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Evaluation of 2-methylalanine and lithium formate monohydrate as alternative ESR dosimeters

This study demonstrates that 2-methylalanine, followed by lithium formate monohydrate, offers superior sensitivity and linear dose response compared to the standard L-alanine for ESR dosimetry in the critical low-dose range below 10 Gy.

Original authors: Samayeh Azariasl, Soma Ohtaka, Kenji Komaguchi, Hiroshi Yasuda

Published 2026-08-13

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CC BY 4.0

Original authors: Samayeh Azariasl, Soma Ohtaka, Kenji Komaguchi, Hiroshi Yasuda

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 Invisible Ink of Radiation

Imagine radiation as a ghostly, invisible ink that stains everything it touches. While we can't see this ink with our eyes, scientists have a special pair of "magic glasses" called Electron Spin Resonance (ESR) that can detect the tiny, permanent scratches—or "radicals"—that radiation leaves behind on certain materials. Think of these radicals like footprints in fresh snow; the more radiation hits the snow, the deeper and more numerous the footprints become. By counting these footprints, scientists can figure out exactly how much radiation a person or object was exposed to.

This is a big deal because radiation is everywhere, from medical X-rays to space travel, but too much of it is dangerous. The current "gold standard" for measuring this invisible ink is a common amino acid called L-alanine. It's like the reliable, sturdy ruler every scientist keeps in their pocket. However, this ruler has a flaw: it's a bit clumsy when measuring very small amounts of radiation. If the dose is low (like a tiny smudge of ink), the ruler's markings are too faint to read accurately. Since the most dangerous health effects often happen at these low doses, scientists are on a treasure hunt for a new material that acts like a super-sensitive ruler, capable of spotting even the tiniest radiation footprints without losing its accuracy.

The Search for a Sharper Ruler

In this study, a team of researchers from Hiroshima University decided to test two new candidates to see if they could outperform the classic L-alanine ruler in the low-dose range. The first challenger was 2-methylalanine, a chemical cousin of the standard alanine, and the second was lithium formate monohydrate, a salt that had shown promise in previous studies. The goal was simple: see which material could "see" the smallest amounts of radiation (between 0 and 10 Gy) most clearly and reliably.

To make a fair fight, the researchers treated all three materials exactly the same. They zapped them with X-rays at doses ranging from 0.5 Gy up to 10 Gy. Then, they used their ESR "magic glasses" to measure the signal, carefully adjusting the settings (like the power of the microwave beam and the strength of the magnetic wobble) to ensure they weren't accidentally blurring the results.

The results were like a race where the underdogs surprised everyone. When the researchers cranked up the microwave power to get a stronger signal, the classic L-alanine got "saturated" (like a sponge that can't hold any more water) and stopped responding well. However, 2-methylalanine was the toughest of the bunch; it resisted this saturation the best, allowing for stronger, clearer readings. Lithium formate came in second, while L-alanine struggled the most at higher power levels.

When it came to actually measuring the radiation doses, the difference became very clear once the dose went above 2 Gy. 2-methylalanine showed the highest sensitivity, meaning it produced the strongest signal for the same amount of radiation. It was also the most obedient, following a perfectly straight line (highly linear) as the dose increased, with a correlation score of 0.9989. Lithium formate was also better than the old standard, but 2-methylalanine was the star of the show. Even more importantly, all three materials held onto their "footprints" perfectly for a whole week. This means that if you get exposed to radiation, you don't have to rush to the lab immediately; you can wait a few days, and the measurement will still be accurate.

The paper concludes that while L-alanine remains the trusted international reference for its long-term stability, 2-methylalanine suggests itself as a powerful new alternative. It shines brightest in the low-dose range where human health is most at risk, offering a sharper, more sensitive way to measure the invisible ink of radiation. The researchers suggest that with more testing on things like long-term durability and consistency, this new material could become a vital tool for keeping people safe from radiation exposure.

