Activation of Mn, Li2O and LiF in the JSI TRIGA reactor to study potential tritium production monitors for fusion applications

Two series of measurements were performed in the JSI TRIGA research reactor in 2014 and 2017 to validate the 55Mn(n,γ)56Mn cross-sections and experimentally investigate the relationship between the 55Mn(n,γ)56Mn reaction and the rate of tritium production through the 6Li(n,t)4He reaction. Indeed, previously observed similarities between the sensitivity profiles of the neutron reaction of tritium production on lithium, 6Li(n,t)4He, and those of the 55Mn(n,γ)56Mn reaction in tritium breeder modules indicated that the latter reaction could be used as an effective monitor of tritium production, at least for short-term monitoring (the half-life of 56Mn being 2.579 h). However, experimental verification, improvements and validation of the 55Mn(n,γ)56Mn cross-sections are needed in order to meet the required accuracy. Foils of certified reference material Al–1%Mn, as well as LiF thermoluminescent detectors and Li2O samples were irradiated, both bare and under cadmium, to study the potential use of the 55Mn(n,γ)56Mn reaction for monitoring tritium production in fusion devices. Additionally, Al–0.1%Au was also irradiated for comparison, the 197Au(n,γ)198Au reaction cross-section being a standard. In order to obtain complementary information for data validation purposes, the irradiations were performed in positions within the JSI TRIGA reactor with different neutron spectra, i.e. in the central channel, the pneumatic tube and the F19 position, both in the outer ‘F’ ring of the reactor core and in the IC-40 irradiation channel located in the graphite reflector surrounding the reactor core. Bare and cadmium-covered irradiations were needed to subtract the contribution of epithermal neutrons to the 55Mn(n,γ)56Mn reaction. Calculations of the reaction rates were performed using the Monte Carlo code MCNP6.1 with a detailed model of the JSI TRIGA reactor, with the samples, the irradiation capsules and covers being modelled explicitly. The uncertainties involved in the measurements and the calculations were carefully evaluated. The principal objective was to study the energy response and correlations between the 55Mn(n,γ)56Mn reaction in irradiated Al–1%Mn and the 6Li(n,t)4He reaction in irradiated LiF and Li2O. Good consistency between the measured and calculated 55Mn(n,γ)56Mn and 197Au(n,γ)198Au reaction rates, in most cases within the uncertainty bars, was observed.


