Development of detectrors for particle identification of RI beam

RI beam production

Radio isotopes (RIs) are produced via fragmentation reactions using stable nuclei or in-flight fissions of uranium. The primary beam of stable nuclei or uranium is accelerated up to 70% of the speed of light by accelerators. Then, RIs are produced by stripping nucleons from primary beam impinged on a beryllium target. The RI beam (secondary beam) has a similar velocity with that of the primary beam.
Since many kinds of RIs are produced by the reactions, separations and identifications of them are essential to maximize the fraction of an objective nuclide among the produced RIs and to efficiently perform the various measurements for the objective nuclide.

Separation of secondary beam

For the separation of the secondary beam, two dipole magnets, slits, and a wedge degrader are generally used as shown in Fig. 1.
First, the magnetic rigidity (A/Z) of the objective nuclide is selected by the combination of the first dipole magnet and slit. However, the nuclei which has similar with that of the objective nuclide cannot be stopped by the slit. Then, the beam passes through the wedge degrader and losses the beam energy in the degrader depending on the atomic number Z. Finally, the objective nuclide (A, Z) is selected by the combination of the second dipole magnet and slit.
In principle, it is possible to extract only the objective nuclide according to the above method. We need to confirm the extraction of the objective nuclide from the particle identifications. The separation is easily achieved for light nuclei but is difficult for heavy nuclei which neighboring nuclei has similar A/Z values. Therefore, for heavy nuclei, we need to set the slit width narrower to select the objective nuclide and the yield of the nuclide becomes lower. In actual experiments, the slit width is not set so narrower to prevent the much loss of the yield and the objective nuclei is identified from the other nuclei in the secondary beam by particle identifications.

Fig. 1. Schematic view of secondary beam separation

Particle identification

Particle identification is the identification of mass-to-charge ratio A/Z (or mass number A) and atomic number Z of each particle using data such like pulse height and timing which are measured by detectors in a beamline. The -TOF-ΔE method is used in the experiments at RIKEN RIBF. In this method, magnetic rigidity , time-of-flight (TOF), and energy loss ΔE are measured. A/Z and Z can be determined from the following relations of magnetic rigidity and energy loss ΔE. The velocity β (= v/c) is calculated from the measured TOF.

Position detector

A dipole magnet and position detector are used to measure the magnetic rigidity . The position detector is placed at dispersive focal plane which the beam position is different depending on the momentum, and the downstream of the dipole magnet. The magnetic rigidity is determined from the measured beam position at the position detector.
PPAC (Parallel Plate Avalanche Counter) is often used as the position detector. PPAC consists of foil electrodes (thin metallized Mylar), wire grids, and a gas of 10 torr. First, ion-electron pairs are generated by interactions between gas atoms and an incident particle. The electrons are accelerated by an electric field between the electrodes and causes electron avalanche. The signal is read out by delay-line or charge division technique. The incident position can be deduced from the time difference or charge ratio of signals at both ends of read out line. PPAC is often used at RIBF. PPAC has disadvantages such like difficulty of making the electrode, complexity of the gas handling to avoid damage to the thin foil electrodes, and the poor detection efficiency in light mass region.

Time-of-flight (TOF) detector

We need to use some detectors to detect the TOF start and stop signals for the TOF measurement. The plastic scintillation detector (Fig. 2) composed of a plastic scintillation material and a photo-multiplier tube (PMT) is generally used for the TOF measurement. The scintillation lights associating with passage of a particle are converted to photoelectrons in the photocathode of PMT. Then, the multiplied electrons in the PMT are detected as a signal. The plastic scintillation detector has fast response time (several ns) and is appropriate for the TOF measurement. Easy making and low cost are also good points of the plastic scintillation detector. However, to increase the time resolution, it is necessary to make the scintillator thicker to gain more light. It causes gain more energy loss of passing particles.

Fig. 2. The plastic scintillation detector with both side read-out (left/right). The aluminum foils are used for increasing the transport efficiency of scintillation lights to the photocathode of PMT. The black sheets and tapes are used for blocking the environmental lights.

Energy loss (ΔE) detector

Ionization chamber (Fig. 3) is often used for the measurement of energy loss. The ionization chamber is composed of a gas of 1 atm. and electrodes. The ion-electron pairs are produced by interactions between gas atoms and a passing particle in the detector. The produced electrons are guided to the electrodes by an electric field and detected as a signal. The ionization chamber is operated under the electric field condition of ionization region which no electron multiplication occurs. Thus, it is possible to know the atomic number Z of a passing particle because the detected charge corresponds the energy loss and is proportional to Z2
. The energy resolution is as good as semiconductor detectors, and operation under high counting rate is possible. The demerits of the ionization chamber are high material density, low S/N ratio due to small signal, and easily occurs pile up due to the long time constant.

Fig. 3. Appearance of our ionization chamber (left) and electrodes installed in the ionization chamber (right). The electrode is made of an aluminized mylar foil.

Total energy (E) detector

NaI(Tl) scintillation detector (Fig. 4) is often used for the measurement of total energy of a beam. The detection principle is similar with that of plastic scintillation detectors. The beam is stopped in a NaI(Tl) crystal and total energy is measured from the produced amount of light. In the case of a short beam line which TOF measurement is impossible, the NaI(Tl) detector is used for identification of mass number A. The wavelength characteristic of NaI(Tl) is proper to use together with a PMT, and thus the time response is good. The energy resolution is high enough for light nuclei. However, for heavy nuclei, it becomes difficult to separate neighboring peaks come from neighboring isotope.

Fig. 4. NaI(Tl) scintillation detector. NaI crystal is a deliquescent. Metal container is needed to avoid exposure the NaI crystal to air.

Requirements for RI beam detectors

Requirements for RI beam detectors
Followings are general requirements for RI beam detectors.

  • High resolution for time or energy
  • Detection efficiency of ~100%    
  • Stable operation under high counting rate (~1 MHz)
  • Stable operation during more than 1 week
  • Low density material to prevent energy loss and straggling of RI beam as possible

The detectors, which we have been developing and using in experiments, don’t fulfil all above requirements. It is essential to develop the detector to be better for advancement of researches with RI beam. We are dealing with the development of detectors for the experiments of mass measurement or cross-section measurement.