🔭 astrophysics

Production of 26Al and 10Be via neutron reactions in SiO2 rocks: implications for burial dating related to the Cradle of Humankind–UNESCO World Heritage Site

This paper outlines the theoretical framework and experimental methodology, including preliminary results from CERN's n_TOF facility, for measuring neutron-induced production rates of 10Be and 26Al in SiO2 rocks to improve burial dating accuracy at the Cradle of Humankind World Heritage Site.

Original authors: Sara Rabaglia, Nicholas Pieretti

Published 2026-07-08

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CC BY 4.0

Original authors: Sara Rabaglia, Nicholas Pieretti

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic Clock and the "Human Cradle"

Imagine the Cradle of Humankind in South Africa as a massive, ancient library. Inside its dark, dusty caves lie the oldest books of human history: the fossils of our ancestors like Homo naledi. But here's the problem: the library is so old and the books are buried so deep that we don't know exactly when they were written or placed on the shelves. Traditional ways of dating these rocks are like trying to guess the publication date of a book by looking at the dust on the cover—it's often inaccurate or impossible.

Scientists have a special tool called Burial Dating to solve this. It works like a cosmic hourglass.

How the "Cosmic Hourglass" Works

The Sand (Cosmic Rays): High above us, space is constantly raining down invisible particles called cosmic rays. When these rays hit rocks on the Earth's surface, they smash into the atoms inside the rock (specifically in a mineral called quartz) and create tiny, rare "time-stamp" atoms called isotopes (specifically Aluminum-26 and Beryllium-10).
The Burial: When a rock falls into a deep cave or gets buried under sediment, it is cut off from the cosmic rain. The "hourglass" stops filling up.
The Decay: The time-stamp atoms that were already inside the rock start to disappear (decay) at a known speed. By measuring how many are left, scientists can calculate how long the rock has been buried.

The Problem: To read the time correctly, you need to know exactly how fast the "sand" was falling before the rock was buried. Currently, scientists have to guess this rate based on computer models. It's like trying to time a race when you don't know the speed of the runners. These guesses create big uncertainties in the dates.

The New Experiment: A "Cosmic Rain Machine"

To fix this, the MUSTAR project (a team from INFN and the University of Bologna) decided to stop guessing and start measuring. They went to CERN (the home of the Large Hadron Collider) to use a special machine called n_TOF.

Think of n_TOF as a super-charged cosmic rain machine.

Instead of waiting for nature to rain down cosmic rays (which takes millions of years to build up enough "sand"), this machine shoots a beam of neutrons at rocks.
It mimics the exact "flavor" of cosmic rays but is millions of times stronger.
In a few hours, it can create the same amount of "time-stamp" atoms that would normally take a million years of natural exposure.

What They Did

The team took real rock samples from the Dinaledi Chamber (where Homo naledi was found) and some pure quartz. They placed these samples in front of the neutron beam at CERN.

To make sure they knew exactly how much "cosmic rain" the rocks received, they used Gold Foils as a reference.

The Analogy: Imagine you are trying to measure how much water falls on a garden. You put a bucket (the rock) under the rain, but you also put a very precise measuring cup (the gold foil) next to it. You know exactly how much water the gold cup catches. By comparing the two, you know exactly how hard the rain was hitting the bucket.
In this experiment, the gold foils caught the neutrons and became radioactive. By measuring the "glow" (gamma rays) coming from the gold, the scientists could calculate the exact number of neutrons that hit the rocks.

The Results So Far

The paper reports on the first steps of this massive experiment:

Uniformity: They confirmed that the "cosmic rain" hit all their rock samples evenly.
Flux Measurement: They successfully calculated the number of neutrons hitting the samples using the gold foils.
Next Steps: The rocks are now being sent to a lab (CIRCE) to count exactly how many new Aluminum-26 and Beryllium-10 atoms were created.

Why This Matters

Once they finish counting the new atoms, they will have a real, experimentally measured number for how fast these isotopes are made by neutrons.

This is a game-changer because it replaces the "guesswork" computer models with hard data. Instead of saying, "We think this fossil is between 200,000 and 500,000 years old," they will be able to say, "Based on real physics measurements, this fossil is exactly 350,000 years old."

This will help scientists finally solve the mystery of when our ancient relatives lived in the Cradle of Humankind, giving us a much clearer picture of our own family tree.

