Double differential ionization of H2O by positron impact

Complete text of publication follows. The study of the intrinsic properties of H2O and of its interactions with various projectiles remains a vigorous multidisciplinary pursuit, spurred by the universal importance of this molecule. In the case of irradiation by positrons (e+), for example, the role that water radiolysis plays in cell destruction (e.g.[1]) is becoming increasingly pertinent due to the widespread usage of positron emission tomography in medical diagnostics. Here, knowledge of integral and differential cross-sections underpins the understanding of the microscopic distribution of energy deposits needed for accurate dosimetry (e.g.[2]). H2O is of special interest due to its strong dipole moment, which has been shown to cause significant forward scattering of electron projectiles [3]. Recently, the fragmentation of H2O has been investigated around 0deg as a function of the energy loss of the positron projectile, at 100 and 153 eV impact energies [4].The main aims of this work have been to investigate whether forward scattering enhances the probability of ECC around 0deg and to measure the intensity ratios of different fragments at various projectile energies. Fig. 1 shows the results for H2O+ fragments at 100 eV positron impact energy. No ECC peak has been found in the double differential electron spectrum, similarly to the earlier results obtained with Ar [5], indicating that the strong dipole moment of the water does not significantly enhance the ECC process for positron projectiles. However, a small shoulder around 28 eV energy loss has been recorded at both projectile energies. This structure may be consistent with the results of high resolution electron momentum spectroscopy ([6] and references therein), identifying 27.1 eV as the onset of a weak shake-up band connected with the 2a1 orbital. This work is sponsored by Engineering and Physical Science Research Council UK (Grant no. GR/S16041/01, EP/E053521/1), Hungarian Scientific Research Foundation (NKTHOTKA, Grant No. 67719, OTKA K73703), the European Union (HPRN-CT-2002-00179 EPIC), and COST-STSM-P9-02555.