Exploring the Frontiers of Nuclear Physics and Space Applications at HIAF

Dr. Lauren Bezzina
Heavy Ion Accelerator Facility
Department of Nuclear Physics and Accelerator Applications
The Australian National University

The Heavy Ion Accelerator Facility (HIAF) in Canberra, Australia, has recently celebrated its 50-year anniversary. Operating almost continually since 1973, HIAF hosts 11 beamlines, with adiverse set of research initiatives from fundamental nuclear physics, accelerator mass spectrometry, to commercial irradiation services for the space sector in Australia. This talk will focus on two of these areas: research into fundamental nuclear reaction dynamics, and the new irradiation beamline.

The formation of superheavy elements (SHEs) by nuclear fusion can be conceptually divided into two steps: capture, and compound nucleus formation. Once captured, the two nuclei may reseparate before fusing to form a compact compound nucleus. This outcome is called quasifission.

Fusion-fission, in many cases, leads to reactions outcomes inseparable from quasifission. Evaporation residue (ER) measurements are therefore the most reliable, direct experimental signature of fusion. ER cross section measurements forming the same compound nucleus, ²²⁰Th, using ¹⁶O, ⁴⁰Ar, ⁴⁸Ca, ⁸²Se and ¹²⁴Sn-induced reactions [1, 4, 2, 3] revealed [1] that fusion was severely suppressed for the more symmetric reactions relative to the 16O-induced reaction.

Measurements of reactions forming ²²⁰Th provide conclusive evidence that the ER cross section is exponentially suppressed as a function of Z_pZ_t. The fission characteristics show no mass-angle correlation, demonstrating that here ER cross sections are suppressed by slow quasifission. 

HIAF’s newest beamline, the Space Irradiation Beamline (HIAF-SIBL) was created to help Australia’s emerging space sector. An often-overlooked challenge is the radiation environment of space, particularly for smaller payloads. In low earth orbits, the trapped protons and electrons of the Van Allen belts pose a particular challenge, while those orbits intercepting the polar regions are exposed to greater numbers of higher-energy galactic cosmic rays. Once the relative safety of the Earth’s magnetic field is left behind, there is an even greater risk of exposure to these high energy heavy ions.

In order to make radiation testing accessible to small companies and research institutions within Australia a new beamline was built at The Australian National University’s Heavy Ion Accelerator Facility (HIAF) with funding from the Australian Space Agency. This new capability takes advantage of the 14UD SSNICS ion source, which can produce ions of any species except noble gases, to emulate parts of the space radiation environment across a wide range of potential mission profiles.

References
[1] D. J. Hinde, M. Dasgupta, and A. Mukherjee. Severe inhibition of fusion by quasifission in reactions forming 220Th. Physical Review Letters, 89(28):282701, dec 2002.

[2] C. C. Sahm, H. G. Clerc, K. H. Schmidt, W. Reisdorf, P. Armbruster, F. P. Hessberger, J. G. Keller, G. Münzenberg, and D. Vermeulen. Fusion probability of symmetric heavy, nuclear systems determined from evaporation-residue cross sections. Nuclear Physics A, 441(2):316–343, aug 1985.

[3] K. Satou, H. Ikezoe, S. Mitsuoka, K. Nishio, and S. C. Jeong. Effect of shell structure in the fusion reactions 82Se+134Ba and 82Se+138Ba. Physical Review C, 65(5):054602, apr 2002.

[4] D. Vermeulen, H. G. Clerc, C. C. Sahm, K. H. Schmidt, J. G. Keller, G. Münzenberg, and W. Reisdorf. Cross sections for evaporation residue production near the N=126 shell closure. Zeitschrift für Physik A Atoms and Nuclei, 318(2):157–169, jun 1984.

Hosted by Prof. Couder