The differential cross section for final state radiation from primary quarks is obtained from a study of isolated energetic photons produced in the reaction e+ e- --> Z0 --> hadrons + gamma, as measured in the DELPHI detector at the CERN LEP collider. When combined with the measurement of the total hadronic width of the Z0, the observed rate determines the electroweak coupling constants of up and down type quarks, i.e., upsilon(1/3)2 + a(1/3)2 = 1.13 +/- 0.29 and upsilon(2/3)2 + a(2/3)2 = 1.65 +/- 0.43. No evidence is seen for additional photon production from anomalous decays of the Z0 or from decays of new particles. This measurement leads to upper limits on the production cross section times branching fraction of (a) the Higgs boson in the reaction e+ e- --> Z0 --> H +gamma, H --> hadrons, (b) an excited quark, q* --> q + gamma, and (c) the contribution of an anomalous decay of the Z0 into a photon and hadrons. These limits, all at the 95% confidence level, vary from 3 to 10 pb as the mass of the intermediate state (H, q* or Z*) varies from 10 GeV/c2 to 80 GeV/c2.
