Neutron Activation Analysis with Radiochemical Techniques

• Neutron activation analysis with radiochemical techniques is a method that induces radioactivity in a sample via neutron irradiation followed by chemical separation for ultra-trace quantification.
• It employs precise irradiation conditions and advanced radiometric detection, such as beta–gamma coincidence spectroscopy, to achieve sensitivities at or below 10⁻¹⁵ g/g.
• The approach is widely applied in nuclear physics and rare-event radiopurity screening, enhancing astrophysical cross section measurements and rare nuclide detection.

Neutron activation analysis (NAA) with radiochemical techniques is a measurement and separation methodology for quantifying ultra-trace elements or determining nuclear cross sections by inducing radioactivity in a sample via neutron irradiation, followed by chemical separation and ultra-sensitive detection of radioactive progeny. It is central to both nuclear physics (e.g., cross section measurements for astrophysics) and ultra-trace radiopurity screening in rare-event physics, where sensitivities at or below the $10^{-15}$ g/g level are pursued for elements such as uranium (U) and thorium (Th) (Barresi et al., 6 Dec 2025, Gyürky et al., 2019).

1. Fundamental Principles

NAA operates on the principle that a sample containing nuclei of interest is bombarded with neutrons, causing a subset of these nuclei to undergo activation reactions (commonly $(n, \gamma)$ processes). The resultant unstable isotopes decay, emitting characteristic particles or photons detectable via radiometric techniques.

The activity $A(t_{\mathrm{irr}})$ accumulated during irradiation is given by:

$$A(t_{\mathrm{irr}}) = N \sigma \Phi \frac{1 - e^{-\lambda t_{\mathrm{irr}}}}{\lambda}$$

where $N$ is the number of target nuclei, $\sigma$ the cross section, $\Phi$ the neutron flux, $\lambda$ the decay constant, and $t_{\mathrm{irr}}$ the irradiation time. Post-irradiation measurement of decay (γ, β, or delayed signals) allows one to reconstruct the original quantity of the element with careful application of decay corrections accounting for irradiation, cooling, and measurement (Gyürky et al., 2019).

Radiochemical techniques, as deployed in "Ultra-trace analysis of U and Th in organic liquid scintillators with high sensitivity," introduce selective extraction and purification steps that isolate the activated species from background and matrix nuclides, enabling unambiguous measurement even at sub-picogram levels (Barresi et al., 6 Dec 2025).

2. Sample Preparation and Neutron Irradiation

The methodology integrates pre-concentration, pre-irradiation cleanup, neutron irradiation under controlled reactor conditions, and post-irradiation chemical separation:

• Transfer of analytes: For organic matrices such as LAB-based scintillators, U/Th are transferred from bulk organic to aqueous phase by repeated liquid–liquid extraction using 1 M HNO$_3$.
• Pre-irradiation radiochemical separation: Selective chromatographic resins (e.g., UTEVA for actinides) are used to pre-concentrate and clean the analyte before neutron exposure, reducing matrix interferences.
• Irradiation parameters: Typically, the samples are irradiated in reactors like the Pavia TRIGA Mark II, employing thermal or epi-thermal neutron spectra. The detailed fluxes: Central Thimble $1.7\times 10^{13}$ n/cm$^2$/s and Lazy Susan $2.2\times 10^{12}$ n/cm$^2$/s are characteristic operational points.
• Irradiation containers: Chemically inert and radiopure vessels, such as PFA centrifuge vials, facilitate low-background sample handling and compatibility with strong acids.

Effective cross sections ($\sigma_{\mathrm{eff}}$) for key actinides in a reactor spectrum, and nuclear reaction chains, for example:

• $$^{238}\mathrm{U}(n,\gamma)\rightarrow ^{239}\mathrm{U} \rightarrow ^{239}\mathrm{Np} \rightarrow ^{239}\mathrm{Pu}$$, $$\sigma_{\mathrm{eff}}(^{{238}}\mathrm{U})\approx2.68$$ b
• $$^{232}\mathrm{Th}(n,\gamma)\rightarrow ^{233}\mathrm{Th}\rightarrow ^{233}\mathrm{Pa}\rightarrow ^{233}\mathrm{U}$$, $$\sigma_{\mathrm{eff}}(^{{232}}\mathrm{Th})\approx7.41$$ b

Irradiation duration is typically 6 h/day, with total times on the order of 24–30 h per rack, enabling activation of the rare actinide isotopes at ultra-low mass levels (Barresi et al., 6 Dec 2025).

3. Radiochemical Separation Processes

Radiochemical separation is fundamental for mitigating matrix and isobaric interferences, especially in complex or bulk-matrix samples. The protocol includes:

• Liquid–liquid extraction: Multi-stage extraction transfers actinides from organic media to aqueous, achieving near-complete recovery of analytes—typical aqueous phase volume following extraction is 200–300 mL.
• Selective extraction chromatography: UTEVA resin captures U and tetravalent actinides selectively. Post-irradiation, TEVA resin is used to isolate $^{239}$Np (from $^{238}$U) and $^{233}$Pa (from $^{232}$Th), with redox adjustment as necessary to enforce actinide +IV oxidation states for optimal resin affinity.
• Stringency in blanks and consumables: Rigorous cleaning protocols and ultra-pure reagents ensure blanks $<$1\,pg actinide per extraction, with ultimate resin-limited blank levels for U of 0.29 pg and for Th of 0.21 pg per 15 mL elution.
• Typical recovery efficiencies: Pre-irradiation UTEVA step yields for U of $86\%\pm12\%$ and Th of $43\%\pm10\%$ (Barresi et al., 6 Dec 2025).

