Tagging of isobars using energy loss and time-of-flight measurements

The technique for tagging isobars in a mixed beam using differences in energy lost in an absorber by different isobars has been tested. As expected, isobar separation does improve by allowing more energy loss (thicker absorbers), but such gains could be realized only after achieving good absorber homogeneity. For heavy ions accelerated to low and moderate energies (<30 MeV/A), we found that when homogenous absorbers are used, the largest impediment to achieving good isobar separation rests with uncertainties in energy caused by straggling in the absorber. Measurements of beam energy loss and energy spread were shown to come close to predicted values when accounting for both collisional and charge-exchange contributions to the calculated energy straggling. Reliable prediction of energy straggling then allowed us to study the efficacy of this method for isobar separation when applied to different mass ranges and beam energies. Time-of-flight was used to measure energy loss since this method allows handling of counting rates in excess of 1 MHz and the demands on timing detector resolution and length of flight path were moderate for all cases under study here. Partial separation in a most difficult case, an analyzed beam of A=132 isobars at energies near 3 MeV/A has been demonstrated. The time-of-flight information can be added to the data stream for events of interest, as an additional parameter (tag) to the online data stream. Such event-by-event tagging enables one to study the effect of difference in isobaric mixture in the beam on the reaction outcome even when isobar separation is not complete.