Two recently introduced GafChromic film models, HS and XR-T, have been developed as more sensitive and uniform alternatives to GafChromic MD-55-2 film. The HS model has been specifically designed for measurement of absorbed dose in high-energy photon beams (above 1 MeV), while the XR-T model has been introduced for dose measurements of low energy (0.1 MeV) photons. The goal of this study is to compare the sensitometric curves and estimated dosimetric uncertainties associated with seven different GafChromic film dosimetry systems for the two new film models. The densitometers tested are: LKB Pharmacia UltroScan XL, Molecular Dynamics Personal Densitometer, Nuclear Associates Radiochromic Densitometer Model 37-443, Photoelectron Corporation CMR-604, Laser Pro 16, Vidar VXR-16, and AGFA Arcus II document scanner. Pieces of film were exposed to different doses in a dose range from 0.5 to 50 Gy using 6 MV photon beam. Functional forms for dose vs net optical density have been determined for each of the GafChromic film-dosimetry systems used in this comparison. Two sources of uncertainties in dose measurements, governed by the experimental measurement and calibration curve fit procedure, have been compared for the densitometers used. Among the densitometers tested, it is found that for the HS film type the uncertainty caused by the experimental measurement varies from 1% to 3% while the calibration fit uncertainty ranges from 2% to 4% for doses above 5 Gy. Corresponding uncertainties for XR-T film model are somewhat higher and range from 1% to 5% for experimental and from 2% to 7% for the fit uncertainty estimates. Notwithstanding the significant variations in sensitivity, the studied densitometers exhibit very similar precision for GafChromic film based dose measurements above 5 Gy.

We demonstrate the use of a multiparticle coincidence technique to image the diffraction of an electron wave whose source is placed at a specific site in a free molecule. Core-level photoelectrons are used to illuminate the molecule from within. By measuring the vector momenta of two molecular fragments and the photoelectron, a richly structured electron diffraction pattern is obtained in a body-fixed frame of the randomly oriented molecule in the gas phase. We illustrate this technique for CO, creating a photoelectron from the C(1s) shell and scanning its energy from zero to 30 eV.

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We have used COLTRIMS to measure the angular distribution of electrons released from the K-shell of N 2 and the carbon K-shell of CO by absorption of one linear polarized photon. For each ionization event which leads to two charged fragments we determine the angle of the photoelectron with respect to the fragment ion momenta. In addition we determine the charge state and energy of the molecular fragments. We find a breakdown of the axial recoil approximation for CO for kinetic energy releases below 10.2 eV, whereas for N 2 that approximation is found to be valid for all fragment energies. Furthermore, the photoelectron emission spectrum for N 2 is found to be the same for the molecular breakup channels producing N + N + and N + N ++ .

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