Speaker
Description
Reactor dosimetry relies on accurate nuclear cross-section data. Activation foils are often the only practical way to measure neutron flux inside a reactor core or near structural components where active detectors cannot survive. Threshold reactions with different energy sensitivities allow unfolding of the neutron spectrum. The high-energy tail is particularly relevant for fusion-fission hybrids, D-T-driven subcritical assemblies, and damage studies in reactor pressure vessels where the 14 MeV component matters such as any future fusion reactors. For reactions leading to isomeric states, the isomeric yield ratio directly affects how foil activity translates to flux. Experimental validation at 14 MeV is therefore needed to benchmark the evaluated libraries and nuclear model codes used in reactor calculations.
NESSA (NEutron Source in UppSAla) is a facility at Uppsala University built around a sealed-tube D-T neutron generator, at present producing up to 5 × 10⁸ n/s at 14.1 MeV. The facility is being developed in phases and recently commissioned with the Sodern Genie 16 generator in December 2025. A higher yield generator of strength 1 x 1010 n/s is scheduled to be commissioned by December 2026. Samples can be placed as close as 3 cm from the target, giving fluence rates of the order of 2.5 x 10⁶ n/cm²/s. Real-time beam monitoring is done with ²³⁵U and ²³⁸U fission chambers, which record the neutron yield throughout irradiation and allow correction for flux variations. The facility is designed to support cross-section measurements, detector testing, electronic damage studies and other work relevant to reactor physics and radiation protection.
As a preliminary study, we have irradiated indium and niobium foils to study several reactions of interest to reactor dosimetry. From indium activation, we observe products from multiple channels: ¹¹³In(n,2n)¹¹²In and ¹¹³In(n,2n)¹¹²ᵐIn (threshold ~9 MeV), ¹¹⁵In(n,n′)¹¹⁵ᵐIn (threshold ~0.5 MeV), ¹¹⁵In(n,α)¹¹²Ag, ¹¹⁵In(n,p)¹¹⁵Cd, ¹¹⁶In via the ¹¹⁵In(n,γ) capture reaction, and ¹¹³In(n,n′)¹¹³ᵐIn. From niobium foil irradiation, we measure the ⁹³Nb(n,2n)⁹²ᵐNb reaction, along with ⁹⁰Y produced via the (n,α) channel. The gamma spectra were acquired with an HPGe detector whose efficiency was determined from FLUKA simulations, and multiple sequential measurements allowed tracking of different decay components.
Cross-section determination requires corrections for neutron self-shielding, gamma-ray self-absorption, and where applicable—feeding from metastable to ground states via isomeric transitions, treated using the Bateman equations. For the ¹¹³In(n,2n)¹¹²In reaction, an additional complication arises from the ¹¹⁵In(n,α)¹¹²Ag channel which produces a 617 keV gamma line overlapping with ¹¹²ᵍIn; the different half-lives (3.13 h for Ag-112 versus 14.88 min for In-112g) allow separation through time-resolved measurements. We have extracted individual cross-sections for both ¹¹²ᵐIn (4⁺, T₁/₂ = 20.67 min) and ¹¹²ᵍIn (1⁺, T₁/₂ = 14.88 min) from the 156.6 keV and 617.5 keV gamma lines respectively. The preliminary results are encouraging—the isomeric ratio σₘ/σg falls in the range 3–4, consistent with TALYS calculations using various level density models and with the expectation from spin statistics that favours population of the higher-spin metastable state. Additionally, activation analysis based on a realistic neutron spectrum at the sample position has been performed using PHITS Monte Carlo simulations, providing an independent check on the expected product yields and helping to validate the experimental methodology.
These first measurements demonstrate that the NESSA facility can produce activation data relevant to reactor dosimetry. The close source-to-sample geometry provides adequate fluence rates, and real-time fission chamber monitoring ensures reliable flux determination. In the presentation, we will report cross-sections and isomeric yield ratios for the reactions studied, compare with evaluated data libraries and TALYS predictions, and discuss the measurement uncertainties achievable with the present setup. Future work will extend to additional dosimetry foils and spectrum-averaged cross-section measurements for reactor and fusion neutron spectra.