Experiments with Sub-Nanosecond Tandem Beam Pulses

The beam pulsing system, which was installed on the Florida State University FN tandem accelerator to prepare the beam for injection into the superconducting linac booster, has been used for a variety of experiments. The beam pulsing system consists of a 48 MHz buncher at the low-energy end and a 24 MHz parallel-plate chopper at the high-energy end, followed by a spiral-loaded cavity phase detector of F.S.U. design. Use of this detector in a phase-locked-loop circuit significantly improves the stability and resolution of the beam pulses. A second high-energy chopper operating at 9.7 MHz was installed to increase the time between pulses from 20 to 100 nsec for experiments. The system has been used to study the 11B(d,n)12C reaction at 12 MeV. Neutrons were detected in a plastic scintillator viewed by two photomultiplier tubes connected to a mean timer circuit. The overall timing resolution of about 500 psec provides adequate time-of-flight resolution at a distance of 2 m. Of particular interest in this reaction are a triplet of states in 12C between 18 and 21 MeV which have recently been studied in several intermediate-energy experiments. Angular distributions have been measured and coincidence experiments are underway to investigate the decay properties of these states. Another experiment has used the pulsed beam in conjunction with the mid-column stripper to search for nanosecond isomers and yrast traps with the 28Si + 56Fe reaction at 103 MeV.