An extendable optical fibre probe survey meter for naturally occurring radioactive material (NORM) and other weak emitters

Abstract

We have developed a radioluminescence-based survey meter for use in industries in which there is involvement in naturally occurring radioactive material (NORM), also in support of those needing to detect other weak emitters of radiation. The functionality of the system confronts particular shortcomings of the handheld survey meters that are currently being made use of. The device couples a LYSO:Ce scintillator with a photodetector via a polymer optical fibre waveguide, allowing for "intrinsically safe" inspection within pipework, separators, valves and other such component pieces. The small-diameter optical fibre probe is electrically passive, immune to electromagnetic interference, and chemically inert. The readout circuit is entirely incorporated within a handheld casing housing a silicon photomultiplier (SiPM) detection circuit and a microprocessor circuit connected to an LCD display. A 15 m long flexible PMMA optical fibre waveguide is butt coupled to an ABS plastic probe that retains the LYSO:Ce scintillator. Initial tests have included the use of lab-based mixed gamma-ray sources, measurements being made in concert with a reference conventional GM survey-meter. Characterization, via NORM sources at a decontamination facility, has shown useful sensitivity, covering the dose-rate range 0.10- to 28 µSv h^-1 (R-squared 0.966), extending to 80 µSv/h as demonstrated in use of a Cs-137 source. The system is shown to provide an effective tool for detection of radioactivity within hard to access locations, in particular for sources emitting at low radiation levels, down to values that approach background.

Figure 1

CAD rendering of (A) enclosure for SiPM board providing alignment of fibre to photosensor and blocking ambient light (B) handheld prototype survey meter with extendable fibre optic probe using Autodesk Fusion 360.

Figure 2

(A) Background noise reaching a maximum amplitude of 5 mV, (B) raw signals from fibre-coupled scintillator captured at threshold level 6.5 mV, and (C) Scintillation signal captured using oscilloscope showing amplified signal (yellow trace) and corresponding comparator output (green trace).

Figure 3

(A) Histogram of cpm readings obtained with the DUT, yielding a mean of 1503 cpm and a standard deviation of 89 cpm; (B) Plot of background cpm data taken over a prolonged counting period.

Figure 4

(A) Cs-137 laboratory source with activity of 10.47 µCi used as main radiation source for measurements with DUT, and (B) Response of DUT in CPM for dose rates between background (0.2 µSv/h) and ~ 80 µSv/h.

Figure 5

(A) Response of DUT in CPM at distances up to 12 cm, and; (B) a jar of monazite acting as the source for this part of the study, obtained from laboratory storage and typically used for educational/training purposes.

Figure 6

Handheld prototype being operated in an actual NORM environment, with the probe (A) positioned inside a tubular; (B) illustration of the miniature size of the probe, wrapped in red tape. (C) Response of fiber optic survey meter in cpm at various dose rates around a NORM sample pack with a 2nd order polynomial fitting, and; (D) Experimental setup in the outdoor environment, with NORM dust (~ 700 g) collected in a pack right after the decontamination process. The probe was placed at various distances from the pack.

Figure 7

Density plots of radiation counts (cpm) showing distribution of recorded values obtained over a prolonged counting period, with use of the FORS system for laboratory and natural sources and with probe and source in fixed positions relative to each other.