Introduction
In future fusion reactors, such as ITER or DEMO, based on the fusion reaction of deuterium and tritium nuclei (the D-T reaction), the required quantities of tritium fuel will be produced by bombardment of lithium nuclei with neutrons. Several types of special tritium breeder modules (TBMs) will be installed in the ITER reactor to demonstrate self-sufficiency of tritium production. Experiments will be necessary to confirm adequate rate of tritium production in the reactor. A direct method of measuring the rate of neutron-induced tritium production consists of the irradiation of LiF and subsequent determination of the tritium concentration in the material.
LiF pellets commercially produced as thermoluminescent detectors (TLDs) can be used to measure tritium production. In the scope of the fusion for energy (F4E) project of the European Commission we proposed to investigate, as an alternative, the potential use of manganese as a detector for monitoring tritium production in fusion machines. Important similarities between the sensitivity profiles of the tritium production reaction in 6 Li and those of the 55 Mn(n,γ) 56 Mn reaction in the TBMs were observed [1][2][3][4], suggesting that the latter reaction could be used to monitor tritium production, at least in the short term (the half-life of 56 Mn being 2.579 h). However, validation and, possibly, improvements of the 55 Mn cross-sections are needed in order to meet the required accuracy in some energy ranges. The target accuracy in determination of the tritium production rate that is relevant for ITER is 5% (1 -σ). Figure 1 displays reaction cross-sections for the 6 Li(n,t) 4 He, 55 Mn(n,γ) 56 Mn and 197 Au(n,γ) 198 Au reactions. Figure 1 also suggests that the epi-thermal contribution to the 55 Mn(n,γ) 56 Mn reaction has to be excluded, and this can be done by comparing the measurements with/without Cd. Differences in the cross-section evaluations for the 55 Mn(n,γ) 56 Mn reaction were observed, which resulted in differences in the calculated reaction rates. Calculated-toexperimental (C/E) ratios for the Frascati Neutron Generator (FNG)-bulk shield [5], FNG-tungsten (W) [6,7] and FNGhelium cooled lithium-lead (HCLL) [3] benchmarks demonstrate that the differences between the results using IRDFF [8] and IRDF-2002 [9] dosimetry nuclear data libraries are up to 5%, up to 10% and around 18%, respectively. Relatively good C/E agreement was found in the FNG-bulk shield experiment which is most sensitive in the 100-1000 eV energy range where the response function uncertainties are low (~5%). On the other hand, large discrepancies between the measured and calculated reaction rates were found for the FNG-W [6,7] benchmark, in which the 55 Mn(n,γ) 56 Mn reaction rates are highly sensitive to the response function above 1 keV, and in the FNG-HCLL benchmark. However, some systematic errors may be present in the last two benchmarks. In the FNG-W benchmark, the exact Mn content in the foils needs to be verified and in the FNG-HCLL benchmark, heterogeneity of 6 Li enrichment in the Li-Pb block was observed. Although the better consistency observed between the 55 Mn(n,γ) 56 Mn and 197 Au(n,γ) 198 Au results seems to indicate better performance of the new data, it is at present still difficult to confirm with certainty the progress achieved.
In the scope of the fusion for energy (F4E) project of the European Commission, a first series of measurements was performed at the JSI TRIGA [10] reactor in 2014 [11]. Aluminium foils containing 1% manganese (Al-1%Mn) or 0.1% gold (Al-0.1%Au) were irradiated in the JSI TRIGA thermal reactor, together with LiF (TLD) and LiPb samples, with the principal objective of studying the energy response of the 55 Mn(n,γ) 56 Mn reaction as well as the correlation between the 55 Mn(n,γ) 56 Mn reaction rates and the tritium production rates in LiF and LiPb. Some inconsistencies were observed between the measured and calculated tritium activity for LiF (TLD) and LiPb samples which were encapsulated in Pyrex glass vials; these were probably caused by the presence of boron in the glass [11].

Experimental campaign
In 2017, a better defined set of experiments was performed at the JSI TRIGA reactor in which foils of certified reference materials Al-1%Mn and Al-0.1%Au and LiF and Li 2 O samples were irradiated. The samples were irradiated in different TRIGA irradiation channels, i.e. in the central channel (CC), the pneumatic tube (PT) in the core periphery (F24 position in the outer 'F' ring of the reactor core), in position F19 and in the IC40 irradiation channel in the graphite reflector. Irradiations with different neutron spectra provide complementary information for data validation. All the irradiations were performed at a reactor power of 25 kW. Figure 2 displays the JSI TRIGA reactor core configuration used for the experiments (core no. 220).
Reaction rates for the 55 Mn(n,γ) 56 Mn and 197 Au(n,γ) 198 Au reactions were measured using Al-1%Mn and Al-0.1%Au foils, respectively. The Al-1%Mn foils were prepared by pressing Al-1%Mn material in wire form; the foil masses were approximately 10 mg and their diameter was approximately 5 mm. The Al-0.1%Au foils were punched out of Al-0.1%Au material in foil form; the foil masses were approximately 6 mg and their diameter approximately 5 mm. Tritium production measurements were performed using two different types of sample, LiF and Li 2 O, developed and produced in the Institute of Nuclear Physics of the Polish Academy of Science in Krakow.
LiF samples consisted of three TLD pellets wrapped in Al foil. The masses of the single pellets were approximately 34 mg. The Li 2 O samples were in powder form, encapsulated in specially designed Al cylindrical containers and closed with a screw cap. The masses of the Li 2 O samples were approximately 120 mg each.
A set of one LiF sample (consisting of three TLD pellets), one Al container with Li 2 O, one Al-1%Mn foil and one Al-0.1%Au foil were packed together into a cylindrical container, 10 mm in diameter and 22 mm high, with a wall thickness of 1 mm. The containers were either made from Al for bare irradiations or from Cd for Cd-covered irradiations. The containers were inserted into dedicated 3D-printed tubes made from polyethylene. Eight such sets of samples were irradiated in the TRIGA reactor. Figure 3 displays a schematic drawing of one set of samples packaged in the containers and a 3D-printed tube, and photographs of the samples, containers and tubes.

