Monitoring reactions for the calibration of relativistic hadron beams

The well-known foil activation technique was used to calibrate an ionisation chamber employed for the on-line beam monitoring of a 120 GeV c−1 mixed proton/pion beam at CERN. Two monitoring reactions were employed: the standard 27Al(p,3pn)24Na and the alternative natCu(p,x)24Na. The parameters on which the technique critically depends and the adopted solutions are thoroughly analysed are the cross-section, the contribution of the competing reactions to the induced activity and the recoil nuclei effect. The experimental results are compared with FLUKA Monte Carlo simulations and with past results obtained with various calibration techniques. The comparison confirms that both reactions can be effectively employed. The natCu(p,x)24Na reaction shows advantages because its cross-section is known at very high energies with a low uncertainty and the production of 24Na is not affected by competing low energy neutron-induced reactions. The contribution of the competing reactions in the case of the 27Al(p,3pn)24Na reaction has been estimated to be 4.3%/100 mg cm−2, whereas the effect of recoil nuclei is negligible.