miniBELEN Commisioning

Commissioning of miniBELEN-10A: A Moderated Neutron Counter with Flat Efficiency for Thick-Target Neutron Yield Measurements

Adapted from: https://upcommons.upc.edu/handle/2117/394217 by N. Mont-Geli, A. Tarifeño-Saldivia, et al.

Introduction

The production of neutrons through α-induced reactions is crucial in various fields, including nuclear astrophysics, underground physics, and nuclear technology. In nuclear astrophysics, (α,n) reactions play a key role in neutron generation for slow neutron-capture nucleosynthesis (s-process) [1] and contribute to the weak rapid neutron-capture process (weak r-process) [2]. These reactions also generate neutron-induced background radiation in underground experiments [3] and are relevant in fission [4], fusion [5], and non-destructive nuclear assays [6][7].

Despite their importance, existing (α,n) reaction data are often outdated, incomplete, or inconsistent. To address these issues, the Measurement of Alpha Neutron Yields (MANY) collaboration was formed. This project utilizes α-particle beams from accelerators at CMAM (Madrid) [8] and CNA (Sevilla) [9] alongside neutron detection systems such as miniBELEN and MONSTER. miniBELEN is a moderated neutron counter using ³He-filled detectors, while MONSTER is a time-of-flight spectrometer employing BC501/EJ301 liquid scintillators [10]. Additional γ-spectroscopy is conducted using LaBr₃(Ce) scintillators and high-purity germanium detectors [11].

The miniBELEN Detector

miniBELEN builds upon previous BELEN-type detectors [12]-[14]. It is a 4π moderated neutron counter consisting of ³He-filled proportional counters embedded in modular High-Density Polyethylene (HDPE) blocks. These blocks can be reassembled to optimize detection efficiency. The detector configuration, miniBELEN-10A, comprises ten cylindrical ³He-filled counters (60 cm length, 1-inch diameter) with varying gas pressures (4, 8, 10, and 20 atm).

To achieve a nearly flat neutron efficiency up to 8 MeV, some counters are partially covered with cadmium filters (2 cm length, 0.5 mm thickness). The HDPE moderator structure is modular, consisting of 7x10x70 cm³ blocks, assembled using stainless steel rods.

Detector Efficiency

The neutron detection efficiency of miniBELEN-10A was determined using Monte Carlo simulations with Particle Counter [16], a Geant4-based application [17]. Simulations modelled an isotropic neutron source at the detector center, yielding efficiency results presented below,

The nominal efficiency (ε) up to 8 MeV is calculated as:

where ε_k represents efficiency per bin and M is the total number of bins.

The uncertainty in efficiency (δε) accounts for fluctuations and is defined as:

Experimental characterization using ²⁵²Cf neutron sources was performed using the Neutron Multiplicity Counting (NMC) technique [18][19], yielding efficiency discrepancies of less than 5% compared to simulations.

Commissioning of miniBELEN-10A

The detector was commissioned by measuring the thick-target neutron yields of ²⁷Al(α,n)³⁰P at CMAM (Madrid). The 5 MV tandem accelerator provided an α-particle beam, which passed through a stainless-steel collimator biased at -100 V to suppress secondary electrons. The target, a high-purity natural aluminium foil, was placed at the detector center. Beam current was measured using an ORTEC-439 digital integrator, while neutron detection data were recorded using a SIS3316 sample digitizer controlled by Gasific 7.0 software [21]. The experimental setup is shown below,

Thick-Target Neutron Yields

The neutron production yields Y(E_α) at a given beam energy E_α were calculated using:

where r_i is the neutron counting rate per detector, ε is nominal efficiency, and I is beam current.

Background contributions were measured using a tantalum dummy target and interpolated using a cubic spline function. At energies above 5 MeV, background contributions were negligible (<1%), but for 3.6-3.7 MeV, they ranged from 16% to 30%. Dead-time effects were corrected for high-counting rates (>10 kHz per channel). The results, shown below, agree with previous measurements within experimental uncertainties (7%).

Summary and Conclusions

miniBELEN-10A is a modular moderated-neutron counter with a nearly flat response up to 8 MeV. Its commissioning through ²⁷Al(α,n)³⁰P yield measurements demonstrated its reliability, with results consistent with previous experiments. Future applications include further α-n reaction studies.

Acknowledgments

This work was supported by the Spanish Ministerio de Economía y Competitividad (grants FPA2017-83946-C2-1/C2-2, PID2019-104714GB-C21/C22, PID2021-126998OB-I00, RTI2018-098868-B-I00), Generalitat Valenciana (PROMETEO/2019/007, co-funded by FEDER), and the SANDA project (H2020-EURATOM-1.1 Grant No. 847552). The authors thank CMAM for beam time and technical support.

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