Activation rate measurements in Al-1%Mn and Al-0.1%Au
After irradiation and appropriate cooling times, the induced activities in the Al-1%Mn and Al-0.1%Au foils were measured using two high-purity germanium (HPGe) detectors manufactured by Ortec (model GEM40P4-PLUS-S, model GEM-C5060P4-B), both operated by an Ortec DSpec 50 data acquisition system. The saturation activities per target atom (which correspond to the reaction rates) were calculated from the determined peak areas in the recorded gamma spectra, the measured sample masses and the quoted concentrations for the reference materials used, the irradiation, cooling and measurement times and the calibrated detection efficiency, using  the JSI-developed SPCACT code. In addition to the saturation activity values, the SPCACT code computes their associated uncertainties, which are combined from the uncertainties in the determined peak areas, the sample masses, the irradiation, cooling and measurement times and the uncertainty in the detection efficiency.

Measurements of tritium production in LiF and Li 2 O
Tritium activity measurements in irradiated LiF (TLD) and Li 2 O materials were performed using the liquid scintillation counting (LSC) technique at the Tritium Laboratory of the Faculty of Physics and Applied Computer Science, UST, Krakow, Poland. Direct LSC measurement of tritium activity in LiF (TLD) or Li 2 O requires conversion of the sample to liquid form through chemical dissolution in an aqueous solution of HNO 3 for LiF, and dissolution in distilled water for Li 2 O [12,13]. Dissolved sample solutions (or fractions thereof) were mixed with the scintillation cocktail Ultima Gold µLLT (Perkin Elmer) in 20 ml polyethylene vials (Packard). Measurements were performed using scintillation spectrometers: TRI-CARB model 2550-TR/AB LSC (Packard) and Quantulus model 1220 (Perkin Elmer). Each single vial was measured for 1000 min in 10 cycles of 100 min each. The measurement yield was no lower than 24% in the tritium channel, which corresponds, for the measurement conditions, to a detection limit no lower than 0.01 Bq per sample (vial). Standardization was performed for each set of samples by use of a dissolved NIST tritium standard, SRM 4926 E B, which we additionally verified in the 9th International Tritium Inter-comparison Exercise (TRIC 2012, IAEA, Vienna). The C/E values with estimated errors are shown in figure 9.

Monte Carlo calculations
Calculations of the 55 Mn(n,γ) 56 Mn, 197 Au(n,γ) 198 Au and 6 Li(n,t) 4 He reaction rates were performed using the Monte Carlo particle transport code MCNP6.1 [14] in conjunction with the ENDF/B-VII.1 nuclear data library [15] and a detailed computational model of the JSI TRIGA reactor. In the calculations, the core configuration and the control rod positions were reproduced and the irradiation capsules, covers and samples themselves were modelled explicitly. Figure 4 displays the calculated neutron spectra in the Li 2 O material in the capsules located in the irradiation channels.
The reactor power and control rod positions were kept constant during the irradiations; however, some small perturbation of the neutron flux and the reactor power due to the insertion of Cd covering the capsules was recorded. These detailed power variations have not yet been taken into account in the present calculations. The MCNP calculations resulted in a considerable overestimation of the effective multiplication factor (around 1.05). This is because we did not take temperature feedback and fuel burnup effects explicitly into account; this should be investigated further. However, this overestimation is compensated for in the normalization of the calculated values with regard to the reactor power level [16] according to equation (1): where R abs is the absolute reaction rate value, R MCNP is the raw calculated result, P is the reactor power level during the irradiation, ν is the average number of neutrons emitted per fission, w is the recoverable energy per fission and k ef f is the calculated effective multiplication factor. The MCNP statistical uncertainties were in general of the order of a few per cent, and up to around 10% for the Cd-covered results. Spectral effects due to fuel burnup were seen as a probable cause for the reduction in the thermal flux of the order of about 10% in the central channel and the first fuel element ring [17].

