Researchers to Develop High-Intensity Gamma Radiation Source at CERN

The “Gamma Factory initiative,” represented by an international research team, is presently investigating an innovative research tool: by using the prevailing accelerator facilities at CERN, the researchers have proposed to create a high-intensity gamma radiation source.

GF concept: laser photons (pink waves) are scattered by PSI circulating in an accelerator ring. The scattered photons propagate in the direction of the ions motion (purple arrows)—resulting in an intense beam of highly energetic gamma rays. Image Credit: Ill./©: Dr Alexey Petrenko.

To achieve this, the researchers will circulate dedicated ion beams in the LHC and SPS storage rings, which will be subsequently stimulated with laser beams to allow them to produce photons. The energies of the photons in the chosen configuration will lie within the range of gamma radiation of the electromagnetic spectrum. This is quite interesting in relation to the spectroscopic study of atomic nuclei.

In addition, the gamma rays will be developed so that it will have an extremely high intensity, whose magnitude will be several orders higher than that of systems being operated today.

In the new issue of the Annalen der Physik journal, the scientists asserted that a “Gamma Factory” built in such a way will result in spectroscopy breakthroughs and also lead to new means of testing the underlying symmetries of nature.

At the core of the Gamma Factory proposal are exclusive ion beams that are composed of heavy elements such as lead in which nearly all the electrons in the outer shell have been removed.

A lead atom usually contains 82 electrons in its shell and 82 protons in the nucleus. When just one or two electrons remain, it results in the so-called “partially stripped ions,” or PSIs for short. In the potential Gamma Factory milieu, the PSIs will circulate in a high-energy storage ring, such as the Large Hadron Collider (LHC) and the Super Proton Synchrotron (SPS) at CERN.

The PSIs provide exclusive opportunities for studying numerous fundamental questions relating to contemporary science. In the field of atomic physics, PSIs act as a type of mini-laboratory to study how systems equipped with a sparse number of electrons behave upon exposure to powerful electromagnetic fields—which, in the example of PSIs, are created by the atomic nuclei themselves.

The primary idea fundamental to the Gamma Factory is to make a laser beam to strike head-on with an expedited beam of PSI. In the “PSI laboratory,” the incident photons are capable of producing excited states by carrying electrons to higher orbits—this represents a perfect test system that will enable in-depth analyses through atomic spectroscopy (that is, primary beam spectroscopy).

Consequently, the laser-stimulated PSIs themselves produce photons, which can be subsequently employed in various other experiments that are “outside” the PSI laboratory (that is, secondary beam spectroscopy).

The gamma ray beam will be defined by high energies of up to 400 MeV, which matches a wavelength of 3 fm. For comparison purposes, the photon energy of visible light has magnitude that is eight orders smaller and a wavelength that is correspondingly higher.

The Gamma Factory that we are proposing offers two immensely exciting prospects: On the one hand, it will be a very intense light source which produces high energy gamma rays at a very specific band of frequencies; at the same time it will act as a giant ion trap where we can use spectroscopy to get a very accurate picture of the PSIs circulating in the storage ring.

Dmitry Budker, Study Author and Professor, PRISMA+ Cluster of Excellence, Johannes Gutenberg University Mainz and Helmholtz Institute Mainz

Budker continued, “In our article, we describe the many possibilities offered by the two approaches. On the other hand, it is important to address the current and future challenges associated with establishing a Gamma Factory like this.”

A few examples of interesting physics applications of primary beam spectroscopy comprise measurement of the impacts of atomic parity violation in PSI—the outcome of feeble communications among subatomic particles—and also detection of the neutron distribution in the nuclei of the PSI. The resultant data would complement some of the most significant research activities that are presently being performed in Mainz.

The secondary, high-energy gamma radiation beams with accurately managed polarization can be used in tandem with “fixed” polarized targets, for instance, to analyze both the structure of atomic nuclei and nuclear reactions applicable to astrophysics.

In addition, the secondary, high-energy gamma radiations can be used for producing powerful tertiary beams, for instance, those of neutrinos, muons, or neutrons.

The Gamma Factory can be optimally operated only by overcoming a range of technological challenges.

So, for example, we need to learn to perform laser cooling of ultrarelativistic PSI in order to reduce their energy spread and obtain a well-defined beam. Whilst the laser cooling of ions at lower energies has already been investigated, at GSI in Darmstadt for example, it has not yet been performed at such high energies as those that will be associated with the Gamma Factory.

Dmitry Budker, Study Author and Professor, PRISMA+ Cluster of Excellence, Johannes Gutenberg University Mainz and Helmholtz Institute Mainz

At CERN, the Gamma Factory is more than a pipe dream because a considerable advancement was made from concept to reality on July 2018. Along with the CERN accelerator experts, the Gamma Factory team managed to circulate beams of hydrogen- and helium-like lead ions in the SPS for several minutes. Later, the hydrogen-like beam was introduced into the LHC, where it subsequently circulated for a number of hours.

The next crucial step is running the dedicated proof-of-principle experiment at CERN's SPS that will hopefully validate the entire Gamma Factory concept,” concluded Dmitry Budker, outlining the next interesting stage.

The Gamma Factory is an aspiring proposal, which is now being investigated within the CERN “Physics beyond Colliders” program.

Journal Reference:

Budker, D., et al. (2020) Atomic physics studies at the Gamma Factory at CERN. Annalen der Physik.

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