 11^{+} band-termination state, of the proton-rich nucleus ^{52}Co (Z=27, N=25). This has in turn enabled a study of mirror energy differences in the A=52 odd-odd mirror nuclei, interpreted in terms of isospin-nonconserving (INC) forces in nuclei. The analysis demonstrates the importance of using a full set of J-dependent INC terms to explain the experimental observations.

Mirror energy differences at large isospin studied through direct two-nucleon knockout

The first spectroscopy of excited states in 52Ni (T(z)=-2) and 51Co (T(z)=-3/2) has been obtained using the highly selective two-neutron knockout reaction. Mirror energy differences between isobaric analogue states in these nuclei and their mirror partners are interpreted in terms of isospin nonconserving effects. A comparison between large-scale shell-model calculations and data provides the most compelling evidence to date that both electromagnetic and an additional isospin nonconserving interactions for J=2 couplings, of unknown origin, are required to obtain good agreement.

Direct observation of long-lived isomers in 212Bi

Long-lived isomers in (212)Bi have been studied following (238)U projectile fragmentation at 670 MeV per nucleon. The fragmentation products were injected as highly charged ions into a storage ring, giving access to masses and half-lives. While the excitation energy of the first isomer of (212)Bi was confirmed, the second isomer was observed at 1478(30) keV, in contrast to the previously accepted value of >1910 keV. It was also found to have an extended Lorentz-corrected in-ring half-life >30 min, compared to 7.0(3) min for the neutral atom. Both the energy and half-life differences can be understood as being due a substantial, though previously unrecognized, internal decay branch for neutral atoms. Earlier shell-model calculations are now found to give good agreement with the isomer excitation energy. Furthermore, these and new calculations predict the existence of states at slightly higher energy that could facilitate isomer deexcitation studies.

Correlations in intermediate energy two-proton removal reactions

We report final-state-exclusive measurements of the light charged fragments in coincidence with (26)Ne residual nuclei following the direct two-proton removal from a neutron-rich (28)Mg secondary beam. A Dalitz-plot analysis and comparisons with simulations show that a majority of the triple-coincidence events with two protons display phase-space correlations consistent with the (two-body) kinematics of a spatially correlated pair-removal mechanism. The fraction of such correlated events, 56(12)%, is consistent with the fraction of the calculated cross section, 64%, arising from spin S=0 two-proton configurations in the entrance-channel (shell-model) (28)Mg ground state wave function. This result promises access to an additional and more specific probe of the spin and spatial correlations of valence nucleon pairs in exotic nuclei produced as fast secondary beams.

Two-nucleon knockout spectroscopy at the limits of nuclear stability

Sudden single-nucleon removal reactions from fast radioactive beams are now key to studies of the structure of rare isotopes. The sensitivity of the heavy residue's parallel momentum distribution to the orbital angular momentum of the removed nucleon is a crucial feature with a high spectroscopic value. Two-nucleon removal reactions provide experimental reach toward the rarest nuclear species. We show that the residue parallel momentum distributions in these reactions offer a clear spectroscopic signal of the angular momentum of the pair of nucleons removed, and thus of the residue final state spins and spectroscopy. Our formalism is applied successfully to new final-state-inclusive measurements of like-nucleon pair removal reactions to states in neutron-rich 36Mg and neutron-deficient 20Mg. We also confront a new final-state-exclusive decomposition of two-proton knockout data to states in neutron-rich 26Ne.

Controlled comparison of the characteristics of long-term benzodiazepine users in general practice

From three general practices, served by 11 principals, 205 long-term benzodiazepine users were identified and matched for age and sex with controls. Benzodiazepine users had significantly higher rates of previous physical illness, consultation and non-psychotropic drug consumption than controls. The characteristics of those receiving prescriptions for benzodiazepine hypnotics alone, anxiolytics alone and anxiolytics plus hypnotics were also investigated. Significant differences emerged between these three groups. Patients receiving hypnotics only were older, had a history of more physical illness and had received more non-psychotropic medication than patients receiving anxiolytics only. The anxiolytic plus hypnotic group had previously received more hypnotics and were currently receiving more medication than the group receiving anxiolytics alone. The results are discussed in relation to current concerns about benzodiazepine dependence and withdrawal.