Results and discussion
The results of the comparison between the calculated and the The displayed C/E uncertainty bars include the experimental uncertainties (most importantly due to uncertainties in the detection efficiency and counting statistics) and computational uncertainties (statistical uncertainties, 5% power normalization uncertainty). For the Al-1%Mn and Al-0.1%Au foils the agreement between the measurements and the calculations is reasonably good, in most cases within the uncertainty bars. Slightly larger differences were observed for the IC-40 location (in the graphite reflector surrounding the reactor core), which are probably due to deficiencies of the computational model (e.g. uncertainties in the graphite reflector composition).     A comparison of the calculated and measured tritium activities expressed as C/E ratios is shown in figure 9. The abscissa denotes the irradiation position and cover. The results indicate a systematic overestimation of the calculations for the LiF (TLD) and Li 2 O samples by roughly 20%, and 40%, respectively.
Several reasons for the observed overestimation were identified. Preliminary analysis of measured values for LiF (TLD) indicated differences occurring within a single package of three LiF samples. Typically, the TLD pellet in the middle of the package shows systematically smaller activity (around two standard deviations), most likely due to neutron absorption in the outer pellets. However, this effect does not represent an important source of uncertainty or bias in the comparison between the experimental and calculated values since (a) the experimental tritium production rates in LiF are in fact averages over the three LiF samples and (b) in the calculations, tallies for the 6 Li(n,t) 4 He reaction were defined in the region comprising all three LiF samples.
For the Li 2 O samples, the measured 3 H activities are subject to an additional systematic error coming from the chemical preparation, and caused by the incomplete recovery of the irradiated Li 2 O samples (up to 10% of the initial mass). This explains the outlying measurement points and irregular C/E values for Li 2 O (figure 9). Another source of the observed differences in tritium activity in LiF and Li 2 O samples is the abundance of 6 Li in the compounds used, which is known with an uncertainty no better than ±2%. Deviations from the certified Li abundances are not uncommon and have already been experienced in some previous studies (e.g. [3]). A possibly lower 6 Li abundance than assumed for natural Li may also contribute to the observed difference. The effects of tritium self-absorption in the irradiated material and tritium escape were not taken into account. Table 1 summarizes the main sources of uncertainty in the experimental and calculated 55 Mn(n,γ) 56 Mn, 197 Au(n,γ) 198 Au and 6 Li(n,t) 4 He reaction rates.

Conclusions
In order to study the potential use of the 55 Mn(n,γ) 56 Mn reaction as a tritium production monitor, a series of foils of certified reference materials Al-1%Mn and Al-0.1%Au as well as LiF (TLD) and Li 2 O samples were irradiated in different irradiation channels in the JSI TRIGA research reactor, both bare and under Cd. The irradiations were performed in different neutron spectra, i.e. in the CC, the PT in position F24 in the outer 'F' ring of the reactor core, in position F19 and in the IC-40 irradiation channel in the graphite reflector. A detailed MCNP6.1 computational model was prepared including the irradiated samples, containers, covers and tubes modelled explicitly.
The experimental and calculated results and their associated uncertainties were studied. Good consistency between   198 Au in the F19 location) and to within 20% for the IC40 location.
In the case of LiF (TLD) and Li 2 O samples, the model calculations overestimate the measurements results of tritium activity by roughly 20%, and 40%, respectively. Systematically lower experimental results obtained for both materials can be attributed in part to incomplete recovery of the irradiated material from the capsule (up to ~10%), abundance and/or isotopic inhomogeneity of 6 Li.
The difficulty of sufficiently accurate radiometric assessment of tritium activity produced in the reaction with 6 Li is a major reason for the search for monitors that can measure tritium production by easier procedures. Gamma spectrometric measurements present a promising alternative, and the 55 Mn(n,γ) 56 Mn reaction is a suitable candidate for monitoring neutron flux at the energies relevant for tritium production. It was demonstrated in this paper that, although not originally planned, Mn foil irradiation can serve to verify and validate other direct measurements of tritium production.
Further studies and laboratory experiments leading to improved assessment of correlation (with reduced uncertainty) between induced tritium activity in 6 Li and neutron fluence measured by use of the 55 Mn(n,γ) 56 Mn reaction can deliver a practical and easy-to-use tool for measuring tritium in the TBM (ITER) project.