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Portable source could produce high-energy muon beams

✇Physics World
Par :Isabelle Dumé
3 novembre 2025 à 10:00

Due to government shutdown restrictions currently in place in the US, the researchers who headed up this study have not been able to comment on their work

Laser plasma acceleration (LPA) may be used to generate multi-gigaelectronvolt muon beams, according to physicists at the Lawrence Berkeley National Laboratory (LBNL) in the US. Their work might help in the development of ultracompact muon sources for applications such as muon tomography – which images the interior of large objects that are inaccessible to X-ray radiography.

Muons are charged subatomic particles that are produced in large quantities when cosmic rays collide with atoms 15–20 km high up in the atmosphere. Muons have the same properties as electrons but are around 200 times heavier. This means they can travel much further through solid structures than electrons. This property is exploited in muon tomography, which analyses how muons penetrate objects and then exploits this information to produce 3D images.

The technique is similar to X-ray tomography used in medical imaging, with the cosmic-ray radiation taking the place of artificially generated X-rays and muon trackers the place of X-ray detectors. Indeed, depending on their energy, muons can traverse metres of rock or other materials, making them ideal for imaging thick and large structures. As a result, the technique has been used to peer inside nuclear reactors, pyramids and volcanoes.

As many as 10,000 muons from cosmic rays reach each square metre of the Earth’s surface every minute. These naturally produced particles have unpredictable properties, however, and they also only come from the vertical direction. This fixed directionality means that can take months to accumulate enough data for tomography.

Another option is to use the large numbers of low-energy muons that can be produced in proton accelerator facilities by smashing a proton beam onto a fixed carbon target. However, these accelerators are large and expensive facilities, limiting their use in muon tomography.

A new compact source

Physicists led by Davide Terzani have now developed a new compact muon source based on LPA-generated electron beams. Such a source, if optimized, could be deployed in the field and could even produce muon beams in specific directions.

In LPA, an ultra-intense, ultra-short, and tightly focused laser pulse propagates into an “under-dense” gas. The pulse’s extremely high electric field ionizes the gas atoms, freeing the electrons from the nuclei, so generating a plasma. The ponderomotive force, or radiation pressure, of the intense laser pulse displaces these electrons and creates an electrostatic wave that produces accelerating fields orders of magnitude higher than what is possible in the traditional radio-frequency cavities used in conventional accelerators.

LPAs have all the advantages of an ultra-compact electron accelerator that allows for muon production in a small-size facility such as BeLLA, where Terzani and his colleagues work. Indeed, in their experiment, they succeeded in generating a 10 GeV electron beam in a 30 cm gas target for the first time.

The researchers collided this beam with a dense target, such as tungsten. This slows the beam down so that it emits Bremsstrahlung, or braking radiation, which interacts with the material, producing secondary products that include lepton–antilepton pairs, such as electron–positron and muon–antimuon pairs. Behind the converter target, there is also a short-lived burst of muons that propagates roughly along the same axis as the incoming electron beam. A thick concrete shielding then filters most of the secondary products, letting the majority of muons pass through it.

Crucially, Terzani and colleagues were able to separate the muon signal from the large background radiation – something that can be difficult to do because of the inherent inefficiency of the muon production process. This allowed them to identify two different muon populations coming from the accelerator. These were a collimated, forward directed population, generated by pair production; and a low-energy, isotropic, population generated by meson decay.

Many applications

Muons can ne used in a range of fields, from imaging to fundamental particle physics. As mentioned, muons from cosmic rays are currently used to inspect large and thick objects not accessible to regular X-ray radiography – a recent example of this is the discovery of a hidden chamber in Khufu’s Pyramid. They can also be used to image the core of a burning blast furnace or nuclear waste storage facilities.

While the new LPA-based technique cannot yet produce muon fluxes suitable for particle physics experiments – to replace a muon injector, for example – it could offer the accelerator community a convenient way to test and develop essential elements towards making a future muon collider.

The experiment in this study, which is detailed in Physical Review Accelerators and Beams, focused on detecting the passage of muons, unequivocally proving their signature. The researchers conclude that they now have a much better understanding of the source of these muons.

Unfortunately, the original programme that funded this research has ended, so future studies are limited at the moment. Not to be disheartened, the researchers say they strongly believe in the potential of LPA-generated muons and are working on resuming some of their experiments. For example, they aim to measure the flux and the spectrum of the resulting muon beam using completely different detection techniques based on ultra-fast particle trackers, for example.

The LBNL team also wants to explore different applications, such as imaging deep ore deposits – something that will be quite challenging because it poses strict limitations on the minimum muon energy required to penetrate soil. Therefore, they are looking into how to increase the muon energy of their source.

