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Towards gamma-ray lasers via super-radiance in a Bose-Einstein condensate of 135mCs isomers

Periodic Reporting for period 1 - GAMMALAS (Towards gamma-ray lasers via super-radiance in a Bose-Einstein condensate of 135mCs isomers)

Reporting period: 2015-12-01 to 2017-11-30

The generation of coherent gamma photons, which would constitute the building block of a gamma-ray laser, has been an active field of research since the demonstration of the first lasers. In fact, the possibility of producing coherent gamma photons would represent a milestone in physics and science, and a game-changer in technology, with long-term impact on society and applications from energy storage to healthcare.
However, the production of coherent gamma photons has been hindered by fundamental mechanisms: 1) Accumulation of sufficient sources (e.g. excited nuclei); 2) Reduction of their emission linewidth, which otherwise prevents any possible increase in the number of photons. In addition, the methods proposed thus far are not achievable with the available technology, require unrealistic nuclear densities, or do not solve the problem of the emission linewidth.
The GAMMALAS objectives are: to identify a feasible approach for the generation of coherent gamma photons; to build a facility for the proof-of-concept demonstration of the approach; and to train a highly specialised PDRA (the Fellow) in a European context, thus maximising the Fellows’s perspectives for future career.
GAMMALAS has successfully identified and theoretically demonstrated a suitable strategy for producing coherent gamma photons, achievable with current technologies. GAMMALAS has also designed, implemented and tested a facility for the experimental demonstration of the controlled production of coherent gamma-rays.
The coherent gamma-ray technology will have a dramatic impact in physics and its applications. For example, novel approaches for nuclear spectroscopy and for investigating the nuclear shape, as well as for tracing of dangerous, explosive or radioactive isotopes can be envisaged. Coherent gamma-rays will also provide new tools for imaging with unprecedented spatial resolution, as well as more precise and effective radiotherapy approaches in oncology and stereotactic surgery for brain tumours. Finally, on-demand coherent gamma photons will have a dramatic impact on energy storage, allowing storing and retrieving energy from isomeric nuclei. This has the potential to revolutionise the batteries technology, with an increase of energy density of several orders of magnitude.
GAMMALAS investigated a suitable mechanism for demonstrating the production of coherent gamma photons with the current technology. The process relies on laser cooling and trapping of 135mCs nuclei: they spontaneously decay in 53 minutes emitting a chain of two gamma photons (846 keV and 787 keV) and – given their alkali structure – they are well-suited for conventional laser cooling. After the excited nuclei are brought around 100 nK, they can exhibit purely quantum properties, in particular spatial coherence (Bose-Einstein Condensate, BEC). The BEC transfers its coherence the emitted gamma photons, triggering a burst of coherent gamma rays. Detailed results are presented in the paper L. Marmugi et al., “Coherent gamma photon generation in a Bose–Einstein condensate of 135mCs”, Phys. Lett. B 777, 281-285 (2018, submitted in December 2016).
GAMMALAS also built a complete experimental facility for production and laser cooling of radioactive Cs isotopes, and for demonstrating production of coherent gamma rays. Cs nuclei are produced by proton-induced fission in U or Th (e.g. 135,135mCs), or fusion-evaporation in BaF2 (e.g. 134,134mCs), at the Accelerator Laboratory of the University of Jyväskylä (Finland, partner of the Action). The particles of interest are extracted as ions, electrostatically accelerated, mass separated, and then routed to the low-energy experimental chamber. Here, ions are neutralised (i.e. transformed in neutral atoms with the addition of an external electron with thin foil implantation), extracted in vapour phase, and cooled down to around 150 microK and trapped in a Magneto-Optical Trap. Further stages of cooling (currently under test at the UCL laboratories), will lead to the BEC and the experimental demonstration of coherent gamma photons production.
The facility has been fully tested and characterised with 133Cs and with 134Cs in a test beam time. These results will be included in a forthcoming scientific publication, currently in preparation. The facility is now ready for the 135mCs beam, tentatively scheduled for spring 2018, pending finalisation of the Accelerator Laboratory timetable.
The Fellow supervised three UCL undergraduate students and four UCL PhD students. The latter are now involved full-time in the research project.
In terms of exploitation, GAMMALAS identified, in collaboration with the University of Surrey, a concrete approach to investigate the nuclear properties, and – in particular – the nuclear shape by using the technology and the facility built during the project. Future investigations will also proceed along this path, and further suitable funding sources are being explored.
Results and information about were disseminated through a series of methods:
- Project website, http://www.ucl.ac.uk/~ucapfre/nuclear.html.
- Social media: dedicated posts on Twitter and Facebook.
- Project advertisement for future students/PhD/CDT candidates.
- Participation to outreach activities at UCL.
- Informal meetings with researchers, potential industrial partners, potential investors.
- Participation to UCL seminars.
- Participation to workshops.
- Participation to scientific conferences.
- Publication of scientific articles (one, plus one in preparation).
The identified mechanism for generating coherent gamma photons was proposed and theoretically demonstrated for the first time within GAMMALAS. As such, results presented in Phys. Lett. B 777, 281-285 are the most recent and advanced proposal in the field. Therefore – to date – they represent the current state-of-the-art.
The experimental facility built within GAMMALAS is the first of its kind for the UK and Finland, and the only one in the world dedicated to isotopes and isomers of Cs, in view of demonstration of collective phenomena in ultra-cold nuclear matter. Other facilities for laser cooling and trapping of atoms are operational at TRIUMF (Canada), Tohoku University (Japan), and INFN-LNL (Italy), but they are dedicated to Fr isotopes.
In addition, some of the technical solutions employed in the experimental facility are targeted evolutions of current technologies. In this view, the GAMMALAS experiment has surpassed the previous state-of-the-art.
Socio-economic impact is expected in the long term, when the demonstrated technology will progressively move towards market.
Societal implications can be divided in two classes. The long-term, widespread effect, which will arise from the availability of the potentially disruptive technology of GAMMALAS, will be observed at a later stage. The short-term impact of GAMMALAS was the creating a new channel for knowledge and personnel exchange between UK (UCL; University of Surrey) and Finland (University of Jyväskylä). In particular, a new multi-disciplinary collaboration was established and will continue after the closure of GAMMALAS. UCL personnel is now permanently present at the Accelerator Laboratory in Jyväskylä. New research and teaching avenues have been therefore opened, whereby UCL and the Fellow integrated the background of the University of Jyväskylä, creating the first cold-atom laboratory of that institution.
Experimental facility at the beginning (2015)/at the end (2017) of GAMMALAS
Simplified sketch of the experimental facility built within the GAMMALAS project.