Radiochemical routes are adapted according to the nuclide's chemical properties and the demands for selectivity and yield (Gyürky et al., 2019).

4. Detection and Measurement Strategies

Radiochemical NAA leverages advanced detection modalities for ultimate sensitivity:

• β–γ coincidence spectroscopy (GeSparK): The primary method for actinide assay is the GeSparK detector, wherein a low-background HPGe is coupled to a liquid scintillator cell, enabling event-by-event coincidence measurements.
  • For $^{233}$Pa (indicative of $^{232}$Th): β decay events ($E_\beta<260$ keV) are accepted only if coincident (±1 μs) with 312 keV ± 10 keV γ in HPGe, suppressing background to $<0.01$ cps.
  • For $^{239}$Np (indicative of $^{238}$U): a delayed-coincidence algorithm identifies a β pulse in LS followed by a delayed (internal conversion or low-E γ) pulse (within 120–1230 ns), coincident with $<300$ keV γ in HPGe. A Bayesian inference framework (JAGS MCMC) fits the expected delayed-time distribution, further suppressing backgrounds by $>10^3\times$.
• Measurement protocols: Extended background runs and blank-subtracted yields are critical for low-count statistics. Table 9 in (Barresi et al., 6 Dec 2025) summarizes achieved detection sensitivities.
• Detection limits: For low background, Currie's formulas are deployed: critical limit $L_C=1.645\,\sigma_B$, detection limit $L_D=3.29\,\sigma_B$ (Gaussian regime), or $L_D=2.71+3.29\sqrt{\mu_B}$ in the non-Gaussian Poisson case.

Complementary detection methods—such as γ spectrometry with HPGe detectors for traditional NAA (Gyürky et al., 2019) or Accelerator Mass Spectrometry (AMS) for long-lived, low-yield isotopes—are also relevant in radiochemical activation studies.

5. Mathematical Framework and Data Analysis

Quantitative analysis relies on precise calibration and correction for nuclear decay, detector efficiency, irradiation conditions, and chemical recovery:

• Mass concentration calculation:

$$C_i = \frac{M_i}{m_S\,N_\mathrm{Av}}\,\frac{R_i}{\sigma_{\mathrm{eff}}\,\Phi\,(1-e^{-\lambda t_{\mathrm{irr}}})\,e^{-\lambda t_{\mathrm{cool}}}\,(1-e^{-\lambda t_{\mathrm{meas}}})/\lambda}$$

where $M_i$ is molar mass, $m_S$ is sample mass, $N_\mathrm{Av}$ is Avogadro's number, $R_i$ is the measured decay rate, and all decay/growth terms are explicitly included (Barresi et al., 6 Dec 2025).

• Decay-factor corrections: The calculation accounts for radioactive build-up and decay during irradiation, post-irradiation cooling, and counting intervals (Gyürky et al., 2019).
• Error propagation: Uncertainties are treated according to the combined contributions from counting statistics, detector calibration, chemical yields, neutron flux, and nuclear data (transition intensities, decay constants) (Gyürky et al., 2019).
• Blank and detection thresholds: Systematic blank runs and statistical analysis determine the practical limits of detection and achievable sensitivities.

6. Achieved Sensitivities and Limiting Factors

The method described attains among the best global sensitivities for U and Th in bulk organic scintillators:

Nuclide	Irradiation Parameters	Sensitivity ($10^{-15}$ g/g)
$^{238}$U	Lazy Susan, 7 days	0.65
$^{232}$Th	Lazy Susan, 30 days	21
$^{232}$Th	Central Thimble, 30 days	1.9

Limiting factors are analyte release from the resin (for U), yielding blank levels that set the ultimate floor, and detector background (for Th), particularly the intrinsic Compton and environmental contributions in the GeSparK system (Barresi et al., 6 Dec 2025). A plausible implication is that further improvements require both reduction of chemical blanks (through alternative resins or direct concentration techniques) and enhancement of detector rejection power, such as improved muon veto or higher-efficiency HPGe.

7. Applications and Prospects for Improvement

Radiochemical NAA is integral to rare-event searches demanding sub-picogram radiopurity, and precise nuclear cross section measurements in astrophysics. The described approach—large-batch pre-concentration, actinide-selective extraction, high-flux TRIGA irradiation, and sophisticated β–γ (and delayed) coincidence detection—embodies the state of the art.

Planned or achievable advances include:

• Replacement of pre-irradiation chromatography with vacuum-concentration ovens to handle $>1$ L samples, which directly increases sensitivity as $1/m_S$.
• Adoption of higher-yield resins (e.g., TRU for Th purification post-irradiation), with manageable blank contributions at this stage.
• High-efficiency HPGe detectors and extended counting times, especially with upgrades to the muon veto system for deep background suppression.
• Transfer of U measurement to the higher-flux Central Thimble contingent upon further blank reduction.

Continuous optimization of actinide chemistry, neutron irradiation geometry, and low-background detection is expected to push attainable sensitivities to or below $10^{-15}$ g/g for both U and Th, supporting future experimental requirements in nuclear and particle physics (Barresi et al., 6 Dec 2025, Gyürky et al., 2019).