Feasibility study of a shielding-independent radiation portal monitor system for revealing 241-Am orphan sources in radiometric surveillance of scrap metal

Abstract

Orphan radioactive sources inadvertently melted in steel plants are a health and economical hazard. Thus, regulation authorities impose strict control measurements to be performed at these plants. However, a recent series of incidents related to the melting of $${}^{241}\hbox {Am}$$ 241 Am sources demonstrated the inadequacy of even state of the art systems in revealing this low-energy gamma ray emitter up to activities of the order of hundreds GBq. However, $${}^{241}\hbox {Am}$$ 241 Am sources are characterized by a weak neutron emission which can be detected by proper monitor systems. This work provides a feasibility study of a shielding-independent neutron-sensitive portal monitor system to be installed in steel plants. Measurements were performed both with a certified $${}^{241}\hbox {Am}$$ 241 Am shielded source and at the weighing area of a steel plant, to assess both the detector performance and the practical feasibility of the system implementation in work-place fields. A risk evaluation analysis demonstrated that with the current control systems $${}^{241}\hbox {Am}$$ 241 Am sources up to 185 GBq could pass undetected with a potential exposure for workers equal to 1.7 mSv, consistent with recent accidents reports. The proposed technique could reveal $${}^{241}\hbox {Am}$$ 241 Am activities down to 2 GBq, decreasing the worker exposure down to 24 $$\upmu \hbox {Sv}$$ μ Sv , without interfering with the standard plant operations.

Calibration of a large-size wide-range neutron spectrometer

“…The second aspect looks a better candidate for explaining the discrepancy and was investigated in better detail. To begin, it is worth considering that the overall amount of helium inside the detector is comparable to the 6 Li content and that, at the energies of interest, the neutron capture of 6 Li has a much lower cross section with respect to 4 He scattering (see Figure 9). Assuming that the scattering reaction between a neutron and a 4 He nucleus can trigger scintillation; the event is counted if the output signal is above the TOT threshold.…”

“…The M800 is a cylinder in which a thin layer of LiF enriched in 6 Li is present. The remaining part of the volume is filled with 4 He at the pressure of 8 atmospheres. The products of the reaction 6 Li(n, α) 3 H excite the gas which de-excites emitting scintillation photons in the UV range, and the collection of the light emitted via silicon photomultipliers (SiPMs) allows the detection of the neutron.…”

“…The tube is divided into eight segments which can be read independently by 24 SiPMs (one triplet per segment). Gamma rejection is obtained accepting only the counts in coincidence of at least two SiPMs belonging to the same segment, and exploiting the amplitude and time characteristics of the pulse recorded [4]. Precisely, the detector allows setting an amplitude threshold and records the time over threshold (TOT) for each pulse, which means that neutrons and γ interactions can be distinguished choosing Fig.…”

An innovative neutron spectrometer for soil moisture measurements

“…The instrument is commercially available and easy to handle (it does not require high voltage, is power supplied by POE, and IP addressable). A detailed explanation of the detector working principle and the outputs produced can be found in [5].…”

“…The Am source is typically used for γ irradiations, but due to its chemical form as an oxide, the nuclear reaction 18 O(α,n) 21 Ne produces a weak neutron emission of about 10 4 n s over 4π ( 18 O is an oxygen isotope with a natural abundance of 0.2%). The Americium source neutron emission has already been measured in a previous campaign [5]. Also in this case, the scattering occurring in the room is expected to alter the neutron energy distribution in the point where the detector is placed, which will therefore be different from…”