Design and Characterisation of an Optical Fibre Dosimeter Based on Silica Optical Fibre and Scintillation Crystal

Abstract

In nuclear power plants, particle accelerators, and other nuclear facilities, measuring the level of ionising gamma radiation is critical for the safety and management of the operation and the environment's protection. However, in many cases, it is impossible to monitor ionising radiation directly at the required location continuously. This is typically either due to the lack of space to accommodate the entire dosimeter or in environments with high ionising radiation activity, electromagnetic radiation, and temperature, which significantly shorten electronics' lifetime. To allow for radiation measurement in such scenarios, we designed a fibre optic dosimeter that introduces an optical fibre link to deliver the scintillation radiation between the ionising radiation sensor and the detectors. The sensors can thus be placed in space-constrained and electronically hostile locations. We used silica optical fibres that withstand high radiation doses, high temperatures, and electromagnetic interference. We use a single photon counter and a photomultiplier to detect the transmitted scintillation radiation. We have shown that selected optical fibres, combined with different scintillation materials, are suitable for measuring gamma radiation levels in hundreds of kBq. We present the architecture of the dosimeter and its experimental characterisation with several combinations of optical fibres, detectors, and scintillation crystals.

Keywords: dosimetry; fibre sensors; gamma radiation measurement; ionising radiation detection; silica optical fibres.

Figure 1

Schematic of the measuring system for ionising radiation measurement. The measuring system contains two variable detection units—SPC and PMT, multimode optical fibre and scintillation crystal hidden in an aluminium box.

Figure 2

The process of grinding/polishing optical fibres with a detailed section, (a) cleaved optical fibre, (b) polished with 30 µ abrasive paper, (c) polished with 6 µ abrasive paper, (d) polished with 3 µ abrasive paper, (e) polished with 1 µ abrasive paper, (f) polished with fine abrasive paper.

Figure 3

Design of the sensor: (a) open sensor construction with scintillation crystal; (b) closed sensor construction with connected optical fibre.

Figure 4

Sample raw data output signals from (a) the SPC and (b) the PMT detectors for 600 s. The graph also shows the average observed countrate.

Figure 5

A comparison of the averaged data from SPC and PMT detectors in the optical fibre dosimeter configuration with an LYSO(Ce) scintillation crystal and FP1500URT optical fibre with a length of 1 m and ID1–ID3 gamma radiation source.

Figure 6

The amount of coupled, transferred, and emitted scintillation radiation depends on the roughness levels for the FP1500URT and FP1000URT. The SPC detector detected the scintillation radiation.

Figure 7

Dosimeter measurement sensitivity comparison using the LYSO(Ce) crystal and different optical fibre links of 1 m length under irradiation of ID1–ID3 sources.

Figure 8

Dosimeter measurement sensitivity comparison with the scintillation crystal coupling to FP1500URT optical fibre with 1.5 mm optical core diameter and 1 m length. (a) The ionising radiation sources: ID1–ID3. (b) The ionising radiation sources: ID4–ID6.

Figure 9

Dosimeter measurement sensitivity decreasing in scintillation radiation delivery with different lengths of the optical fibre link (FP1500URT silica optical fibre) during irradiation of ID1–ID3 sources.

Figure 10

Different conversion efficiencies of ionising gamma rays—⁶⁰Co in different scintillation materials in conjunction with other types of optical fibres and their different lengths.