Experimental Laboratory Setup

In order to evaluate the curve fitting algorithm, experimental spectra need to be obtained in a laboratory setting. When performing measurements with the XRF system there needs to be sufficient shielding protecting the operator

Material Element Peak Observed Channel Theoretical Energy eV Lead Lead Lα 856 10551.20 Lead Lead Lβ 1021 12613.00 Coin Nickel Kα 608 7477.72 Coin Copper Kβ 722 8906.90 Silver Copper Kα 656 8048.11 Silver Silver Kα 1781 22162.99 Silver Silver Kβ 2007 24943.10

Table 3.1: Known peaks identified from calibration samples and their theo- retical energies.

Figure 3.7: The calibration data showing the relationship between observed channel and energy. This data has been fitted with high accuracy.

a household safe was used to house the XRF system during measurements. This also serves as a secure location to store the equipment. The safe is constructed from 6 mm thick steel and has internal dimensions of 598 mm x 408 mm x 360 mm. It has four 16 mm bolt holes in the base, through which cables that are used to supply power and communications can be fed. These holes can also potentially provide a path for X-rays to escape and to alleviate this a lead floor was installed. This floor was designed using lead partitions attached to its underside in such a way that any X-rays that do reach the bolt holes have to undergo multiple reflections resulting in them being attenuated to safe energies. The safe was tested with a Geiger counter while the X-ray tube was producing X-rays and the radiation levels were no higher than background. To help hold the X-ray detector and X-ray tube in position while analysing a sample, a retort stand is used. Figure 3.8 shows the experimental setup in the laboratory.

Figure 3.8: The X-123 and Mini-X in the safe, held in position by a retort stand for laboratory analysis.

housing is required. It is this field deployment that represents the origi- nal contribution involving the described XRF system. This will feature an end-cap which is specially designed to hold the X-ray detector and emit- ter securely in place. Measurements performed in the laboratory will use this end-cap to replicate conditions that will be experienced underwater. It features a thin window through which X-rays pass, the design of which is important to the overall performance of the system. The material used must have low attenuation of X-rays. It must also be thick and strong enough to maintain its waterproof qualities at depth and to deal with the stresses asso- ciated with landing on the seafloor. The seafloor can potentially be abrasive and there is the risk of damage to the cap through impact while landing.

Based on these considerations, the thickness of the window and the ma- terial from which it is made are important design decisions and represent an original research component. The work presented in (Wogman et al. 1975; Wogman and Nielson 1980) use a beryllium window. Beryllium has very low X-ray attenuation (Hubbell and Seltzer 1995). However, it is a brittle ma- terial and has some toxicity, making it problematic to work with. Therefore it has not been considered for use in the work presented in this thesis. A material consisting of carbon, which also has a low attenuation coefficient, potentially represents a viable option for the end-cap material. A plastic known commercially as Delrin, which is a polyoxymethylene homopolymer with a chemical formula of CH2O, was used to construct a prototype. Del-

rin is a strong material with high abrasion resistance. Tests of the level of attenuation using a 50 c Australian coin as a target were conducted using 3 mm of Delrin compared with no material present. This showed that Delrin resulted in a significant reduction in counts detected and also attenuation of peaks associated with the sample. The only peaks present are attributable to the output spectrum of the X-ray tube, both the tungsten peaks and the continuous bremsstrahlung radiation. This indicates that Delrin attenuates the X-rays to such an extent that it is unfeasible as the end-cap material. An alternative is high density polyethylene (HDPE) the chemical formula of which is C2H4. The oxygen atom in Delrin has the largest attenuation

only carbon and hydrogen atoms, will have lower attenuation. Attenuation is still higher than with no material present, but the expected elemental peaks are clearly visible. The number of counts is also significantly higher than the Delrin. Therefore the end-cap has been designed with a 3 mm window made of HDPE. The attenuation of Delrin and HDPE compared to no material present is shown in Figure 3.9.

The housing has been successfully tested in a pressure chamber to a depth of 100 m.

Figure 3.9: Comparison of the spectra obtained of a 50 cent coin target with X-rays being transmitted through Delrin and HDPE. Also shown is the spectrum with no material present.