Performance Evaluation of Time-Measurement System Using Ultra-fast Plastic Scintillation Detectors in BigRIPS (2)*

The Bρ-TOF-ΔE method is often used to identify nuclides of produced secondary beams at the inflight RI beam facilities. To improve the resolution of the mass-to-charge ratio A/Q, it is essential to improve those of the magnetic rigidity Bρ and time-of-flight (TOF). To achieve this requirement on the time resolution of the TOF, the performance-evaluation experiment was performed using the BigRIPS at the RIBF. A secondary beam containing unstable nuclei around 300-MeV/nucleon ¹³⁰Sn, which was produced by irradiating a Be target with a ²³⁸U primary beam, was used for the TOF measurement.

Two TOF detectors were installed at the two focal planes, F3 and F7, respectively (Four in total). A new TOF detector consists of a Elijen EJ-232 plastic scintillator and two HAMAMATSU R13089 PMTs. The EJ-232 has an ultra-fast response with a faster rise time than an EJ-230, which is usually employed at the BigRIPS, instead of reducing the light intensity. The R13089 PMT has a small transit time spread. For the data acquisition, a TAC-ADC system was used in parallel with the existing TDC to evaluate the effect on the intrinsic time resolution from the signal transport and timing jitter of electronic circuit. The time resolution of each TOF detector was extracted from the corresponding TOF and compared. In addition, the resolution of the A/Q was derived from the TOF measured with the newly developed detectors. The achieved TOF resolution and relative A/Q resolution were 50 ps(σ) and 0.034%(σ), respectively.