Characterization of (252Cf-ZrH2) Monte Carlo model for detection of nitrogen and chlorine by thermal neutron-capture PGNAA

Highlights: • MCNP simulations were performed to model ZrH₂ as alternative moderator for C₂H₄ and H₂O. • ZrH₂ was selected due its superior γ-filtering capabilities to model (²⁵²Cf-ZrH₂) PGNAA analyzer. • The (²⁵²Cf-ZrH₂) model has positively identified nitrogen in melamine and chlorine in sodium chloride. • The Minimum Detectable Concentrations (MDC) were (350 ppm) for nitrogen and (640 ppm) for chlorine. • The model may operate for 20000h within the requirements of the annual dose limit (20 mSv). Monte Carlo calculations were conducted to explore the potentials of zirconium hydride (ZrH₂) as possible alternative moderator for high density polyethylene (HDPE) and water (H₂O) in a simple (100MBq) ²⁵²Cf-based Prompt Gamma Neutron Activation Analysis (PGNAA) assembly. The thickness of moderators was optimized against thermal neutron flux (ϕ_th) as well as the ratio of (thermal/fast) neutron flux (ϕ_th/ϕ_fast), calculated at the sample cavity. The capabilities of moderating ²⁵²Cf source neutrons and minimizing the γ-background due to ²⁵²⁵Cf fission γ-rays, ¹H(n_th,γ)²H γ-rays originates in the moderator as well as other (n_th,γ) capture reactions from surroundings were investigated. The optimal selected moderator with sufficient (ϕ_th) at the sample cavity and minimum γ-contamination was selected as ZrH₂ for use in a²⁵²Cf-PGNAA for detection of nitrogen and chlorine in bulk sample simulants of melamine (C₃H₆N₆) and sodium chloride (NaCl). The prompt γ-rays spectra were analyzed to detect the 10.83 MeV and 6.11 MeV prompt γ-peaks released by ¹⁴N(n_th,γ)¹⁵N and ³⁵Cl(n_th,γ)³⁶Cl, respectively. The results showed that the (²⁵²Cf-ZrH₂)-PGNAA model has positively identified nitrogen with net counts (1.25 × 10⁵ n.cm⁻².s⁻¹) and Chlorine with net counts of (4.19 × 10⁶ n.cm⁻².s⁻¹), achieving minimum detectable concentrations of about (350 ppm at 10.83 MeV) and (640 ppm at 6.11 MeV), respectively.