Technical Summary: Production of 26Al and 10Be via Neutron Reactions in SiO2 Rocks

Problem Statement
The Cradle of Humankind World Heritage Site (COHWHS) in South Africa contains a high concentration of hominin fossils, including Homo naledi, preserved within complex dolomitic karst cave systems. Reconstructing the chronology of these sites is critical for understanding early human evolution but is hindered by the limitations of conventional dating methods (e.g., 40Ar/39Ar, paleomagnetism, ESR) which often fail in sedimentary contexts lacking volcanic materials or suffer from geochemical uncertainties.

Cosmogenic nuclide burial dating, utilizing the radioactive decay of 26Al and 10Be, offers a solution for ages up to ~5 million years. However, the reliability of this method is currently compromised by significant uncertainties in the production rates of these isotopes. While muon-induced production is relatively well-constrained, the neutron-induced spallation component relies heavily on theoretical models due to a scarcity of experimental nuclear cross-section data. These uncertainties propagate directly into the calculated burial ages, affecting the precision of paleoanthropological timelines.

Methodology
To address the lack of experimental data, the MUSTAR project (a collaboration between INFN and the University of Johannesburg) initiated a direct measurement campaign at the n_TOF facility at CERN. The experimental approach involves:

Irradiation Setup: Quartz samples collected from the Dinaledi Chamber (COHWHS) were irradiated using the n_TOF white neutron spectrum, which closely resembles the cosmic-ray neutron spectrum in the energy region of interest ( $E_n > 10$ MeV).
Sample Diversity: Three types of samples were irradiated to evaluate systematic uncertainties:
- Chemically purified quartz from Dinaledi.
- Non-purified, mechanically crushed rock from Dinaledi (to assess matrix impurity effects).
- Commercially available ultra-pure quartz.
Experimental Conditions: Irradiations were conducted in three experimental areas (EAR1, EAR2, NEAR) with slightly different neutron spectra. In the NEAR station, samples were tested with and without a $B_4C$ filter to suppress low-energy neutrons and isolate high-energy contributions.
Flux Determination: To quantify the neutron fluence, high-purity $^{197}Au$ foils were placed alongside the quartz samples. The activation of gold isotopes ( $^{196}Au$ , $^{194}Au$ , $^{198}Au$ ) via specific reactions (e.g., $^{197}Au(n,2n)^{196}Au$ ) was measured using High-Purity Germanium (HPGe) detectors. The known cross-sections of these reactions allow for the calculation of the integral neutron flux.
Production Rate Calculation: The production rate ( $P$ ) is defined as the ratio of produced isotopes to the exposure time (equivalent neutron fluence). The ultimate goal is to measure the resulting $^{10}Be$ and $^{26}Al$ concentrations via Accelerator Mass Spectrometry (AMS) at the CIRCE Laboratory.

Key Contributions

First Direct Measurement: This work presents the first direct experimental attempt to measure the production rates of $^{10}Be$ and $^{26}Al$ induced by neutrons in quartz using a spectrum analogous to cosmic rays.
Systematic Uncertainty Analysis: By utilizing multiple experimental areas and varying sample purities, the study aims to quantify systematic uncertainties related to neutron energy distribution and the potential contribution of contaminants in natural rock matrices.
Flux Validation: The use of gold activation foils provides a robust, experimentally derived method to determine the neutron fluence, serving as a critical calibration step before the AMS measurements of the cosmogenic nuclides.

Results
The paper reports preliminary results from the analysis of the gold activation foils:

Irradiation Uniformity: HPGe spectra confirmed that irradiation conditions were uniform across all three sample types.
Flux Estimation: A first estimate of the neutron fluence in the relevant energy region ( $10 \text{ MeV} < E < 1 \text{ GeV}$ ) was successfully derived using the $^{197}Au(n,2n)^{196}Au$ reaction.
Next Steps: These preliminary flux data will be validated using the $^{197}Au(n,4n)^{194}Au$ reaction and compared against calculations based on the nominal proton beam intensity. Once validated, these data will define the equivalent natural exposure time required to calculate the final production rates of $^{10}Be$ and $^{26}Al$ via AMS.

Significance and Claims
The paper positions this work as a foundational step toward an experimentally constrained determination of cosmogenic production rates. By replacing current theoretical approximations with values derived from direct measurement, the study aims to:

Significantly reduce the uncertainties associated with burial dating in the COHWHS.
Improve the accuracy of chronological estimates for fossil-bearing sediments, particularly for resolving the evolutionary placement of species like Homo naledi.
Establish a more rigorous physics-based framework for burial dating that can be applied broadly in archaeology and geochronology, moving beyond reliance on modeled neutron fluxes.

The authors explicitly state that the full production rates will be determined only after the completion of AMS measurements at CIRCE, framing the current results as a crucial validation of the experimental protocol and flux determination methodology.