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Romanian physicists image lead bricks with laser-made muons for the first time
Romanian physicists imaged a pile of lead bricks with a laser-made muon beam in which about 90% of detected particles were muons. The beam is meant to spare muography cosmic-ray scans that take months, a speed gain this first known-target test has not yet quantified.
The Scientist · Science desk
What happened
- At Romania's ELI-NP facility, a laser fired into gas accelerated electrons that struck a solid target, making bursts of light that produced muon pairs.
- A plastic-and-paraffin filter followed by a 2-metre concrete wall blocked the other particles the process created.
- Portable detectors in a van, up to 42 metres from the source, recorded the shadow cast by a pile of lead bricks.
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Why it matters
- constraint Beam purity was established by matching data to simulations, so any field instrument would need its own way to confirm what share of its signal is muons.
- decision For anyone weighing muography for inspection work, the choice of method is unchanged for now: the image is low resolution and the source is an ultra-powerful laser at a national facility.
- capability An on-demand beam would let an inspection start when the operator chooses; the team names ageing infrastructure and nuclear material hidden behind heavy shielding as targets.
The cleverest part of the experiment is that the muon's own penetrating power cleans the beam. Muons are about 200 times heavier than electrons, so they lose far less energy in matter and can cross many metres of rock or metal before being absorbed [3]. The two metres of concrete between source and detector are there to let muons through and block the rest [8]. Separating muons with enough energy for imaging from the particles made alongside them is the step that had held artificial beams back [6]. The paper's title describes the muons as GeV-scale [14].
The target was chosen because its answer is known. A pile of lead bricks has a size and position fixed in advance, so the shadow tests the imaging geometry directly [10]. Whether the particles casting it were muons is a separate question, and here the evidence rests on modelling. The team compared its measurements with detailed computer simulations and concluded that about 90% of detected particles were muons, with energies consistent with a laser-driven source [11]. Roughly one detected particle in ten was something else [1].
The thing this doesn't tell you is whether the method is fast, and speed is the reason to build an artificial beam at all. Natural muons arrive too slowly for muography to be quick and reliable in practical settings [1], and a single cosmic-ray scan can take months [5]. The phys.org account does not report the exposure time, the muon yield per laser shot or the count of muons behind the image. Without those, the lead-brick shadow cannot yet be set against a cosmic-ray scan of the same pile.
The detectors were portable enough to ride in a van [9]. The muons came from an ultra-powerful laser at the ELI-NP facility in Romania [7]. The researchers say both lasers and detectors need to improve before the method can scan more complex objects [12]. They are confident that on-demand beams can eventually work in real-world settings [13].
Cosmic muography, slow as it is, found a hidden chamber inside the Great Pyramid of Giza in 2023 [4]. The laser-driven version, from a team including Madalina Dobre of the Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, is a preprint posted to arXiv [2].
What to watch
- Whether Dobre and colleagues publish exposure time and muons per laser shot, the figures needed to compare the image rate with a cosmic-ray scan of the same bricks.
- Peer-reviewed publication of the arXiv preprint, and whether reviewers accept the simulation-based 90% muon purity.
- A first scan of an object whose interior is not known in advance, after the detector and laser upgrades the team says are needed.