7 Thomson signal and XFI measurements
7.6 Optimisation steps discussion
7 Thomson signal and XFI measurements
0
0 energy [keV]
photo ncount s per 250 eV
Background + XFI-simulation Experiment
20 40 60 80 100100 120 140 160 180 200 0
0 100 100 200 300 400 500 600 700
Figure 7.14: Experimentally measured XFI-spectrum for 18.87 mg/ml GNPs in the mouse phan-tom (green) and the sum of the simulated XFI-scenario and the experimentally measured background (yellow). In this simulation, the incident spectrum was ap-proximated from direct spectrum measurements withµ= 75 keV andσ = 30 keV.
The number of incident photons was 3× 108 in a 60 mm diameter beam with 10 mrad divergence.
7.6 Optimisation steps discussion
Energy 90 keV
Divergence 1 mrad
Number of photons 3×108 Beam diameter 1 mm
Table 7.1: Optimised source parameters for an XFI-scan of the mouse phantom containing 18.87 mg/ml GNPs in the original BOND laboratory setup
tons and electrons. Furthermore, the energy of the electrons colliding with the scattering laser pulse has to be increased by using a different nozzle (bigger diameter), different gas mixtures or injection mechanisms in the laser wakefield acceleration process.
• Collimation of the X-ray beam: so far, the X-ray beam hitting the phantom of inves-tigation was only limited by the diameter of the beam pipe and the opening window in the lead shielding, both having a diameter of roughly 60 mm. Clearly, such a big beam diameter is only useful for a first scan if the overall presence of a fluorescence marker is investigated. For all consecutive measurements, it is necessary to collimate the beam, ide-ally to a pencil beam of 1 mm diameter as the resolution in XFI is only defined by this one parameter. The easiest method to achieve this goal is the design of a copper-collimator with the desired small inner diameter and a thickness of several cm to block all photons outside of the beam volume.
An example XFI-spectrum for the very same setup as simulated in figure 7.14 with the optimised input parameters listed in table 7.1 is shown in figure 7.15. In comparison to the measured spectrum, the Au-fluorescence peaks in the simulated curve can be clearly separated from the Compton scattering background between 70 and 80 keV.
• Shielding of the XFI-setup: as Bremsstrahlung is a very dominant contributor to the total radiation present in the laboratory, it is essential to shield the experimental setup in a way that only the desired Thomson photons can reach the phantom and the detector.
Ideally, the setup has to be positioned far from the electron beam dump where the majority of Bremsstrahlung production happens. This only works at the cost of flux reduction, making this option not very attractive. An alternative method is building a complete lead shielding around the setup with openings only at the beam entrance and exit. Even though different suitable materials exist, lead usually is the element of choice as it offers high stopping power for energetic radiation and its fluorescence lines can easily be separated from the gold lines as they lie several keV apart. Furthermore, a different beam dump
7 Thomson signal and XFI measurements
energy [keV]
photoncounts per 250 eV
Experiment Background + simulation
20 40 60 80 100 120 140 160 180 200 00
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Figure 7.15: The green curve shows the experimentally measured XFI-spectrum for 18.87 mg/ml GNPs in the mouse phantom, while the yellow one is the sum of the simulated XFI-scenario and the experimentally measured background. In this simulation, a monochromatic incident spectrum of 90 keV energy was chosen. The number of incident photons was 3×108in a 1 mm diameter beam with 1 mrad di-vergence.
design has to be considered since the electrons are only deflected into the ground and some lead bricks at the moment, enabling scattering in all directions.
• Flux measurement: a major uncertainty factor in the comparison of measurements with simulations is the number of produced Thomson photons. As the flux on the detector must not be too high in order to still extract a meaningful spectrum, it was not possible to directly measure the produced spectrum for electron charges higher than 2 –3 pC. But since the shape and the mean energy of the spectrum change with different laser powers, it is necessary to determine the exact incident spectrum and also the number of produced photons. Without further optimisation, it was only possible to measure the spectrum at low charges and extrapolate it to higher laser powers, which is only a rough estimate.
Therefore, it would be desirable to obtain a more exact measurement method by e.g.
placing a scintillator crystal into the beam or to use a spectroscopic detector that can cope with a high incident flux. Another option is to place filters made of dedicated materials right in front of the HEXITEC detector to attenuate the photon flux. By knowing the exact thickness of the filters and therefore the attenuation properties it would then be possible to calculate the original spectrum and photon numbers.
88
7.6 Optimisation steps discussion
All of the factors listed above contribute to errors when comparing experimental to simulated data. As neither the beam diameter, nor the mean energy and energy spread of the incident spectrum are known exactly, the number of simulated photons might deviate from the experi-mentally produced flux. Also, the Bremsstrahlung background could not be simulated exactly so far as the laboratory consists of many single components that were not all rebuilt in the GEANT4-scenario. Nevertheless, the results from the simulations under several assumptions still agree well with the experimental data.