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Irradiation facility with thermal neutrons operational


A new facility for the calibration of radiation measuring instruments with thermal neutrons has been set up at PTB. For this purpose, sixteen 241AmBe radionuclide sources were inserted into a graphite block. The entire facility with additional shields has been optimized by means of calculations, and these have been verified via first measurements.

Figure 1 shows the set-up of the irradiation facility located in the basement room 002 of the Chadwick Building.

Figure 1 : Set-up of the irradiation facility with thermal neutrons in Room 002 of the Chadwick Building.

The graphite block has the following dimensions: height = 1.5 m; depth = 1.8 m, and width = 1.5 m. The sources are located in the graphite, at a depth of approx. 1.25 m. Two sources were inserted into each of the 8 boreholes in places located 65 cm and 85 cm, respectively, above the floor. Besides the graphite, also shields made of polyethylene and concrete, as well as the ground soil (due to the fact that the set-up is located in the basement), contribute to neutron moderation.

To reduce the scattered radiation produced by thermal neutrons inside the room and to collimate the useful beam, the upper and front parts of the graphite block are - apart from a surface of 50 cm × 50 cm at the irradiation point where a lead shield was installed to reduce low-energy photon radiation - surrounded by shields of boronated plastic. Another plate of boronated plastic was mounted onto the opposite wall with the aim of minimizing back-scattered neutrons from the useful beam.

The irradiation position (30 cm in front of the set-up and 75 cm above the floor) can be adjusted accurately by means of lasers. In Figure 1, a water phantom is mounted at the irradiation point for the irradiation of personal dose equivalent meters.

Figure 2 shows the schematic set-up as was used for calculations based on Monte Carlo simulations (program PHITS).

Figure 2 : Schematic set-up (top view) of the irradiation facility with thermal neutrons located in the room Chad 002.

The results of the calculations and of first measurements (Bonner sphere spectrometer, gold activation) have yielded the following ambient dose equivalent rate:

                                  dH*(10)/dt = (3.0 ± 0.5) µSv/h

where approx. 85 % are caused by thermal neutrons. This thermalization value is, compared to similar moderator arrangements worldwide, quite high. At IRSN’s SIGMA facility (which has meanwhile been dismantled), only 45 % were attained, and NPL's facility only reached approx. 33 %. Further measurements and calculations are required for a more accurate characterization of the beam and for quality assurance purposes.