Fluence, energy and dose measurements

The total fluence of α-particles and therefore the dose delivered to the dish was ultimately determined by the number of traversals of the dish across the slit. However, the fluence per traversal could also be varied by selecting the aperture directly above the ²³⁸Pu source (25, 4.5. 1.4 or 0.5 mm diameter; the 25 mm aperture is larger than the source with a diameter of ~20 mm) and the width of first collimating slit (currently either 1 mm or 7.5 mm wide; 30 mm long).

The fluence of α-particles across the scanned dish was determined using 25 mm diameter fluorescence nuclear track detector (FNTD) discs (Landauer Inc., Stillwater, OK, USA)⁽²⁰⁾ placed directly on the PET base at the centre of the irradiation dish. The FNTDs were subsequently exposed by traversing the dish over the slit five times for the 7.5 mm wide first collimating slit and 50 times for the 1 mm wide first collimating slit. Following irradiation, the resulting tracks were imaged with a Zeiss LSM 710 confocal microscope using a 63×/1.4 oil objective and a 5 mW HeNe laser (excitation 633 nm, collection 634 nm–755 nm). The 1024 × 1024 pixel (135 × 135 μm²) images were averaged over eight collections per slice with a dwell time of 3.15 μs pixel⁻¹ per collection.

Energy measurements were performed using an A300-17AM Passivated Implanted Planar Silicon (PIPS) surface barrier detector (Canberra Industries Inc., Meriden, CT, USA) coupled to an alpha spectrometer comprised of a Model 2003BT charge sensitive FET input pre-amplifier and a DSA-1000 multichannel analyser (Canberra Industries Inc., Meriden, CT, USA). The detector and spectrometer were calibrated using a three peak (²³⁹Pu, ²⁴¹Am and ²⁴⁴Cm) calibration source (Isotrak QCRB2508, AEA technology QSA, Didcot, UK) in a vacuum chamber. The three dominate peaks produced with the detector at the same angle as the dish correspond to α-particle energies of 5.156 MeV, 5.486 MeV and 5.805 MeV with respective full width half maximum (FWHM) values of 18 keV, 14 keV and 13 keV. Measurements were performed with the surface barrier detector directly behind the PET base of a sample irradiation dish, flushed with helium.

As the α-particles traverse the cell and lose energy, there is a corresponding increase in LET. Therefore, the mean dose to the cell is different to the incident dose to the cell at the PET-cell interface and is dependent on distribution of cell thicknesses. The average dose to the cell monolayer at a given depth for α-particles incident on the cell at an angle θ to the normal of the dish base can be calculated using:

where ρ is the density, Φ is the particle fluence and L is the LET in water [this corresponds to D (Gy) = 0.16 × Φ (particles μm⁻²) × L (keV μm⁻¹)/cos θ, assuming a cell density of 1 g cm⁻³]. For a given incident energy, E_i, the variation in energy and LET along the remaining path of the α-particle as it slows down can be determined using SRIM stopping power data for helium ions in water^{(21, 22)}. Equation (1) can then be used to calculate the relative variation in dose per incident particle of energy E_i as a function depth in a cell monolayer of thickness, t. An estimate of the average dose to the cell monolayer was calculated from the measured total fluence, Φ, and numerically integrated across the measured incident energy spectrum, for a 5 μm cell monolayer thickness⁽⁵⁾.