Pion photoproduction on the nucleon in the quark model

We present a detailed quark-model study of pion photoproduction within the effective Lagrangian approach. Cross sections and single-polarization observables are investigated for the four charge channels, $\gamma p\to \pi^+ n$, $\gamma n\to \pi^- p$, $\gamma p\to \pi^0 p$, and $\gamma n\to \pi^0 n$. Leaving the $\pi N\Delta$ coupling strength to be a free parameter, we obtain a reasonably consistent description of these four channels from threshold to the first resonance region. Within this effective Lagrangian approach, strongly constrainted by the quark model, we consider the issue of double-counting which may occur if additional {\it t}-channel contributions are included.Comment: Revtex, 35 pages, 16 eps figures; version to appear on PR

Method for the direct measurement in the treatment volume of neutron dose from pion stars

Negative pions are presently being used in clinical trials for the treatment of cancer. At the end of their range, negative pions form pionic atoms which disintegrate. These disintegrations, called pion stars, result in the production of neutrons and heavy charged particles. Approximately 80 out of 100 MeV of total kinetic energy is carried by the neutrons. Therefore, the energy deposited in the treatment volume can be significantly affected by the neutron dose, particularly in the case of large treatment volumes. In the past, there has been no technique for directly observing the dose produced only by neutrons in the treatment volume. A method has been developed which measures that dose resulting from neutron-induced hydrogen recoils. This method is described and preliminary experimental data are presented. The results are compared with corresponding calculations