Performance of the improved larger acceptance spectrometer: VAMOS++

TLDR: In this article, a new detection system built for the focal plane of VAMOS is described, which consists of larger area detectors (1000mm×150mm) namely, a Multi-Wire Parallel Plate Avalanche Counter (MWPPAC), two drift chambers, a segmented ionization chamber and an array of Si detectors.

Abstract: Measurements and ion optic calculations showed that the large momentum acceptance of the VAMOS spectrometer at GANIL could be further increased from ∼ 11 % to ∼ 30 % by suitably enlarging the dimensions of the detectors used at the focal plane. Such a new detection system built for the focal plane of VAMOS is described. It consists of larger area detectors (1000 mm×150 mm) namely, a Multi-Wire Parallel Plate Avalanche Counter (MWPPAC), two drift chambers, a segmented ionization chamber and an array of Si detectors. Compared to the earlier existing system (VAMOS), we show that the new system (VAMOS++) has a dispersion-independent momentum acceptance. Additionally, a start detector (MWPPAC) has been introduced near the target to further improve the mass resolution to ∼ 1 / 220 . The performance of the VAMOS++ spectrometer is demonstrated using measurements of residues formed in the collisions of 129Xe at 967 MeV on 197Au.

Figures

Figure 7. Two-dimensional spectrum of energy loss (∆E) vs total energy (∆E + Er) measured over the full focal plane for the 129Xe+197Au system.

Figure 1. Calculated effective solid angle (msr) of VAMOS as a function of the relative rigidity ∆Bρ, for two different sizes of the detection system, where ∆Bρ = ((Bρ/Bρ0) − 1) × 100 and Bρ0 is the reference magnetic rigidity corresponding to particles moving along the axis of the spectrometer. The dashed curve corresponds to the existing size of the focal plane detection system of VAMOS (400 mm × 110 mm). The full curve corresponds to the dimension of the newly constructed focal plane detection system VAMOS++ (1000 mm × 150 mm).

Figure 11. Two dimensional plot of θ as function of relative rigidity for transmitted trajectories. a) Same as in Fig 2b. for a detector size of 400 mm × 110 mm. b) From the present measurement with a detector size of 1000 mm × 150 mm. c) Simulation for an isotropic distribution of particles entering the spectrometer, for a detector size of 1000 mm × 150 mm.

Figure 10. Spectrum showing the correlation between energy and θlab for the 129Xe+197Au system. The dashed line corresponds to the energy of the elastically scattered particles (Q = 0). This can be used to constrain the excitation energy of the detected fragment (see text).

Figure 9. A typical mass spectrum of the fragments detected in the focal plane corresponding to Fig.8. The dominant peak corresponds to the mass of the projectile.

Figure 3. Schematic view of the new VAMOS focal plane detection system along with the MWPPAC near the target. The coordinate system used in the description of a trajectory is shown. The direction for increasing values of Bρ is also indicated.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Performance of the improved larger acceptance spectrometer: vamos++" ?

Compared to the earlier existing system ( VAMOS ), the authors show that the new system ( VAMOS++ ) has a dispersion-independent momentum acceptance. Additionally a start detector ( MWPPAC ) has been introduced near the target to further improve the mass resolution to ∼1/220. 

Q2. What is the effect of the increase in the size of the detectors?

An increase in the size of the detectors, results not only in particles within a larger momentum window being accepted, but also that the effective solid angle distribution becomes more symmetric and uniform. 

Q3. What is the role of a spectrometer in the study of nuclei?

In recent years, magnetic spectrometers in conjunction with Radioactive Ion Beams (RIB) produced from projectile fragmentation, have been used to explore nuclei far from stability [1]. 

Q4. What is the role of a magnetic spectrometer in characterization of nuclei?

In particular, deep inelastic/transfer reactions have been exploited at both LNL and GANIL for characterization of neutron rich nuclei [8,9]. 

Q5. How many polarized horizontal metal wires are placed at the entrance and exit of the DC?

Since the drift distance is large, 6 polarized horizontalmetallic (guard) wires, placed at the entrance and exit of the DC ensure the homogeneity of the electric field in the drift region. 

Q6. What type of detectors are used in the new detection system of VAMOS?

The new detection system of VAMOS is composed of 4 types of detectors: a) Multi-Wire Parallel Plate Avalanche Counter (MWPPAC), b) Drift Chambers (DC), c) Segmented Ionization Chamber (IC) and d) 40 Silicon detectors (Si) arranged in a wall like structure. 

Q7. Why is the use of - coincidences restricted?

The use of γ-γ coincidences for characterizing nuclei far from stability is severely restricted due to the low cross sections for such exotic channels. 

Q8. How many detectors are sufficient to record all transmitted trajectories?

Calculations indicated that detectors of 1000 mm × 150 mm size is suf-ficient to record all transmitted trajectories satisfying the focal condition of the spectrometer. 

Q9. What was the angle between the segment of the clover detector and the reconstructed velocity vector?

The angle between the segment of the clover detector and the reconstructed velocity vector from the spectrometer was used to correct for the Doppler effect on an event by event basis. 

Q10. What is the role of the VAMOS++ spectrometer in the tagging?

The VAMOS++ spectrometer is also expected to be a key device for tagging reaction products with the next generation ISOL facility SPIRAL2. 

Q11. How can the excitation energy of the detected fragment be constrained?

By selecting a suitable kinematic window parallel to the Q = 0 line (Fig. 10), the excitation energy of the detected fragment can be constrained. 

Q12. What was the effect of the VAMOS on the acceptance of the spectrometer?

Ion optic calculations showed that the acceptance of VAMOS was presently being limited by the size of the existing detectors at the focal plane and that it was possible to further improve the acceptance of the spectrometer by increasing their dimensions (mainly along the horizontal direction) [2]. 

Q13. What is the effect of the polarized wires on the drift time?

This reduces the uncertainty in the measurement of the drift time, b) grounded walls have been added in between the various cathode planes (in each drift chamber) to avoid crosstalk (Fig. 5a).