Technical Summary: Evaluation of 2-methylalanine and Lithium Formate Monohydrate as Alternative ESR Dosimeters

Problem Statement
Electron Spin Resonance (ESR) dosimetry is a standard technique for radiation dose assessment, with L-alanine serving as the international reference material due to its tissue equivalence, signal stability, and wide linear dose range (>100 kGy). However, L-alanine exhibits limited sensitivity at low absorbed doses (<10 Gy), a range critical for radiation protection, emergency medical response, and retrospective biodosimetry. The broad ESR spectrum of irradiated L-alanine restricts its practical utility in this low-dose region. Consequently, there is a need to identify alternative materials that offer enhanced sensitivity while maintaining favorable dosimetric properties such as signal stability and linearity.

Methodology
This study systematically evaluated the ESR dosimetric characteristics of two candidate materials—2-methylalanine (2-aminoisobutyric acid) and lithium formate monohydrate—using L-alanine as a reference.

Materials: High-purity samples of L-alanine, 2-methylalanine, and lithium formate monohydrate were prepared in plastic capsules (approx. 100 mg each).
Irradiation: Samples were irradiated with X-rays (160 kV, 6.3 mA) at absorbed doses ranging from 0.5 to 10 Gy. Dose delivery was verified using a built-in ionization chamber with an accuracy of ±0.5%.
ESR Measurement: Spectra were recorded using an X-band spectrometer (9.4 GHz) equipped with a Mn²⁺:MgO internal standard.
Optimization: Microwave power and modulation amplitude were optimized to avoid signal saturation and spectral distortion. A microwave power of 1 mW and modulation amplitude of 0.4 mT were selected for all comparative measurements.
Analysis: The peak-to-peak amplitude of the first-derivative ESR spectra was used to quantify signal intensity. Dose-response linearity and post-irradiation signal stability (over one week) were assessed under controlled ambient conditions (20°C, 32% humidity).

Key Results

Microwave Saturation Behavior: The materials exhibited distinct saturation thresholds. 2-methylalanine demonstrated the highest resistance to microwave saturation, followed by lithium formate monohydrate. L-alanine showed saturation at relatively lower microwave powers, suggesting differences in spin-lattice relaxation properties and radical environments among the three.
Dose Response (0–10 Gy): All three materials showed a dose-dependent increase in ESR signal amplitude.
- Sensitivity: 2-methylalanine exhibited the highest dose sensitivity, followed by lithium formate monohydrate, with L-alanine showing the lowest response.
- Linearity: 2-methylalanine displayed the highest linearity ( $R^2 = 0.9989$ ) and the steepest dose-response slope. Lithium formate monohydrate followed ( $R^2 = 0.9956$ ), while L-alanine showed the lowest linearity ( $R^2 = 0.9803$ ) in this range.
- Low-Dose Performance: The differences in signal intensity became evident above 2 Gy. Below 2 Gy, signals for all materials approached background levels, though 2-methylalanine maintained a more distinguishable and linear response than the others.
Post-Irradiation Stability: Over a one-week storage period, all three materials maintained stable ESR signals with no systematic decrease in intensity. L-alanine showed the highest stability, consistent with its established role, but lithium formate monohydrate and 2-methylalanine also demonstrated excellent short-term stability suitable for delayed readout.

Significance and Claims
The authors conclude that 2-methylalanine holds significant potential as an alternative ESR dosimeter, particularly for the radiologically important low-dose range (<10 Gy). Its superior sensitivity and linearity compared to L-alanine address the limitations of the current reference standard in this specific dose region. Lithium formate monohydrate also presents as a viable alternative with enhanced sensitivity over L-alanine.

The study emphasizes that while L-alanine remains the internationally accepted reference due to its extensive validation and long-term stability, 2-methylalanine offers a promising complementary solution for applications requiring higher sensitivity in the 0.5–10 Gy range. The authors note that further investigation is required to validate practical aspects such as long-term stability, reproducibility, and inter-sample variations before 2-methylalanine can be adopted as a reference dosimeter.

The Invisible Ink of Radiation

The Search for a Sharper Ruler

Technical Summary: Evaluation of 2-methylalanine and Lithium Formate Monohydrate as Alternative ESR Dosimeters

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