Charge-changing reactions of secondary fragments produced in high-energy heavy ion collisions

The authors have begun a program to measure charge changing cross sections of projectile fragments using a quite different technique that is capable of much higher data acquisition rates. The primary beam impinges on a stack of 50 Lucite strips having an average thickness of 3.17 mm, emitting Cerenkov light as its passes through them. Since at a given velocity the intensity of light is proportional to Z2, where Z is the charge of the particle, a fragmentation reaction in a particular strip will be registered as a drop in the light output from that and subsequent strips. The authors use total internal reflection to transport the light to photomultiplier tubes so that there is no wrapping between the strips. Since the energy threshold of the device is approx.1.1 GeV/nucleon, low energy target fragments will not contribute to the signal, a distinct advantage over similar schemes using energy loss to measure the fragment charge. The resolution of the individual strips is typically 0.58 charge units, full width at half maximum, allowing reactions to be well localized even for single unit charge changes. In addition to the C detectors, scintillators and Si(Li) detectors were used to measure precisely the position and charge of the incoming beam particle. The authors have taken data using two beams, 56Fe and 40Ar, at 1.88 and 1.82 GeV/nucleon respectively, and two trigger modes, a free trigger to measure the reaction rate of the incoming beam and an inelastic trigger in which a reaction was required to occur in one of the first 14 C detectors. A total of 909,000 56Fe interactions and 460,000 40Ar interactions have been analyzed so far