The accuracy of Monte Carlo (MC) simulation results relies on validating the MC models used in the calculations. In this work, a MC model for the NACP-02 plane-parallel ionization chamber was built and validated against megavoltage electron backscatter experiments using materials of water, graphite, aluminium and copper. Electron energies ranged between 6-18 MeV and the chamber's air cavity was at the depth of maximum dose, z(max). A chamber model based on manufacturer's specifications resulted in systematic discrepancies of several percents between measured and simulated backscatter factors. Tuning of the MC chamber model against backscatter factors to improve agreement increased the chamber's front window mass thickness by 35% over the reported value of 104 mg cm(-2) in the IAEA's TRS-398 absorbed dose protocol. The large increase in chamber window mass thickness was verified by measurements on a disassembled NACP-02 chamber. The new backscatter factor results based on the tuned MC NACP-02 chamber model matched the experimental results within 1-2 standard deviations. We conclude therefore that for MC simulations near z(max), tuning of the NACP-02 chamber model against experimental backscatter measurements is an acceptable method for validating the chamber model.

A new calorimeter for dosimetry in small and complex fields has been built. The device is intended for the direct determination of absorbed dose to water in moderately small fields and in composite fields such as IMRT treatments, and as a transfer instrument calibrated against existing absorbed dose standards in conventional reference conditions. The geometry, materials and mode of operation have been chosen to minimize detector perturbations when used in a water phantom, to give a reasonably isotropic response and to minimize the effects of heat transfer when the calorimeter is used in non-reference conditions in a water phantom. The size of the core is meant to meet the needs of measurement in IMRT treatments and is comparable to the size of the air cavity in a type NE2611 ionization chamber. The calorimeter may also be used for small field dosimetry. Initial measurements in reference conditions and in an IMRT head and neck plan, collapsed to gantry angle zero, have been made to estimate the thermal characteristics of the device, and to assess its performance in use. The standard deviation (estimated repeatability) of the reference absorbed dose measurements was 0.02 Gy (0.6%).

We critically compare particle induced X-ray emission (PIXE) on the ion microprobe with scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) for the characterisation of gunshot residues (GSR). Samples of gunshot residue from several different firearms were collected. Individual particles of GSR were analysed by SEM-EDS using a 30keV electron beam focussed to ~10nm and PIXE using a 2.5MeV proton beam focussed to ~4 microns. PIXE revealed trace or minor elements undetectable by SEM-EDS, and could discriminate GSR indistinguishable by SEM-EDS. Introduction In order to demonstrate that a suspect in a criminal investigation has fired a weapon, police authorities routinely take swabs from the clothing, skin and hair of the suspect and use electron microscopy to search for gunshot residue (GSR) particles on the swabs. These particles are condensation products of the high temperature, high pressure reactions that occur when a firearm is fired and are known to be deposited at the crime scene and on the shooter [1]. The particles are made up of material from the primer, bullet, bullet jacket, cartridge casing and the gun barrel, are spherical, and are of the order of a few microns in diameter. There are several types of particle that are characteristic of gunshot residue, including those containing Pb, Ba and Sb [2].

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