The post Portable source could produce high-energy muon beams appeared first on Physics World.

Radioactive BEC could form a ‘superradiant neutrino laser’

✇Physics World
Par :No Author
4 octobre 2025 à 14:48

Radioactive atoms in a Bose–Einstein condensate (BEC) could form a “superradiant neutrino laser” in which the atomic nuclei undergo accelerated beta decay. The hypothetical laser has been proposed by two researchers US who say that it could be built and tested. While such a neutrino laser has no obvious immediate applications, further developments could potentially assist in the search for background neutrinos from the Big Bang – an important goal of neutrino physicists.

Neutrinos – the ghostly particles produced in beta decay – are notoriously difficult to detect or manipulate because of the weakness of their interaction with matter. They cannot be used to produce a conventional laser because they would pass straight through mirrors unimpeded. More fundamentally, neutrinos are fermions rather than bosons such as photons. This prevents neutrinos forming a two-level system with a population inversion as only one neutrino can occupy each quantum state in a system.

However, another quantum phenomenon called superradiance can also increase the intensity and coherence of the radiation from photons. This occurs when the emitters are sufficiently close together to become indistinguishable. The emission then comes not from any single entity but from the collective ensemble. As it does not require the emitted particles to be quantum degenerate, this is not theoretically forbidden for fermions. “There are devices that use superradiance to make light sources, and people call them superradiant lasers – although that’s actually a misnomer” explains neutrino physicist Benjamin Jones of the University of Texas at Arlington and a visiting professor at the University of Manchester. “There’s no stimulated emission.”

In their new work, Jones and colleague Joseph Formaggio of Massachusetts Institute of Technology propose that, in a BEC of radioactive atoms, superradiance could enhance the neutrino emission rate and therefore speed up beta decay, with an initial burst before the expected exponential decay commences. “That has not been seen for nuclear systems so far – only for electronic ones,” says Formaggio. Rubidium was used to produce the first ever condensate in 1995 by Carl Wiemann and Eric Cornell of University of Colorado Boulder, and conveniently, one of its isotopes decays by beta emission with a half-life of 86 days.

Radioactive vapour

The presence of additional hyperfine states would make direct laser cooling of rubidium-83 more challenging than the rubidium-87 isotope used by Wiemann and Cornell, but not significantly more so than the condensation of rubidium-85, which has also been achieved. Alternatively, the researchers propose that a dual condensate could be created in which rubidium-83 is cooled by sympathetic cooling with rubidium-87. The bigger challenge, says Jones, is the Bose–Einstein condensation of a radioactive atom, which has yet to be achieved: “It’s difficult to handle in a vacuum system,” he explains, “You have to be careful to make sure you don’t contaminate your laboratory with radioactive vapour.”

If such a condensate were produced, the researchers predict that superradiance would increase with the size of the BEC. In a BEC of 106 atoms, for example, more than half the atoms would decay within three minutes. The researchers now hope to test this prediction. “This is one of those experiments that does not require a billion dollars to fund,” says Formaggio. “It is done in university laboratories. It’s a hard experiment but it’s not out of reach, and I’d love to see it done and be proven right or wrong.”

If the prediction were proved correct, the researchers suggest it could eventually lead towards a benchtop neutrino source. As the same physics applies to neutrino capture, this could theoretically assist the detection of neutrinos that decoupled from the hot plasma of the universe just seconds after the Big Bang – hundreds of thousands of years before photons in the cosmic microwave background. The researchers emphasize, however, that this would not currently be feasible.

Sound proposal

Neutrino physicist Patrick Huber of Virginia Tech is impressed by the work. “I think for a first, theoretical study of the problem this is very good,” he says. “The quantum mechanics seems to be sound, so the question is if you try to build an experiment what kind of real-world obstacles are you going to encounter?” He predicts that, if the experiment works, other researchers would quite likely find hitherto unforeseen applications.

Atomic, molecular and optical physicist James Thompson of University of Colorado Boulder is sceptical, however. He says several important aspects are either glossed over or simply ignored. Most notably, he calculates that the de Broglie wavelength of the neutrinos would be below the Bohr radius – which would prevent a BEC from feasibly satisfying the superradiance criterion that the atoms be indistinguishable.

“I think it’s a really cool, creative idea to think about,” he concludes, “but I think there are things we’ve learned in atomic physics that haven’t really crept into [the neutrino physics] community yet. We learned them the hard way by building experiments, having them not work and then figuring out what it takes to make them work.”

The proposal is described in Physical Review Letters.

The post Radioactive BEC could form a ‘superradiant neutrino laser’ appeared first on Physics World.

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