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Design

This collimator was again designed as an attachment to the C0 collimator. The C0 collimator was maintained as part of the design to shield gamma rays and polyethylene was chosen to address the short comings of the neutron sensitivity map, as discussed in the previous sections. This collimator was designed to be more compact than the C1 collimator and hence suitable for use with the geometric constraints associated with imaging the TRIGA reactor. Although borated polyethylene was found to be a marginally better neutron shield, high-density polyethylene was chosen for its cheapness and availability.

The design of this collimator occurred in many stages. The MCNP model of collimator C0 was altered with several variations of an additional shield, the collimator was characterised and the most ideal variant was chosen. The next iteration was then performed. This method was repeated until a suitable (close to ideal) collimator sensitivity map was obtained.

The first stage was to remove the triple peak structure seen in the characterisation of the C0 collimator. This was achieved by surrounding the probe in a polyethylene tube. The thickness was varied and 2.5 cm was found to be thick enough to significantly reduce the sensitivity of neutrons passing through the thin tungsten wall of the collimator. Clearly a thicker polyethylene tube would further reduce this contribution but the trade-off between the sensitivity function and probe compactness would not be optimal.

The second stage was to design the front face of the collimator. The goal here was to ensure the sensitivity map was a narrow band, as uniform as possible in sensitivity. Here the parameters were the overhang of the polyethylene beyond the face of the collimator and the shape of the slot void. An overhang of 1 cm was determined to be optimal and a tapered slit (increasing with radius) was found to provide the most uniform sensitivity region.

(a) Model of collimator C2 first stage model in the z-plane

(b) 3D visualisation of model

(c) Sensitivity map of collimator C2 first stage design to252Cf neutrons calculated with MCNP

Characterisation

The many stages of the design process are not included for brevity, however the models, 3D visualisations and sensitivity maps from the first and second stage of collimator C2 design are shown in Figs. 4.11 and 4.12 respectively.

(a) Collimator C2 final stage model in the z-plane (b) 3D visualisation of model

(c) Sensitivity map of collimator C2 final stage design to252Cf neutrons

(d) Sensitivity map of collimator C2 final stage design to252Cf gamma rays

Figure 4.12 Geometry and MCNP characterisation of the final stage C2 design.

A video of the system matrix transformations is given in Supplementary Video 1, see appendix A.1, demonstrating the sensitivity of the detector to 2D space during a data collection routine. Additionally the system matrix as a function of the imaged field is also illustrated in Supplementary Video 2, see appendix A.1. This video also demonstrates that the neutron system matrix is slowly varying between 1 and 3 MeV.

For further illustration of the design process, MCNP5-calculated values through elevation

stage collimator C2 design has a narrower width and a high contrast when compared with the other two collimators.

Figure 4.13 Plot of normalised MCNP5-calculated neutron counts with ˆα = 0° as a function

of angle ˆβ when scanning252Cf source for collimators C0, first stage C2 and final stage C2.

Validation

The neutron sensitivity map was validated against experimental results obtained at Lancaster University, UK using the252Cf source discussed in section 3.5.1. The imager was located such that the detector was in the same plane as the252Cf source and was 1 m from the source when placed in the exposed position. The source was moved to the exposed position, data were accumulated and the discrimination parameters were set (identical to Fig. 5.11). Using the method outlined in section 3.3.3, data were collected for ˆα =0°, ˆβ =−135° : 135° in 2°

increments fort_d=100 seconds per point. The MCNP-calculated functions were produced in 1° increments and were plotted with the experimentally-obtained values. The results are shown in Fig. 4.14.

Comments

The experimentally-obtained values show close agreement with the MCNP-calculated values between -90° and 90°. Outside this region MCNP underestimates the sensitivity; this mismatch was due to inconsistencies between the real PE shield and the model. Due to the aluminium structural elements on the side of the probe, the PE could not be kept at a consistent thickness of 2.5 cm all the way around and was approximately 1 cm thick in these regions. This allowed a higher flux of neutrons to pass through to the detector from ˆβ angles

Figure 4.14 Plot of MCNP-calculated neutron counts with ˆα = 0° as a function of angle ˆβ

when scanning252Cf source for collimator C2 and corresponding experimental results.

greater than 90° for some elevation angles (estimated to be fromφ = +30° :−40°). This

feature was non-rotational (not a function of ˆα) and therefore was not easily accounted for in

the sensitivity matrix, due to the reliance on rotational symmetry to produce all the elements from a single MCNP characterisation. The computer time would also be extended by a factor of 90. However, as long as data were collected inside the validated range (and the source distribution not extending more than 90° in azimuth), the sensitivity map and therefore the image solutions would be accurate. 1 cm thick cuboidal PE pieces were placed over the low-shielded region to help reduce the disparity in these regions.